| Thermoelectric effect |
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Applications
Thermoelectric materials · Thermocouple · Thermopile · Thermoelectric cooling · Thermoelectric generator · Radioisotope thermoelectric generator · Automotive thermoelectric generator
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Thermoelectric generators (also called thermogenerators) are devices which convert heat (temperature differences) directly into electrical energy, using a phenomenon called the "Seebeck effect" (or "thermoelectric effect"). Their typical efficiencies are around 5-10%. Older Seebeck-based devices used bimetallic junctions and were bulky while more recent devices use bismuth telluride (Bi2Te3) semiconductor p-n junctions and can have thicknesses in the millimeter range. These are solid state devices and unlike dynamos have no moving parts, with the occasional exception of a fan.
Radioisotope thermoelectric generators can provide electric power for spacecraft. Automotive thermoelectric generators are proposed to recover usable energy from automobile waste heat.
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Thermoelectric generators can be applied in a variety of situations. Usually, thermoelectric generators are used for small applications where heat engines (which are bulkier but more efficient) such as Stirling engines would not be possible.
The use of waste heat in combustion engines promises to be a high volume application field. Not only the exhaust but also cooling agents are targeted. [1]
Two general problems exist in such devices: high output resistance and adverse thermal characteristics. In order to get a significant output voltage a very high Seebeck coefficient is needed (high V/°C). A common approach is to place many thermo-elements in series, causing the effective output resistance of a generator to be very high (>10kOhm). Thus power is only efficiently transferred to loads with high resistance; power is lost across the output resistance otherwise. A generator with very high output impedance is effectively a temperature sensor, not a generator. Secondly, because low thermal conductivity is required for a good TEG, this can severely dampen the heat dissipation of such a device (i.e. TEG's serve as poor heat sinks). For example it is not generally considered wise to place a TEG on an essential IC chip that requires cooling. Because of the low thermal conductivity of a TEG device, that IC is cooled at a slower rate.
Cars produce waste heat, and harvesting it can increase the fuel efficiency of the car. See the article Automotive Thermoelectric Generators for the attempt to take advantage of this.
Space probes to the outer solar system make use of the effect in radioisotope thermoelectric generators for electrical power.
Solar cells use only the high frequency part of the radiation, while the low frequency heat energy is wasted. Several patents about the use of thermoelectric devices in tandem with solar cells have been filed. The idea is to increase the efficiency of the combined solar/thermoelectric system to convert the solar radiation into useful electricity.
Thermoelectric modules can be used for energy recovery of otherwise wasted heat.
Several companies have begun projects in installing large quantities of these thermoelectric devices. Some companies include, Custom Thermoelectric, Marlow Industries, and TEG Power