In many applications, thermoelectric coolers (TEC) can be effectively used as power generators. Actually, in applications where temperatures are less than 500o K, the best choice for power generation are TE cooling modules, either from the perspective of cost or performance. The material available on the subject are not extensively discussed as compared to similar products and this short article is intended to briefly describe the configuration, limitations, and performance of TE coolers as power generators.
For decades, generation of electrical power by thermoelectric devices has been a subject of attention. Thermoelectric power generation is the process of converting heat flow directly into electrical power through what is called the Seebeck effect named after the discovery of the thermoelectric effect by the German physicist Thomas Johann Seebeck in 1821. The Seebeck effect is a phenomenon in which a temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two substances. Historically, high-temperature energy sources have been used because of the inborn higher efficiency at high-temperature differences, ∆T.
Nevertheless, there are numerous low-level energy sources in nature which are candidates for thermoelectric conversion.
For example ocean thermals, PV solar energy, steam and several forms of waste heat. Normally TE modules designed for cooling are the best choice for these applications because they are manufactured from a material of highest efficiency at these nominal temperatures. In fact, they represent the highest imaginable efficiency thermoelectric power generators to be used for low-intensity energy sources.
Here, some of the unique features of these multipurpose devices together with their limitations and precautions are discussed.
Theory of Operation
A thermoelectric cooler contains several N & P pellets electrically connected in series and thermally connected in parallel crammed between two ceramic plates.
The bottom plate is attached to a heat sink and, with the application of DC current of right polarity. The heat is then driven from the top plate to the bottom plate and then into the heat sink, where it is dissipated at the ambient temperature. The resulting is that the top surface becomes cold. The top surface can also supply heat via simply reversing the DC polarity.
Limitations and Precautions
Some significant practical considerations should be made before trying to use TE coolers in the power generation mode. Maybe the most important concern is the question of the viability of the module at the expected maximum temperature. Many standard TE cooling modules are made-up of eutectic Bi/Sn solder which melts at approximately 138°C. Nevertheless, there are some coolers being offered using higher temperature solders designed for operating at high temperatures of 200°C, even approaching 300°C. However, required considerations should always be made regarding the operational lifetime of a TE module used at high temperatures.
Impurities or even combinations of the solder can rapidly spread into the TE material at high temperatures and reduce performance and, in extreme cases, can cause disastrous failure. This process can be controlled by the use of a diffusion barricade onto the TE material. Nonetheless, some manufacturers of TE coolers do not use any barrier material between the solder and the TE material. Although the application of such materials has become a common standard on the “high temperature” TE cooling modules, they are mostly intended for only short-term survivability and may or may not provide acceptable MTBF’s (Mean Time Between Failures) at high temperatures.
In summary, if one expects to operate a TE cooling module in the power generation mode, qualification testing should be done before their practical application to assure their long-term operation at their maximum expected operating temperature.