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: email@example.com, 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 . 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  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...
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
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