SOLAR FLUX INDEX, R

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The present energy receiving apparatus preferably MASS FLOW SOLAR ENERGY RECEIVER takes the form of either a solar energy absorber or

energy emitter. The apparatus comprises a hollow BACKGROUND OF THE INVENTION

member having an open entrance end and a closed, A Field of the Invention

s essentially black body end, the entrance end being The invention relates to energy receiving apparatus, located at the focus of any suitable energy concentratparticularly of the collecting type whereby radiation is ing apparatus, such as a paraboloid mirror. The central absorbed by or emitted from the collecting apparatus. longitudinal axis of the member is generally disposed The invention is particularly useful for efficiently col- coaxially along the concentrating axis of the mirror or lecting solar energy at temperatures sufficiently high to 10 other concentrating apparatus. Light energy entering permit effective conversion of the collected solar en- the absorber, such as from the sun, is absorbed by the ergy to other useful forms of energy.

interior walls of the absorber, often after multiple interB. Description of the Prior Art

nal reflections. The interior walls of the absorber being Solar receivers and collectors of the absorbing type comprised of glass, quartz, glass graded into quartz, or are generally limited in their performance at high tem- 15 any other absorptive material having the desired abperatures by reradiation losses which are directly pro- sorptive characteristics. The exterior walls of the abportional to the fourth power of the temperature of the sorber are silvered in a known fashion to promote interapparatus. Prior art solar energy collecting apparatus nal specular reflections of the non-thermal energy have included "cavity absorbers", such as are disclosed within the cavity of the absorber. The absorber is insuin U.S. Pat. Nos. 3,208,447; 2,793,018; and 2,760,920; 20 lated along its length exteriorly by insulative materials which absorbers are comprised of “silvered" tubular such as metal foils or combinations of metal foils and units having an "entrance" end located at the focus of oxide layers or layers of insulating spheres in a vacuum. an optical system for concentrating the sun's light into A mass flow of a suitabiy chosen fluid is directed either an absorbing "black body" cavity internal of the tubu- externally around the absorbing member or through lar unit. However, reradiation loss from a cavity ab- 25 the cavity itself to extract heat from the walls of the sorber of this type at temperatures sufficiently high to member, the flow of fluid acting to induce a thermal be useful in Carnot engines, turbines, or the like is gradient within the member, the entrance end thereof comparable to the energy entering the absorber due to being at a low temperature relative to the closed end of the T effect mentioned above. Thus, cavity absorbers the member. Thus, the portions of the absorbing mem

30 have proven to be particularly inefficient at the rela- ber which are most capable of radiating longtively high temperatures required for efficient Carnot wavelength radiation, i.e., those portions at or near the cycle operation. U.S. Pat. Nos. 3,217,702 and entrance end, are kept relatively “cool” by transfer of 2,872,915 provide means for reducing reradiation loss absorbed heat to the fluid, the fluid being further by reflecting at least a portion of this loss back into the heated during its flow along the member until the fluid

35 cavity. However, the efficiency of cavity absorbers has is removed from contact with the absorbing member at not been appreciably increased until the conception of the closed end thereof. the present invention wherein reradiation from a cavity Accordingly, it is a primary obect of the invention to absorber is substantially prevented rather than merely cool the entrance end of a high temperature cavity recovered in part. In effect, the present invention pro- 40 absorbing apparatus by the flow of fluid along the vides inexpensive apparatus useful with economical length of the cavity to extract absorbed radiation in the solar concentrating apparatus for efficiently converting form of heat from the walls thereof, the fluid flow creincident solar energy to heat energy at a temperature ating a significant temperature gradient along the ab sufficiently high to perform useful work.

sorbing apparatus, the entrance end thereof being cool SUMMARY OF THE INVENTION

45 relative to the opposite, closed end thereof.

It is another object of the invention to provide insulaIn a simplified form of the invention a cavity absorber tive means for a cavity absorbing apparatus. is cooled at its entrance end by a mass fluid flow It is a further object of the invention to provide a through the entrance end, the Nuid extracting heat solar energy utilization system wherein solar energy from the absorbing walls of the cavity absorber during so collected and concentrated by optical elements is dipassing of the fluid through the cavity. The heated fluid rected into a cavity absorbing member for absorption is removed from the absorber at a closed end opposite by the interior walls of the member, the energy thus said entrance end, the energy in the heated fluid then absorbed being extracted as heat by a fluid flow along being either stored or directly utilized for power gener- the walls and in thermal contact therewith, the fluid ation or to perform work. The fluid flowing through the ss flow acting to cool the entrance end of the absorbing absorber is in thermal contact with the interior walls of member to prevent reradiation loss therefrom and to the absorber and creates a temperature gradient in the remove the energy from the member for storage in a cavity thereof, the temperatures being relatively higher thermal or chemical storage unit or for direct use in a at the closed end of the absorber than at the entrance thermal engine or the like. end. Thus, the entrance end of the absorber can be held 60 Further objects and advantages of the invention will at a relatively low temperature which significantly re- become more readily apparent in light of the following duces reradiation losses from the absorber. While tem- detailed description of the invention. peratures at an near the entrance end of the absorber can be held relatively low, temperatures at the closed

BRIEF DESCRIPTION OF THE DRAWINGS end of the absorber can be held relatively high, thereby 65 FIG. 1 is an elevational view in section of a cavity permitting heating of the mass of Nuid flowing through absorber according to the invention wherein mass Nuid the cavity to a usefully high level while limiting reradia. flow is channeled along the exterior walls of the abtion loss from the cavity.

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4 FIG. 2 is an idealized view in partial section of a absorber. A thickness of approximately 1.0 mm in eithermal energy storage system utilizing a tubular cavity ther case is generally acceptable from an absorptive absorber according to the invention wherein mass fluid standpoint although the wall thickness would normally flow is channeled along the internal walls of the tubular be increased to lend structural integrity. The absorber absorber;

s 10 is open at its entrance end 14 and sealed with a FIG. 3 is an elevational view in section of an embodi- "black" cap 16 at its opposite end. The outer surface ment of the invention illustrating a sealed cavity;

18 of the absorber 10 is coated with a highly reflecting FIG. 4 is a graph illustrating the theoretical model for film 20 such as a silver film or a film of other highly a straight cylindrical tubular absorber with external reflective material such as mercury, nickel, or chrofluid flow and a vacuum interior, the overall efficiency 10 mium. A second member 22 of slightly greater interior being shown as a function of the absorber efficiency dimensions than the external dimensions of the abwhich is determined by the ratio of the solar input sorber 10 surrounds the absorber 10 and is spaced a energy to the thermal input energy;

finite distance therefrom to define an essentially annuFIG. S is a graph illustrating the conversion efficiency lar circulation chamber 24 between the absorber 10 and effective a/e ratio at optimum flow rates for vary. 15 and the member 22. The absorber 10 and member 22 ing solar fluxes; and,

may each be of a cylindrical, rectangular, or other FIG. 6 is a schematic illustrating the conformation of conformation including various cross-sectional geomea particularly efficient absorber entrance end.

tries of a tubular conformation as long as the radial

dimension is smaller than the longitudinal dimension. DESCRIPTION OF THE PREFERRED

20 In practice, the longitudinal dimension is at least ten to EMBODIMENTS

fifteen times greater than the radial dimension. The The present invention provides in the several em- circulation chamber 24 is sealed except for an inlet 26 bodiments thereof apparatus of the energy absorbing near the entrance end 14 of the absorber 10 and an (or emitting) type used as part of an indirect energy outlet 28 near the cap 16 of said absorber. The assemconverter, the thermal output of the apparatus being 25 bly thus described is insulated by high temperature converted into power in a thermal engine or the like. In insulation shown generally at 30, which insulation 30 order for absorbing apparatus to be efficient, black may comprise suitable well-known insulatory materials body characteristics have been thought necessary. or which may comprise layers 32 of metal foil sepaHowever, elevated temperatures necessary for high rated by thin layers 34 of oxide dust, or insulating Carnot efficiency cause loss of absorbed energy by 30 spheres made of hollow glass beads. The layers 32 and reradiation from the absorbing portion of the appara- 34 are shown enlarged relative to the remaining structus. While selectively absorbing surfaces having high ture for clarification of the structure thereof. The insuabsorptivity to emissivity ratios, i.e., ale, have been lation 30 is held against the exterior walls of the memfabricated and exhibit values on the order of 10 for ber 22 and within an evacuated chamber 36 defined by temperatures below 500° C, the present apparatus

ex- 35 vacuum wall 38. The layers 32 of foil may preferably be hibits at its entrance hole (or virtual surface) an effec- greater in number at the end of the member 22 oppo. tive a/e > 500 while producing exhaust temperatures in site the entrance end 14 of the absorber 10, the outerexcess of 1000° C. In principle, those portions of the most layer of foil extending the full length of the mempresent absorber which would radiate relatively long- ber 22 and the innermost layers of foil covering only wavelength radiation are kept cool by transfer of ab. 40 reduced portions of the member 22 near the cap 16 of sorbed heat to a mass flow moving through the ab- the absorber 10. This insulative design may be utilized sorber from the entrance end thereof toward the oppo- to maximize the favorable effect of not only the radial site end of the absorber. The opposite end of the ab- temperature gradient which exists inside the absorber sorber is closed and exhibits essentially black body 10, but also of the axial temperature gradient within characteristics, the mass flow reaching its highest tem- 45 said absorber. The shorter foil layers at the "hot" end perature at this closed end prior to removal from ther- of the member 22 prevent conduction of heat along mal contact with the absorber.

their lengths back toward the "cold” end of the memA first embodiment of the invention is shown in FIG. ber 22. I to comprise a long, thin-walled absorber 10 defining Further discussion of the nature of the member 22 is a central cavity 12, the material comprising the ab- 50 believed to be helpful at this point to insure optimum sorber 10 being in the simplest form glass, quartz, or a operation of the absorber 10. The member 22 may be combination of the two substances such as will be de- comprised of quartz or fused silica having a continuous scribed in detail hereinafter. The absorber 10 could be increase of doping-type substances such as Na,0 (to bored from diamond, sapphire, or quartz as long as about 15%) and Cao (to about 10%) from the closed transparency is maintained. The absorber 10 could also 55 end thereof toward the entrance end 14, thus forming a conveniently be formed of a “hollow" rectangular solid typical soda-lime-silica glass which would absorb infrasuch as would be formed by two rectangular spaced red radiation particularly well at the entrance end 14, plates enclosed about the perimeters thereof. Practi- the member being essentially pure quartz at its hot cally speaking, a glass tube is useful also. Quartz doped portion, i.e., the "closed" end. It is to be understood to yield glass-like properties at the open end portion of 60 that materials other than as specifically described but the absorber and "grading" into a pure quartz at the which exhibit the properties and capabilities described hot portion thereof as will be described is of utility. The herein fall within the scope of the invention due to the thickness of the walls of the absorber can be as thin as teachings herein. is practically possible as long as infrared radiation can The entrance end 14 of the absorber 10 is disposed at be absorbed thereby. Embodiments of the invention 65 the focus of suitable energy collecting and concentratusing external flow of a cooling mass are to be made ing optics, such as a paraboloid mirror 40. When solar thinner as a practical matter than those embodiments energy is to be collected and utilized with the mirror wherein the cooling mass is flowed internally of the 40, the closed end of the absorber 10, i.e., that end

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6 enclosed by the cap 16, is pointed at the sun while the tages brough about by this mass flow in contact with entrance end 14 of the absorber 10 substantially en- the walls of the absorber 10, much of the radiation compasses the image of the sun which is formed by the energy entering the absorber 10 will escape as thermal mirror 40.

radiation if the wavelength selective properties of the In the embodiment of FIG. 1, the closed end of the swalls of the absorber 10 are not properly considered. absorber 10 is pointed at the sun due to the fact that the Only a small fraction of the intense black body radiasolar image entering the open end of the absorber is tion filling the closed end of the absorber 10 can escape formed by the single concentrating mirror 40. If a lens directly, i.e., in a direction axially of the absorber. Even is directly used, for example, then the closed end of the so, much of this directly escaping radiation is reflected absorber 10 would be pointed away from the sun as is 10 back into the absorber by the shadowed" or central shown in FIG. 2. The absorber 10 may be made station- section of the mirror 40. It is therefore to be underary for reasons of economy or may be made to “follow" stood that, for most of the radiation generated by the the sun in a known fashion. In the situation where the walls of the absorber 10 at temperature T, or less, the absorber 10 is rectangular in conformation, the en- walls are to be opaque. Consequently, the intense wall trance end 14 takes the form of a slit or slot and has is radiation at the closed end of the absorber 10 cannot certain inherent “sun-following" characteristics. In “see" the film 20 and “mirror" its way out in a reflectorder to maximize overall efficiency, the open end of ing path as did the solar radiation coming into the abthe absorber 10 must encompass most of the sun's sorber. In a known fashion, glass is a convenient mateimage. A cooling fluid 42 is directed through the inlet rial for the walls of the absorber 10 due to its opacity to 26, filling the circulation chamber 24, and coming into 20 infrared radiation, this opacity being due to absorption thermal contact with the outer surface 18 of the ab- of infrared radiation rather than reflection thereof. In sorber 10. The fluid 42 may be gaseous, such as air, the interest of increased Carnot efficiency, i.e., higher HS, the noble gases, or any heat absorbing gas, or a Tu quartz can be used to replace glass at the closed or liquid, such as water, eutectic sodium and potassium, hot end of the absorber 10, the device being more or mercury (in which case the mercury could form the 25 efficient if the glass “grades” into quartz rather than reflecting film 20 as well as the cooling fluid 42. If, as having distinct glass/quartz regions in the absorber 10. in certain embodiments of the invention, the fluid 42 is If quartz is used as the material composing the walls of flowed internally of the absorber 10, the fluid must be the absorber 10, it could be doped with a well-known transparent to light. Otherwise, the fluid 42 may be infrared absorber near the entrance end 14 in order to chosen as desired for properties other than heat ab- 30 preserve the selective mirroring action of glass. The sorptive capacity, such as for the ability to chemically walls of the absorber 10 are preferably thin, especially react on exposure to the heat generated at the hot end where heat is to be transferred through the walls, but of the absorber 10 or for heavy atomic mass for driving the walls must be thick enough to be opaque to the wall a turbine, etc. While light flux enters the entrance end radiation. Either quartz or glass is capable of absorbing 14 of the absorber at a multiplicity of incidence angles, 35 most of the heat radiation in the absorber 10, quartz an "average" photon is represented by S in FIG. 1 as being particularly more suitable at higher temperaentering the entrance end 14 and being multiply re- tures. However, a grading of these two materials, i.e., flected from the silvered film 20 before being absorbed glass near the entrance end 14 and quartz near the by the walls of the absorber 10 as heat. The thermal closed end of the absorber 10 with a blend or grading of flux incident on the walls of the absorber 10 is much 40 the two materials or with substances approximating the less intense than the flux across the mouth of the en- characteristics of the two materials is desirable. For trance end 14. If the absorber 10 be made sufficiently wavelenghts less than approximately 44, the infrared long such that most of the energy entering the end 14 is transmission “cutofr" for quartz (wavelenghts transabsorbed prior to reaching the end cap 16, then virtu- parent to quartz), quartz does not radiate well. Thus, ally all of the energy is absorbed, i.e., a - 1. The fluid 45 even though some black body radiation may be of suff42 is contact with the outer surface 18 of the absorber ciently short wavelength to be in the glass transmission 10 absorbs this heat energy from the walls of the ab- "window", i.e., s 24, there is virtually no quartz body sorber 10 and, since the fluid 42 is made to flow from radiation in this region. Thus, the wall material at the the vicinity of the entrance end 14 to the outlet 28 at closed or hot end of the absorber 10 is preferably approximately the same temperature as the closed end so formed of a material like quartz having a higher transof the absorber 10, the closed end of said absorber parent-opaque transition wave length (c.8.74 pthan the being filled with essentially black body radiation char- wall material at the entrance end 14, such as glass at acteristic of this temperature which will be referred to 24. Further, the two materials can preferably grade hereinafter as Tl. In practical use, a length to diameter into each other so that for incremental sections of the ratio of approximately 15 is adequate for the absorber 55 absorber 10, a section nearer the entrance end 14, for 10, although it is to be understood that such a ratio is example, will still be a good absorber for the wavenot limiting

length that the next section toward the closed end is The flow of the fluid 42 along the walls of the ab- “becoming" a "bad" emitter of. Stated differently, us sorber 10 removes heat therefrom at a usefully high the transparent-opaque transition wavelength increases temperature, the heat energy in the fluid 42 being 60 with distance from the entrance end 14, any wavethereby utilized in a variety of ways. However, this length radiated well by the relatively hot wall material cooling flow of fluid also serves to prevent reradiation further from the entrance end, and said radiation being loss from the absorber 10 by "cooling" the entrance directed toward the entrance end, will be absorbed well end 14 of the absorber to reduce the reradiation loss by the wall material on which said wavelength is inciwhich is proportional to the fourth power of the tem- 65 dent. Even certain wavelengths not radiated well by the perature. A temperature gradient extending axially more remote hot wall material at the closed hot end of along the absorber 10 thus exists as well as the ex- the absorber 10 are still absorbed well by the wall matepected radial thermal gradient. Even with the advan- rial closer to the entrance end 14. Thus, for example,