Abstract:
A concentrating solar energy device for use in buildings is configured to share structural elements with a building for cost savings. The device incorporates a shallow cylindrical trough mirror comprising mirrored sheets that are conformed to curved rafters, a receiver that is moveable within the cumulative area of focus of the cylindrical trough mirror, a secondary mirror integrated with the receiver that augments solar energy collection, and parallel linear tracking assemblies that move the receiver and that mount to a building&#39;s side walls.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to concentrating solar energy devices, particularly those that are integrated with buildings. These devices can be thermal or electrical in principle, and they provide energy for buildings or to the electrical grid. It is advantageous that these systems are efficient and integrate readily into buildings for cost savings. 
         [0002]    Of the currently deployed concentrating solar energy devices, the majority use relatively large optical elements consisting of parabolic mirrors or Fresnel lenses. These optics yield very high solar concentrations but with correspondingly narrow solar acceptance angles. The latter characteristic requires that the optics and solar energy receiver follow the sun by means of an accurate solar tracker. This considerable moving bulk renders these high concentration devices difficult to integrate into buildings. 
         [0003]    In contrast, a concentrating mirror in the shape of an open-ended shallow trough with circular arc profile, commonly referred to as a cylindrical mirror, can remain fixed as part of a building roof yet provide a medium degree of solar concentration. A cylindrical mirror is often classified in the field of the invention as a non-imaging mirror. Other non-imaging mirrors have been taught in the prior art, such as those with anticlastic or dual-parabolic shapes, but their complex curvatures make them difficult to manufacture and integrate into buildings. A cylindrical mirror, however, is readily made by curving flat reflective sheets and fixing them to supports with pre-cut curvatures. 
         [0004]    A cylindrical mirror projects an oblong area of focus parallel to its axis of curvature. As the sun traverses a cylindrical mirror that is longitudinally oriented east-west, focus changes in both position and concentration ratio. The compiled area of this dynamic focus is the cylindrical mirror&#39;s cumulative area of focus. In order to collect solar energy efficiently, a narrow linear receiver is dynamically positioned by a tracking mechanism within the cumulative area of focus. 
         [0005]    Cylindrical mirrors and other non-imaging primary mirrors in concentrating solar energy systems typically have oblong axes of curvature oriented east-west. In this orientation, the receiver, also aligned east-west, tracks across the mirror&#39;s width as solar declination varies seasonally. The annual extent of solar declination is approximately 47 degrees at latitudes between 30 and 40 degrees north, where promising solar sites abound. This indicates that the receiver need only move tiny increment from day to day, and its position never exceeds the field of the mirror. 
         [0006]    A cylindrical mirror&#39;s peak ratio of concentration varies inversely with its arc. For example, an arc of 72° will yield approximately 31x peak concentration, while a shallower arc of 40° will yield approximately 71x peak concentration. Collecting solar energy at higher concentration by using minors with shallow arcs is advantageous, as the minor requires less material for its construction. A cylindrical mirror with shallow arc profile also integrates more readily into a building roof, and a relatively narrower receiver can be utilized. 
         [0007]    A cylindrical mirror in east-west orientation is ideally inclined toward the equator at an angle from the horizontal approximately equal to the latitude of the cylindrical minor&#39;s location. At this angle, solar declination during an equinox translates to an incidence normal to the cylindrical minor&#39;s chord. As solstice approaches, solar incidence gradually moves off normal. The limit, at solstice, is approximately 23° off normal for locations at latitudes between 30 and 40 degrees. 
         [0008]    As solar incidence moves off normal, concentration ratio of a cylindrical minor decreases. For example, when solar incidence is normal to the chord of a cylindrical mirror of 40° arc, concentration ratio is 71x. When solar incidence is 20° off normal, however, concentration ratio is reduced to 48x. As such, a solar energy receiver just wide enough to encompass a cylindrical minor&#39;s focus at normal incidence would be too narrow to capture the wider focus at 20° off normal. 
         [0009]    A cylindrical minor&#39;s variation in concentration therefore requires careful selection and optimization of a solar receiver. Highly efficient evacuated solar tube collectors are now standard components in parabolic trough concentrating devices. These devices are in widespread use for utility-scale concentrating solar power systems. Evacuated solar tube collectors are most commonly available as a 70 mm diameter absorber tube surrounded by a 120 mm evacuated clear glass tube. 
         [0010]    The high efficiency that makes these solar tube collectors attractive for use in parabolic concentrators also benefits cylindrical mirror concentrating devices. However, when solar incidence is off normal with respect to a building-integrated cylindrical mirror of practical size, focal width will exceed the 70 mm aperture of standard collector tubes. This off-normal focal width could be captured with a double row of collector tubes but at a penalty of double expense and weight. A lower-cost solution is use of additional optics to augment solar energy collection of a single-row of collector tubes. 
         [0011]    Another design challenge for systems with non-imaging concentrating mirrors is finding the optimum path for a receiver to track the minor&#39;s dynamic focus. Two principal tracking methods have been employed in prior art, by mounting the receiver on pivoting arms that direct it in an arc path, or by mounting the receiver to a linear tracking device that drives the receiver along a linear path. 
         [0012]    When these paths were carefully optimized by the inventor, specifically for a cylindrical mirror with 40° arc, differences in annual energy yield was inconsequential. However, a pivoted tracker places the entire receiver weight and torsional load onto relatively small mounting areas, while a linear tracker distributes these loads across a relatively broader area. This is a concern when the mounting areas are wood or concrete block walls, since these materials are prone to fracturing under repetitive stress. 
         [0013]    In prior art, roof-mounting of a tracking mechanism creates several problems, namely, increased roof load, increased solar shading of the primary mirror, difficult access for maintenance and monitoring, and additional roof penetrations. Mounting of a tracking mechanism on a building&#39;s sidewalls obviates these issues. 
         [0014]    A sidewall-mounted tracking mechanism, however, necessitates a receiver long enough to span clear of the building roof. The receiver&#39;s additional thermal expansion and elasticity must be dealt with in order to reduce stress on structural components. This issue has not been adequately addressed in prior art but can be solved with appropriate structural design. 
       SUMMARY OF THE INVENTION 
       [0015]    Accordingly, it is an advantage of the present invention that concentrated solar energy heats a thermal fluid to efficiently provide energy for a building. 
         [0016]    It is another advantage of the present invention that its primary optical component, a cylindrical mirror, is not required to track the sun, thereby reducing moving bulk of the device. 
         [0017]    It is another advantage of the present invention that its cylindrical mirror can be fixed as part of a building roof. 
         [0018]    It is another advantage of the present invention that it&#39;s solar energy receiver incorporates highly efficient, commercially available evacuated solar tube collectors. 
         [0019]    It is another advantage of the present invention that a secondary mirror augments solar energy collection of the receiver while providing it with weather protection. 
         [0020]    It is another advantage of the present invention that its receiver is impelled by a linear tracking mechanism. 
         [0021]    It is another advantage of the present invention that its tracking mechanism can be located at the sides of a building. 
         [0022]    It is another advantage of the present invention, that although its receiver spans clear of the building roof, the receiver is structurally supported at intervals along its length. 
         [0023]    It is another advantage of the present invention that its linear tracking mechanism supports the weight of the receiver across a broad mounting area, thereby minimizing stress fracturing. 
         [0024]    It is another advantage of the present invention that it includes means for reducing structural stresses induced by thermal expansion of the receiver. 
         [0025]    It is another advantage of the present invention that it shares supporting members with a building structure for cost savings. 
         [0026]    The above and other advantages disclosed herein are carried out by the present invention, which is a concentrating solar energy device that can be structurally integrated with a building. In the preferred embodiment, a cylindrical trough mirror, integrated as part of a building roof, concentrates sunlight onto a solar energy receiver that is impelled by a linear tracking mechanism. 
         [0027]    The receiver in the preferred embodiment is of the thermal type, although a photovoltaic type or combined photovoltaic and thermal type could be employed. The receiver is oriented east-west and is positioned by the tracking mechanism optimally within the cylindrical mirror&#39;s focus. The receiver is supported at each end by upright conduits that allow thermal fluid to circulate between the receiver and the building in which the invention is installed. These upright conduits are fastened to carriages of the linear tracking mechanism that run on linear bearings fastened to the building side walls. Carriage position is adjusted by twin screws driven by synchronous stepper motors. With this arrangement, the receiver tracks focus of the cylindrical mirror. 
         [0028]    The receiver incorporates commercially available evacuated solar thermal tubes in a single row series. A secondary mirror array is deployed above this series in order to augment solar energy collection and to protect it from inclement weather. 
         [0029]    In its preferred embodiment, the invention is structurally integrated into a building. During operation, a cylindrical mirror focuses concentrated solar energy onto a moveable receiver that transfers collected heat to a fluid within the receiver. The heated fluid is circulated between the receiver and the building&#39;s heating and cooling equipment to provide renewable space heating, air conditioning, and water heating. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  is a perspective view of a concentrating solar energy device. 
           [0031]      FIG. 2  is a perspective view of a concentrating solar energy device integrated into a building. 
           [0032]      FIG. 3  is a perspective view of a solar thermal tube array and a secondary mirror. 
           [0033]      FIG. 4  is a perspective view of mounting parts for a secondary mirror. 
           [0034]      FIG. 5  shows an assembled solar thermal receiver. 
           [0035]      FIG. 6  is a detail perspective view of a left end attachment for a secondary mirror. 
           [0036]      FIG. 7  is a detail perspective view of intermediate attachments of secondary mirrors. 
           [0037]      FIG. 8  is a detail perspective view of intermediate fixed attachments of secondary mirrors. 
           [0038]      FIG. 9  is a detail perspective view of a right end attachment for a secondary mirror. 
           [0039]      FIG. 10  is a profile view of a cylindrical mirror and a set of impinging solar rays at equinox. 
           [0040]      FIG. 11  is a profile view of a cylindrical mirror and a set of impinging solar rays in summer. 
           [0041]      FIG. 12  is a profile view of a cylindrical mirror and a set of impinging solar rays in winter. 
           [0042]      FIG. 13  is a profile view of a cylindrical mirror and three sets of resultant solar rays. 
           [0043]      FIG. 14  is a profile view of a cylindrical mirror, a set of resultant solar rays, and a receiver. 
           [0044]      FIG. 15  is a profile view of impinging rays on a secondary mirror and reflected rays striking a solar thermal tube array. 
           [0045]      FIG. 16  is a profile view of a cylindrical mirror and its cumulative area of focus. 
           [0046]      FIG. 17  is a profile view of a cylindrical mirror, its cumulative area of focus, a solar receiver, and possible tracking paths. 
           [0047]      FIG. 18  is a perspective view of a tracking mechanism, a receiver, and supporting members for the receiver. 
           [0048]      FIG. 19  is a perspective view of the left assembly of a linear tracking mechanism. 
           [0049]      FIG. 20  is a detail perspective view of one corner of a carriage mount. 
           [0050]      FIG. 21  is a perspective view of the right assembly of a linear tracking mechanism. 
           [0051]      FIG. 22  is a left perspective view of a concentrating solar energy device integrated into a building. 
           [0052]      FIG. 23  is a right perspective view of a concentrating solar energy device integrated into a building. 
           [0053]      FIG. 24  is an exploded perspective view of structural components of a building designed to support a concentrating solar energy device. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0054]      FIG. 1  shows a concentrating solar energy device  31  that can be structurally integrated into a building to provide it with energy. Linear tracking assemblies  37 L and  37 R are symmetrically identical and position receiver  48  optimally within the concentrated focus of cylindrical mirror  32 . 
         [0055]    A cylindrical mirror is herein defined as an open-ended trough that is curved in a circular arc around its longitudinal axis and having a reflective concave field. Advantageous to the invention is that cylindrical mirror  32 , which comprises the primary optical element in the device, need not track the sun in order to capture and concentrate solar energy consistently. Cylindrical mirror  32 , accordingly, remains fixed, which simplifies structural integration of the invention into buildings. 
         [0056]    Cylindrical mirror  32  has a reflective surface  32 A applied to its concave field, which is disposed skyward. Cylindrical mirror  32  is made from sheet metal conformed to cylindrical curvature. Reflective surface  32 A is comprised of commercially available adhesive-backed solar mirror film. 
         [0057]    In  FIG. 2 , concentrating solar energy device  31  is shown integrated into building  34 . Fixed cylindrical mirror  32  focuses solar energy onto receiver  48 , heating a thermal fluid circulated within receiver  48 . This fluid is communicated communally within pipe elbows  35 L and  35 R, supporting pipes  36 L and  36 R, and hoses  33 L and  33 R, which in turn are connected to commercially available heating, cooling, and hot water systems, not shown, for building  34 . Such systems typically utilize fluid pumps, heat pump processes, heat exchangers, and thermal storage. When operatively connected to such systems, device  31  supplies them with thermal energy to drive space heating, process heating, air conditioning, and water heating. 
         [0058]    The invention&#39;s receiver components and assembly are now disclosed.  FIG. 3  shows a collector array  38 , comprised of three identical, commercially available evacuated solar tube collectors  40 A,  40 B, and  40 C welded end-to-end in a linear series. This arrangement permits their mutual communication of thermal fluid. Secondary mirror array  41  is comprised of identical secondary mirrors  42 A,  42 B, and  42 C, which are longitudinally aligned with and proximally centered, respectively, above tube collectors  40 A,  40 B, and  40 C. Secondary mirror array  41  performs two functions relating to collector array  38 , namely, augmentation of solar energy collection and weather protection. 
         [0059]    Secondary mirror  42 A is an oblong, shallow trough mirror made by metal extrusion, roll-forming, or by other means by those knowledgeable in the art of metal fabrication. Reflective surface  43 , comprised of commercially available adhesive-backed solar mirror film, is applied to the lower-disposed concave field of secondary mirror  42 A. Secondary mirrors  42 B and  42 C are identical to secondary mirror  42 A. 
         [0060]      FIG. 4  shows parts used to connect secondary mirrors  42 A,  42 B and  42 C ( FIG. 3 ) to collector array  38 . In  FIG. 4 , connector  45  is an end connector, and connector  46  is an intermediate connector. Each has a cylindrically curved, radially oriented convex top surface which matches the concave cylindrical curvature of the lower fields of secondary mirrors  42 A,  42 B and  42 C ( FIG. 3 ). In  FIG. 4 , connectors  45  and  46  each has a cylindrical void that corresponds respectively with the circumference of the ends of tube collectors  40 A,  40 B, and  40 C ( FIG. 3 ). 
         [0061]      FIG. 5  shows an assembled receiver  49  wherein secondary mirrors  42 A,  42 B and  42 C are attached in a series at four equidistant attachment points above collector array  38 . 
         [0062]    In  FIG. 6 , a detail perspective view, a connector  45  attaches around the circumference of the left exposed end of absorber tube  40 A. Connector  45  also attaches perpendicular to and flush under the left end of the lower field of secondary mirror  42 A. 
         [0063]    Secondary mirror  42 A has parallel mounting slots  44  cut through its thickness at one end of its length and is loosely bolted to connector  45  through slots  44 . This mounting arrangement decouples longitudinal thermal expansion of tube collector  41 A from secondary mirror  42 A during solar exposure. This eliminates structural stresses on secondary mirror  42 A induced by thermal expansion of tube collector  41 A. 
         [0064]      FIG. 7  and  FIG. 9  show, respectively, similar attachments utilizing slots  44  of secondary mirrors  42 B and  42 C. In  FIG. 7 , a detail perspective view, a connector  46  is fixed concentrically around the circumference of the junction of tube collectors  40 A and  40 B. Connector  46  also joins the adjacent ends of secondary mirrors  42 A and  42 B by attaching perpendicular to and flush under the lower fields of secondary mirrors  42 A and  42 B. Secondary mirror  42 A is bolted tightly to connector  46 . 
         [0065]    In  FIG. 8 , a detail perspective view, a connector  46  is fixed concentrically around the circumference of the junction of tube collectors  40 B and  40 C. Connector  46  also joins the adjacent ends of secondary mirrors  42 B and  42 C by attaching perpendicular to and flush under the lower fields of secondary mirrors  42 A and  42 B. Secondary mirrors  42 B and  42 C are bolted tightly to connector  46 . 
         [0066]    In  FIG. 9 , a detail perspective view, a connector  45  attaches around the circumference of the right exposed end of absorber tube  40 C. Connector  45  also attaches perpendicular to and flush under the right end of the lower field of secondary mirror  42 C. 
         [0067]    We will now disclose optical geometry of the invention. When a cylindrical mirror&#39;s axis of curvature is oriented east-west, the sun&#39;s right ascension angle, which corresponds to earth&#39;s daily rotation cycle, has no effect on concentration. The sun&#39;s declination angle, however, is directly related to cylindrical mirror concentration. Declination angle corresponds to earth&#39;s annual cycle around the sun. 
         [0068]    In  FIG. 10 , cylindrical mirror  32  is tilted from horizontal toward the equator at an angle from the horizontal equivalent to installation latitude. Incident solar rays  51  impinge on cylindrical mirror  32  during a summer day. In  FIG. 11 , incident solar rays  52  impinge on cylindrical mirror  32  during a solar equinox, which occurs in spring or fall. In  FIG. 12 , incident solar rays  53  impinge on cylindrical mirror  32  during a winter day. 
         [0069]    Referring to  FIG. 13 , resultant rays  54 ,  55 , and  56  were ray-traced from corresponding incident rays  53  ( FIG. 10 ),  52  ( FIG. 11 ), and  51  ( FIG. 12 ). In  FIG. 13 , a comparison of foci  57 ,  58 , and  59  demonstrates that cylindrical mirror  32  maintains focus in different seasons despite its fixed position. Foci  57  and  59  are larger in area than focus  58 , however, and to maximize capture of focused solar energy in these larger areas, an optical augmentation means will be demonstrated. 
         [0070]      FIG. 14  shows focus  61  during a mid-summer day. Focus  61  represents the average size focus produced by the invention&#39;s cylindrical mirror. It is readily apparent that focus  61  exceeds the diameter of collector array  38 , shown in profile within focus  61 . Approximately two-thirds of the rays in focus  61  miss collector array  38  and are not collected as solar energy. 
         [0071]      FIG. 15  demonstrates solar energy augmentation by secondary mirror array  41 , whereby most said misses are redirected onto collector array  38  as reflected rays  63 , depicted with solid lines. 
         [0072]    In  FIG. 16 , a one year time-lapse depiction of resultant ray traces  64  from cylindrical mirror  32  suggests a cumulative area of focus  65  within which a receiver should travel for optimum solar energy collection. 
         [0073]      FIG. 17  shows receiver  49 , disposed within cumulative area of focus  65  such that secondary mirror  41 , at mid travel between summer and winter seasons, is in parallel opposition to cylindrical mirror  32 . We will consider two paths of travel for receiver  49  within cumulative area of focus  65 . A linear path  62 A, parallel to cylindrical mirror chord  32 B, would result from use of a linear tracking mechanism. A circular path, depicted by dotted line  62 B, would result from use of a pivoted tracking mechanism. A pivoted tracking geometry is conceptualized by elongated receiver support  90  and its stationary pivot  91 , which together would constrain receiver  49  around circular path  62 B. 
         [0074]    An advanced ray-tracing analysis comparing a series of linear receiver paths to a series of circular receiver paths was performed by the inventor. Path distance from a cylindrical mirror of 40° arc was varied in the two series; radius was also varied in the circular path series. Results of this investigation demonstrated little difference in annual energy collected by a receiver traveling the most promising paths of each series. 
         [0075]    However, a pivoted tracking mechanism generating a circular receiver path concentrates the entire weight and torsional load of a receiver onto a relatively small mounting area at the pivot. A linear tracking mechanism, on the other hand, distributes weight and torsional load of a receiver over a much larger mounting area. A linear tracking mechanism can thereby be mounted more reliably to building members constructed of wood or concrete block in consideration of the well known propensity of these materials to develop stress fractures when subjected to concentrated loads. 
         [0076]    We now disclose the invention&#39;s linear tracking mechanism. Turning to  FIG. 18 , receiver  49  is supported by and fixed at each end to, via pipe elbows  35 L and  35 R, the tops of pipes  36 L and  36 R. Pipes  36 L and  36 R are opposed parallel to one another in a plane perpendicular to linear path  62 A. The lower thirds of the lengths of pipes  36 L and  36 R are fastened respectively to carriages  68 L and  68 R. Carriages  68 L and  68 R are symmetrically opposed and perpendicular to the length of receiver  49 . The top edges of carriages  68 L and  68 R are disposed parallel to linear path  62 A. 
         [0077]    In order to stabilize receiver  49  from gravity deflection, diagonal supports  50 L and  50 R are employed. Diagonal support  50 L is an elongated member that connects at its higher disposed end to a connector  46  and at its lower disposed end to connector  70 L. In symmetrical fashion, diagonal support  50 R is an elongated member that connects at its higher disposed end to a connector  46  and at its lower disposed end to connector  70 R. 
         [0078]    When receiver  49  expands longitudinally during solar exposure, the tops of pipes  36 L and  36 R are flexed opposite each other. This could stress components joined directly or indirectly to pipes  36 L and  36 R. As a means of accommodating longitudinal expansion of receiver  49  and reducing said stresses, pipes  36 L and  36 R are comprised of PTFE, a moderately flexible high-temperature plastic. 
         [0079]    In operation of the invention&#39;s linear tracking mechanism, stepper motors  74 L and  74 R are operated synchronously by a commercially-available programmable electronic controller, not shown, to position receiver assembly  49  at precise locations along annual path  62 A, for the purpose of optimizing solar energy collection of receiver  49 . 
         [0080]      FIG. 19  shows linear tracking assembly  37 L. Carriage  68 L is comprised of a flat square plate with its fields disposed vertically and with top edges disposed parallel to linear path  62 A. Pipe  36 L is mounted to the outer field of carriage  68 , bisecting it. Brackets  66  are fixed concentrically around pipe  36 L at intervals along its lower length and are bolted to the outer field of carriage  68 L. Linear bearings  76 L run parallel in a vertical plane and are parallel to linear path  62 A. Carriage  68 L is attached to rollers  77 L that are partially enclosed by and roll within linear bearings  76 L as shown in detail in  FIG. 20 . 
         [0081]    In  FIG. 19 , linear bearings  76 L, stepper motor  74 L, and bearing  82 L must be fixed to a common mounting surface, such as a building&#39;s sidewall, for tracking assembly  37 L to operate. In operation, carriage  68 L can slide on linear bearings  76 L along linear path  62 A. Carriage  68 L has an attached ball nut  81 L that is engaged with acme screw  80 L. Bearing  82 L stabilizes the free end of acme screw  80 L. When stepper motor  74 L rotates acme screw  80 L, screw action impels carriage  68 L along annual path  62 A. 
         [0082]      FIG. 21  shows linear tracking assembly  37 R. The construction and operation of tracking assembly  37 R is symmetrically identical to tracking assembly  37 L with one exception: acme screws  80 R and  80 L ( FIG. 19 ) both have right-hand threads. 
         [0083]    For cost savings, the invention shares structural components with a building. In  FIG. 22 , cylindrical mirror  32  is integrated into building  34  as a partial roof. Tracking assembly  37 L is mounted to building sidewall  84 L via linear bearings  76 L. Linear bearings  76 L are bolted flat against and near the top of the outer disposed field of sidewall  84 L, and are longitudinally disposed parallel to linear path  62 A. 
         [0084]    In symmetrical opposed fashion to tracking assembly  37 L,  FIG. 23  shows tracking assembly  37 R supported by building sidewall  84 R via linear bearings  76 R. 
         [0085]    Use of a linear tracking mechanism rather than a pivoted tracking mechanism is advantageous when the invention is integrated into a building. A linear tracking mechanism distributes weight and torsional load of a receiver and its supports, via linear bearings, across a relatively wide area of a building&#39;s sidewalls. This arrangement minimizes stress fractures developing in the sidewalls, particularly if the sidewalls are constructed of wood or stone. The sidewall mounting location of the invention&#39;s tracking mechanism requires no roof penetrations and simplifies access for monitoring and maintenance of tracking components. 
         [0086]      FIG. 24  shows an exploded view of structural components of building  34  and cylindrical mirror  32 . Rafters  89  are disposed parallel to sidewalls  84 L and  84 R and are distributed at intervals under the length of cylindrical mirror  32  to which they add support and hold shape. Rafters  89  are supported from below by front wall  87  and rear wall  88 . 
         [0087]    The top front surfaces of sidewall  84 L, sidewall  84 R, and rafters  89  are each pre-cut to equal circular arcs with a common axis parallel to the length of building  34 . 
         [0088]    Cylindrical mirror  32  is formed from flexible sheet metal conformed to said pre-cut top front surfaces of sidewall  84 L, sidewall  84 R, and rafters  89 . Cylindrical mirror  32  can be attached to said surfaces with hardware fasteners, not shown. Fascia  90 , comprising a long board or series of boards, is attached flush to the front of rafters  89 , and, as shown in  FIG. 23 , attached flush to the top of the front surfaces of sidewalls  84 L and  84 R. The front edge of cylindrical mirror  32  meets the top rear edge of facia  90  and overhangs front wall  87  as an eave, thereby shading front wall  87  in summer for a passive cooling effect. 
         [0089]    In  FIG. 24 , an exploded perspective view, building  34  has interior walls  85  and  86  disposed parallel to sidewalls  84 L and  84 R and arranged at intervals along the length of building  34 . The front surfaces of interior walls  85  and  86  are joined flush and perpendicular to the rear field of front wall  87 . The rear surfaces of interior walls  85  and  86  are joined flush and perpendicular to the front field of rear wall  88 . 
         [0090]    Interior walls  85  and  86 , front wall  67 , and rear wall  68  can be prefabricated as insulated panels or can be framed on site using construction techniques in common practice. In the preferred embodiment, sidewall  84 L, sidewall  84 R, and rafters  89  are prefabricated from dimensionally stable wood with curvatures cut by a computer-controlled router. 
         [0091]    In an alternate embodiment, the invention is all or partially free-standing and supplies energy for custom needs. Example purposes are process heat, distributed heat, ice making, electric vehicle charging, and utility-scale electricity generation. 
         [0092]    Various materials and manufacturing techniques are described above for which a person familiar with the relevant art could easily find alternatives or substitutes. No material or manufacturing technique described herein is intended to eliminate other materials or methods that could be used to achieve functional end results similar to those described.