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1、Presented ByEric W. GrobNASA Goddard Space Flight CenterEric.W.Grobnasa.gov,Thermo-Electric Coolers NASA GSFC,Thermal & Fluids Analysis WorkshopTFAWS 2011August 15-19, 2011NASA Langley Research CenterNewport News, VA,TFAWS Short Course,Overview,IntroductionPrinciples of OperationSome background in s
2、emiconductorsDesign InformationDesign ConsiderationsWalk-through ExamplesReal Life ProjectsEarly Use on SatelliteHubbleST-8 LHP,TFAWS August 15-19, 2011,2,Acknowledgments,Fundamental information on Thermo-Electric Coolers presented herein has been extracted from several sources, all of which are ava
3、ilable in the public domain. A list of websites used is included in the references.,TFAWS August 15-19, 2011,3,Many thanks go to Dr. Jentung Ku (NASA GSFC) for the information from his papers on this topic, and to David Steinfeld for his garage-built TEC “experiment”.,Overview,The thermoelectric eff
4、ect is the direct conversion of temperature differences to electric voltage and vice-versa. A thermoelectric device creates a voltage when a temperature difference is applied to it. Conversely, when a voltage is applied to it, it creates a temperature difference.,TFAWS August 15-19, 2011,4,Why Use T
5、hem ?,Pros:Unlike common heat pumps (compression/expansion-based and Stirling cycle), these devices have no moving parts and work in any orientationsimple, reliable, compact, low mass, and noiseless, vibration-less operation. Less maintenance - more than 100,000 hours of life for steady state operat
6、ionFunction in environments that are too severe, too sensitive, or too small for conventional refrigerationContains no chlorofluorocarbons or other materials which may require periodic replenishmentTemperature control to within fractions of a degree using appropriate support circuitry.Low thermal ma
7、ss and fast response time of TECs, when combined with an appropriate control loop, can provide precise temperature control. In relatively stable thermal sink environments, TECs achieve 0.01C temperature stability. Such extreme stability is difficult to achieve by other means.The direction of heat-pu
8、mping is fully reversible. Changing the polarity of the DC power supply causes heat to be pumped in the opposite direction, i.e.- a cooler can become a heater.Temperature differences:for a hot side around room temperature, temperature differences of about 72C and 125C can be achieved by single-stage
9、 and multistage TECs, respectively.In situations where the object being cooled generates little or no heat the combination of TE cooling and thermal insulation can produce large temperature differences. Cons:structural integrity of bismuth telluride and soldered joints when subjected to differential
10、 thermal expansion stresses.relatively low COP, particularly with large temperature differences. this can be acceptable when the heat load is small.best suited to situations with modest heat loads, cold temperatures not below 150K, and hot-to-cold-side differences not exceeding 100C.not recommended
11、for use below 130K because of their prohibitively low efficiencies.,TFAWS August 15-19, 2011,5,Thermoelectric Effects,Three separate theories behind the operation of thermoelectric cooling first appeared in the 1800s.,TFAWS August 15-19, 2011,6,Seebeck effect: Alessandro Volta and Thomas Johann Seeb
12、eck (1821) found that holding the junctions of two dissimilar conductors at different temperatures creates an electromotive force or voltage. This is the basis for thermocouples.Peltier effect: Jean-Charles Peltier discovered (1834) a heating/cooling effect when passing electric current through the
13、junction of two conductors. Thomson effect:William Thomson (Lord Kelvin) showed (1851) that over a temperature gradient, a single conductor with current flowing in it has reversible heating and cooling.,Note that the Peltier effect is the inverse of the Seebeck effect.,So What Took So Long To Use Th
14、em - TEC Evolution,Although these principles were discovered in the 1800s, most early work was on metal alloys, not thermoelectric compounds. it wasnt until the introduction of semiconductor materials in the late 1950s, that thermoelectric cooling became a viable technology for small cooling applica
15、tions.The characteristics of TECs make them highly suitable for precise temperature control applications and where space limitations and reliability are paramount or refrigerants are not desired.TECs have several advantages over competing technologies, including:high reliability potentialnoise-free
16、operationvibration-free operationscalabilityorientation-independence and compactness (high energy density). Based on these advantages, TECs now dominate certain applications, and new benefits continue to emerge. TECs in space have become relatively common; they provide temperature control for low no
17、ise amplifiers (LNAs), star trackers, and IR (infrared) sensors.,TFAWS August 15-19, 2011,7,Seebeck Effect,The voltage difference, V, produced across the terminals of an open circuit made from a pair of dissimilar metals, A and B, whose two junctions are held at different temperatures, is directly p
18、roportional to the difference between the hot and cold junction temperatures, THOT TCOLDV = (THOT - TCOLD)where = Seebeck coefficientThe temperature difference, produces an electric potential (voltage) that can drive an electric current in a closed circuit. Using the Seebeck effect, thermoelectric p
19、ower generators convert heat to electricity. Very inefficientUsed when waste heat is readily available, or in remote areas where dependability overrides efficiency,TFAWS August 15-19, 2011,8,Peltier Effect,When an electric current flows through two dissimilar conductors, depending on the direction o
20、f the current flow, the junction of the two conductors will either absorb or release heat.The heat absorbed or released at the junction is proportional to the electrical current. The proportionality constant is known as the Peltier coefficient.Q = *Iwhere = Peltier coefficientand I= junction current
21、Thomson (Lord Kelvin) showed the relationship between the Seebeck and Peltier coefficients as:=TT =temperature of the junction (K) =Seebeck Coefficient (V/K)Semiconductors are materials of choice for producing the Peltier effect.They are more easily optimized for pumping heatDesigner can control the
22、 type of charge carrier employed within the conductorWith semiconductor advancements, thermoelectric modules can now be produced to deliver efficient solid state heat pumping for both heating and cooling.,TFAWS August 15-19, 2011,9,But What Are These Coefficients ?,Remember that the Seebeck Effect -
23、 a voltage is produced when a temperature difference is applied across a junction of dissimilar materials. This applied temperature difference causes charged carriers in the material, whether they are electrons or holes, to diffuse from the hot side to the cold side, similar to a gas that expands wh
24、en heated.The efficiency with which a thermoelectric material generates electrical power depends on several material properties, of which perhaps the most important is the thermo-power, or Seebeck coefficient (). inversely related its carrier density - a higher results in decreased carrier concentra
25、tion and decreased electrical conductivity. Therefore, insulators tend to have very high Seebeck coefficients, while metals have lower values. depends on the materials temperature, and crystal structure and has units of V/K, or V/K.Typically metals have small coefficients because most have half-fill
26、ed bands. Electrons (negative charges) and holes (positive charges) both contribute to the induced thermoelectric voltage thus canceling each others contribution to that voltage and making it small. Superconductors have a zero coefficient because it is impossible to have a finite voltage across a su
27、perconductor, but can be doped with an excess amount of electrons or holes and thus can have large positive or negative coefficients, depending on the charge of the excess carriers. A larger induced thermoelectric voltage for a given temperature gradient will lead to a higher efficiency. There is an
28、 active research effort to find materials that could make cheaper and more efficient thermoelectric power generators.,TFAWS August 15-19, 2011,10,Seebeck Coefficient - Examples,Bismuth telluride (Bi2Te3):287 V/K A gray powder that is a compound of bismuth and tellurium. It is a semiconductor which,
29、when alloyed with antimony or selenium is an efficient thermoelectric material for refrigeration or portable power generation. Furthermore, the Seebeck coefficient of bulk Bi2Te3 becomes compensated around room temperature, forcing the materials used in power generation devices to be an alloy of bis
30、muth, antimony, tellurium, and selenium.Uranium dioxide (UO2): 750 V/K also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that occurs naturally . Used in nuclear fuel rods. A mixture of uranium and plutonium dioxides is used as MOX fuel. Pr
31、ior to 1960 it was used as yellow and black color in ceramic glazes and glass.Perovskite - SrRuO3 (Strontium/Ruthenate):36 V/ KConstantan: 35 V/KThallium tin telluride (Tl2SnTe5): 270 V/K,TFAWS August 15-19, 2011,11,Making a Semiconductor,One process for forming crystalline wafers is forming a cylin
32、drical ingot of high purity monocrystalline silicon is formed by pulling a seed crystal from a melt. A wafer is a thin slice of semiconductor material from these ingots. Wafers are formed of highly pure (99.9999% purity), nearly defect-free single crystalline material.Dopants (impurity atoms) such a
33、s boron or phosphorus can be added to the molten intrinsic silicon, thus changing it into n-type or p-type extrinsic silicon (more on this later).The wafer serves as the substrate for microelectronic devices built in and over the wafer and undergoes many microfabrication process steps such as doping
34、 or ion implantation, etching, deposition of various materials, and photolithographic patterning. Finally the individual microcircuits are separated (dicing) and packaged.,TFAWS August 15-19, 2011,12,Doping to Make P- or N-Type Semiconductors,Semiconductor doping was formally first developed by John
35、 Robert Woodyard working at Sperry Gyroscope Company during World War II.N-Type:Doping pure silicon with Group V elements (nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), bismuth (Bi), extra valence electrons are added that become unbonded from individual atoms and allow the compound to b
36、e an electrically conductive (N-Type). P-Type:Doping with Group III elements (boron (B), aluminum (Al), gallium (Ga), indium (In), thallium (Tl), which are missing the fourth valence electron, creates broken bonds (holes) in the silicon lattice that are free to move. The result is an electrically co
37、nductive p-type semiconductor.,TFAWS August 15-19, 2011,13,Making a Semiconductor,Semiconductor device fabrication is the process used to create the integrated circuits that are present in everyday electrical and electronic, including thermoelectric, devices. Multiple-step sequence of photographic a
38、nd chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material.Silicon is almost always used, but various compound semiconductors are used for specialized applications.The entire manufacturing process, from start to packaged chips
39、ready for shipment, takes six to eight weeks and is performed in highly specialized facilities.When feature widths were far greater than about 10 microns, purity was not the issue that it is today in device manufacturing. As devices became more integrated, cleanrooms became even cleaner. The workers
40、 in a semiconductor fabrication facility are required to wear cleanroom suits to protect the devices from human contamination.In an effort to increase profits, semiconductor device manufacturing has spread from Texas and California in the 1960s to the rest of the world, such as Europe, Middle East,
41、and Asia.,TFAWS August 15-19, 2011,14,Manufacturing a TEC,Semiconductor manufacturing process is beyond the scope of this course (and the instructors capability!), but an quick overview is shown below.,TFAWS August 15-19, 2011,15,Cool Picture,TFAWS August 15-19, 2011,16,Heat Flow in TE Module,The si
42、mplest form of a thermoelectric module:a single semiconductor “pellet is soldered to electrically conductive material (plated copper) on each end.“N-Type” semiconductor material:electrons are repelled by the negative pole of the power supply and attracted by the positive pole. Electrons carry the he
43、at“P-type” semiconductor material:“holes” are repelled by the positive pole of the power supply and attracted by the negative pole. “Holes” carry the heat.It is the charge carriers inherent in the material structure that dictate the direction of the heat flow.This thermoelectric effect and its appli
44、cation in thermoelectric devices involves very complex physics at the subatomic level.,TFAWS August 15-19, 2011,17,For you sub-atomic physicists out there, this is way outside the scope of this course.,How to Use Semiconductors in TECs,Multiple pellets are needed in order to pump an appreciable amou
45、nt of heat through a thermoelectric module.,TFAWS August 15-19, 2011,18,One idea is to connect N-type or P-type material in parallel , both electrically and thermally. Unfortunately, this is not practical.Typical semiconductor pellet is only rated for “tens” of milli-volts.A single pellet can draw 5
46、 amps or more with 60mV applied.A 254-pellet device will draw more than 1000 amps with 60mV applied.The only realistic solution is to wire the semiconductors in series electrically and in parallel thermally.Interconnections between pellets introduce thermal shorting that significantly compromises th
47、e performance of the device.,So how do they make this work ?,Aha !,Use both “N-type” and “P-type” materials.Arrange N and P-type pellets in a “couple” and form a junction between them with a copper tab.Free end of P-type pellet connects to the positive voltage potentialFree end of the N-type pellet
48、connects to the negative side of the voltage.Electrons flow continuously from negative pole of the supply, through the N pellet, through the copper tab junction, through the P pellet, and back to the positive pole of the voltage supply.Configure a series circuit that keeps all of the heat moving in
49、the same direction.The two different types of semiconductor material keep the charge carriers and heat flowing in the same direction through the pellets.,TFAWS August 15-19, 2011,19,Simplest TEC,The simplest TEC consists of two semiconductors:One p-type and one n-type (one couple) semi-conductor, co
50、nnected by a metallic conductor. When a positive DC voltage is applied as shown, electrons pass from the p-type to the n-type element, and the cold-side temperature decreases as the electron current absorbs heat, until equilibrium is reached. Heat is pumped from the cold junction to the hot junction
