Large heat pumps with hot water store in local heating systems-Investigation of operation strategies

In the context of the German energy transition, heat pump technology utilizing heat and electricity from renewable sources is an alternative to traditional fossil fuel based boilers, which enables a sustainable heat supply. Therefore, the transformation of a residential area in Germany into a zero-emissions district employing a heat-pump-based local heating system with hot water store is investigated. The heat pumps are supposed to use the return flow of an existing district heating network as heat source. Most commercially available heat pumps have a standard temperature spread from 3 K to 5 K in the evaporator. In this case, the given limited source flow rate leads to a relatively low heat capacity of the heat pumps (output), which requires an unusual large dimension of the hot water store and impairs the economic efficiency of the heating system. To ensure an optimal store dimension, a possible solution is to install a mixing circuit on the heat source side of the heat pump (with a 5 K temperature spread in the evaporator) to increase the temperature spread of the heat source by remixing the return water, whereby the coefficient of performance for heating becomes smaller. Another solution is to raise the temperature spread in the evaporator directly (design of heat exchanger), also in this case, the coefficient of performance for heating diminishes. Here, we present the theoretical investigation of two heat pump configu-rations as well as operating strategies: a) Installation of a mixing system in the heat pumps with standard temperature spread in the evaporator (5 K) and b) constant water flow through the evaporator with varying temperature spread up to 15 K. The heat pumps were simulated using the program EBSILON (R) Professional. The simulations show that the strategy a) achieves higher coefficient of performance epsilon h, HP within low heating ca-pacity range (300 kW to 750 kW), while the strategy b) has better coefficient of performance epsilon h, HP within high heating capacity range as well as a larger total heating capacity range up to 1750 kW. The result is a better annual coefficient of performance for heating of the strategy b) in the system simulations (heat pumps and hot water store system operation) using the Program TRNSYS. The hot water store decouples heat pump operation from heat load to ensure that the heat pumps always work at the optimum operating points.

Automated summary

In the context of the German energy transition, heat pump technology is explored as an alternative to traditional fossil fuel based boilers for sustainable heat supply. The transformation of a residential area in Germany into a zero-emissions district employing a heat-pump-based local heating system with hot water store is studied. The use of a mixing circuit or raising temperature spread in the evaporator are proposed as solutions to improve the efficiency of the heating system.