Record: heat pump less than 10 g of propane per kW

German researchers have developed a refrigeration circuit integrating a propane heat pump that achieves a heating power of 12.8 kW with only 124 g of refrigerant. 

As part of “LC150” (Low Charge 150 g) project, funded by the German Federal Ministry of Economics and Climate Protection, the Fraunhofer Institute for Solar Energy Systems and a consortium of heat pump manufacturers are developing a low refrigerant charge refrigeration circuit with a propane heat pump. 


Since October 2021, the Fraunhofer ISE project team has been building prototype brine heat pumps, assembling the individual components (evaporator, compressor, condenser, heat exchanger and expansion valve) in a variety of different constellations. In addition to the compressor, the research team also adjusted other factors in order to reduce the amount of refrigerant. For example, the internal volume of the heat exchangers and the amount of oil required were greatly reduced. Additional components such as sensors were also minimised. In addition, piping was kept as short as possible to reduce internal volumes. 


The device with the best constellation of components achieved a maximum heating capacity of 12.8 kilowatts and a coefficient of performance of 4.7 (ratio of heat generated to electricity used) with only 124 grams of propane. This means that the amount of refrigerant required per kilowatt of heating capacity was only 9.7 grams of propane. 


By comparison, commercially available heat pumps use about 60 grams of propane per kilowatt. Propane ihas very good thermodynamic properties and a very low global warming potential. However, since it is flammable, the aim is to use as little propane as possible. 


The “record-breaking” refrigeration circuit is not yet ready for market launch in this configuration, because a semi-hermetic automotive compressor was used, and automotive compressors have not been designed for the high operating hours of a heat pump expected to last about 20 years. 


The final version of the refrigeration circuit will be implemented using a fully hermetic compressor with slightly more refrigerant and a somewhat larger heat exchanger to achieve a more balanced system. The research team is confident that the goals of a refrigeration circuit with an output of 8 to 10 kilowatts and a maximum charge of 150 grams of refrigerant can then be achieved even under real operating conditions.