Thermoelectric Waste Heat Recovery

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: [e-mail], phone: [phone] 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

References: 1. Vazaquez J. et al, “State of the art of thermoelectric generators based on heat recovered from the exhaust gases of automobiles”, Proc. of 7th European Workshop on Thermoelectrics, 2002, Pamplona, Spain 2. Schock H. et al, ”Thermoelectric Conversion of Waste Heat to Electricity in an IC Engine Powered” Vehicle Advanced Combustion Engine R&D FY 2006 Progress Report pp 242-246. 3. LaGrandeur J. et al, ”High-Efficiency Thermoelectric Waste Energy Recovery System for Passenger Vehicle Applications” Advanced Combustion Engine R&D FY 2006 Progress Report pp 232-236. 4. Bell L. E. et al, ”High-Efficiency Thermoelectric Waste Energy Recovery System for Passenger Vehicle Applications” Advanced Combustion Engine R&D FY 2005 Progress Report pp 287-290 5. Birkholz U. et al, "Conversion of Waste Exhaust Heat in Automobile using FeSi2 Thermoelements". Proc. 7th International Conference on Thermoelectric Energy Conversion. 1988, Arlington, USA, pp. 124-128. 6. Serksnis A.W. "Thermoelectric Generator for Automotive Charging System". Proc. 11th Intersociety Conversion Engineering Conference. 1976, New York, USA, pp. 1614-1618. 7. Ikoma K. et al. "Thermoelectric Module and Generator for Gasoline Engine Vehicle". Proc. 17th International Conference on Thermoelectrics. 1998, Nagoya, Japan, pp. 464-467 Fig 7. Temperatures before (Tin) and after (Tout) heat exchanger vs. engine power at 3300 rpm. It seems that the conditions of heat absorption could be significantly improved by introducing changes to the exchanger design (e.g. extending the active surface, decreasing the gas flow speed). In order to estimate the maximum performance of the heat exchanger the following theoretical calculations have been made assuming constant temperatures of the exhaust gases on the exchanger outlet: 100, 150 and 200°C respectively. Fig. 8 Experimental and theoretical power of heat exchanger for assumed temperatures behind heat exchanger Tout at 3300 rpm. The results of the calculations (Fig. 8) indicate that with the above assumptions for the engine power above 10 kW it is possible to recuperate from app. 3 to 5 times more of heat power. Conclusions The performance of the heat exchanger system forms the basis for continuing the process of design optimization. The designed model of heat exchanger allowed for the utilization of 0.6 to 5.0 kW of exhaust gas energy depending on the operating parameters of the engine. However, the analysis of temperature distribution points out that, upon introduction of specific changes into the design, it is possible to recover even 25 kW of heat energy. Assuming the 5% efficiency of the thermoelectric modules it could allow to obtain the