By senior professor Björn Palm, Department of Energy Technology, KTH Royal Institute of Technology, and Operating Agent for HPT Project 64
The topic of this issue of HPT Magazine is Flammable Refrigerants in Heat Pumps. This is a very timely topic as we see the number of products with propane increasing rapidly. According to the Keymark heat pump database, the share of propane-powered products on the EU market rose from 3% in 2021 to 38% in 2024. Statistics on actual sales are not available, but estimates based on subsidy applications in Germany indicate that the market share was about 20% already in 2022. But even if the market for hydrocarbon heat pumps has increased considerably in the last few years, such products have been on the market for a long time. Already in 2008, more than 50 models of hydrocarbon heat pumps were commercially available.
Flammability is not only an issue that needs to be considered for hydrocarbon systems. Most low-GWP synthetic fluids are also flammable, even if the risks are lower. It is therefore important to investigate how systems with flammable refrigerants can be designed to be safe. For heat pumps, there is already the international product standard IEC 60335-2-40, updated in 2024, which can be used as a guideline. In the EU, we are hoping to see a new version of EN 378 shortly. There is also an excellent new book concerning safety with flammable refrigerants, written by Stig Rath and Harald Erös, called Propane, Naturally Safe!
To increase safety, it is important to limit the refrigerant charge. With the old HFC and HCFC refrigerants, reducing charge was not important, as a leak was not a threat to the immediate environment. Systems and components were therefore not designed with charge reduction in mind. With flammable refrigerants, we need to think differently. Several research projects have been focusing on this topic, and it has been demonstrated that it is possible to design and run a 12 kW liquid-to-water heat pump with as little as 120 g of propane or isobutane, i.e., 10g/kW. There are several commercial products on the market with capacities of 6-7 kW, having a charge of 152g, i.e., 21-25g/kW. My expectation is that the specific charges will continue to decrease in the coming years.
Other topics related to safety is how to avoid high refrigerant concentrations in case of a leak. What types of leaks can be expected? What is the mass flow of refrigerant through a given hole size? How will this flow develop over time? How much of the refrigerant will leak out? And how can the releasable charge be limited? How can air circulation in the room be maintained to avoid stratification? Answers to these and similar questions are important to be able to design and install safe systems with flammable refrigerants. Research has already given some answers, but more research is needed, and these results need to be used as a basis for updated standards. In the IEA HPT Project 64 on Safety with flammable refrigerants, we are working on these questions and plan to present a final report within a year. Some of this research is presented as thematic articles in this issue of the HPT Magazine.
Finally, the risks of using propane as a refrigerant should be taken seriously, but we have been using this gas as a fuel in our homes for 100 years. It does not become more dangerous in a hermetic heat pump system!
Senior professor Björn Palm
Operating Agent for HPT Project 64
Department of Energy Technology, KTH Royal Institute of Technology
Bjorn.Palm@energy.kth.se