Patent Publication Number: US-2013240037-A1

Title: Solar cell module and solar generator

Description:
TECHNICAL FIELD 
     The present invention relates to a solar cell module and a solar generator. 
     This application claims the priority of Japanese Patent Application No. 2010-261688, filed Nov. 24, 2010, the entire contents of which are incorporated herein by reference. 
     BACKGROUND ART 
     Existing solar generators generally include a plurality of solar panels that are arranged in a plane so as to be oriented toward the sun. For example, solar generators of a known type include a rack that is mounted on the roof of a building and a plurality of solar panels that are attached to the rack so as to be arranged in a plane. In general, solar panels are made of an opaque semiconductor, and therefore the solar panels cannot be arranged in a stack. Therefore, solar panels having large areas are needed in order that a solar generator can have high electric power. 
     However, due to limitation on an installation space such as a roof, the amount of electric power obtainable by a solar generator has been limited. 
     For this reason, there has been proposed a solar cell including an optical concentrator for guiding incident sunlight to the solar cell (see PTL 1 below). The solar cell described in PTL 1 includes a light-transmitting member having a substantially right-triangular shape in side view. A plurality of V-shaped grooves are formed in the light-transmitting member, and a solar cell is attached to an end surface of the light-transmitting member. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: Japanese Unexamined Patent Application Publication No. 2004-47752 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, with the technology described in PTL 1, in a case where a light-guide member has a large size, an incident light beam is reflected a plurality of times by reflecting surfaces of the V-shaped grooves while the incident light beam propagates through the light-transmitting member before being concentrated on the end surface. Thus, the reflection angle of the incident light beam at the reflecting surfaces changes, the incident light beam fails to satisfy the conditions for total internal reflection at one of the reflecting surfaces, and the incident light beam leaks to the outside. As a result, the efficiency with which light is guided to the solar cell is decreased, and the power generation efficiency is decreased. 
     An object of the present invention, which has been made in order to solve the above problem, is to provide a solar cell module that can prevent decrease in the power generation efficiency and a solar generator including the solar cell module. 
     Solution to Problem 
     In order to achieve the object, a solar cell module according to an embodiment of the present invention includes a light guide module including a first light guide and a second light guide that are disposed so as to face each other and a low-refractive-index layer that is disposed between the first light guide and the second light guide; and a solar cell that receives light emitted from the light guide module. The first light guide has a first main surface, a second main surface, and a first end surface that is connected to the first main surface and the second main surface, and the first light guide allows a first light beam from the outside to enter thereinto through the first main surface, to propagate therethrough, and to be emitted from the first end surface. The second light guide has a first main surface, a second main surface, and a first end surface that is connected to the first main surface and the second main surface, and the second light guide allows a second light beam that has passed through the first light guide to enter thereinto through the first main surface of the second light guide, to propagate therethrough, and to be emitted from the first end surface of the second light guide. The low-refractive-index layer has a refractive index that is lower than a refractive index of any of the first light guide and the second light guide. The solar cell receives the first light beam emitted from the first end surface of the first light guide and the second light beam emitted from the first end surface of the second light guide. The second main surface of the first light guide includes a first reflecting surface that reflects the first light beam and changes a propagation direction of the first light beam, which has entered through the first main surface of the first light guide. The second main surface of the second light guide includes a second reflecting surface that reflects the second light beam and changes a propagation direction of the second light beam, which has entered through the first main surface of the first light guide, has passed through the first light guide, has been refracted by the low-refractive-index layer, and has entered the second light guide. 
     In the solar cell module, the second main surface of the first light guide may include a first light-direction changer that reflects the first light beam and changes the propagation direction of the first light beam, which has entered through the first main surface of the first light guide. The first light-direction changer has a first inclined surface that is inclined at a first inclination angle with respect to the second main surface of the first light guide, and the first inclined surface serves as the first reflecting surface for reflecting the first light beam, which has entered through the first main surface of the first light guide. 
     In the solar cell module, the first main surface of the second light guide may include a second light-direction changer that reflects a third light beam that has entered through the second main surface of the second light guide and that changes a propagation direction of the third light beam. The second light-direction changer has a second inclined surface that is inclined at a second inclination angle with respect to the first main surface of the second light guide, and the second inclined surface reflects the third light beam, which has entered through the second main surface of the second light guide. 
     In the solar cell module, the first inclination angle may be equal to the second inclination angle. 
     In the solar cell module, the second inclination angle may be larger than the first inclination angle. 
     In the solar cell module, the first main surface of the first light guide may be a flat surface, and the second main surface of the second light guide may be a flat surface that is parallel to the first main surface. 
     In the solar cell module, the refractive index of the first light guide may be equal to the refractive index of the second light guide. 
     In the solar cell module, the refractive index of the second light guide may be smaller than the refractive index of the first light guide. 
     The solar cell module may include a spacer that is disposed between the first light guide and the second light guide, the spacer maintaining a distance between the first light guide and the second light guide. 
     In the solar cell module, the low-refractive-index layer may be an air layer. 
     In the solar cell module, the first light guide may have a second end surface that is connected the first main surface and the second main surface and that faces the first end surface. The first light-direction changer may include a first-end-side reflector that reflects a fourth light beam toward the first end surface and a second-end-side reflector that reflects a fifth light beam toward the second end surface, the fourth light beam having entered through the first main surface of the first light guide, and the fifth light beam having entered through the first main surface of the first light guide. 
     In the first light-direction changer of the solar cell module, the area of a reflecting surface of the first-end-side reflector may be equal to the area of a reflecting surface of the second-end-side reflector. 
     The solar cell module may include a plurality of light guide modules each having the same structure as the light guide module, the plurality of light guide modules being disposed so as to face each other. 
     A solar generator according to another embodiment of the present invention includes the solar cell module. 
     Advantageous Effects of Invention 
     With the present invention, a solar cell module that can prevent decrease in the power generation efficiency and a solar generator including the solar cell module can be provided. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a perspective view of a solar generator according to a first embodiment of the present invention. 
         FIG. 2  is a sectional view of the solar generator according to the embodiment. 
         FIG. 3  is a sectional view of a solar cell module according to the embodiment. 
         FIG. 4  illustrates the function of a reflecting surface of the solar cell module according to the embodiment. 
         FIG. 5  illustrates the result of simulation of how sunlight is extracted in the solar cell module according to the embodiment. 
         FIG. 6  is a graph representing the relationship between the absorption wavelength of a solar cell and the intensity/absorption sensitivity. 
         FIG. 7  is a sectional view of a first modification of the solar cell module according to the embodiment. 
         FIG. 8  is a perspective view of a solar cell module according to a second embodiment of the present invention. 
         FIG. 9  illustrates the function of a reflecting surface of the solar cell module according to the embodiment. 
         FIG. 10  is a perspective view of a solar cell module according to a third embodiment of the present invention. 
         FIG. 11  illustrates the function of a reflecting surface of the solar cell module according to the embodiment. 
         FIG. 12  is a perspective view of a solar generator according to a fourth embodiment of the present invention. 
         FIG. 13  is a sectional view of the solar generator according to the embodiment. 
         FIG. 14  is a perspective view of a solar generator according to a fifth embodiment of the present invention. 
         FIG. 15  is a sectional view of the solar generator according to the embodiment. 
         FIG. 16  is a table showing the result of simulation of the sunlight extraction ratio. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     First Embodiment 
     Referring to  FIGS. 1 to 6 , a first embodiment of the present invention will be described. 
     In the present embodiment, a solar generator including a solar cell module that is mounted in a support frame will be described. 
       FIG. 1  is a schematic perspective view of the solar generator according to the present embodiment.  FIG. 2  is a sectional view of the solar generator.  FIG. 3  is a sectional view of the solar cell module.  FIG. 4  illustrates the function of a reflecting surface of the solar cell module.  FIG. 5  illustrates the result of simulation of how sunlight is extracted in the solar cell module.  FIG. 6  is a graph representing the relationship between the absorption wavelength of a solar cell and the intensity/absorption sensitivity. 
     In all of the following figures, components are shown in different scales for ease of viewing the components. 
     As illustrated in  FIG. 1 , a solar generator  1  according to the present embodiment includes a solar cell module  2  and a support frame  8 . The solar cell module  2  has a substantially rectangular shape in plan view. The support frame  8  is attached to the solar cell module  2  so as to surround the four sides of the solar cell module  2 . The support frame  8  is fixed to the solar cell module  2  by using, for example, an acrylic adhesive. 
     In the case of present embodiment, the solar generator  1  is installed, for example, on the roof of a building. When sunlight is incident on the roof, electric power is generated. In addition to the solar cell module  2  and the support frame  8 , the solar generator  1  may include, for example, a storage battery for storing electric power obtained by the solar cell module  2 . The solar generator  1  may be configured to be mounted, for example, in a window of a building or a window of an automobile, instead of on the roof of a building. 
     As illustrated in  FIG. 2 , the solar cell module  2  includes a light guide module  3  and a solar cell  7 . In the solar cell module  2 , light that has entered the light guide module  3  is guided to the solar cell  7 . The solar cell  7  performs photoelectric conversion and outputs electric energy. 
     The light guide module  3  includes a first light guide  4 , a second light guide  5 , and a low-refractive-index layer  6 . The first light guide  4  is a transparent plate having a rectangular shape in plan view. A first light-direction changer  4 S is formed on one side of the first light guide  4 . A light beam L enters from a side of the first light guide  4  opposite to the side on which the first light-direction changer  4 S is formed. Therefore, when installing the solar generator  1  on, for example, the roof of a building, the solar generator  1  is installed in such a way that a side of the first light guide  4  on which the first light-direction changer  4 S is formed faces inward and a side opposite to the side on which the first light-direction changer  4 S is formed faces outward. 
     The second light guide  5  is disposed so as to face the first light guide  4  with the low-refractive-index layer  6  therebetween. Spacers  9  are disposed between the first light guide  4  and the second light guide  5 . The spacers  9  maintain a distance between the first light guide  4  and the second light guide  5 . The refractive index n 1  of the first light guide  4  is equal to the refractive index n 2  of the second light guide  5  (n 1 =n 2 ). 
     The low-refractive-index layer  6 , which is disposed between the first light guide  4  and the second light guide  5 , has a refractive index n 3  that is lower than that of any of the first light guide  4  and the second light guide  5 . The low-refractive-index layer  6  is an air layer. It is not necessary that the layer between the first light guide  4  and the second light guide  5  be an air layer. The layer may be any layer that has a refractive index that is lower than that of any of the first light guide  4  and the second light guide  5 . It is preferable that the layer be composed of a medium having a lower refractive index. 
     The first light guide  4  and the second light guide  5  may be made of an organic material or an inorganic material having high durability and high transparency, such as an acrylic resin, a polycarbonate resin, or glass. However, the material is not limited to these. 
     Any known solar cell, such as an amorphous silicon solar cell, a polycrystalline silicon solar cell, a monocrystalline silicon solar cell, or a compound-semiconductor solar cell can be used as the solar cell  7 . In the present embodiment, a compound-semiconductor solar cell is used as the solar cell  7 . The shape and size of the solar cell  7  are not particularly limited, provided that the solar cell  7  having the shape and size can be disposed within an end surface of the light guide module  3 . The solar cell  7  is bonded to the end surface of the light guide module  3  by using, for example, aGEL (registered trademark) made by Taica Corporation. 
       FIG. 6  is a graph representing the relationship between the absorption wavelength of a solar cell and the intensity/absorption sensitivity. In  FIG. 6 , the horizontal axis represents the absorption wavelength, and the vertical axis represents the intensity/absorption sensitivity. As illustrated in  FIG. 6 , compound-semiconductor solar cells, such as an InGaAs solar cell, a GaAs solar cell, and an InGaAs solar cell, have peaks of the intensity/absorption sensitivity that are higher than that of any of silicon solar cells, such as a crystalline silicon (c-Si) solar cell and an amorphous silicon (a-Si) solar cell, although the light-absorption wavelength ranges of the compound-semiconductor solar cells are narrower than those of the silicon solar cells. Therefore, by using a compound-semiconductor solar cell that has a high peak of the intensity/absorption sensitivity in a specific absorption wavelength range, it is possible to convert sunlight into electricity with an efficiency higher than that of a case where a silicon solar cell is used. 
     Hereinafter, for convenience of description, three of the six surfaces of the first light guide  4  will be referred to as follows. A surface (parallel to the xy-plane in  FIG. 1 ) through which light enters will be referred to as a first main surface  4   a . A surface that faces the first main surface  4   a  and on which the first light-direction changer  4 S is formed will be referred to as a second main surface  4   b . A surface (parallel to the xz-plane in  FIG. 1 ) that intersects the first main surface  4   a  and the second main surface  4   b  and from which light is emitted will be referred to as a first end surface  4   c . Three of the six surfaces of the second light guide  5  will be referred to as follows. A surface (parallel to the xy-plane in  FIG. 1 ) through which light enters and on which a second light-direction changer  5 S is formed will be referred to as a first main surface  5   a . A surface that faces the first main surface  5   a  will be referred to as a second main surface  5   b . A surface (parallel to the xz-plane in  FIG. 1 ) that intersects the first main surface  5   a  and the second main surface  5   b  and from which light is emitted will be referred to as a first end surface  5   c . The first main surface  4   a  of the first light guide  4  is a flat surface, and the second main surface  5   b  of the second light guide  5  is a flat surface that is parallel to the first main surface  4   a.    
     In the case of present embodiment, the first light guide  4  and the second light guide  5  are made of, for example, an acrylic resin. The dimensions of the first light guide  4  and the second light guide  5  are, for example, as follows: the first main surfaces  4   a  and  5   a  and the second main surfaces  4   b  and  5   b  are rectangular and have horizontal dimensions (in the x-axis direction and the y-axis direction in  FIG. 2 ) of 250 mm×250 mm and a thickness (the dimension in the z-axis direction of  FIG. 2 ) of 10 mm. 
     As illustrated in  FIG. 3 , the first light-direction changer  4 S is disposed on the second main surface  4   b  side of the first light guide  4 . The first light-direction changer  4 S reflects light that has entered through the first main surface  4   a  and changes the propagation direction of the light to a direction toward the first end surface  4   c . The first light-direction changer  4 S includes a plurality of triangular-prism-shaped protrusions  4 A that are formed on the second main surface  4   b  of the first light guide  4 . Light beams L 1  that have entered through various portions of the first main surface  4   a  of the first light guide  4  propagate through the first light guide  4  so as to be concentrated on a portion of the first end surface  4   c  on which the solar cell  7  is disposed. 
     The second light-direction changer  5 S is disposed on the first main surface  5   a  side of the second light guide  5 . The second light-direction changer  5 S refracts light that has entered through the first main surface  5   a  and changes the propagation direction of the light. The second light-direction changer  5 S includes a plurality of triangular-prism-shaped protrusions  5 A that are formed on the first main surface  5   a  of the second light guide  5 . Light beams that have entered through various portions of the first main surface  5   a  of the second light guide  5  propagate through the second light guide  5  so as to be concentrated on a portion of the first end surface  5   c  on which the solar cell  7  is disposed. 
     In the case of present embodiment, the light-direction changers ( 4 S,  5 S) are integrally formed with the light guides ( 4 ,  5 ) by processing the light guides. The light-direction changers can be formed by, for example, injection-molding a plastic by using dies having recesses whose shapes are the inverses of those of the protrusions ( 4 A,  5 A). Alternatively, the light-direction changers may be formed by cutting the second main surface  4   b  of the first light guide  4  (and the first main surface  5   a  of the second light guide), which are flat before being cut. 
     The protrusions  4 A are continuously formed on the second main surface  4   b  of the first light guide  4 . The protrusions  5 A are continuously formed on the first main surface  5   a  of the second light guide  5 . The shapes and sizes of all the protrusions  4 A and  5 A are the same. 
     The protrusions ( 4 A,  5 A) are triangular-prism-shaped. As illustrated in  FIG. 3 , the cross-sectional shape of each of the protrusions when the light guide ( 4 ,  5 ) is cut along the yz-plane is not an equilateral triangle or an isosceles triangle but is a scalene triangle. 
     As illustrated in  FIG. 4 , each of the protrusions  4 A of the first light-direction changer  4 S has first inclined surfaces (a steeply inclined surface T 1a  and a gently inclined surface T 1b ). The steeply inclined surface T 1a  has a predetermined inclination angle θ A1  (first inclination angle) with respect to the second main surface  4   b . The gently inclined surface T 1b  has an inclination angle θ A2 , which is smaller than the inclination angle θ m  of the steeply inclined surface T 1a , with respect to the second main surface  4   b . These two first inclined surfaces T 1a  and T 1b  function as a reflecting surface (first reflecting surface) for reflecting (totally internally reflecting) light that has entered through the first main surface  4   a.    
     Each of the protrusions  5 A of the second light-direction changer  5 S has second inclined surfaces (a steeply inclined surface T 2a  and a gently inclined surface T 2b ). The steeply inclined surface T z , has a predetermined inclination angle θ B1  (second inclination angle) with respect to the first main surface  5   a . The gently inclined surface T 2b  has an inclination angle θ 32 , which is smaller than the inclination angle θ B1  of the steeply inclined surface T 2a , with respect to the first main surface  5   a . These two second inclined surfaces T 2a  and T 2b , function as a refracting surface for refracting light that has entered through the first main surface  5   a.    
     As illustrated in  FIG. 4 , when a sunlight beam L 1  (sunlight beam that is incident at a position that is relatively near to the first end surface  4   c ) is incident on the first main surface  4   a  of the first light guide  4  at an incident angle θ 0 , the sunlight beam L 1  is refracted by the first main surface  4   a  at a refraction angle θ 1  when entering the first light guide  4 . Subsequently, the light beam is incident on the steeply inclined surface T 1a  at an incident angle θ 2 , is totally internally reflected by the steeply inclined surface T 1a  at a reflection angle θ 2 , propagates through the first light guide  4  at an angle θ x  to an imaginary plane X that is parallel to the first main surface  4   a , and is emitted toward the solar cell  7 . 
     A sunlight beam L 2  that is incident on the first main surface  4   a  of the first light guide  4  at a relatively remote position (sunlight beam that is incident at a position that is farther from the first end surface  4   c  than a position at which the sunlight beam L 1  is incident) is reflected between the first main surface  4   a  and the second main surface  4   b  a larger number of times than the sunlight beam L 1  while propagating through the first light guide  4 . 
     When the sunlight beam L 2  is incident on the first main surface  4   a  of the first light guide  4  at an incident angle θ 0 , the sunlight beam L 2  is refracted by the first main surface  4   a  at a refraction angle θ 1  when entering the first light guide  4 . The light beam is incident on the steeply inclined surface T 1a  at an incident angle θ 2  and is totally internally reflected at a reflection angle θ 2 . The light beam, which has been totally internally reflected by the steeply inclined surface T 1a  at the reflection angle θ 2 , is reflected a predetermined times between the first main surface  4   a  and the second main surface  4   b . Then, the light beam is incident on the first main surface  4   a  at an incident angle θ 3A , and is totally internally reflected at a reflection angle θ 3A . The light beam, which has been totally internally reflected by the first main surface  4   a  at the reflection angle θ 3A , is incident on the gently inclined surface T 1b  at an incident angle θ 4 , is refracted at a refraction angle θ 5 , and is incident on the first main surface  5   a  (gently inclined surface T 2b ) of the second light guide  5  at an incident angle θ 6 . The light beam, which is incident on the gently inclined surface T 2b  at the incident angle θ 6 , is refracted by the gently inclined surface T 2b  of the second light guide  5  at a refraction angle θ 7  when entering the second light guide  5 . Subsequently, the light beam is incident on the second main surface  5   b  of the second light guide  5  at an incident angle θ 2B , is totally internally reflected by the second main surface  5   b  at a reflection angle θ 2B , propagates through the second light guide  5 , and is emitted toward the solar cell  7 . 
     Here, the incident angle θ 2 , at which the light beam is incident on the steeply inclined surface T 1a  of the first light guide  4 , changes in accordance with the inclination angle θ A1  of the steeply inclined surface T 1a . Therefore, the inclination angle θ A1  of the steeply inclined surface T 1a  is set beforehand so that the incident angle θ 2 , at which the light beam is incident on the steeply inclined surface T 1a , is larger than the critical angle for the interface between the steeply inclined surface T 1a  and air and the light beam is totally internally reflected by the interface. The incident angle θ 4 , at which the light beam is incident on the gently inclined surface T 1b  of the first light guide  4 , also changes in accordance with the inclination angle θ A2  of the gently inclined surface T 1b . 
     The incident angle at which the light beam is incident on the steeply inclined surface T 2a  of the second light guide  5  changes in accordance with the inclination angle θ B1  of the steeply inclined surface T 2a . The incident angle θ 6 , at which the light beam is incident on the gently inclined surface T 2b  of the second light guide  5 , changes in accordance with the inclination angle θ B2  of the gently inclined surface T 2b . In the present embodiment, the inclination angle θ A1  of the steeply inclined surface T 1a  of the first light guide  4  is equal to the inclination angle θ B1  of the steeply inclined surface T 2a  of the second light guide  5  (θ A1 =θ B1 ). The inclination angle θ A2  of the gently inclined surface T 1b  of the first light guide  4  is equal to the inclination angle θ B2  of the gently inclined surface T 2b  of the second light guide  5  (θ A2 = θ   B2 ). 
     To be specific, for example, it is assumed as follows: the inclination angle θ A1  of the steeply inclined surface T 1a  of the first light guide  4  is 24 degrees, the inclination angle θ A2  of the gently inclined surface T 1b  of the first light guide  4  is 21 degrees, the refractive index n 1  of the first light guide  4  and the refractive index n 2  of the second light guide  5  are 1.5, and the refractive index n o  of external air and the refractive index n 3  of the air layer  6  are 1.0. In this case, according to Snell&#39;s law, the critical angle for the interface between the air layer  6  and the steeply inclined surface T 1a  or the gently inclined surface T 1b  of the first light guide  4  is 41 degrees. Here, if the incident angle θ 0 , at which the sunlight beam L 1  is incident on the first main surface  4   a  of the first light guide  4 , is larger than or equal to 27 degrees, the refraction angle θ 1 , at which the sunlight beam L 1  is refracted when the sunlight beam L 1  enters the first light guide  4 , is larger than or equal to 18 degrees. Then, the incident angle θ 2 , at which the light beam is incident on the steeply inclined surface T 1a  of the first light guide  4 , is larger than or equal to 42 degrees (θ 2 =θ 1 +θ A1 ). Because the incident angle θ 2  is larger than or equal to the critical angle (θ 2 ≧41 degrees), the light beam L 1  is totally internally reflected by the steeply inclined surface T 1a . 
     The critical angle for the interface between external air and the second main surface  5   b  of the second light guide  5  is also 41 degrees. Also in this case, if the incident angle θ 0 , at which the sunlight beam L 2  is incident on the first main surface  4   a  of the first light guide  4 , is larger than or equal to 27 degrees, the refraction angle θ 1 , at which the sunlight beam L 2  is refracted when the sunlight beam L 2  enters the first light guide  4 , is larger than or equal to 18 degrees. Then, the incident angle θ 2 , at which the light beam is incident on the steeply inclined surface T 1a  of the first light guide  4 , is larger than or equal to 42 degrees (θ 2 =θ 1 +θ A1 ). Because the incident angle θ 2  is larger than or equal to the critical angle (θ 2 ≧41 degrees), the light beam L 2  is totally internally reflected by the steeply inclined surface T 1a . 
     The light beam, which has been totally internally reflected by the steeply inclined surface T 1a  of the first light guide  4  at a reflection angle θ 2 , is reflected a predetermined times between the first main surface  4   a  and the second main surface  4   b . Then, the light beam is incident on the first main surface  4   a  at the incident angle θ 3A , and is totally internally reflected at the reflection angle θ 3A . Here, if the incident angle θ 3A , at which the light beam L 2  is incident on the first main surface  4   a  of the first light guide  4 , (the reflection angle θ 3A , at which the light beam L 2  is reflected by the first main surface  4   a ) is larger than or equal to 41 degrees and smaller than 62 degrees, the incident angle θ 4 , at which the light beam is incident on the gently inclined surface T 1b  of the first light guide  4 , is larger than or equal to 20 degrees and smaller than 41 degrees (θ 4 =θ 3A −θ A2 ). Because the incident angle θ 4  is equal to smaller than the critical angle (θ 4 &lt;41 degrees), the light beam L 2  passes through the gently inclined surface T 1b . 
     If the incident angle θ 4 , at which the light beam is incident on the gently inclined surface T 1b  of the first light guide  4 , is larger than or equal to 20 degrees and smaller than 41 degrees, the refraction angle θ 5 , at which the light beam L 2  is refracted when the light beam L 2  enters the air layer  6 , (the incident angle θ 6 , at which the light beam L 2  is incident on the gently inclined surface T 2b  of the second light guide  5 ) is larger than or equal to 31 degrees and smaller than 79 degrees. Then, the refraction angle θ 7 , at which the light beam L 2  is refracted when the light beam L 2  enters the second light guide  5 , is larger than or equal to 20 degrees and smaller than 41 degrees. 
     In the present embodiment, the refractive index n 1  of the first light guide  4  is equal to the refractive index n 2  of the second light guide  5  (n 1 =n 2 ). Moreover, the inclination angle of one of the first inclined surfaces of the first light guide  4  (the inclination angle θ A2  the gently inclined surface T 1b ) is equal to the inclination angle of one of the second inclined surfaces of the second light guide  5  (the inclination angle θ 22  of the gently inclined surface T 2b ) (θ A2 =θ B2 ). Furthermore, the first main surface  4   a  of the first light guide  4  is a flat surface, and the second main surface of the second light guide  5  is a flat surface that is parallel to the first main surface  4   a . Therefore, the reflection angle θ 3A , at which the light beam L 2  is reflected by the first main surface  4   a  of the first light guide  4 , is equal to the reflection angle θ 3B , at which the light beam L 2  is reflected by the second main surface  5   b  of the second light guide  5  (θ 3A =θ 3B ). Therefore, the light beam L 2 , which has been incident on the first main surface  4   a  of the first light guide  4  at the incident angle θ 3A  and has been totally internally reflected at the reflection angle θ 3A , is incident on the second main surface  5   b  of the second light guide  5  at the incident angle θ 3B , which is equal to the reflection angle θ 3A , and is totally internally reflected at the reflection angle θ 3B . The light beam L 2 , which has been totally internally reflected by the second main surface  5   b  of the second light guide  5  at the reflection angle θ 3B , propagates through the second light guide  5 , and is emitted toward the solar cell  7 . 
     The above description can be summarized as follows. As illustrated in  FIG. 4 , the light beam L 1 , which is one of light beams that enter through various portions of the first light guide  4 , is incident on the first main surface  4   a  of the first light guide  4  at a position that is relatively near to the first end surface  4   c . The light beam L 1  is incident on the steeply inclined surface T 1a  of one of the protrusions  4 A, is totally internally reflected by the steeply inclined surface T 1a , and is guided to the solar cell  7 . The light beam L 2 , which is one of light beams that enter through various portions of the first light guide  4 , is incident on the first light guide  4  at a position on the first main surface  4   a  of the first light guide  4  that is relatively far from the first end surface  4   c . The light beam L 2  is incident on the steeply inclined surface T 1a  of one of the protrusions  4 A, and is totally internally reflected by the steeply inclined surface T 1a . Along a path toward the solar cell  7 , the light beam L 2  is reflected a predetermined times and fails to satisfy the conditions for total internal reflection. Thus, the light beam L 2  passes through the first light guide  4 . However, the light beam L 2  is totally internally reflected by the second main surface  5   b  of the second light guide  5 , and is guided to the solar cell  7 . If the second light guide  5  were not provided, all the light that has passed through the first light guide  4  would leak to the outside. 
     That is, in the present embodiment, the steeply inclined surface T 1a  of each of the protrusions  4 A of the first light-direction changer  4 S serves as a reflecting surface (first reflecting surface) for reflecting the light beam L 1  and changing the propagation direction of the light beam  1  to a direction toward the first end surface  4   c . The second main surface  5   b  of the second light guide  5  serves as a reflecting surface (second reflecting surface) for reflecting the light beam L 2 , which has passed through the first light guide  4 , has been refracted by the air layer  6 , and has entered the second light guide  5 , and changing the propagation direction of the light beam L 2  to a direction toward the first end surface  5   c.    
       FIG. 5  illustrates the result of simulation of how sunlight is extracted in the solar cell module. As illustrated in  FIG. 5 , some of light beams L that have entered through the first main surface  4   a  of the first light guide  4  propagate through the first light guide  4 , are guided to the solar cell  7 , and contribute to power generation. The remainder of the light beams L are emitted from the first light guide  4 , propagate through the second light guide  5 , are guided to the solar cell  7 , and contribute to power generation. For convenience of drawing, the air layer  6  is not illustrated in  FIG. 5 . 
     Because the solar generator  1  according to the present embodiment includes the first light guide  4  and the second light guide  5 , the solar generator  1  can cause light from the outside to propagate through the first light guide  4  so as to be guided to the solar cell  7 . Moreover, the solar generator  1  can cause light that has passed through the first light guide  4  to propagate through the second light guide  5  so as to be guided to the solar cell  7 . Furthermore, because the low-refractive-index layer  6  is disposed between the first light guide  4  and the second light guide  5 , the refraction angle θ 5 , at which a light beam that has passed through the first light guide  4  is refracted when the light beam enters the low-refractive-index layer  6 , is larger than the incident angle θ 4 , at which the light beam is incident on the gently inclined surface T 1b  of the first light guide  4 . Thus, a light-guide distance, which is the length of a path along which a light beam that has entered the low-refractive-index layer  6  is guided to the second light guide  5 , can be increased. Therefore, the light beam, which has passed through the first light guide  4 , is reflected a smaller number of times between the first light guide  4  and the second light guide  5  while being guided, and thereby the light can be easily guided to the solar cell  7 . Therefore, with the solar cell module  2  and the solar generator  1  including the solar cell module  2 , decrease in the power generation efficiency can be prevented. 
     The refractive index n 1  of the first light guide  4  is equal to the refractive index n 2  of the second light guide  5 . The inclination angle of the first inclined surface of the first light guide  4  is equal to the inclination angle of the second inclined surface of the second light guide  5 . Moreover, the first main surface  4   a  of the first light guide  4  is a flat surface, and the second main surface of the second light guide  5  is a flat surface that is parallel to the first main surface  4   a . Therefore, light guides made of the same material and having the same size can be used as the first light guide  4  and the second light guide  5 . For example, by preparing two first light guides and by inversely placing one of the first light guides so as to face the other first light guide with the low-refractive-index layer therebetween, the one of the first light guides can be used as the second light guide. Therefore, production cost can be reduced. 
     Because the spacers  9  are disposed between the first light guide  4  and the second light guide  5 , the second main surface  4   b  of the first light guide  4  and the first main surface  5   a  of the second light guide  5  do not come into contact with each other, and the low-refractive-index layer  6  having a predetermined thickness can be interposed between the first light guide  4  and the second light guide  5 . Therefore, light that has passed through the first light guide  4  can be prevented from entering the second light guide  5  without passing through the low-refractive-index layer  6 . Therefore, reduction in the power generation efficiency can be stably prevented and the reliability can be improved. 
     Because the low-refractive-index layer  6  is an air layer, it is easy to make the refractive index of the low-refractive-index layer  6  sufficiently low. Therefore, light that has passed through the first light guide  4  can be easily guided to the solar cell  7 . 
     The inventor carried out a simulation of the sunlight extraction ratio in order to demonstrate the effects of the solar generator  1  according to the present embodiment (see  FIG. 16 ). Here, the term “sunlight extraction ratio” refers the ratio (%) of the amount of light that is concentrated on an end surface of the light guide module  3  (at least one of the first end surface  4   c  of the first light guide  4  and the first end surface  5   c  of the second light guide  5 ) to the total amount of sunlight (100%) that is incident on the first main surface  4   a  of the first light guide  4 . The conditions of the simulation in example 1 were as follows: the horizontal dimensions of the first light guide  4  were 250 mm×250 mm, the thickness of the first light guide  4  was 10 mm, the horizontal dimensions of the second light guide  5  were 250 mm×250 mm, and the thickness of the second light guide  5  was 10 mm. The refractive index of the first light guide  4  was 1.5, the refractive index of the second light guide  5  was 1.5, and the refractive index of air was 1.0. For the solar cell module  2  of example 1, when sunlight was incident on the first main surface  4   a  side of the first light guide  4 , the sunlight extraction ratio was 35.996%. 
     The output conditions of the solar cell  7  are standardized with respect to air mass AM1.5, which is specified in JIS. In this case, the incident angle at which sunlight is incident on the first main surface  4   a  of the first light guide  4  is approximately 42 degrees. As a comparative example, the simulation was performed by using only the first light guide  4  and without using the second light guide  5 . The sunlight extraction ratio of the comparative example was 26.326%. Thus, with the solar generator  1  according to the present embodiment, the efficiency with which light is guided to the solar cell  7  was increased to about 1.4 times that of the case where the second light guide  5  was not used. As a result, it has been confirmed that the power generation efficiency can be improved. 
     First Modification of First Embodiment 
     Referring to  FIG. 7 , a first modification of the present embodiment will be described. 
     The basic structure of a solar cell module according to the present modification is the same as that of the embodiment described above. Only the incident direction of sunlight differs from that of the embodiment described above. 
       FIG. 7  is a sectional view of a solar cell module  2  according to the present modification. 
     In  FIG. 7 , the elements the same as those in  FIG. 3  for the embodiment described above will be denoted by the same numerals, and descriptions of such elements will be omitted. In  FIG. 7 , for convenience of drawing, only light beams that enter through the second main surface  5   b  of the second light guide  5  are illustrated. In the present modification, only light beams that enter through the second main surface  5   b  of the second light guide  5  will be described, and a description of light beams that enter through the first main surface  4   a  of the first light guide  4  will be omitted. 
     With the solar cell module  2  according to the present modification, light from the outside is incident not only on the first main surface  4   a  of the first light guide  4  but also on the second main surface  5   b  of the second light guide  5 . 
     As illustrated in  FIG. 7 , the second light-direction changer  5 S is disposed on the first main surface  5   a  of the second light guide  5 . The second light-direction changer  5 S reflects light that has entered through the second main surface  5   b  of the second light guide  5  and changes the propagation direction of the light to a direction toward the first end surface  5   c . The second light-direction changer  5 S includes the plurality of triangular-prism-shaped protrusions  5 A, which are formed on the first main surface  5   a  of the second light guide  5 . 
     Each of the protrusions  5 A of the second light-direction changer  5 S has second inclined surfaces (the steeply inclined surface T 2a  and the gently inclined surface T 2b ). The steeply inclined surface T 2a  has the predetermined inclination angle θ B1  with respect to the first main surface  5   a . The gently inclined surface T 2b  has the inclination angle θ 32 , which is smaller than the inclination angle θ B1  of the steeply inclined surface T 2a , with respect to the first main surface  5   a . These two inclined surface T 2a  and T 2b  function as a reflecting surface for reflecting light that has entered through the second main surface  5   b.    
     Each of the protrusions  4 A of the first light-direction changer  4 S has the steeply inclined surface T 1a  and the gently inclined surface T 1b . The steeply inclined surface T 1a  has the predetermined inclination angle θ m  with respect to the second main surface  4   b . The gently inclined surface T 1b  has the inclination angle θ A2 , which is smaller than the inclination angle θ A1  of the steeply inclined surface T 1a , with respect to the second main surface  4   b . These two first inclined surfaces T 1a  and T 1b  function as a refracting surface for refracting light that has passed through the second light guide  5  and has entered the first light guide  4 . 
     The solar cell module  2  according to the present modification  2  allows light to enter through both the first main surface  4   a  of the first light guide  4  and the second main surface  5   b  of the second light guide  5 . Therefore, as compared with the structure that allows light to enter through one of the surfaces of the solar cell module  2 , the efficiency with which light is guided to the solar cell  7  can be increased and the power generation efficiency can be improved. 
     Second Embodiment 
     Referring to  FIGS. 8 and 9 , a second embodiment of the present invention will be described. 
     The basic structure of a solar cell module according to the present embodiment is the same as that of the first embodiment, and only the refractive index of the second light guide differs from that of the first embodiment. 
       FIG. 8  is a perspective view of a solar cell module according to the present embodiment. 
       FIG. 9  illustrates the function of a reflecting surface of the solar cell module. 
     In  FIGS. 8 and 9 , the elements the same as those in  FIGS. 1 and 4  for the first embodiment will be denoted by the same numerals, and descriptions of such elements will be omitted. In  FIG. 9 , for convenience of drawing, only a light beam L 2  that passes through the first light guide  4  and propagates through a second light guide  15  is illustrated. In the present embodiment, only the light beam L 2 , which passes through the first light guide  4  and propagates through the second light guide  15 , will be described; and a description of a light beam L 1  that does not pass through the first light guide  4  and propagates only through the first light guide  4  will be omitted. 
     In a solar cell module  12  according to the present embodiment, the refractive index n 2  of the second light guide  15  is smaller than the refractive index n 1  of the first light guide  4  (n 2 &lt;n 1 ). The second light guide  15  may be made of a low-refractive index material, such as an amorphous fluoropolymer having a refractive index of about 1.3. As in the first embodiment, the first light guide  4  is made of an acrylic resin (refractive index 1.5), and the low-refractive-index layer  6  is an air layer (refractive index 1.0). 
     As illustrated in  FIG. 9 , a light beam is totally internally reflected by the steeply inclined surface T 1a  of the first light guide  4  at a reflection angle θ 2 , and is reflected a predetermined times between the first main surface  4   a  and the second main surface  4   b . Then, the light beam is incident on the first main surface  4   a  at an incident angle θ 3 , and is totally internally reflected at a reflection angle θ 3 . The light beam, which has been totally internally reflected by the first main surface  4   a  at the reflection angle θ 3 , is incident on the gently inclined surface T 1b  at an incident angle θ 4 , is refracted at a refraction angle θ 5 , and is incident on the gently inclined surface T 2b  of the second light guide  15  at an incident angle θ 6 . The light beam, which is incident on the gently inclined surface T 2b  at the incident angle θ 6 , is refracted by the gently inclined surface T 2b  of the second light guide  15  at a refraction angle θ 7  when entering the second light guide  15 . In this case, according to Snell&#39;s law, the refraction angle θ 7  at the gently inclined surface T 214  is larger than the incident angle θ 4  at the gently inclined surface T 1b  (θ 7 &gt;θ 4 ). The light beam is incident on a second main surface  15   b  of the second light guide  15  at an incident angle θ 8 , and is totally internally reflected by the second main surface  15   b  at a reflection angle θ 8 . In this case, according to Snell&#39;s law, the reflection angle θ 8  of the light beam at the second main surface  15   b  is larger than the reflection angle θ 3  at the first main surface  4   a  (θ 8 &gt;θ 3 ). Subsequently, the light beam, which has been totally internally reflected by the second main surface  15   b  at the reflection angle θ 8 , propagates through the second light guide  15 , and is emitted toward the solar cell  7 . 
     In the solar cell module  12  according to the present embodiment, the refractive index n 2  of the second light guide  15  is smaller than the refractive index n 1  of the first light guide  4 . Therefore, according to Snell&#39;s law, the reflection angle θ 8 , at which the light beam that has passed through the first light guide  4  is totally internally reflected by the second main surface  5   b  of the second light guide  15 , is larger than the reflection angle θ 3 , at which the light beam is reflected by the first main surface  4   a  of the first light guide  4 . Thus, the light-guide distance, which is the length of a path along which the light beam that has entered the second light guide  15  is guided to the solar cell  7 , can be made longer than that of the structure of the first embodiment. Moreover, the number of times the light beam is reflected between a first main surface  15   a  and the second main surface  15   b  can be reduced. Therefore, light that has entered the second light guide  15  can be easily guided to the solar cell  7 . Therefore, reduction in the power generation efficiency can be prevented. 
     The inventor carried out a simulation of the sunlight extraction ratio in order to demonstrate the effects of the solar cell module  12  according to the present embodiment (see  FIG. 16 ). The conditions of the simulation in example 2 were as follows: the horizontal dimensions of the first light guide  4  were 250 mm×250 mm, the thickness of the first light guide  4  was 10 mm, the horizontal dimensions of the second light guide  15  were 250 mm×250 mm, and the thickness of the second light guide  15  was 10 mm. The refractive index of the first light guide  4  was 1.5, the refractive index of the second light guide  15  was 1.3, and the refractive index of air was 1.0. For the solar cell module  12  of example 2, when sunlight was incident on the first main surface  4   a  side of the first light guide  4 , the sunlight extraction ratio was 37.232%. 
     The output conditions of the solar cell  7  are standardized with respect to air mass AM1.5, which is specified in JIS. In this case, the incident angle at which sunlight is incident on the first main surface  4   a  of the first light guide  4  is approximately 42 degrees. As a comparative example, the simulation is performed by using only the first light guide  4  and without using the second light guide  15 . The sunlight extraction ratio of the comparative example was 26.326%. Thus, with the solar cell module  12  according to the present embodiment, the efficiency with which light is guided to the solar cell  7  was increased to about 1.4 times that of the case where the second light guide  15  was not used. As a result, it has been confirmed that the power generation efficiency can be improved. 
     Third Embodiment 
     Referring to  FIGS. 10 and 11 , a third embodiment of the present invention will be described. 
     The basic structure of a solar cell module according to the present embodiment is the same as that of the first embodiment, except that the inclination angle of the second inclined surface of the second light guide differs from that of the first embodiment. 
       FIG. 10  is a perspective view of a solar cell module according to the present embodiment. 
       FIG. 11  illustrates the function of a reflecting surface of the solar cell module. 
     In  FIGS. 10 and 11 , the elements the same as those in  FIGS. 1 and 4  for the first embodiment will be denoted by the same numerals, and descriptions of such elements will be omitted. In  FIG. 11 , for convenience of drawing, only a light beam L 2  that passes through the first light guide  4  and propagates through a second light guide  25  is illustrated. In the present embodiment, only the light beam L 2 , which passes through the first light guide  4  and propagates through the second light guide  25 , will be described; and a description of a light beam L 1  that does not pass through the first light guide  4  and propagates only through the first light guide  4  will be omitted. 
     As illustrated in  FIG. 11 , in a solar cell module  22  according to the present embodiment, the inclination angles of the second inclined surfaces of the second light guide  25  are larger than those of the first inclined surfaces of the first light guide  4 . To be specific, the inclination angle θ B1  of the steeply inclined surface T 2a  of the second light guide  25  is larger than the inclination angle θ A1  of the steeply inclined surface T 1a  of the first light guide  4 ; and the inclination angle θ B2  of the gently inclined surface T 2b  of the second light guide  25  is larger than the inclination angle θ A2  of the gently inclined surface T 1b  of the first light guide  4 . As in the first embodiment, the first light guide  4  and the second light guide  25  are made of an acrylic resin (refractive index 1.5), and the low-refractive-index layer  6  is an air layer (refractive index 1.0). 
     As illustrated in  FIG. 11 , a light beam is totally internally reflected by the steeply inclined surface T 1a  of the first light guide  4  at a reflection angle θ 2  and is reflected a predetermined times between the first main surface  4   a  and the second main surface  4   b . Then, the light beam is incident on the first main surface  4   a  at an incident angle θ 3 , and is totally internally reflected at a reflection angle θ 3 . The light beam, which has been totally internally reflected by the first main surface  4   a  at the reflection angle θ 3 , is incident on the gently inclined surface T 1b  at an incident angle θ 4 , is refracted at a refraction angle θ 5 , and is incident on the gently inclined surface T 2b  of the second light guide  25  at an incident angle θ 6 . The light beam, which is incident on the gently inclined surface T 2b  at the incident angle θ 6 , is refracted by the gently inclined surface T 2b  of the second light guide  25  at a refraction angle θ 7  when entering the second light guide  25 . The light beam is incident on a second main surface  25   b  of the second light guide  25  at an incident angle θ 8 , and is totally internally reflected by the second main surface  25   b  at a reflection angle θ 8 . In this case, because the inclination angle θ B1  of the steeply inclined surface T 2   a  of the second light guide  25  is larger than the inclination angle θ A1  of the steeply inclined surface T 1a  of the first light guide  4 , the reflection angle θ 8  of the light beam at the second main surface  25   b  is larger than the reflection angle θ 3  at the first main surface  4   a  (θ 8 &gt;θ 3 ). Subsequently, the light beam, which has been totally internally reflected by the second main surface  25   b  at a reflection angle θ 8 , propagates through the second light guide  25 , and is emitted toward the solar cell  7 . 
     In the solar cell module  22  according to the present embodiment, the inclination angles of the inclined surfaces of the second light guide  25  are larger than those of the first light guide  4 . Therefore, the reflection angle θ 8 , at which the light beam that has passed through the first light guide  4  is reflected by the second main surface  25   b  of the second light guide  25 , is larger than the reflection angle θ 3 , at which the light beam is reflected by the first main surface  4   a  of the first light guide  4 . Thus, the light-guide distance, which is the length of a path along which the light beam that has entered the second light guide  25  is guided to the solar cell  7 , can be made longer than that of the structure of the first embodiment. Moreover, the number of times the light beam is reflected between a first main surface  25   a  and the second main surface  25   b  can be reduced. Therefore, light that has entered the second light guide  25  can be easily guided to the solar cell  7 . Therefore, reduction in the power generation efficiency can be prevented. 
     The inventor carried out a simulation of the sunlight extraction ratio in order to demonstrate the effects of the solar cell module  22  according to the present embodiment (see  FIG. 16 ). The conditions of the simulation in example 3 were as follows: the horizontal dimensions of the first light guide  4  were 250 mm×250 mm, the thickness of the first light guide  4  was 10 mm, the horizontal dimensions of the second light guide  25  were 250 mm×250 mm, and the thickness of the second light guide  25  was 10 mm. The refractive index of the first light guide  4  was 1.5, the refractive index of the second light guide  25  was 1.5, and the refractive index of air was 1.0. The inclination angles of the second inclined surfaces of the second light guide  25  were larger than those of the first inclined surfaces of the first light guide  4 . For the solar cell module  22  of example 3, when sunlight was incident on the first main surface  4   a  side of the first light guide  4 , the sunlight extraction ratio was 35.976%. 
     The output conditions of the solar cell  7  are standardized with respect to air mass AM1.5, which is specified in JIS. In this case, the incident angle at which sunlight is incident on the first main surface  4   a  of the first light guide  4  is approximately 42 degrees. As a comparative example, the simulation is performed by using only the first light guide  4  and without using the second light guide  25 . The sunlight extraction ratio of the comparative example was 26.326%. Thus, with the solar cell module  22  according to the present embodiment, the efficiently with which light is guided to the solar cell  7  was increased to about 1.4 times that of the case where the second light guide  25  was not used. As a result, it has been confirmed that the power generation efficiency can be improved. 
     Fourth Embodiment 
     Referring to  FIGS. 12 and 13 , a fourth embodiment of the present invention will be described. 
       FIG. 12  is a perspective view of a solar generator according to the present embodiment. 
       FIG. 13  is a sectional view of the solar generator. 
     In  FIGS. 12 and 13 , the elements the same as those in  FIGS. 1 and 2  for the first embodiment will be denoted by the same numerals, and descriptions of such elements will be omitted. 
     As illustrated in  FIG. 13 , the structure of a part of a solar generator  30  according to the present embodiment on the right side (±Y side) of the center line CL is the same as that of the first embodiment. However, the structure of a part of the solar generator  30  on the left side (−Y side) of the center line CL is different from that of the first embodiment. That is, the solar generator  30  is symmetric with respect to the center line CL. In other respects, the fourth embodiment is the same as the first embodiment. 
     As illustrated in  FIGS. 12 and 13 , the solar generator  30  includes a solar cell module  32 , a solar cell  37 , and a support frame  38 . The support frame  38  has a substantially rectangular shape in plan view, and is attached to the solar cell module  32  and the solar cell  37  so as to surround the solar cell module  32  and the solar cell  37 . 
     The solar cell module  32  includes a light guide module  33  and a solar cell  7 . In the solar cell module  32 , light that has entered the light guide module  33  is guided to the solar cells  7  and  37 . The solar cells  7  and  37  perform photoelectric conversion and output electric energy. 
     The light guide module  33  includes a first light guide  34 , a second light guide  35 , and a low-refractive-index layer  6 . The first light guide  34  has a second end surface  34   c   2  that is connected to a first main surface  34   a  and a second main surface  34   b  and that faces a first end surface  34   c   1 . The second light guide  35  has a second end surface  35   c   2  that is connected to a first main surface  35   a  and a second main surface  35   b  and that faces a first end surface  35   c   1 . 
     A first light-direction changer  34 S is disposed on the second main surface  34   b  of the first light guide  34 . The first light-direction changer  34 S reflects light that has entered through the first main surface  34   a  and changes the propagation direction of the light. The first light-direction changer  34 S includes a first-end-side reflector  34 S 1 , which reflects a light beam L 1  that has entered through the first main surface  34   a  toward the first end surface  34   c   1 , and a second-end-side reflector  34 S 2 , which reflects a light beam L 2  that has entered through the first main surface  34   a  toward the second end surface  34   c   2 . In the first light-direction changer  34 S, the area of the reflecting surface of the first-end-side reflector  34 S 1  is equal to the area of the reflecting surface of the second-end-side reflector  34 S 2 . 
     The first-end-side reflector  34 S 1  includes a plurality of triangular-prism-shaped protrusions  34 A 1  that are formed on a part of the second main surface  34   b  of the first light guide  34  on the right side (+Y side) of the center line CL. The second-end-side reflector  34 S 2  includes a plurality of triangular-prism-shaped protrusions  34 A 2  that are formed on a part of the second main surface  34   b  of the first light guide  34  on the left side (−Y side) of the center line CL. The protrusions  34 A 1  and the protrusions  34 A 2  have shapes that are symmetric with respect to the center line CL. Some of light beams that have entered through various portions of the first main surface  34   a  of the first light guide  34  are reflected by the first-end-side reflector  345   1  and propagate through the first light guide  34  so as to be concentrated on a portion of the first end surface  34   c   1  on which the solar cell  7  is disposed. The remainder of the light beams are reflected by the second-end-side reflector  34 S 2  and propagate through the first light guide  34  so as to be concentrated on a portion of the second end surface  34   c   2  on which the solar cell  37  is disposed. 
     A second light-direction changer  35 S is disposed on the first main surface  35   a  of the second light guide  35 . The second light-direction changer  35 S refracts a light beam that has passed through the first light guide  34  and entered through the first main surface  35   a  thereof, and changes the propagation direction of the light beam. The second light-direction changer  35 S includes a first-end-side refractor  35 S 1 , which refracts a light beam that has entered through the first main surface  35   a  toward the first end surface  35   c   1 , and a second-end-side refractor  35 S 2 , which refracts a light beam that has entered through the first main surface  35   a  toward the second end surface  35   c   2 . In the second light-direction changer  35 S, the area of the refracting surface of the first-end-side refractor  35 S 1  is equal to the area of the refracting surface of the second-end-side refractor  35 S 2 . 
     The first-end-side refractor  35 S 1  includes a plurality of triangular-prism-shaped protrusions  35 A 1  that are formed on a part of the first main surface  35   a  of the second light guide  35  on the right side (+Y side) of the center line CL. The second-end-side refractor  35 S 2  includes a plurality of triangular-prism-shaped protrusions  35 A 2  that are formed on a part of the first main surface  35   a  of the second light guide  35  on the left side (−Y side) of the center line CL. The protrusions  35 A 1  and the protrusions  35 A 2  have shapes that are symmetric with respect to the center line CL. Some of light beams that have passed through the first light guide  34  and that are incident on various positions on the first main surface  35   a  of the second light guide  35  are refracted by the first-end-side refractor  355   1  and propagate through the second light guide  35  so as to be concentrated on a portion of the first end surface  35   c   1  on which the solar cell  7  is disposed. The remainder of the light beams are refracted by the second-end-side refractor  35 S 2  and propagate through the second light guide  35  so as to be concentrated on a portion of the second end surface  35   c   2  on which the solar cell  37  is disposed. 
     With the solar generator  30  according to the present embodiment, even when light beams having different angular components are incident on the first main surface  34   a  of the first light guide  34 , the light beams that have entered through the first main surface  34   a  can be reflected toward the first end surface  34   c   1  and toward the second end surface  34   c   2 . Therefore, when installing the solar generator  30 , it is not necessary to consider the direction of the sun. For example, when the solar generator  30  is installed so as to face the east, until the sun reaches the highest point (from morning to noon), the first-end-side reflector  34 S 1  reflects sunlight so that the sunlight can be concentrated on the solar cell  7 . Until sunset (from noon to evening), the second-end-side reflector  34 S 2  reflects sunlight so that the sunlight can be concentrated on the solar cell  37 . In contrast, with a structure including only one of the first-end-side reflector  34 S 1  and the second-end-side reflector  34 S 2 , from sunrise to sunset, light that enters through the first main surface  34   a  is concentrated on only one of the solar cell  7  and the solar cell  37 . Therefore, with the solar generator  30  according to the present embodiment, from sunrise to sunset, light that enters through the first main surface  34   a  can be guided to both of the solar cell  7  and the solar cell  37 . 
     In the first light-direction changer  34 S, the area of the reflecting surface of the first-end-side reflector  34 S 1  is equal to the area of the reflecting surface of the second-end-side reflector  34 S 2 . Therefore, from sunrise to sunset, light that enters through the first main surface  34   a  can be guided to the solar cell  7  and the solar cell  37  in a well-balanced manner. 
     Fifth Embodiment 
     Referring to  FIGS. 14 and 15 , a fifth embodiment of the present invention will be described. 
     The basic structure of a solar generator according to the present embodiment is the same as that of the first embodiment. Only the number of light guide modules differs from that of the first embodiment. 
       FIG. 14  is a schematic perspective view of a solar generator according to the present embodiment. 
       FIG. 15  is a sectional view of the solar generator. In  FIGS. 14 and 15 , the elements the same as those in  FIGS. 1 and 2  for the first embodiment will be denoted by the same numerals, and descriptions of such elements will be omitted. 
     As illustrated in  FIG. 14 , a solar generator  40  according to the present embodiment includes a plurality of (in this example, two) light guide modules  3 , which face each other with a low-refractive-index layer  46  therebetween. The number of light guide modules  3  may be two, three, or more. 
     The solar generator  40  includes a solar cell module  42  and a support frame  48 . The support frame  48  has a substantially rectangular shape in plan view and surrounds the solar cell module  42 . The solar cell module  42  includes a light guide unit  43  and a solar cell  47 . The light guide unit  43  includes a first light guide module  3 A, a second light guide module  3 B, and the low-refractive-index layer  46 . As necessary, spacers may be disposed between the first light guide module  3 A and the second light guide module  3 B. 
     As illustrated in  FIG. 15 , some of light beams that have entered a first light guide module  43 A propagate through the first light guide module  43 A, are guided to the solar cell  47 , and contribute to power generation. The remainder of the light beams are emitted from the first light guide module  43 A, propagate through a second light guide module  43 B, are guided to the solar cell  47 , and contribute to power generation. 
     With the solar generator  40  according to the present embodiment, light from the outside can be made to propagate through the first light guide module  43 A and can be guided to the solar cell  47 . Moreover, light that has passed through the first light guide module  43 A can be made to propagate through the second light guide module  43 B and can be guided to the solar cell  47 . Therefore, reduction in the power generation efficiency can be reliably prevented. 
     The scope of the present invention is not limited to the embodiments described above, and the embodiments can be modified in various ways within the spirit and scope of the present invention. 
     For example, in the embodiments described above, a plate-shaped member is used as the light guide. However, the shape of the light guide is not limited to a plate-like shape and may be, for example, a bar-like shape. The shape may be changed as appropriate. Moreover, the shapes, the dimensions, the numbers, the dispositions, the materials, and the manufacturing method of the elements in the embodiments described above are not limited to those used as examples in the embodiments, and may be modified as appropriate. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be used for solar cell modules or solar generators. 
     REFERENCE SIGNS LIST 
     
         
         
           
               1 ,  30 ,  40  solar generator 
               2 ,  12 ,  22 ,  32 ,  42  solar cell module 
               3 ,  3 A,  3 B,  13 ,  23 ,  33  light guide module 
               4 ,  34  first light guide 
               4   a ,  34   a  first main surface of first light guide 
               4   b ,  34   b  second main surface of first light guide 
               4   c ,  34   c   1  first end surface of first light guide 
               4 S,  34 S first light-direction changer 
               5 ,  15 ,  25 ,  35  second light guide 
               5   a ,  15   a ,  25   a ,  35   a  first main surface of second light guide 
               5   b ,  15   b ,  25   b ,  35   b  second main surface of second light guide 
               5   c ,  15   c ,  25   c ,  35   c   1  first end surface of second light guide 
               5 S,  15 S,  25 S,  35 S second light-direction changer 
               6  low-refractive-index layer 
               7 ,  37 ,  47  solar cell 
               9  spacer 
               34 S 1  first-end-side reflector 
               34   c   2  second end surface of first light guide 
               34 S 2  second-end-side reflector 
               35   c   2  second end surface of second light guide 
             n 1  refractive index of first light guide 
             n 2  refractive index of second light guide 
             T 1a , T 2b  first inclined surface 
             T 2a , T 2b  second inclined surface 
             θ A1 , θ A2  inclination angle of first inclined surface (first inclination angle) 
             θ B1 , θ B2  inclination angle of second inclined surface (second inclination angle)