Patent Publication Number: US-2015083199-A1

Title: Photovoltaic power generation system

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-196242, filed Sep. 20, 2013, the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a photovoltaic power generation system. 
     BACKGROUND 
     In recent years, concerns about environmental issues are boosting the global installation of photovoltaic power generation systems that generate power using sunlight, and mega solar power plants equipped with a large-scale photovoltaic power generation system have been constructed at locations throughout the world. In the photovoltaic power generation system, a number of solar panels are arranged. These solar panels are supported and fixed by a support structure including a rack and a base. The support structure is required to have a strength capable of withstanding wind pressure and the like acting on the solar panels. 
     However, the installation cost of support structures makes up a large proportion of the installation cost of a photovoltaic power generation system. This proportion is larger especially in a mega solar system in which 10,000 or more solar panels are arranged. It is therefore required to reduce the installation costs of the support structures. The reduction of the installation costs of support structures can be achieved by reducing the weight of the support structures. However, it is difficult to reduce the weight of the support structures while ensuring their ability to withstand wind pressure and the like. 
     It is desirable to be able to reduce the installation costs of support structures in a photovoltaic power generation system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side view showing a photovoltaic power generation system according to an embodiment; 
         FIG. 2  is a plan view showing the photovoltaic power generation system shown in  FIG. 1 ; 
         FIG. 3A  is a side view showing an example of the shape of a baffle plate shown in  FIG. 1 ; 
         FIG. 3B  is a side view showing another example of the shape of the baffle plate shown in  FIG. 1 ; 
         FIG. 3C  is a side view showing still another example of the shape of the baffle plate shown in  FIG. 1 ; 
         FIG. 4  is a schematic view showing a state in which the flow of air is changed by a windbreak shown in  FIG. 1 ; 
         FIG. 5  is a plan view showing a photovoltaic power generation system according to Comparative Example 1; 
         FIG. 6  is a plan view showing a photovoltaic power generation system according to Comparative Example 2; 
         FIGS. 7A and 7B  are views showing an analytic model used in numerical analysis; 
       FIGS. BA and BB are views showing wind force coefficient distributions in a photovoltaic array group shown in  FIG. 5  which are obtained by numerical analysis; 
         FIGS. 9A and 9B  are views showing wind force coefficient distributions in a photovoltaic array group shown in  FIG. 6  which are obtained by numerical analysis; 
         FIG. 10  is a plan view showing an example of setting a central region in the photovoltaic power generation system according to Comparative Example 1; 
         FIG. 11  is a plan view showing an example of setting a central region in the photovoltaic power generation system according to the embodiment; 
         FIGS. 12A ,  12 B, and  12 C are views showing results of two-dimensional analysis of the windbreak effect of the windbreak; 
         FIGS. 13A ,  13 B, and  13 C are side views showing examples in which the support structures of the windbreaks shown in  FIGS. 3A ,  3 B, and  3 C are provided with a tilting device; and 
         FIG. 14  is a side view showing an example of arranging the photovoltaic power generation system on a building according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to an embodiment, a photovoltaic power generation system includes a photovoltaic array group and a windbreak. The photovoltaic array group includes a plurality of photovoltaic arrays, each of the photovoltaic arrays including a plurality of solar panels and a support structure which supports the solar panels. The windbreak is arranged behind the photovoltaic array group and includes a curved surface configured to guide at least some of a wind, which blows from a back side of the photovoltaic array group toward the photovoltaic array group, to an upper side of the photovoltaic array group. 
     Concerning a photovoltaic power generation system, JIS (Japanese Industrial Standards) C8955 defines designing a solar panel assuming four kinds of loads: a dead load caused by the mass of a photovoltaic array itself, a wind pressure load caused by wind pressure, a snow load caused by snow accumulated on the surface of a solar panel, and a seismic load caused by a seismic force. The load combination changes depending on the installation environment. The wind pressure load is a load that needs to be taken into consideration in many solar power plants, and an approximation that calculates a wind pressure load applied to a photovoltaic array from a wind velocity is applied. When applying this standard, “in case there is a plurality of racks, a wind force coefficient calculated by the formula may be applied to the peripheral ends, and ½ the value may be applied to the central portion”. However there is no clear definition of what constitutes the central portion. For this reason, when designing a photovoltaic power generation system, it is important to appropriately estimate the region (central portion) where ½ the wind force coefficient at the peripheral ends is used so that safety can be ensured. 
     Embodiments will now be described with reference to the accompanying drawings. In the following embodiments, like reference numerals denote like elements, and a repetitive description thereof will be omitted. 
       FIG. 1  is a side view schematically showing a photovoltaic power generation system  100  according to an embodiment.  FIG. 2  is a plan view schematically showing the photovoltaic power generation system  100 . As shown in  FIG. 1 , the photovoltaic power generation system  100  includes a photovoltaic array group  110  including a plurality of photovoltaic arrays  111 , and a windbreak  120  arranged behind the photovoltaic array group  110 . In the example shown in  FIG. 2 , six photovoltaic arrays  111 - 1  to  111 - 6  are juxtaposed. The photovoltaic arrays  111 - 4  to  111 - 6  are not illustrated in  FIG. 1 . 
     Each photovoltaic array  111  includes a plurality of solar panels  112  which receive sunlight and generate electric power, and a support structure  113  which supports and fixes the solar panels  112 . The support structure  113  includes a rack  114  which supports the solar panels  112  tilting at a given angle from the level surface, and concrete bases  115  which fix the rack  114  on the ground G. Referring to  FIG. 2 , each rectangular block represents one solar panel  112 . In the example of  FIG. 2 ,  20  solar panels  112  connected by conductive connection members are arranged in each photovoltaic array  111 . 
     In general, the solar panels  112  are installed in a tilted state from the viewpoint of power generation efficiency. For example, in regions at high latitudes in the Northern Hemisphere such as Japan, the solar panels  112  are installed while tilted so that their light receiving surfaces  116  face the south. An angle φ made by the level surface and the light receiving surface  116  is determined in consideration of various conditions such as the latitude and environment of the installation location. 
     In this embodiment, a case is assumed where the solar panels  112  are arranged southward. In this case, the six photovoltaic arrays  111 - 1  to  111 - 6  are juxtaposed in a north-south direction. In each of the photovoltaic arrays  111 - 1  to  111 - 6 , the solar panels  112  are arrayed in an east-west direction. The windbreak  120  is arranged on the north side of the photovoltaic array group  110 . Specifically, the windbreak  120  is arranged facing back surfaces  117  of the solar panels  112  of the northernmost photovoltaic array  111 - 1 . 
     The windbreak  120  includes a baffle plate  121  which guides at least some of the wind, which blows from the back side of the photovoltaic array group  110  toward the photovoltaic array group  110  to the upper side of the photovoltaic array group  110 , and a support structure  122  which supports the baffle plate  121  tilting at a given angle from the level surface. The back side of the photovoltaic array group  110  indicates the side facing the back surfaces  117  of the solar panels  112 . In this embodiment in which the solar panels  112  are arranged southward, a wind which blows from the back side of the photovoltaic array group  110  toward the photovoltaic array group  110  indicates a wind including some wind flow from the north to the south, for example, a north wind, a northeastern wind, or a northwestern wind. In the example of  FIG. 1 , the baffle plate  121  is installed such that an upper edge  124  located at a position higher than an upper edge  118  of the solar panel  112 , and a lower edge  125  is in contact with the ground G. 
     The baffle plate  121  may be formed into a planar shape (plate shape) as shown in  FIG. 3A , a curved shape convex in a direction reverse to the side of the photovoltaic array group  110  as shown in  FIG. 3E , or a curved shape convex toward the side of the photovoltaic array group  110  as shown in  FIG. 3C . The baffle plate  121  can be formed from either one member or a plurality of members. Note that the windbreak  120  is not limited to the example shown in  FIG. 1  in which it has a plate member such as the baffle plate  121 . The windbreak  120  can be implemented by any structure having a surface (for example, flat or curved surface) that changes the flow of air so as to guide at least some of the wind, which blows from the back side of the photovoltaic array group  110  toward the photovoltaic array group  110 , the upper side of the photovoltaic array group  110 . 
     The windbreak  120  is arranged behind (that is, on the north side of) the northernmost photovoltaic array  111 - 1 . As shown in  FIG. 1 , a distance Lw between the windbreak  120  and the northernmost photovoltaic array  111 - 1  is set within the range of, for example, 0 to 3 meters. A height Hw of the windbreak  120  is set within the range of, for example, 3 meters or less. An angle θ made by the level surface and the baffle plate  121  is set within the range of, for example, 45° to 60°. When the baffle plate  121  is formed into a curved shape, the angle θ indicates an angle made by the level surface and a line that connects the upper edge  124  and the lower edge  125  of the baffle plate  121 . This arrangement prevents the solar panels  112  from falling in the shadow of the windbreak  120  and also prevents the power generation amount from decreasing due to a decrease in solar irradiation. 
       FIG. 4  schematically shows a state in which the flow of air is changed by the windbreak  120  when a north wind blows. If the windbreak  120  is not provided, some of the north wind blows toward the back surfaces  117  of the solar panels  112 . This wind directly strikes the back surfaces  117  of the solar panels  112 , and a high wind pressure (wind load) thus acts on the solar panels  112 . In general, when the solar panels  112  are installed in a tilted state, the wind that blows from the back side of the photovoltaic array group  110  to the front side makes a higher wind pressure act on the solar panels  112  than a wind that blows from the front side of the photovoltaic array group  110  to the back side. For this reason, when designing the rack  114  and the base  115 , their strengths are determined in consideration of the influence of the wind that blows from the back side toward the photovoltaic array group  110 . 
     However, in this embodiment in which the windbreak  120  is provided, the wind travels along the baffle plate  121  of the windbreak  120 , is lifted obliquely to the upper side, and passes above the photovoltaic array group  110 , as indicated by the arrows in  FIG. 4 . That is, the windbreak  120  prevents at least some of the wind which blows from the back side toward the photovoltaic array group  110  from directly striking the back surfaces  117  of the solar panels  112 . This reduces the wind that directly strikes the solar panels  112  and lowers the wind pressure acting on the solar panels  112 . When the wind pressure acting on the solar panels  112  is reduced, the wind pressure resistance of the rack  114  and the base  115  can easily be ensured, and the rack  114  and the base  115  can be reduced for this reason. This makes it possible to implement cost reductions. To obtain a high windbreak effect, the upper edge  124  of the baffle plate  121  is preferably located at a position higher than the upper edge  116  of the solar panel  112 , as shown in  FIG. 1 . In addition, a width Ww of the baffle plate  121  is preferably larger than a width Wp of the photovoltaic arrays  111 , as shown in  FIG. 2 . In this embodiment, the widthwise direction corresponds to the east-west direction. 
       FIG. 5  schematically shows a photovoltaic power generation system  500  according to Comparative Example 1.  FIG. 6  schematically shows a photovoltaic power generation system  600  according to Comparative Example 2. The photovoltaic power generation systems  500  and  600  shown in  FIGS. 5 and 6  include no windbreak, unlike the photovoltaic power generation system  100  shown in  FIG. 1 . In a photovoltaic array group  510  of the photovoltaic power generation system  500 , each of photovoltaic arrays  511  of six columns includes 10 solar panels  112 . A photovoltaic array group  610  of the photovoltaic power generation system  600  shown in  FIG. 6  includes photovoltaic arrays  611  of five columns, and the number of solar panels  112  changes between the photovoltaic arrays  611 . A photovoltaic array  611 - 1  of the first column located at the northernmost end includes three solar panels  112 , and a photovoltaic array  611 - 2  of the second column adjacent to the south side of the photovoltaic array  611 - 1  includes five solar panels  112 . In this way, the number of solar panels  112  increases by two as the number of columns increases (that is, the position moves southward). In this case, a photovoltaic array  611 - 5  of the fifth column includes 11 solar panels  112 . 
     The present inventors obtained wind force coefficient distributions in the photovoltaic array groups  510  and  610  of the photovoltaic power generation systems  500  and  600  by numerical analysis. Analytic models used in the numerical analysis will be described. In the numerical analysis, elements (for example, a rack and a base) other than the solar panel  112  have little effect on the wind flow and are not taken into consideration. For the photovoltaic power generation system  500 , as shown in  FIG. 7A , a width W of the solar panel  112  is set to 1,500 mm, a depth D is set to 3,000 mm, and a thickness T is set to 100 mm. Additionally, as shown in  FIG. 7B , a height H of the solar panel  112  is set to 500 mm, and the angle φ is set to 30°. As shown in  FIG. 5 , a distance L between the photovoltaic arrays  511  is set to 3,000 mm. The solar panels  112  are arranged southward. The wind directions are set to a direction from the north to the south (direction indicated by an arrow A in  FIG. 5 ) and a direction from the northeast to the southwest (direction indicated by an arrow B in  FIG. 5 ). The wind velocity is set to 30 m/s. 
     For the photovoltaic power generation system  600 , the width W of the solar panel  112  is set to 1,500 mm, the depth D is set to 2,945 mm, and the thickness T is set to 100 mm. Additionally, the height H of the solar panel  112  is set to 730 mm, and the angle φ is set to 10°. As shown in  FIG. 6 , the distance L between the photovoltaic arrays  611  is set to 1,700 mm. The solar panels  112  are arranged southward. The wind directions are set to a direction from the north to the south (direction indicated by an arrow C in  FIG. 6 ) and a direction from the northeast to the southwest (direction indicated by an arrow D in  FIG. 6 ). The wind velocity is set to 30 m/s. 
     A wind force coefficient C is defined by equation (1) below. In equation (1), a direction from the back surfaces  117  of the solar panels  112  to the light receiving surfaces  116  is defined as positive concerning the wind force coefficient C. The wind force coefficient C represents that the larger the absolute value is, the higher the wind pressure acting on the solar panel  112  is. 
     
       
         
           
             
               
                 
                   C 
                   = 
                   
                     ∫ 
                     
                       
                         
                           
                             P 
                             1 
                           
                           - 
                           
                             P 
                             u 
                           
                         
                         
                           
                             ρ 
                              
                             
                                 
                             
                              
                             
                               U 
                               2 
                             
                              
                             A 
                           
                           2 
                         
                       
                        
                       
                          
                         A 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     P l  is the wind pressure acting on the back surface  117  of the solar panel  112 , P u  is the wind pressure acting on the light receiving surface  116  of the solar panel  112 , ρ and U are the density and flow velocity of a fluid (air), respectively, and A is the area of the light receiving surface  116  or back surface  117  of the solar panel  112 . 
       FIG. 8A  shows a wind force coefficient distribution in the photovoltaic array group  510  obtained by numerical analysis in a case where the wind direction is the direction indicated by the arrow A in  FIG. 5  (that is, a case where a north wind is assumed). FIG. SB shows a wind force coefficient distribution in the photovoltaic array group  510  obtained by numerical analysis in a case where the wind direction is the direction indicated by the arrow B in  FIG. 5  (that is, a case where a northeastern wind is assumed),  FIG. 9A  shows a wind force coefficient distribution in the photovoltaic array group  610  obtained by numerical analysis in a case where the wind direction is the direction indicated by the arrow C in  FIG. 6  (that is, a case where a north wind is assumed).  FIG. 9B  shows a wind force coefficient distribution in the photovoltaic array group  610  obtained by numerical analysis in a case where the wind direction is the direction indicated by the arrow D in  FIG. 6  (that is, a case where a northeastern wind is assumed). Referring to  FIGS. 8A ,  8 B,  9 A, and  9 B, the deeper the color is, the larger the value of the wind force coefficient is, and the lighter the color is, the smaller the value of the wind force coefficient is. 
     Referring to  FIGS. 8A and 8B , the wind force coefficient tends to be larger for the solar panel  112  on the windward side in both the wind directions A and B. More specifically, in  FIG. 8A , the wind force coefficients C are maximized in the photovoltaic array  511 - 1  of the first stage and minimized in the photovoltaic array  511 - 2  of the second stage. The wind force coefficients become large toward the photovoltaic arrays  511  on the leeward side. In the photovoltaic array  511 - 1  of the first stage on the windward side, the wind force coefficients are smaller for the solar panels  112  of the first, second, ninth, and 10th columns located at the ends as compared to the solar panels  112  of the third to eighth columns located at the center. In the photovoltaic arrays  511 - 2  to  511 - 6  of the second to sixth stages, the wind force coefficients are large for the solar panels  112  of the first and 10th columns located at the ends as compared to the solar panels  112  of the second to ninth columns located at the center. Referring to  FIGS. 9A and 9B , the wind force coefficient tends to be larger for the solar panel  112  on the windward side in both the wind directions C and D. More specifically, in  FIG. 9A , the wind force coefficients C are maximized in the photovoltaic array  611 - 1  of the first stage and become small toward the photovoltaic array  611  on the leeward side. 
     As described above, the tendency changes between the photovoltaic array group  510  and the photovoltaic array group  610 . In the photovoltaic array group  510 , a north wind swirls at the two ends and at the center of each photovoltaic array  511 . In addition, a northeastern wind strikes the solar panel  112  at the east end (of the 10th column) of each photovoltaic array  511  and then flows through the photovoltaic arrays  511  while being disturbed. On the other hand, in the photovoltaic array group  610 , a wind such as a northeastern wind from an oblique direction easily flows to the center region. The above-described difference in tendency probably occurs due to such a difference in the flow of air. 
       FIG. 10  shows an example of setting a region (central portion)  1001  to which a condition is applied in that ½ of the wind force coefficient at the peripheral ends is used when calculating the wind pressure load in the photovoltaic power generation system  500 . This region will be referred to as a central region. In the example of  FIG. 10 , the central region  1001  is limited to a region located between two line segments passing through the two ends of the photovoltaic array  511  on the rear side (adjacent on the north side) and making an angle of 45° with respect to the photovoltaic array  511  in each of the photovoltaic arrays  511  of the second to fifth stages. In the central region  1001 , the strength of the support structures can be, for example, half that of the support structures at the peripheral ends. 
       FIG. 11  shows an example of setting a central region  1101  in the photovoltaic power generation system  100  according to the embodiment. In this embodiment in which the windbreak  120  is provided, the central region  1101  can be set to a region excluding the peripheral ends of the photovoltaic array group  110 , as shown in  FIG. 11 . In this embodiment, the windbreak  120  prevents the wind from directly striking the solar panels  112  at the peripheral ends of the photovoltaic array group  110 . Since this lowers the wind pressure acting on the solar panels  112 , the central region can be set wider. Specifically, the central region  1101  can be set wider in the photovoltaic arrays  111 - 2  to  111 - 5  other than the photovoltaic arrays  111  (specifically, the photovoltaic arrays  111 - 1  and  111 - 6 ) located on the front and back ends of the photovoltaic array group  110 . For example, in the photovoltaic array  111 - 2 , the strength of the support structure  113  in at least part of regions  1102  that exists outside two line segments passing through the two ends of the photovoltaic array  111 - 1  adjacent on the back side of the photovoltaic array  111 - 2  and making a 45° angle with respect to the photovoltaic array  111 - 1  and that excludes two ends  1103 , can be half that of the support structure  113  at the two ends  1103  of the photovoltaic array  111 - 2 . It is therefore possible to reduce the installation cost of the racks  114  and the bases  115 . 
       FIGS. 12A ,  12 B, and  12 C show the results of two-dimensional analysis of a distance at which the windbreak effect of the windbreak  120  can be obtained. 
       FIG. 12A  corresponds to a case where the baffle plate  121  is formed into a planar shape as shown in  FIG. 3A .
 
 FIGS. 12B and 12C  correspond to a case where the baffle plate  121  is formed into a curved shape as shown in  FIG. 3C . The curvature of a curve mimicking the windbreak  120  changes between  FIGS. 12B and 12C . Referring to  FIGS. 12A ,  12 B, and  12 C, the deeper the color of a line is, the higher the wind velocity is, and the lighter the color is, the lower the wind velocity is. As can be understood from  FIGS. 12A ,  12 B, and  12 C, the distance at which the windbreak effect can be obtained is longer in the curved baffle plate  121  than in the flat baffle plate  121 .
 
     As described above, in the photovoltaic power generation system according to this embodiment, the windbreak is provided on the back side of the photovoltaic array group, thereby reducing the wind pressure acting on the back surfaces of the solar panels. This makes it possible to ensure safety and reduce the weight of the racks  114  and the bases  115 . It is consequently possible to reduce the installation cost of the racks  114  and the bases  115 . 
     The support structure  122  of the windbreak  120  may include a tilting device which controls the tilt of the baffle plate  121 .  FIGS. 13A ,  13 B, and  13 C show states which the baffle plates  121  having the shapes shown in  FIGS. 3A ,  3 B, and  3 C are tilted by a tilting device  1301  so as to make the angle θ small. In this embodiment, the solar panels  112  are arranged southward, and the windbreak  120  is arranged on the north side of the photovoltaic array group  110 . In this case, when a strong south wind blows, the baffle plate  121  of the windbreak  120  receives a high wind pressure. When a strong south wind blows, the wind pressure acting on the baffle plate  121  can be reduced by making the angle θ of the baffle plate  121  small using the tilting device  1301 . In addition, the windbreak may include a storage to store a maintenance tool to be used to maintain the photovoltaic power generation system  100 . 
     The photovoltaic power generation system  100  is not limited to the ground installation example. For example, the photovoltaic power generation system  130  may be installed on a flat roof  1401  of a building  1400 , as shown in  FIG. 14 . In this case as well, the direction of wind that blows from the back side of the photovoltaic array group  110  is changed by the windbreak  120  and passes above the photovoltaic array group  110  as indicated by the arrows in  FIG. 14 . The wind can thus be prevented from directly striking the back surfaces  117  of the solar panels  112 , and the wind pressure received by the solar panels  112  can be reduced. This makes it possible to ensure safety and reduce the weight of the racks  114  and the bases  115 . 
     The arrangement of the photovoltaic array group  110  is not limited to the arrangement example shown in  FIG. 1 . For example, the photovoltaic array group  110  may change the width for each photovoltaic array  111 , like the photovoltaic array group  610  shown in  FIG. 6 . 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.