Researchers at the Freiburg, Germany-based Fraunhofer Institute for Solar Energy Systems (ISE) have achieved what they call a new efficiency record of 12.8kW (3.6TR) heating capacity with only 124g (4.4oz) of propane (R290) – or 9.7g/kW (1.2oz/TR) – in a heat pump.
The record is the result of a multi-stakeholder initiative – called the LC150 (Low Charge 150g) Project – aimed at increasing the efficiency and capacity of low-charge R290 heat pumps.
As part of the project, the Fraunhofer ISE team has been building prototypes for brine heat pumps since October 2021, assembling the individual components in a variety of different “constellations,” according to a recent statement. It recently showed a refrigeration circuit from this project last month at its booth at the Chillventa trade show in Nuremberg, Germany.
The goal of the LC150 project was to achieve a propane charge/heating capacity of 15–30g/kW (1.9–3.8oz/TR) in a heat pump. The record value achieved is even lower than this.
In the search for the ideal heat pump, 26 prototypes have been built so far, 14 of which have already passed through the complete measurement matrix. These modules are measured on test stands 24 hours a day for two weeks at a time. Each prototype is run at between 30 and 150 operating points, and the measured values are recorded by 26 sensors.
The device with the best arrangement of components achieved an efficiency of 4.7 (ratio of heat generated to electricity used), producing the heating capacity of 12.8kW (3.6TR) and with only 124g (4.4oz) of propane. “This is roughly equivalent to the amount of propane in five cigarette lighters,” said Project Manager Clemens Dankwerth of Fraunhofer ISE.
Commercially available heat pumps use about 60g/kW (7.6oz/TR) of propane, or more than six times that of the best prototype, according to ISE.
Not ready for the marketplace
The record-breaking refrigeration circuit is not yet ready for market launch in this form because a semi-hermetic automotive compressor was used, explained ISE. This requires less refrigerant at a higher capacity thanks to its high rotational speed and low oil volume. So far, automotive compressors have not been designed for the high operating hours of a heat pump meant to last 20 years.
“However, the manufacturer is already working on fully hermetic compressors with a longer service life,” noted Dankwerth.
The final version of the record refrigeration circuit would be implemented with slightly more refrigerant and a marginally larger heat exchanger to achieve a more balanced system. “The research team is confident that the goals of the LC150 project – a refrigeration circuit with an output of 8–10kW [2.3–2.8TR] and a maximum charge of 150g [5.3oz] of refrigerant – can thus be achieved even under real operating conditions,” said Fraunhofer ISE.
The second-best refrigeration circuit in the test program so far also met the project’s targets, with a charge of 164g (5.8oz) of propane at an efficiency of 4.8% and a heating capacity of 8.1kW (2.3TR). A conventional fully hermetic compressor was used here.
“My hope is that this project, and this network between the manufacturers, us and the component suppliers, is a way to accelerate the production and the design of heat pump modules for low-charge refrigerant cycles for heat pumps for the European market,” said Lena Schnabel, Head of the Department of Heating and Cooling Technology at Fraunhofer ISE during a presentation in December 2020.
Optimizing components to reduce refrigerant
In addition to the compressor, the research team also adjusted other components to reduce the amount of refrigerant. For example, the internal volume of the heat exchangers and the amount of oil required have been reduced. Additional components such as sensors have also been kept to a minimum. In addition, the piping was kept as short as possible to reduce internal volumes.
The LC150 project, which is funded by the German Federal Ministry of Economics and Climate Protection, runs until March 2023. The researchers aim to design the refrigerant-reduced heat pumps with less effort in the future. To this end, the Technical University of Valencia, Spain, is developing a tool for simulative predictions.
