High efficiency solar panel

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United States Patent

Kapany

54 high efficiency solarpanel

(75) Inventor: Calif.

Narinder S. Kapany, Woodside, 73 Assignee: Kaptron, Inc., Palo Alto, Calif.

(52) U.S. Cl................................... 126/270; 52/306;

United states patents

294, 17 21884 Calver................................. 126/271 622,506 4/1899 Manning......................... 35O1262 X

2,056,861 3/1913 Walker ............................... 26/ E. E. A. - - - - - - - - - - - - - - - a a - a -

3,018,087 if 1962 Steele ............................. 26/271 X

Windows

Es ssee)

Kvdyk sssss &

aparray

Flobansaeat

3,043, l l 2 7/1962 Head............................... 126/270 X

3,780,722 12/1973 Swet ................................... 26/270 Primary Examiner-John J. Camby

Assistant Examiner-Henryy C. Yuen

Attorney, Agent, or Firm-Limbach, Limbach &

Sutton

A window portion is interposed between the incident light and aheat absorbing portion and at least one of the heat absorbing and windowportions has a plurality 615, 618 of spaced apart reflecting surfaces, separate ones of which face each other and transmit the incident light by multiple reflections to the heat absorbing portion. In at least one embodiment, these opposed reflecting surfaces converge in the direction of light travel from the window portion to the heat absorbing portion and infrared reflecting means are interposed between the window portion and the heat absorbing portion to re - -2 f turn infrared light emitted by the heat absorbing por tion back to7theClaims, heat absorbing portion.

15 Drawing Figures

Solar energy

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Solar energy

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Windows 26 345 cccsccesccs)/sekv

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Saksa 23

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Kaka nakaix/kds k$2%

2.2.2.2.2/2 A/A ABSORBER again as s/ 3%2233.3% 23. 23 FIG. I.

Drawings

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with the fluid to be heated, be inexpensive to manufac

HIGH EFFICIENCY SOLARPANEL ture and should reduce convection losses. CROSS-REFERENCE TO RELATED APPLICATION SUMMARY OF THE INVENTION

The present application is related in part to the appli 5 The above described disadvantages of the prior art cant's copending application, Ser. No. 462,856, filed are overcome and the requirements for a high effi Apr. 22, 1974, and entitled HIGH EFFICIENCY ciency solar panel are met by the present invention of LIGHT TRANSMITTING WINDOW. PANEL. a solar panel for converting incident light into heat comprising a heat absorbing portion and a window

BACKGROUND OF THE INVENTION O portion interposed between the incident light and the The invention relates to a solar panel and more par heat absorbing portion with at least one of the heat ticularly to a high efficiency solar panel for converting absorbing and window portions having a plurality of incident solar light into heat. opposed reflecting surfaces which transmit the incident In some prior art types of solar panels, the incident 5 light by multiple reflections to the heat absorbing por light passes through a window to a heat absorber. The tion. By "opposed reflecting surfaces” is meant that window is intended to transmit solar radiation while that separate ones of the plurality of reflective surfaces trapping the re-emitted infrared radiation from the are spaced apart from and face each other, either di absorber. The window also reduces convection losses rectly or obliquely, so that multiple reflections can using "hot house' and "window pane” techniques. The 20 OCC.

window sometimes consists of one or more layers of In some preferred embodiments of the invention, the transparent glass or plastic separated by air. The ab plurality of opposed reflecting surfaces converge in the sorber is designed to be heated by absorption of the overall direction of the light travel from the window energy incident from the sun and to transmit this heat portion to the heat absorbing portion. In one such em bodiment, for example, the opposed reflecting surfaces to a heat transfer fluid, which is typically water or air. 25 are

The absorber is insulated to minimize heat loss to the arranged in V-shaped, parallel grooves. surroundings. Each of the components and more par In some preferred embodiments means are also inter ticularly the window and the absorber can be improved posed between the reflecting surfaces and the heat absorbing portion to reflect infrared light emitted by by the application of optical techniques. the heat absorbing portion back to the heat absorbing The window portion should be higly transmitting to 30 portion.

light having a wavelength in the 0.3 to 2 micron range. one suchFor example, as part of the window portion of Also the transmission of the incident light should be larly shaped wedges area plurality embodiment, provided of parallel, triangu for supporting the independent of the angle of incidence or polarization, thus making the solar panel useful even on a cloudy day opposed reflecting surfaces. Each wedge has one apex pointing away from the heat absorbing portion and while avoiding the use of expensive mechanical track 35 supports ing systems. Infrared re-radiation from the absorbing from the aone pair of reflecting surfaces which diverge apex in the direction toward the heat portion should be trapped within the panel. The win absorbing portion. Thus, in effect, the reflecting sur dow portion should be made of low cost materials with faces facing each other which are supported by each out sacrificing sturdiness. pair of adjacent wedges converge in the direction of Some of these requirements are contradictory. For 40 light travel to the absorbing portion. In this preferred example, in order to make the window portion sturdy embodiment the means for reflecting or in order to make it more effective in trapping infra radiation comprises infrared reflectingthe infrared re red re-radiation, some prior art panels are made with are disposed between the other two apexes ofwhichsurfaces relatively thick window portions which have lower light wedge, that is, the infrared reflecting surfaces areeach ar transmitting qualities than relatively thinner sections. 45 ranged so as to face the heat absorbing portion and are Thus, the light transmitting capability is degraded to supported adjacent to the flat surface of the bottom of improve the capability to trap re-emitted infrared radi each wedge. In other embodiments the opposed reflect ation. Furthermore, when the window portion is made ing surfaces are on self-supporting substrates arranged relatively thick the acceptance angle properties of the in parallel V-groove fashion. Both the V-groove and window portin may be degraded in some prior art ar SO wedge configurations preferably have their longitudinal rangements. axes aligned along the direction of solar travel over the The ideal absorbing portion must have a high absorp panel, for reasons which will be explained in greater tivity for light wavelengths in the 0.3 to 2 micron range detail hereinafter. The infrared reflecting surfaces also and a low emissivity for light wavelengths in approxi may have a variety of shapes such as flat, inverted mately the 10 micron range. For a black body, the ratio 55 V-shaped grooves, or even concave, hemicylindrical of absorptivity to emissivity is approximately one grooves.

whereas for a polished metal surface this ratio is ap The window portion in other embodiments of the proximately three. Ratios on the order of nine have invention comprises a pair of thin transparent sheets been obtained by coating a thin absorption layer over a which are separated by a plurality of ribs made of light reflecting surface. The layer is thick enough to absorb 60 transmitting material arranged to transmit the incident solar radiation but thin compared to the wavelength of light striking the window within a predetermied angle the infrared. In prior art devices of this type, however, of acceptance to the heat absorbing portion by means such coatings tend to be difficult to apply and lose their of multiple total internal reflections. In one embodi high absorptivity to emissivity ratio with age. The dis ment, for example, the ribs take the form of a plurality advantage of a simple polished metal surface is that it is 65 of transparent columns while in other embodiments the highly reflecting with a reflectivity constant of approxi ribs are in the form of transparent zig-zag walls or a mately 0.9. Further, design requirements for such ab honeycomb structure. In these embodiments the win sorbing portions are that they should give good contact dow portion preferably further includes means for seg 7 menting air spaces between the sheets to reduce con FIG. 5b is an enlarged vertical view, in section, of the vection losses and to distribute better the mechanical structure depicted in FIG. 5a, load applied to the window. This form of construction FIG. 5c is an enlarged vertical view, in section, of a has the advantages of lightweight, sturdiness, high light modification of the structure depicted in FIG. 5b, transmission and low cost. FIG. 5d is a perspective view of a second modified In these and other embodiments, the heat absorbing embodiment of the invention, with portions broken portion may include a top panel of reflective surfaces away and in section;

arranged in honeycomb fashion to receive the incident FIGS. 6a and 6b are diagrammatic illustrations for light passing through the window portion and to trans O use in explaining the operation of the window panels mit the light to the heat absorbing portion. The honey depicted in FIGS. 5a-5d;

comb panel has the reflective surfaces preferably ar FIG. 7 is a perspective view of a third modified win ranged to taper downwardly into the heat absorbing dow panel for the embodiment of FIG. 1, with portions portion so as to absorb the energy of the light during broken away; and multiple bounces and to trap the air above the heat 15 FIGS. 8 and 9 are perspective views partly in section absorbing portion, thereby reducing convective heat and with portions broken away of alternative top panels losses. for the absorbing portion.

In still other embodiments the top panel of the ab DETAILED DESCRIPTION OF CERTAIN sorbing portion is provided with a plurality of parallel PREFERRED EMBODIMENTS grooves which may, for example, have a V-cross-sec 20 tional shape. The longitudinal axis of such grooves is Referring now more particularly to FIG. 1, the com preferably oriented along the axis of solar travel over bined solar panel of the invention includes a window the panel for maximum receptivity and absorption of panel portion 10 and a heat absorbing portion 12. The the incident light. heat absorbing portion 12 further includes a fluid heat Many of the above described features of the inven 25 the exchanging chamber 14 and an insulating back 16 for tion may be utilized independently of some of the oth fluid,heat exchanging chamber 14. The heat exchanging such as water or air, is continuously admitted to ers, but they are preferably combined into one embodi the chamber ment which is highly efficient in converting incident hausted from 14thethrough an inlet pipe 15 and is ex chamber by an outlet pipe (not solar radiation into heat.

It is therefore an object of the present invention to 30 shown). In operation, incident solar radiation 18 passes provide a panel which is highly absorbing for incident through the window portion 10 to strike the heat ab sorbing portion 12. The heat generated in the heat light radiation having a wavelength in the range of 0.3 absorbing portion 12 by the solar radiation 18 is trans to 2 microns;

it is another objecct of the invention to provide a heat ferred to fluid passing through the heat exchanging absorbing panel which has a low emissivity at radiation 35 the fluid14.

portion This transfer of solar generated heat into raises its temperature.

wavelengths corresponding to the temperature of the As mentioned above, the window portion must be absorber when the emitted light wavelength equals 5 to designed to accept solar radiation 18 over as wide an 15 microns; angle of incidence as possible. It also must be designed it is a further object of the invention to provide a high to prevent heat loss through reradiation from the heat efficiency panel for converting incident light into heat 40 absorbing portion 12 and heat loss due to convection wherein the panel is lightweight and sturdy; and currents above the heat absorbing portion 12. it is a still further object of the invention to provide a high efficiency heat absorbing panel which has low heat parent sheets 20 and 22 which are pair The window panel 10 includes a of thin, trans spaced apart by a losses due to radiation, convection and conduction. plurality of upstanding columns or ribs 24 which are The foregoing and other objectives, features and 45 also made of lightweight, transparent material and pref. advantages of the invention will be more readily under erably of a material which has high optical transmission stood upon consideration of the following detailed qualities. In practice, the material chosen for the sheets description of certain preferred embodiments of the 20 and 22 and the ribs 24 may be lightweight plastic. In invention, taken in conjunction with the accompanying other embodiment, the top sheet 20 may be glass for drawings. 50 ruggedness.

BRIEF DESCRIPTION OF THE DRAWINGS Means are provided for segmenting the air space between the sheets 20 and 22 to reduce heat loss due to

FIG. 1 is an exploded, perspective view, partly in convection currents which would otherwise develop section and with portions broken away of a solar panel between the two sheets. In the embodiment depicted in according to a first embodiment of the invention; 55 FIG. , the means for segmenting the space comprises FIGS. 2a and 2b are enlarged perspective views, a plurality of transparent bubbles 26 on the upper sur partly in section and with portions broken away of face of the sheet 22. The bubbles 26 may be made of a alternative window panels for the embodiment de transparent material such as plastic. The bubbles 26 picted in FIG. 1; can be pressed flat against the flat sheets 20 and 22 to FIGS. 3a and 3b are enlarged perspective view, with 60 reduce reflection losses at the spherical surface. portions broken away and in section of the rib portions Referring now more particularly to FIGS. 2a and 2b, of the window panels depicted in FIGS. 2a and 2b; other types of columns and air space segmenting are FIG. 4 is an enlarged vertical view, in section, and depicted. In the embodiment of FIG. 2a the window with portions broken away of a window sheet of the sheets 20 and 22 are separated by a plurality of thin embodiment of FIG. 1; 65 sheets 28 turned on edge to form a plurality of parallel FIG. 5a is a perspective view of a modified window ribs which extend the length of the sheets 20 and 22. In panel for the embodiment of FIG. 1 with portions bro the modified embodiment depicted in FIG.2b, the ribs ken away; are a plurality of sheets 30 turned on edge and bent in 8 a zig-zag fashion. In the embodiments of FIGS. 1, 2a sheets 20 and 22. The optical valves 36 have the effect and 2b, the ribs 24, 28 and 30 are made of lightweight, of admitting incident light to the structure but blocking optically transparent material. In still other embodi the escape of a large percentage of the reflected infra ments (FIG. 7), the light transmitting structural sup red radiation emitted by the absorbing portion 12. The port between the sheets may be a honeycomb or other basic structure of each optical valve in the panel 36 is shapes. to provide a plurality of opposed reflecting surfaces The purpose of having the ribs 24, 28 or 30, in addi which converge in the overall direction of light travel tion to separating the sheets 20 and 22 to form an from the window portion 10 to the heat absorbing por insulating air space, is also to provide means for trans 10 tion 12. The reflecting surfaces of each optical valve of mitting incident light striking the top sheet 20 to and ing the panel 36 may be in the form of a plurality of reflect through the bottom sheet 22 so that the transmitted ments V-grooves or in honeycomb cells but both arrange light will eventually strike the absorbing portion 12. By have wider openings at the top surface, that is, providing these ribs, the incident light is accepted over the surface first struck by the incident solar radiation, a wider incident angle than if the ribs were opaque and than at the bottom surface, that is, the surface closest the only light reaching the absorbing portion was light 5 to the heat absorbing portion 12. The optical valve which managed to penetrate both the top and bottom panel 36 may be composed of combinations of metal sheets 20 and 22. From Snell's law that nsin8= nsin8, and/or dielectric materials.

(where 6 and 6 are the incident angle and the re plurality In the embodiment depicted in FIGS. 5a and 5b, a fracted angle, respectively) and the principle of total 20 formed of downwardly converging V-grooves are internal reflection that the angle (6) for light leaving ing, triangularly constructing the panel 36 of upwardly point the material having the higher index (n2) of refraction index shaped wedges 38 of low refractive into a material (such as a coating) of a lower refractive pointing, truncatedinterspersed material between downwardly triangularly shaped wedges 40 of index (na) must be 90 (See FIG. 3b):

high refractive index material. The incident solar radia 25 tion is transmitted through the high refractive index 8 * = sin { (n) - (n) } 2 medium 40 and is reflected at each interface formed where 6, = maximum light acceptance angle between a wedge of high refractive index material 40 max for total internal reflection and a wedge of low refractive index material 38. Thus, these interfaces formed by the discontinuities in the 30 refractive indices are effectively reflective surfaces 42 n1 = refractive index for air or n = 1 for the incident solar radiation. Such a reflecting sur ha = refractive index of material of rib 28, and face 42 is formed on the opposite, downwardly con his = refractive index of a protective dielectric coating verging sides of each wedge 40 and thus light entering 29 on rib 28, the top of the wedge 40 is multiply reflected down where rib 28 is uncoated, n = 1 and 35 wardly towards the heat absorbing portion 12 (not shown in FIG. 5b) which is below the optical valve

In order to prevent the escape of infrared radiation

Note that 6 is measured with respect to the normal of 40 infraredemitted by the heat absorbing portion, a plurality of the top surface of the rib, and therefore the actual reflecting surfaces 44 which face the heat maximum acceptance angle in the above examples is absorbing portion 12 are provided on the bottom of 26. In the case of wall type ribs, such as ribs 28 and 30, each wedge 38. Therefore this optical valve panel 36 is there is some advantage in aligning their longitudinal highly transmitting for incident solar light and highly axis with the direction of travel of the sun over the reflecting for the far infrared which is re-emitted by the panel. When this is done the bulk of the incident light 45 heat absorbing portion 12.

will always be within the acceptance angle since the Referring now more particularly to FIGS. 5c and 5d, acceptance angle for light contained in all planes which metal or multiple dielectric layers are used for reflec contain the longitudinal axis is substantially 180°. tion in place of the wedges 38 and 40 of transparent While the above described mathematical relation material. In particular, in the embodiment of FIG. 5c a ships between the angle of acceptance and the indices 50 plurality of opposed reflecting surfaces 46 are embed of refraction are given with respect to the rib 28, it ing ded in a transparent medium 48. The opposed reflect should be apparent that substantially similar relation surfaces 46 are inclined to converge downwardly ships apply to the ribs 24 and 30. Thus, light is trans towards the heat absorbing portion 12. The infrared ferred through the area of the window portion which is 55 reflecting surfaces 44 are placed on the bottom of the covered by the ribs by means of multiple, total internal panel of material 48 between pairs of opposed converg reflections within the ribs 24, 28 or 30. ing surfaces 46 in a manner similar to the embodiment In some embodiments it is preferable to coat the of FIGS. 5a and 5b, sheets 20 and 22 with a transparent hard coating 32 in In the embodiment depicted in FIG. 5d, thin flat order to prevent scratching and an anti-reflection coat laminates 50 are attached together along one edge to ing 34 to minimize reflection losses (see FIG. 4). Fur 60 form a downwardly opening V-shaped channel 52. A ther, in some embodiments it is preferable to stack a plurality of the V-shaped channels 52 are aligned paral number of window sections 10 to minimize heat losses lel to each other and extend along the length of the due to convection and reradiation from the absorbing solar panel. The opposed reflecting surfaces of each portion 12. adjacent pair of reflecting channels 52 form a pair of Referring now more particularly to FIGS. 5a and 5b, 65 downwardly converging reflecting surfaces as in the still another embodiment of the window section 10 is other optical valve embodiments described above with illustrated. In the embodiment of FIGS. 5a and 5b a reference to FIGS. 5b and 5c. The bottom of each panel of optical valves 36 are located beneath the V-shaped channel 52 is open so that infrared radiation 9 emitted from the heat absorbing portion 12 strikes the While in the above described embodiments, the opti interior surfaces of the V-shaped channel 52 which cal valve panel 36 is illustrated in the figures as com face the heat absorbing portion 12 and is re-reflected prising a plurality of V-grooves, in other embodiments back to the heat absorbing portion 12 by multiple re (FIG. 7), as mentioned above, it has a honeycomb cell flections within the V-shaped channels 52 as indicated structure 60 wherein the opening closest to the incident by the dashed line path in FIG. 5d. In other embodi light is wider than the opening which is closest to the ments, the infrared reflecting surfaces may be flat, absorbing portion 12. The opposed plane surfaces in curved or even corner cubed and are not necessarily side each honeycomb cell of the structure 60 provide a merely the backside of the laminates 50. plurality of opposed reflecting surfaces which operate The reflective surface 46 and 50 may be polished 10 in substantially the same manner as the V-groove ar metal or multiple layers of dielectric coatings over the rangement shown above so that a detailed description channel surfaces to form a reflective laminate. One of the honeycomb arrangement will be omitted. advantage of dielectric coatings over metal surfaces is The honeycomb structure 60 may be situated be that dielectric coatings can be made with a higher re 15 tween the sheets 20 and 22 (as shown in FIG. 7) or it flection coefficient than the metal surfaces alone but may be a separate panel below the sheet 22. The plane generally for smaller angles and wavelength ranges. surface area 62 of the structure 60 below and between One important consideration in the design of these the honeycome cells is preferably a reflecting surface optical valves 36 is the acceptance angle for solar radi for the infrared radiation emitted by the absorbing ation. If the light striking the top of the panel 36 is at portion 12.

too large an angle with respect to the normal to the top 20 One advantage of this structure over the V-groove or of the panel 36, then the incident light will be returned wedge configurations is that it is non-axially direc out the top or refracted out of the panel rather than be tional. The V-groove or wedge structures of FIGS. passed through it by multiple reflections. In all planes 5a-5d preferably are utilized with their longitudinal containing the apex line of the V-groove, all incident 25 panelaxes aligned with the sun's direction of travel over the angles of solar radiation are accepted. Thus, with refer strike so that the bulk of the incident light will always ence to FIG.5a, incident light striking the panel 36 in true because within the incident angle of acceptance. This is a plane which is normal to the panel 36 and which through their longitudinal for light contained in a plane passing passes through the lower apex of any given wedge 40 structures have an acceptance axes the V-groove or wedge will be accepted. 30 angle of nearly 180.

Referring now more particularly to FIG. 6a, which is With the honeycomb structure 60, however, the sun's an enlarged and perspective view of the embodiment direction of travel is not material since light is accepted depicted in FIG. 5d, the apex line is defined as a line 54 in the same manner for any given incident angle within which is centered between the opposed reflecting sur the acceptance angle with respect to the normal to the faces 50 at their closest points and parallel to the top 35 panel for all radial directions about the panel. Another advantage of the honeycomb cell structure surface of the panel 36. For light striking the top of the 60 over some panel 36 in a plane which is normal to the apex line 54, segmenting theother designs is that it is very effective in it can be shown that the acceptance angle of the inci to reduce convection heatabove air space losses.

the absorbing portion dent light is: Referring again more particularly to FIG. 1, as men tioned above, the ideal absorber has a high absorptivity 6ma = sin - in the wavelength range of 0.3 to 2 microns and a low emissivity in the wavelength range of approximately 10 microns (the far infrared). In the embodiment depicted and the full acceptance angle, 20m (i.e. the light strik 45 in FIG. 1, the absorbing portion 12 has a tapered hon ing from either side of a line normal to the top of the eycomb structure 56 embossed directly on the upper panel 36) is: face of the top panel 58 of the heat exchanger 14. The embossed honeycomb structure 56 provides multiple as cir 20na = 2sin d -d- bounce absorption of the incident light ray transmitted through the window portion 10. The multiple bounce 50 absorption is substantially similar in operation to that where d is the width of the smaller end, and d is the described above with reference to the reflecting sur width of the larger end of the V-shaped groove formed faces for the optical valve structure 36, that is, the by the opposed reflecting surfaces 50. For any larger incident light passing through the window 10 is re angle the incident light will not be multiply reflected 55 flected a multiple number of times within the hollows toward the apex but instead will be reflected upwardly surface of the heatstructure of the honeycomb 56 until it strikes the upper exchanger 14 where the incident of the V-groove.

When the groove consists of two different dielectrics, light is converted into thermal energy. The honeycomb structure 56 gives good contact as depicted in FIGS. 6b and 5b, for example, the full between the heat exchanger 14 and the fluid to be acceptance angle can be shown to be: 60 heated and it is inexpensive to manufacture in large quantities. In other less advantageous embodiments, 26 F 2sin -(n' -n'2) 112 the honeycomb structure 56 is not tapered downwardly towards the heat exchanging portion 14 and has a sub stantially uniform cross section. The advantage of hav where n = refractive index of wedge 40, and n' = re 65 ing a multiple bounce absorption structure is that it fractive index of wedge 38. See Fiber Optics, pp. 18-21, provides high absorption with low emissivity of infra by N. S. Kapany, the present applicant, (Academic red. This is because although a polished reflector has a Press, New York 1967). low absorptivity, p, for a single bounce of the incident 10 radiation, the fractional absorption for n bounces is (1 The terms and expressions which have been em - p)". ployed here are used as terms of description and not of For example, if p equals 0.8 and n=3, approximately limitation, and there is no intention, in the use of such 99% of the incident energy is absorbed by the absorb terms and expressions, of excluding equivalents of the ing portion 12. In this manner a high ratio of absorptiv features shown and described, or portions thereof, it ity to emissivity can be obtained while still maintaining appreciable absorption. In addition, the honeycomb being ble recognized that various modifications are possi within the scope of the invention claimed.

structure illustrated in FIG. 1 reduces convection losses What is claimed is:

because the air above the heat exchanging portion 14 is 1. A panel for converting incident light into heat compartmentalized. Furthermore, the top of the hon 10 comprising a heat absorbing portion and a window eycomb structure 56 can be covered with a light trans portion interposed between the indicent light and the mitting panel which may, for example, be the bottom heat absorbing portion, wherein the window portion sheet 22 of the window portion to enhance the com includes a pair of transparent sheets, perpendicular partmentalization of the air above the heat exchanging supports for keeping the sheets spaced apart, the sup ports being made of light transmitting material ar

In other embodiments, the top panel 58 of the ab ranged to transmit the incident light striking the win sorber may have V-grooves similar to those shown for dow within a predetermined angle of acceptance to the the optical valves 36, for example. In still another em heat absorbing portion by means bodiment, referring more particularly to FIG. 8, the top 20 ternal reflections, and means forofsegmenting multiple, total in panel 58 has a plurality of upstanding metallic fibers space between the sheets to prevent heat loss the due air

64. These fibers 64 absorb the heat energy from the incident light and conduct it to the panel 58. They also convective air currents.

2. A panel as recited in claim 1 wherein the supports reduce heat loss due to convection by trapping the air immediately above the panel 58. The fibers 64 are are3.inA the shape of substantially flat ribs. panel as recited in claim 1 wherein the supports preferably coated to more effectively absorb the heat are in the shape of honeycomb cells.

energy, of the light while reducing the infrared emis

SO. 4. A panel for converting incident light into heat The panel embodiments have been described above comprising a heat absorbing portion and a window as generally flat but in other embodiments they may be portion heat interposed between the incident light and the absorbing portion, the window portion having a curved to give a concentrated effect. In such embodi 30 ments, either or both the window and heat absorbing pair of transparent sheets, support columns for keeping portions 10 and 12, respectively, may be curved. Thus, the sheets spaced apart, the support columns being for example, referring more particularly to FIG. 9, the made of light transmitting material arranged to trans honeycomb dimpled top panel 66 of the heat absorber mit the incident light striking the window within a pre 14 may have a curved cross section. 35 determined angle of acceptance to the heat absorbing Although the insulation backing for the heat ex portion by means of multiple total internal reflections. changer 14 is shown as being fiberglass in FIG. 1, in 5. A panel as recited in claim 4 wherein the heat other embodiments the insulation may be composed of absorbing portion comprises a panel of surfaces ar a multilayer structure of alternating sheets of reflective ranged in honeycomb fashion to receive the incident and dielectric material. In still other embodiments the 40 light passing through the window portion and to multi insulating portions may be segmented plastic sheets of ply absorb it.

the type described above in reference to the window 6. A panel as recited in claim 5 wherein the panel of portion 10 with a reflecting surface below. surfaces arranged in honeycomb fashion are reflective While the panel of the invention has been described and taper toward the interior of the heat absorbing above for use in converting solar radiation into heat, it 45 portion.

should be apparent that it may also be used for convert 7. A panel as recited in claim 4 further comprising ing incident radiation from other sources, both natural means for segmenting airk spaces between the sheets. and man made, into heat. ck k -k sk