Natural Refrigerants & Carbon Dioxide

Substances such as air, water, ammonia, hydrocarbons, and carbon dioxide may provide solutions to the problem of finding environmentally acceptable refrigerants.

By Yunho Hwang, Michael Ohadi, and Reinhard Radermacher
Refrigeration and air conditioning play important roles in modern life. They not only provide comfortable and healthy living environments, but have also come to be regarded as necessities for surviving severe weather and preserving food. Unfortunately, accelerated technical development and economic growth in much of the world during the last century have produced severe environmental problems, forcing us to acknowledge that though these technological advances may contribute to human comfort, they also can threaten the environment through ozone depletion and global warming. Aside from cost reduction, these concerns are the biggest driving forces for technical innovation in the field of refrigeration and air conditioning.

Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs)—used as working refrigerants in refrigerators and air conditioners as well as blowing agents in foams—are now being regulated because of their contribution to ozone depletion. Hydrofluorocarbons (HFCs) could be useful as short- and midterm replacements, but may ultimately not be suitable, owing to their high global-warming potential (GWP). Accordingly, a long-term solution will require the use of natural refrigerants. The refrigerator and automotive air-conditioning industries have already begun to address the challenges of replacing HCFCs, including R-22, and, eventually, HFCs on a global level.

The second, more important reason, dealt with thermodynamics. Gerrard says that to reach the operating temperature that they require (–18 DegF) using a CO2 system would have meant using two compressors in order to reach the same energy efficiency as the hydrocarbons.

When choosing propane, they were initially concerned about safety issues. “The first question we had was the flammability of the refrigerant,” he says. “But, we knew that isobutane and propane have approximately the same flammability rating and that in the year 2000, there were about 120 million domestic refrigerators and freezers using isobutane and we weren’t aware of any accidents.”

Unilever undertook a number of risk assessments before the introduction of the hydrocarbon cabinets. The company also did leak testing and found that the leak rates for hydrocarbons in operation were “very, very low,” he says, in line with HFC refrigerated cabinets.. Leak rates are in the range of grams per year, he says, and at this rate it was highly unlikely that flammable mixtures could be formed.

In the US new rules come into effect on Jan. 1, 2010, restricting the use of hydrochlorofluorocarbons in refrigerants for air-conditioning systems, geothermal heat pumps and the chillers used to cool large residential and office buildings.