Waste heat recovery from the exhaust of low-power diesel engine using thermoelectric generators

To meet the increasing world demand for energy, the rate of depletion of non-renewable energy sources must be reduced while developing alternative renewable sources. This can be achieved by increasing the overall thermal efficiency of conventional power plants. One way to do this is by waste heat recovery. Most of the techniques currently available recover waste heat in the form of thermal energy which is then converted to electricity in a conventional steam power plant. Another approach which has received little attention so far is direct conversion of thermal waste energy into electricity. Thermoelectrics is the science dealing with both low-temperature and high-temperature thermal to electrical energy conversion systems. The paper describes a pilot program to investigate the applicability of thermoelectric generators to the recovery of medium-temperature waste heat from a low-power stationary diesel engine. Commercial thermoelectric modules using bismuth telluride based alloys and consisting of 98 couples were used. The modules require a heat source capable of supplying a heat flux of about 8 W/cm². With a temperature difference of 200°C, each module converts 5% of the thermal energy that passes through it into electricity generating 14 W of electrical power. Heat transfer modelling was used to locate the optimum mounting position of the waste heat recovery system (WHRS) on the exhaust pipe. A compact device was then designed incorporating six modules with the exhaust pipe acting as the heat source. The required minimum temperature on the cold side of the modules was provided by using water-cooled heat sinks. Tests have shown that the performance specified by the manufacturer can be reproduced. One promising application for the tested WHRD is as a source of electrical power in motor vehicles in lieu of or in addition to the alternator. A detailed analysis of this application is also presented