The thermoelectric effect can be exploited to convert temperature difference to electric voltage or to convert electric voltage to a temperature difference. And devices incorporating the thermoelectric effect can be built and configured to produce heat when voltage is applied. The effect is distinct from resistive (joule) heating, although in actual practice, both effects may be present in the same apparatus.
The thermoelectric effect can also be configured to measure temperature change or the temperature of a nearby object. The direction of heating and cooling is affected by the polarity of the applied voltage. Consequently, thermoelectric devices can be used as temperature controllers.
Perhaps the most well-known thermoelectric device is the thermocouple. It consists of two dissimilar metals (usually wires) such as iron and copper that are twisted together or fused to create a junction. A difference in temperature on the two sides of the junction will create a voltage and thus current flow. A typical application is the activation of an alarm, for example, if the flame in a gas furnace is extinguished or if the pilot flame goes out. The device requires no external power and is certain to function as long as the thermocouple circuit including junction remains intact.
However, the thermoelectric effect receiving the most attention these days is the Peltier effect in which a temperature difference results from the application of an external voltage. Heat is emitted at one side of the junction and absorbed at the other side. Accordingly, the device can be used for heating or cooling. The effect is named after Jean Charles Peltier, who discovered it in 1834. A frequent application is a simple heat pump that is used to cool sensors in CCD cameras.
Thermoelectric materials make use of the Seebeck effect, described by Baltic physicist Thomas Johann Seebeck: Voltage develops across the two sides of a junction consisting of dissimilar electrically conducting materials when there is a temperature difference between them. The ratio between the voltage and the temperature difference is the Seebeck coefficient.
In different materials, the Seebeck coefficient is not the same. And a spatial gradient in temperature can affect the Seebeck coefficient. If current is driven across this gradient, a continuous version of the Peltier effect will arise. Lord Kelvin (William Thomson) predicted that effect and observed it in 1851. In 1854, work by Kelvin implied that the Thomson, Peltier and Seebeck effects are phenomenologically different instances of the Seebeck coefficient.
The energy conversion efficiency is an important parameter in Peltier modules. Look at a spec sheet for a Peltier module and you’ll probably see that the amount of heat to be transferred through a Peltier module from the cold side to the hot side is denoted Q and is specified in Watts. This parameter typically is either the heat generated by an object to be cooled or the heat conducted to the ambient environment from the hot object. Thus to size a Peltier module, you’ll need to know Q. For a device such as a power transistor, Q is just the power that the transistor handles. (Also note Peltier modules can’t absorb thermal energy. They only transfer it and dissipate it on the hot side of the module.)
The process of sizing a Peltier module is analogous to that for sizing a heat sink. First work out the thermal power to be dissipated, usually just P = IC × VCE in the case of a power transistor. If in doubt use the largest likely value for IC and assume VCE is half the supply voltage. Then find the maximum operating case temperature (Tmax) for the transistor. When Tmax is tough to estimate, a frequent ballpark guess is to assume Tmax = 100°C. Then estimate the maximum ambient temperature, Ta, frequently taken as 25°C.
The temperature difference specified in a Peltier module datasheet (ΔT) is measured on the outside surfaces of the two ceramic plates of the module. It’s also important to figure out if there is any temperature difference between the Peltier module plates and Ta. Some Peltier properties also change with operating temperature, so some Peltier vendors spec their devices for more than one operating temperature.
Thus the important parameters for sizing a Peltier module for cooling are the heat transferred through the module, the temperature across the module, and the Peltier module hot-side temperature (i.e. the semiconductor case temperature). Also note that a heat sink is attached to the Peltier module’s opposite (hot) side. This heat sink must be sized to dissipate both the power transferred through the Peltier module and that due to the electrical power the Peltier module dissipates.
Peltier modules are current-driven similar to LEDs, so the best power supply is a controlled current source. Peltier modules can be driven with voltage sources but at the expense of accurate control of the heat flow and temperature difference across the module.
In recent years, thermoelectric materials have been investigated not only for use as solid-state refrigeration, but also as a means of generating power from waste heat. For these sorts of applications, a dimensionless figure of merit called ZT becomes important: ZT= S2 sT/k, where S is the Seebeck coefficient, s is the electrical conductivity of the thermoelectric material, T is the absolute temperature, and k is the thermal conductivity. A high conversion efficiency requires a high ZT, i.e., large S and s but low k.
The ZT is difficult to directly measure but can be calculated using measured S, s, and k. S and s can be directly measured. The k below room temperature can also be directly measured via specialized instruments.
Unfortunately, several issues limit the use of Peltier modules as thermogenerators. One is that their typical efficiency is only around 5–8%. Modern Peltier devices use highly doped semiconductor materials that can be pricey. Finally, it generally takes a high temperature for a Peltier module to generate much electricity. Alloys based on bismuth and antimony, tellurium or selenium are considered low-temperature thermoelectrics but like to see temperatures above 300°F. Thermoelectrics based on lead alloys handle temperatures up to about 1,000°F, and silicon-germanium thermoelectrics are for temperatures up to about 1,800°F. Consequently, Peltier thermogenerators tend to find use only for low-power remote applications.
But there is great interest in devising thermoelectric devices able to work at lower temperatures and which convert heat to electricity efficiently. Research is progressing in two main areas: materials that generate electricity at lower temperatures, and device structures that convert the infrared radiation given off by hot bodies into electrical current. Researchers say the quest is difficult its tough to improve the ZT because making making improvements in one of the parameters tends to make one or more of the others go in the wrong direction. Consequently, many of the strategies for improving ZT so far only work in a narrow range of temperatures.