1. Introduction

The second law of thermodynamics indicates that in any energy conversion process there is some waste heat the amount of which depends on the efficiency of the thermodynamic engine. The efficiency of typical heat engines is around $40\%$ . In other words, nearly $60\%$ of the energy is wasted in the form of heat [1]. Thermoelectric generators are solid state devices that can be used to convert part of this waste heat into useful electrical energy. Thermoelectricity is currently obtaining large interest, especially from the automobile industry. For example, the BMW car-manufacturer is planning to use a $1~\mathrm{kW}$ thermoelectric generator at the exhaust pipes of its cars for fuel saving (see Fig. 1.1). This interest in thermoelectricity is due to the fact that currently about $60\%$ of the energy in automobiles is wasted as heat, one third of which is through the exhaust pipe [1]. Other than energy harvesting, thermoelectric devices can also be used in a very wide range of applications including solid state refrigeration, semiconductor lasers, aerospace applications, and military applications. However, thermoelectric devices have not yet been used in large scale applications because of their limited efficiency, high costs and material availability. In this chapter, we introduce the basic principles of thermoelectric phenomena and thermoelectric devices, and the importance of using nanotechnologies in order to enhance their efficiency.

Figure 1.1: A $200~\mathrm{W}$ thermoelectric generator is used in BMW's automobile (The 31st international and 10th European conference on thermoelectrics, July 9-12, 2012, Aalborg, Denmark).