Peltier cell

The Peltier cell (also called Peltier devicePeltier heat pumpsolid state refrigerator, or thermoelectric cooler (TEC) and occasionally a thermoelectric battery) is a thermoelectric device consisting of many Peltier effect junctions in series, used both as a cooling and heating device; they are reversible through the Seebeck effect and can also be adopted as generators (for example in solar panels). Peltier cells can be of two types: insulated (if coated with ceramic material) and non-insulated. Its name comes from Jean Charles Athanase Peltier.

The Peltier cell is basically a solid-state heat pump that looks like a thin plate; one surface absorbs heat while the other emits it. The direction in which the heat is transferred depends on the direction of the DC current applied to the ends of the plate itself.

In Seebeck effect solar panels cells are heated on the side exposed to the sun, possibly with the concentrating effect of a Fresnel lens, while on the other side they are cooled by an exchanger crossed by a flow of cold water, obtaining a temperature difference of about 60 °C between the two sides. There is in fact a lower temperature limit in the ambient temperature and an upper limit in the resistance of the cell materials.

Structure of a Peltier cell

A common Peltier cell consists of two doped semiconductor materials of type N and type P, connected together by a copper foil. If a positive voltage is applied to the N-type and a negative voltage to the P-type, the upper lamella cools while the lower one heats up. By reversing the voltage, the thermal energy shift is reversed. On the market there are insulated Peltier cells and non-insulated Peltier cells: the first ones are coated below and above by ceramic material and guarantee higher efficiency than the second ones.

Power supply

Since Peltier cells have to absorb work to transfer heat according to the second principle of thermodynamics, i.e. to establish the stationary temperature difference, they necessarily absorb a large amount of electrical current. A typical 25 W cell of size 30 × 30 × 4 mm typically has a voltage drop of only 8.5 V at its ends and thus absorbs 2.1 A. Furthermore, due to the characteristic voltage-current curve for the junction, one usually supplies the device in current limiting (i.e. constant current).

If a set of cells is used to move a certain amount of heat, as in the case of cooling a laser diode or differently in the case of cooling a sensor, it must be kept in mind that to make the cell work it will obviously be necessary to remove, from the side of the “hot” junction, also the heat related to the power supplied to the junction and dispersed because of the efficiency; due to the modest efficiency, only a limited part of the heat to be removed corresponds to the heat actually moved.

The efficiency of a Peltier cell is maximum when the difference between hot and cold side is very low and the lower the current absorbed. The system is rather inefficient and can have some justification, if well governed, only for the possibility of making a very precise cooling, both punctiform (in the sense of cooling only specific points) and for the temperature range that locally can ensure.

For this reason Peltier cells are mainly used where small amounts of heat need to be moved: they are very useful, for example, to lower the temperature of passive parts (i.e., that do not generate heat).

Operation

Credits[1]

The common use of the cell is the subtraction of heat by adhesion of the cold side to the body to be cooled; the subtraction of heat is favored by the creation of appropriate thermal bridges (thermo-conductive adhesives or, for a better heat transfer, graphite sheets with a thickness of a few tenths of a millimeter) that allow the best conduction. The heat removed is transferred to the hot side, together with the operating heat (which is the majority); from the hot side the heat must be transferred to the external environment.

The main problem is the control of the current intensity to which corresponds the due heat subtraction; if the thermal source changes in value of heat emission, also the subtraction made by the cell must vary accordingly. This variation must be carried out possibly with temperature detectors so that, through a special feedback circuit, the intensity of current supplied to the cell maintains the operation in the permissible temperature range. In fact, it can be verified that:

  • the chilled thermal source decreases heat production or ceases to produce heat. In this case the subtraction of heat from the cell, if not controlled, can lower the temperature below the freezing point in a few seconds. In case the cooled part is for example a CPU of a computer this means that the CPU-Peltier plate complex can freeze and, if exposed to the atmosphere, condense the atmospheric moisture into ice on the component;
  • the thermal source increases heat production. In this case, the rise in temperature of the source, as a function of the heat subtraction performed, raises the temperature of the hot side of the cell. If this temperature exceeds the maximum allowed value, the cell can “cook”, i.e. be irreparably damaged and cease to function; moreover, the damage interrupts the subtraction of heat and therefore the parts that are no longer cooled can also be damaged.

In synthesis, if variable amounts of heat must be subtracted, the operation of the cell must be carefully governed in a variable manner; all energy delivered to the system must be subtracted as heat efficiently and safely from the hot side and dispersed to the outside.

Uses

Peltier cells are used where small amounts of material need to be cooled quickly. They are used for example to freeze biological samples, to cool CCD sensors in telescopes and thermal imaging cameras, in lasers to maintain a stable working temperature and sometimes to cool CPUs or GPUs by using a heat pipe to cool the side of the cell that heats up.

The cooling element is also used in small portable car and RV refrigerators, and in mini cold water dispensers. In the latter case, a heatsink with the finned surface immersed in the liquid is attached to the cell, which on the other side transmits the heat to an active heatsink (i.e. equipped with a fan). Between one component and another it is possible to find common thermoconductive paste. Basically, the cooling system is based on the same principle as CPU, GPU and chipset cooling.

For the reasons mentioned above it is necessary to use suitable power supplies with a current capacity suitable for the cell you intend to use.

The advantages of Peltier effect modules

Peltier effect modules are mainly used because they represent the ideal solution in all those situations where forced air cooling is indispensable, such as in the case of sealed environments/equipment. Other advantages derived from the use of these modules include the following:

  • Precise temperature control and speed of response to temperature changes: for each module operating with a known temperature difference between its hot and cold surfaces, there are well-defined relationships that determine the supply current that must be applied in order to achieve the required heat absorption. High-speed feedback loops allow temperatures to be controlled to an accuracy of a fraction of a degree.
  • Compact form factor and low weight: Peltier effect modules can be very compact, with heights as low as 3 mm. This is a feature particularly appreciated in all applications where weight and size are critical factors.
  • Possibility of cooling to below ambient temperatures: because Peltier effect modules use active type cooling to remove heat, they can employed to cool at temperatures below ambient. For this reason, manufacturers provide performance data for hot surface temperatures of 27 and 50 °C.
  • High reliability due to solid-state design with no moving parts: unlike forced-air cooling systems that use fans equipped with bearings characterized by limited lifespan, Peltier effect modules have no moving parts that can be subject to wear and tear. A typical Mean Time Between Failure (MTBF) value could be around 100,000 hours when operating with a constant temperature difference.
  • Environmental compatibility: since Peltier effect modules do not make use of refrigerants there are no risks from an environmental point of view regarding both emissions during operation and when the equipment is disposed of at the end of its operating life.
  • Possibility of use for cooling or heating: through simple reversal of current flow, Peltier effect modules can be used to pump heat into a system instead of removing heat. They can also be used as thermoelectric generators to store energy from dissipated heat.

References

  1. Heatlord. Wikipedia, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=46942685

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