FIELD OF THE INVENTION
This invention relates to the generation of electrical energy through solar thermal power collection, and, more particularly, to a linear solar energy collection system that employs an array of light-weight solar panels, secondary reflectors, tertiary reflectors and a fixed linear receiver tube in which a heat transfer fluid is circulated.
BACKGROUND OF THE INVENTION
Systems for the generation of electricity by collecting solar thermal radiation were first introduced in 1914, and have become increasingly popular with the rise in fossil fuel costs and concerns over global warming. The majority of solar energy collection systems currently in use are of the type depicted in FIGS. 1 and 2. A generally parabolic-shaped trough 10 is provided having a curved, reflective surface 12 that is typically formed of a number of mirrors. The reflective surface 12 is effective to concentrate and reflect incident sunlight 13 at 30 to 80 times its normal intensity along a focal line that is coincident with a receiver tube 14 mounted by holding bars 16 in a position above the reflective surface 12. The mirrors are carried by a support structure 18 which, in turn, is connected at each end to pylons 20 secured in the ground on a concrete foundation or the like. A motor 22 is drivingly connected to the support structure 12 to pivot it thus allowing the reflective surface 12 to track the progress of the sun across the sky. A local controller 24 may be provided to control the operation of the motor 22 as it pivots the support structure 18 and surface 12 throughout a day, and to monitor certain alarm conditions.
A heat transfer fluid is circulated through the receiver tube 14 which is heated by the sunlight reflected from surface 12. This fluid is used to generate steam which powers a turbine that drives an electric generator. In order to transfer the heated fluid from the receiver tube 14 to a steam generator, a flexible hose 26 is coupled to the receiver tube 14, typically via ball joints 28, and moves with it as the support structure 18 is pivoted. The flexible hose 26 may be connected to a header pipe (not shown), which then connects to the steam generator.
Solar collection systems of the type described above suffer from a number of deficiencies. The mirrors forming the reflective surface 12 typically comprise 4 mm low-iron float glass mirrors thermally sagged during manufacturing into a parabolic shape. These mirrors are very heavy, and are available from only a few manufacturers. They are difficult to install and require robust mounting structure to support in order to provide for accurate positioning of the reflective surface 12 and to resist wind loads. While thinner glass mirrors have been suggested as an alternative, they are more fragile resulting in increased handling costs and breakage losses. Most support structures 18 for the mirrors are formed of galvanized steel which is also heavy, requires precise manufacturing and is expensive to build. Bridge trusses have been employed in more recent designs for the support structures 18, but have proven to be nearly equally expensive to manufacture and often are lacking in torsional stiffness. In addition to these problems, the flexible hoses 26 and ball joints 28 employed to transfer heated fluid from the receiver tube 14 have high thermal losses, and exhibit high failure rates and leaks since they must move with the support structure 18 and reflective surface 12 as they pivot.
The goal of any solar collection system is to reduce the cost of electricity generated. There are fundamentally two ways to do this, namely, reduce the cost of the solar field and annual operating expenses, and, to increase system efficiency. Solar field optical efficiency is dependent upon a number of factors, including, without limitation, sunlight incident angle effects, collector tracking error, the geometric accuracy of the mirrors to focus light on the receiver tubes, mirror reflectivity, cleanliness of the mirrors, the creation of shadows across the mirrors, transmittance of solar energy into the receiver tubes, cleanliness of the receiver tubes, absorption of solar energy by the receiver tubes, end losses and the creation of shadows between rows of mirrors. While current systems produce electricity at a cost in the range of $0.12 to $0.18 per kilowatt-hour, it is desirable to achieve a cost level of about $0.05 per kilowatt-hour to be more competitive with present fossil-fuel based systems.
SUMMARY OF THE INVENTION
This invention is directed to a linear solar energy collection system that improves solar field efficiency, lowers operational and maintenance costs, and therefore reduces the overall cost of generating electricity per kilowatt-hour.
One aspect of this invention is predicated on the concept of providing a simple, modular linear solar energy collection system comprising one or more reflector units each fabricated using light-weight materials arranged in a construction that is highly accessible, easily maintained, and lower in initial cost. In one embodiment, each reflector unit includes a light-weight, generally planar aluminum frame that mounts a number of solar panels in a fixed position at angles progressively increasing from the frame centerline outwardly to its perimeter so as to collectively form a surface having a shape approximating that of a parabola. The focal line of such parabola is coincident with a secondary reflector which receives sunlight incident on the solar panels and reflects such light onto a receiver tube mounted in a fixed position substantially concentric to the centerline of the frame. The frame is supported by truss elements to add rigidity, and is connected to a drive mechanism operative to pivot the frame and truss elements in order to track the position of the sun during the course of a day. A number of individual reflector units may be arranged side-by-side to form a solar energy collection system having a collection field of desired size.
In an alternative embodiment, a reflector unit includes solar panels formed in smaller segments mounted to a number of shafts, which, in turn, are pivotally connected to the frame. The solar panels collectively form a generally parabolic surface as in the previously described embodiment, but may also be tilted by rotation of the shafts in a generally northerly and southerly direction to more directly face the sun as its incidence angle varies with the changing of the seasons.
Still another embodiment of this invention is designed to increase efficiency by reducing the effects that the secondary reflector in the previously described embodiments can have due to blockage of incident sunlight. It has been found that a portion of the sunlight directed to each reflector unit is blocked by the secondary reflector and does not reach the solar panels, and, in turn, the receiver tube. In this embodiment, at least one other secondary reflector is provided in between the first secondary reflector and the receiver tube as well as at least one tertiary reflector. As described below, the secondary reflectors and tertiary reflector(s) cooperate with the solar panels to direct sunlight onto the receiver tube at a higher efficiency.
In all of the embodiments of this invention, each solar panel preferably comprises a honeycomb aluminum section and a highly reflective silver-metallized surface connected together by an adhesive layer. The solar panels are strong, durable, light-weight and efficiently reflect incident sunlight many times its normal intensity onto the secondary reflector. The reflective surface of such panels may be washed to maintain cleanliness, which, in one presently preferred embodiment of this invention, is accomplished by the provision of an in-ground washing system operative to direct cleansing water against such surfaces.
A heat transfer fluid is circulated through the receiver tube for heating by the sunlight directed thereto from the secondary reflector. Because the receiver tube is fixed relative to the pivoting frame, it may be connected to a fixed transfer conduit that communicates with a steam generator and turbine. Since both the receiver tube and transfer conduit are mounted in a fixed position, heat losses resulting from the transfer of fluid out of the receiver tube are minimized and maintenance problems with the moving connections between the receiver tube and transfer conduit that were required in prior art systems, as described above, are substantially eliminated.
BRIEF DESCRIPTION OF THE DRAWINGS
The structure, operation and advantages of the presently preferred embodiment of this invention will become further apparent upon consideration of the following description, taken in conjunction with the accompanying drawings, wherein:
FIG. 1 is perspective view of a prior art solar energy collection system;
FIG. 2 is an end view of the system shown in FIG. 1;
FIG. 3 is a perspective view of one embodiment of a reflector unit for the solar energy collection system of this invention;
FIG. 4 is a perspective view of a solar panel of this invention;
FIG. 5 is an enlarged view of the encircled portion of FIG. 4 showing the solar panel partially disassembled;
FIG. 6 is a perspective view of the receiver tube employed herein;
FIG. 7 is a schematic, end view of the solar panels and secondary reflector of the unit depicted in FIG. 3, illustrating the orientation of the solar panels from the centerline of the frame outwardly;
FIG. 8 is a schematic view of the relationship between the solar panels and secondary reflector of the unit of FIG. 3;
FIG. 9 is a schematic view depicting how the unit herein tracks the position of the sun during the course of a day;
FIG. 10 is a perspective view of the drive mechanism for pivoting the frame and solar panels;
FIG. 11 is an end view of the drive mechanism illustrated in FIG. 10;
FIG. 12 is a perspective view of a solar collection system according to this invention in which a number of reflector units shown in FIG. 3 are oriented side-by-side;
FIG. 13 is a perspective view of the sprinkler system of this invention operating with the solar panels and secondary reflector in a first position before sunrise and a second position after sunset;
FIG. 14 is a perspective view of the frame and solar panel portion of an alternative embodiment of a solar energy collection system according to this invention;
FIG. 15 is an enlarged, side view of a portion of FIG. 14 illustrating the manner in which the solar panels are mounted for tilting movement relative to the frame;
FIG. 16 is a view similar to FIG. 15 showing the solar panels tilted after rotation of the mounting shaft; and
FIG. 17 is a view similar to FIG. 7 depicting a still further embodiment of the solar energy collection system of this invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIGS. 3-12, one embodiment of a solar energy collection system according to this invention is illustrated which may comprise several individual reflector units 30 oriented side-by-side, as discussed below with reference to FIG. 12. The reflector unit 30 is initially generally described, followed by a discussion of individual aspects of the design.
The reflector unit 30 includes a frame 32 having opposed side walls 34, 36, and opposed end walls 38, 40 connected together in a generally rectangular shape as depicted in FIG. 3. The walls 34-40 are preferably formed of aluminum or other light-weight, weather resistant and durable material. The frame 32 is reinforced by a truss structure 42, a portion of which is shown in FIGS. 3 and 12, which is also preferably formed of aluminum or similar material. The truss structure 42 and frame 32 may be supported above ground level by pylons 44 secured on a foundation such as concrete footers (not shown) that can support the weight of the unit 30 and wind loading applied to it. As described in detail below with reference to FIGS. 10 and 11, the frame 32 and truss structure 42 are pivotally mounted to the pylons 44 and may be tilted by operation of a drive mechanism 46 including a drive motor 48.
The frame 32 mounts a number of solar panels 50 and a secondary reflector 52 which collectively form the structure for receiving incident sunlight 53 from the sun 55 and reflecting it onto a receiver tube 54 located in a fixed position at the centerline 56 (See FIG. 7) of the frame 32. The solar panels 50 extend between the end walls 38, 40 and are spaced from one another on either side of the receiver tube 54 in a direction toward the side walls 34, 36. The secondary reflector 52 is located above the solar panels 50, as discussed below, and is supported in that position at each end by rods 58 and tension wires 60 extending from the frame 32.
Referring now to FIGS. 4 and 5, a solar panel 50 according to this invention is shown in greater detail. Each solar panel 50 is generally rectangular in shape having opposed side edges 62, 64 and opposed end edges 66, 68. The panels 50 have a slight concave curvature in a direction from one side edge 62 to the other side edge 64, which may be slightly different from one panel 50 to another as described below. Each panel 50 comprises a base section 70, a top section 72 and an intermediate section 74 sandwiched between the sections 70, 72. The base section 70 is preferably formed of a honeycomb aluminum, or similar light-weight, weather resistant and durable material that may be bent in the slight curvature noted above and shown in FIG. 4. The top section 72 is preferably a highly-reflective, silver-metallized film comprising multiple layers of polymer film with an inner layer of pure silver to provide a reflective surface 76 having high specular reflectance. One suitable material for top section 72 is commercially available from ReflecTech, Inc. of Wheat Ridge, Colo. under the trademark “ReflecTech” solar film. The intermediate layer 74 is preferably a layer of pressure sensitive adhesive. Layer 74 may be affixed on one side to the top section 72 and provided with a peel-off backing (not shown) which is removed prior to attachment to the base section 70.
The receiver tube 54 is a component employed in prior art solar collection systems and is readily commercially available. As shown in FIG. 6, it comprises a hollow, stainless steel housing 78 having a solar-selective absorber surface surrounded by an anti-reflective, evacuated glass sleeve 80. Typically, the housing 78 has a length of 4 meters and a diameter of 70 mm, and the glass sleeve is 115 mm in diameter. A heat transfer fluid such as oil or water is circulated through the housing 78 where it is heated by reflected sunlight, as discussed below. The receiver tube 54 has glass-to-metal seals and metal bellows (not shown) to accommodate differing rates of thermal expansion between the stainless steel housing 78 and glass sleeve 80, and to help maintain the vacuum-tight enclosure. This reduces heat losses at high operating temperatures and protects the solar-select absorber surface of the housing 78 from oxidation.
The solar panels 50 and secondary reflector 52 collectively function to direct incident sunlight 53 onto the receiver tube 54 to elevate the temperature of heat transfer fluid circulating within the receiver tube 54 to a level sufficient to operate a steam generator (not shown) for the production of electricity. The positioning of the solar panels 50 with respect to the secondary reflector 52, and the configuration of the secondary reflector 52, are both important in maximizing the efficiency of the reflector unit 30. The discussion that follows concerns this aspect of the present invention.
A parabola is a geometric shape defined by the locus of points that are equidistant from a point (the focus) and a focal line (directrix) that lie in the same plane. Reflective surfaces having the shape of a parabola have been commonly used in solar power collection systems because incident sunlight may be reflected to collection device located at the focus or directrix of the parabola. The unit 30 of the present invention is designed to take advantage of this property of a parabola, but in a much more efficient, less expensive and practical manner than taught in the prior art.
Referring now to FIG. 7, an end view of the frame 32 and its end wall 40 is shown with the receiver tube 54 depicted within an opening 82 formed in the frame end wall 40, substantially concentric to the frame centerline 56, and the secondary reflector 52 located at a position spaced from the receiver tube 54. A first array 84 of solar panels 50 extends from the receiver tube 54 to the side wall 34 of frame 32, and a second array 86 of solar panels 50 is mounted between the receiver tube 54 and side wall 36. The end edges 66 and 68 of each panel 50 are secured in a fixed position to an end wall 38 and 40, respectively, of the frame 32 by fasteners or other suitable means such as the provision of recesses in the end walls 38, 40 (not shown). The solar panels 50 in each array 84, 86 are oriented at an angle with respect to the secondary reflector 52 so as to direct incident sunlight 53 to a focal line or directrix that is coincident with the surface 96 of secondary reflector 52. As seen in FIG. 7, the angle of the solar panels 50 increases from the centerline 56 of frame 32 outwardly to its side walls 34, 36. In the presently preferred embodiment, the angle of each panel 50 relative to the secondary reflector 52 is chosen to closely approximate the orientation of each of a number of discrete segments 90 “sliced” from a continuous parabola 92, as schematically depicted in FIG. 8. In essence, the solar panels 50 in each array 84, 86 comprise segments of the parabola 92 which are separated from one another, and then individually affixed to the frame 32. Consequently, the solar panels 50 collectively form a reflective, substantially parabolic-shaped surface 94 whose focus and directrix 95 are substantially coincident with the secondary reflector 52.
It should be understood that in a true parabola the distance from every point along its surface to the focal point of the parabola is the same. When a parabola is “cut” into segments 90, e.g. discrete solar panels 50, and then individually mounted to the frame 32 as contemplated in this invention, there must be at least some spacing between the side edges 62, 64 of adjacent solar panels 50 to facilitate mounting and to avoid shadowing or overlap between them. See FIG. 7. The spacing between panels 50, and their linear orientation along the frame 32, both contribute to a change in the distance from the center of each panel 50 to the focus and directrix. Consequently, a slight concave curvature is required in each panel 50, which differs from one panel 50 to another depending on its angulation relative to the secondary reflector 52, in order to ensure that the individual focal point of each panel 50 is substantially the same. Such curvature may be calculated using the standard mathematical equation defining a parabola, namely:
y=x
2/4f
Where: f=the focal point
- x=horizontal distance from the center
- y=vertical distance
As noted above, the first and second arrays 84, 86 of solar panels 50 collectively form a parabolic surface 94 that reflects incident light to a focus or directrix. The secondary reflector 52 is located along the directrix or focal line of surface 94 and is constructed to reflect the light from surface 94 onto the receiver tube 54 to elevate the temperature of heat transfer fluid circulating therein. In one presently preferred embodiment, the secondary reflector 52 is approximately 200 mm to 250 mm in width with a reflective surface 96 in the shape of a hyperbola. The exact geometry of the reflective surface 96 is derived from the Cassegrain Equations for a primary parabolic-shaped reflective surface, which, in this instance, is the parabolic surface 94 collectively formed by the solar panels 50, and a secondary hyperboloid reflective surface. The secondary reflector 52 may be constructed of a honeycomb panel having the appropriate shape noted above connected by an adhesive layer to the same material that forms the top section 74 of solar panels 50.
Referring now to FIGS. 9-11, it is advantageous for the solar panels 50 to be oriented substantially perpendicular to the position of the sun 55 throughout the course of a day in order to maximize the efficiency with which the sunlight is reflected to the secondary reflector 52, and, in turn, to the receiver tube 54. FIG. 9 illustrates this pivotal movement of frame 32, and, in turn, the solar panels 50 and secondary reflector 52, during daylight hours. Such pivotal movement is about an axis which is generally coincident with the centerline 56 of the frame 32.
With reference to FIGS. 10 and 11, and as noted briefly above, the frame 32 is pivoted by a drive mechanism 46 including a motor 48. In the presently preferred embodiment, a support frame 98 is connected to a pylon 44 which rotatably mounts three rollers 100, 102 and 104 spaced approximately 120° apart. These rollers 100-104 receive and support a drive wheel 106 which is connected by a link chain 108, or other suitable drive means such as a belt, to the output shaft of motor 48. The drive wheel 106 is connected by a plate 110 to the rods 58 which support the secondary reflector 52 at one end, and connect to the frame 32 at the opposite end. In response to operation of the motor 48, the drive wheel 106 rotates with respect to the rollers 100-104. The rods 58 and frame 32 rotate with the drive wheel 106, thus pivoting relative to the pylons 44 to assume the positions shown in FIG. 9.
In the presently preferred embodiment, the receiver tube 54 remains in a fixed position with respect to the frame 32 and drive wheel 106 throughout the pivotal motion of the frame 32. As described above, the receiver tube 54 extends through an opening 82 formed in each end wall 38, 40 of frame 32. The protruding end of receiver tube 54 enters a bore 111 formed in the plate 110, and a central bore 112 formed in the drive wheel 106 where it is received and supported by a bearing 114 that allows the receiver tube 54 to remain in a fixed position during rotation of the drive wheel 106. This construction has the advantage of allowing the receiver tube 54 to be connected to a fixed transfer conduit 114, shown in FIG. 3, coupled to a steam generator (not shown). Consequently, the expensive and leak-prone connections between the moving receiver tubes and transfer conduits employed in the prior art, and shown, for example, in FIG. 2, are eliminated in this invention.
The solar energy collection system of this invention is modular in construction. As shown in FIG. 12, a number of individual reflector units 30 depicted in FIG. 3 and described above may be located side-by-side to increase capacity and overall efficiency of the solar field. In such arrangements, a drive mechanism 46 may be located in between adjacent units 30 such that each end of the output shaft of motor 48 may be coupled to the drive wheel 106 of one of the units 30 in the manner described above in connection with a discussion of FIGS. 10 and 11. Further, the receiver tube 54 of one unit 30 may be coupled to the receiver tube 54 of an adjacent unit 30 to transmit heat transfer fluid to one or more conduits (not shown) for the combined collection system.
In another aspect of this invention, an in-ground sprinkler system 116 is provided to help clean the reflective surface 94 of the solar panels 50 and the surface 96 of the secondary reflector 52. As schematically depicted in FIG. 13, one or more first sprinkler heads 118 connected to a source of water (not shown) are positioned to direct streams 119 of water onto the first array 84 of solar panels 50 and a portion of the secondary reflector 52 with the unit 30 in position prior to sunrise, and one or more second sprinkler heads 120 direct streams 121 of water onto the second array 86 of solar panels 50 and the remainder of the secondary reflector 52 when the reflector unit 30 moves to its position at the end of a day. Maintaining the collective reflective surface 94 of the solar panels 50, and the surface 96 of the secondary reflector 52, clean significantly increases the overall efficiency of the reflector unit 30.
An alternative embodiment of a solar energy collection system having one or more reflector units 122 according to this invention is illustrated in FIGS. 14-17. The reflector unit 122 is similar to reflector unit 30 in many respects except for the addition of structure that permits adjustment of the position of solar panels about a second axis. As discussed above, the frame 32 and solar panels 50 of the unit 30 are pivoted as illustrated in FIG. 9 about an axis generally coincident with the centerline 56 of the frame 32. Such motion is in an easterly to westerly direction consistent with the apparent movement of the sun 55 across the sky during the daylight hours. As is well known, the earth tilts on its axis during the course of a year causing the change of seasons and altering the angle of inclination of the sun's rays. The unit 122 of this embodiment of the present invention is designed to not only track the sun's daily path but its annual inclination.
The same frame 32 described above is employed in unit 122, but instead of elongated solar panels 50 extending between the frame side walls 34, 36, a plurality of shorter, segmented solar panels 124 are provided. The solar panels 124 are divided into groups, and each group of panels 124 essentially takes the place of a single solar panel 50 in the embodiment of FIGS. 3-13. As best seen in FIG. 14, one group of several panels 124 is mounted within each of a number of sub-frames 126, e.g. a generally rectangular-shaped structure having opposed ends and opposed sides. These sub-frames 126 are secured in the same fixed positions to the side walls 34 and 36 of frame 32, and at the same angles, as solar panels 50 described above. In one embodiment, the panels 124 within each group may be coupled to a threaded shaft 128, which, in turn, is rotatably mounted to the end walls of a sub-frame 126. A lever arm 130 may extend from each panel 124 and connect to an internally threaded sleeve 132 which threads onto the shaft 128. In response to rotation of a shaft 128, either manually by turning a knob 134 or by operation of a motor (not shown), the sleeves 132 move axially along the shafts 128 causing the panels 124 to tilt. See FIGS. 15 and 16. The direction of rotation of the shaft 128 determines the direction of tilting of the panels 124. In this manner, the panels 124 may be tilted in a northerly direction or in a southerly direction according the angle of inclination of the sun 55. The remainder of the structure and operation of the system 122 is essentially the same as that described above in connection with a discussion of reflector unit 30.
A still further embodiment of this invention is shown in FIG. 17. It has been found that in the embodiments of FIGS. 3-16, wherein a single secondary reflector 52 is employed, a portion of the sunlight may be blocked by the secondary reflector 52 from reaching the solar panels 50 or 124. This can reduce the overall efficiency of each reflector unit 30 or 122. In order to increase efficiency, the reflector unit 150 illustrated in the embodiment of FIG. 17 provides a modified system for reflecting the sun's rays to the receiver tube 54. Except as discussed below, all of the structure of reflector unit 150 is the same as that described above in connection with FIGS. 3-13 or with FIGS. 14-16
The reflector unit 150 includes an upper secondary reflector 52 and two lower secondary reflectors 152 and 154 which are separated by an opening 156. Preferably, the focal line 158 of the upper secondary reflector 52 is centered with respect to the opening 156 and aligns with the receiver tube 54. Each of the lower secondary reflectors 152 and 154 has a generally parabolic, concave surface 160 facing the upper secondary reflector 52, and an opposite, generally parabolic, convex surface 162 which faces at least some of the solar panels 50. Additionally, the reflector unit 150 includes two tertiary reflectors 164 and 166 mounted to the frame 32 in between the first array 84 and the second array 86 of solar panels 50, and beneath the receiver tube 54. Each tertiary reflector 164, 166 has a generally parabolic concave surface 168 facing the receiver tube 54 and an opposite, generally parabolic convex surface 170. The tertiary reflectors 164, 166 are separated from one another by a space 172, for purposes to become apparent below. The lower secondary reflectors 152, 154 and tertiary reflectors 164, 166 are preferably made of the same materials as solar panels 50, e.g. a base section formed of a honeycomb aluminum or similar light-weight, weather resistant and durable material, an adhesive inner layer and an outer layer formed of highly-reflective, silver-metallized film. In the case of the lower secondary reflectors 152, 154, both the concave and convex surfaces 158, 160 are provided with a reflective material, whereas only the concave surface 168 of the tertiary reflectors 164, 166 need have such a reflective surface.
The reflector unit 150 operates as follows. Sunlight incident on the solar panels 50 is reflected in the same manner as described above in connection with a discussion of FIG. 7, except that instead of being directed onto the reflective surface 96 of the upper secondary reflector 52, as in the previously described embodiments, such reflected sunlight is incident on the convex surface 162 of one of the lower secondary reflectors 152, 154. In particular, the solar panels 50 in the first array 84 reflect incident sunlight onto the convex surface 162 of the lower secondary reflector 152, whereas the solar panels 50 in the second array 86 reflect incident sunlight onto the convex surface 162 of the lower secondary reflector 154. As discussed above, the first and second arrays 84, 86 of solar panels 50 collectively form the approximate shape of a parabola and therefore each array 84, 86 is individually effective to reflect incident light toward the focal point 95 of the upper secondary reflector 52. The lower second secondary reflectors 152, 154 are positioned directly in the path of such reflected sunlight. The reflected light from the lower secondary reflector 152, in turn, is directed onto the concave surface 168 of the tertiary reflector 164, and the reflected light from the lower secondary reflector 154 is directed onto the concave surface 168 of the tertiary reflector 166. The two tertiary reflectors 164, 166 are separated from one another by the space 172 so as to align with the reflected sunlight from respective lower secondary reflectors 152, 154, and to avoid being blocked by the receiver tube 54. The positioning and concave, generally parabolic curvature of the tertiary reflectors 164, 166 are chosen so that incident sunlight from the lower secondary reflectors 152, 154 is reflected onto the receive tube 54 to heat the heat transfer fluid therein.
As is apparent from viewing FIG. 17, some of the sun's rays cannot reach the solar panels 50 because they are blocked by the lower secondary reflectors 152, 154. In order to increase efficiency, the reflective concave surface 160 of both of the upper secondary reflectors 152 and 154 is effective to reflect incident sunlight onto the reflective surface 96 of upper secondary reflector 52. As explained above in connection with a discussion of the reflector 52, its parabolic-shaped reflective surface 96 is effective to direct incident sunlight substantially along focal line 158. Since the focal line 158 is centered relative to the opening 156 between the lower secondary reflectors 152, 154 and in alignment with the receiver tube 54, the reflected sunlight from the surface 96 of the upper secondary reflector 52 impinges against the receiver tube 54. In the event some of the sun's rays reflected from the upper secondary reflector 52 do not contact the receiver tube 54, they are reflected thereon by one of the tertiary reflectors 164, 166. Directing sunlight reflected from the lower secondary reflectors 152, 154 to the receiver tube 54 in such a manner, which would otherwise be blocked, increases the efficiency of the solar panel 150 compared to the solar panels 30 and 122 described in the previous embodiments.
While the invention has been described with reference to a preferred embodiment, it should be understood by those skilled in the art that various changes may be made and equivalents substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof.
For example, the receiver tube 54 is depicted in FIGS. 7, 9, 13 and 14 as being positioned at the center of frame 32 and substantially concentric to its centerline 56. As shown in FIGS. 3, 10 and 12, the receiver tube 54 may also be located slightly above the center of frame 32. In both instances, the receiver tube 54 is located at substantially the center of rotation of the frame 32 and generally at the center of gravity of the reflector unit 30. Additionally, whereas the lower secondary reflectors 152 and 154, as well as the tertiary reflectors 164 and 166, are shown as individual elements, it should be understood that a single lower reflector and a single tertiary reflector could be employed with an opening being provided in the lower secondary reflector to allow the passage of reflected sunlight from a single upper secondary reflector 52 to the receiver tube 54.
Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.
Patent Application
20100051018
