Thermoelectric generators (also called Seebeck generators) are devices that convert heat (temperature differences) directly into electrical energy, using a phenomenon called the "Seebeck effect" (or "thermoelectric effect"). Their typical efficiencies are around 5–8%. Older Seebeck-based devices used bimetallic junctions and were bulky. More recent devices use semiconductor p–n junctions made from bismuth telluride (Bi2Te3), lead telluride (PbTe), calcium manganese oxide, or combinations thereof, depending on temperature. These are solid-state devices and unlike dynamos have no moving parts, with the occasional exception of a fan or pump.
Thermoelectric generators can be applied in a variety of applications. 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. Another deciding factor is that while inefficient, 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.
The most common application is the use of thermoelectric generators on gas pipelines. For example, for cathodic protection, radio communication, and other telemetry. On gas pipelines for power consumption of up to 5 kW thermal generators are preferable to other power sources. The manufacturers of generators for gas pipelines are Global Thermo electrics (Canada) and TELGEN (Russia)
Cars and other automobiles produce waste heat (in the exhaust and in the cooling agents). Harvesting that heat energy, using a thermoelectric generator, can increase the fuel efficiency of the car. For more details, see the article: Automotive Thermoelectric Generators.
In addition to automobiles, waste heat is also generated in many other places, such as in industrial processes and in heating (wood stoves, outdoor boilers, cooking, oil and gas fields, pipelines, and remote communication towers). 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. Some companies include TEGPRO (), Thermal Electronics Corp., Custom Thermoelectric(), Marlow Industries, tecteg MFR., wellentech and TEG Power. Other companies are developing consumer-level applications to capture the energy commonly wasted during cooking. A handful of USB cooking products have emerged, such as the BioLite stoves, Hatsuden Nabe thermoelectric cookpot Stealth Power Systems, and the PowerPot. Wood stove TEG12VDC-24AIR and TEG12VDC-24LIQUID TEG Generators producing enough power to trickle charge 12VDC and 24VDC batteries. Thermal Electronics Corp.
Microprocessors generate waste heat. Researchers have considered whether some of that energy could be recycled. (However, see below for problems that can arise.)
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 generators typically have lower efficiency than mechanical generators such as Stirling engines, i.e. they generate less electric power for the same heat flow. For a discussion of the factors determining and limiting efficiency, and ongoing efforts to improve the efficiency, see the article Thermoelectric materials - Device efficiency.
Besides low efficiency and high cost, two general problems exist in such devices: high output resistance and adverse thermal characteristics.
High output resistance - 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 (>10Ω). 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.
Adverse thermal characteristics - because low thermal conductivity is required for a good thermoelectric generator, this can severely dampen the heat dissipation of such a device (i.e. thermoelectric generators serve as poor heat sinks). They are only economical when a high temperature (>200°C) can be used and when only small amounts of power (a few milliwatts) are needed.