Patent Publication Number: US-2021175849-A1

Title: Method of checking pressure in housing unit for photovoltaic power generation apparatus

Description:
TECHNICAL FIELD 
     The present disclosure relates to a method of checking a pressure in a housing unit for a photovoltaic power generation apparatus. The present application claims priority to Japanese Patent Application No. 2018-079652 filed on Apr. 18, 2018, the entire contents of which are herein incorporated by reference. 
     BACKGROUND ART 
     A photovoltaic power generation apparatus that concentrates solar rays and converts solar rays into electric power has conventionally been developed. For example, Japanese Patent Laying-Open No. 2017-73853 (PTL 1) discloses such a concentrator photovoltaic power generation apparatus. 
     This concentrator photovoltaic power generation apparatus includes a plurality of photovoltaic power generation modules. Each photovoltaic power generation module mainly includes a housing, a power generation element arranged in the housing, and a lens member that concentrates solar rays to the power generation element. 
     Such a concentrator photovoltaic power generation apparatus is installed outdoors for necessity of concentration of solar rays, and exposed to wind and rain. When rainwater enters the housing, moisture adheres to a power generation element or wiring, which leads to a problem of corrosion of wires or decrease in amount of power generation. Therefore, it is important to confirm absence of a gap or a hole through which rainwater enters the housing. 
     For example, Japanese Patent Laying-Open No. 2001-205056 (PTL 2) discloses a method of checking for leakage. In PTL 2, whether or not there is leakage through a separation membrane is examined by immersing the separation membrane into an aqueous solution of an organic solvent and thereafter applying a pressure thereto from a secondary side. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Patent Laying-Open No. 2017-73853 
     PTL 2: Japanese Patent Laying-Open No. 2001-205056 
     SUMMARY OF INVENTION 
     A method of checking a pressure in a housing unit for a photovoltaic power generation apparatus according to one manner of the present disclosure includes a step below. 
     A housing unit including a first opening, an internal space in communication with the first opening, a first through hole that allows communication from the outside to the internal space, and a first porous membrane that covers the first through hole and includes pores that allow communication from the outside to the internal space is prepared. A pressure in the internal space is measured while the first porous membrane covers the first through hole and the first opening in the housing unit is closed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view showing a construction of a concentrator photovoltaic power generation apparatus according to one embodiment. 
         FIG. 2  is an exploded perspective view showing a construction of a photovoltaic power generation module included in the concentrator photovoltaic power generation apparatus shown in  FIG. 1 . 
         FIG. 3  is an assembly cross-sectional view showing the construction of the photovoltaic power generation module shown in  FIG. 2 . 
         FIG. 4  is a plan view showing a construction of a housing unit in which a lens member has been removed from the photovoltaic power generation module shown in  FIG. 3 . 
         FIG. 5  is a cross-sectional view showing a state before measurement of an internal pressure in the housing unit for the photovoltaic power generation apparatus according to one embodiment. 
         FIG. 6  is a diagram for illustrating a sealing gasket including a closed-cell structure. 
         FIG. 7  is a flowchart showing a method of checking a pressure in the housing unit for the photovoltaic power generation apparatus according to one embodiment. 
         FIG. 8  is a cross-sectional view showing a state of measurement of an internal pressure in the housing unit for the photovoltaic power generation apparatus according to one embodiment. 
         FIG. 9  is a diagram showing leakage characteristics after reduction in pressure in an internal space in the housing unit for the photovoltaic power generation apparatus according to one embodiment. 
         FIG. 10  is a diagram showing leakage characteristics after increase in pressure in the internal space in the housing unit for the photovoltaic power generation apparatus according to Comparative Example. 
         FIG. 11  is a diagram showing relation between a pressure in the internal space and an amount of warpage of the housing unit during pressure reduction in the internal space in the housing unit for the photovoltaic power generation apparatus according to one embodiment. 
         FIG. 12  is a diagram for illustrating an amount of warpage of the housing unit. 
     
    
    
     DETAILED DESCRIPTION 
     Problem to be Solved by the Present Disclosure 
     A gap or a hole through which rainwater enters the housing may be checked for by adopting a method as described in PTL 2 and immersing the housing into liquid. 
     In this case, however, the housing should be immersed into liquid and the wet housing should be dried. Therefore, man-hours additionally increase and a manufacturing lead time increases. Furthermore, in mass production of relatively large products for a concentrator photovoltaic power generation apparatus, a large space for drying is required. Therefore, it is difficult to incorporate this inspection step into a line for manufacturing photovoltaic power generation modules, and inspection inevitably has to be done off-line. 
     An object of one manner of the present disclosure is to provide a method of checking a pressure in a housing unit for a photovoltaic power generation apparatus that does not require drying of a wet housing, is less in time and efforts for 100% inspection, and allows easy incorporation of an inspection step into a line for manufacturing photovoltaic power generation modules. 
     Advantageous Effect of the Present Disclosure 
     According to the present disclosure, a method of checking a pressure in a housing unit for a photovoltaic power generation apparatus that does not require drying of a wet housing, is less in time and efforts for 100% inspection, and allows easy incorporation of an inspection step into a line for manufacturing photovoltaic power generation modules can be realized. 
     Description of Embodiments of the Present Disclosure 
     Embodiments of the present invention will initially be listed and described. 
     (1) A method of checking a pressure in a housing unit for a photovoltaic power generation apparatus according to one manner of the present disclosure includes a step below. 
     A housing unit including a first opening, an internal space in communication with the first opening, a first through hole that allows communication from the outside to the internal space, and a first porous membrane that covers the first through hole and includes pores that allow communication from the outside to the internal space is prepared. A pressure in the internal space is measured while the first porous membrane covers the first through hole and the first opening in the housing unit is closed. 
     A pressure in the internal space in the housing unit can thus be measured without immersing the housing unit in liquid. Therefore, time and trouble and a large space for drying wet housing units are not required. Incorporation (in-line) of this pressure checking process into a line for manufacturing photovoltaic power generation modules is facilitated and checking can be done in a simplified manner in a short period of time. Time and trouble and efforts for 100% inspection are also less. 
     By measuring a pressure, a gap or a hole in the housing unit other than pores in the porous membrane that allows communication from the outside to the internal space can be checked for. Whether or not there is a gap or a hole through which rainwater enters the housing unit in arrangement of the photovoltaic power generation module outdoors can thus be determined. 
     Since determination above is made based on a pressure value or a rate of variation in pressure, accuracy in checking can be high also for a housing unit in which an internal space communicates with the outside through the first porous membrane. 
     (2) In the method of checking a pressure in a housing unit for a photovoltaic power generation apparatus in (1), in the preparing a housing unit, the housing unit is prepared to include a housing, a bottom plate, a flexible printed circuit board, and a shield plate. The housing surrounds the internal space and includes the first opening at a first end and a second opening at a second end opposed to the first end. The bottom plate is attached to the housing to close the second opening. The flexible printed circuit board is attached to the bottom plate to be located in the internal space, and a power generation element is mounted on the flexible printed circuit board. The shield plate is located above the flexible printed circuit board and includes a light transmission window directly above the power generation element. 
     By checking a pressure in such a state, a gap or a hole that may permit entry of rainwater at a portion where the first porous membrane is attached to the housing and a portion where the bottom plate is attached to the housing can be checked for. 
     (3) In the method of checking a pressure in a housing unit for a photovoltaic power generation apparatus in (2), in the preparing a housing unit, the housing is prepared to include a peripheral wall portion that includes the first opening at the first end and the second opening at the second end and an intermediate bar attached to the peripheral wall portion to separate the second opening into two openings. In the measuring a pressure in the internal space, a pressure in the internal space is measured with a pressure sensor arranged directly above the intermediate bar. 
     By thus providing a pressure sensor at a position directly above the intermediate bar, checking can be done on both sides of the intermediate bar with the same accuracy without any imbalance. 
     (4) In the method of checking a pressure in a housing unit for a photovoltaic power generation apparatus in (2), in the preparing a housing unit, the housing unit is prepared to include a second through hole that is arranged such that the second opening lies between the second through hole and the first through hole and allows communication from the outside to the internal space and a second porous membrane that covers the second through hole. In the measuring a pressure in the internal space, a pressure in the internal space is measured with a pressure sensor arranged at a position intermediate between the first through hole and the second through hole in a plan view. 
     By thus providing a pressure sensor at a position intermediate between the first through hole and the second through hole, the pressure sensor is located equidistantly from the first through hole and the second through hole and checking can be done on both sides with the same accuracy without any imbalance. 
     (5) The method of checking a pressure in a housing unit for a photovoltaic power generation apparatus in (2) further includes reducing a pressure in the internal space in the housing unit before the measuring a pressure in the internal space. 
     By thus measuring a pressure in the internal space after pressure reduction in the internal space in the housing unit, checking can be done with accuracy higher than in measurement of a pressure after increase in pressure in the internal space. A load test of a photovoltaic power generation module is conducted by applying a load onto a rear surface of the bottom plate to warp the bottom plate toward the inside of the module. By reducing a pressure in the internal space, a pseudo load test of the module can also be conducted. 
     (6) In the method of checking a pressure in a housing unit for a photovoltaic power generation apparatus in (5), the pressure in the internal space in the housing unit is reduced to a pressure not lower than −2500 Pa and not higher than −800 Pa. 
     When a pressure in the internal space in the housing unit is higher than −800 Pa (a negative pressure is weaker), a pressure difference between a non-defective product and a defective product is too small and accuracy in checking is poor. When a pressure in the internal space in the housing unit is lower than −2500 Pa (a negative pressure is stronger), the bottom plate may deform and the photovoltaic power generation module may be damaged. 
     (7) In the method of checking a pressure in a housing unit for a photovoltaic power generation apparatus in (5), in the preparing a housing unit, the housing is prepared to include a peripheral wall portion that includes the first opening at the first end and the second opening at the second end and an intermediate bar attached to the peripheral wall portion to separate the second opening into two openings. The method further includes closing the first opening in the housing unit with a plate member. The plate member includes a pressure reduction hole located directly above the intermediate bar while the plate member closes the first opening. In the reducing a pressure in the internal space, the pressure in the internal space is reduced through the pressure reduction hole. 
     By reducing a pressure in the internal space through the pressure reduction hole directly above the intermediate bar, the pressure can be reduced on both sides of the intermediate bar without imbalance and checking can be done with the same accuracy. 
     (8) In the method of checking a pressure in a housing unit for a photovoltaic power generation apparatus in (1), the first opening in the housing unit is closed by pressing a plate member against the housing unit with a sealing gasket being interposed before the measuring a pressure in the internal space. The plate member is pressed against the housing unit such that a thickness of the sealing gasket after the plate member is pressed against the housing unit is at least ¼ and at most ½ a thickness of the sealing gasket before the plate member is pressed against the housing unit. 
     When a thickness of the sealing gasket after the plate member is pressed against the housing unit is larger than ½ the thickness of the sealing gasket before the plate member is pressed against the housing unit, sealing by the sealing gasket is insufficient and checking with high accuracy cannot be done. When a thickness of the sealing gasket after the plate member is pressed against the housing unit is smaller than ¼ the thickness of the sealing gasket before the plate member is pressed against the housing unit, a portion of the housing unit in contact with the sealing gasket may be damaged. In particular when a part of the housing unit is made of an inexpensive resin, there is possibility of damage. 
     (9) In the method of checking a pressure in a housing unit for a photovoltaic power generation apparatus in (8), the sealing gasket includes a closed-cell structure. 
     As the sealing gasket thus includes the closed-cell structure, a plurality of pores in the sealing gasket are independent of one another. Pores in the sealing gasket thus do not allow communication from the outside of the housing unit to the internal space. Therefore, leakage through the sealing gasket can reliably be prevented and checking with high accuracy can be done. 
     Details of Embodiment of the Present Disclosure 
     (Construction of Concentrator Photovoltaic Power Generation Apparatus) 
       FIG. 1  is a perspective view showing a construction of a concentrator photovoltaic power generation apparatus according to one embodiment. As shown in  FIG. 1 , a concentrator photovoltaic power generation apparatus  20  includes a plurality of photovoltaic power generation modules  10 , a pedestal  11 , a plurality of support arms  12 , and a plurality of rails  13 . 
     Pedestal  11  is a portion installed on the ground. Pedestal  11  supports the plurality of support arms  12  and the plurality of rails  13 . Each of the plurality of support arms  12  is arranged to extend vertically. Each of the plurality of rails  13  is arranged to extend laterally. 
     The plurality of photovoltaic power generation modules  10  are arranged in matrix on the plurality of rails  13 . 
     The plurality of support arms  12  and the plurality of rails  13  are movable with respect to pedestal  11 . For example, positions of the plurality of support arms  12  and the plurality of rails  13  with respect to pedestal  11  are varied such that light receiving surfaces of the plurality of photovoltaic power generation modules  10  face the sun during a period from sunrise to sunset. 
     (Construction of Photovoltaic Power Generation Module) 
       FIGS. 2 and 3  are an exploded perspective view and an assembly cross-sectional view showing a construction of the photovoltaic power generation module included in the concentrator photovoltaic power generation apparatus shown in  FIG. 1 , respectively.  FIG. 4  is a plan view showing a construction of a housing unit in which a lens member has been removed from the photovoltaic power generation module shown in  FIG. 3 . 
     As shown in  FIGS. 2 and 3 , photovoltaic power generation module  10  mainly includes a housing  1 , a porous membrane  4 , a bottom plate  5 , a plurality of flexible printed circuit boards  6 , a plurality of power generation elements  7 , a shield plate  8 , and a lens member  9 . 
     Housing  1  mainly includes a peripheral wall portion  2  and an intermediate bar  3 . Peripheral wall portion  2  is in a frame shape that surrounds an internal space  1   d.  Peripheral wall portion  2  includes a first end  2   a  and a second end  2   b  opposite to first end  2   a.  Peripheral wall portion  2  includes a first opening  1   b  at first end  2   a  and a second opening  1   c  at second end  2   b.  Each of first opening  1   b  and second opening  1   c  communicates with internal space  1   d.  Intermediate bar  3  is attached to second end  2   b  of peripheral wall portion  2  so as to separate second opening  1   c  into two openings. 
     A through hole  1   a  ( FIG. 3 ) is provided in housing  1 . Through hole  1   a  passes through housing  1 . Through hole  1   a  thus allows communication from the outside of housing  1  to internal space  1   d.  Through hole  1   a  opens toward second end  2   b  of peripheral wall portion  2 . 
     Porous membrane  4  is attached to housing  1  to cover the entire through hole  1   a.  Porous membrane  4  is located within internal space  1   d  in housing  1 . Porous membrane  4  includes a construction like an open-cell structure. Specifically, a plurality of pores in porous membrane  4  communicate with one another and gas can pass between a front surface and a rear surface. Pores in porous membrane  4  thus allow communication from the outside of housing  1  to internal space  1   d.  Porous membrane  4  is composed, for example, of POREFLON®. 
     Bottom plate  5  is made from a flat plate, and composed, for example, of a metal material. The plurality of flexible printed circuit boards  6  are carried on a surface of bottom plate  5 . The plurality of power generation elements  7  are mounted on the plurality of flexible printed circuit boards  6 . Power generation elements  7  are electrically connected to wires of flexible printed circuit boards  6 . 
     Bottom plate  5  is attached to second end  2   b  of housing  1  by welding. Bottom plate  5  closes second opening  1   c  in housing  1 . While bottom plate  5  is attached to housing  1 , the plurality of flexible printed circuit boards  6  and the plurality of power generation elements  7  are located in internal space  1   d  in housing  1 . 
     Shield plate  8  is made from a flat plate having four edges bent, and composed, for example, of a metal material. Shield plate  8  includes a plurality of light transmission windows (through holes)  8   a.    
     Shield plate  8  is arranged in internal space  1   d  in housing  1  and attached to housing  1 , for example, by a screw (not shown). While shield plate  8  is attached to housing  1 , the plurality of light transmission windows  8   a  are located directly above the plurality of power generation elements  7 , respectively. Shield plate  8  performs a function to prevent a portion other than power generation elements  7  from being irradiated with solar rays. 
     Lens member  9  includes a plurality of lens portions  9   a.  The plurality of lens portions  9   a  correspond to the plurality of power generation elements  7 , respectively. In other words, solar rays concentrated by a single lens portion  9   a  are emitted to a single power generation element  7 . The plurality of lens portions  9   a  are arranged in matrix. Each of the plurality of lens portions  9   a  is, for example, a Fresnel lens. 
     Lens member  9  is attached to first end  2   a  of peripheral wall portion  2  by an adhesive. Lens member  9  closes first opening  1   b  in housing  1 . Solar rays concentrated by each of the plurality of lens portions  9   a  of lens member  9  are emitted to power generation element  7  through each of the plurality of light transmission windows  8   a.  Each power generation element  7  generates electric power in accordance with an amount of light reception upon receiving solar rays concentrated by corresponding Fresnel lens  9   a.    
     As shown in  FIG. 4 , housing  1  may be provided with a plurality of through holes  1   a.  In the present embodiment, for example, two through holes  1   a  (a first through hole and a second through hole) are provided in housing  1 . 
     One (the first through hole) and the other (the second through hole) of two through holes  1   a  are arranged such that second opening  1   c  lies therebetween. Two through holes  1   a  (the first through hole and the second through hole) are arranged at positions, for example, in point symmetry with respect to a center  1   e  of housing  1  in a plan view. One (the first through hole) of two through holes  1   a  is covered with first porous membrane  4  and the other (the second through hole) is covered with second porous membrane  4 . 
     The plan view herein means a point of view in a direction perpendicular to the surface of bottom plate  5 . 
     As shown in  FIG. 2 , a housing unit HU is constructed by assembling housing  1  to which porous membrane  4  is attached, bottom plate  5  to which flexible printed circuit boards  6  and power generation elements  7  are attached, and shield plate  8 . 
     (Apparatus for Checking Pressure in Housing Unit for Photovoltaic Power Generation Apparatus) 
       FIG. 5  is a cross-sectional view showing a state before measurement of an internal pressure in the housing unit for the photovoltaic power generation apparatus according to one embodiment.  FIG. 6  is a diagram for illustrating a sealing gasket including a closed-cell structure. 
     As shown in  FIG. 5 , a pressure in internal space  1   d  in housing unit HU is checked with a pressure checking apparatus. The pressure checking apparatus mainly includes, for example, a press plate  21  (a plate member), a sealing gasket  22 , a pressure sensor  23 , a valve  24 , and a vacuum pump  25 . 
     Press plate  21  is in a shape of a flat plate. Press plate  21  includes a lower surface  21 L located on a side of housing unit HU and an upper surface  21 T opposed to lower surface  21 L. Press plate  21  includes a through hole  21   a  and a pressure reduction hole  21   b  that extend between upper surface  21 T and lower surface  21 L. 
     Sealing gasket  22  is attached to lower surface  21 L of press plate  21 . Sealing gasket  22  is a portion pressed against first end  2   a  of peripheral wall portion  2  in housing unit HU during checking of a pressure in housing unit HU. Therefore, sealing gasket  22  is arranged in a frame shape for superimposition at least on first end  2   a  of peripheral wall portion  2  in the plan view. 
     As shown in  FIG. 6 , sealing gasket  22  is made, for example, of a foam including a closed-cell structure. Specifically, each of a plurality of voids  22   a  (pores) in sealing gasket  22  are disposed independently of one another. Therefore, gas cannot pass through sealing gasket  22  through voids in sealing gasket  22 . Sealing gasket  22  is made, for example, of a soft foam low in density mainly composed of ethylene propylene diene monomer (EPDM) rubber. 
     As shown in  FIG. 5 , pressure sensor  23  is arranged directly above through hole  21   a.  A pressure in internal space  1   d  can be measured with pressure sensor  23  through through hole  21   a.    
     Vacuum pump  25  is connected to pressure reduction hole  21   b  with valve  24  being interposed. Vacuum pump  25  can reduce a pressure in the internal space in housing unit HU. By opening and closing valve  24 , selection between connection and disconnection between vacuum pump  25  and internal space  1   d  can be made. 
     While press plate  21  closes first opening  1   b  in housing unit HU ( FIG. 8 ), through hole  21   a  is arranged at a position superimposed on center  1   e  of housing unit HU in the plan view as shown in  FIG. 4 . Through hole  21   a  is arranged directly above intermediate bar  3 . Through hole  21   a  is arranged at a position intermediate between two through holes  1   a.  Pressure reduction hole  21   b  is arranged directly above intermediate bar  3 . 
     Through hole  21   a  being arranged at a position intermediate between two through holes  1   a  means that through hole  21   a  is located on a straight line that connects two through holes  1   a  (the first through hole and the second through hole) to each other in the plan view and a distance D 1  between one (the first through hole) of two through holes  1   a  and through hole  21   a  is equal to a distance D 2  between the other (the second through hole) of two through holes  1   a  and through hole  21   a.    
     Pressure sensor  23  is arranged to be located at center  1   e  of housing unit HU in the plan view while press plate  21  closes first opening  1   b  of housing unit HU. 
     Pressure sensor  23  is arranged directly above intermediate bar  3 . Pressure sensor  23  is arranged at a position intermediate between two through holes  1   a.    
     Pressure sensor  23  being arranged at a position intermediate between two through holes  1   a  means that pressure sensor  23  is located on a straight line that connects two through holes  1   a  (the first through hole and the second through hole) to each other in the plan view and a distance between one (the first through hole) of two through holes  1   a  and pressure sensor  23  is equal to a distance between the other (the second through hole) of two through holes  1   a  and pressure sensor  23 . 
     (Method of Checking Pressure in Housing Unit for Photovoltaic Power Generation Apparatus) 
       FIG. 7  is a flowchart showing a method of checking a pressure in the housing unit for the photovoltaic power generation apparatus according to one embodiment. 
       FIG. 8  is a cross-sectional view showing a state of measurement of an internal pressure in the housing unit for the photovoltaic power generation apparatus according to one embodiment. 
     As shown in  FIGS. 5 and 7 , in the method of checking a pressure in the present embodiment, housing unit HU is initially prepared (step  51 :  FIG. 7 ). Housing unit HU is prepared to include first opening  1   b,  internal space  1   d  in communication with first opening  1   b,  through hole  1   a  (the first through hole:  FIGS. 3 and 4 ) that allows communication from the outside to internal space  1   d,  and porous membrane  4  (the first porous membrane:  FIGS. 3 and 4 ) that covers through hole  1   a.  Specifically, housing unit HU is prepared by assembling housing  1  to which porous membrane  4  is attached as described above, bottom plate  5  to which flexible printed circuit boards  6  and power generation elements  7  are attached, and shield plate  8 . 
     Thereafter, a cylinder attached to press plate  21  operates. Press plate  21  is thus lowered toward housing unit HU (step S 2 :  FIG. 7 ). 
     As shown in  FIGS. 7 and 8 , press plate  21  is lowered until sealing gasket  22  comes in intimate contact with first end  2   a  of peripheral wall portion  2  in housing unit HU. Press plate  21  is thus pressed against housing unit HU with sealing gasket  22  being interposed to close first opening  1   b  in housing unit HU (step S 2 :  FIG. 7 ). 
     At this time, press plate  21  is preferably pressed against housing unit HU such that a thickness T 2  ( FIG. 8 ) of sealing gasket  22  after press plate  21  is pressed against housing unit HU is at least ¼ and at most ½ a thickness T 1  ( FIG. 5 ) of sealing gasket  22  before press plate  21  is pressed against housing unit HU. 
     Thereafter, while press plate  21  closes first opening  1   b  in housing unit HU, vacuum pump  25  is started up (step S 3 :  FIG. 7 ). Valve  24  is open at this time. As vacuum pump  25  is started up, a pressure in internal space  1   d  in housing unit HU is reduced. The pressure in internal space  1   d  is reduced by exhausting gas in internal space  1   d  by vacuum pump  25  through pressure reduction hole  21   b  located directly above intermediate bar  3 . 
     Whether or not the pressure in internal space  1   d  after pressure reduction has attained to a set pressure is detected (step S 4 :  FIG. 7 ). When the pressure in internal space  1   d  after pressure reduction has not attained to the set pressure, reduction in pressure in internal space  1   d  is continued until the pressure reaches the set pressure. The set pressure is, for example, not lower than −2500 Pa and not higher than −800 Pa. 
     When the pressure in internal space  1   d  after pressure reduction has attained to the set pressure, an operation of vacuum pump  25  is stopped (step S 5 :  FIG. 7 ). Reduction in pressure in internal space  1   d  is thus stopped. 
     Thereafter, a state that reduction in pressure in internal space  1   d  has been stopped is maintained (step S 6 :  FIG. 7 ). At this time, the pressure in internal space  1   d  is negative with respect to the outside. Therefore, when the state that reduction in pressure in internal space  1   d  has been stopped is maintained, air flows in from the outside of housing unit HU into internal space  1   d  through pores in porous membrane  4 . The negative pressure in internal space  1   d  thus gradually becomes weaker. 
     If a gap or a hole is produced between bottom plate  5  and housing  1  or between porous membrane  4  and housing  1 , a rate of negative pressure weakening in internal space  1   d  is higher than in an example where no gap or hole is present. Namely, a rate of flow of air into internal space  1   d  is higher. Therefore, by measuring a pressure in internal space  1   d,  whether or not a gap or a hole is present between bottom plate  5  and housing  1  or between porous membrane  4  and housing  1  can be sensed. 
     Whether or not a set time period has elapsed since stop of reduction in pressure in internal space  1   d  is sensed (step S 7 :  FIG. 7 ) and the pressure in internal space  1   d  at the time point of lapse of the set time period is measured (step S 8 :  FIG. 7 ). 
     After the pressure in internal space  1   d  in housing unit HU is thus reduced and that state is maintained for the set time period, the pressure in internal space  1   d  is measured. The pressure in internal space  1   d  is measured with pressure sensor  23 . Specifically, the pressure in internal space  1   d  is measured with pressure sensor  23  arranged directly above intermediate bar  3 . As shown in  FIG. 4 , the pressure in internal space  1   d  is measured with pressure sensor  23  arranged at a position intermediate between two through holes  1   a.    
     The measured pressure in internal space  1   d  is compared with a threshold value (step S 9 :  FIG. 7 ). By comparing the measured pressure in internal space  1   d  with the threshold value, whether or not a gap or a hole is present in housing unit HU other than the plurality of pores in porous membrane  4  can be determined. Specifically, whether or not a gap or a hole is present between porous membrane  4  and housing  1  or between bottom plate  5  and housing  1  can be determined. 
     In step S 10  including step S 3  to step S 8  in  FIG. 7 , measurement of the pressure in internal space  1   d  in housing unit HU by pressure sensor  23  may continuously be conducted. 
     (Result of Check of Pressure in Housing Unit for Photovoltaic Power Generation Apparatus) 
     The present inventors inspected change in pressure in internal space  1   d  when the pressure in internal space  1   d  was reduced in the pressure checking method.  FIG. 9  shows results. 
     As shown in  FIG. 9 , as reduction in pressure in internal space  1   d  was started, the pressure in internal space  1   d  increased toward a negative pressure side (a minus side). The pressure in internal space  1   d  was maximum on the negative pressure side at the time point of end of pressure reduction. At this time, the negative pressure in internal space  1   d  at the time point of end of pressure reduction was weaker in a defective product than in a non-defective product. This may be because an unintended gap or hole was produced in housing unit HU in the defective product and hence outside air was introduced into internal space  1   d  through this gap or hole during pressure reduction and the negative pressure did not become strong. 
     When housing unit HU was left stand in a state of pressure check after end of reduction in pressure, the negative pressure in internal space  1   d  gradually became weaker. At this time, a rate of negative pressure weakening in internal space  1   d  was higher in the defective product than in the non-defective product. This may be because an unintended gap or hole was produced in housing unit HU in the defective product and hence outside air flowed into internal space  1   d  at a rate higher than in the non-defective product. 
     As set forth above, change in pressure is different between the non-defective product and the defective product. Therefore, by setting a prescribed pressure after lapse of a set time period since start of pressure reduction as a threshold value TH, when the pressure in internal space  1   d  is larger in value on the negative pressure side than threshold value TH, determination as the non-defective product can be made, and when the pressure is smaller in value on the negative pressure side, determination as the defective product can be made. 
     (Comparison Between Reduction and Increase in Pressure in Internal Space  1   d  in Housing Unit HU) 
     The present inventors inspected change in pressure in internal space  1   d  when the pressure in internal space  1   d  was increased instead of being reduced in the pressure checking method.  FIG. 10  shows results. 
     As shown in  FIG. 10 , when the pressure was increased, no noticeable difference as in pressure reduction ( FIG. 9 ) was observed during change in pressure in each of the non-defective product and the defective product. It was thus found that distinction between the non-defective product and the defective product could accurately be made when the pressure was reduced. 
     (Warpage of Housing Unit HU in Reduction in Pressure in Internal Space  1   d ) 
     The present inventors inspected an amount of warpage of housing unit HU when the pressure in internal space  1   d  was reduced in the pressure checking method.  FIG. 11  shows results. 
     The amount of warpage of housing unit HU refers to an amount of warpage of bottom plate  5  of housing unit HU toward internal space  1   d  as shown in  FIG. 12 . 
     As shown in  FIG. 11 , it was found that, as the pressure during pressure reduction was larger in value toward the negative pressure side, the amount of warpage of housing unit HU was larger. It was also found that, under measurement conditions, when the pressure in internal space  1   d  exceeded −2500 kPa, the amount of warpage exceeded 7 mm and bottom plate  5  might be damaged. 
     Effect of the Present Embodiment 
     In the present embodiment, as shown in  FIGS. 4 and 8 , a pressure in internal space  1   d  is measured while first opening  1   b  in housing unit HU including porous membrane  4  is closed. Therefore, change in pressure in internal space  1   d  in housing unit HU can be measured without immersing housing unit HU in liquid. Therefore, time and trouble and a large space for drying wet housing unit HU are not required. Incorporation (in-line) of this pressure checking process into a line for manufacturing photovoltaic power generation modules  10  is facilitated and checking can be done in a simplified manner and in a short period of time. Time and trouble and efforts for 100% inspection are also less. 
     By measuring change in pressure, a gap or a hole in housing unit HU other than pores in porous membrane  4  that allows communication from the outside to internal space  1   d  can be checked for. Whether or not there is a gap or a hole through which rainwater enters housing unit HU in arrangement of photovoltaic power generation module  10  outdoors can thus be determined. 
     Since determination above is made based on a pressure value or a rate of variation in pressure, accuracy in checking can be high also for housing unit HU in which internal space  1   d  always communicates with the outside through porous membrane  4 . 
     In the present embodiment, as shown in  FIGS. 5 and 8 , the pressure is checked while housing unit HU is provided with housing  1 , porous membrane  4 , bottom plate  5 , flexible printed circuit boards  6 , power generation elements  7 , and shield plate  8 . By checking the pressure in such a state, checking in a state that photovoltaic power generation module  10  is close to a completed product can be done. Specifically, a gap or a hole that may permit entry of rainwater at a portion where porous membrane  4  is attached to housing  1  and a portion where bottom plate  5  is attached to housing  1  can be checked for. 
     In the present embodiment, as shown in  FIG. 4 , the pressure in internal space  1   d  is measured while pressure sensor  23  is arranged directly above intermediate bar  3 . Therefore, checking can be done on both sides of intermediate bar  3  with the same accuracy without any imbalance. 
     In the present embodiment, as shown in  FIG. 4 , the pressure in internal space  1   d  is measured while pressure sensor  23  is arranged at a position intermediate between two through holes  1   a.  Therefore, pressure sensor  23  is located equidistantly from two porous membranes  4  and hence checking can be done on both sides with the same accuracy without any imbalance. 
     In the present embodiment, as shown in  FIG. 7 , the pressure in internal space  1   d  is measured after the pressure in internal space  1   d  in housing unit HU is reduced. Thus, as shown in  FIGS. 9 and 10 , checking higher in accuracy than in measurement of the pressure after increase in pressure in internal space  1   d  can be done. A load test of photovoltaic power generation module  10  is conducted by applying a load onto a rear surface of bottom plate  5  to warp bottom plate  5  toward internal space  1   d.  By reducing a pressure in internal space  1   d,  a pseudo load test of photovoltaic power generation module  10  can also be conducted as shown in  FIG. 11 . 
     In the present embodiment, the pressure in internal space  1   d  in housing unit HU is reduced to a pressure not lower than −2500 Pa and not higher than −800 Pa. When the pressure in internal space  1   d  is higher than −800 Pa (a negative pressure is weaker), a pressure difference between a non-defective product and a defective product is too small and accuracy in checking is poor. Since 800 Pa is one of references defined by the International Electrotechnical Commission (IEC), the pressure in internal space  1   d  being not higher than −800 Pa (the negative pressure being larger than 800 Pa) is desirable in conducting the pseudo load test of photovoltaic power generation module  10 . When the pressure in internal space  1   d  is lower than −2500 Pa (a negative pressure is stronger), bottom plate  5  may deform and photovoltaic power generation module  10  may be damaged. 
     In the present embodiment, as shown in  FIG. 4 , the pressure in internal space  1   d  is measured while pressure reduction hole  21   b  is located directly above intermediate bar  3 . The pressure can thus be reduced on both sides of intermediate bar  3  without imbalance and checking can be done with the same accuracy on both sides of intermediate bar  3 . 
     In the present embodiment, as shown in  FIGS. 5 and 8 , press plate  21  is pressed against housing unit HU such that thickness T 2  ( FIG. 8 ) of sealing gasket  22  after press plate  21  is pressed against housing unit HU is at least ¼ and at most ½ thickness T 1  ( FIG. 5 ) of sealing gasket  22  before press plate  22  is pressed against housing unit HU. When thickness T 2  is larger than ½ thickness T 1 , sealing by sealing gasket  22  is insufficient and checking with high accuracy cannot be done. When thickness T 2  is smaller than ¼ thickness T 1 , a portion of housing unit HU in contact with sealing gasket  22  may be damaged. In particular when a part of housing unit HU is made of an inexpensive resin, there is possibility of damage. 
     Since press plate  21  is pressed against first end  2   a  of housing  1  with soft sealing gasket  22  low in density being interposed as described above, sealing gasket  22  can provide sufficient sealing even though there are irregularities on the end surface of first end  2   a.    
     In the present embodiment, as shown in  FIG. 6 , sealing gasket  22  includes the closed-cell structure. A plurality of voids  22   a  in sealing gasket  22  are thus independent of one another. Voids  22   a  in sealing gasket  22  thus do not allow communication from the outside of housing unit HU to internal space  1   d.  Therefore, leakage through sealing gasket  22  can reliably be prevented and checking with high accuracy can be done. 
     It should be understood that the embodiment disclosed herein is illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims rather than the embodiment above and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
     REFERENCE SIGNS LIST 
       1  housing;  1   a,    21   a  through hole;  1   b  first opening;  1   c  second opening;  1   d  internal space;  1   e  center;  2  peripheral wall portion;  2   a  first end;  2   b  second end;  3  intermediate bar;  4  porous membrane;  5  bottom plate;  6  flexible printed circuit board;  7  power generation element;  8  shield plate;  8   a  light transmission window;  9  lens member;  9   a  lens portion;  10  photovoltaic power generation module;  11  pedestal;  12  support arm;  13  rail;  20  concentrator photovoltaic power generation apparatus;  21  press plate;  21 L lower surface;  21 T upper surface;  21   b  pressure reduction hole;  22  sealing gasket;  22   a  void;  23  pressure sensor;  24  valve;  25  vacuum pump; HU housing unit