A heat pump using carbon dioxide as the refrigerant

Tuesday August 27, 2013

In researching and writing about building products for our publication Environmental Building News over the past 20-plus years I’ve 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, has long been the most common refrigerant. But it is being 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).

Using carbon dioxide

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. The Japanese have focused considerable attention on carbon dioxide-based heat pumps, and one Japanese company, Mayekawa, has been selling commercial-scale carbon dioxide-based heat pumps in North America for several years.

Mayekawa offers three different carbon dioxide heat pumps, the EcoCute water-to-water heat pump, the Unimo air-to-water heat pump, and the Sirocco water-to-air heat pump. (The product name, EcoCute, got a little bungled in translation from the Japanese. "Eco" is short for "ecological" in the U.S., while "cute" is derived from a Japanese ky tÿ, meaning "supply hot water.") EcoCute is used generically by a number of Japanese manufacturers, whose development efforts were funded by the government and the utility company TEPCO.

All three of the Mayekawa heat pumps have 25 kilowatt (kW) motors, so they 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 carbon dioxide-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 carbon dioxide being a "transcritical" refrigerant and doesn’t fully change phase like other refrigerants -- I’ll spare you the details here, though I describe it in the article on Mayekawa heat pumps I wrote for the August issue of Environmental Building News, which you can access at the Brooks Library in Brattleboro or access by subscription on BuildingGreen.com.

The EcoCute water-to-water heat pump and the Unimo air-to-water heat pump can produce water at up to 194 degrees Fahrenheit -- far hotter than standard heat pumps. This is significant, because it makes them viable for hydronic (baseboard hot-water) heating. As my friend and energy engineer Marc Rosenbaum, P.E. told me, if this can be done affordably, it will be a "game changer."

One challenge with carbon dioxide-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 temperature.

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

Higher pressure

The other challenge is that carbon dioxide 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 carbon dioxide-based heat pumps.

The future of carbon

dioxide-based heat pumps

I gather that several manufacturers of popular mini-split heat pumps are developing residential-scaled carbon dioxide-based heat pumps and that those heat pumps are currently undergoing testing.

It will be fascinating to see what emerges. What excites me 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 Wilson is the founder of BuildingGreen, Inc. and the Resilient Design Institute (www.resilientdesign.org), both based in Brattleboro. Send comments or suggestions for future columns to alex@buildinggreen.com.


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