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/   

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]. 

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]) 

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. 

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]. 

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.  

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. 

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].  

Heat Pumps (HP) are conversion devices able to transfer heat from a lower temperature heat source into a higher temperature heat sink. HP can be used to 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 of 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. 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, 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 (NEZB), district heating and cooling networks (distributed in every building or central heat pumps), positive energy districts, energy communities and by 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 as a technology for the future. However, they can also be installed in existing buildings, even without deep renovation. 

https://www.kensaheatpumps.com/water-source-heat-pump/ 

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

A water source heat pump (WSHP) works on a similar principle to that of ground source heat pumps (GSHP) by rejecting or extracting heat to a water pipe system (open loop) or a water loop (closed loop). In the first case water is pumped from the source and the heat is extracted from the heat pump before being discharged again to the water source. In the second case sealed pipes with anti-freeze are submerged into the water source. The fluid is heated/cooled by the water and returns to the heat pump. WSHP can be connected to aquifers, rivers, lakes or the sea, and to wastewater, cooling water from industrial systems, or a district heating system [1]. Compared to air source HP, the efficiency of WSHP is increased by the higher heat transfer coefficient of water compared to air, making them more efficient.   Moreover, the temperature of the water is more stable during the year compared to air. In the United States, ASHRAE sets the minimum efficiency requirements for WSHPs to be higher than air source HP being able provide 4 to 6 units of heating for every unit of energy consumed. In Europe the seasonal efficiency requirements set by regulation 813/2013 for water to water heat pump is set to 3.33 and 2.95 for low and medium temperature heat pump respectively [2]. With a water-source heat pump in heating mode, the heat of compression in the refrigerant circuit can be recovered for heating purpose. 

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).  

Borehole used in BTES systems

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]. 

Water tank www.(istockphoto.com) 

This factsheet describes sensible TES for short- and mid-term storage, while long-term is considered in the seasonal TES factsheet. Water energy storage has been used for many years. Today it is considered that TES in water tanks has an important role on the final efficiency of many energy systems, specially those including renewable energy [9,13]. In these storage tanks, attention should be given to ensuring the stratification of the water inside the tank, avoiding heat losses with good insulation of the tank and all auxiliaries, and the temperatures of operation. A way to increase the energy density of water tanks, also ensuring the water stratification, is the inclusion of PCM in the water tanks [14–16]. 

Another sensible TES technology is underground TES, including both aquifer (ATES) and borehole (BTES) [9,13]. The characteristics of these systems are: 

• BTES (borehole TES): It is suitable for soils with rock or water saturated with no or only very low natural groundwater flow. It usually has 30 to100 m depth and a heat storage capacity of 15-30 kWh/m3. The heat is directly stored in the water saturated soil, and it 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. It usually has a heat storage capacity of 30-40 kWh/m3.