Patent Publication Number: US-2019190440-A1

Title: Systems and methods for improving light collection of photovoltaic panels

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 62/591,644, filed Nov. 28, 2017, to U.S. Provisional Application No. 62/616,996, filed Jan. 12, 2018, and to U.S. Provisional Application No. 62/722,733, filed Aug. 24, 2018, the entirety of each of which are herein incorporated by reference. 
    
    
     FIELD 
     The current subject matter pertains to solar photovoltaic (PV) power plants. 
     BACKGROUND 
     PV panels have a front side and a back side. Some PV panels, known as bifacial panels, can collect light from both the front side and the back side. The light incident on the back side of the PV panel can be limited since the back side of the panel may be pointed away from the sun. Power generation of a bifacial PV can be improved by increasing incident light on the backside of the panels. 
     SUMMARY 
     A system comprising a support structure mounted to an underside of bifacial photovoltaic panels arranged in a row is provided herein. The support structure comprises one or more elongated structural members extending along and in a direction parallel to the row. The support structure further comprises one or more pivot arms that rotate about an axle at a top of the support structure, the one or more pivot arms positioned in a perpendicular direction to the one or more elongated structural members, the one or more pivot arms connected to the one or more elongated structural members. The one or more structural elements of the support structure have a reflective outer surface to increase reflected light to the underside of the bifacial photovoltaic panels. 
     A method is provided herein for assembling a system for rotating bifacial photovoltaic panels arranged in a row. In the method, a support structure is assembled for the bifacial photovoltaic panels. One or more pivot arms that rotate about an axle at a top of the support structure are positioned. One or more elongated structural members are positioned to extend along and parallel to the row and in a perpendicular direction to the one or more pivot arms. The one or more elongated structural members are connected to the one or more pivot arms. An underside of the bifacial photovoltaic panels is mounted to the support structure. One or more structural elements of the support structure are coated with a reflective coating to increase reflected light to the underside of the bifacial photovoltaic panels. 
     A system comprising a support structure and one or more bifacial photovoltaic panels arranged in a row is provided herein. The support structure comprises one or more elongated structural members extending along and parallel to the row. The support structure further comprises one or more pivot arms that rotate about an axle at a top of the support structure positioned in a perpendicular direction to the one or more elongated structural members. The one or more pivot arms are connected to the one or more elongated structural members. One or more bifacial photovoltaic panels are arranged in a row. The one or more bifacial photovoltaic panels are affixed to the support structure. The one or more elongated structural members have an outer surface with a reflective coating to increase reflected light to the surface of the one or more bifacial photovoltaic panels. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIGS. 1A and 1B  show two perspective views of a solar tracker with a number of solar panels mounted on it. 
         FIG. 2  schematically illustrates a detailed view of back sides of solar panels and a mounting structure from a solar collector. 
         FIG. 3  schematically illustrates a close-up view of a clamp standoff. 
         FIG. 4  schematically illustrates a clamp standoff in detail. 
         FIG. 5  schematically illustrates a perspective view of a maintenance vehicle that can travel along a row of solar collectors and conduct maintenance processes as it goes. 
         FIG. 6  shows a perspective view of a maintenance vehicle performing a maintenance task on a solar collector. 
         FIGS. 7A and 7B  show perspective views of a maintenance vehicle performing a maintenance task on a solar collector, according to some embodiments. 
         FIG. 8  schematically illustrates another embodiment of a maintenance vehicle that can perform maintenance on a solar collector. 
         FIG. 9  schematically illustrates an alternative embodiment of a solar collector, where this solar collector is being serviced by a cleaning machine. 
         FIG. 10  shows a perspective view of a tracking solar collector. 
         FIG. 11  shows a second perspective view of a tracking solar collector. 
         FIG. 12  shows a third perspective view of a tracking solar collector. 
         FIG. 13  depicts a method for assembling the solar collectors shown in  FIGS. 10, 11, and 12 , according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The current subject matter is directed to a solar tracker structure and related maintenance practices that enable bifacial panels to produce a much higher power output relative to what they would produce using prior art solar trackers and practices. 
       FIGS. 1A and 1B  show two perspective views of a solar tracker  100  with a number of solar panels  102  mounted on it. The solar panels  102  can be supported by two purlins  104 . The purlins  104  can be connected by two pivot arms  106 , which are perpendicular to the purlins. The pivot arms rotate about an axle at the top of a support structure  108 . The support structure can be supported by a ballast foundation  110 . A drive system  112  uses power and torque from a motor (not shown) to rotate the solar panels and to keep them at the correct position. Other foundation designs such as piles driven into the ground or poured concrete can be used too. A variety of support designs can be used as well, such as single poles in a line along the tracker, as shown in  FIG. 12 , instead of the A-frames shown in  FIGS. 1A and 1B .  FIGS. 1A and 1B  represent one solar tracker section that can be coupled with many other solar tracker sections to form a row of trackers. Multiple solar tracker rows can be grouped together for a solar field. 
     Continuing with  FIGS. 1A and 1B , the solar panels  102  on the solar tracker  100  are bifacial panels and collect light from both the fronts and backs of the panels. If the solar tracker orients the panels so that their front sides are facing toward the sun, the front sides of the panels receive light from three sources. First, the front sides of the panels see the direct beam of the sun. Second, the front sides of the panels receive diffuse light that has been scattered by the atmosphere. This can be a significant fraction of the total light during cloudy weather or when there is a large amount of aerosols in the air, such as during a dust storm. A third source of light for the front sides of the panels is light reflected off the ground. This can be higher if the ground is more reflective or if the panels are oriented to have a better view of the ground, such as when the panels are aimed at the sun when it is low on the horizon. 
     The back sides of the panels typically receive light from three sources, diffuse light from the atmosphere, reflected light from the ground, and reflected light from the support structure. The back sides of the panels can also receive light from the direct beam of the sun if the solar tracker is oriented with the backs of the panels facing the sun. However, bifacial panels typically produce more light on the front than the back, and more energy is generally produced overall when the front side has a direct view of the sun. 
     The mounting structure can be designed differently for bifacial panels than for monofacial panels. A first consideration for a mounting structure for bifacial panels is that it allows as much light as possible to reach the back sides of the panels. The panels generate more electricity when more light hits them. A second consideration for a mounting structure for bifacial panels is that the structure allows the light to be distributed as evenly as possible. A panel is composed of a number of cells, for example 72 cells, and these cells are typically electrically connected in series. If one photovoltaic cell is shaded, then it will produce a low amount of current, and the cells wired in series with it will produce the same current as the shaded cell even if these others are not shaded. Therefore, shading one cell disproportionately reduces the power output of the entire panel. 
     A mounting structure can shade the backs of solar cells if structural members are mounted to the back of the solar panels and if they are placed very close to the panels. This can result in reducing the power contribution of the entire back of the panel even if a structural member only blocks some of the cells since the cells are commonly wired in series. 
       FIG. 2  schematically illustrates a detailed view of the back sides of the solar panels  102  and the mounting structure from the solar collector from  FIGS. 1A and 1B .  FIG. 2  shows the solar panels  102  secured to the purlins  104  by clips  202 . A fastener (not shown) holds a clip securely on the solar panel and also securely to the purlin. A standoff  204  increases the gap between the solar panels  102  and the purlins  104  as well as the gap between the solar panels and the pivot arms  106 . The fasteners holding the clips can pass through the standoff  204  or pass near it. The standoff  204  can be made from steel or aluminum or some other metal. An alternative embodiment is that the standoff  204  and the clip  202  can be made as one piece or one assembly as well. By increasing the gap between the solar panel  102  and purlin  104 , the purlin can block much less light on the cells of the solar panels above it. The solar cells that would be shaded have a much better view of the diffuse light from the atmosphere or the light reflected off the ground. 
     Continuing with  FIG. 2 , the structural members can be treated to be highly reflective of light in the solar spectrum. This can be accomplished by painting them white, polishing them, choosing materials that are naturally more reflective, or by other means. For example, structural members can be made of galvanized steel, that when new may have a solar reflectance of 0.6 but can fall to values of 0.2 to 0.3 over time. Coating these parts white with a paint designed for solar reflectance may increase the solar reflectance to 0.85, thereby increasing the reflectance by 3× to 4× over the long term. 
     In one embodiment, all of the structural members are treated to be highly reflective. In another embodiment, only the purlins  104  and pivot arms  106  are treated to be highly reflective. These components are especially important because they are very close to the solar cells. Because of their proximity, they occupy a significant fraction of the view of the solar cells near them. Because the purlins  104  and pivot arms  106  partly shade the back sides of some of the cells, treating them to increase reflectivity preferentially can reflect more light onto those cells that are partly shaded. This can compensate for some shading that would otherwise cause a disproportionate drop in power output from the back sides of the panels. In another embodiment, only the tops and sides of the purlins  104  and pivot arms  106  are treated to be highly reflective because the solar cells have a good view of only these surfaces of these parts. 
       FIG. 3  schematically illustrates a close-up view of another embodiment of an attachment of solar panels to a support structure. In this embodiment, a clamp and a standoff are combined into a clamp-standoff assembly  302 . The clamp-standoff assembly  302  can do three jobs: fasten the solar panel  102  to itself, fasten itself to the purlin  104 , and separate the solar panels  102  from the purlins  104  with a specific gap. This gap can be on the order of a few inches. Providing such a gap can be helpful in increasing incident light on the back side of the solar panels  102 , which can be increase power output of bifacial panels. 
       FIG. 4  schematically illustrates the clamp standoff  302  in detail. The clamp standoff  302  comprises a panel clamp system with a standoff section  402  that provides support between the purlin  104  (shown in  FIGS. 2 and 3 ) and the solar panels  102 . This panel clamp system can comprise the following components: a top bracket  412 , a bottom bracket  404 , a top compliant strip  416 , a bottom compliant strip  418 , a bolt seat  410 , a fastener (not shown), and a nut (not shown). A fastener passes through a series of holes  404  in the clamp components to provide compressive force to clamp onto the solar panel  102  and also to secure the clamp system  204  to the purlin  104  or other structural member. The fastener bears onto the bolt seat  410 , and the bolt seat  410  spreads out the compressive force from the fastener onto a wide area around the top bracket  412 . The top bracket  412  presses downward onto the top compliant strip  416 , which because it is soft, spreads out the force on the solar panel  102  to reduce the pressure. The bottom bracket  414  similarly presses upward on a bottom compliant strip  418  and on the solar panel  102 . The top bracket  412  and the bottom bracket  414  each have rails  406 . The rails  406  serve to locate the compliant strips  506  and  508  since the compliant strips fit around the rails. The rails  406  also serve to locate the top and bottom brackets  412  and  414  with each other. The rails  406  also serve as stops so that in assembly a worker can push the PV panel  102  up against the rail so that the worker has a reference point to properly locate the PV panel  102  before tightening the fastener and nut. 
     The bottom bracket  404  can be formed as a part of the same component as the standoff section  402 . A fastener and nut (not shown) can be used with the series of holes  414  in the assembly to provide compression. Also in this variation, rails  406  can be used to locate the compliant strips  416  and  418  and also to serve as locating features to properly position the solar panels  102  during assembly. A cut-out  408  in the bottom of the standoff section  402  can be used to properly orient and position the clamp-standoff  302  on the purlin  104  (not shown). These features can be paired with the standoff section  402  and associated features to enable separation of the solar panel  102  from the purlins  104 , as in  FIG. 3 . 
     Maintenance processes can be done on solar panels to prevent performance degradation or to increase performance. Maintenance processes could include cleaning solar panels, depositing coatings on solar panels, or performing other suitable task(s). Bifacial panels can benefit from such maintenance processes on both sides of the panels because they collect light from both sides. 
       FIG. 5  schematically illustrates a perspective view of a maintenance vehicle  500  which can travel along a row of solar collectors and conduct maintenance processes as it goes. The maintenance vehicle  500  includes a frame  502 , wheels  504 , and maintenance implements  506 . The maintenance vehicle  500  can have a variety of other components too, such as a control system, a drive system, a wireless communication system, implements for a second maintenance process, storage of consumables, an onboard pretreatment system to treat consumables before depositing them, or other components or systems. The maintenance vehicle  500  can be self-powered or can be pushed or towed by something else like by another vehicle, by a winch, or by workers. The wheels  504  can travel on the solar collector foundation  110 , shown in  FIGS. 1A and 1B , which can double as a track. The wheels  504  can also travel along another track or roll along on the ground.  FIG. 6  shows a perspective view of a maintenance vehicle  500  performing a maintenance task on a solar collector  100 .  FIGS. 7A and 7B  show perspective views of another embodiment of a maintenance vehicle  500  performing a maintenance task on a solar collector  100 . 
       FIG. 8  schematically illustrates another embodiment of a maintenance vehicle  802  that can perform maintenance on a solar collector  100 . In this case, the maintenance vehicle drives underneath the solar collector. It can travel below the support structure  108  on a ballast foundation  110  to perform tasks from below the solar panels  102 . 
     In the embodiments of maintenance vehicles shown in  FIGS. 5 through 8 , the maintenance vehicles each have a particular viewpoint of the solar panels, for example being above them or being below them. 
       FIG. 9  schematically illustrates an alternative embodiment of a solar collector  100  that is being serviced by a cleaning machine  500 . The cleaning machine  500  can clean the front sides of the panels by a combination of one or more of a water spray, brushes, and wipers  902 . The cleaning machine  500  can also clean the back sides of the panels with the water spray  904  that is emitted from the other side of the cleaning machine  500 . The cleaning machine  500  can be arranged to clean the panels when they are tilted, as is shown. The cleaning machine  500  can alternatively be arranged to clean the panels when they are parallel to the earth&#39;s surface, with the water spray  902  pointed upward from the bottom of the cleaning machine  500  while the brushes, wipers, and water spray  902  point downward from the cleaning machine  500  onto the fronts of the panels, which are pointed upward. 
     A perspective view of a tracking solar collector  1000  is shown in  FIG. 10 , and a second perspective view of the same solar collector is shown in  FIG. 11 , according to some embodiments. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The solar collector  1000  can track the sun about one tracking axis  1002 , which is parallel to the ground. The solar collector  1000  includes PV panels  1004 , a structure, and a foundation  1016 . For example, the PV panels  1004  are rectangular in shape. In the solar collector  1000 , the PV panels  1004  are positioned with their long ends parallel to the tracking axis  1002  and with two or more panels  1004  besides one another, transverse to the tracking axis  1002 .  FIGS. 10 and 11  feature embodiments with two panels across, but three or more panels can be arranged in the same manner. The PV panels  1004  can be fastened along their long edges with clips to purlins  1006  that are aligned parallel to the tracking axis  1002 . Three purlins  1006  are shown, and more purlins can be added if more than two PV panels were arranged transverse to the tracking axis  1002 . 
     In another example, some PV panels  1004 , known as bifacial PV panels  1004 , can receive light from both the front and back sides, and the structural members in this solar collector  1000  are positioned so that they do not block light from hitting the back sides of the bifacial PV panels  1004 . The purlins  1006 , pivot arms  1008 , and/or other structural can be treated to be reflective so that additional light can be reflected onto the back sides of bifacial PV panels  1004 . These structural elements can be made reflective by painting them white, by coating them with a reflective coating, by polishing them, by choosing materials that are naturally reflective, or by treating them by other means. 
     In one embodiment, continuing with  FIG. 10 , the purlins  1006  are fastened to two pivot arms  1008 . Each pivot arm  1008  is attached at a pivot point  1010  to two sets of legs  1012 . The pivot points  1010  are used to determine the tracking axis  1002 . Feet  1014  are located at the bottoms of the legs and attach the legs to the concrete foundation  1016 . The concrete foundation  1016  is shown in  FIG. 10  as one piece, but two or four pieces could be used as ballast for the solar collector  1000 . For example, the concrete could be formed by slip-forming, by casting it in place, or by using pre-cast blocks. In another example, the feet  1014  can be located in grooves in the concrete and fastened with an adhesive. 
     In another embodiment, continuing with  FIG. 10 , tracking action is driven by a motor and gearing system that is connected to the drive shaft  1018 , which can be a hollow drive tube. The motor and gearing system rotates the drive shaft, and the drive shaft  1018  uses a pinion gear  1020  to rotate an arc gear  1022 , which is fixed to the pivot arm  1008 . For example, one pinion-gear/arc-gear system is used per leg set  1012 . In another example, one pinion-gear/arc-gear system is used per solar collector. 
     Similar to the solar collector in  FIG. 1 , the concrete ballast  1016  of the solar collector in  FIG. 10  can serve as a track for a driving vehicle. In another example, such a vehicle can be used for transporting people and equipment, performing maintenance tasks such as cleaning, performing diagnostics, and/or making measurements. 
       FIG. 12  shows a perspective view of a tracking solar collector  1200  with a number of bifacial solar panels  1210  mounted on it. The bifacial solar panels  1210  can be supported by a support structure comprising standoffs  1220 , a torque tube  1230 , a bearing  1240 , and posts  1250 . The torque tube  1230  forms an axis about which the solar panels  1210  are rotated. The torque tube  1230  and/or standoffs  1220  are fashioned to be reflective to reflect additional light to the back side of the bifacial solar panels  1210 , such as by painting them white, by coating them with a reflective coating, by polishing them, by choosing materials that are naturally reflective, or by treating them by other means. 
       FIG. 13  depicts a flow diagram  1300  for assembling a system for rotating bifacial photovoltaic panels arranged in a row. At  1302 , a support structure is assembled for the bifacial photovoltaic panels. One or more pivot arms that rotate about an axle at a top of the support structure are positioned at  1304 . At  1306 , one or more elongated structural members are positioned to extend along and parallel to the row and in a perpendicular direction to the one or more pivot arms. The one or more elongated structural members are connected to the one or more pivot arms at  1308 . At  1310 , an underside of the bifacial photovoltaic panels is mounted to the support structure. One or more structural elements of the support structure are coated with a reflective coating to increase reflected light to the underside of the bifacial photovoltaic panels. 
     In the descriptions above and in the claims, phrases such as “at least one of” or “one or more of” may occur followed by a conjunctive list of elements or features. The term “and/or” may also occur in a list of two or more elements or features. Unless otherwise implicitly or explicitly contradicted by the context in which it is used, such a phrase is intended to mean any of the listed elements or features individually or any of the recited elements or features in combination with any of the other recited elements or features. For example, the phrases “at least one of A and B;” “one or more of A and B;” and “A and/or B” are each intended to mean “A alone, B alone, or A and B together.” A similar interpretation is also intended for lists including three or more items. For example, the phrases “at least one of A, B, and C;” “one or more of A, B, and C;” and “A, B, and/or C” are each intended to mean “A alone, B alone, C alone, A and B together, A and C together, B and C together, or A and B and C together.” In addition, use of the term “based on,” above and in the claims is intended to mean, “based at least in part on,” such that an unrecited feature or element is also permissible. 
     The subject matter described herein can be embodied in systems, apparatus, methods, and/or articles depending on the desired configuration. The implementations set forth in the foregoing description do not represent all implementations consistent with the subject matter described herein. Instead, they are merely some examples consistent with aspects related to the described subject matter. Although a few variations have been described in detail above, other modifications or additions are possible. In particular, further features and/or variations can be provided in addition to those set forth herein. For example, the implementations described above can be directed to various combinations and subcombinations of the disclosed features and/or combinations and subcombinations of several further features disclosed above. In addition, the logic flows depicted in the accompanying figures and/or described herein do not necessarily require the particular order shown, or sequential order, to achieve desirable results. Other implementations may be within the scope of the following claims.