Carbon Dioxide Holds Promise As Future Refrigerant In Automobiles

Recent tests conducted at the U. of I.'s Air Conditioning and Refrigeration Center compared the thermal performance of a prototype carbon-dioxide system with one commonly used in automobiles.

"The carbon-dioxide system outperformed the conventional system under most operating conditions," said Predrag Hrnjak, a professor of mechanical and industrial engineering and a researcher at the center. "There is clearly a potential in this technology, which makes it reasonable to look more closely at solving the technical challenges encountered."

Current systems use a synthetic hydrofluorocarbon known as R134a. Developed as an ozone-safe replacement for the chlorofluorocarbon R12, this refrigerant may prove inadequate for future vehicles.

"To reduce global warming and obtain better fuel economy, we must develop more energy-efficient air-conditioning systems," said Clark Bullard, a professor of mechanical engineering and director of the center. "Automakers have encountered a fork in the road. One path would improve the existing R134a system; the other path would design a system around carbon dioxide or some other refrigerant."

Eventually, however, energy-efficient automobile engines may not generate enough waste heat to warm passenger compartments during cold weather. A heat pump, which operates like an air conditioner in reverse, may be required.

"Although R134a is a very good refrigerant, it works poorly as a heating fluid," Bullard said. "Carbon dioxide is much better suited for heat-pump applications. So, if the path of improving the R134a system ultimately dead ends, it makes sense to consider alternative technologies now."

To compare the two systems, the researchers constructed identical environmental test chambers. A variable-speed wind tunnel simulated the range of operating conditions encountered in normal applications. As a basis for comparison, the two heat exchangers had nearly equal volumes and the same pressure drop on the air side. The two compressors were also of similar sizes.

"Our experimental results will be used to validate models for designing next-generation systems that can then be compared experimentally," Hrnjak said. "We are just beginning to learn how to exploit the inherent advantages of carbon dioxide as a refrigerant."

By extracting a lot of basic information from their experiments, the researchers hope to reduce some of the technical uncertainties surrounding the ultimate limits on carbon-dioxide performance.

"Once the industry has a sense of what those realistic goals could be, then automakers can decide which path to take," Bullard said. "In a sense, we are trying to turn these black boxes into crystal balls."

The researchers presented their findings at the International Refrigeration and Compressor Conference, held July 14-17 in Lafayette, Ind.