Imágenes de páginas
PDF
EPUB
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

SOLAR FLUX INDEX, R

T

T

CONVERSION EFFICIENCY, E

CONVERSION EFFICIENCY, E

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

[merged small][ocr errors]

T

L

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][subsumed][merged small][ocr errors]

1

MASS FLOW SOLAR ENERGY RECEIVER

BACKGROUND OF THE INVENTION

A. Field of the Invention

4,033,118

The invention relates to energy receiving apparatus, particularly of the collecting type whereby radiation is absorbed by or emitted from the collecting apparatus. The invention is particularly useful for efficiently collecting solar energy at temperatures sufficiently high to permit effective conversion of the collected solar energy to other useful forms of energy.

B. Description of the Prior Art

5

10

15

25

25

Solar receivers and collectors of the absorbing type are generally limited in their performance at high temperatures by reradiation losses which are directly proportional to the fourth power of the temperature of the apparatus. Prior art solar energy collecting apparatus have included "cavity absorbers", such as are disclosed in U.S. Pat. Nos. 3,208,447; 2,793,018; and 2,760,920; 20 which absorbers are comprised of “silvered" tubular units having an "entrance" end located at the focus of an optical system for concentrating the sun's light into an absorbing "black body" cavity internal of the tubular unit. However, reradiation loss from a cavity absorber of this type at temperatures sufficiently high to be useful in Carnot engines, turbines, or the like is comparable to the energy entering the absorber due to the T effect mentioned above. Thus, cavity absorbers have proven to be particularly inefficient at the relatively high temperatures required for efficient Carnot cycle operation. U.S. Pat. Nos. 3,217,702 and 2,872,915 provide means for reducing reradiation loss by reflecting at least a portion of this loss back into the cavity. However, the efficiency of cavity absorbers has not been appreciably increased until the conception of the present invention wherein reradiation from a cavity absorber is substantially prevented rather than merely recovered in part. In effect, the present invention pro- 40 vides inexpensive apparatus useful with economical solar concentrating apparatus for efficiently converting incident solar energy to heat energy at a temperature sufficiently high to perform useful work.

SUMMARY OF THE INVENTION

30

35

35

45

In a simplified form of the invention a cavity absorber is cooled at its entrance end by a mass fluid flow through the entrance end, the fluid extracting heat from the absorbing walls of the cavity absorber during 50 passing of the fluid through the cavity. The heated fluid is removed from the absorber at a closed end opposite said entrance end, the energy in the heated fluid then being either stored or directly utilized for power generation or to perform work. The fluid flowing through the 55 absorber is in thermal contact with the interior walls of the absorber and creates a temperature gradient in the cavity thereof, the temperatures being relatively higher at the closed end of the absorber than at the entrance end. Thus, the entrance end of the absorber can be held 60 at a relatively low temperature which significantly reduces reradiation losses from the absorber. While temperatures at and near the entrance end of the absorber can be held relatively low, temperatures at the closed end of the absorber can be held relatively high, thereby 65 permitting heating of the mass of fluid flowing through the cavity to a usefully high level while limiting reradiation loss from the cavity.

2

The present energy receiving apparatus preferably takes the form of either a solar energy absorber or energy emitter. The apparatus comprises a hollow member having an open entrance end and a closed, essentially black body end, the entrance end being located at the focus of any suitable energy concentrating apparatus, such as a paraboloid mirror. The central longitudinal axis of the member is generally disposed coaxially along the concentrating axis of the mirror or other concentrating apparatus. Light energy entering the absorber, such as from the sun, is absorbed by the interior walls of the absorber, often after multiple internal reflections. The interior walls of the absorber being comprised of glass, quartz, glass graded into quartz, or any other absorptive material having the desired absorptive characteristics. The exterior walls of the absorber are silvered in a known fashion to promote internal specular reflections of the non-thermal energy within the cavity of the absorber. The absorber is insulated along its length exteriorly by insulative materials

such as metal foils or combinations of metal foils and oxide layers or layers of insulating spheres in a vacuum. A mass flow of a suitabiy chosen fluid is directed either externally around the absorbing member or through the cavity itself to extract heat from the walls of the member, the flow of fluid acting to induce a thermal gradient within the member, the entrance end thereof being at a low temperature relative to the closed end of the member. Thus, the portions of the absorbing member which are most capable of radiating longwavelength radiation, i.e., those portions at or near the entrance end, are kept relatively "cool" by transfer of absorbed heat to the fluid, the fluid being further heated during its flow along the member until the fluid is removed from contact with the absorbing member at the closed end thereof.

Accordingly, it is a primary obect of the invention to cool the entrance end of a high temperature cavity absorbing apparatus by the flow of fluid along the length of the cavity to extract absorbed radiation in the form of heat from the walls thereof, the fluid flow creating a significant temperature gradient along the absorbing apparatus, the entrance end thereof being cool relative to the opposite, closed end thereof.

It is another object of the invention to provide insulative means for a cavity absorbing apparatus.

It is a further object of the invention to provide a solar energy utilization system wherein solar energy collected and concentrated by optical elements is directed into a cavity absorbing member for absorption by the interior walls of the member, the energy thus absorbed being extracted as heat by a fluid flow along the walls and in thermal contact therewith, the fluid flow acting to cool the entrance end of the absorbing member to prevent reradiation loss therefrom and to remove the energy from the member for storage in a thermal or chemical storage unit or for direct use in a thermal engine or the like.

Further objects and advantages of the invention will become more readily apparent in light of the following detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an elevational view in section of a cavity absorber according to the invention wherein mass fluid flow is channeled along the exterior walls of the absorber;

3

4,033,118

FIG. 2 is an idealized view in partial section of a thermal energy storage system utilizing a tubular cavity absorber according to the invention wherein mass fluid flow is channeled along the internal walls of the tubular absorber;

FIG. 3 is an elevational view in section of an embodiment of the invention illustrating a sealed cavity;

FIG. 4 is a graph illustrating the theoretical model for a straight cylindrical tubular absorber with external fluid flow and a vacuum interior, the overall efficiency being shown as a function of the absorber efficiency which is determined by the ratio of the solar input energy to the thermal input energy;

FIG. 5 is a graph illustrating the conversion efficiency and effective a/e ratio at optimum flow rates for varying solar fluxes; and,

FIG. 6 is a schematic illustrating the conformation of a particularly efficient absorber entrance end.

DESCRIPTION OF THE PREFERRED
EMBODIMENTS

5

10

15

4

absorber. A thickness of approximately 1.0 mm in either case is generally acceptable from an absorptive standpoint although the wall thickness would normally be increased to lend structural integrity. The absorber 10 is open at its entrance end 14 and sealed with a "black" cap 16 at its opposite end. The outer surface 18 of the absorber 10 is coated with a highly reflecting film 20 such as a silver film or a film of other highly reflective material such as mercury, nickel, or chromium. A second member 22 of slightly greater interior dimensions than the external dimensions of the absorber 10 surrounds the absorber 10 and is spaced a finite distance therefrom to define an essentially annular circulation chamber 24 between the absorber 10 and the member 22. The absorber 10 and member 22 may each be of a cylindrical, rectangular, or other conformation including various cross-sectional geometries of a tubular conformation as long as the radial dimension is smaller than the longitudinal dimension. 20 In practice, the longitudinal dimension is at least ten to fifteen times greater than the radial dimension. The circulation chamber 24 is sealed except for an inlet 26 near the entrance end 14 of the absorber 10 and an outlet 28 near the cap 16 of said absorber. The assembly thus described is insulated by high temperature insulation shown generally at 30, which insulation 30 may comprise suitable well-known insulatory materials or which may comprise layers 32 of metal foil separated by thin layers 34 of oxide dust, or insulating spheres made of hollow glass beads. The layers 32 and 34 are shown enlarged relative to the remaining structure for clarification of the structure thereof. The insulation 30 is held against the exterior walls of the member 22 and within an evacuated chamber 36 defined by vacuum wall 38. The layers 32 of foil may preferably be greater in number at the end of the member 22 opposite the entrance end 14 of the absorber 10, the outermost layer of foil extending the full length of the member 22 and the innermost layers of foil covering only reduced portions of the member 22 near the cap 16 of the absorber 10. This insulative design may be utilized to maximize the favorable effect of not only the radial temperature gradient which exists inside the absorber 10, but also of the axial temperature gradient within said absorber. The shorter foil layers at the "hot" end of the member 22 prevent conduction of heat along their lengths back toward the "cold" end of the member 22.

The present invention provides in the several embodiments thereof apparatus of the energy absorbing (or emitting) type used as part of an indirect energy converter, the thermal output of the apparatus being 25 converted into power in a thermal engine or the like. In order for absorbing apparatus to be efficient, black body characteristics have been thought necessary. However, elevated temperatures necessary for high Carnot efficiency cause loss of absorbed energy by 30 reradiation from the absorbing portion of the apparatus. While selectively absorbing surfaces having high absorptivity to emissivity ratios, i.e., a/e, have been fabricated and exhibit values on the order of 10 for temperatures below 500° C, the present apparatus ex- 35 hibits at its entrance hole (or virtual surface) an effective a/e >500 while producing exhaust temperatures in excess of 1000° C. In principle, those portions of the present absorber which would radiate relatively longwavelength radiation are kept cool by transfer of ab- 40 sorbed heat to a mass flow moving through the absorber from the entrance end thereof toward the opposite end of the absorber. The opposite end of the absorber is closed and exhibits essentially black body characteristics, the mass flow reaching its highest tem- 45 perature at this closed end prior to removal from thermal contact with the absorber.

55

A first embodiment of the invention is shown in FIG. 1 to comprise a long, thin-walled absorber 10 defining a central cavity 12, the material comprising the ab- 50 sorber 10 being in the simplest form glass, quartz, or a combination of the two substances such as will be described in detail hereinafter. The absorber 10 could be bored from diamond, sapphire, or quartz as long as transparency is maintained. The absorber 10 could also conveniently be formed of a "hollow" rectangular solid such as would be formed by two rectangular spaced plates enclosed about the perimeters thereof. Practically speaking, a glass tube is useful also. Quartz doped to yield glass-like properties at the open end portion of 60 the absorber and "grading" into a pure quartz at the hot portion thereof as will be described is of utility. The thickness of the walls of the absorber can be as thin as is practically possible as long as infrared radiation can be absorbed thereby. Embodiments of the invention 65 using external flow of a cooling mass are to be made thinner as a practical matter than those embodiments wherein the cooling mass is flowed internally of the

Further discussion of the nature of the member 22 is believed to be helpful at this point to insure optimum operation of the absorber 10. The member 22 may be comprised of quartz or fused silica having a continuous increase of doping-type substances such as Na,O (to about 15%) and CaO (to about 10%) from the closed end thereof toward the entrance end 14, thus forming a typical soda-lime-silica glass which would absorb infrared radiation particularly well at the entrance end 14, the member being essentially pure quartz at its hot portion, i.e., the "closed" end. It is to be understood that materials other than as specifically described but which exhibit the properties and capabilities described herein fall within the scope of the invention due to the teachings herein.

The entrance end 14 of the absorber 10 is disposed at the focus of suitable energy collecting and concentrating optics, such as a paraboloid mirror 40. When solar energy is to be collected and utilized with the mirror 40, the closed end of the absorber 10, i.e., that end

4,033,118

5 enclosed by the cap 16, is pointed at the sun while the entrance end 14 of the absorber 10 substantially encompasses the image of the sun which is formed by the mirror 40.

5

10

15

30

In the embodiment of FIG. 1, the closed end of the absorber 10 is pointed at the sun due to the fact that the solar image entering the open end of the absorber is formed by the single concentrating mirror 40. If a lens is directly used, for example, then the closed end of the absorber 10 would be pointed away from the sun as is shown in FIG. 2. The absorber 10 may be made stationary for reasons of economy or may be made to "follow" the sun in a known fashion. In the situation where the absorber 10 is rectangular in conformation, the entrance end 14 takes the form of a slit or slot and has certain inherent "sun-following" characteristics. In order to maximize overall efficiency, the open end of the absorber 10 must encompass most of the sun's image. A cooling fluid 42 is directed through the inlet 26, filling the circulation chamber 24, and coming into 20 thermal contact with the outer surface 18 of the absorber 10. The fluid 42 may be gaseous, such as air, HS, the noble gases, or any heat absorbing gas, or a liquid, such as water, eutectic sodium and potassium, or mercury (in which case the mercury could form the 25 reflecting film 20 as well as the cooling fluid 42. If, as in certain embodiments of the invention, the fluid 42 is flowed internally of the absorber 10, the fluid must be transparent to light. Otherwise, the fluid 42 may be chosen as desired for properties other than heat absorptive capacity, such as for the ability to chemically react on exposure to the heat generated at the hot end of the absorber 10 or for heavy atomic mass for driving a turbine, etc. While light flux enters the entrance end 14 of the absorber at a multiplicity of incidence angles, an "average" photon is represented by S in FIG. 1 as entering the entrance end 14 and being multiply reflected from the silvered film 20 before being absorbed by the walls of the absorber 10 as heat. The thermal flux incident on the walls of the absorber 10 is much 40 less intense than the flux across the mouth of the entrance end 14. If the absorber 10 be made sufficiently long such that most of the energy entering the end 14 is absorbed prior to reaching the end cap 16, then virtually all of the energy is absorbed, i.e., a 1. The fluid 45 42 is contact with the outer surface 18 of the absorber 10 absorbs this heat energy from the walls of the absorber 10 and, since the fluid 42 is made to flow from the vicinity of the entrance end 14 to the outlet 28 at approximately the same temperature as the closed end of the absorber 10, the closed end of said absorber being filled with essentially black body radiation characteristic of this temperature which will be referred to hereinafter as T. In practical use, a length to diameter ratio of approximately 15 is adequate for the absorber 55 10, although it is to be understood that such a ratio is not limiting.

6

tages brough about by this mass flow in contact with the walls of the absorber 10, much of the radiation energy entering the absorber 10 will escape as thermal radiation if the wavelength selective properties of the walls of the absorber 10 are not properly considered. Only a small fraction of the intense black body radiation filling the closed end of the absorber 10 can escape directly, i.e., in a direction axially of the absorber. Even so, much of this directly escaping radiation is reflected back into the absorber by the "shadowed" or central section of the mirror 40. It is therefore to be understood that, for most of the radiation generated by the walls of the absorber 10 at temperature T, or less, the walls are to be opaque. Consequently, the intense wall radiation at the closed end of the absorber 10 cannot "see" the film 20 and "mirror" its way out in a reflecting path as did the solar radiation coming into the absorber. In a known fashion, glass is a convenient material for the walls of the absorber 10 due to its opacity to infrared radiation, this opacity being due to absorption of infrared radiation rather than reflection thereof. In the interest of increased Carnot efficiency, i.e., higher T1, quartz can be used to replace glass at the closed or hot end of the absorber 10, the device being more efficient if the glass “grades" into quartz rather than having distinct glass/quartz regions in the absorber 10. If quartz is used as the material composing the walls of the absorber 10, it could be doped with a well-known infrared absorber near the entrance end 14 in order to preserve the selective mirroring action of glass. The walls of the absorber 10 are preferably thin, especially where heat is to be transferred through the walls, but the walls must be thick enough to be opaque to the wall radiation. Either quartz or glass is capable of absorbing most of the heat radiation in the absorber 10, quartz being particularly more suitable at higher temperatures. However, a grading of these two materials, i.e., glass near the entrance end 14 and quartz near the closed end of the absorber 10 with a blend or grading of the two materials or with substances approximating the characteristics of the two materials is desirable. For wavelenghts less than approximately 44, the infrared transmission "cutoff" for quartz (wavelenghts transparent to quartz), quartz does not radiate well. Thus, even though some black body radiation may be of sufficiently short wavelength to be in the glass transmission "window", i.e., ≤ 2μ, there is virtually no quartz body radiation in this region. Thus, the wall material at the closed or hot end of the absorber 10 is preferably 50 formed of a material like quartz having a higher transparent-opaque transition wave length (e.g.4μthan the wall material at the entrance end 14, such as glass at 24. Further, the two materials can preferably grade into each other so that for incremental sections of the absorber 10, a section nearer the entrance end 14, for example, will still be a good absorber for the wavelength that the next section toward the closed end is "becoming" a "bad" emitter of. Stated differently, as the transparent-opaque transition wavelength increases with distance from the entrance end 14, any wavelength radiated well by the relatively hot wall material further from the entrance end, and said radiation being directed toward the entrance end, will be absorbed well by the wall material on which said wavelength is incident. Even certain wavelengths not radiated well by the more remote hot wall material at the closed hot end of the absorber 10 are still absorbed well by the wall material closer to the entrance end 14. Thus, for example,

35

The flow of the fluid 42 along the walls of the absorber 10 removes heat therefrom at a usefully high temperature, the heat energy in the fluid 42 being 60 thereby utilized in a variety of ways. However, this cooling flow of fluid also serves to prevent reradiation loss from the absorber 10 by "cooling" the entrance end 14 of the absorber to reduce the reradiation loss which is proportional to the fourth power of the tem- 65 perature. A temperature gradient extending axially along the absorber 10 thus exists as well as the expected radial thermal gradient. Even with the advan

« AnteriorContinuar »