Both thermoelectric generators (TEG) and thermoelectric coolers (TEC) are formed from n- and p-type semiconductor materials obtained by doping different foreign atoms into thermoelectric semiconductor materials. In n-type semiconductors, the heat/emf force is carried by free electrons, while in p-type semiconductors, it is carried by holes. A single n or p type semiconductor material that exhibits thermoelectric properties is called a "thermoelement". The thermoelectric effect occurs when p and n type semiconductor materials (thermoelements) are connected in series with each other with conductors and this structure is called "thermocouple". In almost all applications, a single thermocouple cannot meet the desired power generation or the desired heating-cooling requirement. Therefore, thermocouples create thermoelectric modules when they are connected to each other with conductors (copper)electrically in series and thermally in parallel. There is a ceramic layer on the thermoelectric modules. This ceramic layer provides rigidity (strength) to the thermoelectric module, allowing the thermoelements to stay together. Also, since the ceramic material has a high thermal conductivity coefficient and low electrical conductivity, it helps the outer part of the thermoelectric module to provide both electrical insulation and a good heat transfer [1-6]. A cross-sectional view of a typical thermoelectric module is shown in detail below.
Since thermoelectric modules are reversible, in theory, it can be considered that they can be used both as a thermoelectric generator (TEG) and as a thermoelectric cooler (TEC). However, in practice, since TEG is used for power generation and TEC is used for heating-cooling, the type, geometry and solder connection of the semiconductor material inside the module differ depending on the purpose of which the thermoelectric module is used. For example, the materials of TEGs are generally designed to operate at high temperatures, and TEGs that can operate between 300 and 1300 K have been developed in the literature. On the other hand, because TECs are used to dissipate or absorb heat from a surface, cascade TECs have been developed to achieve low temperatures. Shortly, these devices are customized according to the purpose of use and TEG module can be used instead of TEC, or TEC module can be used instead of TEG, but the desired performance cannot be achieved [7]. In addition to all these, TEG and TEC modules can be manufactured in mini sizes, different geometries and even as flexible materials.
In order to obtain more efficiency from thermoelectric modules, depending on the application where the modules are used, additional mechanisms such as heatsinks, fan, heat exchanger can be added on the module surfaces. The devices created in this way are called "thermoelectric systems" and some of these systems are shown below.
References
[1] Ioffe, A. F., Semiconductor Thermoelements and Thermoelectric Cooling, London: Infosearch Limited, (1957).
[2] Rowe, D. M., CRC Handbook of Thermoelectrics, ABD: CRC Press LLC, (1995).
[3] Rowe, D. M., Thermoelectric Handbook: Macro to Nano, ABD: Taylor & Francis Group LLC, (2006).
[4] Goldsmid, H. J., Introduction to Thermoelectricity, Dordrecht: Springer, (2010).
[5] Lee, H., Thermal Design Heat Sinks, Thermoelectrics, Heat Pipes, Compact Heat Exchangers and Solar Cells, John Wiley & Sons, Inc, (2010).
[6] Lee, H., Thermoelectrics Design and Materials, ABD: John Wiley& Sons Ltd, (2017).
[7] Özgün, H., “Termoelektrik Jeneratörlerin Çok Düşük Sıcaklıklarda Teorik ve Deneysel Karakterizasyonu”, Yüksek Lisans Tezi, Enerji Enstitüsü, İstanbul Üniversitesi, İstanbul, (2009).
[8] http://www.peltier-thermoelectriccooler.com/
[9] http://tegway.co/bbs/content.php?co_id=flexible&lang=en