International. The Fraunhofer Institute for Solar Energy Systems (ISE) developed a working model of a propane-based refrigeration system, a refrigerant considered cost-effective and less polluting.
Previously, many manufacturers have offered heat pumps with the use of propane, but mostly for outdoor installation, as the refrigerant is subject to restrictions on indoor use due to its flammability.
For example, if a heat pump in a single-family home with its usual output of five to ten kilowatts exceeds the prescribed maximum of 150 grams of refrigerant charge, it can only be installed with enhanced safety measures.
Now, Fraunhofer ISE has announced that it got a brine heat pump to produce a heating capacity of 11.4 kilowatts with a fully hermetic compressor that uses only 146 grams of propane. With this low refrigerant charge, it is permissible to install the unit inside buildings without taking additional measures.
The unit's specific refrigerant charge is 12.8 grams per kilowatt, which is about one-fifth the propane charge of conventional systems.
"The goal was to develop a heat pump module that would not exceed the limit of 150 grams for indoor use and still provide enough heat for single-family homes", said Lena Schnabel, Head of the Heating and Cooling Department at Fraunhofer ISE.
How was it achieved?
The research team evaluated more than 20 different combinations of heat exchangers and compressors. During the process, they used commercially available components to build the prototype, with asymmetrical plate heat exchangers, which require less refrigerant, being key.
Thus, they reduced refrigerant requirements by decreasing, among other things, the amount of oil in the compressor. In addition, the sensors were kept to a minimum and the pipes were kept as short as possible to reduce the amount of refrigerant needed.
"We have now achieved this goal in cooperation with our industrial partners and provided them with the tools to develop a market-ready heat pump", Schnabel added.
Propane vs F-gases
Commercially available heat pumps typically use fluorinated greenhouse gases as a refrigerant. Although the refrigerant remains in the closed thermodynamic cycle, some of it may be released into the atmosphere during the manufacturing, filling, and repair processes.
The European Union's F-Gas Regulation is currently being amended. Refrigerants with a GWP above 2,500 have been banned since 2020, for example. Propane is a viable alternative thanks to its GWP equivalent of 3. In comparison, refrigerant R410A has a GWP of 2.088.
In addition, propane is available worldwide at low cost and has very good thermodynamic properties that allow for higher efficiencies, compared to conventional heat pumps.
The next step for the Fraunhofer researchers is the development of propane heat pumps for use in multi-family homes.
Together with heating manufacturers and the real estate sector, researchers will develop easy-to-implement and widely applicable solutions to replace gas and oil heating systems in multi-family homes.
In total, three fields of application are covered: underfloor heating systems, indoor-installed central heating systems and heat pumps of higher performance classes installed outdoors.