Thermoelectric generators are devices that convert temperature differences into electrical energy, using a phenomenon called the "Seebeck effect" . Their typical efficiencies are around 5–8%. These are solid-state devices and have no moving parts.
Thermoelectric generators have a wide array of uses. Typically, thermoelectric generators are used for small applications where heat engines such as Stirling engines would not be possible. Thermoelectric generators are more reliable and have a smaller chance of breaking over time and use. Spacecraft are a typical example of an application where maintenance is next to impossible after launch.
Many space probes generate electricity using a thermoelectric generator using a radioactive element as the heat source. Cars and other automobiles produce waste heat. Harvesting it, using a thermoelectric generator, can increase the fuel efficiency of the car.
Waste heat is also generated in many other places, such as heating (wood stoves, outdoor boilers, and the oil and gas industry). Again, the waste heat can be reused to generate electricity. In fact, several companies have begun projects in installing large quantities of these thermoelectric devices.
Thermoelectric generators typically have lower efficiency than mechanical generators such as Stirling engines, though it is widely believed that TEGs will surpass the mechanical generators in the future.
In such devices, two general problems exist besides the low efficiency and high cost: high output impedance and unfavorable thermal characteristics.
High output resistance - in order to get a significant output voltage a very high Seebeck coefficient is needed. A common approach is to place many thermo-elements in series, causing the effective output resistance of a generator to be very high. Thus power is only efficiently transferred to loads with high resistance; power is otherwise lost across the output resistance. A generator with very high output impedance is effectively a temperature sensor, not a generator. This problem is solved in some commercial devices by putting more elements in parallel and fewer in series.
Unfavorable thermal characteristics - because low thermal conductivity is required for a good thermoelectric generator, this can severely discourage the heat dissipation of such a device . TEGs must be used with heat sources above 200°C when many watts are needed, but if only small power (milliwatts) is needed, then almost any temperature difference can be achieved with correct design.
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