Abstract:
The invention provides a solar collector comprising a trough-like reflector for receiving solar rays and for concentrating the rays in a direction generally transverse to the length of the reflector between its ends. Concentrator means is provided for receiving the concentrated rays from the trough-like reflector and for concentrating the rays in one or more of a direction generally along said length and a direction generally transverse to said length.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This is a non-provisional U.S. application claiming priority from U.S. Provisional Patent Application No. 60/893,275 filed on Mar. 6, 2007, the disclosure of which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to solar collectors, and in particular to solar collectors which collect and concentrate solar rays. 
       BACKGROUND OF THE INVENTION 
       [0003]    Solar collectors for collecting solar energy generally fall into one of two categories: concentrating and non-concentrating. Concentrating solar collectors typically comprise a reflector for reflecting and concentrating received solar radiation towards an absorber. The absorber may include a conduit for carrying a heat transfer fluid for absorbing solar thermal energy and/or an array of photovoltaic cells for converting solar energy into electrical energy. The reflector is either in the form of a circular dish with the focal position above the center of the dish, or a trough-like, parabolic reflector which produces a line focus along the length of the reflector. In the latter case, the absorber typically comprises a radiation absorbing tube positioned centrally above the reflector and extending along its length. 
         [0004]    Focussing or concentrating solar collectors typically require some type of sun tracking mechanism and tracking control system to vary the orientation of the collector to maintain the focal position of the solar radiation of the absorber surface. Non-focusing solar collectors generally comprise flat, solar absorbing panels which are fixed in position and do not actively track the sun. 
         [0005]    An example of a trough-like solar collector system is disclosed in WO 2005/090873. The solar collector comprises a parabolic trough-like reflector having a longitudinal absorber positioned above the reflector and mounted thereon by means of a central support upstanding from the reflector. The reflector includes spaced apart ribs fixed to the underside of the reflector panel to help maintain the shape of the reflective surface. The absorber comprises a longitudinal plate having a radiation absorbing surface which may include an array of solar cells mounted thereon. A conduit is positioned adjacent the back of the plate for transferring solar thermal energy into a heat transfer fluid. Transparent panels extend from each side of the absorber to opposed longitudinal edges of the reflector to protect the reflective surface from weathering and to provide additional structural rigidity. 
       SUMMARY OF THE INVENTION 
       [0006]    According to an aspect of the present invention, there is provided a solar collector comprising a trough-like reflector for receiving solar rays and for concentrating the rays in a direction generally transverse to the length of the reflector between its ends, and concentrator means for receiving the concentrated rays from the trough-like reflector and for concentrating the rays in one or more of a direction generally along said length and a direction generally transverse to said length. 
         [0007]    According to another aspect of the present invention, there is provided an asymmetric solar concentrating trough based system, having means for actively tracking the sun on two axes; elevation ( 1 ) with individual troughs and collectively with an array of troughs tracking on azimuth ( 2 ) with a primary ( 3 ) and secondary ( 4 ) mirror for concentrating the sun on two axes. 
         [0008]    According to another aspect of the present invention, there is provided a symmetric solar concentrating trough based system, having means for actively tracking the sun on two axes; elevation ( 1 ) with individual troughs and collectively with an array of troughs tracking on azimuth ( 2 ) with a primary ( 3 ) and secondary ( 4 ) mirror for concentrating the sun on two axes. 
         [0009]    According to another aspect of the present invention, there is provided a two stage reflective solar concentration system where a first primary optical concentration reflector ( 3 ) is a two dimensional symmetric or asymmetric parabolic trough and second optical concentration stage ( 4 ) is a three dimensional modified paraboloid; both designed in combination so as to provide a concentration ratio in the range from about 80 to about 10,000 suns, or more. 
         [0010]    According to another aspect of the present invention, there is provided a two stage concentration system with a third reflective or refractive (e.g. pyramidal frustum) optic stage ( 5 ) designed to accept the concentrated sunlight rays ( 14 ) and mix them with multiple bounces so as to produce a substantially uniform illumination on the target surface within about ±10% to ±30% maximum average illumination levels. 
         [0011]    According to another aspect of the present invention, there is provided a solar concentrating receiver wherein heat is carried away from the concentrated solar area ( 6 ) by heat transfer fluid ( 7 ) running longitudinally through the receiver in close proximity to the focal line ( 8 ) of the secondary 3D paraboloid ( 4 ). 
         [0012]    According to another aspect of the present invention, there is provided a solar concentrator or receiver wherein a high efficiency multi-sun solar cell ( 9 ) is placed at the solar focus area ( 6 ) to simultaneously produce heat and electricity. 
         [0013]    According to another aspect of the present invention, there is provided a solar concentrator receiver wherein a “cold” mirror ( 4 ) is used as the second stage mirror to remove solar radiation at least one of below about 400 nm and above about 700 nm, allowing substantially only the visible light ( 10 ) only to pass through and where a translucent (fiber) optic light conductor ( 13 ) is placed at or near the solar focus area ( 6 ) allowing the transmission of visible light into buildings and/or areas requiring light. The cold mirror prevents heat (infrared (IR) solar radiation) and plastic damaging (Ultraviolet (UV) Solar wave lengths) from entering the fiber optic light conductor ( 13 ), and acts in an analogous way to a band pass filter in the electronics field.) 
         [0014]    According to another aspect of the present invention, there is provided a solar concentrating receiver wherein one or more translucent lens(es) ( 11 ,  12 ) (e.g. planar or focusing translucent plate(s)) are placed in the solar collection beam of light to remove the IR and/or UV solar radiation. 
         [0015]    According to another aspect of the present invention, there is provided a solar concentrating receiver wherein either the cold mirror ( 4 ) or translucent lens ( 11 ,  12 ) are thermally interconnected to one or more UV and/or IR filters to efficiently and simultaneously capture the heat and focus the light into the fiber optic light conductor ( 13 ). 
         [0016]    According to another aspect of the present invention, there is provided a solar concentrator receiver wherein one or more thermal collection path(s) are thermally insulated with a thermal insulating material, e.g. mineral wool or similar high temperature, preferably, non-moisture absorbing insulation. 
         [0017]    According to another aspect of the present invention, there is provided a solar receiver wherein one or more of the fluid path extrusion ( 15 ) and the receiver cover ( 16 ) (if any) are continuous over the length of the primary mirror ( 3 ); and the secondary reflector or concentrator ( 4 ) and the secondary reflector cover ( 16 ) (if any) and the optical mixer ( 5 ) and optical mixer extrusion ( 17 ) are segmented in shorter sections so as to help keep precise alignment between the secondary reflector ( 4 ) and the mixer ( 5 ) during fluid path extrusion ( 15 ) heat up and cool down from about −40 to +100° C. for example, or any other operating temperature range. 
         [0018]    In the above aspects of the invention, reference numbers in parentheses refer to features of the drawings, which are for illustrative purposes only and in no way limiting of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    Examples of embodiments of the present invention will now be described with reference to the drawings, in which: 
           [0020]      FIG. 1A  shows a side view and part sectional view of a solar collector according to an embodiment of the present invention; 
           [0021]      FIG. 1B  shows a front view of the solar collector of  FIG. 1A ; 
           [0022]      FIG. 2A  shows a perspective view of an array of solar collectors according to an embodiment of the present invention; 
           [0023]      FIG. 2B  shows a side view of the solar collector array shown in  FIG. 2A ; 
           [0024]      FIG. 2C  shows a front view of the solar collector array shown in  FIG. 2A ; 
           [0025]      FIG. 2D  shows a top view of the solar collector array shown in  FIG. 2A ; 
           [0026]      FIG. 3  shows a perspective view of a solar collector according to another embodiment of the present invention; 
           [0027]      FIG. 4  shows a perspective, part sectional view of part of the solar collector shown in  FIG. 1 ; 
           [0028]      FIG. 5  shows a perspective view of part of the solar collector shown in  FIG. 4 ; 
           [0029]      FIG. 6  shows part of the solar collector shown in  FIG. 5 ; 
           [0030]      FIG. 7  shows a perspective view of part of the solar collector shown in  FIG. 4 , and further including an optical waveguide; 
           [0031]      FIG. 8  shows a graph of the spectrum of solar radiation versus energy; 
           [0032]      FIG. 9  shows a perspective view of part of a solar collector according to an embodiment of the present invention; 
           [0033]      FIG. 10  shows an example of a radiation mixer and graphs of irradiance as a function of area; 
           [0034]      FIG. 11  shows an example of the geometry of an asymmetric solar collector according to an embodiment of the present invention and examples of the trajectories of solar rays; and 
           [0035]      FIG. 12  shows a cross-sectional view through a mixer according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0036]    Referring to  FIGS. 1A and 1B , a solar collector comprises a trough-like primary reflector  3  for receiving solar rays, a secondary reflector  4  spaced from the primary reflector  3  for receiving solar rays reflected from the primary reflector, and a receiver  20  for receiving solar rays reflected from the secondary reflector  4 . In this embodiment, the solar collector geometry is asymmetric, although in other embodiments, the geometry may be symmetric. 
         [0037]    The primary reflector is shaped to concentrate the reflected radiation towards a focal line  21 , which may be positioned in front of the secondary reflector or mirror  4 , although in other embodiments, the focal line or position may be generally located behind the secondary reflector. The surface of the secondary reflector is curved also to concentrate the solar radiation in a first direction, e.g. x-direction shown by the arrow  22 . The secondary mirror  4  is also curved in an orthogonal direction, along the z-direction (into the page of  FIG. 1 ) to concentrate light in the z-direction (i.e. longitudinal direction). The receiver  20  further includes an optical distributor  24  (e.g. mixer) for receiving concentrated solar radiation from the secondary reflector  4  and more uniformly distributing the solar radiation over a predetermined area. An alternative or additional function of this device is to assist in increasing the amount of received solar radiation from the secondary reflector reaching a predetermined surface. The device may comprise a reflective or refractive element or a combination of both. An embodiment of a reflective version of the device, shown in  FIG. 5 , comprises an element having tapered side walls  25 ,  27 ,  29 ,  31 , and having a first, receiving end  33  defining an aperture for receiving solar rays from the secondary mirror  4 , the side walls tapering inwardly towards an opposite end  35  and defining an area  37 . A solar converter may be positioned at the opposed end  35  for converting solar radiation into electrical energy. The size and shape of the light receiving surface of the converter may substantially correspond to the size and shape of the area  37  so that substantially all available concentrated radiation from the secondary reflector and which enters the distribution device impinges on the converter. The distribution device may have any suitable shape, size and geometry. In the present embodiment, the distribution device  5  is frustopyramidal, having four flat tapered sides. In other embodiments, the distribution device may be conical, thereby forming a circular (or elliptical) area over which solar radiation is distributed, and which may be suitable, for example for circular solar cells. In other embodiments, the distribution device may have any number of sides, e.g. three, five, six, seven, eight, etc. 
         [0038]    In a refractive version of the distribution device, the device may comprise a prism of solid translucent material, e.g. glass or other suitable material and have any suitable shape as described above. 
         [0039]    Referring to  FIGS. 5 ,  6  and  7 , the receiver  20  further comprises a substrate  30  on which a number of distribution devices  5  may be mounted.  FIGS. 5 to 7  show a solar cell  32  mounted on the substrate  30  having a solar radiation collecting area  6  which is registered with the lower area of the distribution device. In this embodiment, the substrate  30  forms part of the wall of a fluid carrying conduit  40 , which in this embodiment has three fluid carrying channels,  41 ,  42 ,  43 . Heat absorbed by the substrate  30  via an optional solar cell  32  is transferred through the substrate to fluid, e.g. liquid flowing through the conduits to a suitable point of use, such as space heating and/or to provide a source of hot water for example. In other embodiments, any number of fluid carrying channels may be provided adjacent the substrate  30 . 
         [0040]    Referring to  FIG. 7 , the receiver includes a light pipe or optical waveguide  13  having a first end  45  which is positioned to receive sunlight from a distribution device  5  and has a second end  47  from which the light is emitted. The light transmitted by the waveguide  13  may, for example, be used to illuminate interior spaces in buildings or other spaces, where needed or desired. The optical waveguide may comprise any suitable material, for example a polymeric or plastic material, glass or any other suitable material. The optical properties of the material should preferably be such as to minimize any light escaping from the sides of the optical waveguide, for example, the refractive index of the material may be such as to provide total or almost total internal reflection. The optical waveguide may comprise a single unitary member or a plurality of individual waveguide members, e.g. a bundle of optical waveguides or fibres. The or each waveguide may have any suitable cross-sectional geometry, including rectangular or circular, and the lower end of the distribution device may be adapted to match the shape of the end  45  of the optical waveguide. 
         [0041]    A means may be provided for filtering one or more parts of the solar spectrum so that only selected wavelengths are admitted to the optical waveguide or other light receiver. Such means may include any one or more of a coating on the primary and/or secondary reflectors  3 ,  4  which selectively absorb certain wavelengths and reflect others, a lens positioned between the primary and secondary reflectors  3 ,  4 , a lens positioned between the secondary reflector and the entrance aperture of the distribution device  5  and/or a coating on the reflective surfaces of the distribution device or a lens between the entrance aperture and the bottom portion of the distribution device. 
         [0042]    Referring to  FIGS. 4 and 7 , the receiver includes a translucent member  49  which is positioned above each distribution device  5  and may engage with the upper peripheral edge defining the entrance aperture of the distribution device to assist in holding each distribution device in place, so that effectively, each distribution device is clamped between the translucent plate  49  and substrate  30 . The translucent plate or lens assists in protecting the receiver from the ingress of external elements such as atmospheric elements, e.g. moisture, dust and other particulate matter and also insects. The translucent plate  49  may also serve as a filter to filter out certain parts of the solar radiation spectrum. Portions of the spectrum which may be filtered using any one or more of the filtering means described above may include ultraviolet light and/or shorter wavelength radiation and/or infrared light and/or longer wavelength radiation. 
         [0043]    In some embodiments, the receiver may include a combination of a fluid conduit and one or more solar cells, without any optical waveguides. In another embodiment, the receiver may comprise a combination of a conduit and one or more optical waveguides in the absence of any solar cells, and in another embodiment, the receiver may include a combination of a conduit, one or more solar cells and one or more optical waveguides. In other embodiments, the receiver may include one or more solar cells in the absence of any conduit or optical waveguide and in other embodiments, the receiver may include one or more optical waveguides in the absence of any conduit or solar cells. 
         [0044]    Referring to  FIG. 1B  which shows a schematic front view of a solar collector of  FIG. 1A , the solar collector includes a plurality of arms or stantions  61  connected to the primary reflector structure along the edge thereof (or at any other suitable position) for supporting the receiver  20 . In this embodiment, the receiver comprises a continuous substrate  30  extending in the longitudinal (i.e. z) direction and which is connected to each stantion either directly or indirectly via a bracket  63  (shown in  FIG. 4 ). The secondary reflector  4  is divided into a plurality of discrete sections along the length of the solar collector, and each secondary reflector section may be connected to the substrate  30  via suitable brackets  65 . The sections  4   a ,  4   b ,  4   c  may be mounted to provide a gap  67  between the ends of adjacent sections to allow the adjacent ends to move towards and away from each other with thermal expansion and contraction. In use, the substrate  20  may be at a higher temperature than the secondary reflectors, and if made of a similar material, the substrate will expand more in the z-direction than the secondary reflector. The difference in movement in the z-direction between the secondary reflector and the substrate on which each distribution device  5  is mounted may be reduced by dividing the secondary reflector into discrete sections so that any differential displacement occurs over a limited length of the secondary reflector. Maintenance of alignment between the secondary reflector  4  and each distribution device is also assisted by connecting the secondary reflector to the substrate  20 . Advantageously, these features allow each secondary reflector associated with a distribution device to remain substantially aligned in the z-direction so that most of the available or substantially all light reflected and focussed by the secondary reflector in the z-direction, as indicated by the broken ray lines  69 ,  70 , are directed into the distribution device entrance aperture. The discrete sections of the secondary reflector may have any desired or predetermined length, and alignment between each secondary reflector and its corresponding distribution device may be improved as the length of each section decreases. In the illustrative embodiment of  FIG. 1B , each section  4   a ,  4   b ,  4   c  spans four distribution devices  5 , although in other embodiments, a section may span any other number of distribution devices, for example one, two, three, five, six or more. Referring to  FIG. 4 , the receiver includes upper, lower and rear housing panels or walls  73 ,  75 ,  77  which enclose the receiver elements, including the substrate, distribution devices and conduit. Insulating material may be provided within the housing in order to thermally insulate the fluid conduit, the substrate and any one or more other components of the receiver. 
         [0045]    In one embodiment, the housing panels and the fluid conduit may comprise extrusions which run continuously from one end to the other of a solar collector. In one embodiment, and with reference to  FIG. 4 , the receiver may include a channel member  79  for seating one or more distribution devices and which may be slidably coupled to the substrate  30  or capable of sliding relative thereto in the z-direction. The channel member may be formed by extrusion. The channel member may also be divided into discrete sections along the length of the receiver and each section may be associated with a corresponding secondary reflector section, for example as shown in  FIG. 1B . At least partially decoupling the mounting for one or more distribution devices from the substrate  30  may also assist in preserving alignment between each secondary reflector and its associated distribution device with changes in temperature. 
         [0046]    Referring to  FIG. 4 , the channel mounting has upper outwardly extending flanges  81 ,  83  for mounting a translucent panel, e.g. filter or lens thereon. The channel section and flanges may all be formed as an integral one piece extrusion. 
         [0047]    Referring to  FIGS. 2A to 2D , one or more solar collectors may be mounted for rotation so that the longitudinal axis of the collector can be maintained substantially perpendicular to the direction of the sun&#39;s rays as the earth rotates. Advantageously, this helps to ensure that the position of focus of the sun&#39;s rays for each secondary reflector in the z-direction remains substantially fixed as the earth rotates to ensure that the rays are reflected into each distribution device and are not offset to one side or the other in the z-direction. This increases the amount of sunlight collected over a daily period. In addition, each solar collector can be mounted to rotate about a longitudinal axis thereof, for example rotational axis  1  shown in  FIG. 1A  so that the solar collector can track the sun as its elevation changes over a daily period. 
         [0048]      FIGS. 2A to 2D  show an array of solar collectors positioned one behind the other and mounted together on a rotary support structure which collectively rotates the array about a vertical axis. The support structure includes a circular ring  201  with a framework positioned within the ring and upstanding therefrom for supporting each solar collector. The ring is supported by a plurality of discrete support members  203  spaced circumferentially around the support ring and which may include one or more bearing members and/or guide members for supporting and guiding the rotary ring as it rotates. Rotation of the support structure may be driven by any suitable means such as a motor via a cable attached at one or two different positions on the support ring or support structure and which is looped about a rotary drum or capstan, driven by the motor. 
         [0049]      FIG. 10  shows an embodiment of a distribution device (e.g. mixer) having a frustopyramidal geometry and a graph showing the distribution of irradiance over the area of its lower aperture. 
         [0050]      FIG. 11  shows the geometry of the primary reflector  3 , secondary reflector  4  and distribution device  5  according to an embodiment of the present invention, with ray lines illustrating the direction of solar rays reflected by and impinging on each element.  FIG. 12  shows a side cross-sectional view through a distribution device illustrating multiple reflections in which each reflection results in forward travel of each ray towards the lower aperture  90  of the distribution device rather than backwards reflection towards the entrance aperture  88 . 
         [0051]    In embodiments of the solar collector, any one or more components may comprise a suitable metallic material, for example aluminum or any other suitable material. Where differential thermal contraction and expansion is an important consideration, components may comprise the same or similar material. 
         [0052]    Other aspects and embodiments of the invention may comprise any one or more features disclosed herein in combination with any one or more features disclosed herein. In any aspect or embodiment of the invention, any one or more features may be omitted altogether or may be substituted by an equivalent or variant thereof. 
         [0053]    Numerous modifications to the embodiments disclosed herein will be apparent to those skilled in the art.