Patent Publication Number: US-2002002972-A1

Title: Solar heating reflecting element suitable for molding

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention generally relates to solar collectors and more particularly to solar collectors which include reflective elements for focusing, incident solar radiation onto a working fluid conduit, for heating the working fluid as it passes through the working fluid conduit. The present invention provides an apparatus and method for forming reflective elements and for assembling a solar collector, of simple construction, using a plurality of molded reflective elements assembled together onto a base to form a unitary solar collecting device, of low cost and easy assembly, suitable, e.g., for heating swimming pool water or the like. This invention specifically provides a method for forming the reflective elements from a ceramic or other moldable material and for treating a surface of the molded reflective element so as to provide a highly reflective surface.  
       [0003] 2. Description of the Prior Art  
       [0004] Numerous, solar collector devices have been proposed for home use, e.g. for heating a working fluid, of water, which may circulate to a conventional domestic hot water heater for supplementing domestic hot water heating, or, for directly heating swimming pool water, or, for heating a working fluid which circulates to a thermal reservoir or storage tank which then flows to a domestic home heating system or the like to supplement conventional heating systems. Many such devices employ heat absorbing conduits fabricated from highly thermally conductive metals such as aluminum and or copper. A working fluid, which may be water, air or another working fluid is circulated through the metal conduit to receive thermal energy absorbed thereby when the conduit is exposed to solar radiation falling thereon.  
       [0005] Many solar collecting devices also include a chamber for surrounding the heat absorbing conduits, to reduce convective heat loss from the heat absorbing conduits. Such a chamber may enclose individual heat absorbing conduits, as shown in U.S. Pat. No. 4,198,955, or may enclose a plurality of heat absorbing conduits, as shown in U.S. Pat. No. 4,397,305. In either case, a transparent material on a surface facing the sun is typically employed to seal the chamber while allowing solar radiation to pass through the transparent material and onto the heat absorbing conduit.  
       [0006] In other examples, the solar collector may also include reflective lenses or the like to focus or otherwise direct incident solar radiation onto the heat absorbing conduit. In one such device, disclosed in U.S. Pat. No. 4,373,513, by Materna, a solar collector of very simple construction is proposed, employing a single non-tracking reflective element in combination with a plurality of heat absorbing conduits which receive reflected solar energy. The Materna device appears to provide a low cost solar collector, however, details of its construction and fabrication are not provided since the &#39;513 patent teaches other aspects of the invention. U.S. Pat. No. 4,309,984 provides yet another example of a reflective lens type solar collector, however, the details of its construction are complex and the materials of metal and glass require labor intensive manufacturing processes and unit assembly.  
       [0007] Another consideration in the fabrication of a solar collector for general use is that some applications may require more or less heating capacity such that there is an advantage to provide a solar collector which may be easily fabricated in various sizes. It is known to combine multiple solar collector device together in order to increase heating capacity, however, these combinations only allow increases in large steps, e.g. a 100% increase in area by combining two collectors of equal area. Such combinations of individual collector elements may not be practical due to available space and or weight considerations, e.g. on a roof installation. As will be detailed below, the present invention allows incremental increases in heating capacity in small increments without substantially increasing the cost or complexity of the installed solar collector.  
       [0008] A need exists in the art for a very low cost solar heating device, especially for heating water directly. A further need exists for a solar heating device of simple enough construction so as to be shipped in an unassembled condition and assembled on sight by an unskilled and untrained user. Such a low cost and easily to assemble device may be employed e.g. for seasonal heating of swimming pool water or the like and may further allow for disassembly and storage of the device in the off season. A need also exists for a solar collector which is easily variable in collecting area without substantially increasing the cost or complexity of installing the solar collector for a particular application.  
       [0009] One way of reducing manufacturing and labor costs, as proposed by the present invention, is to provide a solar collector of modular unitary construction. Examples of such solar collectors are given in U.S. Pat. No. 4,718,404, by Sadler, and in U.S. Pat. No. 4,397,305, by Keefe, wherein molded or otherwise formed heat absorbing conduits take advantage of molding techniques to incorporate additional functionality into the molded parts, thereby reducing the total number of parts as compared with non-modular devices and simplifying assembly by providing interlocking and or interconnecting features molded into the individual elements.  
       [0010] Of particular interest with respect to the present invention is the use of ceramic materials for molding heat absorbing conduit elements as disclosed in U.S. Pat. No. 4,383,959 by Sadler. Ceramic materials offer extremely low raw material and manufacturing costs as compared to a solar collector fabricated from metals, especially copper and aluminum, and provide an advantage over heat absorbing conduit elements fabricated from molded or formed plastic or other conventional molding materials since ceramic materials are impervious to the adverse impervious to the adverse effects of the environment, including excessive temperature and ultraviolet radiation due to prolonged expose to the sun. Furthermore, ceramic materials offer substantially rigid mechanical properties providing structural integrity and thermal stability in comparison to plastic molded or other formed elements, and since ceramic elements are molded or formed, they have the further advantage that additional functionality can be molded into the parts, thereby reducing overall part count.  
       [0011] Accordingly, it is a specific object of the present invention to provide a low cost solar collector for heating water or another working fluid. It is further specific object of the present invention to provide a solar collector of modular unitary construction. It is a specific object of the present invention to provide a solar collector formed by assembling a plurality of substantially identical molded elements onto a base such that the solar collector has sufficient area to meet the heating capacity required by a particular application and furthermore that the area and therefore heating capacity of the solar collector be selectable in accordance with the required heating capacity. It is a still further object of the present invention to provide a method for forming a ceramic element for use as a reflective lens or the like for focusing or otherwise directing incident solar radiation onto a heat absorbing conduit. It is a still further object of the present of the present invention to provide a solar collector of such simple construction so as to be able to be shipped to a sight unassembled and assembled at the sight by an unskilled and untrained user. It is a still further object of the present invention to provide a solar collector which is easily assembled for seasonal use and thereafter disassembled for seasonal storage while requiring a minimum of storage space.  
       SUMMARY OF THE INVENTION  
       [0012] The present invention overcomes the problems cited in the prior by providing a plurality of substantially identical reflective element for reflecting solar energy to a focal axis in a solar collecting apparatus and which can be formed into rows of substantially identical reflective elements. Each reflective element is formed as a substantially rectangular element having opposing side walls and opposing end walls which together form a bottom edge for supporting the reflective element on a flat support such as a sheet metal or mold base. Each reflective element further includes a top wall connected with and supported by the side walls and the end walls. The top wall is formed to provide a reflective surface facing away from the bottom edge and is formed in a shape for reflecting solar radiation falling thereon, from a range of incident angles, to a focal axis. The focal axis of each reflective element is substantially parallel with a longitudinal axis of the reflective element and when the reflective elements are arranged in parallel rows, each row includes a focal axis along its longitudinal length. The reflective surface is coated with a reflective layer such as vacuum deposited aluminum, metalized paint or by other reflective coatings to reflect solar energy to the focal axis. The reflective surface may be formed with a constant radius R with respect to a locus axis, e.g. positioned 4.0 inches above the reflective surface and wherein the focal axis lies equidistant from the reflective surface and the locus axis, e.g., 2.0 inches above the reflective surface. A parabolic reflective surface may also be fabricated to improve the sharpness of the reflective surface focal axis. Ideally, the reflective element is formed by molding a material in a mold such that the mold forms the shape of the reflective surface. In the preferred embodiment of the present invention the reflective element is molded in a press mold from a material comprising hydrated silicates of aluminum which is plastic when wet but which becomes a hard ceramic when fired in an oven.  
       [0013] In another aspect of the present invention a solar energy collecting apparatus for heating a working fluid using the plurality of substantially identical reflective elements arranged in rows is provided. There is provided a working fluid conduit comprising a plurality of fluid carrying members positioned one above each row of reflective elements and substantially coincident with the focal axis of the row. The fluid carrying members are connected together by a plurality of connecting members such that a continuous working fluid conduit passes a working fluid over each row of reflective elements. In another aspect of the present invention the working fluid conduit is movably supported with respect to the base such that each of the plurality of fluid carrying members is movable away from the focal axis when no heating of the working fluid is desired.  
       [0014] There is also provided an adjustable support frame for supporting the solar collecting apparatus in a desired orientation with respect to the sun. The collecting apparatus also includes alignment targets for aligning the shadow of a first target with a second target to optimize the orientation of the solar collector with respect to the sun. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0015] The features of the present invention will best be understood from a detailed description of the invention and a preferred embodiment thereof selected for the purposes of illustration and shown in the accompanying drawing in which:  
     [0016]FIG. 1 illustrates a plan view of an assembled solar collector, not including an outside enclosure according to the present invention.  
     [0017]FIG. 2 illustrates a sectional view taken through section A-A, shown in FIG. 1 and including elements of the outside enclosure according to the present invention.  
     [0018]FIG. 3 illustrates a sectional view taken through B-B of FIG. 1.  
     [0019]FIG. 4 illustrates a sectional view of a molded reflector element according to the present invention and taken through section C-C shown in FIG. 5.  
     [0020]FIG. 5 illustrates a plan view of the molded reflector element according to the present invention.  
     [0021]FIG. 6 illustrates a sectional view of a two piece press mold according to the present invention and taken through section D-D of FIG. 7.  
     [0022]FIG. 7 illustrates a partially cut away front view of the two piece mold according to the present invention;  
     [0023]FIG. 8 illustrates a gang press mold according to the present invention and shown in isometric;  
     [0024]FIG. 9 is an isometric view of a clamp used to secure individual reflector elements to a base;  
     [0025]FIG. 10 illustrates a hydraulic piston assembly according to the present invention;  
     [0026]FIG. 11 Illustrates an adjustable stand for supporting a solar collector at an appropriate angle of incidence with respect to the sun, according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0027] Referring now specifically to FIG. 1, there is illustrated an assembled solar collector in plan view, referred to generally by reference numeral  10 . The solar collector  10  as illustrated in FIG. 1 does not include an outside enclosure to better show the detail of the elements of the present invention, however, an outside enclosure will be detailed below. The solar collector  10  comprises a base  20 , which provides the necessary structural support required for supporting a plurality of rows of reflective elements  60 , which are assembled onto the base  20 . The base  20  also supports, four hydraulic pistons  320  which are provided near each corner of the base  20  and are used to movably support a working conduit  50  above the rows  70  of reflective elements  60 . The working conduit  50  comprises a plurality of hollow tubes  300  connected together by a plurality of connecting tubes  330  for carrying working fluid through the solar collector  10 . The base  20  also provides a mounting surface which may be used to mount the solar collector  10  onto a roof or other structure or to support the solar collector on an adjustable support frame  400 , shown in FIG. 11. The support frame  400  can be adjusted to incline the collector  10  at various angles to increase expose to the sun.  
     [0028] The Enclosure  
     [0029] The present invention may be constructed with or without an outside enclosure. The enclosure, referred to generally by reference numeral  216 , is provided to reduce convective heat lose from the working conduit  50  and the working fluid passing therethrough. Fastened to or formed integral with the base  20  are opposing enclosure side members  214 , shown in FIG. 2, which extending vertically up from the base to form two sides of a four sided enclosure  216 . Similarly, opposing enclosure end members  215 , shown in FIG. 3, form the other two sides of the four sided enclosure  216 . Each side or end member  214 ,  215  may be formed from any sheet metal, e.g. from a weather resistant stainless steel or from anodized aluminum or from any otherwise weather resistant material such a weather treated steel. In one embodiment, the side and end members  214 ,  216  may include a 90 degree bent flange  217  which is fastened to the base  20  by rivets, spot welds or by another suitable means for fastening. In another embodiment, each side or end member may be formed integral with the base by notching the base  20  and bending the side and end members  214 ,  216  vertically up from the base to form a four sided enclosure. Each side or end member  214 ,  216  may also include a feature at a top edge thereof for supporting a transparent enclosure top  220 . The enclosure top  220  seals the top side of the enclosure while still allowing sun light to enter the enclosure  216 . The enclosure top  220  may be formed of PLEXIGLAS, or the like, and should be formed from a material which will not loose transparency after long term exposure to sun light and which will have good weather resistance, e.g. to extreme heat and cold as well as resistance to scratching. One such material is LEXAN ABC manufactured by General Electric Corporation of New York.  
     [0030] In a preferred embodiment, shown in FIGS. 2 and 3, a bracket  222  is fastened to each side and end member  214 ,  216  for supporting the enclosure cover  220  which may be bonded or otherwise fastened to the bracket  222 . Alternately, the bracket  222  may comprise a four sided frame (not shown) for capturing the enclosure top  220  on four edges thereof. The frame may be fastened to one of the side or end members  214 ,  216  by a hinge so that the entire transparent top  220  can be opened to provide access to the inside of the enclosure  216 , or, the four sided frame and cover  220  may be fastened to the side and end members  214 ,  215  to seal the enclosure  216 . In addition, a moisture seal  225  may also be provided to seal the region between the cover  220  and the enclosure  216 .  
     [0031] It is also noted that the base  20  may be made larger to enclose more area. This is advantageous if it is desired to completely enclose the solar collector as well as the all of the working plumbing associated with pumping the working fluid to and from the collector  10 . Such an enclosed unit is shown for illustrative purposes in FIG. 11.  
     [0032] Referring now to FIGS.  1 - 5 , mounted to the base  20  are a plurality of substantially identical reflector elements  60  arranged in a plurality of rows  70  such that each row  70  includes a longitudinal axis  72 . In a preferred embodiment, each reflector element  60  is substantially identically manufactured by a molding, casting or other forming process, or the like, to reduce the manufacturing cost, to take advantage of low cost raw materials and to take advantage of the molding process for, forming the shape of the reflective surface  140 . Each parallel row  70  comprises three reflective elements  60  arranged as a first reflector element  62 , a second reflector element  64  and a third reflector element  66 , such that second reflective element  64  is positioned between the first and third reflective element  62  and  66 , in the row  70 , although, as few as one, or many more than three reflective elements  60  may be used to form each row  70 . Each reflective element  60  includes a longitudinal axis  71  such that when the reflector elements  62 ,  64 , and  66  are positioned end to end to form the row  70 , the longitudinal axis  71  of each of the three reflector elements ( 62 ,  64 ,  66 ) are substantially co-aligned with the row longitudinal axis  72 .  
     [0033] Between the base  20  and the rows  70  there may also be provided an insulating layer  212  which reduces conductive heat loss through the base  20 . Such an insulating layer  212  may be formed e.g. from a single STYROFOAM layer or, from a layer of another insulating materials. If the base  20  is a molded or formed element, the insulating layer  212  may also be incorporated into the base  20 , e.g. if the base  20  is formed using a fiberglass or plastic material. A molded base  20  has certain advantages over e.g. a sheet metal base since the insulating layer  212  as well as other features such mount elements may also be molded or otherwise incorporated with the base  20 . However, the cost of a molding tool is high.  
     [0034] Working Conduit  
     [0035] Supported above each row of reflective elements  70  is the working fluid conduit  50  comprised of the plurality of hollow tubes  300  and a plurality connecting tubes  330  for interconnecting the hollow tubes  300 . Each hollow tube  300  is preferably formed from a round copper tube, e.g. a 1 inch in diameter tube. Alternately, the hollow tubes  300  may also be formed from a black polycarbonite plastic tubing or other tubing materials suitable for carrying a working fluid therethrough. Each hollow tube  300  is supported above the longitudinal axis  72  of a single row  70  and spans across the entire length of the row  72 . The hollow tubes  300  are supported at two ends thereof by two tube support brackets  315 . A single tube support bracket  315  is shown partially cut away in FIG. 2 and both support brackets  315  are shown in end view in FIG. 3. Each support bracket  315  includes a plurality of mounting features  325  for receiving and supporting the hollow tube  300  therein. The mounting features  325 , which in the present embodiment are shown as a plurality of cut-out V shaped notches in FIG. 2, serve to locate each end of the hollow tubes  300  above the center of one of a row  70 . Each tube  300  is fastened to the support brackets  315  by appropriate fastening means such as a clamp or by a snap fit with the mounting feature  325  or by another suitable fastening member.  
     [0036] Each bracket  315  comprises an L-bracket of sufficient thickness so as to provide the appropriate stiffness to support each of the hollow tubes  300  while being supported only at opposing distal ends by two hydraulic pistons  320 . Two brackets  315 , positioned one at each end of the rows  70 , are each supported by a pair of hydraulic pistons  320 . The four hydraulic pistons  320  movably support the two support brackets  315  such that the brackets  315  and the hollow tubes  300 , supported by the brackets  315 , are movable to adjust the distance between the hollow tubes  300  and the reflector elements  60 . As will be detailed below, the four hydraulic piston  320  move in unison to raise and lower the entire working fluid conduit with respect to the reflector element rows  70 .  
     [0037] In order to provide a continuous fluid conduit, the connecting tubes  330  are fitted onto the ends of adjacent hollow tubes  300  to allow the fluid to flow from one hollow tube  300  to an adjacent tube  300  through the connecting tube  330  which connects the adjacent tubes together. The connecting tubes  310  are sized to snugly fit over each hollow tube  300  to prevent the working fluid from leaking. Alternately, hose clamps may be used to further prevent leaking. As shown in FIG. 1, an inlet hose or line  450  is provided between a cold working fluid flow inlet  30  and a first hollow tube  460  to pump a cold working fluid from the pump high pressure line  510  through the solar heater  10 . The cold working fluid is heated by sun light which is reflected onto each hollow tube  300  by each of the reflector elements  60  as the working fluid passes through each hollow tube  300  suspended across each row  70 . An outlet hose  470  exits from a last hollow tube  480  to transport the, now heated, working fluid from the solar collector back to a working fluid reservoir which in the preferred embodiment is a swimming pool. A check valve  350  is provided at the inlet  30  to prevent the working fluid from reversing direction and a pressure relief valve  360  is provided at the outlet hose  40  to allow the working fluid pressure to be relieved, e.g., if the working fluid pressure should exceed about 160 to 180 pounds per square inch. This precaution prevents damage to the solar collector should the pressure of the working fluid exceed safe limits. Additional gate valves or the like may also be provided in the flow inlet  30  or the flow outlet  40  as required.  
     [0038] In the preferred embodiment, the pump  340  may be a swimming pool filter pump which removes water from the swimming pool at a low pressure line  500  and pumps the water through a filter element  520 , the solar collector  10  and then through the outlet line  40  to return the water to the swimming pool. Alternately, a direct return line may also be provided at the filter so that a portion of the water entering the pool filter is returned to the pool directly while the remaining portion of the water leaving the pool filter passes through the solar collector  10 . The portions may be adjusted according to the heating capacity of the solar collector or according to the need for heating.  
     [0039] Hydraulic Pistons  
     [0040] Also provided are two hydraulic piston flow circuits for providing high pressure water to each of the hydraulic pistons  320 . Two inlet lines  370  and  371  connect between the flow inlet  30  and the two hydraulic pistons  320  located at a back side  550  of the solar collector  10 , as shown in FIG. 1. Piston connecting lines  372  and  373  connect the two pistons  320  at the backside  550 , to the two piston  320  at a front side  555  of the solar collector  10 . A return line  370  connects from the two pistons  320  at the front side  555  to the flow outlet line  40 . The pump  340  therefore forces high pressure working fluid through the hydraulic piston flow circuit and into each of the four hydraulic pistons  320 . Each piston  320  is therefore maintained at the pressure of the high pressure pump line  510  as long as the pump  340  is operating. Working fluid at the pressure of the high pressure pump line  510  within each piston  320  raises each piston to its full height thereby raising the entire working conduit to a focal axis of the reflector elements  60 , as will be detailed below, thereby providing the maximum heating effect. Should the pump be turned off, or fail, each piston  320  will lower to a low pressure position thereby lowering the working conduit out of the focal axis of the reflective elements  60  to reduce the amount of solar energy reflected onto the hollow tubes  300 .  
     [0041] A hydraulic piston  320  is shown in cross-section in FIG. 10. Each hydraulic piston  320  comprises a movable piston  375  contained within a cylinder housing  380 . The cylinder housing  380  includes a conduit section  385  for receiving working fluid from the piston conduit  385  and a chamber  390  for receiving high pressure working fluid from the conduit section  385 . A flexible bladder  395  is supported within the chamber  390  and seals the chamber  390  from the conduit section  385  such that as the working fluid pressure within the conduit section  385  increases, the working fluid forces the bladder into the chamber thereby forcing a lower end of the piston  375  upward. Each piston  375  is connected at an upper end thereof to one of the support points  310  for supporting the support brackets  315 . A compression spring  400  surrounds the piston  375  and is captured between a flange  410  formed on the piston, and a flange  415  formed on the cylinder housing  380 . The compression spring  400  forces the piston downward when the fluid pressure in the chamber is below a set pressure. A bottom side  396  of each hydraulic piston  320  is attached to the base  20 . Of course a simple support system such fixedly attaching each of the support brackets  315  to the base  20  such that the working conduit  50  is fixedly supported at the focal axis each reflective element  60  may also be used without deviating from the present invention. Alternatively, electronic solenoids may also be used in place of the hydraulic pistons  320  to electro-magnetically support each of the hollow tube  300  and to lower the hollow tubes  300  when the pump is not operating by using an electronic controller.  
     [0042] In order to improve the thermal absorption properties of the fluid conduit, the surface of the hollow tubes  300  is coated with a black paint or another coating having good thermal absorption properties, e.g. a dendrite coating or other black roughened coating may be used. Likewise, the connecting hoses  330  and the hoses  450  and  470  may also be provided in a black polycarbonite or other black plastic tubing to improve thermal absorption of the working fluid conduit  50 . To further improve performance, each hollow tube  300  is formed from a 1 inch diameter copper tubing having a wall thickness of e.g. 0.06 inches and each copper tube is partially formed to an elliptical shape over the portion of the tube length which is suspended above the row  70 . This leaves a short round portion  560  at each end of the hollow tube  300  to allow mounting of the connecting tubes  330  and to provide a round mounting area for easy mounting with the support bracket  315 . The long axis of the ellipse is supported substantially parallel with the base  20  as is shown in FIG. 4. This feature of the invention serves to place most of the fluid carrying conduit in the plane of the focal axis where the reflected sun light is more highly concentrated. In order to form each tube  300  into an elliptical shape, the tubes may be rolled between formed rollers. Each tube  300  may also be filed with sand during the rolling process to prevent cracking during the rolling.  
     [0043] Reflective Element  
     [0044] Referring now to FIGS. 4 and 5 there are illustrated a side view of a single reflective element  60 , in FIG. 5, and a sectional view of the reflective element  60  taken through section C-C of FIG. 5, shown in FIG. 4. The reflector element  60  includes a hollow cavity  90  on a bottom side thereof, referred to generally by reference numeral  95 . The hollow cavity  90  is open at the bottom side  95  and enclosed by two side walls  100  and two end walls  110  which together form an edge  92  adjacent to the bottom side  95 , for supporting the reflector element on the base  20  or other layer. The hollow cavity  90  is further bounded by a top wall  120 . Top wall  120  is opposed to the bottom side  95  and is formed to provide a reflective surface  140  facing away from the hollow cavity  90 . The top wall  120  is supported by each side wall  100  and by each end wall  110  such that the longitudinal axis  72  of the reflective element  60  is substantially parallel with the side walls  100  which are longer than the end walls  110 .  
     [0045] The reflective surface  140  is formed to reflect sunlight incident onto the reflective surface  140  to a focal axis  132 , which is substantially parallel with the longitudinal axis  72  of the reflective element  60 . As shown in FIG. 4, the reflective surface  140  has a constant radius R about a locus point  130 . In the case where R is constant the reflective surface  140  is cylindrical with a focal axis  132  which is equidistant between the locus axis  130  and the reflective surface  140  such that if the radius R=4.0 inches, the focal axis  132  lies 2.0 inches above the vertex of reflective surface  140 . If a width of the reflective element  60  is approximately 4.0 inches and the radius R is also 4.0 inches, the subtended angle of the reflective surface  140  is approximately 53 degrees. It is noted that by using a cylindrical reflective surface  140  the focal axis of a cylindrical surface will be slightly blurred which can result in less effective heating.  
     [0046] In a preferred embodiment, a parabolic shaped reflective surface  140  is provided. Although a parabolic reflective surface is only slightly altered from a cylindrical surface having the same focal axis, the parabolic surface provides a finer focal axis  132  thereby concentrating more energy onto the hollow tube  300 . Since it is more difficult to fabricate a parabolic surface than a cylindrical surface the cylindrical surface is often used as a close approximation. However, in the present invention each reflective element  60  is molded and the problem of fabricating a parabolic surface need only be addressed once when forming the mold surface. Once the mold is formed to the parabolic shape, each reflective element may then be replicated in the form of a parabolic cylinder.  
     [0047] The points of a parabolic curve in an X-Y coordinate system are given as the set of points M(X, Y) where 
       Y   2 =2 pX   (1) 
     [0048] For a parabolic surface P, a focal axis F is positioned at the point F=(p/2, 0). Thus by setting p=4 inches, a focal axis of the parabolic surface P is positioned at F=(2, 0) or 2.0 inches above the vertex of the parabola just as was given for the cylindrical surface shown in FIG  6 .  
     [0049] Holddown Clamp  
     [0050] The reflective surface  140  is coated to reflect solar energy received thereon to the focal axis  132 , which in the present examples is 2.0 inches above the vertex of the reflective surface. With a width of approximately 4.0 inches each, row of reflective elements  60 ,  61 ,  62  is placed on the base  20  with its side walls  100  parallel to the side walls of an adjacent row so that the rows  70  are spaced slightly more than 4.0 inch centers. Between adjacent rows  70  are mounted sheet metal clamps  200 , shown in the sectioned side view of FIG. 3 and shown in isometric view in FIG. 9. Each clamp  200  is attached to the base  20  by three mounting features  208  which in the present example interlock with the base  20  by a snap fit by applying a downward force to force the locking features  208  into slots provided in the base  20 . The clamp  200  has a length  202  which is substantially equal to the length of a row  70 . The clamp  200  include a plurality of bendable tabs  204  which when the clamp  200  is mounted to the base  20  protrude above the reflective element reflective surfaces  140 . The tabs  204  are then bent over the tops of each reflective element  60 ,  61 ,  62  to hold the elements against the base  20  or the insulating layer  212 , if present. In the case where an insulating layer  212  is provided, each clamp  200  is passed through slots provided in the insulating layer  212 . The clamp  200  provides a total of 12 bendable tabs  204  for holding one edge of six reflective elements, three in a first row  70  and three in an adjacent row  70 . Adjacent tabs  204  are bent in opposite directions to secure one row the first row of reflective elements by the tabs  204  bent in one direction and the second row of reflective elements by the tabs bent in the opposite direction. Each reflective element  60 ,  61 ,  62  is therefore clamped by four bendable tabs  204  which are positioned at each corner of the element. One clamp  200  is also positioned at end of the outer most rows  70  such that the end clamps hold only one row of reflective elements in place. To assemble the rows  70 , the clamps  200  are secured to the base  20  and three reflective elements  60  are positioned onto the insulating layer in a row between opposing clamps  200  and the clamp tabs  204  are bent over to secure each reflective element in place. It is also noted that the reflective elements are easily replaceable if required.  
     [0051] In another embodiment, the clamps  200  may be formed from the base  20  by notching the base  20  to notch out a clamp  200  such as the one shown in FIG. 9 and then bending the clamp  200  by 90 degrees from the base  20  to extend the clamp  200  upward from the base  20 . The reflector elements  60  may then be installed between rows of clamps and secured by bending the tabs  204  over the reflective surfaces  140  as described above. In this case it may be advantageous to apply sections of an insulating layer  212  between the rows of bent up clamps  200  instead of a sheet of insulating layer.  
     [0052] Adjustable Frame  
     [0053] The solar collector  10  may also be mounted onto an adjustable frame  600  shown in FIG. 11. The frame  600  can be used to orient the solar collector  10  to face the sun and set at an angle with respect to the sun which provides maximum heating capacity, e.g. by setting the optimal angle at high noon or optimized at another angle for another condition, e.g. for receiving early morning sun. The frame  600  includes a supporting base  610  having two support members  620 , mounted on pivoting feet  615 , and a plurality of cross members  630  for stiffening the support base  610  and for securing to and supporting the base  20 . A pair of adjustable support legs  635  are pivotally mounted to the support base  610  and also mounted on pivoting feet  615 . At least one cross brace  640  is attached to each of the adjustable support legs  635  near the lower end thereof such that the support legs  635 , the cross brace  640  and the support base  610  form a four sided frame for supporting the solar collector  10 . Each adjustable support leg  635  includes a slot  645  passing therethrough. Two pivot pins  650  each mount to the support base  610  and pass through each slot  645  in the adjustable legs  635 . A locking device  655  attaches to each pivot pin  650  to lock the pivot pin  650  at any desired position along the slot  645  such that the solar collector  10  may be supported at a plurality of inclination angles. One type of locking device  655  may include providing a threaded portion on the ends of the pivot pins  650  and providing a locking nut threaded onto the pivot pin  650  such that when the nut is tightened the adjustable support legs  635  are clamped against the base support  610  to secure the base support  610  at a desired inclination angle. Of course other locking mechanisms and adjustable frame assemblies may be provided without deviating from the present invention.  
     [0054] To assist in selecting an inclination angle, a target  660  is provided on the transparent top  220  and a matching target is provided directly below the target  660  on the base  20 . To set the desired inclination angle of the adjustable frame  600  the shadow of the target  660  is observed on the base  20 . At the appropriate time of day the inclination angle and direction of facing the solar collector  10  is set by adjusting the position of the collector until the shadow of the target  660  provided on the transparent top  220  is positioned directly over the matching target provided on the base  20 .  
     [0055] Fabrication of the Reflector Elements  
     [0056] In order to fabricate each of the reflective elements  60 , there is provided a press mold  74 , shown in FIGS. 6 and 7, having a bottom section  76  and a top section  78 . Each press mold section,  76  and  78  may be formed from a soft material such as plaster of Paris, or the like, of from a harder material such as wood or metal. The bottom section  76  includes a protruding male portion  80  which has the same shape and form of the hollow cavity  90  and which is used to shape the reflector element  60  at the bottom side  95 , thereof. The side walls  82 , of the male portion  80 , are used to form the side walls  100  of the reflector element  60  and are formed with a draft angle  84  which allows the formed reflector element  60  to be easily removed from the press mold  74 . The end walls  86  of the male portion  80  may also be formed with a draft angle and are provided to form the end walls  110  of the reflective element  60 . The press mold top, or female section  78  is used in cooperation with the bottom section  76  and includes a parabolic surface  88  which forms the parabolic reflective surface  140  of the reflective element  60 . The mold parabolic surface  88  may be polished so as to provide a smooth uniformly parabolic surface to the reflective element reflective surface  140 , thereby improving the reflective properties thereof. The top section  78  also includes end surface  89  which cooperates with the end walls  86  of the bottom section  76  to form the end walls  110  of the reflective element  60 .  
     [0057] In order to fabricate a single reflective element  60 , a layer of substantially uniformly thick clay, such as pottery clay or the like, is applied over the bottom section  76 . Such a clay is usually comprised substantially of hydrated silicates of aluminum which is plastic when wet but which becomes a hard ceramic when heated or fired in an oven. To form a clay layer, a clay brick may be rolled into a uniformly thick sheet prior to applying it to the press mold  74 . Once applied over the bottom section  76 , the top section  78  is pressed onto the clay layer with sufficient force to form the clay layer into the shape of the reflective element  60 . The top section  78  is then removed and the formed clay is removed from the bottom section. The press mold  74  may be coated with a mold release layer such as mold soap or the like.  
     [0058] Alternately, a plurality of reflective elements  60  may be formed in a gang mold  89  which includes a bottom section  81  and a top section  83 . The top and bottom sections  81 ,  83  each include a plurality of mold portions  85  having substantially identical characteristics as the single press mold  74 , described above and shown in FIG. 6 and  7 . In this case, a substantially uniform layer of clay may be applied over the entire bottom section  81  of the gang mold  89  with the top section  83  being pressed onto the bottom section  81  thereby forming a plurality of reflective elements simultaneously.  
     [0059] Similarly, one or a plurality of reflective elements  60  may be formed by a slip casting method which is well known. In this case, two mold section having substantially similar characteristics for forming a reflective element  60  as are described for press mold  74 , above, are clamped together and configured with a series of gates through which a liquid clay or slip is poured or pumped into the slip mold such that the liquid clay completely fills each mold cavity thereby forming one or more reflective elements  60  from the liquid clay contained between the mold sections. The liquid clay is then allowed to dry either by air drying or by heating or baking the mold sections in an oven. Once dry, the reflective elements  60  are removed from the mold and may be cleaned of excess clay in the gate areas and separated into individual clay reflective elements  60 .  
     [0060] Once formed, whether by press molding or slip casting, each clay reflective element  60  is heated of fired in a ceramic furnace or oven by techniques which are well known. One example of a firing cycle includes heating the clay reflective elements to a temperature of 1350 degrees Fahrenheit and then slow cooling the elements over a 12 hour period. This cycle may be repeated several time to complete the firing.  
     [0061] In addition to firing, the clay reflective elements are also glazed by applying a liquid clay over the surface of each clay element by spaying, dipping or brushing the liquid clay onto the surfaces of each clay reflective element  60 . The glazing is used to fill any surface voids in the formed elements and to decrease the porosity of each of the element surfaces. The glazing may also be used for coloring. After application of the glaze, the element is again heated or fired in order harden the liquid clay applied on the surfaces.  
     [0062] By firing and glazing the clay elements, each element is transformed into a ceramic reflector element  60 . The ceramic element has excellent properties for use in a solar collector. Such properties include a low coefficient of thermal expansion a low coefficient of thermal conductivity, good resistance to moisture and ultraviolet radiation, excellent mechanical stiffness and little or no maintenance required over the product life. In addition, the raw material, clay, is readily abundant and low in cost. Of course other methods of forming ceramic reflective elements from clay or of forming reflective elements from other plastic materials such as plastics, fiberglass or the like may also be used without deviating from the present invention. However, a clay reflector element  60  is impervious to moisture and will not crack, warp or significantly change its dimensions or shape over temperature extremes caused by prolonged exposure to the sun or by snow and ice. Furthermore, the elements may be easily replaced if damaged.  
     [0063] Once the ceramic reflective elements  60  have been formed, the parabolic reflector surface  140  is then coated with a reflective layer which may comprise a paint which is brushed or sprayed onto the surface  140 . In a preferred embodiment, a metalized layer may be deposited onto the surface  140  such as by electro-plating, vacuum deposition or by other well know metal deposition techniques. In another embodiment, a reflective MYLAR layer may also be bonded onto the surface  140 . Such a MYLAR layer is available in sheets with a highly reflective layer on one surface and an adhesive backing on an opposite surface from 3M Corporation of Minnesota. Ideally, the reflective layer deposited onto the surface  140  will have very high reflectivity and low absorption throughout the full range of infrared wavelengths. In addition the reflective layer should maintain its reflective properties throughout the product life without being significantly affected by environmental elements. In addition, the reflective layer should be scratch and chip resistant. In the preferred embodiment, a layer of metal aluminum is applied onto the reflective surface  140  using a vacuum deposition technique. In this case, one or more reflective elements  60  are placed in a vacuum chamber with each element  60  having a high electrical charge, e.g. a negative charge applied thereto. At the same time, aluminum vapor having a highly opposite charge, e.g. positive, is introduced to the vacuum chamber. In this way a uniform layer of aluminum is applied onto the reflective surface  60  while the other surfaces of the reflective element are masked or otherwise blocked from receiving the aluminum coating.  
     [0064] It will also be recognized by those skilled in the art that, while the invention has been described above in terms of preferred embodiments, it is not limited thereto. Various features and aspects of the above described invention may be used individually or jointly. Further, although the invention has been described in the context of its implementation in a particular environment, and for particular applications, e.g. an infrared sensor assembly and an infrared video camera system, those skilled in the art will recognize that its usefulness is not limited thereto and that the present invention can be beneficially utilized in any number of environments and implementations. Accordingly, the claims set forth below should be construed in view of the full breadth and spirit of the invention as disclosed herein.