Patent Publication Number: US-2013249577-A1

Title: Accelerated lifetime testing apparatus and methods for photovoltaic modules

Description:
FIELD OF THE INVENTION 
     The subject matter disclosed herein relates generally to the testing photovoltaic modules. More particularly, the subject matter is related to methods and apparatus for testing the endurance of photovoltaic (PV) modules over a simulated lifetime. 
     BACKGROUND OF THE INVENTION 
     Currently available accelerated lifetime testers (ALTs) chambers for testing the long-term stability of photovoltaic (PV) devices employ lighting elements positioned at proximate a sunny-side face of a given PV device. In order to test multiple PV panels simultaneously, a light bank of multiple light elements can be employed to illuminate multiple PV devices simultaneously. Additionally, in order to simulate the full light spectrum of the sun (e.g., radiation with a wavelength between about 350 nm and about 800 nm, such as about 360 nm to about 760 nm) and/or the intensity of the sunlight received by the PV device in the field, several light elements can be used. The lighting elements can typically include xenon arc lamps, metal halide lamps, etc., and may have a reflective housing to ensure the light is directed to the PV device(s). 
     However, the lighting elements can become hot during use, and may lead to unnatural heating of the PV devices to temperatures above which would be present in the field, especially when positioned close to the PV device(s) and/or when the light is focused directly onto the surface of the PV device. Thus, the lighting elements are typically spaced sufficiently far from the PV device(s) to reduce the heating effect from the lighting elements. As such, testing multiple PV devices using the light bank of such lighting elements requires a substantial amount of space. 
     Therefore, a need exists for a method and apparatus for performing an accelerated lifetime test of a PV device in a smaller space, in order to reduce the physical footprint required for an ALT chamber. 
     BRIEF DESCRIPTION OF THE INVENTION 
     Aspects and advantages of the invention will be set forth in part in the following description, or may be obvious from the description, or may be learned through practice of the invention. 
     Methods are generally provided for performing an accelerated lifetime test on a photovoltaic device. In one embodiment, the method can include positioning a first photovoltaic device in a first holder adjacent to a light guide such that a transparent surface of the photovoltaic device faces the light guide, directing light emitted from a first light source into the light guide, and redirecting the light emitted from the first light source within the light guide to illuminate the transparent surface of the photovoltaic device. 
     Apparatus is also generally provided for performing an accelerated lifetime test on a photovoltaic device. For example, the apparatus can include a first light source, a light guide positioned to receive light from the light source, and a mounting system configured to hold a photovoltaic device such that a transparent surface of the photovoltaic device faces the light guide. The light guide is generally configured to redirect light emitted from the light source onto the transparent surface of the photovoltaic device. 
     These and other features, aspects and advantages of the present invention will become better understood with reference to the following description and appended claims. The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth in the specification, which makes reference to the appended figures, in which: 
         FIG. 1  shows a perspective view of an exemplary testing chamber according to one embodiment; 
         FIG. 2  shows a cross-sectional view of the exemplary testing chamber of  FIG. 1 , 
         FIG. 3  shows a perspective view of an exemplary testing chamber according to another embodiment; 
         FIG. 4  shows a cross-sectional view of the exemplary testing chamber of  FIG. 3   
         FIG. 5  shows an exemplary light guide for use in the exemplary testing chamber of  FIG. 1 ; 
         FIG. 6  shows an exemplary light guide for use in the exemplary testing chamber of  FIG. 1 ; 
         FIG. 7  shows an exemplary light guide for use in the exemplary testing chamber of  FIG. 1 ; 
         FIG. 8  shows an exemplary light guide for use in the exemplary testing chamber of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Reference now will be made in detail to embodiments of the invention, one or more examples of which are illustrated in the drawings. Each example is provided by way of explanation of the invention, not limitation of the invention. In fact, it will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the scope or spirit of the invention. For instance, features illustrated or described as part of one embodiment can be used with another embodiment to yield a still further embodiment. Thus, it is intended that the present invention covers such modifications and variations as come within the scope of the appended claims and their equivalents. 
     Apparatus and methods are provided for performing an accelerated lifetime test on a PV device (i.e., solar panel). The apparatus and methods can simulate cycles of illumination and dark periods that the PV device is exposed to in the field (e.g., to simulate day and night cycles). Embodiments of the presently disclosed apparatus and methods can allow for multiple PV devices to be tested in a relatively small space. Additionally, embodiments of the presently disclosed apparatus and methods can inhibit and/or prevent heating of the PV devices from the light source(s) used to illuminate the PV devices. 
     One embodiment of an apparatus  100  for performing an accelerated lifetime test on a PV device or module  10  is shown in  FIG. 1 . The accelerated lifetime testing apparatus generally includes a light guide  102  positioned to receive light beams  103  from a first light source  104  and optional second light source  106 . Generally, the light guide  104  is configured to redirect light emitted from the light sources  104 ,  106  onto the transparent surface  11  of the photovoltaic device  10 , with the transparent surface  11  permitting the light to reach the active regions of the photovoltaic device  10 . Additional light sources may also be positioned so that light emitted from such additional light sources can be directed into the light guide. 
     As stated, the light guide  104  can generally be configured to redirect light emitted from the first light source  104 , optional second light source  106 , and any other light sources present in the apparatus  100  onto the transparent surface  11  of the photovoltaic device  10 . The light guide  102  can, in one embodiment, redirect the emitted light from the first light source  104  and optional second light source  106  in a substantially uniform manner onto the transparent surface  11  of the PV device  10 . Thus, the entire surface area of the transparent surface  11  of the PV device  10  can be exposed to substantially the same light, especially in terms of intensity, wavelength spectrum, etc. As such, the PV device  10  can be tested uniformly in the apparatus  100 . 
     As shown, the light guide  102  can redirect light from the light sources  104 ,  106  positioned on a side edge of the light guide  102  in a manner to illuminate the transparent surface(s)  11  of the PV device(s)  10 . Such distribution and redirection of the light in the light guide can be accomplished in a variety of manners, such as through the use of bumps, ridges, and/or diffractive optical elements. For example, diffractive and/or diffusive optical elements can be included within the light guide  102 , and the diffractive and/or diffusive optical elements can have increasing size and/or density within the construction of the light guide  102  as a function of distance away from the light source  104 ,  106 . The use of various configurations of such diffractive and/or diffusive optical elements as part of a light guide  102  is commonly associated with the improved lighting of LCD (liquid crystal display) panels, in terms of, e.g., achieved brightness and uniformity, and such configurations are considered to be within the scope of the present system. 
       FIGS. 5-8  show exemplary light guides  102  that can be used in the embodiments of  FIG. 1 . Although each of these exemplary light guides  102  are discussed in greater detail below, it should be understood that any suitable light guide  102  can be utilized in accordance with the present disclosure. 
     Referring to  FIG. 5 , an exemplary light guide  102  is shown adjacent to a light source  104 . In this embodiment, the light guide  102  generally includes a light guide plate  500 , a reflective plate  502 , a diffusion plate  504 , and a prism plate  506 . As show, the light source  104  generally directs light into the light guide plate  502  at its side surface  501 . The light beams may propagate between a bottom surface  503  and a light emitting surface  505  toward an opposite end surface  507  of the light guide plate  500  by total internal reflection (e.g., as discussed below with respect to  FIG. 6 ), or may be output through the light emitting surface directly. Further, the bottom surface  503  may include structures such as dots formed thereon or facets cut therein and arranged in a pattern (not shown). Light beams encountering any of these structures are diffusely or specularly reflected, so that they are emitted through the light emitting surface  505 . 
     Referring to  FIG. 6 , an exemplary light guide plate  500  (e.g., for use with the embodiment of  FIG. 5 ) is generally shown. The light guide plate  500  comprises a substrate  600  having a light incident surface  501 , a light emitting surface  505  adjacent to the light incident surface  501 , a bottom surface  503  opposite to the light emitting surface  505 , and side surfaces  601 ,  602  and  603 . In one particular embodiment, the light incident surface  501  and the light emitting surface  505  can be provided with anti-reflection films (not labeled), and the bottom surface  503  and the side surfaces  601 ,  602 , and  603  can be provided with reflective films (not labeled). As such, when light beams  103  from the light source  104  are directed on the light incident surface  501  of the light guide plate  500 , most of the light beams pass through the light incident surface  501 , and relatively few light beams  103  are reflected by the light incident surface  501 . This reduces loss of light and enhances the light utilization efficiency of the light guide plate  500 . Likewise, when the internal light beams  103  within the light guide plate  500  reach the light emitting surface  505 , the light can readily pass through the light emitting surface  505 . Alternatively, the reflective surfaces of the bottom surface  503  and the side surfaces  601 ,  602 , and  603  can redirect light within the light guide plate  500  such that nearly all of the light beams  103  received through the light incident surface  501  is eventually directed out of the light emitting surface  505 . 
     Referring again to  FIG. 5 , the light exiting the light emitting surface  505  of the light guide plate  500  then passes through the diffusion plate  504  and the prism plate  506 . The diffusion plate  104  can be, for example, a film or sheet configured to uniformly diffuse the emitted light exiting the light emitting surface  505 . The prism plate  506  can be, for example, ridged with peaks  507  and valleys  508  across the surface  510  oppositely positioned from light guide plate  500 . Thus, the prism plate  506  can collimate the light beams exiting the light guide  102  in order to improve uniformity and brightness across the light guide  102 . 
     In the embodiment of  FIG. 5 , a single prism plate  506  is shown having the peaks  507  and valleys  508  define ridges  512  extending substantially parallel to each other in a first direction in the surface  510 . However, additional prism plates may be present in the light guide  102 . For example, in the embodiment shown in  FIG. 7 , a second diffusion sheet  702  and a second prism plate  704  is shown in the exemplary light guide  102 . In this embodiment, the second prism plate  704  has peaks  706  and valleys  707  that define ridges  708  that are oriented in a second direction that is different than the first direction (e.g., substantially perpendicular). 
     Although shown as separate components, it is noted that the prism plate  506  and  704  (along with the optional diffusion sheets  504 ,  702 ) may form an integral part of the light guide plate  500  (i.e., may form the light emitting surface  505 ). 
       FIG. 8  shows yet another exemplary embodiment of a light guide  102 . In this embodiment, the light source  104  can be positioned near a corner of the light guide plate  500 . In this embodiment, the light emitting surface  505  of the light guide plate  500  is patterned with a plurality of arc-shaped ridges  800  defined by peaks  802  and valleys  804  (i.e., arcuate protrusions of triangular cross-section). Again, although shown as a single component, it is noted that the light emitting surface  505  can be formed with a separate prism plate (along with an optional diffusion sheet), as shown above with respect to  FIGS. 5 and 7 . 
     As stated,  FIGS. 1-2  show an embodiment where the light guide  104  is configured to redirect light emitted from the light sources  104 ,  106  onto a single PV device  10 . However, in other embodiments, the light guide  102  can be configured to redirect light emitted from the light sources  104 ,  106  onto multiple PV devices  10 . For example, as shown, the light guide  102  is configured to redirect light from the light sources  104 ,  106  onto the transparent surfaces  11   a,    11   b,  respectively, of a first PV device  10   a  and a second PV device  10   b.    
     For example, the embodiments of  FIGS. 5-8  can be utilized without a reflective plate or surface and instead with an opposite, second light emitting surface (including, for example, additional diffusion sheets and/or prism plates). 
     As more particularly shown in  FIGS. 2 and 4 , the PV device(s)  10  are shown loaded in a mounting system  110  that is generally configured to hold each PV device  10 , while exposing the transparent surface  11  to light redirected from the light guide  102 . Thus, the mounting system  110  generally can position each PV device  10  such that its transparent surface  11  faces the light guide  102 , while the transparent surface remains exposed. For example, the embodiment shown includes a frame assembly  112  and brackets  114  configured to hold the PV device  10 . However, any suitable mounting system  110  can be utilized to removably hold the PV modules  10 , as long as the transparent surface  11  is substantially unblocked to receive light from the light guide  102  during testing. 
     In one embodiment, the photovoltaic device(s)  10  can be exposed to a series of alternating illumination periods and dark periods in order to simulate day and night cycles found with exposed in the field. As such, the PV device(s)  10  can be exposed to light in a manner that simulates the natural sunlight, as would be found in the field. Additionally, the PV device(s)  10  can be electrically connected to function as if set in actual operation. 
     The light sources  104 ,  106  can be any suitable light source. In one particular embodiment, the light source  104 ,  106  can simulate the light spectrum of the sun (e.g., radiation with a wavelength between about 350 nm and about 800 nm, such as about 360 nm to about 760 nm). For example, suitable light sources  104 ,  106  can include xenon arc lamps, metal halide lamps, fiber optic lighting, LED lamps, fluorescent lamps (e.g., CCFLs), etc., or combinations thereof 
     The light sources  104 ,  106  can be, in particular embodiments, included within a light housing  105 ,  107 , respectively, that can be configured to direct the light emitted from the light sources  104 ,  106  into the light guide  102 . For example, the light housing  105 ,  107  can be reflector housing having a reflective back surface and a front window, thus helping to maximize the use of the light generated by a given light source  104 ,  106 . 
     In the embodiments shown in  FIGS. 2 and 4 , a cooling system  120  is positioned and configured to cool its respective light source  104 ,  106 . The cooling system can, for example, include a fan  122  configured to flow a cooling gas  121  past the light source  104 ,  106  (e.g., between the light source  104  or  106  and the light guide  102  as shown, and/or between the photovoltaic device  10  and the light guide  102 ). The cooling gas  121  can be, in one embodiment, atmospheric air. In one embodiment, the cooling gas can be room temperature. Alternatively, the cooling gas can be passed through a cooling device  124 , in order to reduce the temperature of the cooling gas below room temperature prior to flowing past the light source  104 ,  106 . 
     The apparatus  100  can be utilized in a method of performing an accelerated lifetime test on a photovoltaic device. These methods can replicate a typical lifetime of exposure to the sun in a relatively short and controlled simulation. The testing cycle begins by illuminating the transparent surface  11  of the photovoltaic module  10  using the light guide  102 . Upon turning the light sources  102 ,  104  on, the temperature of the testing chamber may rise due to radiation energy emitted from the light sources  102 ,  104 . As stated, the rate of the temperature rise can be somewhat controlled via a cooling system  120  used in conjunction with the light sources  104 ,  106 . In one embodiment, the temperature of the PV device  10  can be allowed to rise a targeted amount (e.g., can increase 25° C. or less) during an “on” cycle. Once the target temperature is reached, the light sources  104 ,  106  can be turned off (i.e., going dark), and the PV device&#39;s temperature can be reduced back to the initial temperature to complete a testing cycle. 
     The length of the lighted portion (i.e., light sources turned on) and the dark portion (i.e., light sources turned off) of the testing cycles can be adjusted as desired. In one embodiment, the lighted portion (i.e., light sources turned on) of the testing cycle can last long enough to raise the temperature of the PV device about 5° C. to about 15° C. (e.g., about 15 minutes to about 2 hours). 
     This testing cycle can be repeated any number of times to replicate being deployed in the field over an extended period. Once the desired number of testing cycles has been completed, the tester can remove the PV modules  10  for further study. 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.