Integration of Heat Pumps in District Heating networks
Integration of Heat Pumps in District Heating Networks
the substitution of fossil fuels in the heat supply is an essential goal in the energy and heat transition in Germany. The integration of electric heat pumps plays a decisive role in this. This role will also be strengthened with a view to the soon available BEW funding (see our newsletter issue 15). Electric heat pumps use heat at a low temperature level to provide this energy through the use of electric current at a higher temperature level. Electric heat pumps achieve a coefficient of performance (COP) of up to 3 to 8, i.e. 3 to 8 units of thermal energy can be provided with one unit of electrical energy. Heat pumps are also characterized by a very flexible use in terms of source and sink temperature, thus open a wide range of applications for heat supply in buildings and many industrial processes.
District heating networks offer great potential for the integration of heat pumps. In our previous newsletters, we have already reported several times on the integration of heat pumps in buildings (temperature control in heating networks of the 5th generation, heating networks of the 4th and 5th generation). A compact overview of the different integration possibilities of heat pumps in heating networks is provided by the recently published article review on the integration of high-temperature heat pumps in district heating and cooling networks by J. Barco-Burgos et al. from the Universitat Rovira i Virgili in Spain. The article lists a total of 12 different configurations for the integration of heat pumps in thermal networks. The authors classify the integration based on the district heating network generation (3rd, 4th and 5th generation), the location in the heating network (central or decentralized), the technical integration as well as the temperature difference between evaporator and condenser and the associated efficiency of the heat pump.
Central heat pumps in district heating networks
Barco-Burgos et al. distinguish three different places for the integration of heat pumps. The first variant is the central integration of heat pumps. Here, at one point in the network (the energy hub), renewable heat or waste heat is used at a low temperature level to cover the heat demand of the thermal network all year round. This configuration is comparable to conventional district heating networks, where the primary energy source is replaced by heat pumps. On the one hand, these systems require a high temperature difference between evaporator and condenser, but also high outputs of several megawatts of thermal power. Centrally integrated heat pumps can use multiple sources and can be operated both in parallel and in series. As a peak load, further producers, also using fossil fuels, are conceivable in order to reduce investments. Due to the flow temperatures in the system, they are particularly suitable for district heating networks of the 4th or 5th generation and achieve COP values of 2 to 6 depending on the heat source and flow temperature.
Locally integrated heat pumps
Barco-Burgos et al. describe the second possibility for integrating heat pumps as local integration. According to Barco-Burgos, the local integration differs in that, in contrast to the integration described above, the heat pumps directly use the supply or return flow of the heating network as a source and/or sink. All local integrated heat pumps according to Barco-Burgos et al. require another centrally integrated heat source (e.g. CHPs). Locally integrated heat pumps are thus used as booster heat pumps to increase the supply temperature of the thermal network. One option uses the return as a source and the supply as a sink, this option can be used particularly well if the efficiency of the overall system requires low return temperatures and high supply temperatures. Modified options are the use of the supply as a heat sink and heat source. Although the temperature of the supply is increased, a very high temperature spread in the evaporator is necessary. A third possibility is to use the return as a source and heat the return to the temperature level of the supply with the help of an admixture from the condenser. According to Barco-Burgos et al., locally integrated heat pumps can be used for the 3rd, 4th and 5th generation of heating networks, but the applied outputs are lower and range in the range of a few 100 kW to a few megawatts of thermal power. Again, COP values of 2 to 6 are achievable.
Individually integrated heat pumps
Further options for the integration of heat pumps are summarized by Barco-Burgos et al. under "individually integrated heat pumps". Like the locally integrated heat pumps, there must be another energy source in the district heating network in addition to the heat pumps. Depending on the supply temperature in the network (e.g. 5th generation), renewable energies can also be used for this purpose. This category can be further divided into two uses:
- The condenser power of the heat pump is used for direct use in the building or industrial processes.
- The condenser power of the heat pump is used (like the locally integrated heat pumps) to increase the flow temperature or (like the centrally integrated heat pumps) to integrate further power at the same supply temperature level.
To cover the demand of buildings or process heat, heat pumps can be used in heating networks of the 3rd, 4th and 5th generation (necessary in the 5th generation). Depending on the configuration, the heat pumps use the supply or return flow of the district heating network as a source. Regardless of whether flow or return is used as a source, the cooled medium is fed from the evaporator into the return of the district heating network. The great advantage of the supply of individual consumers with heat pumps, which use the network as a source, is the flexible provision of different temperature levels. In conventional networks, the flow temperature depends on the requirements of the worst consumer. This can be avoided with individually integrated heat pumps. The performance of the individually integrated heat pumps for direct use in the building or industrial processes is rather small (a few 100 kW) and, according to Barco-Burgos et al., achieves COP values of 2 to 8.
Especially in heating networks where decentralized heat sources can be used at a low temperature level (waste heat, renewable heat). Individual heat pumps can be used either to increase the supply temperature, to introduce additional power into the heating network in which the heat pump is operated between return and supply, or to carry out a return temperature increase. The return temperature increase is an interesting way to reduce the temperature difference in 3rd generation district heating networks. COP values of 2 to 6 can be realized for this integration option.
The article shows particularly well the possibilities for integrating heat pumps into new and existing heating networks. The use of heat pumps can bring enormous ecological (savings in CO2 emissions) and, in combination with subsidies such as the BEW, economic benefits. In addition, existing heating networks can be efficiently transformed using individual heat pumps.
Further information on the integration of heat pumps in district heating networks
Especially for the concepts of hydraulic integration (hydraulic schemes) and the detailed evaluation of the integration possibilities, we recommend the article by Barco-Burgos et al. in full length. Further information on the funding of heat pumps can be found in our newsletter issue 15. We have compiled information on the individual integration of heat pumps in ultra-low temperature district heating in our showcase ultra-low temperature district heating.
The next issue of our newsletter will be published on 6 April 2022. Until then, please follow us on LinkedIn where we will share smaller application examples and information with you.
