8.7: Applications of Thermoelectrics

Thermoelectric devices are used to cool electronics, food, and people. Computer CPUs, graphics cards, and other types of electronics all generate heat, and these components can be damaged by excessive heat. Small thermoelectric devices can increase the reliability and lifetime of such components. Thermoelectric refrigerators have been used in RVs and submarines [3]. These devices are often less efficient than traditional refrigerators, but they can be small and quiet and require low maintenance. Some butter and cream dispensers in restaurants use thermoelectric devices to keep perishable foods cool [118], and truck-sized thermoelectric refrigerators are used to keep pharmaceuticals cool [118]. Engineers have tried making air conditioning units out of these devices [110]. They are better for the environment than traditional air conditioning units which require freon or other chemicals. However, they are not often used because the efficiencies are a few percent at best [110]. Thermoelectric devices have also been incorporated into military clothing to keep soldiers cool [118].

Thermoelectric devices are used both to make sensors and to control the temperature of sensing circuits. A thermocouple is a small thermoelectric device made from a junction of two materials that is used as a temperature sensor. It converts a small amount of energy from a temperature difference to electricity, and it can be used to measure temperature very accurately. Thermocouples are very common and often inexpensive. Thermoelectric devices are used to cool scanning electron microscopes and other types of imaging devices. Cooling is needed when imaging very small objects because heat causes atoms to vibrate, which can smear out microscopic images. Liquid nitrogen was used to cool imaging devices before thermoelectric devices became available, and it was much less convenient to use. The response of many types of sensors depend on temperature. A thermoelectric device may be part of a control circuit which keeps the sensor at a fixed temperature, so the sensitivity is accurately known.

Thermoelectric devices are used to generate power for satellites and planetary rovers because thermoelectric devices have no moving parts and do not require regular refueling. The Mars rover Curiosity is powered by NASA's Multi-Mission Radioisotope Thermoelectric Generator [119]. Figure \(\PageIndex{1}\) illustrates its major components. This power supply contains around 10 pounds of plutonium 238 in the form of plutonium dioxide. The plutonium decays naturally and produces heat. The heat interacts with a thermoelectric device and produces electricity, and the electricity is stored in a battery until use. The power supply produces around 2 kW of heat and around 120 W of electrical power, so the overall efficiency is around 6% [119]. This technology is not new. The Apollo 12 mission in 1969 used a similar type of power supply, but that supply produced only 70 W and had a lifetime of 5-8 years. Thermoelectric devices have also been used in nuclear power plants as a secondary system to recover some electricity from heat produced [5].

Figure \(\PageIndex{1}\): Labeled pull-apart view showing the major components of the NASA Multi-Mission Radioisotope Thermoelectric Generator. This figure is used with permission [120].

While thermoelectric effects are often fundamental to the operation of sensors and power supplies, the effects are sometimes unwanted [23, p. 457]. Electrical circuits contain junctions of wires made out of different metals. Such a junction occurs, for example, when an aluminum trace on a printed circuit board meets the tin wire of a resistor or when a tin lead solder joint meets a copper wire. The Seebeck effect occurs at all of these junctions. The Seebeck coefficient at a junction of copper and tin lead solder, for example, is \(2 \frac{\mu V}{K}\) [23, p. 457]. These unwanted voltages that develop can introduce noise or distortions into sensitive circuits.

Electrical engineers often think of heat as "wasted energy". Almost every electrical circuit contains resistors which heat up when current flows through them. In some applications, this heating is the desirable outcome. For example, some train stations have heat lamps for the use in winter, and a concert hall on a winter evening fills with people and heats up from the bodies. However, usually the heat is just considered a waste product or a nuisance.

In the long time limit, systems will reach an equilibrium temperature, but on short time scales, temperature differentials often exist. The inside of a car may be at a hotter temperature than the air outside. The air near an incandescent light bulb may be hotter than air elsewhere in a room, and so on. At one time in the past, we assumed that the earth had a nearly infinite amount of petroleum, coal, and other fossil fuels. Today, we know that these resources are finite. Recently, there has been increased interest in energy harvesting both for environmental reasons and for economic reasons, and thermoelectric devices can be used to convert this heat to usable electricity.