2.2 Applications of Thermoelectric Devices

Thermoelectric applications can be generally subdivided by the direction of energy conversion. While the Peltier effect is used within thermoelectric cooling devices, the Seebeck effect is responsible for the conversion of temperature gradients to an electrical voltage.

The initial configuration of thermoelements as illustrated in Fig. 2.1 is used for temperature measurement applications [13,14,15], where the conversion efficiency plays an incidental role beside the linearity between generated voltage and temperature difference within a desired measurement range. Typical materials for thermocouples used in temperature measurement applications are alloys of nickel with chromium as well as aluminum and copper, iron, platinum, and rhodium [16]. Due to the considerable temperature range of about $-270\,\ensuremath{^\circ \ensuremath{\mathrm{C}}}$ to $3000\,\ensuremath{^\circ \ensuremath{\mathrm{C}}}$ covered by available thermoelements, they can be found in measurement applications in almost every process control system in chemical industry.

In spite of the early discovery of the thermoelectric effects almost two centuries ago, a widely spread usage in commercial power conversion applications has not been reached until today due to the low conversion efficiencies. While the most metallic configurations are not suitable because of their low Seebeck coefficients, the introduction of semiconductors as thermoelectric materials enabled maximum conversion efficiencies in the range of $5$ -$10\,\%$ [17,18], whereby theoretically maximum values are predicted in the range of $20\,\%$ [19,20,21,22,23]. The efficiencies are still very low in absolute terms, but it enables a limited economical usage of thermoelectric generators to niche applications, where their outstanding reliability outweighs the low conversion efficiencies. Furthermore, reported power densities are rather low, which is another limitation for some weight sensitive applications.

However, due to their solid state nature and the lack of moving parts, thermoelectric generators convince by good lifetime, extremely long maintenance intervals, and thus high reliability. While these qualities are beneficial in a series of remote applications, under extreme conditions new applications have even been enabled. In the sequel, the most prominent are briefly summarized [24,25,26].

Wherever very cheap energy is available, the drawback of low conversion efficiency is relativized. A good example is the application of thermoelectric generators as power sources for measurement stations in oil and natural gas facilities [27]. While power consumption in measurement applications is relatively low, the power demand can be manifold for cathodic protection of pipelines and reach several hundreds watts. On unmanned offshore oil rigs, thermoelectric generators are used as security backup power source in order to establish a defined state in emergency cases [24].

Some further examples are seismic measurement stations for earthquake prediction as well as early remote communication gear, as radio and TV relay stations [24]. Wherever the waste heat can be used for heating, thermoelectric generation of electric energy can be favorable as well. Thus, some research stations and radio communications gear in the arctic and antarctic region are equipped with thermoelectric generators [28].

Thermoelectric active parts in most fossil fueled thermoelectric generators consist of either lead telluride or one of its alloys, or silicon-germanium alloys due to their fitting operational temperature range. While lead telluride can be used at temperatures of up to $900\,\ensuremath{\mathrm{K}}$ , SiGe enables higher temperatures of up to $1300\,\ensuremath{\mathrm{K}}$ . The biggest limitation for fossil-fueled devices is the availability of fuel, and in some cases also air to maintain proper combustion. An approach to overcome these limitations in even more extreme situations is to use radioisotopes as a heat source. In most reported terrestrial applications, strontium-90 has been applied, which has a half life of about 29 years and thus acts as a continuous power source over a long time. However, the need for proper shielding as well as high costs and, last but not least, security issues and the availability of nuclear fuel limits their use to governmental projects. Probably the most prominent application of radioisotope powered thermoelectric generators is as a power source on space vehicles such as satellites and research gear on several missions [29,30].

M. Wagner: Simulation of Thermoelectric Devices