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Thermoelectric Exhaust Heat Recovery Seminar Report

Thermoelectric devices are being investigated as a means of improving fuel economy for diesel and gasoline vehicles through the conversion of wasted fuel energy, in the form of heat, to useable electricity. In modern engines 60–85% of fuel energy is expelled as thermal energy in the engine exhaust or radiator systems, and while much of this energy is not available to do work on a 2nd law basis, capturing a small portion of the energy that is available with thermoelectric devices can reduce engine loads and or alternator size thus decreasing pollutant emissions and fuel consumption. Thermoelectric devices can also be used in conjunction with traditional CO2 reduction approaches (reducing friction, decreasing pumping losses, and improving combustion efficiencies) to further reduce greenhouse gas emissions. Previous studies have estimated that if the amount of driving energy required for the production of electrical power, approximately 6%, could be recovered this would result in an overall reduction of fuel consumption by 10%. Although challenges exist, it may be possible to recover significant amounts of exhaust waste heat with thermoelectric devices particularly as research of materials and application of heat transfer technologies continue to improve performance of these elements. Continued research in this area is further encouraged as the current national goals and the US Department of Energy’s commitment to reducing America’s dependence on foreign oil gathers more attention.

Several authors have demonstrated the use of thermoelectric elements for automobile heat recovery, installing the devices in the exhaust of vehicles. A summary of the aforementioned studies is presented in Table 1. A close look at Table 1 shows that heat recovery efficiencies are low, both on a system basis (< 2.5%) and on a thermoelectric generator basis ( < 3.5%) despite great progress made over the last decade in the thermoelectric materials figure of merit values ZT. The figure of merit value is a non-dimensional number that provides an overall assessment of the thermo element’s electrical conversion efficiency. Although material selection and ZT values are important to the performance of the thermoelectric, power output also greatly depends on the availability of heat in order to maintain a large temperature difference between the hot and cold side materials of the element.

Exhaust heat recovery from EGR coolers

Availability analyses of exhaust through major engine components have been performed previously by Edwards et al. It was discovered that significant amounts of useful work could be recovered offering the potential of large amounts of energy that could be utilized. An Exhaust Gas Recirculation (EGR) cooler, common on diesel and advanced combustion engines is a well-studied and widely used heat rejection unit which can potentially increase thermoelectric device power output as an effective heat exchanger. This paper presents the results of a heat exchanger similar to an EGR cooler used in conjunction with thermoelectric elements. Results from this study are thought to be applicable for either a heat exchanger system in the engine exhaust solely for exhaust heat recovery or thermoelectric devices or implementation of thermoelectric devices onto existing EGR cooling units.

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