Abstract:
An optical structure is characterized by improving a primary lens of a photovoltaic concentrator system. The optical structure is accomplished by properly dividing the primary lens, determining required optical operational regions, and arranging the optical operational regions basing on an identical location into an annular array, thereby forming the complete optical structure. The optical structure facilitates enhancing uniformity of light distribution throughout the optical operational regions, improving photoelectric conversion efficiency of a solar cell having the optical structure, and reducing operational distance between the primary lens and the solar cell.

Description:
BACKGROUND OF INVENTION 
       [0001]    1. Field of the Invention The present invention relates to an optical structure applicable to a concentrator system in a solar cell. 
         [0002]    2. Description of the Prior Art 
         [0003]    In recent years, due to increasing energy costs and global warming issues, requests for renewable energy bringing less contamination have attracted extensive attention. Especially, solar photovoltaic systems relying on the unfailing solar energy have been developed with various materials and techniques in a worldwide scale for pursuing maximized photoelectric conversion efficiency and reduced power generation costs. Typically, a photovoltaic concentrator system comprises a condensing lens and a high-efficiency solar cell, thereby providing excellent power-generation efficiency with reduced costs of land use per unit area. Besides, such solar photovoltaic systems are not only superior to the traditional thermal power generation solutions in economy but also free from concerns related to waste gas and noise, thus having potential of market growth. 
         [0004]    Conventionally, a Fresnel lens is implemented to substantially focus sunlight on the center of a solar cell. Though the Fresnel lens facilitates photocurrent generation, it nevertheless causes uneven current distribution that results in significant loss of heat from resistors and high operating temperature thereof, thus bringing about deteriorating efficiency of the solar cell. In addition to improving thermal dissipation, another approach to enhancing the photoelectric conversion efficiency in a solar cell is to use a Fresnel lens to provide better uniformity of light concentration. 
         [0005]    Please refer to  FIG. 1  for a primary lens  2  of a typical photovoltaic concentrator system. Therein, a Fresnel lens or a mirror is provided to gather sunlight rays  1  into a concentration region  3 . Optical properties of light vary with wavelengths of light. Hence, variation in the extent of concentration increases markedly when light of a wide range of wavelengths enters the primary lens  2 . 
         [0006]    For instance, there is a great difference in the refractive index of the same plastic material between a light ray with a long wavelength and a light ray with a short wavelength. Under non-total reflection, if light rays with different wavelengths fall on the same optical material at the same incidence angle, the light rays leave the optical material at different emergence angles, depending on wavelength. This can be easily proven by putting an observation plane behind the optical material. 
         [0007]    When applied to collection of light with multiple wavelengths, a solar cell using the traditional primary lens becomes inefficient, because the photoelectric conversion efficiency of the solar cell is highly associated with the range of concentration of light energy involving specific wavelengths of light. Particularly, assuming that different light wavelengths are associated with different concentration ranges, to collect light energy to the full from light rays of all effective wavelengths, a solar cell must has its concentration region made large enough to meet the light wavelength that requires the largest concentration range. However, most of collectable light rays are only available to part of the solar cell, causing inefficient utilization of the solar cell. 
         [0008]    Please refer to  FIG. 2A  for a top view of a conventional primary lens  2  that has been designed and cut into a square.  FIG. 2B  is a partially enlarged view of the primary lens  2  shown in  FIG. 2A .  FIG. 2C  is a polar diagram derived in a conventional illumination test where a light source with a short wavelength at 546.1 nm passes through the conventional primary lens  2 .  FIG. 2D  is a polar diagram showing a light source with a long wavelength at 1300 nm passing through the conventional primary lens  2 . Through  FIGS. 2C and 2D , it is learned that light rays with different wavelengths cause different concentration ranges. 
       SUMMARY OF INVENTION 
       [0009]    An objective of the present invention is to provide an optical structure that comprises a plurality of optical operational regions linked up in an annular array and based at the same location so as to increase focal points. 
         [0010]    Another objective of the present invention is to provide an optical structure that implements a plurality of focal points to distribute light over a photoelectric conversion module so as to maintain a solar cell using the optical structure at a relatively low operating temperature and improve photoelectric conversion efficiency of the solar cell. 
         [0011]    The previously mentioned conventional photovoltaic concentrator system needs a conventional primary lens for collecting sunlight. However, the conventional primary lens fails to accurately concentrate light rays of different wavelengths in the same area but presents a variable concentration region in answering to the light rays with different wavelengths. Hence, the present invention is aimed at improving the conventional primary lens for a solar photovoltaic system so as to enable the improved optical structure to concentrate light rays with different wavelengths in a certain operational region. Besides, the present invention equalizes concentration areas of light rays with different wavelengths so as to allow full use of the light rays, thereby enhancing light uniformity and luminance, and significantly improving efficiency of the solar cell. The optical structure of the present invention can be easily applied to the conventional primary lens and thus is economically beneficial. 
         [0012]    According to a known principle of optics, the smaller the included angle between the direction in which light rays with different wavelengths travel and the normal vector of a solar cell, the closer the locations where the light rays enter the solar cell. Given the aforementioned principle, the present invention appropriately divides an existing primary lens as needed, so as to limit boundaries of concentration areas of light rays with different wavelengths to a certain range. Thus, when ranges required by plural identical primary optical operational regions are all limited, light rays with different wavelengths can be collected in a limited range. From another respect, the present invention features limiting light rays in a certain area where the light rays overlap, thereby improving photoelectric conversion efficiency of the solar cell reasonably. 
         [0013]    In view of this, the present invention involves appropriately dividing a primary lens and determining required optical operational regions. Therein, a plurality of said optical operational regions are linked up in an annular array based at the same location so as to construct a complete optical structure. By the improved optical structure, the present invention facilitates improving uniformity throughout the operational regions and increasing the number of focal points, thereby lowering operating temperature, improving photoelectric conversion efficiency, maximizing the service life of the solar cell, and reducing the operational distance between the primary lens and the solar cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention as well as a preferred mode of use, further objectives and advantages thereof will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein: 
           [0015]      FIG. 1  is a schematic drawing showing light paths of a conventional primary lens; 
           [0016]      FIG. 2A  is a top view the conventional primary lens; 
           [0017]      FIG. 2B  is a partially enlarged view of the conventional primary lenses; 
           [0018]      FIG. 2C  is a polar diagram showing a light source with a wavelength at 546.1 nm passing through the conventional primary lens and presented in a concentration region; 
           [0019]      FIG. 2D  is a polar diagram showing a light source with a wavelength at 1300 nm passing through the conventional primary lens and presented in a concentration region; 
           [0020]      FIG. 3A  is a schematic drawing showing divisional lines on a primary lens according to the present invention; 
           [0021]      FIG. 3B  is a schematic drawing showing four optical operational regions after division jointly forming a complete optical structure of the present invention; 
           [0022]      FIG. 3C  is a partially enlarged vie of the optical structure of the present invention; 
           [0023]      FIG. 3D  is a polar diagram showing a light source with a wavelength at 546.1 nm passing through the optical structure of the present invention and presented in a concentration region; 
           [0024]      FIG. 3E  is a polar diagram showing a light source with a wavelength at 1300 nm passing through the optical structure of the present invention and presented in a concentration region; 
           [0025]      FIG. 4  is a sectional view of the optical structure of the present invention; 
           [0026]      FIG. 5  is a schematic drawing describing a solar cell using the optical structure of the present invention; and 
           [0027]      FIGS. 6A and 6B  are maps of energy distribution measured and plotted against different distances between the disclosed optical structure and a semiconductor chip in the solar cell. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0028]    The present invention is characterized by dividing a typical primary lens  2  into several optical operational regions. To define each said optical operational region, divisional benchmarks are determined taking similar light-entering ranges of light wavelengths. Besides, a divisional angle is determined according to a shape of a concentration region, wherein the angle is derived from dividing 360 degrees by N, where N denotes the number of sides of the polygonal concentration region. Furthermore, the area of the intended concentration region is controlled by a distance between the concentration region and the benchmarks. Afterward, a tip of the optical operational region is taken as a center of rotation so as to form an annular array filling the 360-degree area. Hence, N-1 said regions are integrated into a whole optical structure, thereby accomplishing the present invention. 
         [0029]    Please refer to  FIG. 3A . Therein, a triangular optical operational region  5  is defined on a typical rectangular primary lens  2  along divisional lines  4  adjacent to benchmarks. The optical operational region  5  includes a rough side  52 . At the rough side  52 , a central circle  521  is located at a tip of the optical operational region  5 , and a plurality of refraction portions  522  of concentric arc-shape are arranged on the circumference of the central circle  521 . 
         [0030]    Referring to  FIGS. 3B and 3C , according to the present embodiment, four identical said optical operational regions  5  are arranged in an annular array such that the optical operational regions  5  encircle a center comprising the central circles  521  on the tips thereof, thereby forming an optical structure  6  shaped as a complete square. Boundaries between adjacent said optical operational regions  5  may be realized by any proper connection approach. Of course, the number of the optical operational regions  5  is not to be limited by the present embodiment. Instead, the primary lens  2  may be divided into any number of the optical operational regions  5  as needed. 
         [0031]      FIG. 3D  is a polar diagram derived from a illumination test where a light source with a wavelength at 546.1 nm passes through the optical structure  6  of the present invention. As compared with  FIG. 2C  derived under identical testing conditions, it is learned that the light with the same wavelength presents an evener and more concentrated luminance when passing through the optical structure  6  of the present invention than when passing through the conventional primary lens  2 . 
         [0032]      FIG. 3E  is a polar diagram derived from a illumination test where a light source with a wavelength at 1300 nm passes through the optical structure  6  of the present invention. As compared with  FIG. 2D  derived under identical testing conditions, it is learned that the light with the same wavelength presents an evener and more concentrated luminance when passing through the optical structure  6  of the present invention than when passing through the conventional primary lens  2 . As a whole, the optical structure  6  of the present invention has a compact concentration region with improved concentration uniformity while significantly increasing luminous flux per unit area, thereby improving the photoelectric conversion efficiency of a solar cell using the optical structure  6 . 
         [0033]    Referring to  FIG. 4 , the optical structure  6  of the present invention may be an integrally formed multi-focal Fresnel lens. The optical structure  6  comprises a smooth side  61  and a rough side  62 . Carved at the center of the rough side  62  are a plurality of central circles  621  arranged in an annular array and a plurality of refraction portions  622  of concentric arc-shape relative to the central circles  621  and arranged in a progressive order. These refraction portions  622  are tooth-shaped in a sectional view of the optical structure  6  as shown in  FIG. 4 . The central circles  621  and refraction portions  622  are configured under consideration of light interference and light diffraction and according to required relative sensitivity and reception angle so that light passing therethrough is cast onto a photoelectric conversion module  7  (as shown in  FIG. 5 ), and in consequence multiple focal points positioned differently are provided on the photoelectric conversion module  7 . 
         [0034]    The optical structure  6  is a square transparent plate with the smooth side  61  serving to receive sunlight and the rough side  62  serving to concentrate light rays passing therethrough. Of course, it is feasible that the rough side  62  serves to receive and concentrate sunlight for the smooth side  61  to further cast out the concentrated light rays. Alternatively, the optical structure  6  may be the one shown in  FIG. 3A  where plural identical said optical operational regions  5  are arranged in an annular array relative to a center composed of the central circles  521  on their tips, thereby forming an optical structure  6  shaped as a complete square. 
         [0035]    Referring to  FIG. 5 , a solar cell  10  using the optical structure  6  of the present invention comprises at least one said optical structure  6  and the photoelectric conversion module  7 . The photoelectric conversion module  7  further comprises a frame  71 , a substrate  72 , and a cell  73 . The optical structure  6  is mounted atop the frame  71 . The substrate  72  includes a circuit and is provided below the frame  71  to electrically connect with the cell  73 . Beside, a semiconductor chip  721  is mounted on the substrate  72  to face the optical structure  6 . 
         [0036]    The optical structure  6  may comprise four or more said optical operational regions  5  arranged in an annular array relative to a center composed of the central circles  521  on their tips. Then the optical structure  6  is mounted atop the frame  71  of the photoelectric conversion module  7  and facing the substrate  72  with a predetermined distance H therebetween, wherein the predetermined distance H determines the focal range where the optical structure  6  casts light on the semiconductor chip  721 . 
         [0037]    When light rays enter the optical structure  6 , a focal point generated by the central circles  521  and the refraction portions  522  concentric to the central circles  521  of the optical operational regions  5  is cast on to the substrate  72  so that the light rays are collected on the semiconductor chip  721  of the substrate  72  for photoelectric conversion. Afterward, the resultant electric power is stored in the cell  73  connected with the substrate  72  for being supplied to other powered devices. In the solar cell  10  using the optical structure  6  of the present invention, the semiconductor chip  721  may be a III-V semiconductor chip and the cell  73  may be one of a rechargeable lithium cell and a Ni-MH cell. 
         [0038]    In the solar cell  10  using the optical structure  6  of the present invention, the solar cell  10  composed of the semiconductor chip  721 , namely the III-V semiconductor chip (GaAs, InP, InGaP), has excellent photoelectric conversion efficiency, about 26%˜28%. When made into a multijunctiontandem cell (InGaP/GaAs//InGaAs), the photoelectric conversion efficiency can be increased to about 33.3%. Therefore, the solar cell  10  according to the present invention benefits by the reliability and stability contributed by the III-V semiconductor chip  721 , thus having less tendency to aging and deterioration even working outdoor and being less sensitive to temperature variation. 
         [0039]    The characteristic of photovoltaic concentrator has close relationship with the light concentrating factor (C) and resistance (Rs), which can be represented by the following mathematic formulas: 
         [0000]      Current: I L =CI L,1 ; 
         [0000]      Voltage:  V   OC,C   =V   OC,1 +( nkT/e )InC; 
         [0000]      Power:  P=CP   1   +CI   L,1   ΔV   OC,C   −C   2   I   L,1   2   Rs;   
         [0040]    Wherein, I L,1  is the current before the light is concentrated; V OC,1  is the voltage before the light is concentrated; k is the Boltzmann constant value; T is the absolute temperature. 
         [0041]    In the other hand, by improving the uniformity of the light focused on the semiconductor chip  721 , the dark current can also be reduced, the conversion efficiency can be increased, and the operating temperature of the photoelectric conversion module  7  can also be improved. The conversion efficiency of the semiconductor chip  721  of photoelectric conversion module  7  and the temperature have the following mathematic relationship: 
         [0042]    Short-Circuit Current: the relationship between I SC  and temperature is: 
         [0000]    
       
         
           
             
               
                 I 
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                           nkT 
                         
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                     ] 
                   
                 
               
             
             ; 
           
         
       
     
         [0043]    Wherein, T is the temperature; Eg is the energy gap of semiconductor. 
         [0044]    Open-Circuit Voltage: the relationship between V OC □I SC  is: 
         [0000]    
       
         
           
             
               V 
               OC 
             
             ≈ 
             
               
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                         J 
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                         J 
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                 . 
               
             
           
         
       
     
         [0045]    Taking the solar cell  10  composed of the III-V semiconductor chip  721  as example, the photoelectric conversion efficiency thereof decreases by about 0.067% when the temperature increases by about 1° C. Thus, the multi-focal optical structure  6  also facilitates maintaining the optimal temperature for the semiconductor chip  721  by effectively lowering the peak temperature of the semiconductor chip  721  during light concentration. 
         [0046]    In the present embodiment, the optical structure  6  may have four optical operational regions  5  as shown in  FIG. 3B  so as to generate four different focal points at the same time when passed by light rays and evenly distribute the four focal points over the semiconductor chip  721  (III-V semiconductor chip), thereby maintaining the semiconductor chip  721  at a relatively low temperature and thus ensuring the photoelectric conversion efficiency. In other words, the photoelectric conversion efficiency of the semiconductor chip  721  is ensured from being adversely affected by the excessive temperature happening in a single-focal optical structure. 
         [0047]    Similarly, with quantitative increase of the optical operational regions  5  of the optical structure  6 , the focal points generated by the optical operational regions  5  on the semiconductor chip  721  increase in a proportional manner while being evenly distributed over the semiconductor chip  721 . Of course, a plurality of said optical structures  6  may be provided on the frame  71  of the photoelectric conversion module  7  to face and correspond to a plurality of said semiconductor chips  721  on the substrate  72  so as to further enhance the photoelectric conversion efficiency of the solar cell  10 , thus achieving prompt charging the cell  73 . 
         [0048]    Reading  FIGS. 6A and 6B  with reference to  FIG. 5 , distribution of energy of light is measured and plotted against different distances between the disclosed optical structure  6  and the semiconductor chip  721 . 
         [0049]    As shown in  FIG. 6A , when the distance H between the optical structure  6  of the solar cell  10  and the semiconductor chip  721  is relatively small, the four focal points draw light rays pass therethrough close to the center of the semiconductor chip  721 . At this time, since the four focal points are partially overlapped due to the relatively small distance, the light rays are collected on the semiconductor chip  721  with enhanced uniformity and concentration while thermal energy generated by the concentrated light rays is evenly distributed over the semiconductor chip  721 , but not rivet on the center of the semiconductor chip  721 . 
         [0050]    As can be seen in  FIG. 6B , when the distance H between the optical structure  6  of the solar cell  10  and the semiconductor chip  721  is relatively large, the four focal points evenly distribute light rays passing therethrough to four corners of the semiconductor chip  721 . At this time, owing to the increased distance, the focal range is enlarged and the multiple focal points evenly distribute thermal energy generated by the concentrated light rays over the semiconductor chip  721 , thereby maintaining the semiconductor chip  721  relatively cool and ensuring the photoelectric conversion efficiency. 
         [0051]    However, it is to be noted that the distance H between the optical structure  6  and the semiconductor chip  721  is associated with the area of the optical structure  6  that receives illumination. In other words, the larger the area of the optical structure  6  receiving light is, the longer the focal length between the optical structure  6  and the semiconductor chip  721  is, rendering the larger distance between the optical structure  6  and the semiconductor chip  721 . 
         [0052]    On the contrary, the smaller the area of the optical structure  6  receiving illumination is, the shorter the focal length between the optical structure  6  and the semiconductor chip  721  is, rendering the smaller distance between the optical structure  6  and the semiconductor chip  721 . Similarly, when the optical structure  6  with a fixed area of illumination works with photoelectric conversion modules  7  in different sizes, variable focal lengths would be achievable, so as to provide the optimal focal efficiency at the semiconductor chip  721  on the substrate  72 . 
         [0053]    It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.