Solar thermal receiver

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Drawing sheet — no readable text.

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SOLAR THERMAL RECEIVER

the copper tube and the extrusion. A clear silicone (or other transparent low conductivity material) layer with a prismatic upper surface and a triangular-faceted lower

This is a division of application Ser. No. 701,308 filed surface interlocking with the top surface of the upper Feb. 13, 1985, now U.S. Pat. No. 4,672,949. aluminum extrusion, is placed on top of the upper alumi FIELD OF THE INVENTION num extrusion. The improved thermal receiver con struction disclosed herein provides a less expensive and

The present invention relates generally to solar en more efficient thermal energy receiver which dramati ergy collectors and more specifically to an improved cally reduces heat loss from the receiver to the environ thermal receiver for use in solar energy collectors. 10 ment, significantly reduces the front surface reflection BACKGROUND OF THE INVENTION loss of the receiver, and minimizes reflection losses from the black-painted absorber.

In the past, solar energy collectors, such as the one shown in the U.S. Pat. No. 4,069,812, which is included BRIEF DESCRIPTION OF THE DRAWING herein by reference, have provided a very effective and 15 A better understanding of the present invention may efficient means of converting solar energy into both useful thermal energy by transferring heat to a working be had from the following detailed description, when read in connection with the accompanying drawings, in fluid and/or useful electrical energy by means of solar which:

cells. As the state of the art advances in solar energy collection, new and more efficient methods of heat 20 torFIG.

and 1 is a cross-sectional view of the thermal collec receiver of the present invention; and collection and solar conversion are constantly being sought. Generally, improved collection systems have theFIG. 2 is a cut-away, cross-sectional view showing details of the thermal receiver.

either increased the efficiency of the optical system for transforming light rays into usable energy, or improved DETAILED DESCRIPTION OF THE tracking systems for tracking the sun, or improved ther 25 PREFERRED EMBODEMENT mal systems for collecting and retaining as much heat by a solar receiver as possible. The present invention is Referring in detail to FIG. 1, there is shown a thermal primarily concerned with the more efficient collection solar collector 2 similar to the collector disclosed in the and the retention of thermal solar energy by providing above-referenced U.S. Pat. No. 4,069,812, including an improved thermal energy receiver for use within a 30 collector housing walls 42, 44 and 46 and a linear Fres solar collector device. nel lens 22 supported by the walls 42, 44 and 46. A Thermal inefficiencies in solar collectors can result pre-formed insulation material 7 is supported by the from many causes. For example, while solar energy walls 42, 44 and 46 and provides a space into which a may be efficiently collected and focused upon a thermal thermal receiver device 4 is placed. The collector 2 is receiver, much of that energy may subsequently be lost 35 arranged to focus incident solar radiation along a focal to the environment surrounding the receiver by radia axis 6 which extends along the longitudinal axis of the tive, convective and conductive heat transfer, since the collector 2.

receiver will normally operate at a much higher temper In FIG. 2, a copper (or other suitable metal) tube 1. ature than the environment. Additionally, reflection for transporting a heat exchange medium is disposed losses are normally experienced at the black-coated along the focal axis 6. The tube 1 is placed within a receiver and at the surfaces of transparent covers over lower aluminum extrusion 3 and connected thereto with the receiver. Even when the solar receiver or absorber a metal-filled silicone adhesive 5. The lower aluminum is black-coated, reflection losses still occur, albeit to a extrusion 3 rests upon a layer of isocyanurate urethane lesser extent. Moreover, prior attempts to alleviate heat (or other suitable low conductivity material) insulation losses in solar thermal receivers have more often than 45 7 which fills the volume between the lower aluminum not resulted in a more expensive product which is some extrusion 3 and the housing of a solar collector such as times deficient in other respects. For example, in order the housing walls 42, 44 and 46 of the U.S. Pat. No. to reduce radiative heat losses, some receivers have 4,069,812.

utilized black chrome selective coatings which are less An upper aluminum extrusion 9 is constructed to be durable and more expensive than low-cost black paint. 50 placed upon the top of the copper tube 1. In the present SUMMARY OF THE INVENTION example, the upper aluminum extrusion includes detents 11 and 13 for receiving corresponding bosses 15 and 17

An improved solar energy thermal receiver is herein of a lower aluminum extrusion or support structure 3. after described which provides significantly increased The upper extrusion 9 includes a black-painted, triangu efficiencies in transforming solar radiation into useful 55 lar-faceted top surface 19. The peaks and valleys of the heat transported by a thermal fluid. A thermal fluid faceted top surface 19 are extended along the top sur conducting copper (or other metal) tube is placed upon face of the upper extrusion 9 and run parallel to the a lower aluminum extrusion. A metal-filled silicone (or longitudinal axis of the collector 2. other thermally conductive) adhesive may be placed A clear silicone (or other transparent material with between the tube and the extrusion. The lower alumi low thermal conductivity) layer 21 has its lower surface num extrusion rests upon a pre-formed volume ofisocy faceted to correspond to and engage with the faceted anurate urethane (or other low conductivity material) upper surface 19 of the upper aluminum extrusion 9. insulation which fills the space between the lower alu The silicone layer 21 includes a prismatic upper surface minum extrusion and the solar collector housing. An 23 with the longitudinal axes of the prisms running upper aluminum extrusion is secured to the upper half 65 across the collector, i.e. perpendicularly to the longitu of the copper tube and includes a black painted, triangu dinal axes of the triangular facets engaging the lower lar-faceted upper surface. A metal-filled silicone (or surface of the silicone layer 21 and the upper surface of other thermally conductive) adhesive is placed between the upper aluminum extrusion 9.

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The thick layer of optically clear silicone polymer 21 sive 5 may also be placed between the copper tube 1 and above the metal absorbing surface of the top aluminum the lower aluminum extrusion 3. extrusion 9 transmits most of the concentrated incident In the present example, the silicone layer 21 was sunlight (coming from the linear Fresnel lens concentra formed with the General Electric Silicone RTV-615. tor 22) to the black-painted aluminum absorbing surface Also in the exemplary embodiment, the following base 19, and dramatically reduces heat loss to the environ line dimensions were implemented for the new thermal ment. The heat loss reduction is due to the very low receiver: Silicone layer 21 thickness-0.5 inch; Silicone thermal conductivity of the silicone material (about 0.1 layer 21 width-1.6 inches; Receiver tube 1 diamete BTU/HR-FT-DEG.F at room temperature), combined r-1.0 inch; and isocyanurate insulation 7 thicknes with its relatively large thickness (about 0.5 inch). It is O s-1.0 inch (nominal). Also, in the present example, the noted that silicone polymer is opaque in the infrared metal-filled silicone adhesive 5 was used both between portion of the spectrum, and effectively blocks thermal the copper tube 1 and the upper or top aluminum extru radiation emissions from the metal absorbing surface 19. sion 9. In the present example, the collector 2 utilizes a Thus, the dominant heat loss mechanism is simple con 15 linear Fresnel lens 22 which is 36 inches wide in aper duction, which can be controlled and optimized by the ture dimension.

proper choice of silicone layer thickness. Based on mea The embodiments of the present invention in which sured optical properties, the silicone layer 21 only ab an exclusive property or privilege is claimed are defined sorbs about 12% of the incident sunlight per one-half as follows:

inch of silicone thickness, and about two-thirds of this 20 1. A radiant energy collector including a housing for absorbed sunlight is thermally conducted to the re supporting means for focusing along a focal axis of the ceiver after absorption. Thus, the net energy through collector incoming radiation onto a thermal energy put efficiency of a 0.5 inch thick layer of the silicone is receiver, the housing supporting the means for focusing about 96% thereby allowing the receiver to have an in a position to receive the incoming radiation and also excellent optical efficiency. supporting along the focal axis the thermal energy re The prismatic structure on the upper (exposed) sur 25 ceiver, the thermal energy receiver comprising: face of the silicone layer 21 can be configured as simple support means mounted within said housing gener equilateral triangles in cross-section. The prisms effec ally along the focal axis of the collector; tively eliminate the front surface reflection loss nor a conduit positioned in the support means in a posi mally experienced with receiver glazing. Rays of light 30 tion extending along the longitudinal axis; reflected from the external surface of one prism are an energy absorber in thermal contact with said con transmitted directly to the adjacent prism and proceed duit and positioned between said means for focus directly to the absorbing surface 19 of the top aluminum ing and said conduit, said energy absorber have a extrusion 9. Thus the prismatic outer surface is an excel black surface exposed to the means for focusing; lent, yet simple, anti-reflection device. The prisms are 35 a dielectric layer of a thermally insulating material easily molded directly into the silicone material. transparent to incoming radiation and in thermal The triangular-faceted surface 19 on the metal ab contact with the black surface of the energy ab sorber or top aluminum extrusion 9 is painted with a sorber and positioned between the means for focus black paint to minimize reflection losses from the ab ing and said energy absorber to receive the focused sorber 9. The facets 19 can be simple equilateral trian radiation and transmit such radiation to said ab gles extruded into the metal absorber 9. Solar rays re sorber, said layer having a thickness between said flected from the black-painted surface of one facet, means for focusing and said energy absorber re-impinge onto the adjacent facet, thereby minimizing greater than several wavelengths of the incoming reflection losses. For example, if a paint with only a radiation to minimize thermal losses from the ab 90% absorptance were used on the faceted absorber 45 sorber.

surface, 10% of the incidence sunlight would be re 2. The radiant energy collector as described in claim flected by each facet onto the adjacent facet, which in 1 wherein the dielectric layer has a faceted surface turn, would absorb 9% of the reflected 10%. Thus, the facing said means for focusing.

net absorptance of the faceted surface would be 99% 3. The radiant energy collector as described in claim for a poorly performing 90% paint. Since the triangular 50 1 wherein said energy absorber has a faceted surface in facets can be manufactured by direct profile extrusion thermal contact with said dielectric layer. of aluminum, no added cost will be incurred. Moreover, 4. The radiant energy collector as described in claim low cost black paint can be used in place of expensive, 1 wherein the dielectric layer is composed of a silicone less durable black chrome selective coatings with no polymer.

loss in performance because of the elimination of the 55 5. A radiant energy collector including a housing for thermal radiation loss from the receiver by the infrared supporting means for focusing along a focal axis of the absorbing silicone layer 21. collector incoming radiation onto a thermal energy The exemplary embodiment includes corrosion receiver, the housing supporting the means for focusing resistant copper (or steel) tubing for the heat exchange in a position to receive the incoming radiation and also flow passage 1, while using lower-cost aluminum extru supporting along the focal axis the thermal energy re sions 3 and 9 to provide the relatively complex receiver ceiver, the thermal energy receiver comprising: structure. Either alumina-filled or metal-filled silicone support means mounted within the housing in a posi adhesives 5 are used to bond the copper tube 1 to the tion substantially aligned with the focal axis; upper aluminum extrusion 9 with excellent thermal a fluid conduit positioned in the support means along performance results. The metal-filled silicone adhesive 65 the focal axis of the collector; 5 acts as a thermal conductor to minimize the tempera an energy absorber in thermal contact with said con ture gradient between the upper aluminum extrusion duit and positioned between said means for focus and the copper tube 1. The metal-filled silicone adhe ing and said conduit, said energy absorber having a

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black surface exposed to the means for focusing; absorber to minimize thermal losses from the ab and sorber.

a dielectric layer of a thermally insulating material 6. The thermal energy receiver as set forth in claim 5, transparent to incoming radiation and in thermal wherein said dielectric layer has a faceted surface facing contact with the black surface of the energy ab said means for focusing.

sorber and positioned between the means for focus 7. The thermal energy receiver as set forth in claim 5, ing and said energy absorber to receive the focused radiation and transmit such radiation to said ab wherein said radiation-absorbing element has a faceted sorber, said layer of a thermally insulating material surface in thermal contact with said dielectric layer. having a high transmittance for incoming radiation, 10 8. The thermal energy receiver as set forth in claim 5, a high absorptance for infrared radiation, a low wherein said dielectric layer is composed of a silicone thermal conductivity, and having a thickness be polymer.

tween said means for focusing and said energy k is is