Transcritical CO2 Heat Pumps: CO2 as R744 Refrigerant: Commercial, Industrial, and Residential Applications

     CO2 heat pumps are typically air source heat pumps that use CO2 as the refrigerant instead of CFCs or HFCs, which deplete the ozone layer and trap heat as excellent greenhouse gases. CO2 has some advantages over other refrigerants.

“CO2’s thermophysical properties means it takes less energy to move heat around.”

     CO2 heat pumps heat water more efficiently than other heat pumps. The standard measure of heat pump efficiency is the coefficient of performance, or COP. COP is a ratio of the amount of heat energy moved versus the amount of electricity used to move it. An average air source heat pump can hit a COP of 3. CO2 heat pumps’ COP varies due to ambient temperature, but the range is between 2.6 and 5.5, mostly significantly better than air source heat pumps can do, but only under certain circumstances, such as when water heating is utilized.  When combined with hot water heating, a CO2 heat pump can use the hot water to store energy for later use (at much higher temperatures than home water heaters typically get) when electricity prices are high. When there is no hot water heating, these systems operate at efficiency just slightly below air source heat pumps. Thus, without water heating, they are less economical than cheaper-to-install heat pumps using HFCs or other refrigerants. It is also totally non-flammable.

     CO2 heat pumps began to be manufactured and used in Japan in the 2000s, including some for industrial use. As a refrigerant, CO2 is much cheaper than other refrigerants, maybe five times cheaper.

CO2 is a “transcritical” or “supercritical” refrigerant, meaning it doesn’t fully change phase like other refrigerants. It has high specific heat, excellent heat conduction, and low kinematic viscosity, which means it flows well. CO2 operates at higher temperatures and pressures, where it can act both as a gas and as a liquid. Components of the pump systems, like tubing and compressors, must be designed to handle those higher temperatures and pressures.

     Whether CO2 heat pumps are economical or not compared to alternatives depends on usage, local power costs, ambient temperatures, etc. They are being deployed for niche uses, such as supermarkets in Australia and supermarkets, hotels and resorts in Denmark. They are being deployed mostly in Asia and Europe. The CO2 volumes used are small, so they don’t really provide any CO2 sequestration potential.

     U.K. company Clade makes CO2 heat pumps. Below, they explain how they work and some of the advantages.

     Company Quality Air Care, in an informative article, explains more about the advantages of CO2 heat pumps.

“The key components of a CO2 heat pump include a compressor, gas cooler (instead of a condenser), expansion valve, and evaporator. CO2’s excellent heat transfer properties allow these systems to operate efficiently even in extreme temperature conditions, making them particularly valuable in cold climates where conventional heat pumps struggle.”

“CO2 heat pumps can achieve higher temperatures than conventional systems, with water heating capabilities up to 90°C (194°F) without auxiliary heating elements. This high-temperature performance makes them especially suitable for industrial processes and domestic hot water applications.”

     CO2 has a much lower global warming potential than other refrigerants in common use. The CO2 refrigerant in use for heat pumps is known as R744.

     CO2 heat pumps also exhibit high efficiency in low ambient temperatures, something other air source heat pumps struggle with. As noted, they are especially efficient at water heating applications, beating out traditional water heaters by a large margin. This gives them advantages for niche commercial and industrial applications, which are noted below.

   CO2 heat pumps do have some challenges and limitations. Below is a list of manufacturers and models available, noted challenges and limitations, more comparisons with other heat pump types, and future tweaks that are being pursued.

 

References:

 

CO2 Heat Pumps: Benefits, Performance, and Applications. Quality Home Air Care. May 3, 2025. CO2 Heat Pumps: Benefits, Performance, and Applications – Quality Home Air Care

Why CO2 Heat Pumps Are The Future Of Cooling. Jon Okun, Matt Ferrell and Sunny Natividad. Undecided.  September 12, 2023. Why CO2 Heat Pumps Are The Future Of Cooling - Undecided with Matt Ferrell

CO2 Heat Pumps. Clade Engineering Systems. CO2 Heat Pumps | Clade Engineering Systems