History and introduction

       J.T. Seebeck (1770-1831) found out in 1821 that when different metals are attached at two different connections, a minimal amount of voltage in terms of micro-volts will build up provided that the two connections are held at different temperatures. This effect is the basis for thermocouple thermometers, and familiarly known as “Seebeck effect”.

After more than a decade, 1834, the inverse of the Seebeck effect which is known as the “Peltier effect” was discovered by a scientist named Peltier. In his experiment, he found out that if a voltage is induced in a thermocouple, a temperature variation is developed between the connections which in effect results in a small heat pump. This phenomenon was later referred to as thermo-electric cooler (TEC).

Today, functional TECs use more than a few thermocouples in series to result in a significant amount of heat transfer. The thermocouples are made of N-type and P-type semiconductor materials bonded together. Semiconductors are heavily doped by means of adding impurities to produce an N-type semiconductor (excess electron carrier) or P-type semiconductor (lack electron carrier). The most famous combination used for thermocouples are Bismuth and Telluride semiconductors.

Thermoelectric Cooling

       Peltier effect is used for thermoelectric cooling to generate heat flux between the connections of two different types of materials. Some of these instruments are called TEC (thermoelectric cooler), Peltier diode, Peltier cooler, heater, thermoelectric heat pump, Peltier heat pump, Peltier device, cooling diode or solid state refrigerator. These instruments are solid-state active heat pumps which transfer heat from one side of the connection to the other side against the temperature ramp (from cold to hot),which in effect consumes electrical energy. Peltier devices are most commonly used as a cooling device since heating is more simply and efficiently achieved by so many other processes. But when a single device is needed for both cooling and heating, Peltier device is more advantageous to use. This process is achieved by connecting the device into a DC voltage which will cause one side to cool and the other side to warm. The efficiency of this process in moving the heat away from the cold side is a function of the current applied and effectiveness of heat removal from the hot side.

Uses of Peltier Coolers

- Peltier devices are usually used in portable and camping coolers and for cooling small instruments and electronic components. Some of the electronic equipment for military which is designed purposely in the field is thermoelectrically cooled. Dehumidifiers which extract water from air, use the concept of Peltier heat pumps cooling effect.

- The usual component of thermal cylinders are Peltier elements which is used for DNA synthesis by PCR (polymerase chain reaction), a common molecular biological technique which involves the fast cooling and heating of the reaction mixture for enzymatic synthesis cycles, denaturation and primer annealing.

- Peltier effect is also utilized in satellites and spacecrafts to oppose the effect of the direct sunlight on one side of the craft. This is done by dissolving the heat over the cold shaded side of the craft, upon which the heat is dissipated into the space by thermal radiation.

- Peltier elements are used as a cooling device for photon detectors such as CCDs in a sophisticated digital cameras or astronomical telescope. In effect it lessens the dark counts which are the result of thermal noise. (Dark count is an incident wherein a pixel gives a signal not because it received a photon but misinterpreted the thermal fluctuation as a photon. As a result it occurs as speckles or “pixel noise” in digital photos taken at low lights.)

- Thermoelectric coolers are used to keep the temperatures of the components of a computer within the design limits without the noise of the fan or to maintain its stable operation when overclocking. In fiber optic applications, where the wavelength of the device is a function of temperature, Peltier cooler are used together with a thermisror configured in a feedback loop to sustain a constant temperature which in that way will stabilize the wavelength of the device. A Peltier cooler with waterblock or heat sink can cool integrated circuits to below temperature condition.

- One of the latest additions to USB gadgets and toys are the USB drink coolers and chillers where Peltier devices are used. These gadgets are directly powered from the USB port and keep the drinks chilled while some can keep the drinks warm.

A quick look at typical peltier elements

CPU Peltier Coolers

       The challenge for today's CPUs having a power dissipation of over 100W lies on developing a Peltier CPU cooler implementing only a Peltier element and a heat sink, considering that a ready-made Peltier coolers are limited and very expensive. Such kind of coolers can dissipate over 200W of heat. So for modern CPUs, combination of Peltier with watercooling is much better. But in any case, the by product cooling system will still be expensive to use because of its high power consumption and its environmental issues. The large power dissipation will even require powerful and loud fans

The problem of this device is the destructive effect when the cooling of the Peltier element fails (e.g. fan failure or pump failure in the case of water cooling) which is much worst than the effect of a conventional cooling system failure. Because even if the CPU has a thermal protection that automatically shuts down the system if temperature gets too high, Peltier may still destruct it by continuing to heat up even after it has shut itself down.

Another problem associated to Peltier cooling is condensation. By using Peltier elements it is possible to cool down components below ambient temperature thus condensation may happen, which should be prevented because water and electronics don't go well together.

Advantage of Peltier elements

       Peltier cooling systems have still the biggest advantage despite of the problems associated with it. Unlike other cooling systems which allow cooling below ambient temperate (vapor phase refrigeration), Peltier comes inexpensive and more compact. Peltier elements are solid-state devices which require no maintenance, very reliable and no moving components.