Abstract:
An symmetric solar collector system is disclosed which comprises one or more reflectors in the shape of an asymmetrical vertically-biased parabolic trough, which allows for the reflectors to be stacked vertically, and have a zero footprint. The reflectors each include a reinforced absorber comprising two or more tubes attached to each other in truss-like fashion, with a sag to length ratio of less than about 1/500. In addition, although the vertically-biased trough shape lessens the amount of surface area available for water or ice to accumulate, the reflector surface is partially coated with a material that is highly absorptive of solar wavelengths, and thus, heats any accumulated water/ice to the point of evaporation.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a device and method for collecting solar energy. In particular, the present invention relates to an asymmetric solar collector system with a zero footprint, comprising a means to minimize water-based interference. 
       BACKGROUND 
       [0002]    The total amount of energy available through solar thermal collection is limited by the size of the available area and the efficiency of the collectors. With moveable mirrors to track the position of the sun, parabolic trough solar collectors maximize the energy collected by using wide mirror apertures, while minimizing the heat loss with small area absorbers. Increasing the amount of energy collected requires increasing the area of the collector installation. Thus, the size of the area available for the collectors limits the total amount of energy collectable for a given location and installation. Maximizing the available area available for solar collectors is therefore paramount to increased energy collection. 
         [0003]    Traditional concentrator designs typically employ symmetric or “horizontally biased” asymmetric mirrors where much of the sunlight is reflected up to an absorber. These systems are often used in horizontal arrays, where the collectors are arranged spatially on a horizontal or near-horizontal surface. In such arrays, the vertical profile of the collectors is low, or the space between collectors large, to minimize the amount of light blocked by adjacent collectors when the sun is low in the sky. The opposite is true for a vertically mounted array of solar collector where the collector&#39;s horizontal profile is minimized to reduce the amount of light blocked by vertically adjacent collectors when the sun is high in the sky. 
         [0004]    Additionally, symmetrical or horizontally-biased designs deployed in arrays cover significant horizontal surface area. An example of a trough-like reflector covering a large horizontal surface area is disclosed in WO2007109900A1 (Gerwing et al.). Similarly, WO9857102A1 (Karlsson et al.) disclose an asymmetric parabolic reflector which is horizontally-biased, and thus, requires a large horizontal surface area. These systems have a large “footprint”. The horizontal surface areas are not commonly available in urban settings. Tall buildings, such as apartment blocks, have significantly greater ratios of vertical surface area to horizontal area, and thus are limited in their solar energy collection capability by the symmetrical or horizontally biased market options. Such locations are optimal for vertically-biased collectors. 
         [0005]    One factor related to efficient solar energy collection is the method of absorption of solar rays reflected from a parabolic trough. The principle energy gathering components of parabolic trough concentrating solar collectors are a parabolic trough mirror (concentrator mirror) that reflects sunlight into a narrow line at the mirror focus, and an absorber placed along the focal line of the mirror to intercept and absorb the reflected sunlight. The narrow absorber is required to span relatively large distances between supports and must have minimal deflection (sag) due to gravity in order to stay within the focal region of the concentrator mirror. Absorber sag in concentrating solar collectors pulls the absorber away from the focal line to where it fails to intercept all of the concentrated sunlight. 
         [0006]    In traditional designs, it is common for absorbers to be constructed of a single tube or a multiplicity of tubes (as those found in flat plate solar collectors), with thermally conductive fins, or additional structures to cover the focal region of the collector, the configuration of which is uniquely determined by each design. The absorber must intercept the maximum amount of reflected sunlight at, or near, the focus of the reflector. Often these absorbers are found to have either a back plate or structural members that are employed to provide support. Unfortunately, in many cases, extra plates and additional structure lead to higher thermal loss factors due to increased absorber area, and the necessary temperature gradients to transfer energy from the tips of fins to the heat transfer fluid regions. It is recognised that some of these structural elements also mitigate losses by inclusion of insulation or geometric features to reduce the heat loss. Ultimately, these added complexities and necessary design features result in additional costs. 
         [0007]    An absorber, heated by concentrated light, loses energy to its surroundings through black body radiation, forced and natural convection, and conduction. All of these terms scale with the area of the absorber. To maximize efficiency through the absorber area, the ratio of the absorber surface area to the light gathering aperture area of the concentrator is kept to a minimum. Designing to minimize absorber area is difficult, and reducing the size of the absorber usually reduces the structural rigidity of the absorber along its length. Single tube absorbers made from common metals (like steel, copper and aluminium), when supported at distant endpoints, sag significantly, with the absorber falling below the lower edge of the focal line over the absorber length, especially when heated. This draws the absorber out of the focal region of the concentrator mirror. The ratio of absorber length to diameter, for single tube absorbers, with most concentrators makes it impossible for the absorber to span large distances between supports without significant sag. 
         [0008]    While, for example, U.S. Pat. No. 4,156,419; CN20131814; DE19925531; WO199964795; and WO2008090461A2 all disclose absorbers comprising multiple tubes, none address the problem of sagging in a satisfactory manner. 
         [0009]    Another disadvantage of symmetrical or horizontally-biased collector designs is their susceptibility to the accumulation of water phases (e.g. solid and/or liquid, depending on the ambient temperature) and particulate matter. In particular, mirror surfaces with near-horizontal slopes less than 45 degrees from horizontal accumulate different phases of water and debris, which impair the specular reflectivity of the mirror(s), and reduce the light gathering efficiency. 
         [0010]    In some designs, the mirrors employed in parabolic trough and trough-like concentrators are unprotected and subject to outdoor environmental conditions. High reflectivity mirrors, used in concentrating solar radiation collectors, have surface temperatures that do not increase significantly above ambient temperatures. On mirror surfaces having an upward facing component to their slope, water can accumulate as either solid or liquid and may remain on the surface of the mirror for sustained periods. The presence of water reduces the specular reflection of the mirror, lowering the performance of the solar collector. Typical mirrored collector designs employ mirrors that reflect 95% of the incident sunlight radiation, leaving just 5% as heat absorbed by the mirror surface. The low absorptivity that makes the mirror a good reflector, combined with conductive and convective losses, leaves very little residual energy in the mirror to the raise the mirror temperature significantly above the ambient temperature. 
         [0011]    Depending on the season and daily time-dependent location of the sun, the mirror may have a primarily upward facing surface for at least part of its curved surface. Such upwards facing surfaces may then be subject to accumulation of water. This is especially problematic in the winter when the water is often in the solid forms of ice or snow. Once on the surface, the area of the mirror that is covered by water is reflectively impaired and no longer performs as designed, with reduced specular reflectivity. Although U.S. Pat. No. 4,015,585 discloses a solar heating device for melting snow, it is rather complex and expensive, as it requires the use of a plurality of pipes underlying a reflector, and circulating fluid there through. 
         [0012]    There is thus a need for a device and method of solar collection with minimal vertical footprint, employing high-efficiency absorbers and means for minimizing water-phase accumulation. 
       SUMMARY 
       [0013]    The present invention addresses the aforementioned issues through the use of a vertically-biased solar collector which provides for: zero-footprint; a shallow collector depth; the ability to stack collectors; minimization of component of horizontal mirror surface area; and proximity to a sold vertical surface area to reduce wind-loading leading to mechanical simplicity. 
         [0014]    The invention in its general form will first be described, and then its implementation in terms of preferred embodiments will be detailed hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification. 
         [0015]    In one aspect of the present invention, there is provided a novel, vertically-biased design of a solar collector. The cross-section of the mirror is an asymmetric section of a parabola. This asymmetry tilts the aperture vertically relative to designs using a symmetric mirror aperture which is perpendicular to the sun&#39;s rays. The mirror is referred to as “vertically-biased” for these reasons. 
         [0016]    In another aspect of the present invention, there is provided a solar collector comprising: a) a reflector assembly for receiving solar radiation; and b) an absorber positioned for receiving solar radiation reflected from the reflector assembly, wherein the reflector assembly comprises a reflector surface having a longitudinal cross-sectional shape of an asymmetric parabola with a vertical bias. 
         [0017]    In a further aspect of the present invention there is provided a device for solar energy collection comprising a plurality of the solar collectors described above mounted on a plurality of rows adjacent to a vertical or near-vertical surface. 
         [0018]    In yet another aspect of the present invention, there is provided a solar absorber having a truss-like structure, comprising two or more tubes and a means of joining the tubes, wherein the absorber has a sag to length ratio of about 1/500 or less. 
         [0019]    The present invention comprises a shallow asymmetric parabolic solar collector. By vertically biasing the collector mounted on a vertical surface, the depth of the collector is reduced. The depth is defined as the distance from the vertical surface, upon which the collector is mounted, to the point on the collector furthest from the vertical surface. The vertical bias of the present invention requires less space for mounting nearer the support wall than similar sized symmetrical or horizontally-biased collectors, while still permitting the collector to track the sun year round, preferably for latitudes greater than 23 degrees North or South. 
         [0020]    The present invention provides for a “zero-footprint” design, with no need for a horizontal footing and taking only the wall space at a depth for a single row of collectors when installed as an array. A deployment of a plurality of rows to create an array of collectors requires no horizontal space beyond that of the first row. 
         [0021]    The vertical bias of the collectors provides for a low horizontal profile, and makes the collectors ideal for use in vertical arrays. Collector arrays using the zero-footprint design are suited, for example, in densely populated areas where horizontal real estate is at a premium and vertical real estate is plentiful. The zero-footprint design benefits, for example, apartment block deployments due to the stack-ability, and minimal collector depth. 
         [0022]    A vertically-biased mirror increases the fraction of the year when the entire mirror has little near horizontal slope, and therefore minimizes the build-up of efficiency-impairing water (solid or liquid) and debris on the mirror surface, thus improving overall operational efficiency. Mirror life is prolonged with reduced particle accumulation and longer cycle times between cleaning. 
         [0023]    The solar collector of the present invention is designed to be mounted on a solid vertical or near vertical surface. This takes advantage of the proximity to the solid surface for buffeting from wind loads. The application to wall mounting means that there is no need for wind load protection beyond that inherent in the design. Thus, a design of the present invention is light-weight Infrastructure supports this light-weight device with no requirement for special reinforcement. The relative low weight of the present invention, translates into lower infrastructure, installation, and material costs. 
         [0024]    The solar collector of the present invention can be integrated into structural or architectural features. Examples of these would be but not limited to building curtain walls, overhangs, steep roofs, fences, and geographic features. 
         [0025]    The truss absorber of the present invention is designed to be positioned along the focal line of the reflected sunlight for a parabolic trough solar collector, the region where the line width of the concentrated sunlight is a minimum. The absorber must be optimized to cover the focal line height at the concentrator focus while being as small as possible to minimize heat loss to the surroundings. Only the smallest amount of design area is apportioned to the absorber height to account for sag. With a simple two-tube absorber design built as a truss, the absorber is greater and more rigid than a single tube absorber. This reduces the absorber sag to a tolerable amount. A simple two-tube absorber can also be easily pre-stressed during fabrication to counter the sag and reducing sag effectively to zero when installed. Two tubes are used in the preferred embodiment of the present invention; however a similar truss structure of alternate embodiments can be made with more than two tubes, if necessary, rigidly joined together to make a truss absorber that therefore meets the design requirement and intercepts all of the reflected sunlight. 
         [0026]    The simple multi-tube truss design of the present invention is also a fin-less design. With fins, heat must be conducted along the fins to the channel containing the heat transfer fluid. The higher temperature at the edge of the fin raises the heat loss of the absorber. The fin-less design results in minimal temperature gradient as the heat is conducted directly through the wall of the tube (about a distance of 1 mm). In addition, there are no back plate, nor extra-structural elements, and therefore no additional loss factors. 
         [0027]    Whereby a standard copper tube has a sag to length ratio of about 1/65 (i.e. the sag is about 1/65 of the length of the tube), the absorber truss of the present invention has sag to length ratio of about 1/500 or less. For example, a truss absorber of the present invention spans a collector&#39;s design distance of 2.45 m with minimal sag less than 5 mm peak. 
         [0028]    In addition, the truss absorber can, if desired, be employed to support a non-structural features without appreciably increasing the sag. The truss absorber is preferably externally finished with a selective coating with high absorptivity at visible (solar) wavelengths and low emissivity at thermal infrared wavelengths. 
         [0029]    The solar collector of the present invention further comprises a solar energy-absorbing area integrated onto, or adjacent to, the sun facing surface (mirror) of the solar collector appliance. During operation, light is passively absorbed as heat by area(s) with high absorptivity at solar wavelengths, and conducted to the mirror surface of the collector. The solar energy absorbing area provides sufficient heat energy to increase the temperature of the surrounding material to evaporate, sublimate, liquefy or otherwise facilitate removal of water from the mirror surface, clearing the mirror of optical impairments. 
         [0030]    The solar energy absorbing areas, once integrated onto the mirror&#39;s surface, covers less than about 5% of the total mirror surface area. This is a considerable improvement when compared to the typical loss of energy transfer due to presence of water which is of the order of 20 to 50% of total mirror surface area. Since water accumulation on vertical surfaces is usually minimal, the absorptive material need only be applied to the relatively small fraction of the vertically-biased mirror that has a low slope. The 5% coverage of the absorbent material over the mirror need only apply to the low slope regions of the mirror. 
         [0031]    The solar energy absorbing areas operate when the solar collector is in operation. Raising surface temperature of the solar collector by a few degrees above ambient temperature is all that is required to begin the evaporation, sublimation, or liquefaction process. While the solar energy absorbing areas are used on vertically-biased mirrors, they are also applicable to existing mirrored systems used to collect solar energy. For example, the solar energy absorbing areas may be extended to glazed systems flat panels or evacuated glass tubes where impairments due to water impact performance of transmitted solar radiation. 
         [0032]    The solar energy absorbing areas may be integrated directly onto a mirror or affixed as an appendage using a stable, thermally conductive product. 
         [0033]    The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by the description of the preferred embodiments which follow. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]      FIG. 1  illustrates a side view of a vertically-biased asymmetric parabolic trough concentrating solar collector. 
           [0035]      FIG. 2  illustrates a front view of a vertically-biased asymmetric parabolic trough concentrating solar collector, showing the offset of the absorber to the lower region of the collector. 
           [0036]      FIG. 3  illustrates a partial isometric view of water accumulation on the lower edge of a solar collector. 
           [0037]      FIG. 4  illustrates a side view of a stacked deployment of vertically-biased collectors. 
           [0038]      FIG. 5  illustrates a side view of a stacked deployment of vertically-biased collectors during a seasonally high sun angle. 
           [0039]      FIG. 6  illustrates an elevation of a truss absorber with periodic joints between adjacent tubes. 
           [0040]      FIG. 7  illustrates a close-up view of a joint between two tubes. 
           [0041]      FIG. 8  illustrates an elevation of a truss absorber with a continuous joint between adjacent tubes. 
           [0042]      FIG. 9  illustrates a close-up view of a continuous joint between two tubes. 
           [0043]      FIG. 10  illustrates an end-on cross-sectional view of truss absorber tubes and joint. 
           [0044]      FIG. 11  illustrates a close-up view along the length of truss absorber tubes which are each rolled flat at the mid section. 
           [0045]      FIG. 12  illustrates cross-section of rectangular area of the tubes at the flattened section. 
           [0046]      FIG. 13  illustrates the elevation of two lengths of tubes with mid sections rolled flat configured and joined at an angle offset θ from one another. 
           [0047]      FIG. 14  illustrates a cross-section of rectangular area of the tube at the flattened section assembled and joined at an angle offset θ to one another. 
           [0048]      FIG. 15  illustrates a cross-sectional view with concentrated sunlight on one side of the truss absorber shown  FIG. 10 . 
           [0049]      FIG. 16  illustrates a cross-sectional view of a truss absorber supporting a back plate and providing for insulation and exposed absorber area reduction to reduce heat loss. 
           [0050]      FIG. 17  illustrates a cross-section of a truss absorber encapsulated in a partially transparent partial back plate apparatus. 
           [0051]      FIG. 18  illustrates a cross-sectional view of a truss absorber encapsulated in an apparatus comprised of a clear transparent film or glazing to minimize convective losses. 
           [0052]      FIG. 19  illustrates a partial isometric view of water accumulation on the lower edge of a solar collector, with a solar energy absorption area affixed to the mirror. 
           [0053]      FIG. 20  illustrates a plurality of passive solar energy absorption areas integrated onto the surface of a solar collector. 
       
    
    
     DETAILED DESCRIPTION 
       [0054]    Wherever ranges of values are referenced within this specification, sub-ranges therein are intended to be included within the scope of the invention unless otherwise indicated. Where characteristics are attributed to one or another variant of the invention, unless otherwise indicated, such characteristics are intended to apply to all other variants of the invention where such characteristics are appropriate or compatible with such other variants. 
         [0055]    The following is given by way of illustration only and is not to be considered limitative of this invention. Many apparent variations are possible without departing from the spirit and scope thereof. 
         [0056]    A preferred embodiment of the solar collector of the present invention is shown in  FIG. 1 . The solar collector ( 2000 ) is vertically-biased, has a zero-footprint, and can be vertically stacked. 
         [0057]    The mirror  200  is constrained to follow a parabolic arc between the upper end block  300  and lower end block  301 . The distance between the upper end block  300  and lower end block  301  is precisely defined by the cords  400 . The angles between the edges of the mirror  200  and the cords  400  (edge angles) are set by the upper and lower end blocks  300  and  301 . The mirror  200  is fixed to the upper end block  300  and lower end block  301  via the upper mirror support  201  and lower mirror support  202 . The parabolic shape of the mirror is achieved by matching the distance between the upper and lower end blocks  300  and  301 , and the edge angles of the mirror  200  set by the upper and lower end blocks, with the arc length of the mirror. 
         [0058]    The asymmetric parabolic shape in the mirror  200  is achieved by making the edge angle of the mirror  200  at the upper end block  300  different from the edge angle of the mirror  200  at the lower end block  301 . In the vertically-biased mirror  200  shown in  FIGS. 1 and 2 , the edge angle of the mirror  200  set by the upper end block  300  is smaller than the edge angle of the mirror  200  set by the lower end block  301 . This makes the arc length for the portion of the mirror  200  between the vertex of the parabola and the upper end block  300  longer than the arc length for the portion of the mirror  200  between the vertex of the parabola and the lower end block  301 . For noon fall and winter sun angles a below 45 degrees, this configuration is vertically-biased in that the cord  400  is closer to vertical than it would be for a symmetrical mirror. 
         [0059]    The angle β between the sunlight  100  and the normal to the cord  800  is a measure of the degree of vertical-bias in the mirror  200 . β is 0 degrees in a symmetrical parabolic mirror. When measured in the counter clockwise direction from the normal to the cord  800 , vertically-biased mirrors have positive values of β. The mirror  200  in  FIG. 1  has a β angle that is approximately 30 degrees. In a preferred embodiment, the forward vertical-bias angle β ranges from 5 degrees to 45 degrees, preferably between 15 and 25 degrees; and more preferably between 18 and 22 degrees. The solar collector of the present invention tracks the sun and maintains the angle β during normal light collecting operations. 
         [0060]    The mirror material is that which is typically found in the art. For example, the mirror material may be: highly polished anodized aluminium with a surface protected by a micro coating that prevents oxidation; or aluminium foil over a substrate; or aluminized film over a substrate; a mirrored flexible sheet material (for example, mirrored polycarbonate, mirrored acrylic or mirrored fibreglass); or a mirrored surface or back side reflectively coated material (for example, the mirror may be silver, aluminium or even stainless steel). 
         [0061]    The collector  2000  rotates about the axis of rotation  600  in the preferred embodiment to track the incident sunlight. Axis of rotation  600  could be located differently in alternate embodiments. The absorber  500  is located at the focal line of the parabola. The absorber lies in the focal region and adjacent the axis of rotation in manifestations of this design where the absorber is fixed in place. The front/aperture plane of the collector  2000  is defined by the cord  400 , upper mirror support leading edge  201  and the lower mirror support leading edge  202 . The solar collector is driven by actuator(s) (not shown) that both move and hold the apparatus in place. The actuators are controlled by a controller designed to track the sun&#39;s movement and adjust the collector accordingly. 
         [0062]      FIG. 2  illustrates the view looking at the front plane of the collector  2000 . The absorber  500 , in both  FIGS. 1 and 2 , is deployed along the focal region of the mirror  200  to intersect the reflected sunlight. The absorber  500  is shown situated in the lower half of the collector as shown due to the vertically-biased asymmetry of the collector mirror  200 . 
         [0063]      FIG. 3  illustrates a partial isometric view of the vertically-biased asymmetric parabolic trough concentrating solar collector mirror  200  and depicts the accumulation of water  720  on the lower mirror surfaces near the lower mirror support  202  where, although present, the amount of water accumulation is minimized with the reduction of near horizontal surfaces by the vertical bias of the present invention. 
         [0064]    The vertical stackability of collectors  2000  is shown in  FIG. 4  with the present invention connected via a support structure  2001  to a vertical structure  3000 . Here the deployed angle of the collector is depicted in an operationally seasonal mode with a lower sun angle. 
         [0065]      FIG. 5  illustrates the collectors  2000  deployed on vertical structure  3000  depicting a higher sun angle  101  and also depicting minimal shadowing  102  by the collector that is physically higher than the row below, where most of the sunlight  103  passes to the next collectors in subsequently lower rows. Blockage can be reduced with increased vertical spacing between collector  2000  rows, however the high southern (low northern for southern hemisphere) component of the sun elevation occur only in the early morning or late afternoon for latitudes above 40° North (below 40° South) in the spring and summer. The scenario of blockage in this case is not considered a problem since the majority of energy is collected within a few hours of mid day when, in the summer season, the southern (northern) component of the sun elevation is at its lowest daily elevation for a South (North) facing collector, reducing the light blockage during the peak collection period of the day. The slight degradations in efficiency, due to blockage at the extreme early and late periods of the day, therefore, impact only minimally the total daily energy collection. 
         [0066]      FIG. 6  illustrates an elevation of a truss absorber with periodic joints between adjacent tubes supported near the end points symbolized by two triangles ( 199 ). The truss absorber  500  of  FIG. 6  comprises two or more tubes  510  and a means to rigidly join the tubes  520  (or  550  from  FIG. 8 ) in such a fashion as to yield a rigid absorber in the direction between the tubes  520  when deployed in a horizontal fashion length-wise and stacked one tube above another tube in a vertical or nearly vertical arrangement as shown. 
         [0067]    A gap  610 , of a preferred embodiment of the present invention is shown in  FIGS. 7 and 10 , between tubes  510  or  560  creates separation at the absorber ends to allow for adaptors to be fitted to the tubes to establish mating interconnection with additional absorbers or system plumbing. This gap also permits fixing points to be established between adjacent tubes along the truss&#39; length. These fixing points enable ease of attachment of sensors, back plates, or full encasement coverings. An alternate embodiment may not include the gap  610 , securing the tubes to one another without a gap  610 . 
         [0068]    Detail  520  and  550  from  FIGS. 6 through 9  are any material capable to be employed as a rigid means of attaching tubes and maintain rigidity beyond temperatures of a minimum of 25° C. Non-limiting examples include solder (e.g. melting tin/antimony between the two tubes), brass (e.g. the process of brazing), or a suitable high temperature epoxy. Details  520  and  550  are attached to the tubes by conventional means, for example (but not limited to) welding, soldering, epoxy, or a combination thereof. Details  520  and  550  are not applied within about 0.5% to about 10%, preferably about 1% , from the ends of the tubes  510  to avoid interference with the means of connecting the tubes to other systems. 
         [0069]    Another form of tubes used to form a truss as introduced in  FIG. 6  are shown in  FIGS. 11 through 14 . Here the tubes  510  have been transformed to become partially flattened tubes  560  giving a tube an approximately elliptical or rectangular profile for most of the tube&#39;s functional length. The tubes are not so flattened as to restrict or stop the flow of heat transfer fluid within the tube. These tubes are also not flattened within about 0.5% -10%, preferably about 1% from the ends of the tubes in order to facilitate means of connecting to other systems. In  FIGS. 11-14 , use of partially-flattened tubes yields an even stronger truss in the direction between the two tubes (vertical). 
         [0070]      FIG. 12  shows a cross-sectional view of the truss absorber  410  with the flattened tubes where the elongated widths of both tubes  560  are aligned vertically. This truss  410  configuration yields a stronger truss  410  in the vertical direction than that of truss  500 . 
         [0071]      FIGS. 13 and 14  show an application of the tubes  560  forming truss  415  where the wider width dimension of one tube is aligned vertically and the wider width of the second tube is not aligned vertically but rather at an offset angle θ to the first tube. 
         [0072]    A truss configuration  415  of  FIGS. 13 and 14  employs two tubes ( 560 ) vertically adjacent to each other to provide the strength to resist sagging, while the second more horizontally-biased tube  560  at and offset angle θ to the first tube meets the design requirement to occupy the focal region and absorb the concentrated sunlight. Conversely for a similar horizontally biased arrangement, application of truss absorber  500  or  410  would not benefit from the strength of the truss  415  configuration. Employing truss absorbers  500  or  410  to a horizontal configuration, as this in  415 , would result in “sag”. 
         [0073]    Concentrated sunlight  570  ( FIG. 15 ) is reflected to the ‘front’ face of the truss absorber  500  where it is intercepted. Concentrated sunlight is incident upon roughly 50% of the absorber surface. Absorbed energy is efficiently conducted through the tubing walls to the working fluid. Direct sunlight  580  strikes roughly 50% of the absorber  500  area on the ‘back’ face of the truss. The truss absorber in this case can be any of the preferred embodiments of  500 ,  410  or  415 , absorber  500  embodiment is shown for illustration here. 
         [0074]    A non-structural back plate  590  ( FIG. 16 ) may be added to the design to improve efficiency. The truss absorber  500  in this case is fully capable of supporting the back plate. To help minimize losses, insulation  595  can be added to the design between the truss absorber and the back plate. The back plate  590 , while itself continuous end-to-end following the absorber, is attached only periodically along the length of the truss to minimize conduction of heat away from the truss absorber to the back plate. The truss absorber in this case can be any of the preferred embodiments of  500 ,  410  or  415 , absorber  500  embodiment is shown for illustration here. 
         [0075]    An apparatus  620  of  FIG. 15  encapsulates the truss for its entire length and is fully supported by the truss absorber  500 . This apparatus  620  can be added to the design to reduce convective and radiation losses at both the front, where an optically transparent covering  630  transmits concentrated sunlight to the absorber, and the back, where a plate  640  shields the absorber from forced convection. The apparatus enshrouds the truss design and is attached only periodically along the length of the truss to minimize conduction of heat to the apparatus, away from the absorber. The apparatus  620  can also be insulated  595  on the back side. 
         [0076]      FIG. 18  shows optically transparent apparatus  650  encapsulating the truss along the truss&#39; length. This embodiment permits sunlight, both reflected from the concentrator mirror  570  and directly from the sun  580 , to strike the absorber while minimizing convective heat losses. The truss absorber in this case can be any of the preferred embodiments of  500 ,  410  or  415 , absorber  500  embodiment is shown for illustration here. 
         [0077]      FIG. 19  shows a partial isometric view of the lower edge of the mirror of a concentrating solar collector&#39;s mirror surface  200  with a significant component of upward facing surface area and having depicted herein the accumulation of water  720  on the surface. Passive solar energy absorption area  900  is adjacent to the mirror and affixed to the mirror in a thermally conductive fashion. 
         [0078]      FIG. 20  shows a partial isometric view of the concentrating solar collector&#39;s mirror surface  700  with a significant component of upward facing surface area and having depicted herein the accumulation of water  720  on the surface. A plurality of passive solar energy absorption areas  730  is integrated onto the surface to distribute the heat over the primarily horizontal region of mirror surface. Heat is conducted away from the absorption areas via the mirror material. The preferred embodiment of the thermally absorption areas  730  are not limited to circular or oval shapes, but could be realized in any pattern, such as a matrix of small dots, linear areas running parallel to the straight edge of the mirror  740 , or vertical linear stripes perpendicular to the straight edge of the mirror  740 . The surface area of mirror to be integrated with the solar energy absorption area shall not exceed 5% of the total mirror area on the portion of the mirror that is susceptible to accumulation of water. For a vertically-biased mirror this is the lower portion of the mirror, for a symmetrical or horizontally biased mirror this absorbent area is applied to the entire mirror surface. 
         [0079]    A matrix of thermally absorptive ‘dots’ or islands or a strip may be applied via a spraying process directly to the surface requiring heating. Alternatively, a strip of thermally absorptive material may be placed adjacent to and bridged thermally to a surface which will benefit from the heating. 
         [0080]    Examples of the thermally absorptive material include, but are not limited to: black paint; highly-absorptive, low-emissive coating material; anodizing of a thermally absorptive compound; film; etching, or a combination thereof. The thermally absorptive material raises the temperature a few degrees above ambient to start the process of liquefaction, sublimation or evaporation. 
       CONCLUSION 
       [0081]    The foregoing has constituted a description of specific embodiments showing how the invention may be applied and put into use. These embodiments are only exemplary. The invention in its broadest, and more specific aspects, is further described and defined in the claims which now follow. 
         [0082]    These claims, and the language used therein, are to be understood in terms of the variants of the invention which have been described. They are not to be restricted to such variants, but are to be read as covering the full scope of the invention as is implicit within the invention and the disclosure that has been provided herein.