Thermoelectric cooling

Thermoelectric effect
Principles Thermoelectric effect Seebeck effect Peltier effect Thomson effect Thermopower Seebeck coefficient Ettingshausen effect Nernst effect
Applications Thermoelectric materials Thermocouple Thermopile Thermoelectric cooling Thermoelectric generator Radioisotope thermoelectric generator Automotive thermoelectric generator

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). They can be used either for heating or for cooling (refrigeration), 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 main advantages of a Peltier cooler (compared to a vapor-compression refrigerator) are its lack of moving parts or circulating liquid, and its small size and flexible shape (form factor). Its main disadvantage is high cost and poor power efficiency. Many researchers and companies are trying to develop Peltier coolers that are both 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.

Peltier element (16x16 mm)

Free convection 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.

A USB-powered beverage cooler

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 DC 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.

Advantages and Disadvantages

Some benefits of using a TEC are:

• No moving parts, fluid, or refrigerants
• Flexible shape (form factor); in particular, they can have a very small size
• Has a long life, with mean time between failures (MTBF) exceeding 100,000 hours
• Is 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 TEC will typically produce a maximum temperature difference of 70°C (158°F) between its hot and cold sides. The more heat you wish to move 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.

Thermoelectric junctions are about 4 times less efficient in refrigeration applications than 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.^{[citation needed]}

Uses

Consumer products

Peltier elements are 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 below the ambient temperature. With feedback circuitry, peltiers can be used to implement highly stable temperature controllers that keep desired temperature within +/-0.01 Celsius. Such stability may be used in precise laser applications to avoid laser wavelength drifting as environment temperature changes. Climate-controlled jackets are beginning to use Peltier elements.

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.

The effect is used in satellites and spacecraft to counter the effect of direct sunlight on one side of a craft by dissipating the heat over the cold shaded side, whereupon the heat is dissipated by thermal radiation into space.^{[citation needed]} 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, lasting several decades, fueled by the decay of high energy radioactive materials.

Photon detectors such as CCDs in astronomical telescopes or very high-end digital cameras are often cooled down with Peltier elements. This reduces dark counts due to thermal noise. A dark count occurs when a pixel registers an electron because of a thermal fluctuation rather than because it has received a photon. On digital photos taken at low light these occur as speckles (or "pixel noise").^{[citation needed]}

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

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

These universal IDs clearly indicate the size, # of stages, # of couples, and current rating in amps, as seen in the diagram below.

Peltier elements all conform to a universal identification specification

See also

• Bismuth telluride, an efficient thermoelectric material
• Freezer
• H₂Ceramic cooling - a hybrid combination of thermoelectric and liquid cooling
• Heat transfer
• Joule's law
• Pyroelectric effect - the creation of an electric field in a crystal after uniform heating
• Seebeck effect
• Thermionic emission
• Thermoelectrics
• Thermoelectricity
• Thermopower
• Thermotunnel cooling
• Thermoacoustics

External links

• Thermoelectrics at the Open Directory Project