Fraunhofer ISE develops propane heat pump using only 146 g of refrigerant


Propane heat pumps for indoor use in single-family homes are now one step closer to commercialization. Germany’s Fraunhofer ISE has successfully developed a model of a heat pump refrigeration circuit using less than 150 g of propane.

“Many heat pump manufacturers offer propane heat pumps but mostly for outdoor installation only, since the refrigerant is subject to extensive safety regulations for in-door use due to its flammability,” Fraunhofer ISE said in a statement. “If a heat pump in a single-family home with its usual 5 kW to 10 kW of power exceeds the prescribed maximum of 150 grams of refrigerant charge, then it can be installed only by carrying out increased safety measures.”

One of the best results of the Low Charge 150 g (LC150) project is a brine heat pump with a heating capacity of 11.4 kW using only 146 g of propane, making it suitable for indoor use without undergoing extensive safety precautions. The refrigerant charge of the unit is 12.8 g/kW, which is about one-fifth the propane charge of commercially available systems, according to the German research institute.

“The project goal was to develop a nearly market-ready heat pump module which uses the climate-friendly refrigerant propane, does not exceed the 150-gram limit for indoor use, and yet still provides sufficient heat for single-family homes,” said Dr. Lena Schnabel, head of the heating and cooling department at Fraunhofer ISE. “We have now achieved this goal in cooperation with our industry partners and have given them the tools to develop a market-ready heat pump.”

The research consortium set up, measured, evaluated, and optimized more than 20 different combinations of heat exchangers and compressors. One of the key components in decreasing the required amount of propane was the use of an asymmetrical plate heat exchanger. Other factors included using less oil in the compressor and shortening the pipes’ length.

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In October 2022, Fraunhofer ISE developed a heat pump with a heating capacity of 12.8 kW using only 124 grams of propane. However, its refrigeration circuit wasn’t suitable for commercialization because it used a semi-hermeneutic automatic compressor that couldn’t operate for enough hours.

The research institute now wants to develop low-charge propane heat pumps for use in multi-family homes, as part of the LC290 project running until June 30, 2025. The consortium aims to develop heat pump solutions for floor heating systems, indoor central heating systems, and high-performance, outdoor, heat pumps.

The projects are funded by the German Federal Ministry for Economic Affairs and Climate Action.

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