Green infrastructure means introducing vegetation on/in man-made buildings and constructions to provide environmental, economic, and social benefits (i.e., clean air and water, climate regulation, food provision, erosion control, and places for recreation) [1]. Green roofs advantages over traditional solutions include reduction of surface runoff in large cities, reduced urban heat islands, support to biodiversity, improvement of the durability of waterproofing materials, increase of energy savings in buildings, and enhanced carbon sinking capacity [2]. 

Left: intensive green roof in Singapore; right: extensive green roof in Lleida (Spain) [2]

Green roofs are usually made of the following layers (from top to bottom): vegetation layer, substrate layer (usually topsoil or garden soil), filter layer (usually polypropylene or polyester geotextile membranes), drainage layer (being polyethylene or polystyrene modular panels, or porous stone materials with some retention capacity), protection layer, and root barrier and waterproofing layer [2,3]. 

Energy balance in a green roof system [1] 

Green roofs can be extensive or intensive. Extensive green roofs have a thin substrate layer, are light weight, do not feature irrigation systems, and are planted with resistant species. Intensive green roofs have larger substrate thicknesses, include an irrigation system, and the plant species used are those of typical traditional gardens. 

A careful selection of plants in view of the local climatic conditions, building characteristics, and type of system configuration is very important [3,4]. Selecting an appropriate type of plant will greatly affect the performance of the system because different types of plants have different characteristics, including plant trait, leaf area index, foliage height, albedo, and stomatal resistance. In addition to that, the selection of plants also depends on a few factors, such as preferred visual effect, availability of plant species, and requirement of an irrigation system. 

MATURITY:  

Green roofs are mature and in the market (TRL 8-9). Nevertheless, there is research going on to improve some of their components (TRL 3-6). 

Passive building design means providing passive heating, passive cooling, and natural ventilation to maintain comfortable indoor conditions with no need for energy, by taking advantage of location (climate), orientation, massing, shading, material selection, thermal mass, insulation, internal layout and the positioning of openings to allow the penetration of solar radiation, daylight, and ventilation in the desired amounts [1–8]. When duly applied, passive design strategies are a designer’s first opportunity to increase a building’s energy efficiency, without adding much less front-end cost to a project as compared to active design strategies. Efficient passive design results in smaller heating and cooling loads (so that the building’s mechanical system – if any – can be downsized) and smaller electric loads for lighting through the use of daylighting design strategies.  

Beyond local climate, building orientation is a key aspect for passive design. The most energy-efficient designs are facing south or north to allow better solar energy management and better quality of daylighting. Building shape is also very relevant in the design, as an elongated and narrow plant (with south or north facing façade) allows for more of the building to be receive daylight. Shading strategies properly combined with other passive design strategies are also required, especially in hot climates [9,10]. Since the main difficulty in designing natural ventilation systems driven by buoyancy and wind is the simultaneous estimation of ventilation airflows and indoor temperatures, solar chimneys are used [11,12]. A solar chimney is a vertical shaft utilizing solar energy to enhance natural ventilation. 

Passive heating can be achieved by capturing the heat from the sun inside the building. Tweaking the window-to-wall ratio and the building exposure to the sun, all the while controlling for the thermal mass, heat flows and insulation allows to effectively store, distribute and retain the heat. The thermal mass defines the capacity to absorb, store and release heat. Heavyweight construction materials like concrete, brick and stone exhibit large thermal mass that can be used to effectively store the heat over peak hours and release it overnight. 

Passive building design. Figure from

Passive cooling is a set of design strategies to reduce heat gains and favour heat dispersion. Many methods exist and include using solar shadings as well as designing openings in such a way to allow good ventilation (such as solar chimneys). Shading can either be operable (external louvres, blinds, and deciduous trees) or fixed (e.g. eaves, overhangs, fences and evergreen trees). 

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Passive design strategies are rated in different standards, such as PassivHaus (Passive House), BREEAM, LEED or WELL. 

The literature shows that today there are many net-zero, nearly-zero energy, and certified Passive House buildings worldwide, in different climate or geographic regions. Most are in Europe and North America, followed by New Zealand, Kore, Japan, China, and India [1]. Literature also shows that is it possible to achieve at least the Passive House energy standard of performance in all climate zones [13]. 

Photovoltaic (PV) systems represent the most used technology to convert solar radiation into electricity. A PV module is a grid of cells of semiconductor material, which is able to convert sunlight (photons) to electricity (voltage potential). The electricity from a module is generated in the form of direct current (DC), hence an inverter is required to convert it to alternating current (AC). A single PV cell is small (6”x6” square) and can produce only few Watts of power (1/2 Watts) of direct current electricity (DC), however, most solar panels on today's market produce between 250 and 400 Watts of power. The efficiency of a PV cell is affected by the material of the semiconductor and the PV cell technology. Most PV modules are based on crystalline silicon solar cells, which combine good efficiency and relatively low cost. Other PV cells technologies available on the market include thin films, which consist of thin layers of semi conductive material for enhanced flexibility and lightweight, suitable for portable applications or to be used as a building integrated PV system (BIPV). Some types of thin-film PV cells are easier to manufacture and scale up compared to silicon cells but they are characterized by a lower efficiency [1]. Other technologies include III-V single- and multi-junction solar cells made from elements from the groups III and V, and next generation cells which are manufactured using organic materials, quantum dots and perovskites (hybrid organic inorganic material) [2].  

PV modules can be connected and arranged in a PV array. 

In addition to the PV panels, a PV system includes wires to connect modules, junction boxes, mounting hardware, power electronics to manage the PV array’s output and a charge controller to manage the energy storage (i.e. batteries) [3].All those components are part of the Balance of system (BOS).  A PV system can be grid connected or off-grid.  Most typically, PV panels are installed on roofs or on the ground. In the latter case, PV arrays can be mounted in a fixed position (usually facing south at EU latitudes), or on devices that track the sun to increase the energy production. 

Source https://www.renergy.com.mt/projects-domestic-pv-panels-malta/   

Buses are an integral part of urban public transport systems. However, urban buses are responsible for about 8% of the road transport Green House Gas emissions in EU [8]. Buses with internal combustion engines are responsible also for emissions of fossil carbon dioxide (CO2) as well as air pollutants particulate matter (PM) or nitrogen oxides (NOx). Thus, zero emission buses, have a significant potential in the reduction of transport related emissions in urban areas. 

Zero emission buses use an electric motor with batteries or fuel-cell with hydrogen tank for propulsion. Electric buses use an electric propulsion motor and the energy is provided either by batteries (Battery electric vehicles, BEV) and also, mainly in older installations by overhead wires (trolleybus) or inductive systems embedded in the road.  The battery trolleybus is a less used solution that can still use overhead wires for propulsion but can ride independently, on a part of its route which is not equipped with wires using a battery [2]. Different charging technologies are available for electric buses: manual plug-in connection and automatic connections, like pantograph-type connections from the top or the bottom of the vehicle [3]. Moreover, different BEV charging approaches can be used, including overnight depot charging, alone or supported by opportunity charging (e.g. at the end of the line or at stops) during the daily use. 

Fuel-cell electric buses contain a hydrogen tank, and the fuel cell which converts hydrogen into electricity then supplied to the electric motor, directly or after being stored into the on-board battery. Fuel cell provides all of the energy for the vehicle, while batteries offer peak power for the motor during the acceleration and regenerative braking phases.  [4].  

The European Clean Bus Platform offers E-Bus Decision Support Toolkit [6] and the Operators’ guide to fuel cell bus deployment [5] developed by the JIVE project for HFC buses bring together European cities, transport authorities and operators to share experience, exchange of knowledge and support deployment of zero emission buses. 

The European Commission’s software tool VECTO, can be used to calculate the energy consumption and range, to support the purchasing process of new buses [11]. 

An electric car uses electric power instead of an internal combustion engine powered by liquid or gaseous fuels like diesel fuel, gasoline, LPG or methane. It has been increasingly recognized that electric cars provide an opportunity to reduce global GHG emissions [3] and greatly increase air quality.  

Even though the emissions by a vehicle determine only a part of total emissions related to mobility (apart from emissions during production of energy, production of a vehicle and its shipping and maintenance etc.), the transformation to net zero emission electric car fleet, combined with green electricity, could reduce CO2 emissions 10–12 times comparing to fossil-fuel vehicles [5]. Their widespread adoption also may help to decrease noise pollution in cities [6].  

The adoption of electric car accelerates – the share of battery electric vehicles sales reached 5.4% in 2020 and 9,1% of new vehicles in 2021, while plug-in hybrid respectively 5,1% and 8,9%. A rapid growth is expected in major markets [2], although in some markets sales already reached high percentages (e.g. 80% of new sales in Norway or 30% in the Netherlands), proving that these vehicles are mature for most applications. However, a broader adoption of electric cars still faces several obstacles.  

The main challenges for electric car adoption are high purchase costs in the lower vehicle segments, the still insufficient charging infrastructure in some parts of Europe and their charging time on longer trips.  

The purchasing cost of an electric car shall not be confused with its Total Cost of Ownership (TCO) that includes the purchasing price, but also running costs, repairs, taxes, maintenance, depreciation and resale values. While purchasing cost can be still unaffordable for lower-income customers, also due to the increases in the prices of critical minerals which are crucial for battery (in particular cobalt and lithium [4]), the TCO can be already equal or lower than an internal combustion engine for customers driving high yearly mileage.  The range of electric cars can cause ‘range anxiety’ in drivers mainly due to lack of awareness of recent improvements in performance (most current vehicles have running ranges from 300 to 500km) and deployment of recharging infrastructures (in fact, in some countries this problem is much less perceived than in other). Whoever has access to a private garage or parking place at home or at the workplace can easily cover 90% of the needs by installing a wallbox there, while public infrastructure is essential to allow other users to charge.  

A parallel deployment of vehicles and their infrastructure is essential to accompany the growth of this market, but an increased popularity of electric cars can in principle also create an overload risk for existing electricity grid in the unlikely case that all vehicles charge at peak time, but work is already underway to support the balancing, demand-management and resilience of a ‘smart’ grid via Vehicle to Grid (V2G) features. Thus, electric car mass-market requires some investment in grid infrastructure to meet this increased demand (World Economic Forum, 2022).  

Therefore, new solutions need to be developed such as: 

• Reduce initial cost and improve convenience by improving battery energy density and reducing the cost per kilowatt-hour (kWh) of batteries (cost already decreased by almost 90% in last decade, with a 7-fold increase in density) [1]; 

• smart and flexible charging in public parkings, i.e. schedule charging based on power constraints, price and priority, selling unused energy back to the grid; 

• smart energy management, improving electric cars charging and stationary load management, reducing the risk of grid overload and using electric cars as grid energy storage; 

Source: https://afdc.energy.gov/vehicles/how-do-all-electric-cars-work 

In order to speed up electric cars adoption, cities can implement different measures, including provision of free or preferential parking for electric cars, developing a wide range of publicly available chargers (mostly low power, but including also a small share of fast chargers), facilitating the installation of private charging points in residential and office buildings (right to charge) offering access to priority lanes for electric cars (e.g. limited to shared electric cars fleets), introducing zero-emission zones, electrifying the municipal vehicle fleets, simplifying administrative processes to build charging points, providing local subsidies for electric cars purchase or tax write-offs for companies or citizens willing to install charging points, facilitating zero emissions car-sharing schemes, multimodal integration with public means via park & ride or long distance travel. 

DESCRIPTION:  

Connected, Cooperative and Automated Mobility (CCAM) refers to the development of user-centered, inclusive and shared mobility concepts, for people and goods, that are based on automation and connectivity. These innovative mobility concepts aim to complement and integrate the EU transport system with the aim to make it safer, smarter and more sustainable. CCAM has the potential to transform the way we move and the way we travel: it can decrease transport accidents while increasing performance, make traffic more efficient, reduce congestion, and enhance the inclusiveness and resilience of mobility services. From (4) “In road transport, it can provide more safety, better social inclusion and higher efficiency; in railways, it enhances the performance of the overall system, including train operations, traffic management, maintenance and it creates opportunities for new mobility services; in waterborne, it can improve the safety of shipping and the efficiency of transport and logistics as well as benefit the environment”. The integration of CCAM solutions into the overall transport ecosystem, coupled with smart traffic management, could increase road infrastructure capacity, thereby reducing transport emissions, congestion and pollution. In rail, CCAM could offer safer and more efficient systems leading to optimised automated urban rail operations, fully leveraging the benefits of digitalisation and automation. Regarding inland waterway transport, CCAM could reduce the risk of human error in logistics operations, which account for a large share of accidents, by improving vessel technologies and systems, as well as enhancing data integration and connectivity. In urban air transport, CCAM relates mainly to urban air mobility, through concepts such as on-demand transport of people and goods using aerial drones.  

Figure 1: Tram perception system concept from the TAURO project (5) 

Most importantly, when thoroughly planned, CCAM solutions could make mobility fair and accessible to all, particularly to vulnerable user groups, which includes women, children, disabled persons, the elderly, but also people from low-income backgrounds and people living in rural or peri-urban areas. Nevertheless, a fragmented and incoherent deployment of CCAM could have negative rebound effects at both societal and environmental level (i.e. low acceptability, increase in urban sprawl, rise in GHG emissions, and lack of synchronisation of mixed traffic situations between automated and conventional vehicles)(5). A long-term vision and strategy, from research to regulation, as well as proper governance models will be key to deploy meaningful CCAM systems and technologies on our roads. Making CCAM solutions sustainable and futureproof requires an alignment between public and private efforts at local, national and international level, as well as a coherent and harmonised R&I agenda. Advancing and maturing CCAM technologies will enable trustworthy interactions between all traffic participants, allow a better use of public space and improve the infrastructure capacity of our cities.  

As we move towards climate-neutrality in Europe by 2030, the development and large-scale deployment of CCAM has to answer to individual and collective needs, values and expectations, to ensure a positive impact. Proactive planning, anticipation, reflexivity and responsiveness, in line with local policy goals, will help to identify potential bottlenecks (at technical, policy or regulatory level) and create user-centered, sustainable and inclusive mobility solutions for all (4). 

Figure 2: Autonomous shuttles part of pilot trials in the SHOW project (7)

Note: For the description of Cooperative Connected and Automated Mobility, the term vehicle will be used to refer to any means of transport (road vehicles, vessels, trains).  Whenever necessary to refer to a particular type, the transport mode will be mentioned. 

MATURITY:  

The maturity of CCAM technologies varies greatly depending on the transport mode and level of automation.  

• Road transport: connected and autonomous driving for road vehicles has been intensively researched, with many projects carrying out field tests and pilot trials, and real life case studies. Such is the case for the CoEXist (a), AVENUE (b), AUTOPILOT (c), SHOW (d), ENSEMBLE (t) and L3PILOT (u) projects.  

• Vessels: The AUTOSHIP project (n) has tested autonomous vessels at TRL 7, with TRL 9 expected after 2023, while the EGNSS Hull-to-Hull (o) project has developed systems to be used in such vessels, with TRL 8. 

• Railway: Automated underground metro systems are already commercially available and deployed in various cities. Automated surface rail operations are been researched by the Shift2Rail Joint Undertaking (7), especially within the TAURO (f), X2RAIL-4 (g), SMART2 (h) and CONNECTA-3 (i)  projects, and will be continued through the Europe’s Rail Joint Undertaking programme (8). The technologies are currently in TRL 4 to 6, and are expected to reach TRL 8 or 9 by 2030.    

Solar thermal panels are devices that convert solar radiation into heat and transfer it to a heat transfer fluid (typically water or an anti-freeze fluid) to be stored or to reach the point of use. Solar thermal systems are usually implemented with a water storage tank, to reduce the production-demand fluctuations, ensure high operational safety and good performance, and guarantee at least 25 to 30 years of lifetime.  The most common application is solar heating to produce thermal energy and domestic hot water (DHW) which represents more than 90% of the applications on the global market [1]. In this case, solar thermal panels are included in a solar system, where collectors are usually coupled with a water storage tank. Solar thermal systems can also be employed in large scale heating systems including district heating networks [2,3]. Solar thermal collectors can be mainly divided in two types: tracking and stationary solar collectors [4].Tracking solar collectors are able to track the sun and concentrate the direct solar radiation through lenses and mirrors to the receiver. These collectors are suitable for medium-to-high temperature applications and include parabolic trough collectors, linear Fresnel reflectors, parabolic dishes, and central receiver systems [5]. On the other hand, stationary solar collectors are installed in a fixed position and are more suitable for low-temperature thermal energy. The most typical solar thermal stationary modules are Flat-Plate Collectors (FPC) and Evacuated Tube Collectors (ETC).  

Source: https://www.onosisolar.com/solar-collectors/flat-plate-solar-thermal-collector/ 

FPC is the main solar collector technology installed in Europe with more that 2 million m2 installed per year [6]. FPCs are suitable for low-medium temperature applications including domestic hot water, heating, preheating and combined systems. FPC consists of tubes carrying a heat transfer fluid placed in an insulated, weather-proof box with a dark absorber material and thermal insulation material. FPC can be installed as a thermosiphon system, in which the circulation of the heat transfer fluid is induced by the density difference caused by the increase in temperature, or as an active system with a pump [1]. The simplicity of construction makes FPCs a relatively low-cost solution, which can be installed as single modules on roofs, or manufactured in a larger format for roof or facade integration or for ground-mounted systems. For the production of FPC, different materials can be used including copper, aluminium, staineless steel combined with a glazing to achieve a high efficiency. 

Source: https://www.viessmann.co.uk/products/solar-thermal/tube-collectors/vitosol-300-tm  

  
In ETC systems, the absorbing plate and heat pipe are located in vacuum-sealed glass tubes to improve solar radiation absorption and reduce heat transfer losses, thus achieving a greater performance that allows heat production even in winter with low-light conditions and cold ambient temperatures. Both FPC and ETC are insulated to prevent heat losses, however ETC has a greater performance ratio(up to 80% [7])  due to its design with vacuum insulated glass tubes.  Moreover, ETCs are suitable to hot, mild, cloudy or cold climates while FPCs have some limitations [5].There are two types of ETC systems. One type allows the heat transfer fluid to flow in and out of each tube absorbing directly the solar radiation, while in the other type, a copper heat pipe is used inside the tubes as efficient thermal conductor, which contains a small amount of non-toxic fluid as heat transfer medium that undergoes an evaporating–condensing cycle releasing heat from the tip of the heat pipe to the heat transfer fluid. 

Solar thermal panels are devices that convert solar radiation into heat and transfer it to a heat transfer fluid (typically, water or an anti-freeze fluid) to storage or the point of use. Solar thermal systems are usually implemented with a water storage tank, to reduce the production-demand fluctuations, and have high operational safety, a great performance ratio, and can account for at least 25 to 30 years of lifetime.

Solar thermal collectors can be mainly divided into two types: tracking solar collectors and stationary [1]. The first is able to track the sun concentrating it through lens and mirrors to the receiver using direct solar radiation. These collectors are suitable for medium to high-temperature applications and include parabolic trough collectors, linear Fresnel reflectors, parabolic dishes, and central receivers [2]. Stationary solar collectors are, on the other hand, in a fixed position and are more suitable for low-temperature thermal energy (<100ºC). The most typical solar thermal stationary modules are Flat-Plate Collectors (FPC) and Evacuated Tube Collectors (ETC). This factsheet in particular focuses on ETC. 

Components of evacuated tubes solar collectors 

Source: Supankanok, R., Sriwong, S., Ponpo, P., Wu, W., Chandra-Ambhorn, W., & Anantpinijwatna, A. (2021). Modification of a Solar Thermal Collector to Promote Heat Transfer inside an Evacuated Tube Solar Thermal Absorber. Applied Sciences, 11(9), 4100. 

  
In the ETC, the absorber plate and heat pipe are located in vacuum-sealed glass tubes to improve solar radiation absorption and reduce heat transfer losses, achieving a greater performance ratio that allows heat production even in winter with low-light conditions and cold ambient temperatures. Both FPC and ETC are insulated to prevent heat losses, ETC has a greater performance ratio to its design with vacuum-insulated glass tubes. Moreover, ETCs are capable to work in hot, mild, cloudy, or cold climates while FPC has some limitations [2]. 

 
There are two types of ETC, the first one allows the heat transfer fluid to flow in and out of each tube absorbing directly the solar radiation, while the second feature a copper heat pipe inside the tubes as an efficient thermal conductor, which contains a small amount of non-toxic fluid as heat transfer medium that undergoes an evaporating–condensing cycle releasing flowing and releasing heat at the tip of the heat pipe to the heat transfer fluid. 

Evacuated tubes solar collectors in rooftop installations 

Source: https://www.viessmann.co.uk/products/solar-thermal/tube-collectors/vitosol-300-tm 

 
ETC, are usually combined with auxiliary systems (e.g. biomass boiler, heat pumps, etc.) to supply the needs when no solar radiation is available. ETC can be used for domestic applications for space heating and domestic hot water (DHW) supply (60-90ºC). For process heating in industries, ETC can reach higher temperatures (150ºC/180ºC) combined with high-temperature heat pumps. ETC combined with adsorption/absorption can be used for cooling supply. The main drawbacks are its mechanical fragility and mostly economic due to a greater initial investment cost [3]. 

Hydrogen used as a fuel has emerged as an alternative fuel with multiple benefits. It is commonly used in its gaseous form to power a fuel cell, generating electricity, where it is completely emission free at point of use, with zero tailpipe emissions. Fuel cells are especially useful in-vehicle applications where the energy and power density of batteries is not sufficient, with also the advantage of low refuelling times.  

In urban transport, there are some vehicle categories that benefit the most with the use of hydrogen, it being mostly related to the need for long autonomy, or the need for fast refuelling, as compared to the longer recharging times for battery powered vehicles. Such is the case for buses, waste collection trucks, taxi fleets, delivery trucks and inland waterway vessels in which BEVs encounter challenges and FCEVs show great promise.  

The successful deployment of hydrogen-powered transport for the reduction of emissions in cities depends on some factors. Firstly, the market availability of mature, efficient, and less costly technology related to hydrogen/fuel cell-powered vehicles and vessel technology is essential. Moreover, the availability of hydrogen refuelling stations, with high safety standards and reliability, is necessary to enable refuelling of fleets. Finally, the way that hydrogen is produced determines its environmental impact. Fossil-based hydrogen is mainly produced from natural gas, emits CO2 during its production, and thus is not environmentally friendly. This production method can be more environmental friendly if it is combined with carbon capture techniques, known as fossil-based hydrogen with carbon capture. Finally, renewable hydrogen is produced by water electrolysis with renewable energy. 

In the EU, the majority of research was carried out under the Fuel Cells & Hydrogen (FCH) Joint Undertaking I and II (for the 7th Framework Programme and Horizon 2020), which is followed by the Clean Hydrogen Joint Undertaking for Horizon Europe (1). 

Thermal energy storage (TES) systems can store heat or cold to be used later, under varying conditions such as temperature, place, time, or power. The main use of TES is to overcome the mismatch between energy generation and energy use [1]. The main requirements for the design of a TES system are high energy density in the storage material (storage capacity), good heat transfer between the HTF and the storage material, mechanical and chemical stability of the storage material, compatibility between the storage material and the container material, complete reversibility of a number of cycles, low thermal losses during the storage period, and easy control. Moreover, one design criteria could be the operation strategy, the maximum load needed, the nominal temperature and enthalpy drop, and the integration into the whole application system. 

Already in 2011, Arce et al. [2] calculated the potential of load reduction (L), energy savings (E), and climate change mitigation (as CO2 emissions reduction – RCO2) of TES in buildings in the EU:

There are three technologies of TES systems, each one with a different performance, which will drive which technology each one is more appropriate. Moreover, each technology is in a different maturity status. Sensible TES is when the energy is stored by increasing or decreasing the temperature of a material (i.e., water, air, oil, bedrock, concrete, brick). Latent TES uses the phase transition, usually solid-liquid phase change, of a material (e.g., water turns into ice). The materials used in latent TES are therefore called phase change material (PCM). The last technology includes sorption and chemical energy storage and is usually known as thermochemical TES.  

Several reviews can be found in the literature on TES for building applications, such as PCM for heating and domestic hot water (DHW) [3], PCM for air conditioning [4], PCM in building envelopes [5,6], adsorption for cooling in buildings [7], TES in hybrid systems [8], TES for seasonal storage [9], or more general about the use of TES in building applications [10–12]. Moreover, TES systems also have an important role in district heating and cooling systems [13]. 

This factsheet describes latent TES for active systems. Common applications are their inclusion in HVAC systems, thermal load management, peak shaving both of electricity and thermal energy, etc. [14]. Since PCMs absorb and release heat at nearly constant temperatures, they are particularly attractive for building applications [15]. 

The most common PCM is water/ice but other PCMs include organic (i.e., paraffin, fatty acids, esters) and inorganic (i.e., salt hydrates, salts) [15–17]. Today push towards sustainability has driven research towards the development of bio-based PCMs [18] or the use of wastes as PCM [19]. The optimal PCM to be used depends on the physical, chemical, and technical requirements of the application. The materials use the latent heat between the solid and liquid phase change; therefore, they must be encapsulated, that is included in a container or entrapped in another material such as gypsum, concrete, or polymer. Moreover, different device designs and system configurations can be adopted for using PCMs, depending on the chemical and physical compatibility of the storage material and the heat transfer fluid (HTF) and the application requirements. 

Latent TES can be included in heat pump systems, since it balances system operation and allows heat pumps to operate at full capacity throughout the year, increasing the seasonal performance factor (SPF) [20,21]. Another usual application of PCM is its inclusion in (solar) water tanks [6,22]. PCMs are also used in conjunction with photovoltaic (PV) panels to increase their performance by reducing the panel temperature [23]. Ice storage is widely used in Asia, mostly in big buildings to produce cold during the night and use it during the day when the electricity is more expensive [24]. Space heating and space cooling are also common applications where PCM is used [4]. 

Thermal energy storage (TES) systems can stock heat or cold to be used at a later time under varying conditions such as temperature, place, time, or power. The main use of TES is to overcome the mismatch between energy generation and energy use [1]. The key requirements for the design of a TES system are high energy density in the storage material (storage capacity), good heat transfer between the heat transfer fluid and the storage material, mechanical and chemical stability of the storage material, compatibility between the storage material and the container material, complete reversibility after a number of cycles, low thermal losses during the storage period, and easy control. Moreover, one design criteria could be the operation strategy, the maximum load needed, the nominal temperature and enthalpy drop, and the integration into the whole application system. 

Already in 2011, Arce et al. [2] calculated the potential of load reduction (L), energy savings (E), and climate change mitigation (as CO2 emissions reduction – RCO2) of TES in buildings in the EU. The applications considered were seasonal solar thermal systems (L=25,287 MWth; E=46,150 GWhth; RCO2=12,517,676 tons), district and central heating systems (L=1,453,863 MWth; E=2,326,182 GWhth; RCO2=630,957,558 tons), solar short-term systems (L=416,180 MWth; E=319,269 GWhth; RCO2=86,599,153 tons), and passive cold systems (L=9,944 MWth; E=18,148 GWhth; Ee=6,481 GWhe; RCO2=3,085,135 tons). The subscript “th” stands for “thermal” and the subscript “e” stands for electric. 

Three TES technologies can be identified depending on whether sensible heat, latent heat, or thermochemical concepts are used. Each technology comes with a different performance that dictates the best-fitting application. Moreover, each technology is in a different maturity status.  Sensible TES is realized when the energy is stored by increasing or decreasing the temperature of a material (i.e., water, air, oil, bedrock, concrete, brick). Latent TES uses the phase transition, usually solid-liquid phase change, of a material (i.e., water turns into ice). The materials used in latent TES are therefore called phase change materials (PCMs). Finally, sorption and chemical energy storage  is usually known as thermochemical TES. 

Several reviews can be found in the literature on TES for building applications, i.e., PCM for heating and domestic hot water (DHW) [3], PCM for air conditioning [4], PCM in building envelopes [5], adsorption for cooling in buildings [6], TES in hybrid systems [7], TES for seasonal storage [8], or more general building applications [9–11]. Moreover, TES systems also have an important role in district heating and cooling systems [12]. 

This factsheet is devoted to seasonal TES. Seasonal TES are normally used to store thermal energy produced in an array of solar collectors during the summer months for its use in winter, but other energy sources can also be used [9]. The available technologies are sensible TES (with large water tanks, in underground TES systems –aquifer (ATES) and borehole (BTES)) and thermochemical TES [9]. The main characteristics are: 

• Water tank TES: the tank can be built almost independently from the geological conditions and should be placed to avoid groundwater as much as possible. It is usually 5 to15 m deep and features a heat storage capacity of 60-80 kWh/m3. The tank is usually made of concrete, stainless steel, or fibre reinforced polymer, with a coating layer inside the tank surface and an insulation layer outside. 

Water tank from Jenni Energietechnik being implemented in a building for water seasonal TES [9] 

• Pit TES: the tank can be built almost independently from the geological conditions and should be placed to avoid groundwater as much as possible. It is usually 5 to15 m deep and features a heat storage capacity of 30-50 kWh/m3 when a mixture of water and gravel is used as storage material. 

• BTES (borehole TES): It is suitable for soils with rock or saturated water with no or only very low natural groundwater flow. It is usually 30 to100 m deep and features a heat storage capacity of 15-30 kWh/m3. The heat is directly stored in the water-saturated soil, and it is injected in it with U-pipes. 

• ATES (aquifer TES): It uses aquifers with high porosity, ground water, and high hydraulic conductivity, as well as small flow rate. Typically, it exhibits a heat storage capacity of 30-40 kWh/m3. 

ATES systems for seasonal storage [16] 

Thermochemical energy storage can be divided into sorption TES and chemical-reaction TES (which are commonly used at high temperatures, therefore with limited applications to buildings). Sorption TES can be further divided into solid-adsorption TES or liquid-adsorption TES. In a sorption system, a liquid sorbate (usually water) interacts with a solid or liquid sorbent (i.e., zeolites, silica gels, activated carbons, salts, salt composites). Adsorption systems ae more compact but exhibit lower energy storage efficiency [13]. These systems can be used for cooling, heating, and dehumidification, and are typically adopted for long-term storage (seasonal storage) [14,15]. 

MATURITY:  

Sensible TES systems for seasonal storage are quite mature (UTES TRL=9, water tank TES=8-9, pit seasonal storage TRL=7), while thermochemical TES is going through conceptualisation and lab validation (TRL=2-4). 

DESCRIPTION:  

The new generation of district heating and cooling networks are based on low and/or ultra-low temperature sources of renewable energy and waste heat for heating as well as high-temperature cooling. Such networks provide key options for decarbonizing the thermal energy needs of the building sector and allow supporting strategies for climate neutrality in the urban context for reaching net-zero greenhouse gas emissions while increasing energy security. This factsheet aims to provide knowledge, information, and recommendations on advancing district heating and cooling networks into the new generation based on low and/or ultra-low temperature district heating solutions. The factsheet provides perspectives on establishing the complete network from energy generation to substations toward climate neutrality in urban areas. 
 
Each successive generation of district heating and/or cooling technologies has involved an improvement in supply temperatures and efficiencies. For heating, whereas the current generation of third generation (3G) district heating networks involves supply temperatures at 100°C and below, distribution efficiencies and grid losses are further improved in the newer generation of fourth (4G) or fifth generation (5G) networks. The common point of such networks is the utilisation of locally available renewable energy sources at temperature levels as close to the actual demand temperature for heating and cooling of connected end-users as possible. There are both similarities and differences between these networks and relevant technologies can co-exist. In the more well-established 4G definition, supply temperatures are a maximum of about 60°C for heating and below [1]. As a subcategory concept, depending on definition, bi-directional ultra-low temperature district heating with supply temperatures at 50°C and below are also used to refer to 5G networks [1]. Some networks operate at near-ambient and ambient temperatures with different network-substation configurations. For district heating and cooling networks operating at 10-30°C , thermal grid losses can be about two-thirds less over 3G networks while requiring extra electrical energy for driving the pumps in the network distribution and substations [2].    

Overall, the new generation of district heating and cooling networks is contextualised within smart energy systems where smart thermal grids support higher penetrations of renewable energy sources in the energy system [3]. Smart thermal grids also include large-scale heat pumps that are powered by excess renewable power from intermittent energy sources of solar and wind that would otherwise be curtailed [4]. The ability to integrate low and/or ultra-low temperature renewables and waste heat from multiple sources with or without booster heat pumps defines a central aspect of the new generation of district heating and cooling networks (also see Figure 1 and a complementary factsheet on “Technologies and applications for low/high temperature heat recovery in district heating”). In addition, the new generation of district heating and cooling networks require buildings that are compatible with using low or ultra-low temperature district heating and high-temperature district cooling. This will require building renovation, if not sufficiently compatible, or booster heat pumps. Locally, solutions can be directed to also addressing energy poverty in the urban context and raising thermal comfort. Seasonal thermal storage can support the performance of new generation district heating and cooling networks, including aquifer or borehole thermal energy storage that can provide long-term thermal energy storage [1]. 

Figure 1. Generations of district heating and cooling networks with 4G covering both low and ultra-low temperature district heating networks. Low-temperature refers to supply temperatures of about 50-60°C and maximum of 70°C for heating. Ultra-low temperature refers to supply temperatures below 50°C for heating, overlapping with 5G networks [1]. Two options that relate to another solution factsheet for heat recovery from data centers and supermarkets are marked.   

Technically, different configurations of district heating and cooling networks can lead to different efficiencies, flexibilities, and integration of renewable energy and waste energy sources [5]. As a representative comparison, Table 1 provides an overview of the typical technical specifications for district heating and cooling networks that are labelled as 5G. For example, some have both central and distributed designs for the shared thermal source operating at fixed or variable system temperatures and single and/or two pipe distribution systems with or without pipe insulation and thermal storages. It is common for heat pumps to be placed in substations or the side of end-users in buildings with prosumers. This can minimize upfront investment cost for utilities while potentially increasing the initial investment for the end-users. 
 

New generation district heating and cooling networks are envisioned to be more flexible in the way energy is exchanged, not relying on the central provision of heat and cold in part or whole, and with diverse connections to the network. A much more specialised form of new generation district heating and cooling networks is defined based on thermal energy supply grids using water or brine as a carrier medium that operates at temperatures close to the ground temperature and is supported by hybrid substations and water source heat pumps [6]. Yet different definitions for the same concept can often overlap, including bi-directional low temperature networks and even anergy networks [6].  Networks can also involve free-floating network temperatures with bi-directional and decentralised energy flows and active substations with prosumers. 

Table 1. Technical specifications of district heating and cooling networks labelled with the 5G concept (adapted from [1]) 

Vertical green infrastructure (VGI) has a great potential to support climate transition of compact urban areas. VGI does not compete for land use, but is able to support microclimate regulation through cooling capacities, to reduce noise pollution, to recycle and upcycle rainwater and grey water, to produce food or habitat for species, and to generate pleasant green spaces, with a significant aesthetic value. From a built environment perspective, these solutions can enhance buildings’ performance by protecting and increasing the durability of wall coverings [1], reducing energy consumption, improving photovoltaics panels’ efficiency [2] and reducing sound transmission [3]. 

Green walls and roofs can use the functional benefits of nature to maximize buildings performance and contribute to a sustainable strategy for urban regeneration and retrofitting of buildings. In this sense, depending on the city morphology and plans, green walls can also have a greater potential than green roofs, considering that in urban centres the extent of facade greening can be double the surface of roofs [4]. 

Green wall, Liverpool, United Kingdom. Urban GreenUP. Source: https://www.urbangreenup.eu/cities/front-runners/liverpool.kl#lg=1&slide=3. 

VGI can be of various form, ranging from green fences and barriers to green walls to vertical mobile and living gardens. Each technology can adopt multiple design features such as different plant species, substrate compositions, among others. The design has impact on the performance of the final solution, also depending on local climate, buildings’ orientation and urban morphology.  

Generally speaking, there is a clear distinction between green facades, where climbing plants grow along the wall and cover it, and the concepts of living walls and vertical mobile gardens, which include materials and technology to support a wider variety of plants, creating a uniform growth along the surface. These solutions bring a wider variety of plant species to green walls, allowing the integration of shrubs, grasses and several perennials as long as their watering and nutrient needs are taken into account [5]. Climbing plants are considered a cheap solution of vertical greening, while vertical living walls allow for exploration of new species and vegetation, also exploring the use of patterns, variations in colour, texture, foliage shape and density, vitality and growth. Examples of VGI projects can be found in Naturvation, Urban GreenUP, ProGIreg, and Vertical Green 2.0. 

Green façade, Valladolid, Spain. Urban GreenUP. Source: https://www.urbangreenup.eu/cities/front-runners/valladolid.kl#lg=1&slide=9. 

Vertical green infrastructures can also be integrated with other solutions, such as low-carbon, efficient, and sustainable building materials – reducing GHG emissions by combining energy efficiency with CO2 capture [Green INSTRUCT]. 

Vertical Garden, Torino, Italy. ProGIreg. Source: https://progireg.eu/nature-based-solutions/green-walls-and-roofs/. 

Community Garden, ProGIreg. Source: https://progireg.eu/nature-based-solutions/community-based-urban-farms-and-gardens/. 

Community gardens can be considered as part of urban land gardened collectively by a group of people. Community gardens utilize either individual or shared plots on private or public land to produce fruit, vegetables, and/or plants [2]. In some cases, cities could take the opportunity to turn unused urban land into productive community gardens and assign them through open calls or through public private partnerships with local association. Community gardens can have a positive impact on locals, contributing to improved mental and physical health through exposure to nature and healthy sources of food and a sense of belonging to a community. 

Floating garden, Liverpool, United Kingdom. URBAN GreenUP. Source: https://www.urbangreenup.eu/cities/front-runners/liverpool.kl#lg=1&slide=11. 

Allotment gardens consist of separate parcels of land assigned to individuals or households for personal use [2]. 

Floating gardens are floating green ecological units integrated in a water environment, which can provide habitats for various aquatic and terrestrial species. Floating gardens can take many forms including pontoons, floating platforms and barges (URBAN GreenUP). Floating gardens may provide habitat for varied marine/terrestrial species, opportunities for urban agriculture and climate change mitigation. 

Telheiras allotment garden, Lisbon, Portugal. NATURVATION. Source: https://una.city/nbs/lisboa/telheiras-allotment-garden

Flooding, along with related storms, is the most important natural hazard in Europe in terms of human and economic loss [12] and the intensity of flood events has grown in the last years. In urban areas, the waterproofing of soils is incrementally increasing flood risk due to the presence of roads, industries and houses. The intensity and the frequency of flood events will increase in the next years due to global warming and the consequent change in the global water cycle. The resulting increased flood risk poses challenges to society, physical infrastructure and water quality.  
 

Floodable parks can be designed to control flow rates and decrease flow peaks by storing excess floodwater and releasing it slowly once the risk of flooding has passed. They are innovative sustainable urban drainage systems that can play a particularly important role in mitigating potential impacts caused by surface run-off water from rain, flash floods, or from small and medium sized water courses (URBAN GreenUP). 

Corktown Common Park, Toronto, Canada (UrbanToronto, image George Broen College). Source: https://urbantoronto.ca/news/2014/07/waterfront-toronto-officially-opens-corktown-common.  

In this context, the implementation of nature-based solutions, such as floodable parks, instead of traditional grey infrastructure, can provide numerous benefits such as ecological restoration and biodiversity benefits (e.g., shelter for aquatic birds) [1], leisure and recreation spaces for better human health and wellbeing, and support to water cycle (water storage and reuse) [2].  

For example, the park Le Jardin de Niel in Toulouse was designed with multiple functions such as i) leisure for open and public use with rest areas, picnic areas, and playgrounds, and ii) flood prevention using Flexbrick (open or wide-seamed ceramic tiles) to favour soil drainage, preventing drainage water loss, and reducing the impact of floods caused by heavy rain (Urban Nature Atlas). 

Le Jardin de Niel, Toulouse, France. Urban Nature Atlas. Source: https://una.city/nbs/toulouse/niel-garden  

Another good example is Enghaveparken – Climate park that has a 22.600 m3 water reservoir to handle Copenhagen’s current and future challenges with water. The rainwater is stored and used for watering plants and trees during dry spells and can even be used to clean the city streets. At the same time, the rainwater is handled above ground in the multifunctional reservoir and a dike. The park offers opportunities for recreation, exercise, and sensuous experiences [17]. 

SUDS can be considered more sustainable than traditional drainage methods mostly due to the co-benefits they can bring to urban areas such as recreational benefits, biodiversity and habitat creation, and carbon storage and sequestration. SUDS are also environmentally beneficial because they cause minimal or no long-term damage. 

The main types of SUDS are the following: 

Hard-drainage flood prevention can be considered as a hybrid nature-based solution (NBS) that focuses on the delivery of additional permeability with respect to traditional engineered approach to water management. It supports water infiltration thus reducing pluvial flood risk in urban areas and improving the quality of water within sewerage systems [2]. 

Grassed swales are shallow, flat bottomed, vegetated open channels designed to convey, treat and often attenuate surface water runoff (Urban GreenUP).  

Grassed swales in Izmir, Turkey. URBAN GreenUP. Source: https://www.urbangreenup.eu/cities/front-runners/izmir.kl#lg=1&slide=17. 

Water retention ponds provide additional capacity to retain storm water continuously. In dry periods, they hold water, providing a water source that can be accessed. They can improve the water quality (e.g., with downstream infiltration). For example, in UNaLab, a retention pond has been built in Vuores, Finland to enable the treatment (retention and sedimentation) of urban runoff from a new housing estate. It complements the existing measures in the Vuores area for enhanced stormwater quality and quantity management. 

Retention pond in Vuores, Finland. UNaLab. Source: https://unalab.eu/en/retention-pond. 

Floodable parks can be designed to control flow rates and decrease flow peaks by storing excess floodwater and releasing it slowly once the risk of flooding has passed. This solution can mitigate potential impacts caused by surface run-off water from rain, flash-floods or from small- and medium-sized watercourses. Other potential benefits are reducing the water flow entering the public sewerage system and delivering amenity and biodiversity benefits (Urban GreenUp). 

Water-retentive pavements, including hard drainage pavements and green parking pavements, are permeable pavements that are commonly used on roads, paths and parking lots. These solutions can help control storm water, reduce stagnation of runoff and surface water, and improve water quality via filtration. 

Constructed wetland is a NBS mostly designed to increase water quality and/or support wastewater treatment, which recreates the removal processes developed in natural wetlands, exploiting complex biochemical, physical, and physiological removal processes (ReNature). 

Rain garden is a bio-retention shallow basin designed to collect, store, filter and treat water runoff. To optimise its functions, it must include a porous soil mixture, native vegetation and some hyperaccumulator plants, capable of phytoremediation (cleaning up contaminants). For example, see the rain garden developed in the Gavoglio Urban Park in Genova (part of the UNaLAB project). 

A digital twin presents a digital replica of a real object or process or a system and uses data from the real environment to represent, analyse, validate and simulate present and future behaviour [11]. Typically, connecting real objects with its digital twin enables testing new scenarios or models in real time without interfering with the real objects. Using digital twins, various forms of data analysis can be performed in the digital realm and results can be visualised, e.g., predictive analysis, cause-effect analysis, or analysis of what-if scenarios, and that makes local digital twins (LDTs) of cities, regions or communities an ideal tool for awareness raising, planning and decision-making. Such a solution can play a critical role for climate neutrality initiatives where monitoring, analysis and predictions or forecasting can provide citizens, planners, policy and decision makers, necessary data-driven intelligence on which appropriate actions or interventions can be introduced.  

There are common misconceptions about [19] digital twins, such as that it has to be an exact 3D model of a physical thing or confusing the different levels of integration between physical and real systems. The latter is clarified by the following three different definitions [11] [19]: i) a digital model requires manual exchange of data between real objects and digital objects and a change in physical object has no impact on its digital replica; ii) a digital shadow is at next level where there is one-way data exchange and change in physical system is reflected in its digital object; and, iii) a digital twin is expected to have bidirectional exchange of data which means change in one should result in a change in its corresponding model.  

There are several digital twin solutions and most of them fall within the digital model or digital shadow category. Most solutions focus on a specific problem domain such as industry [33], manufacturing [23][32], health [12] or medicine [24]. Digital twins have also gained attention in the smart city domain [29][27][15] with numerous examples of LDTs either developed or in development at local, city, regional or national level in the EU and in other parts of the world. In terms of EU-funded research projects several domain-specific digital twin solutions are in development in the urban context such as construction [22][BIMPROVE][COGITO][SPHERE], water [DWC][SCOREWATER], green infrastructure including agriculture & farming [RESET][FinEst GreenTwin][21][13] mobility [DUET][LEAD][MOVE21][AI4CITIES], energy [TwinERGY][SPHERE][AI4CITIES], planning, policy-making and decision support [CUTLER][RESET][Smarticipate][DUET][URBANAGE].  

Source: RUGGEDISED project, Deliverable 6.6 

These solutions rely on various technologies including IoTs, AI and other but all are grounded on data from many sources: local platforms, big data, (non-)spatial and temporal data streaming, and require  interoperability, harmonisation and access through web services (e.g., RESTful service interface), federated data spaces and others complying to service interoperability, data privacy and security, data processing based on trusted machine learning and AI models, and 2D/3D and immersive visualisation and simulations.  

LEAD platform deployment 

Ecosystem of Digital Twins: The key for climate neutrality challenge is to perform data analysis at a city or city-regional scale and it requires cross-disciplinary data fusion and knowledge generation that is often needed across multiple levels of governance. Hence, there are already ongoing efforts on defining principles for an ecosystem of digital twins where results and data sharing across digital twin nodes will be key enabler for deriving much needed intelligence [25]. 

For assistance in the implementation of LDTs, cities can benefit from programmes and initiatives under the DIGITAL Europe Programme [35]. 

DUET Technology architecture  

E-participation [1][2][3][4][5] enables citizens to use digital technologies or platforms, e.g., combination of geographic information systems (GIS), Web 2.0 and mobile technologies (including video, mobile messaging and Internet access), for communication, engagement and deliberation on policy or planning challenges.  

Engagement and participation are vital tools in climate adaptation and environmental decision making as these entail increased community acceptance, support for climate actions, and provide new insights based on local knowledge [12]. Citizens can be consumers as well as producers of useful data for policy development and decision making (WeGovNow, Smarticipate, AI4PublicPolicy). 

There are multiple degrees of citizen participation ranging from passive, i.e., being simply informed, to responsive, i.e., contribute to consultation, to active, i.e., being fully empowered by having final decisions delegated to them (see Arnstein’s ladder [6]) [7]. In e-participation initiatives, both top-down (i.e., issues identified by public authority) and bottom-up (i.e., citizens led initiative) approaches can be applied. As multiple actors (i.e., different departmental units) are involved in the provision of e-participation, cross-organizational issues related to ownership and accountability may arise [3].  

Technologies supporting government processes (GovTech) can add great value to participatory processes (e.g., access to sensor kits, web portals and data), as shown by examples of Madrid (Decide Madrid), Bristol (Bristol Approach to citizen sensing – e.g., air quality, solid fuel burning etc.) [7], and Brussels (Curieuzenair). E-participation is usually considered part of e-government [5]. 

E-Government (or Electronic-Government) [1][2][8] refers to the application of Information and Communication Technologies (ICT) to government functions and procedures with the objective to increase efficiency of government agencies, enhance delivery of public services, and facilitate low cost and faster public engagement with public authorities. A comparative survey [8] of global e-government performance of municipalities highlights the best e-governance practices. It uses five categories of measures: privacy and security, usability, content, service and citizen and social engagement. For citizen and social engagement category Shanghai, Auckland, Seoul, Madrid, Paris, and Lisbon are ranked top cities for year 2018-19. 

Open Governance [9] is about transparency of and access to government data and decision making process so that innovative forms of collaborative actions (i.e. bottom-up and top-down) can be applied to solve policy problems, raise awareness, increase public participation, change behaviour, promote e-democracy, and revolutionise traditional service provision [10][11]. It is closely associated with open government data that can provide new insights about issues and services as well as offers the opportunities to participate, comment and influence plans and policy agenda to foster greater citizen participation. 

E-participation solutions range from responding to planning e.g., top-down to bottom-up urban regeneration [Smarticipate] or policy challenge [WeGovNow] or reporting a local problem (e.g., Bristol’s FixMyStreet); or bottom-up budget planning (e.g., Helsinki’s participatory budgeting) or accessing open data (e.g., Hamburg’s Transparency portal).  

There are several e-participation initiatives where various ICT tools are used to deliver different public services. For instance,  

• provision of integrated and inclusive public services [inGov],  

• citizen-centric policy development [AI4PublicPolicy] [PolicyCLOUD] [DECIDEO],  

• smart decision-making through digital twins [DUET, TwinERGY],  

• citizen-based planning intervention and calculated feedback [13] [Smarticipate],  

• user-centric services [UserCentriCities],  

• co-creating public services through open data [O4C] [WeLive],  

• co-creating public services for senior citizens [Mobile Age, URBANAGE], and  

• civic engagement, communication and collaboration to solve local policy challenges [WeGovNow] [CO3].  

Cross border e-governance initiatives such as [ACROSS], [DE4A] and [GLASS] go beyond one city’s public administrative level (even at EU level and beyond [iKaaS]) and deal with cross-border interoperable, mobile [mGov4EU] and privacy-aware public services.  

MATURITY: 

Many e-government and e-participation tools are available at higher TRL and are already being used by municipalities for public services and e-participation, e.g., open source Consul platform is being used in 35 countries by 135 institutions; Similarly, Organicity tools are used for over 35 experiments in various cities. 

Some of the example solutions fall under validation and demonstration category such as DUET and Smarticipate.  

Circular design of textiles involves designing textiles in ways that make them last longer, create less waste during their entire life-cycle, are made from non-toxic material and can be recycled (2). 

The development and take-up of such solutions can benefit from the use of co-creation approaches in urban environment involving multiple stakeholders, such as designers, manufacturers, researchers, and behavioural scientists. Innovative technologies for customer-driven design and on-demand production can further contribute to lowering the industry’s environmental footprint, especially if coupled with local production and green distribution models (6) (2). 

Source: (5) 

LONGEVITY and REUSABILITY 

Circular design principles in the fashion industry greatly impact product longevity as 40% of all reasons for consumers discarding clothes are linked to functional changes of garments, such as holes or tears, worn-out appearance, loss of elasticity or shape, stains, colour changing or fading (5) (7). The use of appropriate materials and the development of innovative fibres and new construction methods of yarns and fabrics can positively influence the quality of the final product, and hence its durability and reusability.  

DIGITAL SOLUTIONS FOR TEXTILE WASTE PREVENTION 

Design-driven digital precision technologies and AI software can reduce pre-consumer waste, limit the high percentage of returns of items bought online, and encourage on-demand custom manufacturing, leading to efficiencies of industrial processes and lowering the GHG emissions related to the fashion industry and e-commerce (4). Examples of projects applying and developing digital solutions are Refream and Rodinia. 

RECYCLABILITY and NEW MATERIALS 

Circular design strategies improve the disassembly and recyclability of textiles, by opting for quality materials, avoiding fibre blending, limiting the application of non-textile accessories and components or making them easily removable. Solutions for recyclability in the fashion industry include also the development of innovative biodegradable materials, both for textile and non-textile products, and disintegrating stitching for easy disassembly (3) (5) (9). Representative cases are the projects Naturella and the innovative stitching developed by the project CIRCTEX. 

Fashion and textile design can further contribute to circularity in the industry through innovative use of recycled pre- and post-consumer materials, creation of new regenerated fibres from pre-consumer and post-consumer waste, and development of novel biomaterials. Examples are provided by the Trash2Cash and VegeaTextile projects. 

Source: VegeaTextile commercial scale first production testing 

Circular textiles and fashion design favours the use of sustainable printing technologies, and sustainable, energy- and water-efficient finishing processes (e.g. bleaching, dyeing), which can help achieve not only improved recyclability of the end-to-life product but also an overall lower environmental impact of the manufacturing (3). H2COLOR-AUX provides a good example of an innovative sustainable dyeing product. 

Users of such solutions can be:  

• Businesses (B2B): use of fibres, yarns, fabrics and non-textile materials – e.g. shoe manufacturers, the accessory industry, the furniture industry, the car upholstery industry, and the clothing industry. 

• Consumers (B2C): customisation of products and use of end-products. 

• Public administrations (B2G): public procurement as a driver for market demand for circular designed workwear and uniforms (6). 

Currently, at the end of a building’s life the majority of its materials are sent to landfill or down-cycled into products of much lower value. To maximize the recovery and valorisation potential of existing materials in buildings and minimize waste in construction, optimal waste management at the end of life of buildings is essential. The right treatment and disposal of materials at the end of life can significantly reduce the construction sector’s environmental footprint. 

There are several approaches to better manage materials from demolition of buildings: 

Urban mining – the process of recovering and reusing a city’s materials – can offer a viable solution to improve waste management in the construction sector. The aim is also to increase the value of waste, so that recovered materials can be used in applications similar to those for which they were originally designed. 

Using the recovered materials for new building projects has environmental advantages. By keeping the materials in the city/district, long supply chains can be prevented and emissions thereby reduced. Urban mining also limits the demand for new building materials, which in turn reduces greenhouse gas emissions. 

Building phases (BAMB project) [6] 

Urban mining can be facilitated by the use of material passports, which offer information about the types of materials present in a building. Material passports were successfully piloted in BAMB, where material passports where used in designing and constructing a new office building [11]. The use of innovative technology-enabled solutions can further facilitate the recovery of used materials, such as the BIM-based “smart pre-demolition audits and waste management” tested in the HISER project [13]. 

In addition, designing buildings for deconstruction and reuse allows better access to materials at the end-of-life of a component or a building. Design for de-construction or disassembly (i.e., a reverse construction process) allows the building to be de-constructed as much as possible, without damaging the remaining functional parts and components [BAMB]. This increases potential for renovation and reduces landfill waste [Houseful]. The design should allow simple disassembly and preferably avoid multiple types of structural systems [12]. In designing for reuse, a BIM (Building Information Model) can be a useful digital tool to store the information for the lifecycle of the building. For instance, the design for re-use was piloted in BAMB and in Houseful. 

Life cycle of building [5] 

Lease contracts and buy-back agreements for materials encourage recycling and re-use of materials. Suppliers conclude a contract with the buyers, committing to eventually buy back the product. The benefit for buyers lies in overall reduced prices; for the suppliers, the material flow for recycling is guaranteed. The lease contracts are still in early piloting phase. 

Currently, the construction sector is mostly following linear economy principles – consuming non-renewable virgin materials, producing landfill waste, and producing an overall negative environmental impact. The construction industry is the biggest global consumer of resources and raw materials, consuming e.g., roughly 50% of the total steel production. In addition, 3 billion tonnes of raw materials are used yearly to manufacture building products worldwide [8][9]. In Europe, the construction and demolition waste is the biggest waste stream (measured in weight), accounting for 32% of the total waste generated [12]. In addition, the construction sector is energy intensive and thus generates CO2 emissions. To mitigate these problems, a transition to a circular economy model is needed.  

Building phases in circular construction (BAMB project) [15] 

The re-use of local building and demolition waste focuses on short-distance supply routes for materials. In order to match the local building waste to the local need, different technologies can be applied, such as building information modeling (BIM) software and radio-frequency identification (RFID) tags. Spare parts warehouses and material banks for second-hand use play a vital role in local reuse of building and demolition waste. In order to make the material banks and information flow as efficient as possible, local public-private partnerships are important. For instance, circular transition projects in Leuven clearly proved the importance of data governance driven by enhanced stakeholder interactions, and showed how it should be implemented in the circular transition process [10]. 

Building information modeling (BIM) software is a tool that is used to load, store, edit, and manage virtual building data over its entire life cycle. Usually, new buildings are designed with BIM models, whereas existing buildings typically do not have them. Currently, there are tools available to scan the material or structure properties and integrate it into the BIM models, e.g., via RFID tags. Other tools allow to estimate the recyclability and reusability of recovered materials. Until now, they have been mostly focusing on steel, timber and concrete structures [5][6].  ,

Re-using local waste (ReCreate), A pilot building located in Sweden built of 99% of reused material [18] 

MATURITY:  

Examples of solutions already commercialised or close to commercialisation: 

• Madaster has a cloud-system for categorisation of materials in buildings. The platform creates material passports. The data is owned by the owners or managers of the buildings and they can share the information with stakeholders. 

• The BC materials is a commercial player who transforms pure earth of construction sites to local building materials such as clay plasters, compressed earth blocks and rammed earth for walls and floors [20].  

Examples of pilot cases:  

• FISSAC demonstrated manufacturing processes (e.g. transforming waster into raw materials, manufacturing at industrial scale), product validation (e.g. eco-design of eco-innovative construction products in pre-industrial processes and at real scale), and industrial symbiosis (software platform).  

• Houseful set out to demonstrate and validate a methodology to quantify the degree of circularity of buildings at lab scale and, in four selected European buildings, at large scale.  

• BAMB implemented several pilots, such as: 

• The Circular Retrofit Lab (Vrije Universiteit Brussel campus) tested and implemented different scenarios for the reuse and refurbishment of the VUB Campus’ prefabricated student housing. Strategies have been explored for internal transformations, external transformations, and the module multiple functional reconfigurations. Eight student rooms were renovated using demountable, adaptable and reusable building solutions, creating as little demolition waste as possible [21].  

• In Heerlen, Netherlands, a Green Transformable Building Lab was developed around a multifunctional and reversible steel frame filled with interchangeable, independent and reversible floor, façade and roof elements [22] 

• The New Office Building pilot project in Essen was built close to Zeche Zollverein, a former coalmine industrial complex. Focusing on cradle-to-cradle design approaches, the new office building will host over 200 high-quality office spaces and a rooftop garden [23]. 

The Residual Value Calculator is a model that provides insights into the residual financial value of building products, such as the interior and exterior facades of homes and offices. The residual financial value is the financial value of building products or elements after their first life cycle. Compared to a linear business models, the financial benefits of re-use of building products and elements are determined by calculating their residual value [Residual Value Calculator][4]. 

The Residual Value Calculator gives estimates about the monetary value of a certain building product or component. The model estimates the residual value after certain years of use by using factors like quality of the material, detachability, price of raw materials, and costs of transport, maintenance and repairs. This provides an incentive to disassemble and reuse materials and components, as the economic value is transparently calculated for product suppliers, project developers, housing corporations and building project financiers [Residual Value Calculator]. 

The residual value calculation also enables better understanding of different business models. The traditional linear model of selling products or services down the supply chain can be compared with, for example, buy-back or lease business models. In addition, the model improves understanding of how design-for-recycling compares with the linear model [2]. 

Since this tool needs information on materials, material passports are essential for residual value calculations to return realistic results [1]. Material passports include information about materials and construction products. 

The built environment industry mostly follows a linear economy model: from extraction of non-renewable virgin materials, through manufacturing and constructing, to disposal in landfills. Registration and documentation of products and materials used in the built environment can help the transition to a circular economy, encouraging their reuse and eliminating waste, thus reducing the negative environmental footprint of the building sector [2]. 

Online Material and/or Building Registries are a new approach to the concept of “Buildings as Material Banks”. The idea of “Buildings as Material Banks” is based on material identity; all materials and components in the building are documented and updated when renovation/upgrading occurs. Typically, the documentation can be stored in a Building Information model (BIM). Online registries can and should include statistical and measured specification, performance data gathered from manufacturers, certifiers and verifiers, as well as sensors, inspections and measurements. This data, usually collected in Material and/or Buildings Passports, can be both an input to and an output of Building Information Modelling (BIM), Life Cycle Assessment (LCA) and other types of certifications, such as Environmental Product Declarations (EPD), financial incentives frameworks (including smart insurance), multi-criteria evaluation models, policy development (including open data web platforms), and deconstruction/reuse processes. 

Concept of the material and component bank [1]

A key barrier to the viability of secondary construction material markets is logistics. By creating digital and physical platforms to coordinate efforts, cities can accelerate private actors’ contributions to a circular construction sector. A main concern is accessing information on materials with potential to be a resource or input to other processes. 

A material passport includes digital data, such as the history of building materials, the extent to which building components can be reused, how this can be done, and  the potential for recycling and biodegradation. Digital data can be implemented also in BIM (Building Information Models), which can be adopted in the design phase serving for the whole life cycle, or can be created for existing buildings, based on data capture, building surveys, and pre-existing information [2]. 

The material and component bank organizes the transfer of materials and components, which are extracted from demolished or deconstructed structures and which can be used in new ones. The bank helps the sustainable planning of demolition and deconstruction, extraction and collection of recyclable and reusable materials and components, as well as assessment and improvement of quality [1]. 

Main businesses of material and component bank [1] 

The challenge of managing and evaluating the amounts of data required is best met through digital tools. Sophisticated digital tools such as BIM-supported Material and Building Passports collected in online open-source registries can support decision-making for circularity from the planning phase, through the building use phase, including retrofit cycles, and up to the disassembly and end of (first) life. The newest approach is to include also the building residual value to the material bank information. This helps understand the financial value of the existing material and encourages recycling. 

There is an informational and communicational gap between supply and demand projects. For example, research and building data are decentralized and scattered throughout all types of mediums. To help bridge these gaps, the BAMB project created a geospatial mapping and a BIM (Building Information Model) system.  

The CE-LCC includes properties of circular economy products, such as different lifespan of parts and components, design for repair, or disassembly. This means that costs for components of a building, such as a façade made out of different materials, can be calculated with different and multiple use cycles. A use cycle usually corresponds to the timespan in which the object meets the functional demands of the user. A multiple use cycle allows to reuse/recycle the component after its end of use [1]. In addition, the methods include processes taking place after the end of use phase, such as recycling or dismantling.  

The CE-LCC model helps to evaluate the different options for components of ‘circular’ buildings, for instance, 1) the structure is made from reclaimed materials, 2) the structure is made from modules that can be changed, updated or reused, and 3) the structure is made from bio-based and biodegradable material. The CE-LCC model helps decision-makers to choose a suitable ‘circular’ option for their building [4]. 

In CE-LCC, the product or material life is considered as a loop. Environmental performance often improves the most by combining circular design options to narrow, slow, and close cycles simultaneously, instead of focusing on one [3]. Narrowing loops means to reduce resource use or to increase resource efficiency. Slowing loops means to lengthen the use of a building, component, part, or material. Closing loops is to (re)cycle materials from end-of-life back to production. 

The components, whose service life is short, benefit more from slow and close cycles. In addition, the components with longer service life benefit more from reducing resources and slowing loops [3].  

Examples of CE-LCC tool, the overall structure of use cycles of a part in the model [1] 

Greywater is the wastewater generated in households or office buildings without serious contaminants, such as water from baths, sinks and washing machines. Greywater can be separated from blackwater (water from toilets or kitchens) and then treated on-site for direct reuse in toilets recharge or irrigation. Greywater is a relevant secondary source of water and nutrients. Many studies have analysed the environmental, economic, and energetic benefits of the reuse of greywater [1][2][6][7]. Greywater treatment systems can be introduced in new buildings or in existing buildings with retrofitting measures. There are different greywater treatment systems: diversion and filtration, diversion and treatment (using chemicals), or nature-based solutions (NBS). 

Industrial symbiosis refers to an association between industries where companies’ waste or side products turn into input materials for another company, creating economic gain for both [8]. The word “symbiosis” is usually associated with relationships in nature, where two or more species exchange materials, energy, or information in a mutually beneficial manner. In the case of industrial symbiosis, it is industrial players or companies, often from different sectors but located close-by or even in the same cluster or industrial park that form partnerships to exchange and revalue goods. In many cases, this includes public-private partnerships. In one example, industrial waste heat and CO2 from a sugar refinery were used to heat a greenhouse nearby and the CO2 was used to speed up tomato growing [6]. 

Local or wider co-operation in industrial symbiosis can reduce the need for virgin raw material and waste generation, thereby closing the material loop – a fundamental feature of the circular economy and a driver for green growth and eco-innovative solutions. It can also reduce emissions and energy use and create new revenue streams.  

For example, Kalundborg Symbiosis is a partnership between 14 public and private companies in Denmark, which share excess energy, water and materials, and are connected via more than 20 different flows of resources. For example, the Kalundborg community waste material goes to Argo energy plant as fuel and the Kalundborg heat pump uses the wastewater from other industries in the area to generate heat [9]. 

Different service providers offer services to help customers improve their buildings energy performance. These are typically called energy service companies (ESCOs). They can help achieve energy savings by implementing energy-efficiency and renewable energy projects [e.g. Houseful]. Typically, the service is delivered through Energy Performance Contracting (EPC), a business model where the solution provider guarantees the performance and receives a performance-based remuneration from the client. The use of EPC in the public sector, and partially in industrial and commercial buildings, has been consolidating in the past few decades, whereas it is still not very common for residential buildings [7] [9]. 

Energy-saving measures implemented through EPC can be related to e.g., boiler and chiller systems, lighting, HVAC, roofing, insulation, windows and building management systems, as well as deep renovation [9]. ESCOs contracting models can differ, presenting various financing terms, repayment options and different allocation of risk between the service provider and the customer. Examples of EPC are provided in the projects STARDUST and STUNNING. 

Source:  OnePlace [11] 

One of the most common is the EPC Guaranteed Savings model, where the ESCO guarantees a certain level of energy savings to the client and takes on any eventual performance risk. In this case, the customer assumes the credit risk, obtaining a bank loan or using his own equity to repay the debt and the contractually determined fees to the ESCO for the duration of the contract. In this case, the customer will use the savings as repayment, and any savings exceeding the guarantees are split between the consumers and the ESCO according to contractual provisions. In case the energy savings were not sufficient to cover the debt, ESCOs would have the obligation to cover the difference [4][5]. Clients will actually start to benefit from energy and cost savings after the end of the contract. Countries with ESCOs that use this as the main financing model include Czech Republic, Denmark, Canada and Thailand [4]. For instance, in Denmark the municipality of Middelfart implemented energy saving measures in approximately 100 public buildings [13]. The contracted guaranteed savings were 21%, but the actual results have showed savings up to 24% of the total energy use. 

Another example is the EPC Shared Savings model, in which cost savings are split between the ESCO and the customer for the duration of the contract, based on a pre-determined percentage. The ESCO assumes the financial risk, by covering investment and implementation costs, as well as the technical risk – which can be of value to the client as it avoids the investment costs [4][5]. This model requires the ESCO to be creditworthy and to have sufficient revenue streams to pay back the loan. Examples of countries where this model is used are India, Chile, and Greece [4].  

Source: STUNNING project, funded by H2020 (2017-2019) 

SuperESCOs are governmental entities created to serve the public sector, develop the capacity of private ESCOs, and facilitate project financing. Existing programmes designed to engage clients with ESCOs –  such as energy audits programs, rebates, direct install programs, demand side management bidding, or standard offer approach – rarely provide sufficient funding for implementation costs such as engineering, procurement and installation costs. As energy efficiency projects do not seem to be an investment priority for many businesses, even those with financial means, supporting ESCOs creditworthiness is important to increase the adoption of energy-saving measures [4]. Super ESCOs are typically bigger companies or energy utilities [13]. 

ESCOs services can produce even more interesting results when combined with active buildings. Active buildings typically have a passive design, smart monitoring capability, can generate renewable energy on-site as well as store it, can integrate electric vehicles, and can intelligently manage integration in micro-grids and national grids [1]. Active building Energy Performance Contracting (AEPC) builds on the EPC model by exploiting energy demand-response systems and demand-side flexibility. This means that active consumers are able to change energy consumption patterns in response to market signals, such as prices or incentives, or depending on their self-generation and storage assets [8]. The building can thus adapt the use of energy in the slots where energy is most available and cheaper. In addition, the building can feed the excess energy to the grid when self-generated energy surpasses building consumption. AEPC usually targets cluster of buildings, which makes it more attractive to the ESCO companies and allows application in the residential sector. For instance, in Belgium [AmBIENCEe], active energy performance contracting has been piloted, through the dynamic simulation of the building behaviour and the setting of active control targets [12]. 

Additional information can be found in a European Commission’s Joint Research Centre (JRC) ESCO library, including different financial models [10]. 

The two pillars of the decarbonisation of the buildings sector in Europe are a) improving energy efficiency and b) supplying with renewable energy sources both new and existing buildings. Recent developments show that energy developments at the neighbourhood or district scale can accelerate and improve the required quality to meet the requirements of the Paris agreement (1,2).  Acting at the neighbourhood level permits to better consider the energy interactions between the buildings and the local energy system. The concept of zero-energy districts is based and benefits from highly energy efficient buildings and local generation and consumption of renewable and low carbon energy systems (3,4).  

Figure 1: A building belonging to the ZERO-PLUS project settlement in Voreppe, France (20). 

Positive Energy Districts (PEDs) are neighbourhoods with annual net zero energy import and net zero CO2 emissions, generating more renewable energy than they consume yearly. PEDs are characterized by a) net-positive renewable energy production on a yearly basis, b) high energy efficiency, c) flexibility d) diversified renewable energy technologies, and e) focus on providing inclusive, affordable, and sustainable lifestyles rather than on economic advantages. They usually include energy storage and EV charging solutions and require integration of different systems and infrastructures and interaction between buildings, the users and the regional energy, mobility and ICT systems. 

Three categories of PEDs have been identified. The difference lies in the ability to interact with energy networks, consumers, and producers outside the geographical boundaries of the PEDs.  

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Figure 2: Plus energy settlement in Freiburg, Germany (21).

Autonomous PEDs have clear geographical boundaries and on-site renewable energy generation. They may not import energy from the external electricity grid and district heating/gas network but can export the excess renewable energy. Dynamic PEDs have clear geographical boundaries and annual on-site renewable energy generation and can furthermore interact with other PEDs, external electricity grids, and district heating/gas network. On the contrary, virtual PEDs allow the implementation of (virtual) renewable energy systems and energy storage solutions outside their geographical boundaries.  

In a net-zero (NZED) or positive energy district, the central energy system should be completely based on renewable energy sources and should accommodate and provide the energy for the whole community. Further, community-level financial aspects and environmental impacts (including GHG emissions) should be critical parts of the decision process (6). 

Figure 3: Net Zero Energy Buildings Cluster / retrofit projects. Renovated district in Bad Aibling, Germany (22). 

NZEDs and PEDs present several important advantages compared to Net Zero Energy Buildings, (NZEBs): 

1. They promote full sharing of energy needs, costs, and resources among the community buildings, that is very beneficial to a cost-effective management (7). 

2. The potential mismatch between energy demand and energy generation, which is very common at the building scale, can be avoided at the neighborhood/district level as communities can ensure a more balanced management (8-9).   

3. At the community level, oversizing of the energy system can be avoided by proper assessment and sharing of the total energy demand (10).  

4. The management of the various available and used energy resources in a centralized energy system is more efficient and offers the flexibility to balance the demand and supply by using efficient energy storage systems (13). This offers more flexibility and a potential for more energy services like electric mobility.  

5. the process of generation and distribution of energy runs in parallel, allowing the minimization of the distribution and surplus losses (11-12). 

6. In high-rise buildings, NZEB concepts come with specific challenges that are much better addressed when the energy system is generated at the community level (14).    

7. The implementation and use of a microgrid at community level can supply excess energy to the energy grid resulting in additional financial benefits for the tenants (15). 

To realise the net-zero/positive concepts at the district level, three main energy components have to be optimized (Figure 1): 

1. The thermal load of the building has to be minimized using energy conservation technologies at the building scale, to minimize the heating, cooling and lighting loads (16-17).  

2. The energy generation and supply system have to be based on clean and renewable energy technologies to supply the whole district. 

3. The outdoor climate has to be properly considered to achieve the best possible conditions during the whole year (18-19). 

The main purpose of this document is to provide knowledge, information and a reference for the city authorities, building professionals and stakeholders to design and implement NZEDs/PEDs across different climatic and location-based conditions. 

Figure 4: Portland Net Zero Energy Community (23). 

The design and operation of zero or very low energy buildings started in the 2000s and even earlier. The basic idea is to use energy conservation technologies combined with renewable energy systems to decrease as much as possible the energy use of buildings (see Figure 1). Nearly-Zero Energy Buildings (NZEBs) have continuously gained acceptance by the market and the research community and several thousands of such buildings have been built and monitored.  

The NZEB principle has been adopted as the current and future policy by the European Union. Indeed, according to the EU Directive on Energy Performance of Buildings (EPBD) [1], new buildings occupied by public authorities had to be NZEBs by December 31, 2018 and all new buildings had to comply by December 31, 2020. NZEBs are defined as buildings with a very high energy performance, as determined in accordance with Annex I of the EPBD. The nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources produced on-site or nearby. The EPBD does not prescribe a uniform approach for implementing NZEBs. It states that Member States shall detail NZEB definitions, reflecting national, regional or local conditions, and including a numerical indicator of primary energy use expressed in kWh/m2/year. As such, the requirements show heterogeneity across the various Member States [2]. On average, NZEBs energy performance levels appear 10% higher than the benchmark levels recommended by the Commission in 2016 [3] in most Member States. 

Most common NZEB technologies include both passive solutions (e.g. sunshade, natural ventilation and lighting, night cooling), and active solutions (e.g. mechanical ventilation with heat recovery, heat pumps in combination with efficient lighting, appliances, and envelope). The typical envelope U-values fall in the ranges 0.15 – 0.20 W/m²K (walls) and 0.10 – 0.25 W/m²K (roofs). Regarding renewable energy sources, PV and solar thermal are frequently used [4]. 

Numerous building design procedures and principles are employed to achieve the nearly zero energy target resulting in a variety of potential solutions that present significant cost differences. Some favour excessive use of photovoltaics or other renewables and almost an insignificant use of energy conservation technologies, disregarding the primal objective of high energy performance and thus minimal energy needs.  

However, the Commission’s proposal to revise the EPBD [5] (December 2021) makes a step forward from current NZEB to zero-emission building (ZEB), aligning the energy performance requirement for new buildings to the longer-term climate neutrality goal and “energy efficiency first principle”. According to the directive’s proposal, a ZEB is defined as a building with a very high energy performance, with the very low amount of energy still required fully covered by energy from renewable sources generated on-site, from  renewable energy community or from a district heating and cooling system. The ZEB requirement should apply as of 1 January 2030 to all new buildings, and as of 1 January 2027 to all new buildings occupied or owned by public authorities. 

Positive Energy Buildings, PEB, is a relatively new and more ambitious energy target than the Nearly Zero Energy buildings, NZEB. Currently, there is no official definition at EU level. However, there are several projects investigating the PEB concept. According to the EU EXCESS project (1,2), ‘‘a positive energy building (PEB) is an energy efficient building that produces more energy than it uses via renewable sources, with high self-consumption rate and high energy flexibility, over a time span of one year. A high-quality indoor environment is an essential element in the PEB, maintaining the comfort and well-being of the building occupants. The PEB is also able to integrate future technologies, such as electric vehicles with the motivation to maximize the onsite consumption and also share the surplus renewable energy”. As mentioned in (3): ‘Technically, a PEB is a Net ZEB with an increased capacity of the renewable energy generation inside the boundary of the building in order to surpass the annual equality of the net energy balance’.   

The EU energy and environmental policy framework aims to make Europe the first climate neutral continent. The development of advanced energy technologies and the massive retrofitting of the building stock as foreseen by the European Strategic Energy Plan and the European Green Deal create new advanced targets regarding energy and emissions status of the current and mainly of the future buildings leading to increased spread of PEBs (4,5). 

The PEB concept is based mainly on the use of intensive energy conservation measures to decrease as much as possible the energy needs of buildings combined with  renewable energy systems installed in the buildings’ boundaries, producing excess energy that either is stored within the building area, is transferred to the grid, or both. The concept of PEBs is illustrated in Figure 1 (2). 

The decrease of the energy demand of the buildings through the intensive and efficient use of energy conservation measures, is the core requirement of a PEB, in line with the "energy efficiency first" principle (6).  Decisions on the level of the selected energy conservation measures as well as options for on-site renewable energy sources should be based on cost optimality, as proposed by the Energy Performance of Buildings Directive (2).  

The heat island phenomenon raises the temperature of cities, increases the energy demand for cooling and deteriorates comfort conditions in the urban environment. To counterbalance the impact of the phenomenon, important mitigation techniques have been proposed and developed. Absorption of solar radiation by the materials of the building envelope and the city structure contribute to increasing urban overheating. Use of solar reflective materials, known as cool materials, helps to decrease the surface temperature by up to 20 °C and the ambient urban temperature by up to 2 °C (1). In parallel, it can contribute to decreasing the cooling load of buildings by up to 40 %, the peak indoor summer temperature by up to 5-6 °C, the concentration of harmful pollutants and in particular of ground level ozone, and the heat related mortality and morbidity, all the while improving indoor and outdoor thermal comfort (2). 

Reflective, cool materials present a high reflectance to solar radiation combined with a high emissivity factor, thus these materials do not absorb much and can easily lose the stored energy. White materials present a high reflectivity that can reach 0.95 both in the visual and infrared solar spectrum; however, also coloured reflective materials exist that present a high reflectance in the infrared part of the solar spectrum, to cater to different users’ needs and local architectural requirements (3).  

Cool materials can be used either on the envelope of the buildings, as cool roofs or in the urban structures as cool pavements and cool streets. When applied on roofs, materials of very high reflectivity can be used. However, when used on vertical surfaces, pavements and streets, reflectivity should not exceed 50 %, to avoid problems of glare and contrast. In Europe, most of the cool materials are accredited and their performance is certified by the European Cool Rood Council. A list of the certified materials can be found in (4). The various types of existing cool materials for buildings and pavements can be found in (1,5). Examples of successful projects, as well as data on the accredited materials and other useful information on reflective materials can be found in the final report of the Cool roofs council project of EU (6). 

COOL ROOFS

LONGEVITY and REUSABILITY 

Cool materials present similar mechanical properties as the conventional ones. Optical ageing of cool materials can be a significant problem especially for the non-certified products. Information on the long-term reflectance of cool materials is provided for all certified products by the European Cool Roof Council (4). The use of materials presenting a minimum loss of solar reflectance is highly recommended. When paints or coatings are used, water-based materials containing zero or minimum VOCs have to be selected.  

COOL PAVEMENTS 

Reflective Cool asphalt in Athens. Source (9) 

RECYCLABILITY and NEW MATERIALS 

Ceramic, asphalt, or concrete based cool materials can be recycled as any other conventional material of the same nature. Especially concrete pavements should be fully recycled and reused as a filling material for new pavements or other building processes. Most of the used cool membranes can be also fully recycled. 

The urban air mobility (UAM) concept revolves around the use of highly automated aircraft (e.g. drones) to transport goods and people in urban and suburban areas. These aircraft fly at low altitudes, and are usually associated with the use of electric motors for propulsion, to achieve sustainable flights with no emissions during operation. 

Urban air mobility is a new concept which, although still in research phase, is rapidly evolving and taking shape. To support it, the SESAR Joint Undertaking has defined the U-Space Blueprint (1), “a set of new services and specific procedures designed to support safe, efficient and secure access to airspace for large numbers of drones”, which should eventually help to enable UAM and drone operation in urban environments in the EU. Moreover, UAM is supported by the Urban-Air-Mobility Initiative Cities Community (UIC2) (3), while the European Union Aviation Safety Agency (EASA) has started developing a regulatory framework for it (4). 

Figure 1:UAM ecosstem for the AiRMOUR project (5)

There are different types of use cases for UAM, with the most prominent ones being (2): 

• Air taxis: this application foresees the use of automated drones for fast and efficient passenger transport for short distance, within the urban context (a)(b). 

• Goods deliveries: delivery of goods using automated drones, also for faster deliveries, while reducing road congestion (5). Delivery of medical supplies (e.g. samples and drugs) is an important application for this use case (c). 

• Infrastructure inspection: assessing the integrity of core infrastructure can be complemented with the use of drones, especially in the case of difficult to access locations, which can lead to reduced cost of operations (d). 

• Public safety and security: drones can be used for police and firefighter emergency services, when a fast response is required, and when there might be the need for assessment of safety in dangerous situations, before a ground crew intervenes (d). 

Challenges for full deployment of UAM includes integrating its infrastructure elements, such as vertical ports (vertiports) and landing pads, into the city especially from the point of urban planning, as well as the air traffic management systems and infrastructure needed to keep drone operations safe and reliable. Another issue of concern is the noise generated by the aircraft, which should be at an acceptable level for the urban environment.  

Figure 2: AMU-LED airspace structure (6)

Urban rail transit (e.g. light rail, rapid transit, commuter rail, metro rail) is an important component of urban mobility and transport. With relatively low lifecycle emissions, electrified rail transport is considered to be key for the reduction of emissions in transport in general, and also in urban areas.  

Railway networks in urban and suburban areas play a prominent role in major cities and highly dense urban areas, serving the daily needs of citizens and offering an attractive alternative to the use of private car in congested and polluted areas. Due to the way that rail infrastructure works, rail transport is usually associated with multimodal journeys, with commuters making a long part of their urban and suburban journey on it.  

Challenges for urban rail transit include the need to offer increased capacity to an increasing number of passengers, which can be achieved through improved system capacity, enhanced traffic management and automation concepts, as well as new high-capacity rolling stock. Moreover, increasing the attractiveness of urban rail transport is another major challenge, taking into account the cost of service, comfort, and security.  It is expected that improvements in IT solutions, alongside adaptation of rail communication protocols to platforms dedicated to multimodal services, can enable innovative services and thus improve the attractiveness and provide an incentive for a modal shift from more polluting transportation means (7). 

Most of the rail research in the EU for the 7th Framework Programme and Horizon 2020 has been carried out within the Shift2Rail Joint Undertaking, which was succeeded by the Europe’s Rail Joint Undertaking for Horizon Europe (2), with several pilots in urban and suburban settings.  

The EC Communication on a European strategy on Cooperative Intelligent Transport Systems [1] acknowledges the key role of digital technologies in the current transformation of the transport sector. Intelligent Transport Systems (ITS) apply information and communication technologies such as journey planners, eCall (a system that automatically alerts emergency services in case of an accident), short-range communications for improving passenger safety and reducing road fatalities, and connected and automated mobility concepts for passenger and goods. Their aim is to make mobility safer, more efficient and more sustainable [5]. The Sustainable and Smart Mobility Strategy highlights the role of ITS to achieve the digital transformation of the transport sector, including milestones such as deploying and connected automated mobility at large scale by 2030 and eliminating fatalities in all transport modes by 2050 [6]. Moreover, by providing route guidance, they enable trip optimisation, reduce travel distances and increase multimodal transport and use of more sustainable modes. Furthermore, they also help to reduce emissions and improve air quality [7]. All the advantages and innovative solutions are particularly important for inhabitants of urban areas.  

The Directive on the framework for the deployment of Intelligent Transport Systems [3] and its amendment [4] stresses the importance of Intelligent Transport Systems for real-time traffic management, multimodal travel information, the improvement of transport safety and comfort, the uptake of zero-emission vehicles and MaaS solutions, as well as the deployment of Cooperative, Connected and Automated Mobility.  

Several detail solutions can be implemented within ITS, alone or in a combination of. Among of others, they include: 

- Digital twins are digital replicas of a physical, e.g. urban, space [8]. They use mathematical modelling methods to analyse the transport network and develop proposals for solving transport problems: optimisation of traffic and pedestrian flows, public transport, traffic management, optimisation of traffic lights, and investment justification in the construction of transport infrastructure [9][c, d] 

- Dynamic platoons of vehicles, which uses connectivity and automation to allow group vehicles to travel together in order to increase road capacity and decrease emissions [a, u] 

- Geopositioning which permits to precisely define position of a vehicle [b]. Recent project [e] tackled the problem of positioning errors which occurs in particular in deep urban valleys. 

- Intelligent traffic lights, variable speed limits and dynamic lanes to improve traffic safety [k], decrease congestion and improve air quality [10] 

- Cooperative forms of communications between vehicles that allow for better perception, sensoring and anticipation of traffic scenarios to improve passenger safety and reduce road fatalities. 

ITS infrastructures take also advantage of latest developments in artificial intelligence, big data and internet of things solutions [j].  

However, solutions implemented so far represent a small percentage of what has been proposed by the scientific community [2] and ongoing and future research and innovation initiatives may significantly widen ITS infrastructure applications.  

Source: https://www.researchgate.net/profile/Luz-Santos-Jaimes/publication/304998579_Ontology_Driven_Reputation_Model_for_VANET/links/57884a5008ae95560407bf01/Ontology-Driven-Reputation-Model-for-VANET.pdf [12] 

Waterborne transport greenhouse gas (GHG) emissions are on the rise, and they represent today almost 3% of global GHG emissions [1]. Ships entering EU ports emit 13% of the total EU transport GHG emissions [2], while inland waterway transport in the EU is estimated to emit 3.8 million tonnes of CO2 emissions per year [3]. Apart from GHG emissions, shipping is responsible for water degradation, air pollution and noise pollution. All these impacts have a negative impact on cities and their inhabitants.  

The International Maritime Organization (IMO), the United Nations agency focusing on waterborne transport, has developed an initial strategy on the reduction of GHG emissions from ships [5]. The strategy presents several measures to achieve its aims, including optimization of logistic chains and their planning, focus on power supply from renewable sources and development of infrastructure to support supply of alternative fuels and innovative technologies to further enhance the energy efficiency of ships. 

The European Commission is contributing to the task with several initiatives. The EU Maritime transport strategy 2009-2018 [8] included a set of environmental objectives for international shipping, such as the reduction of GHG emissions, NOx, and SOx, and the promotion of alternative fuels in ports. The roadmap for a resource efficient transport system set an objective of 40% reduction for EU CO2 emissions from maritime transport by 2050 compared to 2005 levels [9]. In the framework of the European Green Deal, EU climate target plans include the introduction of a high share of alternative fuels, such as renewable and low carbon liquid fuels [10]. To enable the uptake of alternative fuels, the Commission proposed a regulation requiring ports to address the demand for decarbonised fuels. At the same time, docked ships will be required to use shore-side electricity [11]. 

The Partnership Proposal for Zero-Emission Waterborne Transport focuses on six parallel activities, covering the use of sustainable alternative fuels, electrification, energy efficiency, design and retrofitting, digital green, and ports [4]. Also, the Clean Hydrogen Partnership [12] covers research on hydrogen and fuel cells for maritime applications, e.g. development and validation of a vessel running on liquid hydrogen (l, m), a vessel for hydrogen storage (n) or fuel cells, and hydrogen technologies developed for ports (o). Finally, energy-efficient and zero-emission vessels are also one of out of the five main topics of European waterborne transport research in H2020 [6]. Their successful deployment cannot happen without support from port cities.  

Within the framework of Mission Innovation, an industry roadmap for zero-emission shipping was developed. Three main pillars are considered the foundation for a zero-emission shipping future: ships, fuels, and fuelling infrastructure [7]. The latter is the most important, from the perspective of cities. 

The biggest potential for the reduction of emissions from waterborne transport is a transition towards a new generation of fuels and the preparation of appropriate fuelling infrastructure. There are several potential alternative fuels that can lead toward zero-emission vessels, including hydrogen and ammonia. These fuels can be used either in combustion engines (for long-distance shipping) or in fuel cells combined with electric motors (for shorter distances) [2]. Depending on the type of waterborne transport different solutions are necessary [4]: 

• ferries – the most suitable option for waterborne transport electrification, since they operate between fixed points in a limited range. Apart from battery packs, fuel cells and internal combustion engines powered by alternative fuels might be also used as an energy source; 

• short sea shipping – vessels that operate in a range of up to 200 nautical miles should enable to use battery packs, fuel cells, hybrid propulsion systems with electric and alternative fuels as well as propulsion systems using on-board renewable energy sources - as an individual energy source or in a combination of them; 

• inland waterway transport – limited range and operation with easily available recharging and refueling infrastructure enable to implement and test a wide variety of zero-emission solutions. Temporarily, as a transition phase, for currently operating vessels, retrofitting and usage of HVO (Hydrotreated Vegetable Oil, produced from renewable and sustainably sourced vegetable fats and oils) might be considered; 

• long-distance, international cargo ships, offshore vessels, and cruise ships - they mostly operate far from ports, nevertheless several solutions can be applied to reduce the impact on the environment (air, water, and noise pollutions), including in the vicinity of cities. Their high energy requirements make their transition towards zero-emission the most challenging but also necessary in the nearest future. Options include alternative fuels, electric and hybrid engines, renewable energies, etc. 

Regardless of solutions implemented for particular vessels, all relevant infrastructure (for recharging, refuelling etc.) in ports needs to be prepared and all the operations needs to be managed according to specific needs for new types of vessels. 

Heat Pumps (HP) are conversion devices able to transfer heat from a lower temperature heat source into a higher temperature heat sink. HP can provide heating, cooling, or domestic hot water both for residential and non-residential applications. There are several types of HP (electric compression heat pumps, gas driven HP, heat driven HP, etc.), with electric HP being by far the most common. An electric HP is made of five main components: an evaporator, a compressor, a condenser, an expansion valve and a refrigerant.  The working principle is based on the use of a refrigerant that release heat to the working fluid (air or water). The refrigerant is compressed using electrical energy and the heat of the refrigerant is released during the process of condensation (heat sink), passing from gas to liquid state. The refrigerant then returns then to its gas state passing through an expansion valve and using a cold source (energy source) to reject the heat. When the HP is working in heating mode, the energy source is cooled down and heat is provided to the energy sink of the process, while in cooling mode the process work in reverse.  An air source heat pump uses ambient air as heat sink. The parameter that has to be considered in the efficiency of a heat pump is the coefficient of performance (COP).  Typical air source HP require one unit of final energy (electricity) to provide 3-5 units of heat output and 2-4 units of cooling [1]. This parameter expresses the ratio between thermal power generated and the electricity consumed. Due to its versatility and high energy efficiency, the development of HP technology will be crucial in this transition towards electrification and decarbonisation of cities. 

Source: https://www.energy.gov/energysaver/air-source-heat-pumps 

Heat pumps have traditionally used refrigerants with high global warming potential (GWP) releasing greenhouse gases to the atmosphere (through leakages). The use of low-impact refrigerants is a need already covered by regulations, and expected to increase in the future. In fact, F-Gas regulation imposes a series of restrictions on the use of refrigerants until 2030, phasing out some higher GWP refrigerants soon, which paves the way for the use of natural refrigerants (e.g. CO2, propane and ammonia). 

Heat pumps have characteristics that make them very interesting for use in near-zero energy buildings (NZEB), district heating and cooling networks (distributed in every building or central heat pumps), positive energy districts, energy communities and in combination with other sources (geothermal, solar, etc.). NZEB are designed to have a very low energy demand, which is largely covered by energy from renewable sources, including self-production of renewable energy. In this context and considering residential energy consumption, the HP is imposed as a technology for the future. However, they can also be installed in existing buildings, even without deep renovation. 

Source: https://live.staticflickr.com/4061/4629658452_12c3717f49_b.jpg 

New developments of HPs include for decarbonisation of industries, electrification of district heating and cooling networks, utilization of waste heat (to upgrade it and inject it in networks) and use of both condenser and evaporator at the same time (using a dual source heat exchanger, see example below). 

Building Integrated Solar Thermal (BIST) can be considered a typology of solar thermal collectors, which are directly integrated to the building and used to convert solar radiation into useful heat that can be used in different applications.  

BIST typologies can be classified using different criteria, most commonly by the heat transfer fluid used in the solar thermal system which can be air, water or refrigerant. BIST using air as heat transfer fluid can be used for example to pre-heat the air passing through a heat recovery ventilator or an air coil of a heat pump [2]. The air-based BIST consists of an air gap between a glazed or opaque cover (or also PV panels) and the building fabric. Air-based BIST has the main advantage of avoiding freezing problems and corrosion, low cost, and simple structure. Water-based BIST can be employed in a building to produce domestic hot water or space heating. In this case, flat plate collectors are the main technology used but evacuated tubes can also be employed due to the higher efficiency but with the disadvantage of fragility.   

Refrigerant as heat transfer fluid can be used in systems coupled with heat pumps and PV/thermal systems. In this case, the BIST can operate as the evaporator of the heat pump increasing the efficiency using the rejection heat from the PV/thermal systems.  

BIST can be made in different designs to be integrated as architectural element that can be integrated in roofs, façades, windows, or used as shading element or to substitute frames and balustrades [3].  

Source: https://siko.at/       

The integration of BIST has a strong influence on the building aesthetics, therefore colour and the shape, and support elements have a fundamental role. Moreover, the integration of solar thermal elements  must offer structural integrity, protection from wind, rain and moisture [3]. 

20% of all food produced in the EU is currently wasted. 70% of this waste stems from households and processing. Although actions are primarily needed to reduce food waste, existing waste can be recycled in several ways to capture valuable nutrients [1]. 

When selecting recycling solutions for food waste, the food waste hierarchy can be used as an indicator as to which solution to give preference (see visual) [2]. Reuse for consumption and high-value products, followed by recycling of nutrients, should be preferred, where possible, over incineration for energy recovery only.  

Figure 1 Food waste hierarchy [2]

Several solutions exist to recycle food and nutrients: 

Using waste products and leftovers from food production to create new products 

Waste products, especially stemming from processing or unsold production can be used as input for new products instead of being thrown away. For instance, old bread from bakeries can be used for beer production, spent grain from whiskey and beer manufacturing for flour production (AGRAIN), whey in cheese production for protein powder, and fruit pits for protein powders and cosmetic products (KERN TEC). More examples for circular food products can be found in EIT Food’s Rising Food Star Association, WaysTUP and Greenovate!Europe.  

Figure 2 VALUEWASTE biowaste valorisation [VALUEWASTE] 

Extracting nutrients from consumer food waste  

When biowaste consists of different products mixed together, it becomes more challenging to reuse it. Extraction from household biowaste requires first the collection of the waste, and then the technologies to extract nutrients or other valuable parts of the waste. 

DECISIVE uses micro-scale anaerobic digestion (AD) and solid state fermentation (SSF) to create a circular metabolism for biowaste in cities. Nutrients and energy from biowaste are returned to urban farms in the city [DECISIVE]. 

Companies in the SCALIBUR project treat household and production food waste in cities. They extract sugars, create biopesticides and bioplastics from household waste through enzymatic hydrolysis and fermentation. Black soldier fly larvae are used to digest food waste from restaurants and turn it into biomass from which protein, fats and chitin can be extracted and used for many industrial applications and the organic residues can be used to improve the quality of soil [SCALIBUR]. 

WaysTUP explores different product value chains for biowaste, including coffee oil production from spent coffee grains, insect proteins from insect feeding on biowaste as well as extraction of flavours, carotenoids, gelatines and other products [WaysTUP]. 

The HOOP project will offer more insights on circular bioeconomies for cities [SCALIBUR]. 

Figure 3 WaysTUP Transforming Urban Biowaste into new products [WaysTUP] 

To decrease the heat gains of buildings, reflective or cool materials can be applied on the roof or the facades of buildings (1-3). Reflective materials are characterized by high solar reflectance (SR) combined with a high thermal emittance value (4).  

Numerous reflective white or light-colored materials are currently commercially available for buildings presenting solar reflectance values ranging from 0.4 to 0.9, and emissivity values close to 0.9. Reflective materials present a much lower surface temperature than conventional materials of dark color. For example, under solar conditions of about 1000 W/m2, an insulated black surface with solar reflectance of 0.05 and under low wind speed conditions, presents a surface temperature up to 50 °C higher than ambient air temperature, while for a white surface with solar reflectance of 0.8, the temperature rise is about 10 °C (5).  

Cool Materials 

Figure 1: Visible and infrared images of cool [1,2] and standard [3,4] black coatings applied on concrete tiles, (15) 

Reflective coatings can contribute to reducing the surface temperature of a concrete tile by 7.5 °C, and it can be 15 °C cooler than a silver-grey coating [6,7]. 

Parallel to the development of white reflective materials, a new technology of colored infrared reflective materials has been developed and is commercially available, (8,9). Use of infrared reflective materials increases the solar reflectance of the commercially available dark products from 0.05–0.25 to 0.30–0.45, (9).  

Recently, the development of daytime radiative photonic cooling technologies has permitted to decrease the surface temperature of the building materials at sub ambient levels, (10). Photonic materials coolers exhibiting an extraordinary solar reflectance combined with a high value of emissivity in the atmospheric window, can operate at sub ambient surface temperatures, (11). Sub-ambient photonic materials are already available for building applications. A review of the developments and recent achievements in the field of daytime radiative cooling technologies is given in Ref. [12]. 

Cool Roofs 

Figure 5: A cool roof on an industrial building in the Netherlands. Visual and infrared image after the implementation of the cool roof, (17). 

Use of reflecting materials contributes to lowering the surface temperature of the building materials since solar radiation is reflected rather than absorbed. As a result, the heat penetrating into the building is considerably decreasing, indoor temperatures are lower and the need for air conditioning is significantly reduced.  

The European Commission’s Renovation Wave strategy​ [1]​ states that “The Commission will introduce Digital Building Logbooks that will integrate all building related data provided by the upcoming Building Renovation Passports Smart Readiness Indicators, Level(s) and EPCs to ensure compatibility and integration of data throughout the renovation journey”. 

A digital building logbook is a common repository for all relevant building data. Functioning as a dynamic tool that allows a variety of data, information and documents to be recorded, accessed, enriched and organised under specific categories​ [2]​. The purpose of this novel concept is to ensure all relevant data is stored and made available to various stakeholders to support their decision-making process. For the building owner, it can enrich a Building Renovation Passport (i.e. a renovation plan for the specific building) and improve the likelihood the renovation decision is well-informed in terms of economic, environmental and social parameters. It can improve circularity by enabling construction and waste companies to identify certain materials and effectively dismantle the building at the end of its lifetime. Furthermore, it can help local authorities to plan for carbon neutral (or positive energy) districts as the logbook comprises detailed information on all the buildings. The logbook can therefore allow the local authorities to more accurately match local energy demands with energy supply (e.g. how many buildings must be renovated for low-temperature district heating to be feasible in a specific district). The instrument can also be used to drive local collective decarbonisation efforts. For example, by aggregating demand for energy renovation solutions or renewable energy installations. Furthermore, the digital building logbook can also help local authorities to more easily and accurately monitor the mitigation progress of the building stock.  

(Source: BPIE for Horizon 2020 project X-tendo, 2021 ​[2]​) 

The sub-type benefits the Digital Building Logbook can bring to enable smart and sustainable cities, are several including,:  

Drive local collective decarbonisation efforts. The decarbonisation efforts of buildings and cities need to become more harmonised for the EU to reach its long-term objectives. The Digital Building Logbook could be used to aggregate demand for renovations in a district, where renovation needs more easily could be identified. The Digital Building Logbook could be a useful tool in the transformation towards positive energy districts. [3] 

Support local authorities. The Digital Building Logbook can be used to monitor the progress towards climate and sustainability goals. For example, it is used in Flanders (Belgium) to monitor the progress of the building stock towards the long-term target. Furthermore, the Digital Building Logbook could be used to support local authorities to develop better energy and climate plans, where the logbook provides detailed information about every buildings, including which measures are required in the future. [3] 

Support local one-stop shops for energy renovations with data. Deep renovation is a complex process that involves a complete overhaul of a building. Most people are aware that better insulation of walls, roofs and basements will lower the energy consumption of the household but they have little information on what the benefits entail, who to contact, what measures to prioritise or which order they should be implemented. Digital Building Logbooks can enrich one-stop shops with data, enabling them to provide a stronger case for why the building owner should invest in an energy renovation. [3] 

Facilitate circularity in the building sector. The Digital Building Logbook can enable circularity in construction and buildings through deconstruction, reuse and recycling of materials. The logbook can include information on which materials the building comprise and where these are places, which will facilitate a more effective recycling process. Furthermore, it can improve traceability of materials and chemical substances. [3] 

Support the development of Building Renovation Passports. A building renovation passport provides a long-term, tailored renovation roadmap for a specific building, following a calculation based on available data and/or an on-site audit by an expert. The available data in the Digital Building Logbook allows for a more precise and cost-effective Building Renovation Passports to be developed. [5] 

(Source: VEKA – Flemish Energy and Climate Agency. Illustration of their operational Woningpas, which is the main inspiration for the digital building logbook)  ​​​​​​​

Buildings represent the largest energy consumer in Europe. Deep energy renovation of the existing stock to reduce its energy consumption seems to be the most appropriate measure and a key policy to achieve emissions reduction targets. The largest part of the European building stock was built way before strict energy requirements were set and, given the long life span of buildings, it is expected that between 85 -95 % of the existing stock will be still under use by 2050 [1]. Given the importance of the global climate change and the commitments under the Paris agreement, the European Union, (EU), has proposed in the Climate Target Plan 2030 to reduce the greenhouse gas emissions by at least 55% by 2030 compared to 1990 (2). 

Figure 1: Deeply renovated residential buildings in Leida, Spain (7). The Spanish demo of Bellpuig consists of a recently built multifamily house with poor performance. The east-oriented main façade is retrofitted by installing timber prefabricated façade modules, including new windows with shading system, decentralised ventilation machines with heat recovery and PV panels. 

However, according to a recent study performed on behalf of the European Commission (3), ‘the average total annual energy renovation rate of residential buildings, namely the sum of all different levels of energy renovation depths from “below threshold” to “deep renovations”, for the period 2012-2016 based on floor area is estimated to be at around 12% for EU28 as a whole. For residential buildings, the annual weighted energy renovation rate was estimated close to 1% within the European Union. This is in line with other estimations of the European Commission (0.4-1.2% depending on the Member State) and highlights the insufficient progress in the building sector in terms of moving towards decarbonisation of the building stock’. Therefore, the weighted annual energy renovation rate in the EU is as low as 1% while the annual rate of deep renovation is only 0.2% and 0.3% in residential and non-residential buildings, respectively (3). To break down the energy renovation barriers, the Commission introduced the Renovation Wave strategy (2), which aims to at least double the annual energy renovation rate by 2030 and to foster deep renovation. 

Figure 2: Energy renovation of a building in Bulgaria (8). The project involves renovation of the existing envelope including additional insulation and high quality windows, new HVAC systems and a better ventilation and control system. 

The Energy Performance of Buildings Directive (EPBD, 2010/31/EU) is the main legal tool to enhance the energy efficiency in buildings across the EU (4). To support the implementation of the Renovation Wave strategy, the Directive was revised in 2021. The proposal defines deep renovation as a renovation that transforms buildings into Nearly-Zero Energy Buildings (NZEBs) in a first step. As of 2030, deep renovation will transform existing buildings into Zero-Emission Buildings (ZEBs) (5). 

Figure 3. Energy renovation of a commercial building in Germany (9). The building comprises a single volume with four rectangular courtyards and a publicly accessible ground floor that provides a new pedestrian connection between downtown Munich and the museum district. Floor-to-ceiling windows and a smart spatial organization allow employees to have visual connection to their colleagues throughout the building, while various open areas act as meeting spaces where people can collaborate across departments. Thanks to a holistic approach to sustainable design, the new building consumes 90% less electricity and uses 75% less water than its predecessor. Heating, ventilation, and air conditioning systems can be adjusted by employees, and thanks to the company’s smart building technology, data from 30,000 points allow for a comprehensive insight into the daily energy performance of the building. 

The deep or NZEB renovation level vary across the Member States. Common measures identified in deep renovation include thermal insulation to achieve U-values of 0.10 – 0.20 W/(m²K) for walls and 0.10 – 0.20 W/(m²K) for roofs, passive technologies (shading devices, natural ventilation, night cooling, thermal mass), active technologies (mechanical ventilation with heat recovery, condensing boilers, district heating) and renewable energy from photovoltaics (PVs) and solar thermal (6). 

The main purpose of this document is to provide knowledge, information and a reference for the city decision makers, building professionals and building stakeholders to implement appropriate energy renovation measures across the EU Member Countries.   

Enhancing the thermal performance of building envelopes by improving the thermal properties of construction systems is possible by acting upon not just the insulation level, but also the thermal inertia through proper selection of the construction materials [1–3]. The building structure itself constitutes a source of thermal energy storage and plays a key role in buffering heat and in reducing indoor temperature swings [4]. Literature claims that it is impossible to design energy-efficient buildings using only a U-value-based approach (that is, only with insulation) and that the role of thermal inertia, i.e., the positive effect of thermal capacity, appears to be relevant in particular for moderate climates [5,6]. 

Historical buildings using stone masonry or rammed earth are characterized by high thermal inertia [7–10]. Studies based on dynamic simulations and on-site monitoring proved that massive envelopes are able to ensure a considerable reduction of indoor thermal discomfort, especially during summer in cooling dominated climates [7]. The use of alveolar bricks or hollow bricks also increases the thermal inertia of buildings compared to tradition brick construction [11,12]. 

Effect of the addition of thermal inertia (i.e., addition of PCM) in the building envelope [4]  

Modern architecture has moved to more innovative constructions systems using stone masonry or rammed earth. Today, stone is used by many designers to build single layer, multiple layers, composite load-bearing masonry walls, and self-supporting masonry envelopes [7]. Rammed earth construction is also moving towards multi-layers walls (i.e., including insulation) [13] and considers embodied energy and the use of sustainable materials (i.e., by-products) [14]. 

Another addition to modern architectural practices sensitive to the role of thermal inertia is the incorporation of phase change materials (PCM) in walls [4]. When PCM is added to the building envelope, the envelope itself becomes a source of thermal energy storage (TES) and it plays a key role in buffering heat and in mitigating the dynamicity of outdoor thermal oscillations, while displacing the heat penetration in time so that the heat reaches the indoors when it is most needed (peak load reduction and offset). Therefore, the building wall/structure acts as a heat sink during warm/hot periods (and a heat source during cool/cold periods).  

Use of sustainable materials in modern rammed earth architecture [14] 

Finally, another material largely studied for its thermal capacity in building envelopes is concrete. Today, studies involve increasing the circularity of concrete by developing new formulations (i.e., geopolymers) [15,16] or adding by-products (i.e., fly ash, steel slag) [17,18] or involve increasing its thermal capacity/inertia by adding PCMs in the concrete formulation [19,20]. 

Solar thermal panels are devices that convert solar radiation into heat and transfer it to a heat transfer fluid (typically water or an anti-freeze fluid) to be stored or to reach the point of use.

Source: Jafari S, Sohani A, Hoseinzadeh S, Pourfayaz F. The 3E Optimal Location Assessment of Flat-Plate Solar Collectors for Domestic Applications in Iran. Energies. 2022; 15(10):3589. https://doi.org/10.3390/en15103589 

FPC is the main solar collector technology installed in Europe with more that 2 million m2 installed per year [1]. FPCs are suitable for low-medium temperature applications including domestic hot water, heating, preheating and combined systems. In buildings FPC can be used both as a direct system or an indirect system. In the first case, cold water in a storage tank is pumped and heated directly through the collector. However, in passive system (thermosiphon) can be employed avoiding the use of the pump and using the force of gravity and the buoyancy effect of water at different temperatures. In these systems a horizontal tank are employed and placed above the collector. In an indirect system a heat transfer fluid in a closed loop is heated by the collector and used to transfer thermal energy to the water through a heat exchange. FPC can be also used in cities to provide renewable heating at district level being an effective solution when they operate at low temperatures [2] .  FPC consists of tubes carrying a heat transfer fluid placed in an insulated, weather-proof box with a dark absorber material and thermal insulation material. FPC can be installed as a thermosiphon system, in which the circulation of the heat transfer fluid is induced by the density difference caused by the increase of temperature, or as an active system with a pump [3]. The simplicity of construction makes FPCs a relatively low-cost solution, which can be installed as single modules on roofs, or manufactured in a larger format for roof or facade integration or for ground-mounted systems. For the production of FPC, different materials can be used including copper, aluminium, stainless steel combined with a glazing to achieve a high efficiency. 

Source: https://www.onosisolar.com/solar-collectors/flat-plate-solar-thermal-collector/  

Heat Pumps (HP) are conversion devices able to transfer heat from a lower temperature heat source into a higher temperature heat sink. HP can provide heating, cooling or domestic hot water both for residential and non-residential applications. There are several types of HP (electric compression heat pumps, gas driven HP, heat driven HP, etc.), with electric HP being by far the most common. An electric HP is made of five main components: an evaporator, a compressor, a condenser, an expansion valve and a refrigerant.  The working principle uses a refrigerant to release heat to the working fluid (air or water). The refrigerant is compressed using electrical energy and the heat of the refrigerant is released during the process of condensation (heat sink), passing from gas to liquid state. The refrigerant then returns to its gas state passing through an expansion valve and using a cold source (energy source) to reject the heat. When the HP is working in heating mode, the energy source is cooled down and heat is provided to the energy sink of the process, while in cooling mode the process works in reverse. Typical HP require one unit of final energy (electricity) to provide 3-5 units of heat output and 2-4 units of cooling [1]. The ratio between thermal power generated and the electricity consumed is usually referred to as the coefficient of performance (COP) which is the main parameter to express HP efficiency. Due to its versatility and high energy efficiency, the development of HP technology will be crucial in this transition towards electrification and decarbonisation of cities.  

Heat pumps have traditionally used refrigerants with high global warming potential (GWP), releasing greenhouse gases to the atmosphere through leakages. The use of low-impact refrigerants is a need already covered by regulations, and expected to increase in the future. In fact, the F-Gas Regulation imposes a series of restrictions on the use of refrigerants until 2030, phasing out some higher GWP refrigerants soon, which paves the way for the use of natural refrigerants (e.g. CO2, propane and ammonia). 

Heat pumps have characteristics that make them very interesting for use in near-zero energy buildings (NZEB), district heating and cooling networks (distributed in every building or central heat pumps), positive energy districts, energy communities and in combination with other sources (geothermal, solar, etc.). NZEB are designed to have a very low energy demand, which is largely covered by energy from renewable sources, including self-production of renewable energy. In this context and considering residential energy consumption, the HP is an important technology for the future. However, they can also be installed in existing buildings, even without deep renovation. 

New developments of HPs include decarbonisation of industries, electrification of district heating and cooling networks, utilisation of waste heat (to upgrade it and inject it in networks) and use of both condenser and evaporator at the same time (using a dual source heat exchanger, see example below). 

Ground source heat pumps (GSHP) extract the energy from the ground, achieving a higher efficiency compared to air source HP. This makes them highly relevant from a climate mitigation perspective. However, GSHP are more expensive in terms of up-front cost. This makes them particularly relevant in an urban context for apartment buildings, or for shared schemes among groups of houses where the drilling costs can be shared, or for district heating and cooling networks. 

In a conventional GSHP, a horizontal or vertical collector is used to extract thermal energy from the ground which is transferred to the HP refrigerant through an external working fluid (brine or water) working in a closed loop. However, direct expansion of the refrigerant of the heat pump can also be done removing the external closed loop. This technology usually refers to direct expansion GSHP (DX-GSHP) and is considered more efficient and cheaper. Nevertheless, DX-GSHP has more complications during the design and needs a high refrigerant charge. Moreover, it may lead to environmental problems such as ground pollution [2]. 

MATURITY:  

Nowadays, GSHP are widely installed in both residential and non-residential buildings. The annual rate of installation is estimated to increase of 10-12% every year [3]. 

Thermal energy storage (TES) systems can  store heat or cold to be used at a later time under varying conditions such as temperature, place, time, or power. The main use of TES is to overcome the mismatch between energy generation and energy use [1]. The key requirements for the design of a TES system are high energy density in the storage material (storage capacity), good heat transfer between the heat transfer fluid (HTF) and the storage material, mechanical and chemical stability of the storage material, compatibility between the storage material and the container material, complete reversibility after a number of cycles, low thermal losses during the storage period, and easy control. Moreover, the most important design criteria are the operation strategy, the maximum load needed, the nominal temperature and enthalpy drop, and the integration into the whole application system. 

Integration of active TES systems in a building [10] 

Already in 2011, Arce et al. [2] calculated the potential of load reduction (L), energy savings (E), and climate change mitigation (as CO2 emissions reduction – RCO2) of TES in buildings in the EU. The applications considered were seasonal solar thermal systems (L=25,287 MWth; E=46,150 GWhth; RCO2=12,517,676 tons), district and central heating systems (L=1,453,863 MWth; E=2,326,182 GWhth; RCO2=630,957,558 tons), solar short-term systems (L=416,180 MWth; E=319,269 GWhth; RCO2=86,599,153 tons), and passive cold systems (L=9,944 MWth; E=18,148 GWhth; Ee=6,481 GWhe; RCO2=3,085,135 tons). The subscript “th” stands for “thermal” and the subscript “e” stands for electric. 

There are three technologies of TES systems, each one with different performance, which will drive for which technology each one is more appropriate. Moreover, each technology is in a different maturity status.  Sensible TES is when the energy is stored increasing or decreasing the temperature of a material (i.e., water, air, oil, bedrock, concrete, brick). Latent TES uses the phase transition, usually solid-liquid phase change, of a material; the materials used in latent TES are called phase change materials (PCMs). The last technology includes sorption and chemical energy storage  and it is usually known as thermochemical TES. 

Integration of PCM in a TABS system: PCM in concrete slab with exchange with air for free-cooling in summer and free-heating in winter [15] 

Several reviews can be found in the literature on TES for building applications, i.e., PCM for heating and domestic hot water (DHW) [3], PCM for air conditioning [4], PCM in building envelopes [5], adsorption for cooling in buildings [6], TES in hybrid systems [7], TES for seasonal storage [8], or more general building applications [9–11]. 

This factsheet focusses on the use of PCM for heat recovery and free-cooling and free-heating. In this context, core activation, thermally-activated building systems (TABS), suspended ceilings, and ventilation systems are used [10]. In free-cooling, coldness from the night is captured and stored to be used when cooling is needed in the building [12]. Free-heating is a similar concept, when heating from waste heat recovery or available solar energy are captured and stored to be used at a later time, in the heating system. PCMs help in increasing the amount of coolness or heat that can be stored; moreover, PCMs help storing the thermal energy at the desired temperature. These technologies may use ventilation systems to improve their performance [13]. Heat recovery can be done from any waste energy source available in the building. An example is the use of PCMs to recover the rejected heat from air-conditioning equipment [14].  

Phase change materials integration in a ceiling (https://www.building.co.uk/news/phase-change-materials/3111760.article) 

Even if these technologies cannot provide thermal comfort by themselves, and some supplementary equipment might be needed, they allow removing/downsizing the mechanical ventilation systems, while providing better thermal conditions, circulating fresh air, reducing energy costs, and improving demand side management. 

Heat recovery in district heating networks is relevant for climate-neutral targets due to the replacement of fossil fuels in proportion to the use of waste heat alongside renewable energy sources. The exchange of heat and coolth among different buildings and sources represents an interactive way of satisfying thermal energy needs for space heating. Such exchanges can include those based on the rejected waste heat from the refrigeration of supermarkets that can be used in local residential blocks. In addition, heat recovery from local data centers, underground (metro) shafts, and wastewater can be integrated into new generation district heating and cooling networks. These can be ambient loop systems that operate at ambient temperatures with booster heat pumps located at each connected end-user or building [1] (also see the solution factsheet on “From third generation (3G) to fifth generation (5G) district heating and cooling networks (energy generation to substations” that provides complementary information). These forms of interaction can be used to define a transition from a heat market that used to be only consumer driven to a heat marked that is based on distributed prosumers [2]. 

Based on the Handbook for Increased Recovery of Urban Excess Heat [3], the potential for urban heat recovery is relatively large and is estimated with a potential of meeting about 10% of the heating and cooling demands of the EU. The largest heat volumes of waste heat come from wastewater and the lowest from metro systems [3]. Within the total heat recovery potential of about 1.41 EJ, 44% comes for wastewater, 21% from service sector buildings, 19% from data centers, 8% from residential buildings and 3% from metro systems [3]. The recovery/reuse of unexploited low-temperature heat in urban areas span data centres, transport systems, facilities, supermarkets, offices, shops, wastewater sewage networks, electrical substations, and natural sources in blue spaces, such as harbours, rivers, lakes, and seawater. Options for heat recovery from data centers, metro stations, food production facilities and retail stores, service sector buildings, such as hospitals, residential buildings, and wastewater treatment plants also involve different temperature ranges. Table 1 summarises the options that can be found in urban areas based on the temperature ranges and constant or variable temporality of supply. 

Figure 1. European Waste Heat Map that provides a geospatial representation of waste heat sources at each data point with an example being shown for a supermarket. The map involves thousands of points, each with waste heat potential [7]. 

Table 1. Summary of types, temperature ranges, temporality and the heat pump conversion for waste heat sources [3]  

In Europe, heat sources that are located within 2 km of a district heating network as a reasonable proximity involve [3]:  

• 3,982 wastewater treatment plants (accessible heat recovery potential of 625 PJ per year) 

• 997 data centers (accessible heat recovery potential of 271 PJ per year) 

• 20,171 food retain stores with refrigeration to preserve perishable foods (60 PJ per year) 

• 1,852 metro stations (accessible heat recovery potential of 49 PJ per year) 

• 669 food production units (accessible heat recovery potential of 4.8 PJ per year) 

Such sources of urban heat can be used directly in low-temperature district heating networks or indirectly by using a booster heat pump that brings the heat source to the expected temperature, especially in high-temperature district heating networks [3]. For example, the GrowSmarter project [4] involved waste heat recovery based on wastewater, waste heat from data centres, vacuum waste systems, and waste heat from fridges and freezers in supermarkets. In Stockholm, the district heating concept and business model was renewed based on “open district heating” for feed-in of waste heat. A heat recovery ratio of 73% allowed recovering ~3.1 GWh of thermal energy per year. In the data center, surplus heat from the cooling units is integrated into the existing district heating network. Plug and play heat pumps are installed at the facilities of the waste heat producer that allows this ability. In total, the data center is expected to recover about 1MW heat. Quantitatively, this can be equivalent to heating more than 1,000 apartments [5]. The recovered heat is suitable for being consumed in the supply line after a temperature lift based on efficient heat pumps using renewable electricity [5].  

Figure 2. Depiction of integrating waste heat from data centers and an underground (metro) ventilation shaft into district heating and cooling networks with distributed booster heat pumps in different buildings as well as thermal storage [1]. 

Heat recovery in the ReUseHeat project also provided a win-win situation where a data center reduces its cooling costs and the district heating company obtains heat that increases its heat capacity. The data center provides warm water at about 25°C that is piped to an energy station where the temperature is further increased through a heat pump [3]. In Berlin, the waste heat recovery from the ventilation units of a metro system is fed-in to a low temperature network at 50°C [3].  
 
In certain contexts, low-temperature district heating networks can be established as branch connections to existing high-temperature district heating networks. In such cases, the energy cascade that the low-temperature district heating network provides decreases the return temperature of the high-temperature district heating network, thereby increasing system efficiency. In one case, heat from the return pipe could satisfy about 25% of the heat demands of end-users [6].

Figure 3. Integration of heat recovery from the cooling load of supermarkets into new generation district heating and cooling networks with ultra-low temperature warm pipes and high temperature cold pipes and prosumer buildings [8].  

Currently, 75% of European houses are energy inefficient, which means that much of the energy needed is simply lost [7]. 

There are several passive and active solutions to improve energy efficiency in buildings. 

Passive solutions typically refer to solutions that reduce the amount of energy lost. They include façade isolation, roof isolation, insulation of heating tubes, energy-efficient windows, structural air tightness on doors and windows and other materials improving energy efficiency. The windows can have properties for thermal insulation or prevention of excess solar gain, avoiding overheating. In STARDUST, MySMARTLife and EU-GUGLE, both active and passive solutions were implemented. 

Active solutions refer to solutions that monitor and control the use of energy, or even solutions that can generate energy. 

The active solutions considered can range from energy management and monitoring solutions, to renewable energy production – such as solar thermal panels, shared photovoltaic systems, and ground source heat pumps. In some cases, it also includes energy storages like batteries or thermal storages [1]. The active solutions can be also service-based, in which case the owner of the building is leasing the solution [1] [3]. Also, thermal storages, thermally activated building structures and phase change materials can be a part of the solution [2] [4] [5].  

The solutions can be implemented in new buildings or in renovation of existing buildings [e.g. MySMARTLife - Nantes, Houseful - El Mestres]. Typically, the energy saving actions include improvement of thermal insulation and ventilation energy efficiency, as well as integration to low carbon energy sources like photovoltaic panels, ground source heat pumps, or integration into the local district network. The actions will reduce energy use, thus decreasing CO2 emissions. In addition, the actions will improve thermal comfort and air quality. 

Source: Energy efficient building, active and passive solutions [10]  

As renovations or implementation of active measures can prove a considerable financial burden, low-income households could benefit in the short-term from the adoption of temporary passive measures that can be implemented quickly and without the need for extensive technical expertise. Examples are radiator reflector panels that can be attached to the wall behind the radiator, insulation of tubes, water-saving showerheads, weather strips to close gaps between windows and walls, and hydronic balancing to optimize the distribution of water in the heating and cooling system to achieve maximum energy efficiency [18].  

In commercial buildings, the heating, ventilation, and air conditioning (HVAC) systems are typically more advanced compared to those present in apartment buildings. In addition, in commercial buildings the cooling typically consumes a considerable amount of energy. Therefore, the choice of the most impactful measures for energy efficiency has to necessarily be informed by the typology of building. 

Concrete is the most-used manufactured substance on the planet in terms of volume. It is used to build homes, schools, hospitals, workplaces, roads, railways and ports, and to create infrastructure to provide clean water, sanitation and energy [1]. Concrete is manufactured by mixing cement, water, aggregates and small quantities of chemical admixtures to improve its properties and meet specific product requirements. The direct CO2 emissions related to concrete largely come from cement production. The cement used today, called Ordinary Portland cement (OPC), is made from finely ground Portland clinker and gypsum. Clinker is produced by heating crushed limestone, clay and sand to 1450 °C [2]. Within the clinker production, there are two main sources of CO2: direct emissions (60%) stemming from the decarbonation of limestone and indirect emissions (40%) from burning fuel [3]. The European cement industry has actively worked on reducing emissions for a long time. In 2013, CEMBUREAU elaborated a Roadmap, which was revised in 2018 to ensure the objective of carbon neutrality down the cement and concrete value chain set for 2050 are met. A schematic illustration of 2050 road map is shown in Figure 1 [2]. 

Figure 1. CEMBUREAU 2050 roadmap. CO2 reductions along the cement value chain (5Cs: clinker, cement,  concrete, construction, re-carbonation) Source: Cembureau [2]

SUSTAINABILITY  

OPC typically contains 95% clinker and 5% gypsum. A very effective strategy of reducing cement footprint is to substitute part of the Portland clinker with other materials for which the footprint is low or even zero. Such substitutes are called supplementary cementitious materials (SCMs) and include: granulated blast furnace slag (GBFS), a by-product of pig-iron production in blast furnaces, fly ash (FA), a by-product from coal-fired power plants, natural pozzolanic materials obtained from volcanic compounds, sedimentary rocks (limestone), clays, agricultural residues (rice husk ash), silica fume (a by-product of silica and ferro-silica alloy production processes) and mixes thereof [3,4,5]. OPC clinkers partially substituted with one or more SCMs are referred to as blended cements. Reducing the clinker content in cement saves both energy-related and embodied CO2 [3]. Examples of projects applying this concept are LC3 and EnDurCrete. 

INNOVATION/NEW MATERIALS 

Alternatives to Portland cement can achieve significant emission savings as they rely on different raw material mixes and require lower firing temperatures. Belite-rich Portland clinkers are produced with the same process as OPC clinkers, but with less limestone in the clinker raw material mix, thus the CO2 generation is reduced. However, this emission reduction of around 10% is rather modest relative to OPC [3].  

A promising lower-carbon alternative is calcium sulphoaluminate (CSA) clinker. It contains ye’elimite as the main constituent, which reduces direct CO2 emissions by 44% [1]. Unfortunately, their cost also increases significantly at the same time, because higher ye’elimite content requires more expensive aluminium-rich raw materials [3]. Belite calcium sulphoaluminate (BCSA) clinker is able to circumvent the high raw material costs of CSA clinkers. They are preferably referred to as “belite–ye'elimite–ferrite” (BYF) cements. Such clinkers have CO2 savings of 20% or greater per unit of clinker in the cement due to the lower limestone content with additional savings coming from the lower firing temperatures and 30-50% lower electricity demand for grinding as they are more friable [1]. Projects applying and developing these new types of materials include Ecobinder for BYF cements. Commercially available BYF clinkers Aether and Ternocem have the potential to replace OPC clinker in many major applications [3].  

The carbonated calcium silicates (CACS) clinkers, primarily consist of wollastonite cured with CO2. Process CO2 emissions generated in CACS clinker making are, in principle, re-absorbed during the CO2 curing process [1]. Solidia cements (Solid life) have recently been commercialised for fabricating certain cement-based products, but the CO2 gas for curing currently comes from industrial gas suppliers. The long-term goal is to use recycled industrial CO2 from industrial flue gases, promoting emission reductions and circularity [3]. 

Cements based on magnesium oxides derived from magnesium silicates (MOMSs) are able to counterbalance and absorb the CO2 released in the manufacturing process while curing. If the magnesium oxides are coming from natural magnesium sources free of carbon, such as magnesium silicate rocks, they would yield net negative CO2 emissions, having a true environmental advantage [1]. CO2MIN is a good example, focusing on identifying such rocks. 

Figure 2. CO2 savings with LC3 cement  Source: LC3 [11] 

RECYCLING and RE-CARBONATION 

Concrete reabsorbs a significant amount of CO2 over its normal service lifetime and also after demolition. The recarbonation of recycled cement paste is a rapid process at normal pressure and temperature, showing a substantial CO2 sequestration potential. This concept reduces the CO2 footprint of cement, promotes recycling of end-of-life concrete and circularity as the carbonated material can be used as SCM for the production of new blended cements [2]. Examples of projects applying and developing recycling technologies are C2CA, TRACK4REUSE and RE4. 

Users of such solutions can be:  

-Industries, universities and research institutions investigating durability, sustainability, alternative cement materials, recycling and re-carbonation along concrete value chain 

-Potential customers in the target sectors: concrete markets and manufacturers of new concrete technologies  

-Recommendations and standardisation experts

Product labels for circularity can encourage consumers to make more environmentally friendly purchasing choices, thus nudging manufacturers towards a more sustainable production (16);  help extend product lifespan by promoting repair over disposal (4); and ensure correct sorting and recycling of end-of-life products. 

They can include various lifecycle aspects, such as a product’s origin, energy efficiency, lifespan (i.e. durability, reparability and upgradeability), or its recyclability – although most consumer-oriented information is currently related to upstream and end-of-life aspects (16).  

Indeed, product lifespan labels for circular economy are still in the early stages of development, and are mostly private labels or voluntary schemes. Similarly, labels and certificates for used goods are relatively scant and tend to be limited to certain product groups (16). Other consumer-oriented labels are waste separation labels, which can improve the waste stream both at consumer and sorting plant level (9), and recycled content labels, which promote the use of recycled materials (16).  

In an increasingly digital and IoT-enabled economy, some manufacturers turn to interactive and smart labels. These solutions help engage consumers by providing an immersive consumer experience, maintain control over the supply chain and grant traceability, and fight counterfeiting (17).  

Product labels and Digital Product Passports, as proposed in the European Commission’s Sustainable Products Initiative (2), can play an important role in the transition to a circular economy, especially if paired with eco-labels and certification schemes. Their impact can be maximised by the integration with novel technologies such as functional materials and inks, Radio-frequency identification (RFID) and Near-field communication (NFC) technologies, blockchain, and IoT (11). 

Smart Labels for circularity, durability, reparability, and recyclability 

Traditional optical markers (e.g. 1-dimensional barcodes, QR codes) are widely used to return static information about a product. Enabled by barcode scanning apps, they can redirect the consumer towards information on how to e.g. recycle a specific product or packaging (9), or disassemble, repair, and replace a component (21). 

Further evolutions of product labelling are represented by RFID tags, novel QR codes with functional components (e.g. smart inks and indicators (14)), printable electronics, and new flexible chips (such as CAPID) – which can provide dynamic, context- and item-specific information (12) usually accessible through a smartphone. 

Depending on the technology, smart labels can thus carry information about origin and composition (20), environmental footprint, instructions for repair, recycling and disposal (18), but can also be sensitive to environmental conditions and provide real-time information and feedback.  

Examples of innovative solutions for smart labelling are TagItSmart tags and On Track RFID-tags. 

TagItSmart concept overview 

Smart Labels for zero-waste – food industry 

The experimentation with and use of smart inks and labels is more advanced in the sector of fast-moving consumer goods (FMCG), especially for perishable food products. The use of such labels can maximise shelf life of products and help contribute to the reduction of food waste by monitoring the freshness of the product, detecting leakages, and monitoring after-opening time. Freshness smart labels can provide visual or tactile feedback, such as the IFLFSCTM, T-Sense Cold, and Mimica Touch temperature-sensitive labels. Alternatively, they can require an NFC reader (smartphone and dedicated app) to access the information, like in the case of INNPAPER printed electronics and GLOPACK RFID bio-sensors.  

GLOPACK RFID Tag 

Sustainable and recyclable labels 

As both traditional and smart labels need not only to enable circularity but also to be sustainable themselves, more and more packaging and labelling solutions try to make use of recycled and recyclable materials. Examples are the 100% compostable packaging compatible with compostable labels by Nature Fresh, and Mimica Touch caps and tags matching the material of the package for easy sorting.  

With regard to RFID tags, newer generations using printable electronics are more sustainable than traditional ones (8), as they can offer biodegradable or recyclable solutions and, especially if combined with addictive manufacturing techniques, help reduce e-waste (22). Examples are INNPAPER paper-based smart label and T-Sense Cold indicators. 

Urbanisation and industrialisation have led to reduced vegetative cover and decreased water storage in the subsurface, as well as the concentration and accumulation of surface runoff in sewage systems due to reduced infiltration into the soil [1] [2] [5]. Until now, no universally applicable urban flood management solution has been developed. However, hybrid measures that combine nature-based solutions (NBS) with conventional engineering solutions/grey infrastructure have been identified as the optimal mix of security provided by grey infrastructure with the multiple co-benefits of nature-based solutions [3]. 

The typical engineering solutions are rainwater systems collecting rainwater via drainage pipes, channelled to the urban rainwater system, and further to the water utilities to be purified. In older cities, rainwater is typically collected in the same water systems as sewage water, which creates large volumes of water for the local water utilities to purify.  

There are a number of nature-based solutions for rainwater retention [4]: 

GREEN ROOFS 

Green roofs are associated with three main positive effects: 1) cooling and evapotranspiration that leads to lower local temperatures, 2) storing capacity of the excess water, and 3) sunlight absorption.  

GREEN ROOFS [4] 

Green roofs can be categorised as follows:  

Intensive green roofs have both ecological and aesthetic requirements, as they are usually designed also for recreational purposes. The growth media is relatively thick and deeper than in extensive green roofs. Typical plants for intensive green roofs are trees, shrubs and perennials. Intensive green roofs need regular irrigation and fertilization. The water storage capacity is 30-160 l/m2. 

Extensive green roofs are light weight systems, typically characterized by minimum maintenance and management. Typical plants for extensive green roofs are low growing, rapidly spreading and shallow-rooting. Extensive green roofs are defined as a roof for which the capacity to store water is at least 25 l/m2, typically between 20-50 l/m2 . 

Smart roofs represent an extension of conventional green roofs because the system is equipped with a drainage system under the vegetation layer. The drainage layer retains storm water.  

Constructed wet roofs connect green roofs and constructed wetlands for domestic wastewater (grey water) treatment. Wet roofs are most commonly planted evenly with wetland or marsh plants, but can also have height differences. In addition, constructed wet roofs retain storm water for some time, and gradually release rainwater and reduce the overall runoff. Constructed wet roofs have positive impacts on the microclimate and cool down the local climate. 

VERTICAL GREENING in building integrated solutions 

Vertical greening in building integrated solutions can be either facade-bound or ground-based. These solutions have lower capacity of water retention compared to green roofs. Since pure vertical soil is not found in nature, nearly all types of vertical greening need maintenance [4].  

FAÇADE GREENING SOLUTIONS [4] 

BIOSWALES 

Bioswales are vegetated linear and low-sloped pits constructed near buildings or roads to reduce flood risk after heavy rains. Bioswales absorb, store, and convey surface water runoff.  

BIOSWALE [4] 

The basic principles in urban rain water management and retention is to mimic nature, meaning to stay as close as possible to the natural water cycle. In an ideal case, infiltration can store water and then release it as needed. However, in urban environments this is not always possible, therefore, rainwater should be stored in a buffer and gradually released to the natural environment [7].  

Alternative solutions for rainwater management and retention are for example [13]: 

• Infiltration reservoir roadway runs alongside the road and stores excess water from the road. 

• Infiltration trenches are areas close to roads or parking areas allowing rainwater run and preventing road flooding. 

• Swales collect storm water from roads, driveways, parking lots and other hard surfaces. 

• Soakaways are pits in the ground into which rainwater drainage runs. 

• Infiltration basins, vegetated depressions storing runoff on the surface and infiltrating it gradually into the ground. They are dry except in periods of heavy rainfall. 

• Permeable surfaces let the rainwater penetrate the surface. 

• Green roofs are roof structures with vegetation. 

• Retention ponds are used to hold and distribute rain runoff, which in turn helps prevent flooding. 

Infiltration reservoir roadway [7] 

Infiltration trenches [7] 

Swales [7] 

Basins, detention pond [7] 

Permeable surfaces [7] 

Well-functioning rainwater management prevents water concentration and runoff. The water concentration can be harmful for the building and road foundations. Runoff can cause road flooding and difficulties in road traffic. In addition, runoff water can damage agriculture and spoil the harvest. The runoff can be easily avoided with water buffers. The integration of the water into urban areas can have both recreational and functional purposes.  

Water management platforms collect the information about the water use of irrigation, water distribution and water consumption. In addition, the platforms enable a holistic analysis of all water use and where the water use is located in the city or region. The water management platforms serve the whole water system, and the rainwater system is a part of it. Water management systems provide better situation awareness and real-time information of the water system functions [14]. 

The use of renewable energy sources to produce electricity is one of the key actions of the energy transition for reducing carbon emissions to the atmosphere. On-site generation of electricity from renewables can help local municipalities, cities and communities to have substantial environmental, economic and social benefits. Indeed, generating electricity in the urban environment is possible to reduce transmission and distribution losses, and increase flexibility and energy security. The main technologies used for renewable electricity generation which can be adopted in cities are:  

• Wind power: wind represents one of the renewable sources which gained momentum in recent years with a substantial increase in wind power installed worldwide and a decrease in cost [1]. Wind energy is converted into electricity using wind turbines which are a mature technology available in different sizes and energy capacity. Today, small wind turbines are available to provide electricity for small-scale applications [2]. Wind turbines applied to cities can be stand-alone or building-integrated wind turbines. The turbines can be either horizontal-axis or vertical-axis [3].   

• Solar photovoltaics (PV): Photovoltaics (PV) represent the most used technique to convert solar radiation into electricity. A PV module is made of cells of semiconductor material which can convert light (photons) to a voltage potential (electricity). The most common installations of PV modules are roof or ground-mounted PV panels. PV modules can be also integrated into the building envelope (building-integrated PV, BIPV) acting as real construction products necessary for the integrity of the building functionality [1]. The efficiency of PV modules can be increased using PV concentrators (CPV) which use optical devices with cheap and suitable technology to concentrate the light on small and highly efficient PV solar cells. Another promising solution is the installation of PV modules in water bodies including oceans, lakes, reservoirs, irrigation ponds, and wastewater treatment plants. A floating PV (FPV) plant consists of a pontoon or separate floats anchored to the bottom of the water, to the shore or to adjacent structures. 

• Geothermal power [4]: The thermal energy contained in the deep earth crust can be used both for heating/cooling and to produce electricity. In the latter case, the steam captured in deep ground reservoirs is directly used to drive turbines and generate electricity. Dry steam, flash steam or binary cycle can be employed to generate electricity. Another technology under development is enhanced geothermal systems (EGSs) where e a subsurface fracture is created and a heat transfer fluid is injected and heated by the rocks and pumped to the surface to generate electricity.  

• Biomass, waste-to-energy, and biofuels [3,5] where electricity is produced from local biomass waste (e.g., from municipal solid, wood, agricultural wastes, sewage, and plant material). Waste-to-energy can incentivize waste recycling activities in cities promoting circular economy and and minimising the decrease in volume of usable waste, which is necessary to ensure a reliable and constant supply of energy. Biomass feedstocks can be burned producing steam to feed turbines that most commonly generate heat and electricity (combined heat and power - CHP). Biomass can be also converted in combustible oil or biofuels in a gasification process in a low-oxygen environment which can be more efficient compared to the burning of solid biomass (anaerobic digestion). Biogas can be captured from landfills and other facilities such as wastewater and manure treatment plants. The methane produced from the anaerobic digestion can be burned in a combustion process to produce electricity. 

• Hydropower [6,7]: it is a mature technology which converts the kinetic energy from falling or running water to electricity using turbines. Nowadays, hydropower is the largest source of renewable electricity worldwide and it can be available in different sizes. Small hydropower systems can be also integrated into municipal water facilities or irrigation ditches. 

Another technology which can be considered for on-site renewable electricity production are fuel cells [8]. These devices are able to generate electricity through a chemical reaction of oxygen and hydrogen. Since pure hydrogen does not exist in nature it can be generated from other sources including the reformation of natural gas or biogas or by electrolyzing water. The process of electrolysing water, which needs electricity, can be done through renewable sources. 

The use of renewable energy sources to produce thermal energy is one of the key actions of the energy transition for reducing carbon emissions to the atmosphere. On-site generation of thermal energy from renewables can help local municipalities, cities and communities harness substantial environmental, economic and social benefits. The main technologies used for renewables thermal energy generation which can be adopted in cities are:  

• Solar thermal – It represents the most common solution to produce thermal energy [1].  Solar thermal collectors are devices that convert solar energy into thermal energy available in a working fluid. Solar thermal collectors can be work for both small-scale and large-scale applications, depending on the technology. Solar thermal collectors can also be integrated into the building envelope (BIST). The easiest way to classify solar collectors is to divide them into concentrated and non-concentrated solar collectors. Under the first category, solar thermal collectors can be distinguished according to the heat transfer fluid used, which is typically air, water, or a refrigerant. The most common uses include: 

• Solar water heating systems: used for residential and non-residential applications to cover space heating or domestic hot water (DHW) demands. 

• Solar district heating: large scale solar collectors can be employed to provide heating to district heating networks.  

• Solar thermal cooling: solar thermal collectors can be coupled with absorption chillers to provide cooling energy for residential use. Due to the high temperature to drive chillers, evacuated tubes or concentrators are usually employed. 

• Solar heat for industrial processes: solar thermal collectors can be used to supply heat for different industrial processes including textile, food, and agricultural applications. 

• Geothermal [2]: The thermal energy contained in the deep earth crust can be used both for heating/cooling applications. The most common use of geothermal energy for thermal energy production are geothermal heat pumps (GSHP), which extract the energy from the ground to support the evaporation/condensation of the refrigerant achieving a higher efficiency compared to air source heat pumps. In a conventional GSHP, a horizontal or vertical collector is used to extract thermal energy from the ground which is transferred to the HP refrigerant through an external working fluid (brine or water) working in a closed loop. However, direct expansion of the refrigerant of the heat pump can also be realised by removing the external closed loop. Low- to mid-temperature geothermal energy (from 10°C to 150°C) can be directly used into distric heating or cooling networks in lieu of fossil fuels. In this case, wells around 2 km of depth can be drilled without affecting the visual landscape. 

• Biomass, waste-to-energy and biofuels [1,3]: local solid biomass can come from waste including municipal solid, wood, agricultural wastes, sewage, and plant material. Waste-to-energy can incentivize waste recycling activities in cities thus promoting circular economy and minimising the decrease in volume of usable waste, which is necessary to ensure a reliable and constant supply of energy. Biomass feedstocks can be burned producing steam to feed turbines that most commonly generate heat and electricity (combined heat and power - CHP). The main direct combustion technologies include mass burning, modular incineration and refuse-drived fuel. Small scale biomass such as wood pellets, briquettes or wood stoves are commonly employed for residential use. In direct combustion applications flue gases and ashes have to be monitored. Biomass can also be converted in combustible oil or biofuels in a gasification process in a low-oxygen environment which can be more efficient compared to the burning of solid biomass (anaerobic digestion). Biogas can be captured from landfills and other facilities such as wastewater and manure treatment plants. The methane produced from the anaerobic digestion can be burned in a combustion process to produce thermal energy. 

Thermal energy storage (TES) systems can store heat or cold to be used later under varying conditions such as temperature, place, time, or power. The main use of TES is to overcome the mismatch between energy generation and energy use [1]. The main requirements for the design of a TES system are high energy density in the storage material (storage capacity), good heat transfer between the HTF and the storage material, mechanical and chemical stability of the storage material, compatibility between the storage material and the container material, complete reversibility of a number of cycles, low thermal losses during the storage period, and easy control. Moreover, one design criteria could be the operation strategy, the maximum load needed, the nominal temperature and enthalpy drop, and the integration into the whole application system. 

Already in 2011, Arce et al. [2] calculated the potential of load reduction (L), energy savings (E), and climate change mitigation (as CO2 emissions reduction – RCO2) of TES in buildings in the EU. The applications considered were seasonal solar thermal systems (L=25,287 MWth; E=46,150 GWhth; RCO2=12,517,676 tons), district and central heating systems (L=1,453,863 MWth; E=2,326,182 GWhth; RCO2=630,957,558 tons), solar short-term systems (L=416,180 MWth; E=319,269 GWhth; RCO2=86,599,153 tons), and passive cold systems (L=9,944 MWth; E=18,148 GWhth; Ee=6,481 GWhe; RCO2=3,085,135 tons).  

There are three technologies of TES systems, each one with different performance, which will drive for which technology each one is more appropriate. Moreover, each technology is in a different maturity status. Sensible TES is when the energy is stored increasing or decreasing the temperature of a material (i.e., water, air, oil, bedrock, concrete, brick). Latent TES uses the phase transition, usually solid-liquid phase change, of a material (i.e., water turns into ice). The materials used in latent TES are therefore called phase change material (PCM). The last technology includes sorption and chemical energy storage and is usually known as thermochemical TES. 

Several review can be found in the literature on TES for building applications, such as PCM for heating and domestic hot water (DHW) [3], PCM for air conditioning [4], PCM in building envelopes [5], adsorption for cooling in buildings [6], TES in hybrid systems [7], TES for seasonal storage [8], or more general about the use of TES in building applications [9–11]. Moreover, TES systems also have an important role in district heating and cooling systems [12]. 

Zeolite used as sorption material 

This factsheet describes thermochemical TES. Thermochemical TES uses reversible chemical reactions or physical/chemical sorption, with potential to yield higher energy densities and minor thermal losses. This technology can be divided into sorption TES and chemical-reaction TES (which are commonly used of high temperatures, therefore with limited applications to buildings). Sorption TES can be further divided into solid-adsorption TES or liquid-adsorption TES. In a sorption system, a liquid sorbate (usually water) interacts with a solid or liquid sorbent (i.e., zeolites, silica gels, activated carbons, salts, salt composites). Adsorption systems are more compact but with lower energy storage efficiency [13]. These systems can be applied to cooling, heating, and dehumidification, and are much used in long-term storage (seasonal storage) [14,15].  

Mechanical energy storage (or electromechanical energy storage) systems are devices which convert electrical energy into kinetic or potential energy which can be reconverted into electricity at a later stage. Mechanical energy storage systems can be used in the grid to balance peak periods and to provide ancillary services including frequency, primary and voltage control to the power grid. The main technologies include pumped hydro energy storage (PES), flywheels, compressed air energy storage (CAES), and liquid air energy storage (LAES).  

PES: This technology comprises two reservoirs at different elevations connected by either pipes or tunnels and exploits the potential energy of water to store energy. In those systems, water is pumped to the upper reservoir during periods of low electricity demand when there is an excess of electricity available and released to power turbines when more electricity is needed. The efficiency of this system is estimated to be between 70-85% [1]. 

Flywheels: they store kinetic rotational energy through a rotor which is charged and discharged electrically using a dual-purpose motor/generator. Flywheels are characterized by fast response times, long cycle life, and high power density. The most common applications include transportation, frequency regulation and stability in electric grids, balancing fluctuations in renewable generation and peak management. The storage capacity of the flywheel is proportional to the mass of the rotor, which requires making these systems very heavy (e.g., metallic or composite materials) [2]. 

Source: https://en.wikipedia.org/wiki/Flywheel_storage_power_system 

Source: https://arena.gov.au/ 

CAES: this technology stores electrical energy in the form of compressed air during periods of excess electricity or low energy demand. In these systems, air is stored in underground mines or caves created inside salt rocks. To reconvert the pressurized air into electricity two main technologies can be used which differentiate the two main CAES typologies: diabatic CAES (D-CAES) and adiabatic CAES (D-CAES) [3]. In systems belonging to the first typology, the pressurized air is fed into a combustion chamber using natural gas or fuel and then expanded into a turbine to generate electricity. The efficiency of these systems can be estimated around 55% [1]. On the other hand, in A-CAES systems no combustion chamber is used. In this case, the heat produced during the compression is recovered and stored as thermal energy and used to increase the temperature of the air before the expansion in the turbine. Because of the heat recovered, the efficiency of these systems can exceed 70%. CAES is a technology suitable for peak shaving, load shifting, or for stabilising the grid through frequency regulation, contingency reserves, voltage support, or black start [4]. 

Source http://www.apexcaes.com/technology-overview 

LAES: this technology is similar to the concept of an adiabatic CAES yet, in this case, the pressurized air (or nitrogen) is liquefied using a Linde or Claude process and then stored in an unpressurized vessel. The main advantage compared to CAES is the higher energy density of the liquefied air (or nitrogen) which is then evaporated and expanded in a turbine when electric power is needed. LAES can be also combined with waste heat and waste cold sources to increase the process efficiency. However, also in a stand-alone configuration, the heat of compression and the cold released before the expansion in the turbine can be recovered to improve both the liquefaction and discharging process reaching a round trip efficiency of around 60%. LAES can be integrated into the grid with the same functionality as CAES with the ability to provide also cold thermal energy in a co-generation asset [5].  

Source Borri E, Tafone A, Zsembinszki G, Comodi G, Romagnoli A, Cabeza LF. Recent Trends on Liquid Air Energy Storage: A Bibliometric Analysis. Applied Sciences. 2020; 10(8):2773. https://doi.org/10.3390/app10082773 

Another technology is pumped heat electrical storage (PHES) which is similar to PES, but in this case, heat is pumped between two thermal storages at different temperatures using a reversible heat pump/heat engine. However, this technology is in the development stage. 

Ocean contains a vast renewable energy potential, which could support sustainable long-term development and can be a crucial component in the world’s emerging blue economy. Ocean energy technologies are utilizing different resources. They can provide both reliable and stable electricity, as well as support other components of the blue economy, such as aquaculture and desalination. 

The four main technologies developed are: 

• Wave energy: Utilizes the movement of waves to generate electricity. Depending on the design, they can take advantage of wave motion, the height difference or they can use trapped air to drive turbines. Multiple designs have been suggested, from ocean deployed, large capacity devices to smaller capacity devices deployed in larger numbers in coastal sites, such as ports.  

• Tidal energy: Two main technologies have been developed to produce electricity. Tidal range that harvests power from the water level differences between high and low tide and tidal stream that utilizes the kinetic energy of the tidal current. Tidal range applications are not currently being pursuit at large due to their limited deployment location, costs and environmental impacts. Tidal stream devices can be both placed on the seabed or on the sea surface.    

• Salinity gradient energy: Based on the difference of salt concentration between the freshwater of a river and the saltwater of the sea where the river empties.  

• Ocean thermal energy conversion: Utilizes the difference of temperature between the sea surface and the seawater in depths 800-1000m. These devices require a temperature difference between the sea surface and the deep sea of at least 20-25o C, making them viable only in tropical regions. 

The ocean energy sector has significant potential to contribute to the energy mix and therefore to the decarbonisation of the EU, having a theoretical potential in Europe of about 2800TWh for wave and 50TWh for tidal energy annually (Magagna D, 2020). The highest potential exists along the Atlantic coast followed by the Baltic and the Mediterranean. 

The heat island phenomenon rises the temperature of cities, increases the energy demand for cooling and deteriorates comfort conditions in the urban environment. To counterbalance the impact of the phenomenon, important mitigation techniques have been proposed and developed.  

It is widely accepted that an increase of the urban green infrastructure, and particularly of the tree cover, improves urban resilience [1-2]. Trees provide urban overheating reduction, pollutant removal, carbon sequestration, retention and detention of storm water runoff, while improving residents’ health [3-8]. The American Forestry Association in 1989 estimated that the value of an urban tree is close to $57,000 for a 50 years mature specimen. This value includes the benefits for air conditioning, soil protection, air pollution and wildlife habitats (9). 

The present fact sheet aims to provide information and recommendation on the impact of urban greenery on the urban climate and in particular on the potential decrease of the urban overheating. It does not discuss the impact of urban greenery on the specific energy demand of buildings. The fact sheet considers four types of urban greenery: Urban parks, street parks, green roofs and vertical green infrastructure attached to buildings. 

Greenery contributes to decreasing the ambient temperature through evapotranspiration and shading. In parallel, when impervious surfaces like asphalt and concrete are replaced by vegetation, the stored heat during the day and the emitted sensible heat during the day and night are significantly reduced. The potential temperature decrease because of the increased green infrastructure fraction depends on the difference of the thermal balance between the non-vegetated control/reference scenario and the vegetated one. It is mainly affected by the specific climatic conditions, the availability of soil moisture, the type of vegetation and the way it is distributed in a city. As a result, increased vegetation fractions may contribute to reducing the ambient temperature during the night or the day or both, or even cause some warming effects under specific conditions. 

To increase the urban green infrastructure, additional trees placed in urban parks or streets as well as building integrated greenery like planted roofs and vertical greenery systems may be used.  

Urban Parks 

Figure 2 : Bergpark Wilhelmsshohe Germany. Picture from : Bergpark Wilhelmshöhe - Urban Park in Germany - Thousand Wonders.  

Green roofs are partially or completely covered by plants over an engineered planting substrate on specialized build up of polymer materials. They are differentiated by the type of plants they may support. Extensive type green roofs are covered by low vegetation while intensive green roofs may support growing of shrubs and small trees. Important benefits are associated with the use of green roofs. Because of the solar and heat protection they provide, they contribute to lowering the energy consumption of buildings, while through latent heat processes, they decrease the surface temperature of the roofs and reduce the release of sensible heat to the atmosphere. In parallel, green roofs help with the storm water runoff management, provide better air quality, reduce noise, prevent erosion and increase the durability of the roof materials (10). 

Green Roofs 

Figure 3 : Green Roof in Athens, (9) 

There is an acceptable correlation between additional tree cover and the ambient temperature drop during the peak day period and at night. The average maximum drop of the peak daily temperature may not exceed 1.8 ◦C, when the tree fraction reaches its maximum, while the average night-time maximum mitigation potential is much higher and close to 2.3 ◦C. Given the serious spatial limitations in cities, a potential increase of the tree cover by 20% may initiate a temperature decrease close to 0.3 and 0.5 ◦C during the daytime and night-time, respectively. 

The local landscape and climate conditions highly affect the potential temperature drop in a city; however, the order of magnitude of the temperature drop remains close to the previously mentioned levels. Given that most of the cooling demand is during the peak daytime period, the shift of the maximum mitigation potential of trees during the night reduces the expected cooling contribution. Potential problems of night warming and serious increase of humidity levels reported by numerous studies should not be neglected, especially when additional trees are deployed in urban canyons, reducing the sky view factor, and when additional green infrastructure is planned in humid climates. 

There is a statistically significant correlation between the peak daily temperature decrease caused by higher green infrastructure fractions and heat-related mortality. When the peak daily temperature drops by 0.1 ◦C, then the percentage of heat-related mortality decreases on average by 3.0% 

Urban greenery and in particular trees contribute seriously to urban cooling 10 to 15 years after plantation. However, urban trees and the rest of the urban greenery systems present a very long-life span and may contribute to urban cooling for more than 30-50 years.  

Street Trees 

Figure 4: Street Trees. Picture is taken from : 20 Incredible Benefits of Urban Street Trees (arbor1.com).  

Vertical Greening 

Figure 5 : Vertical Greening in Volksbank Austria. Picture is taken from: Green Buildings - Volksbank Blog.

Several European cities increase the percentage of urban green infrastructure to improve the urban microclimate. The greenness of European cities has increased by 38% over the last 25 years, with 44% of Europe’s urban population currently living within 300 metres of a public park, (11). On average, some 40% of the surface area of European cities is made up of urban green infrastructure, with around 18.2 m2 of publicly accessible green space per inhabitant, (11). Figure 1, gives the available public green space per inhabitant in Europe, (12). 

Figure 1. The available public green in European cities per inhabitant, (12) 

Alternative cooling strategies based on improved thermal protection of the building envelope, and on the dissipation of a building's thermal load to a lower temperature heat sink, appear to be very effective.  

These strategies and techniques known as passive and hybrid cooling have already reached a serious level of acceptance from both the technical and scientific experts. 

The present document aims to inform and provide knowledge and recommendations on the use of artificial solar control devices in cities.  

Technologies dealing with the solar control of open spaces is one of the most important strategies for heat gain prevention. Their role focuses on the prevention of solar gains on landscape surfaces, occupant clothing or skin. The way outdoor spaces are used as well as the duration and intensity of the use depends highly on how comfortable they are. It is possible to control the climate of outdoor spaces by modulating the heat flows that determine thermal comfort levels, like the direct or diffuse solar radiation on the human body as well as the infrared radiation received by the human body by the surrounding surfaces, Figure 1.  

Water scarcity is a concern not only due to a changing climate and a rise in demand, but also due to non-accountable losses within water distribution networks. In some parts of the EU, up to 80 % of water pumped into distribution systems is lost [Life SmartWater]. Leakages in large diameter main pipelines represent the highest share of the total water losses. However, the issue has been poorly addressed thus far, for example due to lack of access to pipelines. In addition, there is a lack of leakage control and management of water systems [1]. 

Water pipe networks are vulnerable to losses due to the geographically wide range of water networks, permanent material wear on the pipes, and many access points. In addition, usually there is no availability of real-time information about network conditions nor the water flow for the water network operator. The undetected pipeline damages, possible illegal withdrawal, and potential contamination in the water network can result in damage to the environment [Life SmartWater]. 

Active management of water distribution networks through data and optimisation is not yet common. Currently, most of the water networks are operated based on mid-term planning. Leakages are often detected very late, with a delay of weeks or months. The detection is usually done manually. Therefore, smart water management systems, which can prevent water losses, have the potential to gain major savings in cost, and protect the environment [Life SmartWater].