制冷原理与装置》讲义 What are thermoelectric devices used for? Thermoelectric the military for night vision equipment, electronic Refrlgeration quipment cooling, portable refrigerators, and e inertial guidance systems 陈江平 上海交通大学制冷研究所 8rmoeloct ic devico? The History of Thermoelectrics Thermocouples, Generators and Refrigerators
1 Thermoelectric Refrigeration 陈江平 上海交通大学制冷研究所 《制冷原理与装置》讲义 What are thermoelectric devices used for? the military for night vision equipment, electronic equipment cooling, portable refrigerators, and inertial guidance systems What is a Thermoelectric device? Thermoelectric modules are small, sturdy, quiet heat pumps operated by a DC power source. They usually last about 200,000 hours in heating mode or about 20 hours if left on cooling mode. When power is supplied, the surface where heat energy is absorbed becomes cold; the opposite surface where heat energy is released becomes hot. If the polarity of current flow through the module is reversed, the cold side will become the hot side and vice-versa. Thermoelectric modules can also be used as thermocouples for temperature measurement or as generators to supply power to spacecrafts and electrical equipment. The History of Thermoelectrics In 1821, Thomas Seebeck discovered that a continuously flowing current is created when two wires of different materials are joined together and heated at one end. This idea is known as the Seebeck Effect1 (Figure 4). The Seebeck effect has two main applications including temperature measurement and power generation. Thirteen years later Jean Charles Athanase reversed the flow of electrons in Seebeck.s circuit to create refrigeration. This effect is known as the Peltier Effect.1 This idea forms the basis for the thermoelectric refrigerator. Scottish scientist William Thomson (later Lord Kelvin) discovered in 1854 that if a temperature difference exists between any two points of a current-carrying conductor, heat is either evolved or absorbed depending upon the material.6 If such a circuit absorbs heat, then heat may be evolved if the direction of the current or of the temperature gradient is reversed. Thermocouples, Generators, and Refrigerators Thermoelectric modules can also be used as hermocouples for measuring temperature or providing the temperature-sensing element in a thermostat. To measure temperature the thermoelectric circuit is broken so the current quits flowing. When the current ceases, voltage is measured by a voltmeter1 (Figure 5). The voltage generated is a function of the temperature difference and the materials of the two wires used. Two wires used to measure temperature in this manner form a thermocouple. Thermocouples are the most prevalent device for temperature measurement. Thermoelectric modules can also be used as power generators. A thermoelectric generator (Figure 6) has a power cycle closely related to a heat engine cycle with lectrons serving as the working fluid. Heat is transferred from a high temperature heat source to a hot junction and than rejected to a low temperature sink from the cold junction. The difference between the two quantities is the net electrical work produced. The voltage output has been increased significantly with the use of semiconductors instead of metal pairs. Some use n-type and p-type materials connected in series for greater efficiency (Figure 7). N-type materials are heavily doped to create excess electrons, while p-type materials are used to create a eficiency of electrons. Melcor, the world.s first manufacturer of thermoelectric coolers, utilizes processed bismuth telluride to yield semiconductors with thermoelectric properties3. The couple is connected in series electrically and in parallel thermally then integrated into modules. The modules are placed between ceramic plates to offer optimum stability, electrical insulation, and thermal conductivity. The modules can be either mounted in parallel to increase the heat transfer effect or stacked to achieve high differential temperatures
Temperature Range It is theoretically possible to get a tempera bout 75 degrees Celsius working against the hot side temperature of 35 degrees Celsius. 2 This will only happen system. Typical pplications yield about half of the eached by using multiple lermoelectrie modules. Since thermoelectric modules ts ean be caculate b& will not perform as well in colder temperatures, their rature range becomes much smaller. Features and Advantages Features Advantages Assembly ccurate temperature control I coolIng depending on 2. Quite and no bradon 3. Solld-state devise hang no mowing shear so excess loading should be 3. Loaltzed coolIng. Small size and lghbwelght. adg3 hs. per sq inch of「mmmm如曲m 5. No CFC aues or refrigerants require Future research Example problem devices, reducing the cost of producing them and increasing their applications. predict their reliabilify and longterm deare toelectric research in ial uses. Professor um coefficient od because it offers a convenient earth friendly alternative fo normal power systems 1一“口算
2 Global makes a thermoelectric generator5 (Figure 8). In the center of the generator is a thermoelectric module, which contains lead-tin-telluride semiconductor elements. On one side of the module there is a gas burner. The other side has aluminum cooling fins or a heat pipe to keep it cool. The hot side maintains a temperature of 540 degrees Celsius, while the cold side stays at about 140 degrees Celsius. Thermoelectric devices can also be used as refrigerators on the bases of the Peltier effect.1 To create a thermoelectric refrigerator (Figure 9), heat is absorbed from a refrigerated space and than rejected to a warmer environment. The difference between these two quantities is the net electrical work that needs to be supplied. These refrigerators are not overly popular because they have a low coefficient of performance. The coefficient of performance for thermoelectric refrigerators can be calculated by dividing the cooling effect by the work input Temperature Range It is theoretically possible to get a temperature range of about 75 degrees Celsius working against the hot side at a temperature of 35 degrees Celsius.2 This will only happen if there is no thermal load, which will not happen in a real system. Typical pplications yield about half of the theoretical temperature difference. More extreme temperatures can be reached by using multiple thermoelectric modules. Since thermoelectric modules will not perform as well in colder temperatures, their temperature range becomes much smaller. Features and Advantages Features Advantages 1. Accurate temperature control possible. 1. Fast temperature response. 2. Heating and cooling depending on current direction. 2. Quite and no vibration. 3. Solid-state devise having no moving parts. 3. Localized cooling. 4. Small size and lightweight. 4. Can be used in any direction. 5. Environmental friendly. 5. No CFC gases or refrigerants require Assembly Thermoelectric modules are nstalled2 through mechanical clamping, epoxy bonding, and solder bonding (Figure 10). While the modules are strong in compression, they are weak in shear so excess loading should be avoided. Maximum recommended compression loading is 350 lbs. per sq. inch of module surface Future Research Researchers are working on improving the efficiency of thermoelectric devices, reducing the cost of producing them and increasing their applications. Researchers are trying to maximize the electricity output for a given heat source by changing the materials used in construction. They are also studying materials so they can predict their reliability and long-term behavior. The Japanese government is funding thermoelectric research in the fields of space technology, and domestic and industrial uses. Professor Michael Rowe proved that the amount of heat contained in the water leftover from a bath would provide enough electricity to power a color television for an hour.9 Electric power was produced through a series of thermocouples squeezed in between a few hot and cold-water channels. The power produced was about 100 watts. Research in the field of thermoelectrics is bound to continue because it offers a convenient, earth friendly alternative to normal power systems. Example Problem Question: A thermoelectric refrigerator removes heat from a refrigerated space at .5 degrees Celsius at a rate of 130 Watts and ejects it to an environment at 20 degrees Celsius. Determine the maximum coefficient of performance this thermoelectric refrigerator can have and the minimum required power input?