Heat pump using carbon dioxide as a refrigerant

By researching and describing products related to the construction industry for our Environmental Building News for the past twenty-plus years, I have been able to cover some interesting breakthrough products and technologies. One of these technologies that I wrote about a few weeks ago is the use of carbon dioxide as a working fluid for heat pumps.

But let me go back a little in the context of refrigerants. These are liquids used in refrigerators, air conditioners, and heat pumps to transfer heat from one place to another when cooling or heating a space. This “vapor compression cycle” equipment uses the principle that gas absorbs heat when compressed, and when it expands it removes heat - so this is a way of moving heat from one place to another.

Since the compression and expansion cycle leads to a phase change (turning a liquid into a gas or vice versa), a large amount of heat can be absorbed and released.

Refrigerant problems
In the last 35 years, refrigerants have been under fire - both for their impact on the protective ozone layer of the Earth and for their global warming potential (GWP). HCFC-22 (R-22), hydrochlorofluorocarbon, has long been the most common refrigerant. But at present, its use is being discontinued in accordance with the international treaty for the protection of the ozone layer of the Earth.

And this is good, since R-22 is both a strong ozone depleter and a strong greenhouse gas. The HFC refrigerants (hydrofluorocarbons) that replaced HCFC-22 are much better in terms of ozone depletion (the ozone depletion potential, or ODP, is 0), but they are still very visible greenhouse gases (high GWP).

Using CO2 as a refrigerant
These problems with HCFC and HFC have led to interest in other chemicals that can be used as refrigerants, one of which is carbon dioxide (CO2). The Japanese have focused heavily on CO2-based heat pumps, and one Japanese company, Mayekawa, has been selling industrial-scale CO2-based heat pumps in North America for several years.

Mayekawa offers three different CO2 heat pumps, an EcoCute water-water heat pump, a Unimo air-water heat pump, and a Sirocco water-air heat pump. (The name of the product, EcoCute, came from a combination of words. “Eco” is short for “environmental” in the US, while “cute” comes from Japanese kyūtō, which means “hot water supply”). The term "EcoCute" is used collectively by a number of Japanese manufacturers whose development efforts were funded by the government and utility company TEPCO.
All three Mayekawa heat pumps have 25 kW motors, so they are significantly larger than the heat pumps used for homes.
High efficiency is an important advantage of such systems; they work with a coefficient of performance (COP) of about 4.0. If they are configured to provide space cooling in addition to hot water (water-water and air-water models only), the COP can reach 8.0.

Higher output temperatures
In terms of performance, the big difference between CO2-based heat pumps is that they can produce much higher outlet temperatures. Why they can do this is difficult to understand and is due to the fact that CO2 is a "transcritical" refrigerant and does not completely change the phase like other refrigerants - I will spare you the details here, although I explained this in an article on heat Mayekawa pumps, which I wrote for the August issue (requires BuildingGreen.com subscription).

EcoCute water-to-water and Unimo air-to-water heat pumps can produce water at temperatures up to 194 ° F (90 ° C) - much hotter than that produced by standard heat pumps. This is of great importance because it makes them viable for hydraulic (baseboard) heating. As my friend and energy engineer Mark Rosenbaum, P.E., told me that if this can be done at an affordable price, then it will “change the rules of the game.”

One of the problems with CO2-based heat pumps is that they need a fairly large rise in temperature during operation. This is the temperature difference in the thermal circuit between the supply and return temperatures.

Standard gas or fuel oil boilers can provide water for plinth heating with a temperature of 180 ° F (82 ° C), and return water to the circuit with a temperature of 150 ° F (66 ° C) after delivering its heat through plinth radiators. Thus, the boiler must “raise” water from 150 ° F to 180 ° F. For a CO2 heat pump, this lift is not enough. EcoCute requires a minimum lift of approximately 45 ° F for efficient operation.

Higher pressure
Another problem is that CO2 refrigerant cycles operate at much higher pressures than equipment with a standard vapor compression cycle. On the evaporator side, the pressure can be approximately 600 psi, and in the gas cooler (which replaces the condenser of the unit with a standard compression cycle), the pressure can be between 1,500 and 1,800 psi.
Higher pressures and the need for more reliable (and more expensive) components have slowed the development of CO2-based heat pumps.

The future of CO2 heat pumps
I conclude that several manufacturers of mini-split heat pumps are developing CO2-based heat pumps for residential applications, and that these heat pumps are currently being tested.

It will be interesting to see what happens. What excites me is that such heat pumps increase the potential to meet most of our energy needs through the use of electricity generated by the sun as an alternative to burning fossil fuels. There are problems, of course, but such products could help us move in the future to solar energy.

Alex is the founder of BuildingGreen, Inc. and Executive Editor of Environmental Building News (https://www.buildinggreen.com/ecommerce/ebn.cfm). In 2012, he founded the Resilient Design Institute (http://www.resilientdesign.org/). To keep up with Alex’s latest articles and thoughts, you can follow him on Twitter http://twitter.com/atwilson