This type of system is used to move heat from one area to another by use of electrical energy. The electrical energy, rather than the refrigerant, serves as a ‘carrier’. The essential use of thermoelectric systems has been in portable refrigerators, water coolers, cooling of scientific apparatus used in space exploration, and in aircraft. The main advantage of this system is that there are no moving parts. Therefore, the system is compact, quiet, and needs little service.
Thermoelectric coolers are solid state equipment used in applications where temperature stabilization, temperature cycling, or cooling below ambient are required. There are many products using thermoelectric coolers, including CCD (charge coupled device) cameras, laser diodes, microprocessors, blood analyzers and portable picnic coolers.
Thermoelectrics are based on the Peltier Effect, discovered in 1834, by which DC current applied across two dissimilar materials causes a temperature differential. The Peltier effect is one of the three thermoelectric effects, the other two being known as the Seebeck effect and Thomson effect. Whereas the last two effects act on a single conductor, the Peltier effect is a typical junction phenomenon. The three effects are connected to each other by a simple relationship (Godfrey, 1996).
The typical thermoelectric module is manufactured using two thin ceramic wafers with a series of P and N doped bismuth-telluride semiconductor materials sandwiched between them. The ceramic material on both sides of the thermoelectric adds rigidity and the necessary electrical insulation. The N type material has an excess of electrons, while the P type material has a deficit of electrons. One P and one N make up a couple, as shown in Figure 3.69. The thermoelectric couples are electrically in series and thermally in parallel. A thermoelectric module can contain one to several hundred couples. As the electrons move from the P type material to the N type material through an electrical connector, the electrons jump to a higher energy state absorbing thermal energy (cold side). Continuing through the lattice of material, the electrons flow from the N type material to the P type material through an electrical connector, dropping to a lower energy state and releasing energy as heat to the heat sink (hot side).
Thermoelectrics can be used to heat and to cool, depending on the direction of the current. In an application requiring both heating and cooling, the design should focus on the cooling mode. Using a thermoelectric in the heating mode is very efficient because all the internal heating (Joulian heat) and the load from the cold side is pumped to the hot side. This reduces the power needed to achieve the desired heating.