Abstract:
Solar concentrating wedge can potentially deliver twice the output of a CPC trough. The wedge uses a new optic that turns light sharply allowing the collector to have a very compact profile. In this way, the focal zone is made shorter and hotter for solar thermal applications and the maximum flux density is doubled for photovoltaics. The collection optics are self-cooling. A simple and sturdy non-tracking frame ensures that collected light will follow the intended path to the absorber.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to the collection of sunlight and specifically to improvements in a stationary solar concentrating collector. 
     Practical stationary solar concentrators include the well known compound parabolic concentrator (CPC) trough and the linear wedge. These collectors focus light onto an absorber without following the sun, thus eliminating the expense of rotating machinery. 
     The CPC is elegantly simple, though very tall compared to its width. CPC height poses an engineering challenge since, if the collector is made larger to gather more light, its reflective wall cross section must be increased disproportionately to overcome the greater effects of gravity and wind loading. By comparison, the wedge is short and wide, a profile that is stable and easily scaled-out to collect a larger area of sunlight. Solar Concentrating Wedge, U.S. Pat. No. 8,333,480, Murtha, uses advanced collection optics to achieve the low profile. 
     SUMMARY OF THE INVENTION 
     The primary object of this invention is to introduce a linear wedge that has an unexpectedly high geometric concentration ratio. 
     Accordingly, the primary object is accomplished in the following manner: modified collection optics are placed in an stepped arrangement that allows the new wedge to become ultra-compact, resulting in a much shorter and hotter focus. 
     Another object is to present a compact wedge that has self-cooling collection optics. 
     Other objects and advantages will become apparent from the following detailed description and accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an end view of a prior art wedge. 
         FIG. 2  is an end view of a compact wedge of the present invention. 
         FIG. 3  is an end view of the ultra-compact wedge. 
         FIG. 4  is an end view of the stepped collection optics. 
         FIG. 5  is an end view of the preferred embodiment collection optics. 
         FIG. 6  is a isometric view of the ultra-compact wedge and frame. 
     
    
    
     DESCRIPTION OF THE INVENTION 
     Turning now to  FIG. 1 , there is shown a prior art solar concentrating wedge  2  (U.S. Pat. No. 8,333,480) with a 24° acceptance angle. Collection optic  4  is a flat array of abutting prisms that directs light into the wedge toward the absorber. Hollow fin-type absorber  6  is irradiated on both sides for a geometric concentration ratio of 4.5×. It should be noted that 4.5× is also the concentration ratio that a truncated CPC will produce, having the same collector width, the same fin absorber height and a 24° acceptance angle. 
     In  FIG. 2 , compact wedge  8  of the present invention, is shown for the first time. Both sides of the collector are identical, where one side has been turned 180° in relation to the other. The hollow collector is bounded by an inclined row of stepped prisms  10  on top and compound reflectors  12  on the collector bottom. In the focal zone, fin-type absorber  14  extends vertically from the collector apex to the collector bottom.  FIG. 1  and  FIG. 2  collectors are the same width, however absorber  14  is a third shorter than the absorber in  FIG. 1 , which increases the geometric concentration ratio to 6.8×. 
     In  FIG. 3 , ultra-compact wedge  16  is shown for the first time. Upwardly curved bottom reflectors  18  reduce the size of the focal zone. Stepped prisms  10  are arrayed in an arch and each individual prism is rotated slightly so that light will have a clear path to absorber  20 . Absorber  20  is half the height of the  FIG. 1  (and CPC) absorber, producing a concentration of 9×. With this new level of heat production, it is fortunate that the collection optics are self-cooling. Heat from the absorber creates a convection current that pulls cool air  22  through the gaps between prisms. The air circulates, picks up momentum and leaves through the collector apex as warm air  24 . All of the prisms are passively cooled by air  22 . 
       FIGS. 1 ,  2  and  3  are drawn to scale. The reader can verify each geometric concentration ratio by measuring the collector from tip to tip and dividing by the absorber height. 
     Solar thermal: The value of the collector is that pure water, for human consumption and agriculture, can be obtained from any brackish water source. Solar thermal energy boils the water which becomes steam and the steam condensate is captured as purified water. Concentrated light accelerates the distillation process. From a different perspective, solar generated steam can also be used to power a steam engine without air pollution. For either process, it is the collector surface area that will determine the volume of steam produced and the steam can be plentiful since the wedge is scalable. 
     Concentrating photovoltaics: The compact wedge should be useful for illuminating solar cells. Flux density is high and the delivery angle from the collection optics to the cells is direct compared to the CPC. 
     In  FIG. 4  the collection optics are arrayed at an incline and each prism is separated from its neighbor by an air gap  26 . Each optic is an assembly of a substantially triangular clear acrylic or glass prism  10  and aluminum sheet reflector  28 . Sheet  28  provides the initial light bounce, but each reflection after that is a total internal reflection. Prism  10  can be a single part or it can be made of several pieces that are held together with index matching glue. 
     Incident ray  30  enters through prism inlet surface  32 . Ray  30  transmits prism bottom surface  33  and is then sent up diagonally by reflector  28  for a total internal reflection  34  at a predetermined angle before exiting the prism large end outlet surface  36 . Each large end  36  is overshadowed  40  by a cantilever pointed end  38  of the next higher prism, so that light is not lost through gap  26 . 
     A preferred embodiment of the collection optic is shown in  FIG. 5 . In transverse cross-section, prism  10  has the general appearance of a cornucopia, having a pointed end and a large end. Reflector  28  is disposed adjacent the prism bottom surface. The optic combines the acceptance angle of a prism and the directionality of a reflector, collecting all light between equinox ray  42  and solstice ray  44  and having that light cleanly turn-the-corner into the wedge. The optic has a 24° acceptance angle that collects sunlight three months before and three months after summer solstice, the brightest six months in the northern hemisphere. 
     In this embodiment, the prism incorporates a total internal reflection comb  46  which straightens out some of the rays before they exit the prism. Comb  46  is a stack of individual acrylic blocks  48  that are designed to maintain thin air spaces between the blocks so that total internal reflection can occur. The blocks are inserted into recess  50  that has been molded into prism  10 . Long sealing beads of clear silicone (not shown) will keep moisture out of the air spaces while maintaining the block positions. 
       FIG. 6  is a perspective view of the  FIG. 3  ultra-compact wedge  16 . Stationary frame  52  is constructed of square tubular steel. Identical optics  10  are used on both sides of the collector. Bottom reflector  18  and absorber  20  run the full east-west length of the collector. The top of absorber  20  is nearly coincident with wedge apex  23 . For any given latitude, the collector should be pointed  54  half way between the solstice and equinox positions of the sun. 
     SUMMARY 
     The reader has been shown a solar concentrating wedge that is potentially more powerful than a CPC trough. The wedge uses new, highly effective optics for light collection. The stationary collector has no moving parts and therefore fewer maintenance and labor costs. Collector scalability allows lower manufacturing costs overall. There has always been a need for a cost effective non-tracking solar concentrator and now the compact wedge has all the right features.