Reversible CO2 (R744) heat pumps, in concert with a transcritical CO2 booster refrigeration system, are being used in multiple European retail locations for heating and air-conditioning, according to Jonas Schönenberger, Head of Research and Development at Swiss refrigeration engineering consultancy Frigo-Consulting.

Schönenberger provided these examples during his presentation in an HVAC and heat pumps session at the ATMOsphere (ATMO) Europe Summit on natural refrigerants. The conference took place November 15–16 in Brussels and was organized by ATMOsphere, publisher of

“End users choose CO2 as a refrigerant because it is future-proof, non-flammable, non-toxic and doesn’t include TFA [trifluoroacetic acid],” said Schönenberger. “It is also very efficient and can compete with all other solutions on a life-cycle basis.”

“End users choose CO2 as a refrigerant because it is future-proof, non-flammable, non-toxic and doesn’t include TFA [trifluoroacetic acid]. It is also very efficient and can compete with all other solutions on a life-cycle basis.”

Jonas Schönenberger, Frigo-Consulting

At a 20,700m2 (222,813ft2) Migros shopping mall in Lucerne, Switzerland, two reversible CO2 heat pumps were installed in 2019 to provide heating and air-conditioning. The two air-to-water units replaced the mall’s existing oil-fired system and serve 50 shops spread across two 45-year-old buildings.

According to Schönenberger, the mall’s retail operations use a CO2 booster system with ejectors and parallel compression, in addition to the heat pumps.

The two heat pumps offered a cooling capacity of 1.1MW (312.8TR) and a heating capacity of 1.3MW (369.7TR), the latter of which also utilizes heat recovery from the retail booster system.

In an analysis of the Migros shopping center, conducted by Frigo-Consulting in partnership with the Swiss Federal Office of Energy during 2020 and 2021, the heat pumps had a heating COP of 3.33 and a cooling COP of 5.32.

The analysis stated that “this pilot and demonstration project showed that it is possible to supply [a shopping mall] with the required heat and cooling energy by two reversible air-to-water CO2 heat pumps.”

Another example presented by Schönenberger was a 14,500m2 (156,077ft2) Selgros cash-and-carry market in Warsaw, Poland. The site uses two reversible R744 heat pumps to provide 620kW (176TR) in air-conditioning capacity and 850kW (242TR) of heating capacity. The market also uses two CO2 booster systems with ejector and parallel compression for its refrigeration. Waste heat from this system is recovered by the heat pumps for heating.

In Bern, Switzerland, a reversible CO2 heat pump is used to provide 80kW (23TR) in air-conditioning and 210kW (60TR) in heating for a 3,500m2 (37,674ft2) Coop supermarket and residential units that are part of the same building. Waste heat from the supermarket’s CO2-based refrigeration system is recovered by the heat pumps for domestic hot water and space heating of the building’s communal areas.

“CO2 is very good at producing high-temperature tap water, especially in combination with facility heating; it can be a very interesting solution,” explained Schönenberger.

In these examples, he noted that the use of a CO2-based heat pump has also helped to reduce the number of contractors required as the same person that services the sites’ CO2 booster systems can also service the heat pumps.

‘Paradigm change’ needed

These examples belie the notion that CO2 can’t be applied efficiently in a variety of applications, said Schönenberger.

“There used to be many rumors that it wouldn’t be possible to apply CO2 in an efficient manner,” he added. “However, we all now know that CO2 may be used in commercial applications in a very efficient way, and we see many benefits from these systems.”

Still, to support the wider adoption of R744-based HVAC systems, CO2 “requires a paradigm change,” he said.

For example, governmental policy needs to adopt a long-term perspective with clear signs as to which direction the market should be heading. End users must also consider the entire life cycle of a system’s costs and impacts and for the market to develop more natural refrigerant-based systems with the necessary installation and maintenance capacity, he added.

Moreover, to increase the efficiency of R744-based systems, he said that improvements need to be made at the system level.

“All the components have reached quite a high level of efficiency; now we have to put these components together, into a heat pump for example, and then apply it in a very efficient manner to optimize the benefits,” he added.