Abstract:
The present invention relates to an improved solar energy concentrating system. The system comprises a dual axis sun tracking paraboloid dish collector on a polar mount, with a re-reflecting mirror in top of the paraboloid dish, which reflects the concentrated solar irradiation into an opening in the paraboloid dish into a light pipe and with a movable third mirror redirects the light into a second light pipe along the polar axis, which with a fourth fixed mirror, sends the concentrated solar irradiation into a third light pipe to the cavity receiver. The invention replaces the need of flexible connectors to accommodate the movement of the mirror, with a combination of mirrors and light pipes, transferring the solar irradiation to a cavity-receiver. Dual axis tracking systems can capture more solar energy, on a more constant basis throughout the day and the year, and by reflecting directly into the cavity-receiver, thermal losses are minimized.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    For the last century, numerous researchers and inventors have come up with clever ideas to harness the solar irradiation in an efficient and inexpensive way to produce electricity. The pace of research has substantially increased during the last decades with high fossil fuel prices and awareness of the implications of releasing CO 2  into the atmosphere. 
         [0002]    The amount of solar irradiation impinging daily the earth is huge, yet the resource is feeble and constantly changing and despite best efforts, even the last commercial installations are still too expensive and have low inefficiencies. Notwithstanding the substantial price reduction of photovoltaic panels (“PV panels”), without inexpensive storage, PV panels do not offer a solution. 
         [0003]    Solar thermal with thermal storage appears to be a better option, but the price is still too high to be competitive. Most solar thermal commercial operations utilize trough sun tracking parabolic mirrors, but in 2012, two large plants utilizing flat heliostats hitting a central receiver in a tower were installed in Ivanpah, Calif. and Crescent Dunes, Nev. A third option, utilizing a parabolic dish, with a Stirling engine at the focal point started operation in Maricopa, Ariz. in 2010, but filed for bankruptcy protection in 2012. 
         [0004]    Fixed collectors cannot reach high temperatures even for a few hours and therefore are not used to generate electricity. Sun tracking parabolic trough consists of long lines of collectors, held horizontally, oriented North-South tracking the sun&#39;s movement from East to West. Unfortunately such arrangement suffers substantial cosine losses, especially in the winter. Inclining the collectors to ameliorate the cosine losses, poses insurmountable problems. The structure will have to be heavy and rigid to withstand wind, yet light so that the mirror could be moved to track the sun. State of the art plants require about 10,000 m 2  per installed MW. 
         [0005]    Heliostats are relatively flat mirrors with dual axis sun tracking. Each individual mirror moves independently, aiming to reflect sunlight into the central power. The particular cosine losses of each mirror depend on the position of the mirror with respect to the tower and the location of the sun. Overall, they are more efficient than the single axis sun tracking parabolic collectors, requiring about 7,000 m 2  per installed MW, but since each mirror is only about 15 m 2 , it requires a large number of mirrors, each one with its own tracking controller and sensors. The Ivanpah&#39;s installation has 173,000 mirrors for a total area of 2,600,000 m 2 . 
         [0006]    Dual axis sun-tracking paraboloid dishes have several advantages: normal direct irradiation is higher than horizontal irradiation (there are no cosine losses) and it is more evenly distributed both during the day and during the year. Overall, dual axis sun tracking paraboloid dishes could capture about 36% more solar energy on a yearly basis. 
         [0007]    However, they have several disadvantages: (a) to capture meaningful amounts of energy the mirror needs to be large which adds complications to the sun tracking mechanism and offers more wind resistance; (b) to achieve high temperatures, a high concentration ratio is required, which implies a very accurate tracking mechanism, and; (c) the arrangement requires flexible yet leak proof connections for transporting the working fluid into and away from the focal point. After constructing a couple of demonstration units, the consensus reached was that the paraboloid dishes were not a very promising avenue. Mounting Stirling engines on top of a paraboloid dish simplified the tracking accuracy and the need of flexible connections, but difficulties with the Stirling engines failed to offer a competitive solution. 
         [0008]    The present invention relates to the use of a dual axis sun tracking paraboloid dish collectors with a re-reflecting mirror above the focal point of the paraboloid dish, re-reflecting the concentrated light into an opening on the paraboloid dish, where the light is transmitted via light pipes to a cavity-receiver operating at high temperature. Several collectors clustered together could feed a single cavity-receiver and generate hundreds of kW. 
       PRIOR ART 
       [0009]    Efforts to capture solar energy to produce steam are more than 150 years old (August Mouchot-1860). Sun-tracking parabolic reflectors are more than 100 years old. Frank Shuman applied for patent U.S. Pat. No. 1,240,890 in Sep. 30, 1912 for a Sun Boiler that comprises, among other things, a sun tracking parabolic collector. 
         [0010]    More recently, Niedermeyer (U.S. Pat. No. 4,340,031-Jul. 20, 1982) proposed a way of constructing a concave paraboloid reflector surface supported on a plurality of parabolically shaped segments extended radially, mounted on a base that rotates with a plurality of wheels along a track and with means of changing the horizontal inclination of the reflector, with an absorber mounted on the focal point, with fluid flowing inside the absorber support base and transferring the heat captured to heat transfer equipment located within the base. Since no details are offered as to the mechanism to transfer the hot working fluid from a tilted reflector, it is assumed that the solution is a flexible connector. 
         [0011]    Even more recently, Cordy (U.S. Pat. No. 5,347,986-Sep. 20, 1994) provided a detailed analysis of paraboloid dish collector problems and proposed a light weight point focus reflector in a light weight cradle which is gimbal mounted with a receiver cavity mounted at the focal point and a rather long plumbing system, its length calculated to provide the desired flexibility without flexible joints, with a hybrid tracking system with four photo detectors in the central part of the collector dish to provide correction, if needed, to the tracking mechanism. 
         [0012]    Last year, Coffey et al (U.S. Pat. No. 8,479,515-Jul. 9, 2013) offered another parabololid dish system, fitted into a polar mount and with a mast on the dish where a boiler is mounted, with an insulated supply line and an insulated hose inside the mast carrying the water and steam to a turbine below. 
         [0013]    Finally, Falcey&#39;s recent patent (U.S. Pat. No. 8,752,379-Jun. 17, 2014) offers an interesting possibility for a hybrid system, using a solar concentrator and fiber optic cables that transfer the collected solar radiation to a solar thermal converter which is used to augment the heating of a working fluid in a boiler. Falcey recognizes that there is a substantial loss of energy in the fiber optic and immerses it in an optical cooling system that pre-heats the working fluid. 
         [0014]    While these patents offer interesting insights and possibilities, they still have to contend with a moving reflector and the need to have flexible connectors to transfer the working fluid into the receiver and the steam to the heat exchanger apparatus. 
       OBJECTIVES AND ADVANTAGES 
       [0015]    The object of this invention is to enhance: (i) the amount of energy that can be captured by a given area; (ii) minimize the thermal losses for a more efficient operation, and; (iii) reach high temperatures for improved efficiency of electricity generation or other uses requiring high temperatures, such as thermo-chemical or photo-chemical reactions. 
         [0016]    It is an object of the present invention to provide for a simpler and accurate tracking mechanism, which should be able to track the sun with accuracies of less than 0.1° (1.7 mrads). 
         [0017]    It is a further object of the present invention to construct optically efficient large mirrors, built with a plurality of small (about 1 m 2 ) mirrors with spacing between the mirrors to ameliorate the wind resistance and with a light structure capable of withstand strong winds without deforming. 
         [0018]    It is a further object of the present invention to eliminate the need of flexible connectors to transfer the working fluid back and forth to the focal point, by re-reflecting the concentrated solar irradiation to a fixed point where it can be utilized by a heat exchanging apparatus. 
         [0019]    It is a further object of the present invention to utilize light pipes to direct the concentrated solar irradiation to the desired location, by providing strategically placed mirrors along the path to change the direction of the light rays. 
         [0020]    It is a further object of the present invention to redirect the concentrated solar irradiation into a solar cavity or solar furnace where the solar irradiation could be used to boil a working fluid to generate electricity by conventional ways, or to use it in other applications requiring high temperature. 
       SUMMARY OF THE INVENTION 
       [0021]    The present invention provides for a practical and economical way of meeting the objectives listed above while minimizing its limitations. 
         [0022]    A paraboloid dish mirror (geometric form created by rotating a parabola along its main axis) concentrates incoming radiation into a focal point. The amount of solar energy captured by any collector is proportional to its area. Given the expected efficiencies, a small parabolioid dish (1 m radius) should be able to capture sufficient energy to generate about 4 kWh/d. Such small capture is not really worth the effort. On the other hand, a 6 m radius paraboloid dish might be able to capture some 160 kWh/d, or almost 20 kWh if operating for a period of eight hours, but now presents a big challenge to track the sun accurately and to resist the wind. While these problems will eventually be overcome, a paraboloid dish with a radius of about 4 m with an area of about 50 m 2 , capable of generating some 72 kWh/d appears to be a practical compromise. For a given area, the shape of the mirror depends on the eccentricity of the parabola. Small eccentricity produces flatter mirrors with longer focal length, which requires better tracking accuracy. Within limits, a 4 m radius mirror with a focal point at about 2 m represents a reasonable compromise. 
         [0023]    The main challenge with the proposed re-reflecting parabolid dish collector (“RPDC”) is the accuracy and precision required. The misalignment tolerance for high concentration ratios is very small but on the other hand it is desirable to have the re-reflecting mirror and consequently the light pipes as small as possible which implies higher concentrations. If the radius of the small mirror is 10% of the radius of the main mirror, the theoretical geometric concentration ratio (“CR”) would be  100  and will require a tracking accuracy of about 0.25° (4.4 mrads), while if the radius of the reflecting mirror was only 5% of the radius of the main mirror, the possible CR would be 400 but the tolerance to deviations on the tracking accuracy is reduced to 0.1° (1.7 mrads). For the proposed 4 m radius mirror, if the radius of the reflecting mirror is 5% of the main mirror, the re-reflecting mirror and light pipe will have a radius of 20 cm (40 cm or 16″ diameter). 
         [0024]    A small misalignment of 0.25° (4.4 mrads) could result in the re-reflecting rays missing the opening in the main mirror, and in subsequent zigzagging of the light rays inside the light pipe. A good mirror with 95% reflectivity will lose 5% of the potential energy captured with each bounce. Obviously, minimizing the number of bounces is critical. Great accuracy and precision is required in the tracking mechanism, in building a near perfect mirror that will not be deformed by the wind and in the construction of the mirror. 
         [0025]    To ameliorate the wind effects, the main mirror will be constructed with a plurality of paraboloid mirror segments, each about 1 m 2 , with some spacing between the segments and even some small circular holes in the mirror segments, to allow the air to pass through the mirror and thus reducing the size and weight of the structure needed for maintaining the shape of the mirror. 
         [0026]    There are two methods used for tracking the sun movements: (a) an altitude-azimuth mechanism, or; (ii) a polar mount mechanism. The polar mount mechanism is simpler and more predictable and likely to require less maintenance, but requires: (i) accurate aiming to True North, the exact place that represents the location of the axis of rotation of the Earth (the Polar Star is slightly off True North); (ii) an inclined plane, and; (iii) is usually mounted on a pole. The altitude-azimuth requires either sensors and/or controller to move almost simultaneously the PRDC along the altitude and azimuth axes and while the tracking mechanism usually results in uneven stepwise movement, its main advantage is that it is usually based on a horizontal platform. A main advantage of the polar mechanism is that it is easier to redirect the reflected rays along the two axes to a fixed point and from there to the cavity-receiver. 
         [0027]    While a simplified embodiment of the invention have been enunciated and will be further described in detail below, it will be apparent to those of skill in the art that various modifications and substitutions may be made thereto and that the invention intends to cover all such modifications and substitutions that fall within the scope of the appended claims as might be understood from the foregoing written description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The present invention will become more fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration only and thus are not limitative of the present invention and wherein: 
           [0029]      FIG. 1  illustrates a slanted view of the re-reflecting paraboloid dish solar collector (“RPDC”) with a re-reflecting mirror above the mirror and a light pipe segment at the bottom of said paraboloid dish 
           [0030]      FIG. 2A  illustrates a ray diagram superimposed on a lateral view of the RP DC and a re-reflecting mirror, showing the rays impinging on the main dish being reflected into the top re-reflecting mirror and re-reflected to the bottom of the dish, where they are collected into the light pipe and then with a 45 degree mirror, sent into another pipe. 
           [0031]      FIGS. 2B, 2C and 2D  shows similar ray diagrams with the sun coming 0.3° away from the axis of the paraboloid, with minute changes on the location and the eccentricity of the re-reflecting mirror. 
           [0032]      FIG. 3  illustrates a lateral view of the RPDC on the polar mount, showing the support base and the paraboloid dish (it is shown in three tilted positions to cope with the annual change of declination as it moves with the seasons), and the way the light pipes redirect the concentrated sun rays to the fixed center on the arm of the support base and then along the fixed line to the foundation of the structure, where it is finally redirected to the desired location. 
           [0033]      FIG. 4  shows an exploded view of the moving mechanism to rotate the RP DC along the polar axis to follow the sun&#39;s movement on the celestial sphere and  FIG. 7C  shows a lateral view of a segment of the polar base with the complementary mechanism. 
           [0034]      FIG. 5  shows an exploded view of the declination moving mechanism, with an arc guide attached at the base of the mirror and a displacement bar to move the mirror along the guide.  FIG. 7C  shows the complementary moving mechanism with a servo motor attached to the axis base. 
           [0035]      FIG. 6A  shows a side view of the main and re-reflecting mirror depicting the shade provided by a shadowing ring and the location of photo-detectors to assist in the tracking accuracy while  FIG. 6B  shows a top view of said arrangement. 
           [0036]      FIG. 7A  shows a front view of a light pipe and  FIG. 7B  shows a ray diagram of light traveling inside the light pipe when there is a slight angle of the rays with respect to the angle of the pipe. 
           [0037]      FIG. 8A  shows a composite top view of a cavity receiving the cavity light pipes and  FIG. 8B  shows a composite side view of said cavity. 
           [0038]      FIG. 9  illustrates a top view of the proposed integration of the paraboloid dish depicting the shape and method of forming the metallic structure to support the paraboloid dish and  FIG. 10  shows a magnified view of a portion of the mirror to illustrate the opening between the glass mirror and the frame and perforations in the mirrors to minimize the resistance to wind. 
           [0039]    Finally,  FIG. 11  shows a sketch of a plurality of RPDC sending the concentrated solar irradiation to a central cavity to feed a power station. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]      FIG. 1  shows a simplified sketch of a tilted view of the RPDC  10 , the object of the invention. The system consists of a paraboloid dish mirror or main mirror  12 , formed by a plurality of segments of paraboloid mirrors  17  constructed and supported with a metallic structure  16  to maintain the shape of the main mirror even through strong winds, with a re-reflecting mirror  11  supported above the paraboloid dish mirror by a plurality of metallic struts  15  attached to the outmost ring of said metallic structure. The metallic structure is attached and supported by a central support disk  14  which holds the metallic structure together. The central support disk is a large disk, about 20% of the radius of the main mirror, which might be flat in the back and concave in the front facing the main mirror, and with engraved channels where the inner part of the metallic structure is placed and fastened, providing support and rigidity. A central light pipe  13 , fully described on  FIG. 7  is attached to a perforation at the center of the main mirror, passes through a hole in the central support disk and goes to the polar axis of rotation of the RPDC. Details of construction of the main mirror are given in  FIG. 9 . 
         [0041]    In the ideal case with rays coming parallel to the axis of the paraboloid dish mirror, with a perfect mirror and perfect alignment, the eccentricity and location of the re-reflecting mirror is predefined mathematically, based on the dimension and eccentricity of the paraboloid dish mirror. The desired theoretical geometric concentration ratio of the system is defined by ratio of the area of the main mirror to the area of the re-reflecting mirror. The eccentricity of the re-reflecting mirror is proportional to the eccentricity of the main mirror times the ratio of the radius of the main mirror to the radius of the re-reflecting mirror. Finally, the re-reflecting mirror is located along the line connecting the focal point of the main mirror with the center of the said main mirror, at such distance that corresponds to the summation of the focal points of the two mirrors. 
         [0042]      FIG. 2A  shows a simplified ray diagram of a lateral view of the RPDC, herein referred as  20 . Common elements with the RPDC  10  shown in  FIG. 1  have the same reference number. Sun rays  24  impinging in the main mirror are shown almost vertically, but not parallel. The sun rays coming at the left hand side of the main mirror are assumed to be coming at an angle of 90.25° with respect to an horizontal line (not shown) connecting the lips of the main mirror, while the sun rays coming from the right hand size of the main mirror are coming at an angle of 89.25° with respect to the same horizontal line, to mimic the sun&#39;s size. The impinging solar rays are shown with a greater density at the extremes of the main mirror than in the center, to represent that solar radiation is constant over an area and this is a two dimensional drawing. The reflected rays  25  are the inclined rays shown to be emanating from the main mirror converging at a point slightly below the re-reflecting mirror. The re-reflected rays  26  are the almost vertical rays coming down from the re-reflecting mirror which pass through the opening or hole  27  in the main mirror, into the central light pipe  13  (not shown, but inferred) and continue down until the re-reflected rays hit the axial mirror  23  that modifies and directs the re-reflected rays in the new direction in the axial light pipe  28  (not shown but inferred). 
         [0043]      FIG. 2A  shows the idealized case, where the sun rays are parallel to the axis of the parabola or perpendicular to the horizontal line connecting the lips of the main mirror, with the re-reflecting mirror having the pre-defined eccentricity and such re-reflecting mirror is located at the exact distance defined as the summation of the two mirrors focal points.  FIGS. 2B to 2D  show examples of possible scenarios when the sun rays are coming only 0.3° (about 5.25 mrads) of the axis of the paraboloid dish.  FIG. 2B  has the location and eccentricity of the re-reflecting mirror at the pre-defined position, while  FIG. 2C  has the re-reflecting mirror moved slightly forward (less than 0.25% of the combined focal lengths) and  FIG. 2D  has the re-reflecting mirror in the position used in  FIG. 2C  but also has modified the eccentricity of the re-reflecting mirror (about 10% of the predefined eccentricity). The labels of  FIG. 2A  are applicable to  FIGS. 2B to 2D , but are not included to avoid cluttering the figures. 
         [0044]      FIGS. 2B to 2D  are illustrations only of the effects that small misalignment would have on the RP DC and the possibility of cures that could be used a priori to prevent or correct unavoidable slight misalignments or deviations. The collectors illustrated in  FIGS. 2B, 2C and 2D  collect 57%, 82% and 92%, respectively, of the incoming rays. The need for accuracy and precision are exacerbated with higher concentration ratios. 
         [0045]    Comparing the ideal ray diagram shown in  FIG. 2A , with some possible outcome in the event that there is a small misalignment, shown in  FIGS. 2B-2D  also illustrate that the conservation of angular momentum could create new problems (bouncing of the light rays on its trajectory along the light pipe) and it is obvious that the best strategy is to minimize misalignments. While a misalignment of 0.25° appears minor, that misalignment coupled with the divergence of the sun rays that are not totally parallel and the need to re-reflect the majority of the rays even if there is a small misalignment, might suggest that having the re-reflected mirror concentrate the rays slightly could be a good solution. However, because of conservation of angular momentum, after the new focus, the rays will start again diverging. Placing additional mirrors or lenses always implies the penalty that some of the energy will be absorbed by the additional mirror or lenses, rather than in the cavity-receiver. 
         [0046]    Constructing a one piece large paraboloid dish is difficult, expensive and would represent a big transportation challenge. While constructing and assembling a multi-segmented precise mirror in the field also poses great difficulties, assembling prefabricated tightly fitting precise parts might allow achieving the desired tolerances. 
         [0047]      FIG. 9  shows the main mirror formed by a plurality of paraboloid mirror segments. In this illustrative embodiment, the main mirror is composed of four concentric donuts, each donut containing a plurality of similar paraboloid mirror segments. The structure or frame supporting the mirrors consists of five concentric circles and a plurality of radial struts connecting the concentric circles alongside each of the mirror segments. Each circle is formed by a plurality of arc segments attached to connectors. Some connectors might be T-connectors, attaching two arc segments with a strut, but other connectors might be symmetric X-connectors connecting two arc segments but also with a lower and an upper strut. In this illustrative embodiment, the main mirror is composed of six similar internal inner circle mirror segments  91 , forming a circle with a radius of about 1.4 m followed by nine first donut mirror segments  92  of the first donut formed by a structural first circle  93  and a structural second circle  94  of about 2.2 m of radius, followed by a second donut with  15  second donut mirror segments  95 , formed by the structural second circle and a structural third circle  96  of about 2.9 m of radius, and the external donut with  18  outer circle mirror segments  97 , formed by the structural third circle and a structural external circle  98  with a radius of about 3.8 m. While other dimensions can work, the illustrative embodiment shows that only four shapes of mirrors are required. Each mirror, a segment of the chosen paraboloid is about 1 m 2 , made out of thermally shaped glass, as typically made by the automotive industry with conventional mirror and protective layers attached to the back thereto. The main mirror structure is formed by the structural inner circle  99  and structural circles  93 ,  94 ,  96  and external circle  98 , with each circle formed by identical arc segments and by a plurality of identical (for each donut) radial struts connecting each of the five circles. The struts are a segment of the parabola that defines the main mirror. Radial struts connecting two circles might have different shape or overall length, but the struts between two circles will be identical, thus all struts  102 ,  103 ,  104  and  105  are identical among themselves, but designated strut  102  is different from other numbered struts. The connectors will be similar within a concentric circle, but different connectors will be needed for each concentric circle. 
         [0048]      FIG. 10  along circle  101  on  FIG. 9  shows some details of the connectors and preparation for placing the mirror in the structure. To allow the wind to pass through the mirror, each mirror segment has a plurality of holes  100  and in addition, there are spaces or openings between the structure and the mirror segments, with concentric spaces  106  between the arcs and the mirror segments and radial spaces  107  between the struts and the mirror segments purposely left surrounding the mirror. The space or void, is provided by purposely using longer mirror holding lips  109  attached to either one of the concentric circles or one of the struts. Depending on the location on the mirror, the holding lips can attach two mirror segments, as illustrated by the double holding lip  109 , but could also attach to only one mirror segment and a concentric ring (not shown). As shown in this portion of the embodiment, by attaching the double or single holding lips to pre-drilled perforations on the arc segment of one of the structural circles  96  or one of the radial struts  102  or  103  composing the structure with appropriate fasteners  111 , and then attaching the mirror segments into the pre-drilled preparations on said lips with appropriate fasteners  110  to receive the mirror segment, a large mirror can be constructed. Small compression springs (not shown) can be placed between the arcs or struts and the mirror segments fastened to the holding lips, to allow for minute adjustments of the mirror pointing direction, to assure that the alignment meets the desired tolerance. Each concentric circle will be formed by identical arc segments, which will be inserted into the corresponding T or symmetrical X connector (not shown) attached to the corresponding struts fastened by screws into the pre-drilled and tapered holes. The main mirror so constructed with identical pieces, with precise drillings for joining the pieces together, will allow the construction of a precise dish paraboloid collector that will be light yet strong, rigid yet flexible. 
         [0049]    The sun tracking mechanism consists of two movements: a polar movement and a declination movement. The emphasis is in a simple and predictable movement to be able to achieve the needed accuracy.  FIG. 3  shows a lateral view of the proposed RPDC  30  on a polar mount and depicting the intended declination movement for the RPDC. It consists of a large concrete base  34 , a proper foundation to provide the desired stability, supporting and anchoring a vertical base  35  which is shown here as the preferred embodiment as being made out of a large diameter pipe segment (some 20-30 inches), which is anchored to the concrete base by flanges and bolts (not shown) with either compression springs, small wedges or a hard rubber like gasket (not shown) to insure that the vertical base is truly vertical. The holes in the flange for the bolts of the vertical base are made slightly larger than the size of the bolts to allow slight movement of the vertical base for exact orientation. The vertical base is truncated in the upper part at an angle that is preferably as close as possible to the site&#39;s latitude. Attached to the vertical base is an axis base  36 , preferably in this embodiment as another pipe segment of similar diameter as the vertical base, which can be fastened to the vertical base with the plane connecting the axis base pipe segment precisely oriented at such angle that corresponds to the site&#39;s latitude and pointing to True North, shown as the broken line  31 , by special flanges (larger than customary to attach the truncated ellipse with a circle), using standard means of fastening (not shown), again using compression springs, wedges or a gasket for adjusting the axial base into its final position and orientation. The upper portion of the axis base will include the polar sun tracking mechanism enclosed within the sun tracking polar axis cap  37 . Attached to the upper part of the sun tracking polar axis cap, there are beam supports  38  holding the main mirror  10  in one extreme and a counterbalancing weight  39  on the other extreme. Also shown on  FIG. 3 , superimposed for clarity, although not actually visible, are the three needed light pipes: (a) the central light pipe  13 , attached to the metallic structure, the inner ring of the main mirror and the support base of the main mirror that receives the re-reflected rays from the re-reflecting mirror and terminating shortly before the axial mirror  141 , (not totally visible because it is located between the beam supports); (b) the axial light pipe  32  that receives the concentrated rays from the axial mirror and delivers the concentrated rays to the cavity mirror  142  (the axial light pipe is not visible while inside the axial base); and (c) the cavity light pipe  33 , that receives the concentrated rays from the axial light pipe via the cavity mirror and delivers it into the cavity-receiver (the cavity light pipe is not visible because it is to be located inside the inferred trench in the concrete base). 
         [0050]    The axis base needs to be oriented precisely to True North (the exact point corresponding to the Earth&#39;s axis of rotation, near the Polar Star). Since the focusing tolerance available is so small, precise construction and erection is needed. The vertical base needs to be truly vertical, the axis base needs to be attached at the proper angle and the unit must point directly to True North. Rather than relying on perfect parts and erection, it is safer to provide for strong yet flexible adjustment. The adjustable mechanism needs to be strong enough to support the moving collector without drifting during the day or lifetime. The proposed solution is probably the less expensive alternative that meets those requirements. 
         [0051]      FIG. 4  shows an exploded view of the polar sun-tracking mechanisms, looking inside the polar axis cap  37 . The polar axis cap has an outer lip  40  with a diameter slightly larger than the axis base  32  and an inner lip  41  with a diameter slightly larger than the diameter of the axial light pipe (not shown). A large roller bearing  43  is firmly attached to the outside wall of the inner lip with the inner ring  44  of the roller bearing while the outer ring  45  is firmly attached to the inner wall of the axis base. An axis gear  46  is attached to the outmost extreme of the inner lip. The rest of the mechanism is shown in  FIG. 7C  as a cut-out along area  47  of  FIG. 4 . It consists of a complementary gear  53  attached to a servo-motor  52  which is fastened to the axial support wall  50  of the axis base. The servo-motor is fastened to the axial support wall with flexible means of fastening  51 , including tension springs (not shown) to absorb pressure from the axis gear if the cap twists, maintaining tension in the connection but preventing damage to the gears. Furthermore, the servo-motor includes means of disengaging the gears such as a disengaging clutch  54  which will allow decoupling the gears in the event of heavy winds, allowing the mirror to swing freely around the polar tracking mechanism. To assist the roller bearing in maintaining the paraboloid dish collector in place despite wind gusts, radial rubber segments  48  can be attached to either the outmost portion of the axial base or to the inside of the cap&#39;s outer lip, which will dampen the bending movement on the cap, relieving stress on the roller bearing. 
         [0052]    The polar sun-tracking mechanism is simple yet robust and includes the elements to dampen sporadic and unpredictable twists that wind gust will be imposing on the mechanism. The servo motor, controlled by a micro-processor or other means, provides the needed steady rotational movement. The signals to the servo will be timed following apparent time (actual local time corrected by the equation of time and the needed corrections to adjust for sidereal/solar time). Since the mirror might have to be stopped when the wind velocity exceeds a prudent threshold or during the night, the micro-processors would have to re-orient the mirror to the desired position every morning or after the wind subsides. 
         [0053]    If the objective was tracking a star, the polar tracking mechanism would suffice. Unfortunately, the sun is much closer and the earth moves around the sun in a path that is inclined with respect to the celestial equator (the ecliptic) and the sun appears to be north of the celestial equator during the summer and below the celestial equator during the winter. To properly trace the sun with a polar mount, the paraboloid dish collector needs to change its declination to point north of the equatorial celestial plane during the summer and south during the winter. The sinusoidal movement is periodic and predictable, but not necessarily constant, with small correction needed during the solstices and almost hourly adjustments during the equinoxes to maintain the desired precision. 
         [0054]      FIG. 5  shows an exploded lateral view of the declination tracking mechanism  60 . The mechanism is attached to the support beams  38  that are attached to the polar axis cap on top of the polar axis tracking mechanism. To compensate for the weight of the mirror, at the other extreme of the support beams, a counterbalancing weight  39  is fastened. A radial guide  61  (there are two symmetric systems; one on each of the support beams but the description is in singular, as seen with the RPDC system at the right hand side). The radial guide is attached to the support beam by a plurality of fasteners  66 . The purpose of the radial guide is to provide for a circular motion to the mirror base  69  that is attached to the central support dish, in a circle that has as the center a point that is in the axis of the polar tracking mechanism to assure that as the mirror moves, the light pipe (not shown) attached to the mirror also moves, and the axial mirror (not shown) redirects the concentrated rays into the axial light pipe. The mirror base is rectangular, dimensioned to fit between the two radial guides and is either welded or fastened to the central support dish. Attached to the mirror base are three pegs or prongs, located in an arc with the same radius of the arc of the circular guide  65  (the perforation or hole in the radial guide). The two outer prongs  162  have a ball bearing that fits inside such circular guide. The central prong  163  also passes through the circular guide and has a ball bearing, but the ball bearing moves inside the vertical guide  68 . The movement of the mirror is produced by a servo-motor  62  that is coupled to a endless threaded shaft  64  that rotates freely with ball bearings (not shown) inside a declination guide box  63  that is fastened to the radial guide with fasteners  67 . The movement of the endless threaded shaft causes the vertical guide to move up or down, forcing the mirror to move along the circular guide  65 . The radial guide and the circular guide are not symmetric. In theory, the mirror only needs to swing up and down 23.45° from the center to accommodate the declination changes, but the radial guide and the circular guide are slightly larger on the lower side (the lower arc angle should correspond to the latitude angle), to allow the mirror to be placed vertically in the event of high winds. 
         [0055]    The vertical guide allows the ball bearing attached to the central prong to be displaced horizontally in and out, depending on the position along the circle. The horizontal movement corresponds to the radius of the circular guide and the cosine of the angle the central prong is displaced with respect to the center of said circular guide. To facilitate the movement of the mirror but also to provide a stabilization force against wind gusts, two shock absorbers, springs or other means of dampening movement (not shown) are connected from the center point of the beam support that corresponds to the axis of the polar tracking mechanism and one of the concentric circles that is part of the metallic structure that forms the main mirror. Both shock absorbers are connected to the same concentric circle, one above the support beam and the second one below the support beam. Both the mirror base and the mirror move freely in circular paths centered in the same spot, and thus properly tensed and/or dimensioned dampening means will dampen any movement produced by the wind. 
         [0056]    During periods of high winds, the main mirror might need to be moved to such position that offers less wind resistance. The proposed solution is simple and economic, using the existing infrastructure. A first measure, discussed above, consists of using a longer lower arc in the circular guide which would allow placing the main mirror vertically, to minimize possible lift. A second measure consists of disengaging the polar axis tracking mechanism which would allow the main mirror to move freely around the ball bearing on the polar axis. Naturally, the main mirror will either face or back the wind, depending on the way the main mirror was facing the wind when the polar axis mechanism was disengaged. Backing the wind is the preferred alternative. Therefore, prior to disengaging the polar axis mechanism, the mirror needs to be turned to a point that a portion of the back faces the wind. A third measure is needed because placing the main mirror perpendicular to the wind minimizes the exposed area. This is accomplished by using a small, 1 m 2  rudder shaped wind vane (not shown) mounted on top of the counterweight structure, at the other extreme of the main mirror. The wind vane would normally move freely with the wind, but that can be locked, and moved with a signal into the desired position, assisting in turning the mirror. Once the main mirror is perpendicular to the wind it will offer less resistance and together with the wind vane, will self correct with shifting gusts. 
         [0057]    The invention requires a minimum of four mirrors: (a) the paraboloid dish mirror or main mirror whose main purpose is to capture the solar irradiation; (b) the re-reflection mirror, a smaller paraboloid mirror located above the main mirror that captures the reflected rays from the main mirror and re-reflects them to the central light pipe; (c) the axial mirror, located at the end of the central light pipe to re-direct the concentrated rays into the axial light pipe, inside the polar tracking mechanism, and; (d) the cavity mirror that redirect the rays coming from the axial light pipe into the cavity light pipe. The axial and cavity mirrors should be preferably plane, but a carefully selected slight curvature (concave) to reduce dispersion of the concentrated rays might be useful. 
         [0058]    Two mirrors need to be moved: (a) the main mirror moved along the two axes, the polar axis and the declination axis, and; (b) the axial mirror, so that when the main mirror is moved along the declination axis, the axial mirror is moved to reflect the incoming irradiation into the axial light pipe. There are two simple alternatives to move the axial mirror: (a) a simple mechanism with an electric motor that will be receiving simultaneously, a proportional signal to the signal sent to the axial declination servo to adjust the main mirror, so that when the declination servo receives a signal to move, the axial mirror moves at the same time, or; (b) a simple, gravity actuated weight that moves a gear which in turn, moves a complementary gear to move the mirror. The re-reflecting mirror and the cavity mirror are fixed at the desired position. 
         [0059]    The invention also requires a minimum of three light pipes: (a) the central light pipe connected to the center of the main mirror and directing the light to the axis mirror; (b) the axis light pipe, inside the axial base, receiving the light from the axis mirror and transporting the light to the cavity mirror, and; (c) the cavity light pipe, inside a trench on the concrete base, that transfers the light from the cavity mirror to the cavity. The length of the pipes is different. The cavity light pipe should be oriented towards the cavity-receiver to avoid having to re-direct again the concentrated rays. To capture some of the heat lost as some of the concentrated rays hit the light pipes, it might be advisable in an alternative embodiment, to enclose at least the exposed portion of the axial light pipe and the cavity light pipe inside another pipe with the chosen working fluid flowing the annular area in the same direction of the light, pre-heating the working fluid. Counter-current flow might be thermally more effective, but co-current flow minimizes the thermal losses of the pre-heated working fluid and centralizes the hot area around the cavity. 
         [0060]      FIG. 7A  shows a frontal view of a light pipe  70 . It is formed by a glass pipe  71  to which a mirror substance  72  (silver or aluminum compounds) has been deposited on the outside surface of the glass pipe following standard mirroring techniques (silvering, sputtering or vacuum depositing) and an additional paint, substrate or a passivation-protective layer  73  to protect the mirror substance. There is experience in the industry to cover curved surfaces with mirror like substances. In the alternative embodiment of placing the light pipe inside another pipe with the working fluid, having the mirror or silvering method used in the inside of the glass pipe and covering the mirror substance with a protective transparent substance, would ease concerns about corrosion by the working fluid. 
         [0061]      FIG. 7B  shows a composite view of a portion of a light pipe  76  presenting a ray diagram inside the mirror as if said mirror was transparent, with a multitude or concentrated rays  75 , assuming that the rays are coming slightly off the longitudinal axis of the pipe, and focusing around the middle of the segment, and therefore some of the concentrated rays hit the mirror substance at a small angle and get reflected back into the pipe, to hit against the opposite wall of the mirror some distance downstream of the pipe. Even if the bulk of the concentrated rays are perfectly aligned with the longitudinal axis of the pipe, since the rays are not perfectly parallel but maintain the angular momentum of the sun&#39;s rays, some rays will be bouncing back and forth from the walls of the light pipe. The ray diagram does not show small deviations due to the glass index of refraction. The effect is minimal given the large ratio between the light pipe diameter and the thickness of the glass. With each bounce some energy is transformed into thermal energy. Small misalignment might result in thermal energy building up in the light pipe which could be recovered, if enclosed with another pipe carrying working fluid as mentioned in an alternative embodiment. 
         [0062]    Despite the simplicity of the predictable movements with the sun tracking mechanism, means to provide additional adjustments are needed with a separate control loop that will be utilizing photo-detectors to correct, if needed: (a) faulty signals from the micro-processor to move the RPDC along either one of the axes; (b) any misalignment of the base and polar axis due to settlement or wear, or; (c) restore the RPDC to its intended position following a shut-down for maintenance or expected high winds. Four photo-detectors, aligned with the four cardinal points, nearby the central light pipe, with a simple control mechanism, would provide corrective signals to orient the RPDC. To protect the photo-detectors from concentrated solar irradiation, it is preferable to put a shadow disk around the re-reflecting mirror and place the photo-detectors outside the shadow ring, exposing said photo-detectors only to direct solar irradiation, as shown in  FIG. 6A , which shows a lateral view of the arrangement. The re-reflecting mirror has a shadow ring  81  surrounding said re-reflecting mirror and extending some 5 cm out. The shadow ring is part of the metallic structure that is holding the re-reflecting mirror in place and will be supported by the metallic struts  15  (not shown) attached to it. The shadow ring will cast a shadow on the main mirror, surrounding the concentrated rays that are aimed to the central light pipe. A plurality of photo-detectors  84  will be placed radially from the center point of the central light pipe, near the edge of the shadow projected by the shadow ring, on the main mirror, and thus will have direct solar irradiation hitting them directly. If there is a misalignment, one or more of the photo-detectors will be first covered by the shadow and will transmit lower voltage to a controller that would trigger movement of the mirror.  FIG. 6B  shows a top view of the location of the photo-detectors on the main dish, with the shadow cast by the shadow ring and a truncated portion of the metallic struts, herein designated as  15 A. The shadows show areas of umbra  82  and penumbra  83 . The controlling mechanism is shown in Cordy&#39;s patent. 
         [0063]    A cavity or solar furnace, herein referred as the cavity-receiver, is a well insulated enclosure with small openings or apertures, to let the radiation in. To assure a better distribution of the captured heat and avoid problems with overheating unevenly the tubes where the working fluid will be circulating, in the preferred embodiment, the solar irradiation coming in from the light pipes is used to heat directly the working fluid in the cavity-receiver.  FIG. 8A  shows a top view of a sketch of the cavity-receiver  120  (without the top), consisting of a cylindrical pressure vessel  121 , surrounded by a thick layer of insulation  134 . The cavity-receiver has a plurality of cavity ports or openings  124  where the incoming concentrated solar irradiation coming inside the cavity light pipes  33  enters the cavity-receiver. The path of the cavity light pipes should be properly aligned so that none of the ports should face another port on a straight line. Each of the ports is covered by a thick silica glass  125 .  FIG. 8B  shows a sketch of a lateral view of the cavity-receiver. The cavity-receiver is fed by a liquid working fluid line  128 , which is controlled by a valve and level signals (not shown), and an outlet line  129 , that takes the steam or vapor of the working fluid to the power center. A demister  131 , in top of the cavity-receiver, separates entrapped liquid drops of working fluid from the vapor or steam of the working fluid. A different cavity-receiver for thermal or photo-chemical reactions would be needed but it is outside the scope of this invention. 
         [0064]    Finally,  FIG. 11  shows a potential embodiment of the invention with a plurality of RPDC  110  connected via the axial light pipes that are connected to the cavity light pipes into a cavity-receiver  115  where steam is produced and is transferred to a power center  114  via steam pipe line  113  where the steam is used to move a turbine. A condenser (not shown) condenses the spent steam and returns it to the cavity-receiver via recirculation pipeline  112 . 
         [0065]    Other embodiments are possible, utilizing lenses and mirrors to have many of the cavity light pipes merge the light into a plurality of light manifolds and the light manifolds delivering further concentrated solar irradiation to the cavity-receiver, or utilizing the concentrated rays into the cavity-receiver to heat molten salts or metals to store energy for night use, or using the concentrated rays in thermo or photochemical reactions to produce hydrogen. 
         [0066]    Certain features of this invention might sometimes be used to advantage without a corresponding use of other features. While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. It is, therefore, apparent that there is provided in accordance with the present invention, a system and method for solar energy capture and conversion. While this invention has been described in conjunction with a number of embodiments, it is evident that many alternatives, modifications and variations would be or are apparent to those of ordinary skills in the applicable arts. Accordingly, this invention intends to embrace all such alternatives, modifications, equivalents and variations that are within the spirit and scope of this invention.