Thermoelectric generator

A thermoelectric generator (TEG), also called a Seebeck generator, is a solid state device that converts heat (driven by temperature differences) directly into electrical energy through a phenomenon called the Seebeck effect (a form of thermoelectric effect). Thermoelectric generators function like heat engines, but are less bulky and have no moving parts. However, TEGs are typically more expensive and less efficient. When the same principle is used in reverse to create a heat gradient from an electric current, it is called a thermoelectric (or Peltier) cooler. Thermoelectric generators could be used in power plants and factories to convert waste heat into additional electrical power and in automobiles as automotive thermoelectric generators (ATGs) to increase fuel efficiency. Radioisotope thermoelectric generators use radioisotopes to generate the required temperature difference to power space probes. Thermoelectric generators can also be used alongside solar panels.

History In 1821, Thomas Johann Seebeck discovered that a thermal gradient formed between two different conductors can produce electricity. At the heart of the thermoelectric effect is that a temperature gradient in a conducting material results in heat flow; this results in the diffusion of charge carriers. The flow of charge carriers between the hot and cold regions in turn creates a voltage difference. In 1834, Jean Charles Athanase Peltier discovered the reverse effect, that running an electric current through the junction of two dissimilar conductors could, depending on the direction of the current, cause it to act as a heater or cooler.

George Cove had accidentally invented a photovoltaic panel, despite intending to invent a thermoelectric generator with thermocouples, in 1909. He notes that heat alone didn't produce any power, only incident light, but he had no explanation for how it could be working. The operational principle is now understood to have been a very simple form of Schottky junction.

Efficiency The typical efficiency of TEGs is around 5–8%, although it can be higher. Older devices used bimetallic junctions and were bulky. More recent devices use highly doped semiconductors made from bismuth telluride (Bi2Te3), lead telluride (PbTe), calcium manganese oxide (Ca2Mn3O8), or combinations thereof, depending on application temperature. These are solid-state devices and unlike dynamos have no moving parts, with the occasional exception of a fan or pump to improve heat transfer. If the hot region is around 1273K and the ZT values of 3 - 4 are implemented, the efficiency is approximately 33-37%; allowing TEG's to compete with certain heat engine efficiencies. As of 2021, there are materials (some containing widely available and inexpensive arsenic and tin) reaching a ZT value > 3; monolayer AsP 3 {\displaystyle {\ce {AsP3}}} (ZT = 3.36 on the armchair axis); n-type doped InP 3 {\displaystyle {\ce {InP3}}} (ZT = 3.23); p-type doped SnP 3 {\displaystyle {\ce {SnP3}}} (ZT = 3.46); p-type doped SbP 3 {\displaystyle {\ce {SbP3}}} (ZT = 3.5).

Construction

Thermoelectric power generators consist of three major components: thermoelectric materials, thermoelectric modules and thermoelectric systems that interface with the heat source.

Thermoelectric materials Thermoelectric materials generate power directly from the heat by converting temperature differences into electric voltage. These materials must have both high electrical conductivity (σ) and low thermal conductivity (κ) to be good thermoelectric materials. Having low thermal conductivity ensures that when one side is made hot, the other side stays cold, which helps to generate a large voltage while in a temperature gradient. The measure of the magnitude of electrons flow in response to a temperature difference across that material is given by the Seebeck coefficient (S). The efficiency of a given material to produce a thermoelectric power is simply estimated by its "figure of merit" zT = S2σT/κ. For many years, the main three semiconductors known to have both low thermal conductivity and high power factor were bismuth telluride (Bi2Te3), lead telluride (PbTe), and silicon germanium (SiGe). Some of these materials have somewhat rare elements which make them expensive. Today, the thermal conductivity of semiconductors can be lowered without affecting their high electrical properties using nanotechnology. This can be achieved by creating nanoscale features such as particles, wires or interfaces in bulk semiconductor materials. However, the manufacturing processes of nano-materials are still challenging.

Thermoelectric advantages Thermoelectric generators are all-solid-state devices that do not require any fluids for fuel or cooling, making them non-orientation dependent allowing for use in zero-gravity or deep-sea applications. The solid-state design allows for operation in severe environments. Thermoelectric generators have no moving parts which produce a more reliable device that does not require maintenance for long periods. The durability and environmental stability have made thermoelectrics a favorite for NASA's deep space explorers among other applications. One of the key advantages of thermoelectric generators outside of such specialized applications is that they can potentially be integrated into existing technologies to boost efficiency and reduce environmental impact by producing usable power from waste heat.

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