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Technologies and applications for low/high temperature heat recovery in district heating (heat from district heating return pipelines, data centers, etc.)

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]  

 

 
 
 
 
 
 
 

Waste heat source 
 
 
 
 

Recovery type 
 
 
 
 

Temperature range °C 
 
 
 
 

Temporality 

(seasonal) 
 
 
 
 

Heat pump conversion type 
 
 
 
 

Data centre 
 
 

Server room air cooling systems 
 
 

25–35 
 
 

Principally constant 
 
 

Air to water 
 
 
 
 

Metro stations 
 
 

Platform ventilation 

exhaust air 
 
 

5–35 
 
 

Variable 
 
 

Air to water 
 
 
 
 

Food production facilities 
 
 

Rejected heat from 

refrigeration processes 
 
 

20–40 
 
 

Principally constant 
 
 

Liquid to water 
 
 
 
 

Food retail stores 
 
 

Rejected heat from 

refrigeration processes 
 
 

40–70 
 
 

Principally constant 
 
 

- 
 
 
 
 

Service sector buildings 
 
 

Central cooling devices 
 
 

30–40 
 
 

Variable 
 
 

Liquid to water 
 
 
 
 

Residential sector buildings 
 
 

Central cooling devices 
 
 

30–40 
 
 

Variable 
 
 

Liquid to water 
 
 
 
 

Wastewater treatment plants 
 
 

Post-treatment sewage water 
 
 

8–15 
 
 

Principally constant 
 
 

Water to water 

 

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].  

 

MATURITY: 

 

Urban waste heat recovery systems involve system innovation and do not have high maturity at a system level [3]. Awareness, capacity and know-how across stakeholders who could otherwise be involved in urban waste heat systems are lacking. This affects the process of identifying waste heat sources, system design, implementation, and customers who will be utilizing the waste heat. The collaboration of a wide-range of stakeholders are necessary, including district heating network operators, the owners of urban waste heat, urban planners, architects and installers. Due to the lack of awareness and collaboration based on suitable business models, there is mostly low demand for heat recovery solutions. For this reason, It is recognized that technological maturity is not the main barrier of utilizing urban waste heat - it is awareness on a largely unknown potential and the relatively lower maturity of a collaborative concept for sharing thermal energy [3]. 

CO-BENEFITS
KEYWORDS
PRE-CONDITIONS & ENABLING CONDITIONS
CONSTRAINTS/BARRIERS FOR IMPLEMENTATION
DRAWBACKS/ADVERSE IMPACTS OF THE SOLUTIONS after implementation
EXTERNAL LINKS
IMPACTS (Indicators + DNSH)
INSTRUMENTS/Processes for implementation
EXAMPLES

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Grey water treatment (including Nature Based Solutions) and reuse

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). 

 

Traditional greywater treatment 

Greywater treatment by mechanical systems is typically based on filtration or treatment with chemicals. In filtration, the aim is to remove impurities using filters, with typically a few or several filters in a row in order to guarantee good results. In a purifying process done with chemicals, the aim is to add chemicals that bind impurities, which are then removed from the water, for example, by filters. The mechanical treatment can start with a settlement tank, where coarse particles settle in the bottom of the tank and are then removed. After that, the greywater flows through filters, typically first gravel and sand and then biological filters like wood or peat. Last, if needed, ultraviolet light or chemicals are used to remove potential bacteria. 

Green roof and greywater treatment [7] 

 

Grey water recovery system [10] 

The first filter is a biofilter, which removes the fats and oil. The sand and gravel filter removes small particulates and other impurities. 

 

NBS-based greywater treatment 

Nature-based solutions (NBS) applications are typically constructed wetlands, green roofs, and green walls [1][2][3]. 

 

Several studies have shown that NBS-based greywater treatment has high removal performances [1][5], indicating the suitability of these systems in treating domestic greywater. Planning and design parameters should be carefully considered when implementing NBS; high residence time of water can be especially important for grey water treatment efficiency (see e.g., [1]). To optimize the removal processes in NBS, appropriate plant species and substrates (as growing material), optimal hydraulic parameters, and suitable operating conditions are needed.  

 

The decentralized process consists of several stages: (i) greywater separation, (ii) storage, (iii) treatment by innovative NBS as multi-level green walls/green façades, or by mechanical systems as multi-layer filters and activated carbon, and (iv) final disinfection using commercial O3/UV systems (Ozone and Ultraviolet).  

MATURITY:  

 

Some building-level solutions for grey water treatment and reuse (including nature-based) are commercially available, for example:  

 

  • Aqua Gratis is a technology to capture and reuse the bath and shower water for flushing toilets. The solution is at the stage of initial market commercialisation. The technology development of Aqua Gratis was funded by the EU. 

  • REDI gives a solution for single-family houses, where the treated greywater can be used for watering the garden. 

  • Disinfection can be done with commercially available solutions, e.g. with ozone and ultraviolet (UV) light. 

 

Some examples of pilots are: 

 

  • Greywater treatment with nature-based solutions for indoor or outdoor modules in multi-level green walls/green façades was carried out in Houseful. The project tested also ozone and ultraviolet light for disinfection.  

  • Water management systems and how to monitor and collect water condition information for urban water management platforms were piloted in UNaLab. 

  • Green walls and constructed wetlands were piloted in NAWAMED, with a focus on grey water treatment from a public building, a parking area, and a refugee camp. 

  • A service model for grey water treatment with NBS was tested in Houseful. The service model considers a leasing contract and a payment fee per m3 of water treated and reused. 

 

The nature-based grey water systems have been tested in a rather short period of time (e.g., some months to 1–2 years). Since the operating time of grey water treatment should be closer to 15-20 years, a further full-scale testing is still needed. [1]. 

Passive building design strategies: building orientation, passive heating and cooling

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). 

 

 '

Shading devices for north-facing openings. 

Figure from https://www.yourhome.gov.au/passive-design 

 

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]. 

 

MATURITY:  

 

Although individual passive techniques are already commercial, their holistic implementation in buildings is still at TRL=4-6. 

Citizen Participation Platforms

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,  

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.  

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Last update: June 14, 2023

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