Thermoelectric cooling

Thermoelectric cooling uses the Peltier effect to create a heat flux between 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.

Operating principle

Peltier element schematic. Thermoelectric legs are thermally in parallel and electrically in series.

Free convection thermoelectric cooler (Peltier cooler) with heat sink surface temperature contours, and rising warmer air and falling cooler air flow trajectories, predicted using a CFD analysis package, courtesy of NCI.

Main article: Peltier effect

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.

Construction

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. 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.

Some benefits of using a TEC are:

• No moving parts so maintenance is required less frequently
• No chlorofluorocarbons (CFC)
• Temperature control to within fractions of a degree can be maintained
• Flexible shape (form factor); in particular, they can have a very small size
• Can be used in environments that are smaller or more severe than conventional refrigeration
• Long life, with mean time between failures (MTBF) exceeding 100,000 hours
• Controllable via changing the input voltage/current

Some disadvantages of using a TEC are:

• Only a limited amount of heat flux is able to be dissipated
• Relegated to applications with low heat flux
• Not as efficient, in terms of coefficient of performance, as vapor-compression systems (see below)

Performance

A single-stage TEC will typically produce a maximum 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, as well as 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.

$\ Q=PIt$

where P is the Peltier Coefficient, I is the current, and t is the time. The Peltier Coefficient is dependent on temperature and the materials the TEC is made of.

In refrigeration applications, thermoelectric junctions have about 1/4th 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 semi-conductors 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.

Uses

A USB-powered beverage cooler

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.

Consumer products

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 replace heat sinks for microprocessors. They are also used for wine coolers.

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.

Identification

Peltier elements all conform to a universal identification specification

The vast majority of TECs have an ID printed on their heated 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.

See also

• Thermoacoustics
• Thermotunnel cooling

External links

• Template:Dmoz
• tec-microsystems.com, Thermoelectric Coolers Basics - answers for the most common questions