*5.1. Thermoelectric Generators*

Energy can be derived from heat using thermoelectric generators (TEGs) based on the thermoelectric effect. It is also known as the Seebeck effect, according to which electricity is generated by the temperature gradient between two conductors. A TEG can be attached to the body to convert the temperature difference between a body skin and the surrounding environment into voltage. This concept was launched in 1999, where the first wristwatch supplied by body heat was invented [92]. TEGs can be used as an efficient power supply for wearable devices when the human body and the surrounding environment have a temperature difference of 5 to 10 degrees.

The electric potential of a TEG is expressed by Equation (10)

$$V\_{\rm TEG} = \mathbb{S} \cdot \Delta T \tag{10}$$

where *S* is the Seebeck coefficient of the material used and (∆*T*) is the temperature difference across the TEG.

A thermally powered wearable device that incorporates an accelerometer to sense falls was developed in [93]. In this application, the device generated 520 µW of output power at 15 ◦C, which charged a capacitor and a power management unit, included to link the thermal source and a sensor node.

An in-depth analysis of thermoelectric generation technology was recently presented in [94], illustrating the working principles of TEGs and their applications. Nevertheless, the development of thermoelectric materials with acceptable power factors remains a major challenge, for which various techniques have been investigated to achieve better efficiencies.
