Patent Publication Number: US-2015059819-A1

Title: Solar power generation device

Description:
BACKGROUND OF THE  INVENTION 
     1. Technical Field 
     The present invention relates to a solar power generation device converting solar energy into electric energy. 
     2. Description of Related art 
     There is a solar power generation device, as a related-art example, converting solar energy into electric energy by collecting solar light by a reflecting mirror to solar cells arranged on a cooling pipe (for example, refer to Patent Literature 1). 
     In the solar power generation device described in Patent Document 1, solar energy is converted into electric energy by collecting solar light reflected by a reflecting mirror  301  to a solar cell  302  as shown in  FIG. 12 . In this case, as the solar light includes heat rays, the solar cell  302  is heated and the temperature thereof is increased. Accordingly, in the solar power generation device described in Patent Document 1, a thermoelectric conversion element  304  is disposed between the solar cell  302  and a cooling pipe  303 , and thermoelectric conversion is performed by utilizing the temperature difference between the solar cell  302  and the cooling pipe  303 , thereby improving power generation efficiency. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP2004-271063A 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the present invention, there is provided a solar power generation device including a polygonal cylindrical cooling pipe, plural thermoelectric conversion elements installed on respective side surfaces of the cooling pipe, plural solar cells installed on the thermoelectric conversion elements respectively and an insulation covering side surfaces of the solar cells and the thermoelectric conversion elements. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a perspective view showing a solar power generation device according to Embodiment 1 of the present invention; 
         FIG. 2  is an A-A cross-sectional view of  FIG. 1 ; 
         FIG. 3  is an enlarged view of a part “B” of  FIG. 2 ; 
         FIG. 4  is a perspective view showing a thermoelectric conversion element according to Embodiment 1 of the present invention; 
         FIG. 5  is an enlarged view of a part “C” of  FIG. 3 ; 
         FIG. 6  is a schematic view showing part of a power generation unit according to Embodiment 1 of the present invention; 
         FIG. 7  is a view showing another form of  FIG. 4 ; 
         FIG. 8  is a partial cross-sectional view of a solar power generation device according to Embodiment 2 of the present invention; 
         FIG. 9  is a block diagram snowing part of a system configuration of the solar power generation device according to Embodiment 2 of the present invention; 
         FIG. 10  is a flowchart for explaining the operation according to Embodiment 2 of the present invention; 
         FIG. 11  is a schematic view snowing the relation between the temperature of the solar cell and power generation efficiency according to the embodiments of the present invention; and 
         FIG. 12  is a schematic view showing a related-art solar power generation device. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the present invention will be explained with reference to the drawings. 
     Embodiment 1 
     As shown in  FIG. 1 , a solar power generation device  1  according to Embodiment 1 has a structure of collecting solar light to a power generation unit  100  by a reflecting mirror  101 . The reflecting mirror  101  has a semicylinidrical shape in cross section. The power generation unit  100  is arranged in the vicinity of a focal line of the reflecting mirror  101 . The reflecting mirror  101  and the power generation unit  100  are supported by a frame  11  provided to stand on a base  10 . 
       FIG. 2  is a cross-sectional view taken along A-A of  FIG. 1 . As shown in  FIG. 2 , the reflecting mirror  101  has a trough shape in which a cross section perpendicular to the longitudinal direction is a semicylindrical shape. 
       FIG. 3  is an enlarged view of a part “B” of  FIG. 2 . As shown in  FIG. 3 , the power generation unit  100  is provided with thermoelectric conversion elements  103  on side surfaces of a cooling pipe  102  and provided with solar cells  104  on upper surfaces of the thermoelectric conversion elements  103 . The cooling pipe  102  has a polygonal cylindrical shape a cross section of which is an octagon and has rectangular flat surfaces on side surfaces. Cooling water is circulated inside the cooling pipe  102 . The cooling water cools the solar cells  104  through wail surfaces of the cooling pipe  102  and the thermoelectric conversion elements  103 . The cooling pipe  102  is an example of a cooling portion. The solar cells  104  are installed on the upper surfaces of the thermoelectric conversion elements  103  through a paste  202 . The paste  202  is, for example, a highly-thermal conductive paste having highly thermal conductive characteristics. 
     The solar cells  104  according to the embodiment are cooled by the cooling water flowing in the cooling pipe  102  through the thermoelectric conversion elements  103 . Moreover, thermal energy is converted into electric energy in the thermoelectric conversion elements  103  due to the temperature difference generated between the solar cells  104  and the cooling pipe  102  as the thermoelectric conversion elements  103  are interposed therebetween, as a result, a power generation amount can be increased. 
     Effects of cooling the solar cells  104  will be explained. As shown in  FIG. 1  and  FIG. 2 , solar light is reflected by the reflecting mirror  101  and collected to the power generation unit  100 . As the solar cells  104  are disposed on the surface of the power generation unit  100 , the collected solar light is converted from solar light energy into electric energy by the solar cells  104 . The solar light collected by the reflecting mirror  101  has a high energy density and includes heat rays, therefore, the surface temperature of the solar cells  104  installed so as to face the reflecting mirror  101  may be increased to the vicinity of 200° C. in the summer season. It is known that power generation efficiency of the solar cells  104  is reduced as the temperature is increased. For example, a material of the solar cells  104  is crystalline silicon, the power generation efficiency is reduced approximately 4% when the temperature in a photoelectric conversion portion of the solar cell  104  is increased 10° C. That is, it is desirable to cool the solar cells  104  when the power generation efficiency is reduced in the case where the temperature of the solar cells  104  is increased in the solar power generation device  1 . Accordingly, an apparatus with stable power generation efficiency can be realized by providing the cooling pipe  102  as in the solar power generation device  1  according to the embodiment. 
     The thermoelectric conversion element  103  is a device converting thermal energy into electric energy. The thermoelectric conversion element  103  is formed by mounting a P-type thermoelectric conversion element  103   p  in which Sb or the like is added as a dopant to an alloy of a bismuth telluride system and an N-type thermoelectric conversion element  103   n  in which Se or the like is added as a dopant on a wiring substrate  105  so as to be electrically connected in series. The thermoelectric conversion element  103  is formed by the wiring substrates  105  in which upper and lower surfaces thereof are flat as shown in  FIG. 4 . Since the cooling pipe  102  has the polygonal cylindrical shape having flat surfaces on side surfaces, the contact area between the cooling pipe  102  and the thermoelectric conversion element  103  becomes large when the thermoelectric conversion element  103  is installed on the side surface of the cooling pipe  102 . Accordingly, heat generated on the surface of the solar cell  104  is efficiently transmitted to the cooling pipe  102  through the thermoelectric conversion element  103 . 
     The solar cells  104  are made of crystalline silicon or crystalline compound semiconductors, amorphous silicon and so on. The solar cells  104  directly convert light energy into electric energy by using a light electromotive force of the semiconductor. 
     An insulation  201  is arranged between adjacent two solar cells  104  as well as between adjacent two thermoelectric conversion elements  103 , respectively. As shown in  FIG. 3 , the insulation  201  is packed, for example, so as to completely fill in a space between adjacent two solar cells  104  as well as a space between two thermoelectric conversion elements  103 . It is desirable that the insulation  201  is packed so as not to protrude from the surface of the solar cell  104 . This is because, if the insulation  201  protrudes from the surface of the solar cell  104 , solar light is reflected by the insulation  201  and concentrated to part of the solar cell  104 , which may hasten the deterioration of the solar cell  104 . 
     The insulation  201  according to the embodiment functions as an insulative heat insulating material as well as functions as an antireflection member. That is, the insulation  201  blocks heat transmission between adjacent two solar cells  104  as well as prevents reflection of solar light incident between adjacent two solar cells  104 , thereby suppressing occurrence of scattered light. 
     As the insulation  201  functions as the heat insulating material and the heat transmission between adjacent two solar cells  104  is prevented, heat of the solar cells  104  is transmitted to the cooling pipe  102  efficiently through the thermoelectric conversion elements  103  arranged on undersurfaces of the solar cells  104 . Accordingly, cooling efficiency by the cooling pipe  102  can be increased as the insulation  201  functions as the heat insulating material. 
     Here, materials for the insulation  201  are, for example, insulative heat insulating materials mainly made of calcium sulfate, calcium silicate, glass wool and so on. 
       FIG. 5  is an enlarged view of a part “C” of  FIG. 3 .  FIG. 5  is also a cross-sectional view taken along D-D of  FIG. 6 , which shows part of the power generation unit  100 . As shown in  FIG. 5 , the solar cell  104  has a larger area than the thermoelectric conversion element  103 , the solar cell  104  has a protruding portion  114  protruding from an end surface of the thermoelectric conversion element  103 . Due to the protruding portion  114 , a distance between a solar cell  104   a  and a thermoelectric conversion element  103   b  arranged on an undersurface of an adjacent solar cell  104   b  becomes long, therefore, neat of the solar cell  104   a  is not easily transmitted to the thermoelectric conversion element  103   b.  Then, an under surf ace of the thermoelectric conversion element  103   b  is not easily affected by the heat of the solar cell  104   a,  therefore, the temperature difference between an upper surface and the under surface of the thermoelectric conversion element  103   b  becomes large and a generation amount of electric energy in the thermoelectric conversion element  103   b  is increased. 
     Moreover, as the insulation  201  is packed between adjacent two solar cells  104  as well as the solar cells  104  have the protruding portions  114  protruding from end surfaces of the thermoelectric conversion elements  103 , it is possible to prevent infiltration of moisture such as rainwater into the thermoelectric conversion elements  103 . 
     The cooling pipe  102  is formed so as to be rotated around the central axis with respect to the longitudinal direction thereof. As the cooling pipe  102  is rotated, a position of the solar cell  104  facing the reflecting mirror  101  and a position of the solar cell  104  not facing the reflecting mirror  101  can be exchanged. As the solar cell  104  facing the reflecting mirror  101  receives the collected solar light having high energy density, the deterioration may proceeds more rapidly than in the solar cell  104  not facing the reflecting mirror  101 . Accordingly, the lifetime of the solar power generation device  1  can be extended by exchanging the positions of the solar cell  104  facing the reflecting mirror  101  and the solar cell  104  in the counter side periodically by rotating the cooling pipe  102 . 
     Also in the solar power generation device  1  according to the embodiment, the thermoelectric conversion elements  103  and the solar cells  104  are disposed so as to be divided with respect to side surfaces of the cooling pipe  102 , the thermoelectric conversion elements  103  and the solar cells  104  are not required to have a large area. Accordingly, yields of thermoelectric conversion elements  103  and the solar cells  104  can be increased, and they can be easily replaced in the case of deterioration, therefore, the solar power generation device  1  having excellent maintainability can be provided. 
     It is sufficient that the insulation  201  covers side surfaces of the solar cell  104  and the thermoelectric conversion element  103  when considering only heat-insulation performance. In this case, the insulations  201  are coated on the side surfaces of the solar cells  104  and the thermoelectric conversion elements  103  in advance, thereby simplifying manufacturing processes. Additionally, when a gap exists between adjacent two insulations  201 , a layer (air layer) formed by air with high heat insulation performance is formed in the gap, which further increases the heat insulation performance between the solar cell  104  and the adjacent thermoelectric conversion element  103 . 
     In order to stabilize heat distribution between adjacent two thermoelectric conversion elements  103 , as shown in  FIG. 7 , the thermoelectric conversion elements  103  preferably have a trapezoid shape which becomes gradually small from the solar cell  104  side toward the cooling pipe  102  side. Due to such trapezoid shape, side surfaces of adjacent thermoelectric conversion elements  103  are parallel to each other and heat distribution between them can be stabilized. 
     The reflecting mirror  101  may be formed by combining many flat mirrors as well as may be formed by combining a plurality of parabolic reflecting mirrors. 
     It is desirable that the reflecting mirror  101  is directed to a direction directly facing the solar light for utilizing solar energy at the maximum, and a tracking device may be used for following the movement of the sun. 
     The cooling pipe  102  preferably has a polygonal cylindrical shape having fiat side surfaces, for example, polygonal cylindrical shapes such as a triangular shape and a square shape in cross section. 
     Embodiment 2 
     A solar power generation device  201  according to Embodiment 2 is the same as Embodiment 1 shown in  FIG. 1  in the entire structure of the device, however, a power generation unit  200  differs from the power generation unit  100 . 
       FIG. 8  is a cross-sectional view of the power generation unit  200 . As shown in  FIG. 8 , the power generation unit  100  has a cooling pipe  102 , thermoelectric conversion elements  103  ( 103   a  to  103   h ) installed on side surfaces of the cooling pipe  102  and solar cells  104  ( 104   a  to  104   h ) respectively installed on upper surfaces of the thermoelectric conversion elements  103 . 
     The solar power generation device  201  according to the embodiment can increase the power generation efficiency of the solar cells  104  by switching functions of the thermoelectric conversion elements  103  between a power generation function and a cooling function based on the temperature of the solar cells  104 , which will be described in detail. Specifically, the solar power generation device  201  according to the embodiment measures the temperature of the solar cells  104  to determine whether the temperature is equal to or lower than a set temperature or not. When the temperature of the solar cell  104  is equal to or lower than the set temperature, the solar power generation device  201  uses the thermoelectric conversion element  103  as the power generation function to thereby increase the power generation amount, and when the temperature of the solar cell  104  is higher than the set temperature, the solar power generation device  201  uses the thermoelectric conversion element  103  as the cooling function to thereby prevent the reduction of power generation efficiency. As a result, the power generation efficiency of the entire solar power generation device  201  can be increased. 
       FIG. 9  is a block diagram showing part of a system configuration of the solar power generation device  201  according to Embodiment 2. As shown in  FIG. 9 , the solar power generation device  201  includes temperature sensors  206  ( 206   a  to  206   h ) and controllers  207  ( 207   a  to  207   h ). The temperature sensors  206   a  to  20   6   h  are provided for measuring temperatures of the solar cells  104   a  to  104   h  respectively so as to correspond to respective solar cells  104 . As the temperature sensor  206 , for example, a thermocouple can be used, which can be provided by being adhered to a back surface or a side surface of the solar cell  104 . The controllers  207   a  to  207   h  correspond to the respective thermoelectric conversion elements  103   a  to  103   h  to control the respective thermoelectric conversion elements  103   a  to  103   h.  The controllers  207   a  to  207   h  switch functions of respectively corresponding thermoelectric conversion elements  103   a  to  103   h  to the power generation function or the cooling function based on the temperature of the solar cells  104   a  to  104   h  measured by the temperature sensors  206   a  to  206   h.    
     That is, the thermoelectric conversion elements  103  according to the embodiment have both Seebeck effect and Peltier effect. The power generation function in the embodiment means a function of generating power by this Seebeck effect. In the embodiment, power generation by Seebeck effect is performed by utilizing the temperature difference between the solar cells  104  heated by solar light and the cooling water circulating in the cooling pipe  102 . Here, the Seebeck effect is a phenomenon in which an electromotive force is generated in accordance with the temperature difference by bonding different kinds of metals or semiconductors to give the temperature difference to a bonded portion. The cooling function in the present invention means a function of cooling by utilizing heat absorption action in the Peltier effect. In the embodiment, the heat is transmitted from the solar cells  104  heated by solar light to the cooling pipe  102  by supplying the power to the thermoelectric conversion elements  103  to thereby cool the solar cells  104 . Here, the Peltier effect is a phenomenon reverse to the Seebeck effect, in which absorption and release of heat dependent on the direction and size of electric current occur when different kinds of metals or semiconductors are bonded and electric current is allowed to flow. 
     It is also preferable to calculate temperatures of the solar cells  104   a  to  104   h  by measuring voltage values generated by the thermoelectric conversion elements  103   a  to  103   b  without using the temperature sensors  206   a  to  206   h.  As the temperatures of the solar cells  104  can be measured without the necessity of using the temperature sensors  206   a  to  206   h  in this case, the number of components in the solar power generation device  201  can be reduced. 
     The control performed during the operation of the solar power generation device  201  according to the embodiment will be explained with reference to  FIG. 10 . 
     As shown in  FIG. 10 , the solar power generation device  201  first measures temperatures of corresponding solar cells  104   a  to  104   h  by using respective temperature sensors  206   a  to  206   h  under control of the controllers  207   a  to  207   h  in Step S 10 . 
     Next, the solar power generation device  201  determines whether the temperatures of corresponding solar cells  104   a  to  104   h  are equal to or lower than the set temperature by the controllers  207   a  to  207   h  in Step S 11 . Here, when the temperatures of the solar cells  104   a  to  104   h  are equal to or lower than the set temperature (Yes in S 11 ), the process proceeds to Step S 12 , where power generation is performed by using corresponding thermoelectric conversion elements  103   a  to  103   h  as the power generation function. On the other hand, when the temperatures of corresponding solar cells  104   a  to  104   h  are higher than the set temperature (No in S 11 ), the process proceeds to Step S 13 , where the solar cell  104  is cooled by using the thermoelectric conversion element  103  installed on the undersurface of the solar cell  104  having a higher temperature than the set temperature as the cooling function. For example, when only the temperature of the solar cell  104   e  is higher than the set temperature, only the thermoelectric conversion element  103   e  installed on the undersurface of the solar cell  104   e  is used as the cooling function, and other thermoelectric conversion elements  103   a  to  103   d,  and  103   f  to  103   h  are used as the power generation function. Accordingly, when the temperature of part of the solar cells  104  is higher than the set temperature, cooling is performed individually only by the corresponding thermoelectric devices  103 , thereby performing control in accordance with characteristic variations and states among plural solar cells  104 , and uniformizing power generation efficiency of the solar cells  104 . As described above, when the temperature of the solar cell  104  is equal to or lower than the set temperature, power generation is performed by utilizing the temperature difference between the solar cell  104  and the cooling pipe  102 , therefore, the power generation amount of the entire solar power generation device  201  can be increased. 
     Here, when the thermoelectric conversion element  103  is used as the cooling function, it is necessary to allow electric current to flow in the thermoelectric conversion element  103  to be operated, as Peltier, therefore, the power generation efficiency of the entire solar power generation device  201  is reduced if an increased amount of the power generation efficiency by the cooling is increased more than electric current to flow. Accordingly, it is required in the present embodiment that a boundary temperature at which an improvement of the power generation efficiency by the cooling in the solar cell is increased more than the electric current to flow in the thermoelectric conversion element is calculated and that the temperature is set in advance as a desired set temperature, for example, by performing an experiment so as to correspond to characteristics of the solar cells to be used. 
       FIG. 11  is a graph showing the relation between the temperature of the solar cell (device temperature) and the power generation efficiency (conversion efficiency) obtained when the thermoelectric conversion element  103  is used as the power generation function. As shown in  FIG. 11 , when the temperature of the solar cell is increased, the power generation efficiency is reduced. For example, when the material of the solar cell is crystalline silicon, the power generation efficiency is reduced approximately 4% when the temperature of the thermoelectric conversion portion of the solar cell is increased 10° C. 
     To use the thermoelectric conversion element by switching between the cooling function and the power generation function based on whether the temperature is equal to or lower than the desired set temperature or not as in the present invention is effective also in a solar power generation device not having the reflecting mirror. However, as the power generation efficiency of respective solar cells  104   a  to  104   h  can be uniformized in the solar power generation device  201  having the reflective mirror  101 , the present invention is preferably applied to the solar power generation device  201  having the reflecting mirror as described above. 
     if goes without saying that the present invention can be used by combining the above various embodiments.