Patent Application
20070256725
This invention relates to solar power generators, more particularly to concentrating-type photovoltaic devices.
Photovoltaic solar energy collection devices used to generate electric power generally include flat-panel collectors and concentrating solar collectors. Flat collectors generally include PV cell arrays and associated electronics formed on semiconductor (e.g., monocrystallinc silicon or polycrystalline silicon) substrates, and the electrical energy output from flat collectors is a direct function of the area of the array, thereby requiring large, expensive semiconductor substrates. Concentrating solar collectors reduce the need for large semiconductor substrates by concentrating light beams (i.e., sun rays) using, e.g., a parabolic reflectors or lenses that focus the beams, creating a more intense beam of solar energy that is directed onto a small PV cell. Thus, concentrating solar collectors have an advantage over flat-panel collectors in that they utilize substantially smaller amounts of semiconductor. Another advantage that concentrating solar collectors have over flat-panel collectors is that they are more efficient at generating electrical energy.
A problem with conventional concentrating solar collectors, such as solar collector 50, is that they are expensive to operate and maintain. The solar collector optics (e.g., primary mirror 53 and secondary mirror 54) used in conventional collectors to focus the light beams are produced separately, and must be painstakingly assembled to provide the proper alignment between focused beam 55 and PV cell 51 (i.e., such that focused beam 55 is centered on PV cell 51). Further, over time, the reflectors and/or lenses can become misaligned due to thermal cycling or vibration, causing focused beam to become misaligned, and become dirty due to exposure to the environment, thus reducing the intensity of focused beam 55. Maintenance in the form of cleaning and adjusting the reflectors/lenses can be significant, particularly when the reflectors/lenses are produced with uneven shapes that are difficult to clean.
Because PV cell 51 accounts for a significant portion of the overall cost of concentrating solar collector 50, there is a significant incentive to minimize the size (and, hence, the production cost) of PV cell 51. In this regard, a problem with conventional PV cell 51 is the inefficient conversion of circular solar light (e.g., focused beam 55; see
What is needed is a concentrator PV (CPV) device that avoids the expensive assembly and maintenance costs associated with conventional concentrator-type PV cells, provides a compliant and optically stable package for the PV cell, and more efficient use of the active area of the PV cell.
The present invention is directed to a Cassegrain-type CPV device that induces utilizes a solid glass or plastic optical structure having a relatively large convex (protruding) lower surface, a central cavity defined in the lower surface, and an upper aperture surface having a relatively small centrally-located concave (curved) surface (e.g., a depression). Cassegrain-type primary and secondary mirrors are respectively disposed on the convex lower surface and in the central depression such that the reflective surfaces face into the optical structure. A lower cover includes a central portion disposed over the central cavity, and one or more peripheral portions extending from the central portion over the backside surface of the primary mirror. A PV cell is mounted on the central portion of the lower cover such that the PV cell is disposed inside the central cavity. In one embodiment, the convex and concave surfaces are associated conic (e.g., hyperbolic and/or parabolic) surfaces arranged such that the portion of light passing through the aperture surface onto any point on the primary mirror is reflected to a corresponding point on the secondary mirror, which in turn re-reflects the light, and focuses the light into the central cavity and onto the PV cell. Because the optical structure is solid (i.e., because the convex and concave surfaces remain fixed relative to each other), the primary and secondary mirrors remain permanently aligned, thus maintaining optimal optical operation while minimizing maintenance costs. Moreover, the loss of light at gas/solid interfaces is minimized because only solid optical material (e.g., low-iron glass) is positioned between the primary and secondary mirrors.
In accordance with an aspect of the present invention, the space inside the central cavity between the molded optical element and the PV cell is filled with a resilient, optically transmissive material suitable for withstanding the intense light at and in proximity to the cell. The resilient, optically transmissive material precludes open cavities that can scatter light, disrupt thermal conduction, and become points of failure over time. In one embodiment, the resilient, optically transmissive material is a gel that assumes a liquid state when uncured, and a relatively solid state when subsequently cured. An advantage to using a curable gel material is that it can be introduced into the cavity in an uncured liquid state, which allows the gel material to till the complex shapes surrounding the PV cell. A further advantage of using such a gel material is that mechanical movement of the parts (e.g., the PV cell and its supporting structure) during assembly and operation can be accommodated without the formation of cracks or voids in the critical cell joint region. Preferably, the gel material is silicone because such materials are mechanically compliant, are known to be optically stable over temperature and humidity, and are capable of withstanding extended exposure to ultraviolet radiation without yellowing. In accordance with another aspect, the gel material is light transmissive at least down to the longest wavelength (lowest photon energy) converted by the PV cell. This longest wavelength is determined by the smallest band gap in the cell, which may be a multi-junction cell.
In accordance with another aspect of the present invention, a PV cell of the CPV device includes contact structures disposed on the four corners of the upper (exposed) surface, whereby the contact structures define a central, substantially circular active area. Because the focused beam generated by a Cassegrain-type optical system is circular, the PV cell of the present invention provides efficient use of the PV cell active area, thus reducing production costs by minimizing the overall “chip” size of the PV cell.
In accordance with a further aspect of the present invention, the metallization pattern includes gridlines in a radial and azimuthal pattern. Because the light forms a circular spot centered on the cell, the photo-generated current will increase radially from the center to the edge of the cell. Gridlines that extend radially inward from the corners of the cell toward the center are optimally suited to the current generation of the cell. Optional azimuthal extensions on the radial gridlines may be added to further reduce the distance carriers need to migrate within the semiconductor before reaching a gridline. Further optimization of the precise spacing and number of gridlines as well as their dimensions is dependent on the efficiency of the cell and the level of concentrated light reaching the cell and is routinely undertaken by those skilled in the art of gridline optimization.
In accordance with another aspect of the present invention, the PV cell is mounted onto a heat slug that is then mounted into the cavity formed in the optical element. Given that the optical element is preferably made of glass, and the heat slug is preferably made of metal, it is expected that the two objects will have different coefficients of thermal expansion. Therefore, it is desirable for the photovoltaic cell and the heat slug to form an assembly that self-centers itself into the central cavity. Accordingly, in one embodiment, the heat slug includes several resilient fingers that facilitate self-alignment of the heat slug in the cavity, thereby self-aligning the PV cell with the focal point of the optical system associated with the optical element. The PV cell is mounted onto the backside surface of the heat slug, which defines a circular opening positioned over the circular active area of the PV cell. The emitter (topside) contacts of the PV cell are electrically connected (e.g., soldered) to the heat slug, which provides a conductive path between the PV cell and a first external conductor. The base (bottom side) contact of the PV cell is electrically connected to a second conductor. In one embodiment, the first and second conductors are provided on the lower cover that is conformally mounted onto a backside surface of the optical element.
These and other features, aspects and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings, where:
The present invention relates to a concentrator PV (CPV) device that avoids the expensive assembly and maintenance costs associated with conventional concentrator-type PV cells, provides a compliant and optically stable package for the PV cell, and more efficient use of the active area of the PV cell. In particular, the present invention provides structures and methods that improve the attachment and performance of PV cells in solid dielectric solar concentrators, such as that disclosed in co-owned and co-pending U.S. patent application Ser. No. 11/110,611 entitled “CONCENTRATING SOLAR COLLECTOR WITH SOLID OPTICAL ELEMENT”,which is incorporated herein by reference in its entirety.
Optical element 110 is a solid, disk-like, light-transparent structure including an upper layer 111, a relatively large convex surface 112 protruding from a lower side of upper layer 111, a substantially flat aperture surface 115 disposed on an upper side of upper layer 111, and a relatively small concave (curved) surface (depression) 117 defined in aperture surface 115 (i.e., extending into upper layer 111). In order to minimize material, weight, thickness and optical adsorption, upper layer 111 may be vanishingly small. In one embodiment, optical element 110 is molded using a low-iron glass (e.g., Optiwhite glass produced by Pilkington PLC, UK) structure according to known glass molding methods. Alternatively, clear plastic may be machined and polished to form single-piece optical element 110, or separate pieces by be glued or otherwise secured to form optical element 110. In a preferred embodiment, optical element 110 is 5 to 12 mm thick and 20 to 40 mm wide. This thickness helps to ensure that the heat conduction path from the backside convex surface 112 to aperture surface 115 does not become too resistive as it would be if optical element 110 were either thicker or hollow.
PV cell 120 is located in a central cavity region 113 that is defined in the center of convex surface 112. PV cell 120 is connected by way of suitable conductors 122 and 124 (indicated in
Primary mirror 130 and secondary mirror 140 are respectively disposed on convex surface 112 and concave surface 117. Primary mirror 130 and secondary mirror 140 are shaped and arranged such that, as shown in
Lower cover 150 includes a central portion 151 and a curved peripheral portion 152 extending outward from central portion 151. In one embodiment, lower cover 150 includes a material having relatively high thermal conductivity, and includes a thickness selected such that a lateral thermal resistance of lower cover 150 (i.e., measured in a radial direction from central portion 151 to the outer edge of peripheral portions 152) is less than a transverse thermal resistance of optical element 110 (i.e., measured from the convex surface 112 to the aperture surface 115), as described in co owned U.S. patent application No. ______, entitled “PASSIVELY COOLED SOLAR CONCENTRATING PHOTOVOLTAIC DEVICE” [Ally Docket No. 20060255/XCP-069], which is filed on the same date as the present application and is incorporated herewith by reference in its entirety. In another embodiment, lower cover 150 includes a protective layer (e.g., Tedlar®, which is a trademark of the DuPont Corporation, or TPT (Tedlar, polyester, Tedlar), which is a three-ply material engineered for strength, stability and dielectric breakdown strength) that is disposed over cavity 113, and is secured to the back (non-reflecting) surface of primary mirror 130 using a suitable adhesive (e.g., Ethylene vinyl acetate copolymers (EVA)). PV cell 120 is mounted on an inside surface of central portion 151 such that PV cell 120 is disposed inside cavity 113. Lower cover 150 is laminated onto optical element 110, for example, using lamination techniques disclosed in co-owned and co-pending U.S. patent application Ser. No. ______, entitled “LAMINATED SOLAR CONCENTRATING PHOTOVOLTAIC DEVICE” [Atty Dkt No. 20060351-US-NP (XCP-071)], which is co-filed with the present application and incorporated herewith by reference in its entirety.
Referring again to
In one embodiment, the gel material (e.g., silicone) utilized for optically transmissive material 128 is in a gel form that has a liquid state when uncured, and a relatively solid state when subsequently cured. A suitable curable silicone gel is produced by Dow Corning under the model name/number Sylgard 3-6636. An advantage to using such a curable gel material is that it can be introduced into cavity 113 in an uncured liquid state, which allows the gel material to fill the complex shapes surrounding PV cell 120. A further advantage of using such a gel material is that mechanical movement of the parts (e.g., the PV cell and its supporting structure) during assembly and operation can be accommodated without the formation of cracks or voids in the critical cell joint region. Another advantage to using silicone for the gel material is that is mechanically compliant, are known to be optically stable over temperature ranges (e.g., below 100° C.) and humidity ranges typically experienced by CPV devices, and are capable of withstanding extended exposure to ultraviolet radiation without yellowing. In accordance with another aspect, the gel material is light transmissive at least down to the longest wavelength (lowest photon energy) converted by PV cell 120. This longest wavelength is determined by the smallest band gap in PV cell 120, which may be a multi-junction cell.
In accordance with another embodiment of the present invention, a method for producing CPV device 100 includes mounting PV cell 120 a heat slug 160 that is then mounted into central cavity 113 of optical element 110. Heat slug 160 includes a metal substrate 161 that is in thermal contact with PV cell 120, and serves to transfer heat from PV cell 120. In one embodiment, one or more openings 165 are formed in metal substrate 161 that align with corresponding openings 155 in central portion 151 of lower cover 150 to facilitate the passage of current from PV cell 120, e.g., by way of conductors 122 and 124 In another embodiment, current is transmitted to and from PV cell 120 by way of lower cover 150 or primary mirror 130 in a manner similar to that disclosed in co-owned and co-pending U.S. patent application Ser. No. 11/110,611 (cited above). Metal substrate 161 also defines one or more passages 169 that align with corresponding passages 159 defined in central portion 151 of lower cover 150 to facilitate the flow of optically transmissive material 128 out of cavity 113 during assembly. In one embodiment, the assembly including heat slug 160 and PV cell 120 is mounted onto central portion 151 of lower cover 150 prior to assembly onto optical element 110. In this embodiment, optically transmissive material 128 is inserted in a relatively liquid state into cavity 113 such that optically transmissive material 128 fills or nearly fills cavity 113, and then the assembly including lower cover 150, heat slug 160 and PV cell 120 is mounted such that PV cell 120 and heat slug 160 are inserted into cavity 113, thereby forcing excess optically transmissive material 128 through passages 159 and 169. Subsequent to this assembly process, a curing process is performed to convert optically transmissive material 128 from its relatively liquid state to a relatively solid cured state.
In accordance with a further aspect of the present invention, PV cell 120A further includes metal gridlines 126A1 to 126A4 that extend in a radial and azimuthal pattern from contact structures 124A1 to 124A4, respectively. Because the light forms a circular spot centered on PV cell 120A, the photo-generated current will increase radially from center to the edges of the cell. As depicted in
As indicated in
In accordance with another aspect of the present invention, PV cell 120A is mounted onto a metal substrate 261 of heat slug 260, which is then mounted into cavity 213 of optical element 210. Given that optical element 210 is preferably made of glass, and heat slug 260 is preferably made of metal, it is expected that the two objects will have different coefficients of thermal expansion. Therefore, it is desirable for PV cell 120A and heat slug 260 form an assembly that self-centers itself into cavity 213. Accordingly, heat slug 260 includes a several resilient fingers 263 that have a fixed end integrally formed with or otherwise fixedly connected to metal substrate 261, and a curved body extending between its fixed and free ends. Resilient fingers 263 are shaped to facilitate self-alignment of the heat slug in cavity 213, thereby self-aligning PV cell 120A with the focal point F of the optical system formed by primary mirror 230 and secondary mirror 240 on optical element 210. For the purposes of this invention, the use of the term focal point refers both to concentration by imaging and non-imaging elements.
It is an aspect of this invention that the walls of cavity 213 are tapered. This tapering serves several useful functions. First, it accommodates the molding process. Second, in operation, should the assembled components within the expand due to thermal expansion, the tapered walls enable the components to accommodate the volume change by gliding out along the tapers.
In one embodiment, which is depicted in
Although the present invention has been described with respect to certain specific embodiments, it will be clear to those skilled in the art that the inventive features of the present invention are applicable to other embodiments as well, all of which are intended to fall within the scope of the present invention. For example, the primary and secondary mirrors may be preformed and then mounted to the optical element using a suitable adhesive, but this approach may substantially increase production costs. In yet another alternative embodiment, the curved surface utilized to form the secondary mirror may be convex instead of concave, thus being in the form of a classical Gregorian type system. In yet another alternative embodiment, the curved surfaces utilized to form the primary and secondary mirrors may be elliptical, ellipsoidal, spherical, or other curved shape.
