Automobiles without air conditioners are more the exception than the rule nowadays. To many, a summer of driving without A/C is unthinkable. Air conditioners even play a role in winter drivingthey make defrosters work better, for instance. But air conditioners also reduce engine performance and diminish fuel economy, especially in smaller cars.
That comfort penalty grows in importance when it is factored into efforts to improve fuel economy and range for new automobile technologies such as hybrid vehicles. Fang Chen and Vince Mei, researchers in the Energy Division, found a way to reduce the comfort penalty. They devised a recuperative heat exchange concept and adapted it to the cooling cycle of an automotive air conditioner, resulting in a substantial improvement in its efficiency.
Their invention, called the liquid-overfeed air conditioner system, is essentially an additional heat-exchanger loop that overfeeds refrigerant through the system's evaporator, increasing its cooling capacity. Excess liquid leaving the evaporator is routed to the heat exchanger, protecting the unit's compressor and decreasing its work load, which in turn lessens the load on the engine, which must power the compressor as well as the drive train when the A/C is on.
The environmentally mandated shift to refrigerants that don't contain chlorofluorocarbon (CFC) refrigerants has increased interest in their work because most of the non-CFC refrigerants don't cool as effectively as CFCs. "We're trying to find ways to improve efficiency in air conditioners," Chen says. "We're also studying the thermodynamic cycle to find ways to counteract the drop in efficiency encountered with non-CFC refrigerants."
If automakers do become interested in modifying their cooling system designs to include the liquid-overfeed heat exchanger, the task shouldn't be too difficult because the device doesn't take up much space and is put together from readily available components. Some automobiles already have an accumulator in their cooling systems; adding the liquid-overfeed feature would be a simple matter.
Currently, environmental damage (ozone depletion and global warming) caused by refrigerant leakages from automobile air conditioners receive more attention than concerns over energy efficiency. Chen, however, points to an energy cost that is greater than it appears on the surface. If a car has 100 horsepower, as many do, the five-horsepower cost in engine power for running the air conditioner doesn't seem that great. However, the same car at cruising speed requires only 25 horsepower at the most. Then, relatively speaking, that five horsepower represents a much larger demand on the engine.
Energy-efficient mobile air conditioning will become more important for electric as well as hybrid vehicles, Chen says, explaining that every kilowatt-hour of battery energy used by air conditioning is one less kilowatt-hour to power the electric vehicle. Operating an air conditioner would drop the 75- to 100-miles-per-recharge range of an electric vehicle considerably, putting its feasibility in question.
As designers face those restrictions, the liquid overfeed system has attracted notice in the field: It won this year's Inventors Club of America International Hall of Fame Award. More recently, the liquid overfeed air conditioner was part of a winning University of Tennessee (UT) entry in the 1995 Hybrid Electric Vehicle Challenge at Auburn Hills, Michigan. The 1995 Chrysler Neon that a group of UT engineering students modified to run on alternate fuels was tested in the Large-Scale Climate Simulator at ORNL's Buildings Technology Center.
It has been estimated that if the liquid overfeed technology were fully implemented by U.S. automakers, the device could save 340 million gallons of fuel per year, cut carbon dioxide emissions, and reduce the consumer's cost of driving. The system could also help extend the range of the hybrid-electric vehicle, bringing it closer to the marketplace.
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