Principles of Direct Thermoelectric Conversion

Abstract

The aim of this chapter is to present some fundamental aspects of the direct thermoelectric conversion. Thermoelectric systems are solid-state heat devices that either convert heat directly into electricity or transform electric power into thermal power for heating or cooling. Such devices are based on thermoelectric effects involving interactions between the flow of heat and electricity through solid bodies. These phenomena, called Seebek effect and Peltier effect, can be used to generate electric power and heating or cooling. The Seebeck effect was first observed by the physician Thomas Johann Seebeck, in 1821, when he was studying thermoelectric phenomenon. It consists in the production of an electric power between two semiconductors when submitted to a temperature difference. Heat is pumped into one side of the couples and rejected from the opposite side. An electrical current is produced, proportional to the temperature gradient between the hot and cold sides. The temperature differential across the converter produces direct current to a load producing a terminal voltage and a terminal current. There is no intermediate energy conversion process. For this reason, thermoelectric power generation is classified as direct power conversion. On the other hand, a thermoelectric cooling system is based on an effect discovered by Jean Charles Peltier Athanasius in 1834. When an electric current passes through a junction of two semiconductor materials with different properties, the heat is dissipated and absorbed. This chapter consists in eight topics. The first part presents some general considerations about thermoelectric devices. The second part shows the characteristics of the physical phenomena, which is the Seebeck and Peltier effects. The thirth part presents the physical configurations of the systems and the next part presents the mathematical modelling of the equations for evaluating the performance of the cooling system and for the power generation system. The parameters that are interesting to evaluate the performance of a cooling thermoelectric system are the coefficient of performance (COP), the heat pumping rate and the maximum temperature difference that the device will produce. It shows these parameters and also the current that maximizes the coefficient of performance, the resultant value of the applied voltage which maximizes the coefficient of performance and the current that maximizes the heat pumping rate. To evaluate the power generator performance it is presented the equations to calculate the efficiency and the power output, as well as the operating design that maximizes the efficiency, the optimum load and the load resistance that maximizes the power output. The last part of the chapter presents the selection of the proper module for a specific application. It requires an evaluation of the total system in