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Thermoelectric cooling uses the Peltier effect to create a heat flux at the junction of two different types of materials. A Peltier cooler, heater, or thermoelectric heat pump is a solid-state active heat pump which transfers heat from one side of the device to the other, with consumption of electrical energy, depending on the direction of the current. Such an instrument is also called a Peltier device, Peltier heat pump, solid state refrigerator, or thermoelectric cooler (TEC). It can be used either for heating or for cooling, although in practice the main application is cooling. It can also be used as a temperature controller that either heats or cools.
This technology is far less commonly applied to refrigeration than vapor-compression refrigeration is. The primary advantages of a Peltier cooler compared to a vapor-compression refrigerator are its lack of moving parts or circulating liquid, very long life, invulnerability to leaks, small size, and flexible shape. Its main disadvantages are high cost and poor power efficiency. Many researchers and companies are trying to develop Peltier coolers that are cheap and efficient. (See Thermoelectric materials.)
A Peltier cooler can also be used as a thermoelectric generator. When operated as a cooler, a voltage is applied across the device, and as a result, a difference in temperature will build up between the two sides. When operated as a generator, one side of the device is heated to a temperature greater than the other side, and as a result, a difference in voltage will build up between the two sides (the Seebeck effect). However, a well-designed Peltier cooler will be a mediocre thermoelectric generator and vice versa, due to different design and packaging requirements.
Thermoelectric coolers operate by the Peltier effect (which also goes by the more general name thermoelectric effect). The device has two sides, and when a DC electric current flows through the device, it brings heat from one side to the other, so that one side gets cooler while the other gets hotter. The "hot" side is attached to a heat sink so that it remains at ambient temperature, while the cool side goes below room temperature. In some applications, multiple coolers can be cascaded together for lower temperature.
Two unique semiconductors, one n-type and one p-type, are used because they need to have different electron densities. The semiconductors are placed thermally in parallel to each other and electrically in series and then joined with a thermally conducting plate on each side. 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 moved to the other side of the device where the heat sink is. Thermoelectric Coolers, also abbreviated to TECs are typically connected side by side and sandwiched between two ceramic plates. The cooling ability of the total unit is then proportional to the number of TECs in it.
Current semiconductors being explored for TEC applications are antimony and bismuth alloys. So far, they are the materials that have led to the largest efficiency TEC systems. This is because they have a combination of low thermal conductivity and high electrical conductivity. These two factors, when combined, increase the system’s figure of merit (ZT), which is a measure of the system’s efficiency. The equation for ZT can be found below, where alpha is the Seebeck coefficient.
There are very few other materials that could be used for TEC applications since the relationship between thermal and electrical conductivity is usually a positive correlation. If these two values decrease or increase together, however, the overall effect is a net zero and the ZT value would remain too low for commercial applications.
Strengths and weaknesses
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There are many factors motivating further research on TEC including lower carbon emissions and ease of manufacturing. However, several challenges have arisen.
One of the most significant benefits of TEC systems is that they have no moving parts. This lack of mechanical wear increases the lifespan of the system and lowers the maintenance requirement. Current technologies show the mean time between failures (MTBF) to exceed 100,000 hours at ambient temperatures. Additionally, concerns such as fatigue and fracture become far less applicable to a stationary system.
Another benefit of TEC is that it does not use refrigerants in its operation. Some refrigerants such as chlorofluorocarbons (CFCs) were once used liberally in many cooling technologies, and prior to their phaseout contributed significantly to ozone depletion. Many refrigerants also have significant global warming potential.
The fact that TEC systems are current-controlled lead to another series of benefits. The first is that temperature control to within fractions of a degree can be achieved. This accuracy is a result of the device being powered using standard DC current, allowing the user to adjust exactly how many hole-electron pairs are recombining and, therefore, how much heat or cooling is being produced. It also facilitates the use of the system itself and makes its size more variable. TEC devices are much 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 main disadvantage of TEC systems is that only a small amount of heat flux can be dissipated. These systems are used in environments where the temperature difference between either end of the semiconductors is so small that it would not be possible to generate a large heat flux from it. This means that they can only be used for applications that require small flux. In other words, any large scale cooling would be more efficiently done by other technologies. Lastly, TEC systems are not currently as efficient as competing vapor-compression systems. This topic is further discussed in the performance section below.
A single-stage TEC will typically produce a maximal temperature difference of 70 °C between its hot and cold sides. The more heat moved using a TEC, the less efficient it becomes, because the TEC needs to dissipate both the heat being moved and the heat it generates itself from its own power consumption. The amount of heat that can be absorbed is proportional to the current and time.
where P is the Peltier coefficient, I is the current, and t is the time. The Peltier coefficient depends on temperature and the materials the TEC is made of.
In refrigeration applications, thermoelectric junctions have about 1/4 the efficiency compared to conventional means (they offer around 10–15% efficiency of the ideal Carnot cycle refrigerator, compared with 40–60% achieved by conventional compression-cycle systems (reverse Rankine systems using compression/expansion).) Due to this lower efficiency, thermoelectric cooling is generally only used in environments where the solid-state nature (no moving parts, low maintenance, compact size, and orientation insensitivity) outweighs pure efficiency.
Peltier (thermoelectric) cooler performance is a function of ambient temperature, hot and cold side heat exchanger (heat sink) performance, thermal load, Peltier module (thermopile) geometry, and Peltier electrical parameters.
Requirements for thermoelectric materials:
- Narrow band-gap semiconductors because of room-temperature operation
- Heavy elements because of their high mobility and low thermal conductivity
- Large unit cell, complex structure
- Highly anisotropic or highly symmetric
- Complex compositions
Common thermoelectric materials used as semiconductors include bismuth telluride, lead telluride, silicon germanium, and bismuth-antimony alloys. Of these bismuth telluride is the most commonly used. New high-performance materials for thermoelectric cooling are being actively researched.
Thermoelectric coolers are used for applications that require heat removal ranging from milliwatts to several thousand watts. They can be made for applications as small as a beverage cooler or as large as a submarine or railroad car. TECs have limited life time. Their health strength can be measured by the change of their AC resistance (ACR). When a TEC gets "old" or worn out, the ACR will increase.
Peltier elements are commonly used in consumer products. For example, Peltier elements are used in camping, portable coolers, cooling electronic components and small instruments. The cooling effect of Peltier heat pumps can also be used to extract water from the air in dehumidifiers. A camping/car type electric cooler can typically reduce the temperature by up to 20 °C (36 °F) below the ambient temperature. Climate-controlled jackets are beginning to use Peltier elements. Thermoelectric coolers are used to augment heat sinks for microprocessors. They are also used for wine coolers.
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, automotive, mini refrigerators or incubators, military cabinets, IT enclosures, and more.
Science and imaging
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. Since 1961, some unmanned 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.
Photon detectors such as CCDs in astronomical telescopes, spectrometers, or very high-end digital cameras are often cooled by Peltier elements. 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").
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.
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.
The vast majority of TECs have an ID printed on the cooled side.
These universal IDs clearly indicate the size, number of stages, number of couples, and current rating in amps, as seen in the adjacent diagram.
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