Thermoelectric cooling

Two unique semiconductors, one n-type and one p-type, are used because they need to have different electron densities. The alternating p & n-type semiconductor pillars are placed thermally in parallel to each other and electrically in series and then joined with a thermally conducting plate on each side, usually ceramic, removing the need for a separate insulator. When a voltage is applied to the free ends of the two semiconductors there is a flow of DC current across the junction of the semiconductors, causing a temperature difference. The side with the cooling plate absorbs heat which is then transported by the semiconductor to the other side of the device.

The cooling ability of the total unit is then proportional to the total cross section of all the pillars, which are often connected in series electrically to reduce the current needed to practical levels. The length of the pillars is a balance between longer pillars, which will have a greater thermal resistance between the sides and allow a lower temperature to be reached but produce more resistive heating, and shorter pillars, which will have a greater electrical efficiency but let more heat leak from the hot to cold side by thermal conduction. For large temperature differences, longer pillars are far less efficient than stacking separate, progressively larger modules; the modules get larger as each layer must remove both the heat moved by the above layer and the waste heat of the layer.

ZT values for various materials and bismuth alloys.[3]

Requirements for thermoelectric materials:[4]

• Narrow band-gap semiconductors because of room-temperature operation;
• High electrical conductivity (to reduce electrical resistance, a source of waste heat);
• Low thermal conductivity (so that heat doesn't come back from the hot side to the cool side); this usually translates to heavy elements
• Large unit cell, complex structure;
• Highly anisotropic or highly symmetric;
• Complex compositions.

Materials suitable for high efficiency TEC systems must have a combination of low thermal conductivity and high electrical conductivity. The combined effect of different material combinations is commonly compared using a figure of merit known as ZT, a measure of the system's efficiency. The equation for ZT is given below, where ${\displaystyle \alpha }$ is the Seebeck coefficient, ${\displaystyle \sigma }$ is the electrical conductivity and ${\displaystyle \kappa }$ is the thermal conductivity.[5]

$${\displaystyle \mathbb {Z} \mathrm {T} =(\alpha ^{2}\sigma \mathrm {T} )/\kappa }$$

There are few materials that are suitable for TEC applications since the relationship between thermal and electrical conductivity is usually a positive correlation. Improvements in reduced thermal transport with increased electrical conductivity are an active area of material science research. Common thermoelectric materials used as semiconductors include bismuth telluride, lead telluride, silicon–germanium, and bismuth antimonide alloys. Of these, bismuth telluride is the most commonly used. New high-performance materials for thermoelectric cooling are being actively researched.[6]

For decades, narrow bandgap semiconductors, such as bismuth, tellurium and their compounds, have been used as materials of thermocouples.

Peltier elements all conform to a universal identification specification

The vast majority of thermoelectric coolers have an ID printed on the cooled side.[7]

These universal IDs clearly indicate the size, number of stages, number of couples, and current rating in amps, as seen in the adjacent diagram.[8]

Very common Tec1-12706, square of 40 mm size and 3–4 mm high, are found for a few dollars, and sold as able to move around 60 W or generate a 60 °C temperature difference with a 6 A current. Their electrical resistance will be of 1–2 ohm magnitude.

A significant benefit of TEC systems is that they have no moving parts. This lack of mechanical wear and reduced instances of failure due to fatigue and fracture from mechanical vibration and stress increases the lifespan of the system and lowers the maintenance requirements. Current technologies show the mean time between failures (MTBF) to exceed 100,000 hours at ambient temperatures.[9]

The fact that TEC systems are current-controlled leads to another series of benefits. Because the flow of heat is directly proportional to the applied DC current, heat may be added or removed with accurate control of the direction and amount of electric current. In contrast to methods that use resistive heating or cooling methods that involve gasses, TEC allows for an equal degree of control over the flow of heat (both in and out of a system under control). Because of this precise bidirectional heat flow control, temperatures of controlled systems can be precise to fractions of a degree, often reaching precision of milli Kelvin (mK) in laboratory settings.[10]

TEC devices are also more flexible in shape than their more traditional counterparts. They can be used in environments with less space or more severe conditions than a conventional refrigerator. The ability to tailor their geometry allows for the delivery of precise cooling to very small areas. These factors make them a common choice in scientific and engineering applications with demanding requirements where cost and absolute energy efficiency are not primary concerns.

Another benefit of TEC is that it does not use refrigerants in its operation. Prior to their phaseout some early refrigerants, such as chlorofluorocarbons (CFCs), contributed significantly to ozone depletion. Many refrigerants used today also have significant environmental impact with global warming potential[11] or carry other safety risks with them.[12]

TEC systems have a number of notable disadvantages. Foremost is their limited energy efficiency compared to conventional vapor-compression systems and the constraints on the total heat flux (heat flow) that they are able to generate per unit area.[10] This topic is further discussed in the performance section below.

Peltier elements are commonly used in consumer products. For example, they are used in camping, portable coolers, cooling electronic components, mattress pad sleeping systems and small instruments. They can also be used to extract water from the air in dehumidifiers. A camping/car type (12V) electric cooler can typically reduce the temperature by up to 20 °C (36 °F) below the ambient temperature, which is 25 °C if the car reaches 45 °C under the sun. Climate-controlled jackets are beginning to use Peltier elements.[15][16]

Thermoelectric coolers can be used to cool computer components to keep temperatures within design limits or to maintain stable functioning when overclocking. A Peltier cooler with a heat sink or waterblock can cool a chip to well below ambient temperature.[17] Some Intel Core CPUs from the 10th generation and onwards are capable of using the Intel Cryo technology,[18] which uses a combination of thermoelectric cooling and a liquid heat exchanger to deliver a much greater cooling performance than normally possible with standard liquid cooling. Local environment conditions are electronically monitored to prevent shorting from condensation.[19]

Thermoelectric coolers are used in many fields of industrial manufacturing and require a thorough performance analysis as they face the test of running thousands of cycles before these industrial products are launched to the market. Some of the applications include laser equipment, thermoelectric air conditioners or coolers, industrial electronics and telecommunications,[20] automotive, mini refrigerators or incubators, military cabinets, IT enclosures, and more.

In fiber-optic applications, where the wavelength of a laser or a component is highly dependent on temperature, Peltier coolers are used along with a thermistor in a feedback loop to maintain a constant temperature and thereby stabilize the wavelength of the device.

Some electronic equipment intended for military use in the field is thermoelectrically cooled.

Peltier elements are used in scientific devices. They are a common component in thermal cyclers, used for the synthesis of DNA by polymerase chain reaction (PCR), a common molecular biological technique, which requires the rapid heating and cooling of the reaction mixture for denaturation, primer annealing, and enzymatic synthesis cycles.

With feedback circuitry, Peltier elements can be used to implement highly stable temperature controllers that keep desired temperature within ±0.01 °C. Such stability may be used in precise laser applications to avoid laser wavelength drifting as environment temperature changes.

The effect is used in satellites and spacecraft to reduce temperature differences caused by direct sunlight on one side of a craft by dissipating the heat over the cold shaded side, where it is dissipated as thermal radiation to space.[21] Since 1961, some uncrewed spacecraft (including the Curiosity Mars rover) utilize radioisotope thermoelectric generators (RTGs) that convert thermal energy into electrical energy using the Seebeck effect. The devices can last several decades, as they are fueled by the decay of high-energy radioactive materials.

Peltier elements are also used to make cloud chambers to visualize ionizing radiation. Just by passing an electric current, they can cool vapors below −26 °C without dry ice or moving parts, making cloud chambers easy to make and use.

Photon detectors such as CCDs in astronomical telescopes, spectrometers, or very high-end digital cameras are often cooled by Peltier elements that may be arranged in a multi-stage,[22] cascade refrigeration configuration.This reduces dark counts due to thermal noise. A dark count occurs when a pixel registers an electron caused by thermal fluctuation rather than a photon. On digital photos taken at low light these occur as speckles (or "pixel noise").

They are used in Energy Dispersive Spectrometers to cool the sensor crystals, eliminating the necessity of large liquid nitrogen dewars.