Passing electric currents through two dissimilar metals (wires made from copper and bismuth), Peltier noticed that a change in temperature occurred at the junctions between them. One junction point of the wires got hot while the other got cold. This phenomenon of the heat transfer of an electrical current at the junction of two dissimilar metals, known as the Peltier Effect, is the basis for thermoelectric cooling.
Conventional cooling systems like air conditioners and air-to-water heat exchangers rely on chemical refrigerants or water to cool, or remove heat from, enclosures. In addition to refrigerants, air conditioners use compressors, evaporators, condensers and fans to provide cooling.
Other than a fan, thermoelectric coolers do not need any of the things required by air conditioners and air-to-water heat exchangers to effectively cool industrial enclosures. The Peltier Effect allows thermoelectric coolers to provide cooling without refrigerants, water or components such as compressors and coils. A typical thermoelectric cooler contains a “Peltier element,” a fan, and a power supply.
Peltier elements are constructed of two bonded semiconductors, often consisting of bismuth doped with telluride and copper. The semiconductors used in Peltier elements are referred to as “N-type” and “P-type.” An N-type semiconductor has an overabundance of electrons and the P-type semiconductor lacks a full set of electrons.
This electron deficiency creates “holes” in the P-type semiconductor’s molecular structure that are ready to accept the extra electrons moving over from the N-type semiconductor. These electrons move when the electric current from the power supply is passed through the semiconductor, carrying heat with them and leaving behind a cool surface. The fan then blows across this cooled surface and circulates cooled air throughout the enclosure.
When we speak about efficiency, we typically refer to the amount of “work” produced by a machine in relation to the amount of power that is put into it. Efficiency in cooling is described using the term “coefficient of performance.” The Coefficient of Performance (COP) is the ratio of heat energy that is removed to the amount of electrical power supplied. For the past several years, most of the thermoelectric coolers available on the market operated with a COP as low as 0.3. This means that for every 1 W of electrical energy invested, only 0.3 W of heat energy were removed, which is not very efficient at all. Thanks to design improvements, the COP of today’s thermoelectric coolers can be as high as 1.2.
A key design improvement that has helped to increase the efficiency of thermoelectric coolers is active control, and one of the best control methodologies for efficient energy usage (and improved lifespan) is Pulse Width Modulation (PWM). With PWM, power to the thermoelectric cooler is quickly switched on and off at a constant frequency.
Switching the voltage on and off in this manner will allow the thermoelectric cooler to deliver greater cooling capacity and efficiency – doing more work while consuming less power.
As previously discussed, since thermoelectric coolers work electronically – using the Peltier elements to generate cooling – the only moving parts are the fans used to circulate cool air throughout the enclosure.
This provides a number of benefits when compared to enclosure air conditioners. With fewer active components, there is a lower risk of component failure and less required maintenance. Along with solid-state construction, this leads to higher reliability, lower maintenance costs and consistent cooling.
Fewer moving parts also produce less vibration, acoustic and electrical/EMI noise. Many applications that might be impractical for air conditioners or air-to-water heat exchangers because of noise or vibration concerns can be cooled effectively with thermoelectric coolers. For example, thermoelectric coolers are ideal for applications that involve high-precision processes, sensitive or intricate electronics, or integral sensors that are susceptible to vibration and noise interference.
A smaller physical size (both in linear dimension and weight), thanks in part to limited internal components, creates an immediate advantage when compared to most air conditioners and air-to-water heat exchangers before the unit is ever turned on. This relatively small size – as small as 16 x 5 x 6 in. (H x W x D), weighing as little as 5 lb – allows thermoelectric coolers to be used in ways that other cooling devices cannot.
Thermoelectric coolers can cool pendant arm-mounted and otherwise suspended HMI enclosures without dramatically affecting the weight load placed on the arm or pendant system. They can also be used in multi-unit configurations.
Judith Koetzsch Product Manager for Climate Control Products and Mark Madden is the Lead Applications Engineer for Climate Control Products, both at Rittal Corp. This article is excerpted from the Rittal White Paper 304: Thermoelectric Cooling for Industrial Enclosures.
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