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Thermo-Electric Coolers NASA GSFC

Thermo-Electric Coolers NASA GSFC

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Thermo-Electric Coolers NASA GSFC

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  1. TFAWS Short Course Thermo-Electric CoolersNASA GSFC Presented ByEric W. Grob NASA Goddard Space Flight Center Eric.W.Grob@nasa.gov Thermal & Fluids Analysis WorkshopTFAWS 2011August 15-19, 2011 NASA Langley Research CenterNewport News, VA

  2. Overview • Introduction • Principles of Operation • Some background in semiconductors • Design Information • Design Considerations • Walk-through Examples • Real Life Projects • Early Use on Satellite • Hubble • ST-8 LHP TFAWS – August 15-19, 2011

  3. Acknowledgments 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”. Fundamental information on Thermo-Electric Coolers presented herein has been extracted from several sources, all of which are available in the public domain. A list of websites used is included in the references. TFAWS – August 15-19, 2011

  4. Overview The thermoelectric effect 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

  5. Why Use Them ? Pros: • Unlike common heat pumps (compression/expansion-based and Stirling cycle), these devices have no moving parts and work in any orientation • simple, reliable, compact, low mass, and noiseless, vibration-less operation. • Less maintenance - more than 100,000 hours of life for steady state operation • Function in environments that are too severe, too sensitive, or too small for conventional refrigeration • Contains no chlorofluorocarbons or other materials which may require periodic replenishment • Temperature control to within fractions of a degree using appropriate support circuitry. • Low thermal mass 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 0.01°C temperature stability. Such extreme stability is difficult to achieve by other means. • The direction of heat-pumping 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 72°C and 125°C can be achieved by single-stage 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 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 150°K, and hot-to-cold-side differences not exceeding 100°C. • not recommended for use below 130°K because of their prohibitively low efficiencies. TFAWS – August 15-19, 2011

  6. Thermoelectric Effects Note that the Peltier effect is the inverse of the Seebeck effect. Three separate theories behind the operation of thermoelectric cooling first appeared in the 1800s. Seebeck effect: Alessandro Volta and Thomas Johann Seebeck (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 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. TFAWS – August 15-19, 2011

  7. So What Took So Long To Use Them - TEC Evolution Although these principles were discovered in the 1800’s, most early work was on metal alloys, not thermoelectric compounds. it wasn’t until the introduction of semiconductor materials in the late 1950’s, that thermoelectric cooling became a viable technology for [small] cooling applications. 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 potential • noise-free operation • vibration-free operation • scalability • orientation-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 noise amplifiers (LNAs), star trackers, and IR (infrared) sensors. TFAWS – August 15-19, 2011

  8. 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 proportional to the difference between the hot and cold junction temperatures, THOT – TCOLD V = α(THOT - TCOLD) where α = Seebeck coefficient The temperature difference, produces an electric potential (voltage) that can drive an electric current in a closed circuit. Using the Seebeck effect, thermoelectric power generators convert heat to electricity. • Very inefficient • Used when waste heat is readily available, or in remote areas where dependability overrides efficiency TFAWS – August 15-19, 2011

  9. Peltier Effect When an electric current flows through two dissimilar conductors, depending on the direction of 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 = π*I where π = Peltier coefficient and I= junction current Thomson (Lord Kelvin) showed the relationship between the Seebeck and Peltier coefficients as: π = αT T = temperature of the junction (°K) α = Seebeck Coefficient (V/°K) Semiconductors are materials of choice for producing the Peltier effect. • They are more easily optimized for pumping heat • Designer can control the type of charge carrier employed within the conductor With 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

  10. But What Are These Coefficients ? Remember that the Seebeck Effect - 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 when 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 concentration and decreased electrical conductivity. Therefore, insulators tend to have very high Seebeck coefficients, while metals have lower values. • depends on the material's temperature, and crystal structure and has units of V/°K, or μV/°K. Typically metals have small coefficients because most have half-filled bands. Electrons (negative charges) and holes (positive charges) both contribute to the induced thermoelectric voltage thus canceling each other's contribution to that voltage and making it small. • Superconductors have a zero coefficient because it is impossible to have a finite voltage across a superconductor, 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 active research effort to find materials that could make cheaper and more efficient thermoelectric power generators. TFAWS – August 15-19, 2011

  11. Seebeck Coefficient - Examples Bismuth telluride (Bi2Te3): 287 μV/°K • A gray powder that is a compound of bismuth and tellurium. It is a semiconductor which, 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 bismuth, antimony, tellurium, and selenium. Uranium dioxide (UO2): 750 µV/°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. Prior to 1960 it was used as yellow and black color in ceramic glazes and glass. Perovskite - SrRuO3 (Strontium/Ruthenate): 36 μV/° K Constantan: 35 µV/°K Thallium tin telluride (Tl2SnTe5): 270 µV/°K TFAWS – August 15-19, 2011

  12. Making a Semiconductor One process for forming crystalline wafers is forming a cylindrical 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 as 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 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

  13. Doping to Make P- or N-Type Semiconductors Semiconductor doping was formally first developed by John 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 be 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 conductive p-type semiconductor. TFAWS – August 15-19, 2011

  14. 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 and 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 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 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, and Asia. TFAWS – August 15-19, 2011

  15. Manufacturing a TEC Semiconductor manufacturing process is beyond the scope of this course (and the instructor’s capability!), but an quick overview is shown below. TFAWS – August 15-19, 2011

  16. Cool Picture TFAWS – August 15-19, 2011

  17. Heat Flow in TE Module • The simplest 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 heat • “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 application in thermoelectric devices involves very complex physics at the subatomic level. For you sub-atomic physicists out there, this is way outside the scope of this course……. TFAWS – August 15-19, 2011

  18. How to Use Semiconductors in TECs • 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 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 the performance of the device. So how do they make this work ? Multiple pellets are needed in order to pump an appreciable amount of heat through a thermoelectric module. TFAWS – August 15-19, 2011

  19. 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 potential • Free end of the N-type pellet 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 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

  20. Simplest TEC The simplest TEC consists of two semiconductors: • One p-type and one n-type (one "couple") semi-conductor, connected 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. The net cooling is diminished by the effects of Joulean losses generated by the current, and heat conduction through semiconductor material from the hot to the cold junction (back conduction). TFAWS – August 15-19, 2011

  21. Brilliant ! Using this special properties of the thermoelectric “couples”, many pellets can be arranged in rectangular arrays to create practical thermoelectric modules. • The device can pump appreciable amounts of heat • Series electrical connection is suitable for commercially available DC power supplies. Most common thermoelectric devices • 254 alternating P and N-type pellets • Use 12 to 16 VDC supply • Draw 4 to 5 amps TFAWS – August 15-19, 2011

  22. Thermal Gradients Within a Thermo-Electric Couple Same as “couples” just covered, except packaging ceramic layers added. Temperature extremes are at the “junctions”. TFAWS – August 15-19, 2011

  23. Constructing a TEC A typical single stage cooler consists of two ceramic plates with an array of these p-type and n-type semiconductor materials (bismuth telluride alloys) between the plates. These multiple elements of semiconductor materials are connected electrically in series and thermally in parallel. 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. The heat absorption (cooling) is proportional to the current and the number of thermoelectric couples. This heat is transferred to the hot side of the cooler, where it is dissipated into the heat sink and surrounding environment. TFAWS – August 15-19, 2011

  24. From Schematic to Packaging So, the most efficient scheme is to connect our P-N pellets electrically in series, but thermally in parallel. Metalized ceramic substrates provide the platform for the pellets and the small conductive tabs that connect them. The pellets, tabs and substrates thus form a layered configuration. TFAWS – August 15-19, 2011

  25. Real TEC Modules - Single A typical thermoelectric module consists of an array of Bismuth Telluride semiconductor pellets that are “doped” with P and N pellets,. Module size varies from less than 0.25” x 0.25” to approximately 2.0” x 2.0”. TFAWS – August 15-19, 2011

  26. Real TEC Modules - Multiple • Multistage TEC's (also called cascade or stacked thermoelectric modules) are used where larger delta T's are required. Some applications require: • multiple single-stage: • or stacked multistage modules. TFAWS – August 15-19, 2011

  27. Peltier Videos • Illustrates the basic function, i.e.- create a temperature difference to generate voltage (power). • http://youtu.be/pgIOUXKyzFE • And this one illustrates applying a voltage to get cooling. • http://youtu.be/u46VGSw9v1I TFAWS – August 15-19, 2011

  28. THAT’s great, but how do I use ALL this KNOWLEDGE? Design Guide Online Application S/W Desktop Software TFAWS – August 15-19, 2011

  29. Some Terminology Temperature Difference ( T): hot side temperature of the module minus the cold side temperature of the module. Temperatures are referenced at the ceramic substrates of the module. Qc: total rate of heat being removed from the cold side of the module. Waste Heat: this is the rate of heat that the heat sink attached to the hot side of the module must dissipate. It is the sum of the heat removed from the cold side of the module plus the Joulean losses from the power input to the module plus parasitics. Input Voltage: the voltage applied to the module. Input Current: the current the module will draw at a particular input voltage. Coefficient of Performance (COP): ratio of heat removed to power input. QMAX: The amount of heat that a TEC can remove at a 0°temperature difference when the hot‐side of the elements within the thermoelectric module are at 300°K (27°C). IMAX: The current that produces Tmax when the hot‐side of the elements within the thermoelectric module are held at 300°K (27°C). VMAX: The voltage that is produced at DTmax when Imax is applied and the hot‐side temperature of the elements within the thermoelectric module are at 300 K.  TMAX: The maximum obtainable temperature difference between the cold and hot side of the thermoelectric elements within module when Imax is applied and there is no heat load applied to the module. This parameter is measured with the hot‐side of an element at a temperature of 300°K (27°C). . • In reality, it is virtually impossible to remove all sources of heat in order to achieve the true  Tmax. Therefore, the number only serves as a standardized indicator of the cooling capability of a thermoelectric module. TFAWS – August 15-19, 2011

  30. What Do All These Terms Mean? The terms Imax, Vmax , Qmax and ΔTmax never cease to confuse people. All these terms are defined at a given hot side temperature Thot of the TEC. Imax, Vmax and ΔTmax occur at the same time with Q = 0. Imax, Vmax and Qmax occur at the same time with ΔT = 0. Qmax and ΔTmax are shown on the performance curve as the end points of the Imax line. TFAWS – August 15-19, 2011

  31. Illustration of Terms Pump Analogy ? IMAX corresponds to the current that gives TMAX at QC = 0. QMAX: is the amount of heat that a TEC can remove at a 0°temperature difference when the hot‐side of the elements within the thermoelectric module are at 300°K (27°C). TMAX is the maximum obtainable temperature difference between the cold and hot side of the thermoelectric elements within module when Imax is applied and there is no heat load applied to the module. This parameter is measured with the hot‐side of an element at a temperature of 300°K (27°C). TFAWS – August 15-19, 2011

  32. Another View of Same Type of Data Melcor TFAWS – August 15-19, 2011

  33. What Does Imax Mean ? Imax is the direct current level which will produce the maximum possible ΔT, (i.e. ΔTmax), across the TEC with no heat load (Q=0). . Imax Operating below Imax there is insufficient current to deliver the ΔTmax Operating above Imax the power dissipation within the thermoelectric begins to elevate the system temperature and diminish ΔT. Imax and Vmax occur at the same operating point, i.e. Imax is the current level produced by applying Vmax to the thermoelectric device. Imax is not especially temperature-dependent. It tends to be fairly constant throughout the operating range of a thermoelectric device. Imax is NOT the maximum current that the TEC can withstand before failing. Thermoelectrics generally operate within 25% - 80% of the maximum current. TFAWS – August 15-19, 2011

  34. What Does Vmax Mean ? Vmax is the DC voltage which will deliver the maximum possible ΔT (i.e. ΔTmax), across the TEC with no heat load (Q = 0) At voltages below Vmax, there is insufficient current to deliver ΔTmax At voltages above Vmax, the power dissipation within the thermoelectric begins to elevate the system temperature, and diminish ΔT. Imax and Vmax occur at the same operating point, i.e. Imax is the current level produced by applying Vmax to the thermoelectric device. Vmax is temperature-dependent. The higher the Thot , the higher the Vmax rating for a specific device. Vmax is NOT the maximum voltage that the thermoelectic device can withstand before failing. TFAWS – August 15-19, 2011

  35. What Does Qmax Mean ? Qmax is the maximum load to the thermoelectric device that produces ΔT = 0. Qmax, Imax, and Vmax occur at the same operating point to produce ΔT= 0. It specifies an end point on the load line. Qmax does not project the maximum heat that can be handled by the thermoelectric device. • If the load goes beyond, the TEC will still pump the heat, but the thermal load simply winds up at an above-ambient temperature. (i.e. you have to look at different performance curve.) TFAWS – August 15-19, 2011

  36. What Does Tmax Mean ? ΔTmax • Ia the maximum possible ΔT across the thermoelectric device at a given Th. • ΔTmax always occurs at Q = 0. • ΔTmax always occur at Imax, and Vmax • ΔTmax specifies another end point on the load line. TFAWS – August 15-19, 2011

  37. TEC Performance Curve • The performance chart can be used to define all four engineering parameters providing that two are known or defined by a given cooling requirement. • Generally, Thot, Tcold, and Qc are known and the I and V needed to produce this cooling is of interest. • You can use this to analyze a test result when V, I, Thot, and Tcold were measured, and you want to know Qc. • Start with the known parameters and graph them as shown in the example. TFAWS – August 15-19, 2011

  38. Let’s Try It !! • If you know the desired T and QC: • Draw in the curve for the desired QC. • Draw the horizontal line for the desired ΔT. • Draw a vertical line downward from the intersection of the ΔT line and the QC curve on the upper graph into the lower graph. • Draw the horizontal V line, the placement being determined by calculating the ratio of AB/BC = EF/FG as shown on the graphs above. • So, this example shows that for the selected TEC, to achieve 20W of cooling with a 36°C ΔT, requires 3.9A at 9.5V. 1 2 2 3 1 3 4 3 4 TFAWS – August 15-19, 2011

  39. TEC Selection - Process Calculate heat loads In choosing a TEC, it is vital to understand the thermal load. There are two components in the thermal load in a thermoelectric system • Active load: Heat that is generated by the electrical components that require cooling – active heat dissipation from components • Passive load: Passive heat loads are parasitic in nature and may consist of radiation, convection, or conduction. Define Temperatures • Know your TEC’s sink temperature: example - must account for temperature drop from TEC to radiator and size/design to maintain the temperature you prescribe…..more parasitics!! Determine number of stages required • Function of overall T, rather that heat load. • Generally one stage will suffice for many applications, but multi-stages are possible, depending on the overall T. Select a TEC from vendor data/catalog using performance curves Call vendor to discuss !!! TFAWS – August 15-19, 2011

  40. TEC Selection – Example (Marlow Guide) TFAWS – August 15-19, 2011

  41. TEC Selection – Example (Marlow Guide) TFAWS – August 15-19, 2011

  42. TEC Selection – Example (Marlow Guide) TFAWS – August 15-19, 2011

  43. TEC Selection – Example (Marlow Guide) TFAWS – August 15-19, 2011

  44. TEC Selection – Example (Marlow Guide) See next page for Excerpt from Marlow Catalog TFAWS – August 15-19, 2011

  45. Select From Marlow Single-Stage Small size (<15mm) for lower heat load applications. Reduced power consumption Large size (>15mm) for medium to high heat load applications. Reliable in many applications with moderate on/off cycles, temperature cycles and for short term high temperature requirements. Unique diffusion barrier and solder type extends the operating conditions beyond standard commercial TEC Modules. Small size, high performance, telecom grade performance. Low power consumption using highest performance MAM thermoelectric material. Designed for qualification to Telcordia reliability assurance. TFAWS – August 15-19, 2011

  46. TEC Selection – Example (Let’s Try One !!) 2.0 0.4 0.0 0.6 3.0 0.0 -10.0 10.0 TFAWS – August 15-19, 2011

  47. TEC Selection – Example (Marlow Guide) TFAWS – August 15-19, 2011

  48. TEC Selection – Example (Marlow Guide) 10.0 67.0 0.15 0.15 0.85 TFAWS – August 15-19, 2011

  49. TEC Selection – Example (Marlow Guide) 0.15 0.85 TFAWS – August 15-19, 2011

  50. TEC Selection – Example (Marlow Guide) 3.0 0.15 20.0 3.0 0.85 3.5 RC6-4-01 20.0 3.7 8.2 RC3-2.5-01 6.0 2.5 3.6 See next page for Excerpt from Marlow Catalog TFAWS – August 15-19, 2011