Thermoelectric Power Generation and Refrigeration Systems ME 372 Instructor: Jesse Adams March 1, 2001 By: Ann-Marie Vollstedt
Thermoelectric Power Generation and Refrigeration SystemsWhat are thermoelectric devices used for? Thermoelectronic devices are used in a variety of applications. They are used bythe military for night vision equipment, electronic equipment cooling, portable refrigerators, and inertial guidance systems. Military quality night vision binoculars 8 (Figure 1) retail at about $500. These products are useful to the military during war and training because they are reliable, small, and quiet. Another advantage to these Figure 1: Night vision binoculars. thermoelectric products is that theycan be run on batteries or out of a car lighter. The medical community usesthermoelectric applications for hypothermia blankets, blood analyzers, and tissuepreparation and storage4. The main advantage of thermoelectric devices to the medicalcommunity is that the devices allow doctors precise temperature control, which is usefulin handling tissue samples. Hypothermia blankets are pads that patients rest on duringsurgery to keep their body at a certain temperature. Many people have thermoelectricproducts in their homes such as beer keg coolers, wine cellar chillers, water coolers, andpicnic basket coolers. Water Coolers like Advanced Thermoelectric Product’s “M-5”7 (Figure 2) retail for about $350. The M-5 holds 0.74 gallons of water and weighs 14.7pounds without the bottle. Consumers like the M-5 because it constantly keeps water atan ideal drinking temperature of 39-52 degrees Fahrenheit.
Thermoelectric devices are probably most wellknown for their contribution to powering spacecrafts likethe Voyager (1980). Radioisotope ThermoelectricGenerators provided all of the on- board electrical powerfor NASA’s Voyager. The Thermoelectric devices provedreliable since they were still performing to specification 14years after launch. The power system provided theequivalent of 100-300 watts electrical power and multiplesthereof.9 NASA is now requiring higher efficiency ratesout of smaller units. Figure 2: M-5 water coolerWhat is a Thermoelectric device? A thermoelectric device is one that operates on a circuit that incorporates boththermal and electrical effects to convert heat energy into electrical energy or electricalenergy to a temperature gradient. Thermoelectric elements perform the same coolingfunction as Freon -based vapor compression or absorption refrigerators. Energy is takenfrom a region thereby reducing its temperature. The energy is than rejected to a heat sinkregion with a higher temperature. Thermoelectric elements are in a totally solid state,while vapor cycle devices have moving mechanical parts that require a working fluid.3
Thermoelectric modules (Figure 3)are small, sturdy,quiet heat pumpsoperated by a DCpower source. Theyusually last about Figure 3: Thermoelectric module200,000 hours inheating mode or about 20 hours if left on cooling mode. When power is supplied, thesurface where heat energy is absorbed becomes cold; the opposite surface where heatenergy is released becomes hot. If the polarity of current flow through the module isreversed, the cold side will become the hot side and vice-versa. Thermoelectric modulescan also be used as thermocouples for temperature measurement or as generators tosupply power to spacecrafts and electrical equipment.The History of Thermoelectrics In 1821, Thomas Seebeck discovered that a continuously flowing current iscreated 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 mainapplications including temperature measurement and power generation.
Thirteen years later Jean Charles Athanase reversed the flow of electrons inSeebeck’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 Figure 4: Seebeck effect: Two wires of different temperature difference exists metals are connected at both ends to create a closed circuit. If one end is heated a current will flow between any two points of a current- continuously. carrying conductor, heat is eitherevolved or absorbed depending upon the material.6 If such a circuit absorbs heat, thenheat may be evolved if the direction of the current or of the temperature gradient isreversed.Thermocouples, Generators, and Refrigerators Thermoelectric modules can also be used as thermocouples for measuringtemperature 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 Figure 5: The voltage generated is a function of the temperature difference and the materials of the temperature difference and the the two wires used.
materials of the two wires used. Two wires used to measure temperature in this mannerform a thermocouple. Thermocouples are the most prevalent device for temperaturemeasurement. 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 electrons 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 Figure 6: A simple thermoelectric generator 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 deficiency 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 Figure 7: A Thermoelectric generator using n and p-type materials.
plates to offer optimum stability, electrical insulation, and thermal conductivity. Themodules can be either mounted in parallel to increase the heat transfer effect or stacked toachieve high differential temperatures. Global makes a thermoelectric generator5 (Figure 8). In the center of thegenerator is a thermoelectric module, whichcontains 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 heatpipe to keep it cool. The hot side maintains atemperature of 540 degrees Celsius, while the cold Figure 8: Globals thermoelectricside stays at about 140 degrees Celsius. generator Thermoelectric devices can also be used as refrigerators on the bases of the Peltiereffect.1 To create a thermoelectric refrigerator(Figure 9), heat is absorbed from a refrigeratedspace and than rejected to a warmer environment.The difference between these two quantities is thenet electrical work that needs to be supplied.These refrigerators are not overly popular becausethey have a low coefficient of performance. Thecoefficient of performance for thermoelectricrefrigerators can be calculated by dividing the Figure 9: A Thermoelectriccooling effect by the work input as shown in the refrigerator based on the Peltier effect.
example on page 10.Temperature RangeIt is theoretically possible to get a temperature range of about 75 degrees Celsius workingagainst the hot side at a temperature of 35 degrees Celsius.2 This will only happen if thereis no thermal load, which will not happen in a real system. Typical applications yieldabout half of the theoretical temperature difference. More extreme temperatures can bereached by using multiple thermoelectric modules. Since thermoelectric modules will notperform as well in colder temperatures, their temperature range becomes much smaller.AdvantagesThermoelectric devices are advantageous because they are reliable, light in weight, small,quiet, and inexpensive.2 They will function in environments that are too severe, toosensitive, or too small for conventional refrigeration. These environmentally friendlydevices offer precise temperature control, while requiring minimal maintenance becausethey have no moving parts. Thermoelectric devices are most useful for small coolingjobs where a compressor based system would be impractical. These devices are alsouseful because they can heat as well as cool depending on the polarity of the powersource.Assembly Thermoelectric modules are installed2 through mechanical clamping, epoxybonding, and solder bonding (Figure 10). While the modules are strong in compression,
they are weak in shear so excess loadingshould be avoided. Maximumrecommended compression loading is350 lbs. per sq. inch of module surface.Future Research Researchers are working on Figure 10: Thermoelectric Installation guideimproving the efficiency ofthermoelectric devices, reducing the cost of producing them and increasing theirapplications. Researchers are trying to maximize the electricity output for a given heatsource by changing the materials used in construction. They are also studying materialsso they can predict their reliability and long-term behavior. The Japanese government isfunding thermoelectric research in the fields of space technology, and domestic andindustrial uses. Professor Michael Rowe proved that the amount of heat contained in thewater leftover from a bath would provide enough electricity to power a color televisionfor an hour.9 Electric power was produced through a series of thermocouples squeezed inbetween 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 aconvenient, earth friendly alternative to normal power systems.
Example ProblemProblem 10.73 from Thermodynamics, An Engineering Approach.Question: A thermoelectric refrigerator removes heat from a refrigerated space at –5degrees Celsius at a rate of 130 Watts and rejects it to an environment at 20 degreesCelsius. Determine the maximum coefficient of performance this thermoelectricrefrigerator can have and the minimum required power input?Solution:Recall: The standard comparison for refrigeration cycles is the reversed Carnot cycle.The performance of refrigerators is expressed in terms of the coefficient of performance(COP). CoolingEffect QHCOP= = WorkInput Wnet −in 1The COP for Carnot refrigerators is: TH ( − 1) TLa. Maximum coefficient of refrigeration: 1 1COP max = COP r = = = 10.72 TH 293K ( − 1) ( − 1) TL 268Kb. Minimum required power input: . . QL 130WW in = = = 12.1W COPmax 10.72
Works Cited 1. Cengal,Yunus, and Michael Boles. Thermodynamics: An Engineering Approach. Hightstown: McGraw Hill, 1998. 2. “An Introduction to Thermoelectrics.” Tellurex Corporation. February 8, 2001. < http://www.tellurex.com/> (1 February 2001). 3. “Thermoelectric History/General Information.” Melcor, Thermoelectric Engineering Handbook. < http://www.melcor.com/handbook.htm> (1 February 2001). 4. “Thermoelectric Applications.” Melcor, Thermoelectric Engineering Handbook. < http://www.melcor.com/handbook.htm> (1 February 2001). 5. “About Generators.” Global Thermoelectrics.< http://www.globalte.com/genabout-frames.htm > (1 February 2001). 6. “Thermoelectrics.” The Columbia Encyclopedia. < http://www.bartleby.com/65/th/thermoel.html > (1 February 2001). 7. “The M-5 Water Cooler.” Advanced Thermoelectric Products. < http://www.electracool.com/products.htm> (27 February 2001). 8. “Night Vision Binocular.” Night Vision Optics. < http://www.nightvisionoptics.com/> (27 February 2001). 9. “Thermoelectric Generators: Seebeck effect used for conversion of heat energy into electric power.” Global Techno Scan. http://www.globaltechnoscan.com/31jan-6feb/generators.htm (1 February 2001).