Solar Direct-Conversion Power Systems-KMM

8/8/2019 Solar Direct-Conversion Power Systems-KMM

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1962 Cooley: Solar Direct-Conversion Power Systems 91in the choice of power systems based on components kw level, solar turbine, solar Stirling engine, nuclearwhich will compete with solar-cell systems in the future. turbo, solar thermionic and nuclear thermionic systems.Much of the "state-of-art" as of the beginning of 1961 is Their conclusions fo r each case of power level was thatgiven in the literature.1 3 no system had a predominant advantage when evalu-Choice of a given power system requires an inquiry ated with respect to appropriate weighting of reliability,into the advantages and disadvantages when faced with weight, availability, growth potential, cost, hazards,the following requirements: and life. The analysis was somewhat cursory but served1) period of time needed to provide a beginning fo r further analysis. One area2) level of power which would not receive a similar weighting by other3) orbital conditions analyzers is in materials. Those devices operating at4) reliability of components very high temperatures (such as thermionic diodes at5) interaction with spacecraft, payload and launch 1500'C to 2500C) are involved in a seriously difficultvehicle materials problem.6) cost Because of the growth in launch-vehicle capability7) weight now in progress, reliability probably will become more8) hazards important than weight in design considerations, es-9) availability pecially fo r operational spacecraft. Another two to four10) growth potential. years of research and development will be needed beforesufficient engineering and scientific data are available to

Fisher and i\Ienetrey7 have made an interesting com- make firm decisions regarding choice of the power sys-parison of several systems for a 3- and 30-kw output. tems to standardize.At 3 kw, they compared photovoltaic, solar turbine,solar Stirling engine, solar thermoelectric, solar thermi- ACKNOWLEDGMENTonic and nuclear-fission turbo systems and, for the 30- The author would like to thank R. Karcher of Gen-eral Electric for help on certain calculations and Fig. 4,7 J. Fisher and WV . R. Menetrey, "Comparison of energyconversion and W. Scott of the National Aeronautics and Spacesystems," Proc. 14th Annual Power Sources Cnof., May, 1960, pp. Agency, Washington, D. C., fo r comments in the sec-94-100; (PSC Publication Committee, P.O. Box 891, Red Bank,N. J.) tion on Design Consideration.

Solar Direct-Conversion Power Systems*WILLIAM C. COOLEYt

Summary-A survey is made of the present status of technology I. INTRODUCTIONof solar photovoltaic, photoemissive, thermoelectric and thermionicpower systems for spacecraft. The subjects of radiation damage to HE USE of photovoltaic solar cells has been asolar cells, power-system design, and solar simulation are reviewed. basic factor contributing to the success of manyVarious types of solar power systems are discussed and compared long-duration spacecraft missions. In the nearwith respect to weight, availability, environmental tolerance, an d future, photovoltaic power systems will continue tocost. It is concluded that solar photovoltaic and solar thermionicsystems are most desirable for power levels up to 3 kw . However, play a dominant role However, other types of solarthe life capability of thermionic converters has no t yet been estab- direct-conversion power systems are under developmentlished. Solar thermoelectric and photoemissive systems will be less which may prove superior fo r certain applications. Thisdesirable because of their lower efficiency and, therefore, larger area paper reviews the present status and future prospectsper unit power output, except for missions where radiation resistance fo phtvlac phteisv,slr-hroeti nor economy ar e paramount considerations.soa-hrincpwryte .* Received by the PGMIL, October 2, 1961.t Exotech, Inc., Alexandria, Va. Prior to October 1, 1961, Chief,Advanced Technology Program, NASA, Washington 25, D. C.

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92 IRE TRANSACTIONS ON MILITARY ELECTRONICS JanuaryII . PHOTOVOLTAIC POWER SYSTEMS in th e base material, which prevents holes which are

A. Solar-Cell Design and Operation produced deep in the crystal by infrared photons fromreaching the p-n junction. Therefore, radiation causes aThe invention of the silicon photovoltaic cell by Bell decrease in response of a solar cell to the infrared end ofTelephone Laboratories in 1954 occurred at a fortunate th e solar spectrum.time in that it has permitted the development of silicon A recombination center called the silicon A center hassolar cells as long-duration power sources fo r nearly all been identified as the substitution of an oxygen atomspacecraft since 1957. for silicon in the lattice, which results from associationA photovoltaic solar cell produces electrical power by of riaion-produce latticevancs with oxygenseparating charges (electrons and holes) by a built-in impurities. These centers are believed to be responsibleelectrostatic field in the vicinity of a p-n junction in a forith era ntdamagesenitvito n-onsolasemiconductor. The junction is located near the front cel whe mat dempte siliconw ontaisurface of the solar-cell crystal, at a depth of the order cen.hicheresearom needed ton wich cainsof one micron, in order to take advantage of the high oxygen. Further research is needed to identify and, ifofeonermiron, i ordhoeeertron padvanta sofa ptheohi possible, to reduce the formation of recombination cen-generation rate of hole-electronl pairs by solar photons ters in n-type silicon which lead to performance de -which are absorbed exponentially with distance into the gratin n-type s olar cl Research o ecell Th bl ean ulravoletphoonsare bsobedgradation of p-on-n-type solar cells. Research on thecell. The blue and ultraviolet photons nature of recombination centers is proceeding at variousnear th e surface, while the red and infrared photons places by the use of electron spin resonance techniquespenetrate more deeply. and other methods.

In order to maximize th e cell efficiency, the diffusion At the present time, the best p-on-n-type silicon solarlength for minority carriers in the base material (holes c m win n-type material and electrons in p-type material) te same ritancertoriaion dame asxtebeshold e lng odertha caries podued eepinthe same resistance to radiation damage as the bestshould be long in order that carriers produced deep in n-on-p cells. It is estimated that either cell, protected bythe crystal by photons at the infrared end of the solar .n cels It is esiae thtete 'el,poetdb0 mils of synthetic sapphire for shielding, will degradespectrum have a chance to diffuse back up to the P-n by 25 per cent in power output in a period of approxi-junction and be separated. In order to obtain a long mately three months due to proton irradiation withinminority-carrier diffusion length, crystal perfection theinnerVanAllenbelt. If thecellsareprotectedbytheinnghanimpuritlevels Iftelow.aeprtctdbust be high and impurity levels low. cover slides of less than 30-mil thickness, the degrada-The present production processes fo r 1 X 2-cm gridded tion will be correspondingly much more rapid. Fig. 1silicon solar cells tvpically result in a distribution of cell s t c1 ~~~~~~~~shows typical curves of ef:ficiency vs proton radiationefficiencies (for example, from about 9 to 14 per cent). dose fo r both p-on-n and n-on-p cells. (It may be seenCells of the highest efficiency can be selected and sold at that the short-circuit current decreases linearly with thea price which depends primarily on th e market demand logarithm of dose after an initial period.) Potentialfor th e large yield of lower efficiency cells. methods fo r improving the radiation resistance of siliconsolar cells are based on 1) improving their response tothe blue end of the spectrum and intentionally sacrific-Exposure of a semiconductor crystal to particle radia- ing initial infrared response, 2) eliminating the forma-tion (such as the electrons and protons in the Van Allen tion of recombination centers by control of impurities orbelts) results in displacement of atoms from their nor- addition of chemical dopants, or 3) building in an elec-mal sites in the crystal lattice. Th e resulting vacancies trostatic drift field to sweep minority carriers up to theand interstitial atoms can diffuse and may become p-n junction.associated with other lattice defects such as impurityatoms to form "recombination centers" fo r electronsand holes. Any semiconductor device which depends forits operationi on the diffusion of either electrons or holes \ALLIUMthrough a significant distance in the crystal will have ARSIDEits performance affected by the introduction of recom-bination centers. For example, solar cells, diodes and .5L/CONpower transistors fall in this category.In the case of arsenic-doped n-type silicon, high-qual- o SILICON/P^r _ ~~~~~AWERAGL)\. \it y single crystals ar e available which have a minority- ecarrier diffusion length of 50 to 100 microns. This is \adequate so that p-on-n solar cells with an initialJ, l, ,,, .,,l,efficiency of 12 to 14 per cent can be made. The intro- RADIATION DOS (PROTONS/CM2)duction of recombination centers by particle radiationFi.1Efcof9Meprtnadtonncauses a reduction in minority-carrier diffusion length various types of solar cells.



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1962 Cooley: Solar Direct-Conversion Power Systems 93In the case of solar cells made from gallium arsenide,the high absorption coefficient for solar photons leadsto absorption of most photons near the front surface ofthe cell, close to the p-n junction, and therefore a longminority-carrier diffusion length is not required, even ifattainable. Therefore, the reduction of minority-carrier

diffusion length in gallium arsenide under irradiationshould produce only a small effect on solar-cell perform-ance. For this reason, gallium arsenide appears to be apromising material fo r radiation-resistant solar cells.Recent tests show that GaAs cells have much greater Fig. 2-Orbiting solar observatory with 25-w solar-cell array.resistance than silicon cells. However, present produc-tion costs of GaAs are quite high as compared to silicon.Fig. 1 shows radiation data fo r GaAs cells.Extensive research has been performed on cadmiumsulfide solar cells and efficiencies of five to seven percent have been achieved with single crystals and 2 to2.5 per cent with large area (2 0 cm2) thin film cells. Itappears that the operation of these cells may dependpartially on photoemissive as well as photovoltaicprocesses. Further research is needed to understand th ecomplicated phenomena involved. If photoemission is asignificant factor, cadmium-sulfide solar cells may havegood promise as radiation-resistant devices, since minor-ity-carrier lifetime would be of little or no importance.C. Power System Design

Figs. 2 and 3 show typical sun-oriented solar-cellarrays which are used in spacecraft. Table I summarizesthe major design parameters for various power systems.It may be noted that most of the solar-cell systems listedwill produce an average power of approximately 1w/lbof system (including conversioni equipment, but exclud- Fig. 3-One of th e 10-ft2 solar-cell panels made by Iloffmanin g the battery weight). Electronicis for th e Ranger Lunar Spacecraft.TABLE I

DESIGN PARAmETERS FOR ORIENTED SOLAR CELL ARRAYSTotal poe Watts System Weight _PowerSpacecraft Orbit Area Powtpu (Excluding bat- SstmWih(ft2) Output (ft2) teries) lb/ft2 SystemW/eight

Orbiting solar 300 Nautical Miles 4 25 w avg 6.4 -_observatory CircularRanger Lunar Flyby 20 194w max 9. 7 max 2.5 3.9 max157 w min 7.8 min 3.1 minNimbus meteorological 500 Nautical Miles -|45 200 w avg 4.5 avg 2.2 2.0 avgsatellite CircularOrbiting geophysical 150 Nautical Miles 414 w max 5.5 max 1.65 maxobservatory Perigee 75 3.360,000 Nautical Miles 250 w avg 3.3 avg 1.0 avgApogee l_l_l_l_lOrbiting astronomical 475 Nautical Miles 115 600 w max 5.2 max 2.1 maxobservatory Circular 250 w avg 2.2 avg 2.5 0.9 avgTwo-Man space |-300 Nautical Miles - 1500 w - _laboratory Circular



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94 IRE TRANSACTIONS ON MILITARY ELECTRONICS JanuaryThe design of a solar-cell array which is to be sun- Aoriented in space requires that the solar cells be mounted

on a panel and coninected with sufficient cells in series togive the required battery-charging voltage. Enoughstrings are connected in parallel to give the desired cur-rent, with redundanicy to compensate for any open-cir-cuit failures. Blockinig diodes in series with each stringof cells prevent reverse current flow from the storagebatteries in case of short circuits or during periods ofdarkness. The solar cells in various strings should havematched I-V characteristics and operating points shouldbe selected which make allowance fo r possible circuitfailures or degradation of solar-cell output by radiationldamage or other effects during the desired system life.The cells must be mounted on a panel which issufficiently rugged to withstand the launching vibra-tionis anid accelerationis. The thermal conductivity Fig. 4-Ranger panel with plane mirror concentratinig system de-through the panel should be high and the emissivity veloped by Electro-Optical Systems, Inc., for the NASA Je tof the back surface and of the solar-cell covers should be Propulsion Lab.high to reduce cell operating temperature and therebyincrease efficiency. The solar-cell cover slides may in-clude an anti-reflecting front surface and an ultra-violet-reflecting back surface in order to maximize theuseful radiation at the cell and to reject useless solarradiation which would only heat the cells excessively orcause darkening of the cement which bonds the coverslide to the cell. The cover slides should be made ofquartz, synthetic sapphire or a glass which does notdarken under exposure to the relatively high flux of low-energy particles in the Van Allen belts. The thermal ex -pansion coefficient of the cover slide should be matchedto that of the solar cell to minimize thermal stresses onthe adhesive bond. Fig. 5-Concentrating solar-cell array developed by th e Boeing Co .In order to increase the power output per unit area ofsolar cells, it is possible to use plane or concave mirrors solar concenitrators requires more accurate control ofto increase the illumination intensity on the cells. Fig. sun-orientationi than a nonconicenitratilng array.4 shows a concenitrating system developed by Electro- The orientationi of the solar-cell array toward the sunIOptical Systems, Inc., which uses an arrangement of requires a sun-sellsor and a servo control system whichplane mirrors mounted on a Ranger solar-cell panel. applies torques to the panel. If the solar array isTests at sea level indicated that the power output of the mounted rigidly to the spacecraft, the entire systemarray at Mars could be doubled by adding the reflecting may be oriented by use of reaction jets, reaction wheels,concenitrators. The mirrors are made by stretching interaction with the earth's magnetic field, or solar-aluminized Mylar plastic on a metal frame. radiation pressure. If the array is mounted on bearingsAnother concept fo r attaining solar concentration is from the spacecraft, friction torques may be overcometo use maniy small mirrors so that each solar cell or a and control torques introduced by means of drivesmall patch of cells has its owII concentrator. For ex - motors. Disturbing torques which must be consideredample, one configuratiofi which is under development by include those due to gravity gradient, aerodynamiiicthe Boeing Companiy is shown in Fig. 5. These designs moments, interactioni of internal currents with thenot only increase the power output per unit investment ambienit magnetic field, solar-radiation pressure, im-in solar cells, but may also lead to lower weight per unit pacts by micrometeoroids, friction in bearings and slip-power output. When using solar concentrators, there is rings, and effects of internial moving parts, such as tapea strong incenitive to improve the heat-rejection system recorders.to avoid excessive loss of efficiency due to high operatingtemperatures. Improvements in performance cani be D. Solar Simulationattained by the use of mirror surfaces with spectrally One of the serious impediments to the development ofselective reflection properties. Generally, an array with solar cells and power systems has been the lack of an



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1962 Cooley: Solar Direct-Conversion Power Systems 95adequate solar simulator which would duplicate the sulated from the cathode. Spicer has estimated [6]intensitv vs wavelength distribution of solar radiation that vacuum spacings of the order of 0.01 cm would beoutside the earth's atmosphere. The use of actual sun- necessary to minimize electron space-charge effects, andlight at th e earth's surface or even at altitudes up to he concludes that the optimum converter would be com-10,000 feet is rather unsatisfactory and requires detailed posed of a (Cs)Na2KSb surface as an emitter and anmeasurement of the intensity-wavelength distribution Ag-O-Cs surface as a collector. Neglecting space-chargebecause of the variability of atmospheric absorption. effects, conversion efficiencies of two per cent areThe use of a tungsten filament at 2800K is inadequate calculated for "average" cathodes. Efficiencies might bebecause the peak intensity occurs at a wavelength of 1 doubled by the use of optimum surfaces.micron (in the infrared) instead of 0.45 micron as in Since efficiencies of two to seven per cent are consider-sunlight outside the earth's atmosphere. None of the ably below those attainable with photovoltaic devices,existing sun simulators gives an intensity which ac - the potential of photoemissive devices is limited to thecurately simulates the solar spectrum at all the wave- possibility of lowering the weight or cost per unit powerlengths from 0. 3 to 1.2 microns where solar cells have output. Practical applications will require developmentsignificant response. Most sun simulators use an arc of deployment techniques to expose large areas to thelight source, such as xenon, mercury and xenon, or sun. Furthermore, the materials and geometry mustcarbon. It may eventually be possible to develop a permit solar radiation to strike the photocathode with-black body source with a temperature near 6000K, but out appreciable absorption in the electrically conductingthe technological problems are very difficult. collector.

Presently available arc lights generally do not match As a result of recent research on electron tunnelingth e solar spectrum so that tailoring of the spectral through thin dielectric films of less than 100-A thick-distribution is required. This may be accomplished by ness, it becomes feasible to consider such films to re-the use of spectrally selective filters or mirrors or by place a vacuum as the insulator in photoemissive de-spectral analysis of the light, followed by selective vices. Such a "dielectric tunnel solar cell" would haveattenuation and remixing. the advantage of greater radiation resistance than semi-If an accurate solar simulator of small area could be conductor photovoltaic cells and appears to be mechan-developed, it would be useful in solar-cell develop- ically more practical than a vacuum-insulated photo-ment, production control, analysis of radiation effects, emissive device.and in selection of matched cells fo r use in solar-cellarrays. In particular, the use of an accurate sun simula- IV . SOLAR-HEATED POWER SYSTEMStor would permit rapid measurement of the integrated Solar-heat collectors may be used with thermoelectriceffect of radiation damage without the necessity fo r or thermionic "heat engines" to provide power by directmeasuring spectral response with a spectrophotometer, conversion. Flat plate collectors with spectrally selec-then determining performance under a standard light tive coatings can provide hot junction temperatures upsource (e.g., 2800K tungsten) and finally calculating to about 500i F for use with thermoelectric converters.integrated response for space sunlight conditions. Efficiencies are predicted to be of the order of two toIt seems unlikely that an accurate solar simulator four per cent so that large area devices are required, ascan be developed at reasonable cost which can be used for photoemissive systems. Higher efficiencies are attain-to test large area solar-cell arrays. Therefore, such test- able by raising heat input temperatures by the use ofing is usually done in sunlight at a high-altitude site solar concentrators.such as Table Mountain, Calif. Consideration is also The use of large mirrors with a cavity receiver tem-being given to th e use of high-altitude balloons or aircraft perature above 1300F makes it feasible to consider thefo r test purposes in order to minimize atmospheric use of lithium hydride as a thermal energy storage mate-attenuation of solar radiation. rial fo r satellite applications. If a satisfactory contain-

Another impediment in solar-cell development and ment system can be developed fo r lithium hydride, ittesting has been the inadequacy of measuring techniques will be applicable to both solar-thermoelectric and solar-and calibration standards for determining easily the mechanical power systems and will permit a lowerintensity vs wavelength distribution at various points weight system than is possible using presently availableof an illuminated area. Improved spectrophotometer batteries.equipment is needed. Thermionic converters require geometrically accuratemirrors of high concentration ratio in order to attainIII. PHOTOEMISSIVE DEVICES the temperatures of 2000F to 3000F which are neededA photoemissive solar-energy converter utilizes a fo r eficient converter operation. Fig. 6 shows a 16-ftphotocathode from which electrons are emitted by diameter concentrator which was fabricated by GE fo rphoton absorption. Generally, the electrons are collected use in ground tests of a 500-w thermionic generator.by a plane collector electrode which is vacuum-in- Figs. 7 and 8 showT a 135-w unit using a 5-ft diameter



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96 IRE TRANSACTIONS ON MILITARY ELECTRONICS January

Fig. 9-Five-ft miirror for the SET-I solar energythermionic system.

Fig. 6-Sixteen-ft (iamiciitr paraboloid concentrator for uise in GEsoldtr thermioniic systemis for grounid tests at P'hoeniiix Ariz.

~~~~~~i ~~~~~~~~Fig. 10-Five-ft mirror made by the Boeing Co. by vacuumideposition and honieycomb fabricatioii techniiques.

miirror which is under developmenit for poteintial applica-tioni in palanetary spacecraft. Fig. 9 shows an all-mietal____________________________________ 5-ft mirror fo r this system (SET-I) which is beingdeveloped by Electro-Optical Systemis, Inic., for theNASA jet Propulsion Laboratory. The nirror weighs 12Fig. 7-SET-1 system in foldedI positioni. pounids (0.6 lb/ft2) anld is made by electroforminigtechniques as a replica of a standard searchilight miirror.~~~~~~~ ~~~~~Fig 10 shows a 5-ft diamieter paraboloid mnirror miadeby the Boeinig Comipaniy by vacuum depositioni methods~~~~ ~~~fromi a searchlight miirror. The structure is alumiinumihonecycomb with reiniforced plastic cover sheets.s~~v~ri~n Figs. I11 and 12 show two views of a foldable all-mietalFresnlel miirror of 4-ft diamieter which was mnade for theU. S. Air Force by the Allison Division of GeneralMotors. The weight of the mirror as showni is 0.46lb/ft2.The use of solar-thermionic systems in satelliteswould be comiplicated by the thermaizl cycling of theconiverters from suni to shade and by th e lack of technzol-ogy for thermal energy storage materials with high heatof f us io n and the propernelting poit. Recent resultsFig. 8-SET-Isystem in operationdl position. by the GE Compandy oniani NASA contract indicate



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1962 Cooley: Solar Direct-Conversion Power Systems 97r0 rrB r & S L r i Q r m: 52w2a (SOLAR POWER SUPPLIES INCLUDE STORAGE FOR LOW OR8IT SATELLITE)

POWllYER SOLAR-STERLL S CYCLE URS EWERIIT 15010F- SWWER o

SOLAR-TUELECT RICALPOWER ( TLOWATS}l l~~~~~Fig. 13-Estimated weights of space-power systems as a function_ ~~~~~~~~~~~~~~o power output.ig . 11-Four-ft diameter foldable all metal Fresnel reflector madeby the Allison Div. of General Motors by electroforming tech- lower weight than shielded nluclear-reactor systems. The

solar-thermionic systems are particularly desirable fo rpower levels up to 3 kw, assuming that a long life can_ ~~~~~~~~~~beemonstrated and that the accurate sun-orientation_l ~~~~~~~~~~~requirements can be fulfilled.

VI. CONCLESIONS~~~~~~~~~~~A review of the present status of technology and_ l _ ~~~~~~~~~futureequirements for solar direct-conversion power__ ~~~~~~~~~~systems o be used in spacecraft indicates that silicon|_ ~~~~~~~~~~solar-cell arrays combined with batteries will continlueto meet most requirements in the near future. The needfor higher power systems of 500- to 3000-w capacity willprovide incentives for the developmenlt of larger areasolar cells as well aIS concentratinlg solar-cell arrays and__ ~~~~~~~~~~solar-thermionlic systems in order to reduce costs, im-_ ~~~~~~~~~~~proveeliability and decrease weight. For 24-hr satel-_ ~~~~~~~~~~~lites,olar cells should be protected against electronradiation damage by about a 0.065-in thickness of non-Fi.12-Rear view of Allison Fresnel mirror showing foldable brwig lasFosteies ihnteinrVnig.~ letoome uuarfae Allen belt (about 600 to 5000 miles altitude), adequateshielding against p)rotons is impractical and the effic-iency of silicon solar cells will degrade appreciably in athat molten silicon can be contained fo r a limited time few months. The use of improved radiation resistantin a boron nitride or pyrolytic graphite container, solar cells made of gallium arsenide appears to be aThese studies have also shown that certain mixtures of promlising solution, although present p)roduction costsoxides have high heats of fusion and the proper melting ar e high.points and can probably be contained in refractory The large-scale testing of solar cells and the conductmaterials for longer periods of time than silicon, of research on radiation damage effects mnake it desirable

to develop improved solar simulators and spectro-V. (COMPARISON OF POWER SYSTEMS photometers.Fig. 13 shows a comparison of the estimated weight of Recent research on photoemissive power conversionvarious power systems as a function of power output. It indicates that ultimate efficiencies of only two to fourmay be seen that for power levels up to about 1 to 3 per cent may be attainable. Even though they offerkw in earth-satellite applications, the s ol ar di re ct - potential weight and cost savings, as well as radiationconversion systems are generally of lower weight than resistance, the need fo r deployment of larger areas pernuclear power systems, with the exception of radio- watt of output may introduce considerable complexityisotope-thermionic systems. In the power range from 3 and unreliability in a space application.to 30 kw, the solar-mechanical systems appear to be of The development of thermoelectric materials has



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98 IRE TRANSACTIONS ON MILITARY ELECTRONICS Januaryproceeded to the point where o ve r- al l s ol ar conversion primarily on a comparison of availability, reliabilityefficiencies of five per cent may be attainable. The for use in the particular mission environment, and cost.large collector area required as compared to solar cells ACKNOWLEDGMENTwith 10 to 14 per cent efficiency makes the solar-thermoelectric systems less desirable, except in missions The author wishes to acknowledge the assistance ofwhere radiation resistance or economy are paramount W. C. Scott and H. W. Talkin, National Aeronauticsconsiderations. and Space Administration, in reviewing this paper.The development of thermionic converters is proceed- BIBLIOGRAPHYing rapidly and it is expected that solar-thermionic sys- [1] N. W. Snyder, Ed., "Energy conversion for space power," intems with over-all conversion efficiencies of 10 per cent "Progress in Astronautics and Rocketry," Academic Press, Inc.,

or higher can be produced*in th e next few years. If long New York, N. Y. , vo l. 3 ; 1961.Or higher can be produced in the next few years. If long [2] N. W. Snyder, Ed., "Space power systems," in "Progress inendurance can be attained, they show promise fo r Astronautics an d Rocketry," Academic Press, Inc., New York,weigt-saingand economy in planetary spacecraft 11N. Y., vol. 4; 1961.weight-saving "nd economy in plalletary spacecraft [3] "Coatings for Solar Cells," Final Report by Hoffman Sciencewhere continuous exposure to sunlight is possible. Center on Jet Propulsion Laboratory, Pasadena, Calif., P. 0.Practical applications in earth satellites will, require M-42848; February 28, 1961.pelrequire [4 ] F. W. O'Green, "A Unique Experiment on Midas II for De-solution of thermal cycling problems or development of termining Solar Cell Performance in Space," Lockheed Missilesthermal energy storage materials an d containment and Space Division, Sunnyvale, Calif. Rept. No. SSM-C-T61-8(LMSD-447875); January 19, 1961.methods which will operate at temperatures near 2500F. [5] M. Wolf, "The present state-of-the-art of photovoltaic solarThe potential advantages of solar-thermionic systems energy conversion," Solar Energy, vol. 5, pp. 83-94; July-September, 1961.as compared to solar-cell arrays include lower weight, [6] W. E. Spicer, "Considerations of photoemissive energy con-lower cost and insensitivity to both radiation damage verters," RCA Rev., vol. 22 , pp. 71-81; March, 1961.[71 J. A. Raicker and B. XV . Faughnan, "Radiation Damage toand to high ambient temperatures (e.g., on the moon). Silicon Solar Cells," RCA Labs., Princeton, N. J. ; July 31, 1961.However, sun-orientation accuracy of about 0.1 degree This work wa s a summary report on NASA under ContractNo. NAS 5-457.is required when using a paraboloidal collector of high [8] W. C. Cooley, "Spacecraft Power Generation," presented toconcentration ratio. AGARD Combustion and Propulsion Panel, Pasadena, Calif.;August 24-26, 1960. Published in "Advanced Propulsion Tech-A comparison of various solar direct-conversion power niques," S. S. Penner, Ed., Pergamon Press, Inc., New York,systems shows they are all closely competitive fo r N. Y.; 1961.[9 ] W. C. Hulten, WV . C. Honaker, and J. L. Patterson, "Irradia-power levels from 50 to 3000 w which will meet most of tion Effects of 22 and 240 Mev Protons on Several Transistorsthe spacecraft requirements during the next 10 vears. an d Solar Cells," Rept. No . NASA TN D-718; April, 1961.10] Personal Communication from Dr. Joseph A. Baicker of theTherefore, system selection will probably be based RCA David Sarnoff Research Center, September 19 , 1961.

The Cesium- 137 Thermoelectric Generator*H. 0. BANKS, JR.t AND W. W. T. CRANEt

Summary-Designed fo r th e initial application of supplying power INTRODUCTIONfor an under-sea seismic station to be supervised by Lamont Geo-logical Observatory researchers, this cesium generator incorporates HE isotopic-fueled thermoelectric generator hasan external shell that will withstand compressive forces at the great- come into being, after three years effort by special-est of ocean depths and a power-conversion system employing lead- ists in this new field of power supply. Its safety,telluride thermocouples mounted on a printed circuit board. A com- versatility, relative independence to environment, asbination radiation shield and heat sink surrounded by a specialof~~thrarelcosadcmatdislto,uiis well as reliability over long periods of unattendedcomplex oprtin suggests immediatesapliaton wheretetheripractically al l of the decay heat. oealn ugss1mdaeaplaln hr teA very high degree of safety from radioactivity is assured, due to electrical power sources have proven inadequate todouble encapsulation of fuel increments, an ultrastrong radiation operational requirements. Obtaining its heat from theshield for containment of the latter, and an external shell that will spontaneous decay of a selected radioisotope, thewihtn 1600pi *rsue generator employs nuclear-fission products previouslyconsidered as waste material from spent reactor-fuel* Received ~~~~~~~~elements. While storage of this waste was once an ex-*eevdby the PGMIL, November 29, 1961.pesv an difcldsoalrbem itow an et Royal Research Corporation, Hayward, Calif. esv an dficldspa roem tow anb

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Solar Direct-Conversion Power Systems-KMM