July 4, 2000 WEST LAFAYETTE, Ind. – Researchers are making progress in perfecting automotive and portable air-conditioning systems that use environmentally friendly carbon dioxide as a refrigerant instead of conventional, synthetic global-warming and ozone-depleting chemicals.
It was the refrigerant of choice during the early 20th century but was later replaced with manmade chemicals. Now carbon dioxide may be on the verge of a comeback, thanks to technological advances that include the manufacture of extremely thin yet strong aluminum tubing.
Engineers will discuss their most recent findings from July 25 to 28, during the Gustav Lorentzen Conference on Natural Working Fluids, one of three international air-conditioning and refrigeration conferences to be held concurrently at Purdue University. Unlike the two other conferences, the biannual Gustav Lorentzen Conference, which is being held for the first time in the United States, focuses on natural refrigerants that are thought to be less harmful to the environment than synthetic chemical compounds.
"The Gustav Lorentzen Conference focuses on substances like carbon dioxide, ammonia, hydrocarbons, air and water, which are all naturally occurring in the biosphere," says James Braun, an associate professor of mechanical engineering at Purdue who heads the organizing committee for all three conferences. "Most of the existing refrigerants are manmade."
Purdue engineers will present several papers detailing new findings about carbon dioxide as a refrigerant, including:
• Creation of the first computer model that accurately simulates the performance of carbon-dioxide-based air conditioners. The model could be used by engineers to design air conditioners that use carbon dioxide as a refrigerant. A paper about the model will be presented on July 26 during a special session sponsored by the U.S. Army in which researchers from several universities will present new findings.
• The design of a portable carbon-dioxide-based air conditioner that works as well as conventional military "environmental control units." Thousands of the units, which now use environmentally harmful refrigerants, are currently in operation. The carbon dioxide unit was designed using the new computer model. A prototype has been built by Purdue engineers and is being tested.
• The development of a mathematical "correlation," a tool that will enable engineers to design heat exchangers – the radiator-like devices that release heat to the environment after it has been absorbed during cooling – for future carbon dioxide-based systems. The mathematical correlation developed at Purdue, which will be published in a popular engineering handbook, enables engineers to determine how large a heat exchanger needs to be to provide cooling for a given area.
• The development of a new method enabling engineers to predict the effects of lubricating oils on the changing pressure inside carbon dioxide-based air conditioners. Understanding the drop in pressure caused by the oil, which mixes with the refrigerant and lubricates the compressor, is vital to predicting how well an air conditioner will perform.
Although carbon dioxide is a global-warming gas, conventional refrigerants called hydrofluorocarbons cause about 1,400 times more global warming than the same quantity of carbon dioxide. Meanwhile, the tiny quantities of carbon dioxide that would be released from air conditioners would be insignificant, compared to the huge amounts produced from burning fossil fuels for energy and transportation, says Eckhard Groll, an associate professor of mechanical engineering at Purdue.
Carbon dioxide is promising for systems that must be small and light-weight, such as automotive or portable air conditioners. Various factors, including the high operating pressure required for carbon-dioxide systems, enable the refrigerant to flow through small-diameter tubing, which allows engineers to design more compact air conditioners.
More stringent environmental regulations now require that refrigerants removed during the maintenance and repair of air conditioners be captured with special equipment, instead of being released into the atmosphere as they have been in the past. The new "recovery" equipment is expensive and will require more training to operate, important considerations for the U.S. Army and Air Force, which together use about 40,000 portable field air conditioners. The units, which could be likened to large residential window-unit air conditioners, are hauled into the field for a variety of purposes, such as cooling troops and electronic equipment.
"For every unit they buy, they will need to buy a recovery unit," Groll says. "That's a significant cost because the recovery unit is almost as expensive as the original unit. Another problem is training. It can be done, but it's much more difficult than using carbon dioxide, where you could just open a valve and release it to the atmosphere."
The recovery requirement would not apply to refrigerants made from natural gases, such as carbon dioxide, because they are environmentally benign, says Groll, who estimates that carbon dioxide systems probably will take another five to 10 years to perfect.
Carbon dioxide was the refrigerant of choice a century ago, but it was later replaced by synthetic chemicals.
"It was actually very heavily used as a refrigerant in human-occupied spaces, such as theaters and restaurants, and it did a great job," says Groll, who is chair of the Gustav Lorentzen Conference.
But one drawback to carbon dioxide systems is that they must be operated at high pressures, up to five times as high as commonly seen in current technology. The need to operate at high pressure posed certain engineering challenges and required the use of heavy steel tubing.
During the 1930s, carbon dioxide was replaced by synthetic refrigerants, called chlorofluorocarbons, or CFCs, which worked well in low-pressure systems. But scientists later discovered that those refrigerants were damaging the Earth's stratospheric ozone layer, which filters dangerous ultraviolet radiation. CFCs have since been replaced by hydrofluorocarbons, which are not hazardous to the ozone layer but still cause global warming.
However, recent advances in manufacturing and other technologies are making carbon dioxide practical again. Extremely thin yet strong aluminum tubing can now be manufactured, replacing the heavy steel tubing.
Carbon dioxide offers no advantages for large air conditioners, which do not have space restrictions and can use wide-diameter tubes capable of carrying enough of the conventional refrigerants to provide proper cooling capacity. But another natural refrigerant, ammonia, is being considered for commercial refrigeration applications, such as grocery store display cases, Groll says.
Engineering those systems is complicated by the fact that ammonia is toxic, requiring a more elaborate design in which the ammonia refrigerant is isolated from human-occupied spaces. The first ammonia systems are currently being tested in Europe, and results will be presented during the Gustav Lorentzen Conference, Groll says.
Groll's work is funded by the U.S. Army, Air Force and the American Society of Heating, Refrigerating and Air-Conditioning Engineers, as well as the Air Conditioning and Refrigeration Technology Institute.
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