2.3 Future Applications

Beside the already established niches, further increase of efficiency and power density as well as the ongoing development of energy costs can open a series of additional applications in economically competitive areas to conventional energy sources. Wherever a lot of waste heat is produced in a considerable temperature range, a good chance exists to establish increased efficiencies by thermoelectric devices [31].

In the automotive sector, it is thinkable to replace the alternator by thermogenerators driven by waste heat at both radiator and exhaust gas system [32,33]. Depending on the size of the vehicle, an increase of the fuel efficiency of up to $5\,\%$ can be achieved with today's technology [34]. Furthermore, the emerge of hybrid vehicles opens a wide application of thermoelectric devices as additional energy source for battery charging [35].

Sufficiently cheap thermoelectric modules could be used to increase the efficiency in stationary industry and power generation facilities beside other measures such as coupling to district heating systems. Solar energy is thinkable as a clean power source for an alternative to caloric power plants. Thermoelectric devices could be used to increase the efficiency of solar cells. Materials having their maximum figure of merit at relatively low temperatures such as bismuth telluride and its alloys could be used for geothermal power sources.

In the cooling sector, Peltier modules with sufficiently high efficiencies would be an attractive alternative to conventional thermodynamic machines [36]. Refrigerators would benefit from several advantages of solid state cooling such as silence and long life times as well as the absence of environmentally hazardous coolants. On a much smaller scale, within integrated devices such as power amplifiers and processors, the steady increase of power densities requires sophisticated thermal management both within the device as well as on a packaging level [37,38,39,40].

Beside the replacement and improvement of established technologies, thermoelectrics is one of the technologies having the potential to open completely new applications. The lifetime of mobile and integrated devices is mostly limited by their energy sources. Therefore, a replacement of classical batteries by according self-recharging systems is highly desired. Ongoing reduction of the power consumption of VLSI circuits makes them compatible to available natural energy sources incorporated in the environment of usage.

Recently, the term "energy harvesting" has been established based on the idea of energy conversion from even very small ambient sources to electrical energy. It incorporates the exploitation of several present forms of energy by according converters [41,42]. While mechanical vibrations can be harvested by piezoelectric as well as electrostatic and electromagnetic devices [43,44], especially wireless compact devices are thinkable to power themselves from ambient radio frequency electromagnetic radiation. Light serves as the energy source converted by solar cells and thermoelectric devices gain potential from ambient temperature differences.

Thereby, the power characteristics with respect to time of each conversion mechanism has to be adapted to the needs of the application. Thermoelectric generators are thinkable to continuously harvest small amounts of energy for charging a battery or capacitor, which itself provides comparable high power over a short time. Especially when different technologies are combined, power management has to be applied. Furthermore, the operating cycles of the application can be adapted to the energy available [45].

Despite of the single technologies possible power densities, the ideal power source varies from application to application. For example, personal radio communication gear, mobile computing devices, health monitoring systems [46], or simple wristwatches [47] could be recharged by a combination of piezoelectric and thermoelectric converters within clothes. While available mechanical energy during walking can be converted by shoe inserts [48,49], the temperature difference between the human body and its ambiance can serve as the energy source for thermoelectric generators [50,51].

Another energy harvesting application for thermoelectric devices is the exploitation of natural temperature differences between air and soil [52,53]. Thereby, the soil's thermal capacity causes a delay in temperature evolution and the temperature difference changes its sign between daytime and nighttime. In contrast to photovoltaic devices, there is always a potential for harvesting, even in the absence of sunlight.

The slowly closing gap between the energy consumption of single applications, especially in low power microelectronics, and available devices for providing the energy needed on a regenerative basis is documented by an impressive number of publications. However, there is still a long way to go in order to make thermoelectrics competitive for a wider range of applications by continuous efforts on increasing efficiency and power densities.

M. Wagner: Simulation of Thermoelectric Devices