Design of Thermoelectric Generators for Automotive EGR Applications

Licentiate thesis, 2014

Rising energy prices and greater environmental awareness along with stringent emissions legislation in the automotive industry make it possible to introduce techniques in the aftertreatment system that have previously been unprofitable. One such technique, studied in this work, is heat recovery in the exhaust gas recirculation (EGR) cooler using thermoelectric generators (TEG). CFD and thermoelectric simulations are supported with measurements in order to build models that describe the phenomena in a correct manner. Models describing thermoelectric effects, i.e., the Seebeck, Peltier and Thomson effects, together with Joule heating and Fourier conduction, are well known and describe the phenomena satisfactorily when using temperature-dependent material data. A thermoelectric module contains interfaces between different materials and these are never in perfect contact. Consequently, at material junctions, non-ideal contacts will lower the thermoelectric performance. How large these thermal and electrical contact resistances are is not possible to determine analytically since they depend on several parameters that are dependent on the different materials and process parameters, as well as on contact pressure over the junctions. A method for determining contact resistances that combines measurements of commercial modules and simulations is developed and is shown to predict resistances with excellent results, even for geometrically different modules. Furthermore, measurements and simulations on different levels are performed, from detailed studies of a single thermocouple to a full scale study of a thermoelectric heat exchanger prototype. It is shown that the greatest heat transfer resistance is located on the gas side, and it is of great importance to improve the heat transfer in the gas to achieve good overall efficiency. At the same time, it is also of great importance to maintain a low pressure drop in the exhaust gas flow. Consequently, when integrating TEG in EGR coolers, for heat recuperation, it is also important to focus on the design of the heat exchanger to achieve high efficiency, and not only focus on developing new thermoelectric materials and reducing contact resistances. It is concluded that thermoelectric simulations combined with CFD allow fast and inexpensive concept evaluations, and different potential TEG designs can be evaluated and compared with high accuracy.