The thermoelectric effect is that phenomenon, whereby electric voltage directly results from a difference in temperature. Conversely, a change in temperature can also result in electric voltage. A thermoelectric scale will create a voltage, when each side has a different temperature.
With atoms, applying a temperature difference, makes charge carriers in the material (electrons or holes), to diffuse from the hot region, to the cold region. This is just like a classical gas that expands on heating, and explains the thermally-induced current.
The thermoelectric effect can be applied in many situations – generate electricity, measure temperature, heat objects, cool them, and even cook them. Considering the direction of heating and cooling is determined by the sign of the applied voltage, thermoelectric equipment can be conveniently used, to control temperature.
Thermoelectricity and the thermoelectric effect actually comprises of three distinct phenomena:
- the Seebeck effect
- the Peltier effect
- the Thomson effect
The thermoelectric effect is often called the Peltier-Seebeck effect. This comes from the independent studies of Jean Charles Athanase Peltier, a French physicist, and Estonian German physicist, Thomas Johnann Seebeck.
Related in a sense, but not entirely to the thermoelectric effect, is Joule heating – the heat generated as a voltage differential is applied across a resistive material. Joule heating is typically not referred to as the thermoelectric effect, but it is usually regarded as a loss mechanism, due to non-ideality in thermoelectric systems. Whereas the Peltier-Seebeck and Thomson effects can be reversed, Joule heating is irreversible.
The Seebeck effect involves converting temperature differentials directly into electricity. Seebeck noticed a compass needle would be deflected, when a closed loop was formed of two metals, joined in two places. These two places had an established temperature difference.
The reason for the needle deflecting is that the metals respond in different ways, to the temperature difference, thereby creating a current loop and producing a magnetic field. Seebeck was not aware an electric current was involved, and accordingly called his observation, the thermomagnetic effect, on the thinking that both metals were magnetically polarised, by the temperature gradient. Hans Christian Ørsted was instrumental, to the adoption of the eventual term, “thermoelectricity”.
This results in a voltage or thermoelectric EMF (Electromotive Force), created in the presence of a temperature gradient, between the two different metals or semiconductors. Where there is a complete loop, this causes a continuous current in the conductors. The resultant voltage is of the order of several microvolts per kelvin difference. A common combination of metals is the copper-constantan couple, with a Seebeck coefficient at room temperature of 41 microvolts per kelvin.
In a typical circuit, taking the Seebeck coefficients SA and SB as fairly constant, over the measured temperature range, the resulting voltage can be represented as:
V = (SA – SB). (T1 – T2), where T1 and T2 are the respective temperatures, of the two metals.
The Seebeck coefficients are also called thermoelectric power or thermopower. They are a non-linear function of temperature, and owe much dependence to the material, molecular structure, and absolute temperature of the metal conductors.
One common implementation of the Seebeck effect is the thermocouple device – thermocouple, because it is made from a coupling or junction, of (usually metal) materials. The thermocouple either measures a temperature difference directly, or measures an absolute temperature, using the technique of setting one end to a known temperature. A number of thermocouples connected in series are referred to as a thermopile. One reason a thermopile is constructed is to increase the output voltage, since the voltage induced over each individual couple is minimal.
The Seebeck effect is also responsible for thermal diodes and thermoelectric generators like radiosiotope thermoelectric generators, or RTGs. These are used to create power, using heat differentials.
Two effects make the Seebeck effect happen – charge carrier diffusion and phonon drag. Where both connections are held at the same temperature, with one connection periodically opened and closed however, and temperature-dependent AC voltage is measured. This Kelvin probe application is often relied upon, to argue that the underlying physics only needs one junction. This effect is still visible if the wires only come close but never really touch, eliminating the need for diffusion.
Thermoelectric power or thermopower is a material’s Seebeck coefficient. It measures the size of an induced thermoelectric voltage, in response to a temperature difference, across the material. Thermopower is measured in V/K (volts per kelvin). In practice though, microvolts per kelvin is more common. Values spanning hundreds on µV/K, either side of zero (0), are what you find with good thermoelectric materials.
The thermopower term is quite misleading, as it measures voltage or electric field, induced on the basis of a temperature difference, and not electrical power. Applying a temperature difference makes the charge carriers in a material – electrons or holes – to diffuse across the material, from the hot side to the cold side, just as a classical gas expands on heating.
- Thermoelectric generators
The Seebeck effect is used in thermoelectric generators, which function similarly to heat engines. They are however less bulky, more expensive and less efficient. They are used in power plants, to convert waste heat into electrical power – akin to energy recycling – and in motor vehicles, as automotive thermoelectric generators (ATGs), for improving fuel efficiency. Space probes usually employ radioisotope thermoelectric generators, using the same mechanism, but with radioisotopes, to generate the required heat difference.
- Peltier effect
The Peltier effect is beneficial in building compact refrigerators, with no circulating fluid or moving parts. Such refrigerators are useful where using them is more advantageous than any downsides they may have. The Peltier effect is used in many laboratory systems, for amplifying DNA via the polymerase chain reaction (PCR), and thermal recyclers. PCR needs the cyclic heating and cooling of samples, to predefined temperatures. Including many thermocouples in a small space, enables the amplification of many samples in parallel.
- Temperature measurement
Thermocouples and thermopiles using the Seebeck effect, can measure the temperature difference between any two objects. Thermocouples are accurate in measuring high temperatures, keeping the temperature of one junction constant, or by cold junction compensation, measuring it independently. Thermopiles use many thermocouples connected in series using electricity, for very sensitive measurements, of tiny temperature differences.
- Thermoelectric refrigeration
Thermoelectric (solid state) refrigeration aims to reduce the temperature of an insulated chamber to less than that of the surrounding air, by pumping heat energy out of an insulated chamber. It uses the Peltier principle described within this article, to pump heat electronically.
Thermoelectric cooling is rapidly gaining ground (even while not viable for every refrigeration), compared with traditional refrigeration systems, designed based on refrigerants and compressors. The reason is because they significantly perform better, than traditional refrigeration systems.
Thermoelectric refrigeration has an integrated chip design, no moving parts or hazardous gases, and operate silently. There are no bulky compressors involved, and have more precise temperature stability.
Refrigeration by thermoelectricity has the downsides of being less efficient than conventional refrigeration, even though it is more cost-effective and more practical, than conventional refrigeration technology, where there is transfer of thermal energy, away from a solid or liquid. Efficiency concerns, have limited its adoption in refrigeration trucks or vans, or refrigerated containers, especially in the cold chain, where it is important.
The thermoelectric effect is obviously of high regard in industry, and can at least be used to minimise heating up, within the peripheral systems of refrigeration trucks, and other refrigerated units or vehicles, involved in logistics operations.