Topical Article: Who Wants to Be a Millionfold Deployed Refrigerant?

Heat pumps are a key technology to accelerate the de-fossilization of all sectors. Especially in residential applications, heat pumps serve as exchange technology for conventional heating technologies. The operation of heat pumps requires a refrigerant that significantly determines the system’s efficiency. However, the refrigerant choice is complex due to interactions with all components and nonlinear thermodynamic behavior. Therefore, refrigerant selection is widely discussed in the literature. This work introduces the refrigerant selection problem and applies a simple screening method to identify proper refrigerants in residential heat pumps. The application reveals future work to improve the refrigerant selection aiming for the optimal refrigerant in heat pumps concerning their application.

In nine out of ten cases, the heat supply of these buildings is based on the combustion of oil and gas in conventional heating systems. The combustion yields that 30% of total CO2 emissions in Germany are attributable to the building sector, with almost 80% of these emissions being emitted in the provision of space heating and domestic hot water. The lower the GWP and the refrigerant charge, the lower the direct emissions in the event of a leakage. Basically, the higher the efficiency, the less electricity must be used to provide heat, whereby the efficiency strongly depends on the selected refrigerant. The choice of refrigerant is complex due to interactions in the process and thermodynamic nonlinearities.

In order to offer sustainable and legally compliant heat pumps for the provision of heat for residential buildings in the long term, only refrigerants with a GWP of 150 or less should be used in the future. Despite the limitations, the efficiency of a heat pump must not be negatively affected by a change of refrigerant, as this reduces the potential to be a sustainable technology. Changing the refrigerant to a low GWP refrigerant can also reduce the indirect emissions of a heat pump. In addition, the cost of the refrigerant should be low not to inhibit the market penetration of heat pumps. Then, after the preselection, a detailed analysis of the heat pump circuit is performed to provide an evaluation of the refrigerants.

Non-flammable refrigerants of safety class A1 cannot meet either the political specifications or the technical requirements for an air-to-water heat pump in the future. With the help of the various selection stages, the number is reduced to 31 fluids, which fulfill both the political and the operational specifications for air-water heat pumps. Due to the high pressure, there is a higher density at the compressor inlet, which positively affects the delivered mass flow and thus the heat capacity in the condenser. In addition, it can be shown that the A3 refrigerants tend to have higher efficiencies.

Christian Vering, Christoph Höges, Dirk Müller, RWTH Aachen University, Germany

The text has been shortened by the HPC team
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