Working fluid selection for heat pumps in solar district heating systems

In the context of the global energy transition, solar energy holds a promising potential as an energy source for a sustainable heat supply. Solar thermal collector in combination with heat pump technology utilizing electricity from renewable sources, such as photovoltaic, opens the possibility for the transformation of many existing residential areas into zero-emissions districts. However, according to the EU F-Gas Regulation No. 517/2014, the use of many commercial working fluids with high GWP is restricted in future refrigeration systems. Here, we present the results of current research on existing heat pump applications in Germany with a heating capacity over 20 kW, and then we evaluate possible refrigerants for a typical heat-pump-based solar district heating system with a tank store in the scenario of a residential area in Germany considering two different heat pump configurations: system A (two-stage compression with an open-flash-economizer) with nine candidate refrigerants, and system B (low-temperature-cycle and high-temperature-cycle in series) with 90 pairs of refrigerants as candidates. Two criteria - the coefficient of performance for heating COPh and the volumetric flow rate of the suction gas (V) over dot(Sg) - were applied to assess optimal working fluids. The simulation results demonstrate a trade-off between low GWP and good safety properties (nonflammability, nontoxicity). Based on the simulation results, we also discuss the corresponding restrictions and necessary precautions for the application of our refrigerant candidates considering safety issues and potential environmental effects. For the heat-pump-based solar district heating system (referred to in the following as "HP-SDH system ") in this exemplary residential area, the commercially available refrigerant R1234ze(E) exhibits the best combination of thermodynamic, environmental and safety properties; it is mildly flammable, and its application must comply with the safety and environmental requirements of the relevant German and European regulations.