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].
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