WASTE HEAT RECOVERY FROM AUTOMOBILE EXHAUST USING THERMOELECTRIC GENERATOR

Abstract

In recent years, an increasing concern of environmental degradation due to automobile exhaust emissions and the limitations of energy resources has resulted in extensive research for novel technologies of generating electrical power. A thermoelectric generator using the exhaust waste heat from an automobile has the potential to replace the existing alternator system in an automobile, and thus improve fuel economy and reduce emissions. The conversion efficiency of modern thermoelectric materials has increased more than three-times in the last twenty years. But there are many challenges in the thermal design of Exhaust based Thermoelectric Generator (ETEG) systems, such as increasing the efficiency of the heat exchangers (hot box and cold plate) maintaining a sufficient temperature difference across the thermoelectric module during different operating conditions and reducing thermal losses through the system as a whole. Firstly, the whole ETEG system is mathematically analysed in this thesis. The concept of mismatching is also presented when modules are connected in series or parallel configuration. A working prototype of exhaust based thermoelectric generator with single module is demonstrated. The conversion efficiency and power produced by module are estimated on the basis of Seebeck effect, Fourier’s law of heat conduction and Newton’s law of heat convection. After this, methods to increase conversion efficiency of TEG system are discussed. One is the use of DC-DC boost converter along with TEG. This boost converter steps up the voltage induced by module. Single stage and multi-stage conversion network of DC-DC converter are also discussed which are used according the need of output voltage. The other methods of increasing conversion efficiency are development of thermoelectric material with higher Figure of merit value (approx. 1) and thermal optimization of heat exchanger. Thermal optimization of heat exchanger includes the design of heat exchanger with some specific internal fins. The flow of exhaust gas become more turbulent due to this internal design of heat exchanger which cause a significant increase in hot side temperature of TEG. Ultimately, the conversion efficiency increases due to higher temperature gradient. In the end, results are discussed by graphical interpretation of conversion efficiency with respect to various related parameters in TEG device such as temperature gradient, heat transfer coefficient, engine speed etc.