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A counterintuitive but promising path to reducing the loads imposed by automotive air conditioning systems is to use heat—specifically the waste heat generated by engines. This can be an abundant source of energy, since most light-duty vehicles with combustion engines are only about 30% efficient at best. With that degree of thermal efficiency, an engine releases 70% of its fuel energy as waste heat through the coolant, exhaust gases, and engine compartment warm-up. During much of a typical drive cycle, the engine efficiency is even lower than 30%. As efficiency decreases, the amount of waste heat increases, representing a larger potential energy source. NREL's Vehicle Ancillary Loads Reduction (VALR) team is investigating a number of heat generated cooling technologies including thermoacoustics, metal hydride heat pumps, zeolite systems, absorption refrigeration, and thermoelectrics.
A promising device being researched is the thermoacoustic technique, which uses waste heat to generate sound. As the sound waves cycle through compression and expansion, it drives a heat pump that cools the car. NREL is performing optimization studies for a standing wave thermoacoustic cooler, and has completed the design for the multiple heat exchangers within such a device.
In the past two decades, thermoacoustic effects have been increasingly used in the design of engines and refrigeration cycles. Recently, thermoacoustic refrigerators have flown on the space shuttle and cooled electronics in a Navy destroyer. A thermoacoustic system, compared to a A/C system, may be reliable, have a long lifetime, and be environmentally safe, but it currently has a low energy density, therefore taking up a large volume. It also may be limited in terms of how much energy it could produce.
A heat pump system could operate through an endothermic process in which hydrogen is released (or desorbed) from a metal hydride. As the vehicle is cooled through this process, waste heat would be used to regenerate the hydride for continuous or future use. A disadvantage is that hydride systems are relatively costly and low in performance efficiency. On the plus side, such a system would require no traditional refrigerants, which contribute to climate change. Development on hydrides to date has been done mainly by companies researching nickel-metal-hydride batteries and solid-state storage applications.
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Absorption refrigeration resembles today's vapor compression refrigeration systems, but the compressor is replaced by an absorber and generator. The generator creates a pressure differential to circulate vapor. Absorption systems are more complex than conventional air conditioning, but they offer a potential increase in efficiency if high temperature waste heat is available. A disadvantage is that they might operate on lithium bromide or ammonia, which would pose a safety issue.
Zeolite systems have good potential for use in vehicle cooling, and favorable efficiencies have been proven. The materials are more readily available than metal hydrides. A disadvantage is that the working fluid becomes ice during cooling and may present vehicle integration issues as well as a lower limit on the temperature. Research is needed to develop suitable small components. Companies outside of the United States are currently developing home zeolite systems on a larger scale, such as 10 kW.
Learn more about the VALR team's other research and development areas.
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