Thermoelectric Waste Heat Recovery

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Study of Recovery of Waste Heat From the Exhaust of Automotive Engine K. Wojciechowski1, J. Merkisz2, P. Fuć2, P. Lijewski2, M.Schmidt1 1 Faculty of Materials Science and Ceramics AGH University of Science and Technology, Al. Mickiewicza 30, 30-059 Cracow, Poland 2 Institute of Combustion Engines and Transportation, Poznan University of Technology, Piotrowo 3, 60-965, Poznan, Poland e-mail:, phone: (+48)12-61-73-442 Abstract Automotive engines reject a considerable amount of energy to the ambience through the exhaust gas. Significant reduction of engine fuel consumption could be attained by recovering of exhaust heat by using thermoelectric generators. One of the most important issues is to develop an efficient heat exchanger which provides optimal recovery of heat from exhaust gases. The work presents a design and performance measurements of a prototype thermoelectric generator mounted on self-ignition (Diesel) engine. Using the prototype generator as a tool, benchmark studies were performed for improvements in the heat exchanger including determination of temperature distribution and heat flux density. Introduction Contemporary car engines exchange app. 30-40% of heat generated in the process of fuel combustion into useful mechanical work. The remaining heat is emitted to the environment through the exhaust gases and the engine cooling systems. Therefore, even partial use of the wasted heat would allow a significant increase of the overall combustion engine performance. Changing the heat energy of the exhaust gases into electric power would bring measurable advantages. Modern cars equipped with combustion engines tend to have large numbers of electronically controlled components. The observed tendency is to replace mechanical components with the electronic ones. This increases the demand for electric power received through the power supply systems of the vehicle. This tendency will undoubtedly remain at least due to the legal regulations connected with the on-board diagnostic systems, which force a more comprehensive control of operation of the vehicle components in the respect of safety improvement and emission control. This leads to the significant increase of demand for electric power in the vehicle which has to be generated by the alternator. It is predicted that if only 6% of the heat contained in the exhaust gases was changed into electric power, it would allow to lower fuel consumption by 10% due to the decreased waste resulting from the resistance of the alternator drive [1]. Power generation system using the thermoelectric generator should generally consist of the following components: heat exchanger, thermoelectric module, cooling system and DC/DC voltage converter. One of the most important design issues related to the construction of the thermoelectric generator TEG is to develop an efficient heat exchanger, which should provide optimal recovery of heat from exhaust gases. The heat exchanger delivers heat power received from the exhaust gases to the structure of TE modules. Due to the high speed of the exhaust gases flux, the heat exchange surface area in the heat exchanger should be increased by using the ribbing, grooving and protrusions which would introduce a turbulent flow allowing the increased flow of heat due to convection [4,5]. Heat absorption from gases should occur on a relatively short distance, due to the possibility of increasing the back pressure which would contribute to the changing operating conditions and limiting the engine power. The paper [4] puts together the comparison of testing designs of thermoelectric generators and exchangers using the exhaust gas heat. On the basis of test results of the thermoelectric generators under research, the design issues can be divided into three groups: a) related to design of the heat exchanger allowing the absorption of the sufficient amount of heat energy from the gases, b) related to the selection of materials for the...
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