Cryogens are effective thermal storage media which, when used for automotive purposes, offer significant advantages over current and proposed electrochemical battery technologies, both in performance and economy. An automotive propulsion concept is presented which utilizes liquid nitrogen as the working fluid for an open Rankine cycle. When the only heat input to the engine is supplied by ambient heat exchangers, an automobile can readily be propelled while satisfying stringent tailpipe emission standards.
Nitrogen propulsive systems can provide automotive ranges of nearly 400 kilometers in the zero emission mode, with lower operating costs than those of the electric vehicles currently being considered for mass production. In geographical regions that allow ultra low emission vehicles, the range and performance of the liquid nitrogen automobile can be significantly extended by the addition of a small efficient burner. Some of the advantages of a transportation infrastructure based on liquid nitrogen are that recharging the energy storage system only requires minutes and there are minimal environmental hazards associated with the manufacture and utilization of the cryogenic "fuel." This paper describes the fundamental concepts of cryogenic automotive propulsion. This includes the thermodynamic theory, working cycle , individual system components, Finally, conclusions which shows that zero emission vehicle (ZEV) is the best alternative to replace existing gasoline vehicle. Also requirement of nitrogen as a fuel lowers the cost of fuel and improves overall efficiency.
INTRODUCTION
In September of 1990, in an effort to improve local air quality, the California Air Resources Board enacted the Low Emission Vehicle (LEV) program. The LEV program established several categories of emission standards for cars and light trucks. The most stringent of these categories was for the zero-emission vehicle (ZEV). The LEV program requires that, by 2003, each of the seven largest automobile manufacturers (Chrysler, Ford, General Motors, Honda, Mazda, Nissan and Toyota) produce and offer for sale ZEVs at a rate equal to 10% of the automobile sales each company has in the state, or about 110,000 cars per year. Similar mandates have also been adopted by New York and Massachusetts.
The impetus for the LEV legislation is the desire to reduce air pollution. In urban areas of southern California, vehicles account for over 50% of the air pollution emitted. In 1995, the South Coast Basin (which includes Los Angeles, Orange, and parts of San Bernardino and Riverside counties) experienced 98 days in which the EPA health standard for ground-level ozone was exceeded. Ground-level ozone can cause aching lungs, wheezing, coughing and headaches. Serious health problems can also arise for those people with asthma, emphysema and chronic bronchitis. Children appear to be at particular risk. A 1984 study conducted at USC showed that children raised in the South Coast Basin suffered a 10% to 15% decrease in lung function.
The deleterious effects of gasoline and diesel powered vehicles are not limited to air quality in southern California. In half of the world's cities, tailpipe emissions are the single largest source of air pollution. Worldwide, automobiles account for half of the oil consumed and a fifth of the greenhouse gases emitted. This situation is not expected to improve in the near future, as the number of cars and light trucks in the world – over 500 million – is expected to double in the next thirty years. Most of this growth will occur in developing countries which have little or no emission controls.
Currently, the battery powered electric vehicle is the only commercially available technology that can meet ZEV standards. However, electric vehicles have not sold well. This is primarily due to their limited range, although anemic performance, slow recharge and high initial costs are also contributing factors. All of these issues can be traced directly to the limitations of electrochemical energy storage, particularly lead-acid batteries. Lead-acid remains the dominant technology in the electric vehicle market, but only exhibit energy densities in the range of 30-40 W-hr/kg. This compares with about 3,000 W-hr/kg for gasoline combusted in an engine running at 28% thermal efficiency. Lead acid batteries can take hours to recharge and must be replaced every 2–3 years. This raises the specter of increased heavy metal pollution, were a lead-acid powered electric fleet ever to come to pass.
Even advanced battery systems, such as nickel-metal hydride, zinc-air, and lithium-ion suffer from slow recharge and high initial cost. Nickel-metal hydride batteries, often touted as the heir-apparent of lead-acid, still contain a heavy metal and must realize dramatic reductions in cost in order to be truly competitive. Lithium-ion batteries, considered by many to be the third-generation solution, must also contend with cost and demonstrate their safety to a wary public.
Another energy storage medium will be required to make ZEVs the non-mandated automobile choice of the car-buying public. If certain technical challenges can be overcome, that energy storage medium may well be liquid nitrogen. Since 1993, the University of Washington has been researching the technical challenges involved in building and operating a vehicle powered by liquid nitrogen. Issues pertaining to frost-free heat exchanger performance, cryogenic equipment, cycle analysis, drive train selection and vehicle configuration are being investigated.
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CRYOGENIC AUTOMOTIVE PROPULSION
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