Danish controls manufacturer Reftronix has developed a cost-effective CO2 (R744) heat pump capable of replacing an oil or gas furnace used in older residential homes for radiator-based space heating and domestic hot water.

Reftronix, which produces electronic controls and cloud solutions for smaller CO2 refrigeration systems like condensing units, partnered with Danish refrigeration contractor Temp-Tech to create a proof-of-concept CO2 heat pump prototype, with funding support from Energy Cluster Denmark, a government group.  Reftronix is seeking an OEM partner to manufacture the CO2 heat pump, said Henrik Christensen, Managing Director for Reftronix.

The CO2 heat pump is designed for older residential homes in Europe. “There are lots of old homes using oil or gas for hot water,” said Christensen. “In some European countries, they have to get off fossil fuels by 2030. So they need an option to replace the water boilers.” The current war in Ukraine has only heightened the need to seek an alternative to oil and gas provided by Russia.

Christensen explained that older homes, absent expensive insulation, require water temperatures of 70°C (158°F). However, conventional air-water heat pumps using f-gases or even propane (R290) as a refrigerant only meet European efficiency standards (a COP over 3.0) up to 55°C (131°F), which is sufficient in modern homes.

This leaves an opportunity for a CO2 heat pump, which isn’t efficient at 55°C but can leverage the transcritical cycle to efficiently produce 70°C water temperature. OEMs have regarded such CO2 heat pumps as too expensive to produce, but Reftronix believes its prototype offers a cost-effective design.

Reftronix’s main goal in the design of the CO2 heat pump was to keep it simple. “There are rumors that CO2 systems are way too complex and expensive ,” said Fred Schmidt, CO2 Process Control Expert for Reftronix, who designed the controls for the system.  “So we said, ‘let’s make it simple and reduce the components to a minimum to keep the price down.’”

Benefiting from Japanese experience

Reftronix’s CO2 prototype benefited from the availability of small compressors used for two decades by Japanese manufacturers of Eco-Cute CO2 heat pump water heaters. The prototype, based on a converted R32 heat pump, uses a two-stage (booster) 2HP Panasonic compressor. The company plans to build and field-test another unit with a 4HP compressor.

The prototype also features a brazed-plate gas cooler, four-way valve, four-inlet evaporator, four open-economizer receivers, Reftronix controller and Invertek inverter. The receivers prevent high pressures and remove the need for a safety valve. An integrated cloud link enables the unit to be run at off-peak times and reduced the load on the electrical grid, cutting utility costs.

The prototype CO2 heat pump delivers a COP of more than 3.0 at 60°C (140°F) water temperature, and a COP of 2.5 at 70°C; in both cases insulation, which would increase COP, was not used in order to facilitate the development of the prototype. (It would be used in a final product.)

“We said, ‘let’s make it simple and reduce the components to a minimum to keep the price down.'”

Fred Schmidt, Reftronix

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