A Heat Pump Using Carbon Dioxide as the Refrigerant

Editor’s note: We updated this 2013 blog post on April 15, 2024, to reflect more recent advancements in CO₂-based heat-pump water heaters. To learn more about another such product, see the April 2024 article by Brent Ehrlich “On-demand Water Heat Goes All Electric with Heat Pumps, CO₂.”

In researching and writing about building products for The BuildingGreen Report over thirty-plus years, I had an opportunity to cover some fascinating breakthrough products and technologies. One such technology I was writing about a few weeks ago is the use of carbon dioxide as a working fluid for heat pumps. 

But let me back up with a little context about refrigerants. These are the fluids used in refrigerators, air conditioners, and heat pumps that transfer heat from one place to another in cooling or heating a space. This “vapor-compression-cycle” equipment takes advantage of the principle that compressing a gas absorbs heat and expanding it releases heat—so it’s a way to move heat from one place to another.

When this compression and expansion cycle results in a phase change (converting it from liquid to gas or vice versa), significant heat can be absorbed and released.

Problems with refrigerants

Over the past 35 years, refrigerants have come under fire—both for their impact on the Earth’s protective ozone layer and for their global warming potential (GWP). HCFC-22 (R-22), a hydrochlorofluorocarbon was long the most common refrigerant. But it was phased out according to the international treaty to protect the Earth’s protective ozone layer.

That’s a good thing, as R-22 is both a significant ozone depleter and a significant greenhouse gas. The HFC (hydrofluorocarbon) refrigerants that have replaced HCFC-22 are much better from an ozone-depletion standpoint (ozone depletion potential or ODP of 0), but they are still very significant greenhouse gases (high GWP), which is why the Kigali Amendment to that original treaty was put in place. After years of resistance, the U.S. government is now implementing that phaseout as well.

Using CO₂ as a refrigerant

These concerns with HCFC and HFC refrigerants have led to interest in other chemicals that can be used as refrigerants, one of which is carbon dioxide (CO₂). Manufacturers in Japan have focused considerable attention on CO₂-based heat pumps, and one Japanese company, Mayekawa, has been selling commercial-scale CO₂-based heat pumps in North America for several years.

Mayekawa offers many different CO₂ heat pumps for multiple applications, including three types in its unimo line: air to water, water to water, and a hybrid of both air and water heat sources.  These products are considerably larger than the heat pumps used for homes.

High efficiency is an important benefit of such systems; they operate at a coefficient of performance (COP) of about 4.0. If they are configured to provide space cooling in addition to hot water (just the water-to-water and air-to-water models), the COP can be as high as 8.0.

Higher output temperatures

From a performance standpoint, the big difference with CO₂-based heat pumps is that they can produce much higher-temperature output. Exactly why they can do this is complex and has to do with CO₂ being a “transcritical” refrigerant that doesn’t fully change phase like other refrigerants—described in detail in an article on Mayekawa heat pumps that I wrote in 2013.

One challenge with CO₂-based heat pumps is that they need a fairly large lift temperature to operate. This is the difference in temperature in a heating loop between the supply and return temperatures.

A standard gas- or oil-fired boiler may deliver 180°F water for hydronic heating, and return water in the heating loop at a temperature of 150°F after delivering it’s heat through baseboard radiators. So the boiler has to “lift” the water from 150°F to 180°F. That isn’t enough lift for a CO2-based heat pump. The EcoCute needs a minimum of about 45°F of lift to function effectively.

Higher pressure

The other challenge is that CO₂ refrigerant cycles operate at far higher pressure than standard vapor-compression-cycle equipment. At the evaporator side, the pressure can be about 600 pounds per square inch (psi), while in the gas cooler (which replaces the condenser in a standard compression-cycle device), the pressure can be 1,500 to 1,800 psi.

The higher pressure and the need for more robust (and more expensive) components to contain that pressure has slowed the development of CO₂-based heat pumps.

The future of CO₂-based heat pumps

What excites me about the future of carbon dioxide as a refrigerant is that such heat pumps increase the potential of providing more of our energy needs using electricity generated by sunlight as an alternative to burning fossil fuels. There are challenges, certainly, but such products could help us transition to a solar future.

Alex is founder of BuildingGreen, Inc. He retired from the company at the end of 2023. Alex also founded the Resilient Design Institute in 2012, where he still serves as Executive Director.