US20090032086A1

US20090032086A1 - Terrestrial solar array including a rigid support frame

Info

Publication number: US20090032086A1
Application number: US12/131,556
Authority: US
Inventors: Mikhail Kats; Scott Elman; Gary Hering
Original assignee: Emcore Corp
Current assignee: Solaero Solar Power Inc
Priority date: 2007-07-30
Filing date: 2008-06-02
Publication date: 2009-02-05
Also published as: US20090032084A1; KR20090013001A; CN101359884A; GR1006269B; ES2340562B2; CN101359884B; AU2007219267B1; ITMI20071833A1; FR2919758A1; PT103890B; DE102007044477A1; PT103890A; FR2919758B1; US7381886B1; KR101003539B1; ES2340562A1

Abstract

A concentrator photovoltaic solar cell array system includes a central support mountable on a surface and a solar cell array including triple junction III-V semiconductor compound solar cell receivers and a support frame coupled to the solar cell array and carried by, and rotatable with respect to, the central support about an axis orthogonal to the central longitudinal axis. The support frame can include (i) a first frame assembly coupled to the solar cell array and (ii) a second frame assembly coupled to the first frame assembly arranged to increase the rigidity thereof. The system also has an actuator for rotating the central support and the support frame as well as pivoting the support frame so as to adjust its angle with respect to the earth's surface, so that the solar cell array is maintained substantially orthogonal to the rays from the sun as the sun traverses the sky.

Description

REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part application of and claims priority to U.S. application Ser. No. 11/830,636, filed on Jul. 30, 2007, now U.S. Pat. No. 7,381,886, which is incorporated herein by reference. This application is related to co-pending U.S. application Ser. No. 12/024,489 filed Feb. 1, 2008, which is a divisional of U.S. application Ser. No. 11/830,636.
This application is also related to co-pending U.S. patent application Ser. Nos. 11/500,053 filed Aug. 7, 2006, and U.S. patent application Ser. No. 11/849,033 filed on Aug. 31, 2007 by the common assignee.

BACKGROUND

This disclosure relates generally to a terrestrial solar power system for the conversion of sunlight into electrical energy, and, more particularly to a solar cell array using IV-V compound semiconductor solar cells for unitary movement to track the sun. Compound semiconductor solar cells, based on III-V compounds, have 28% efficiency in normal operating conditions. Moreover, concentrating solar energy onto a III-V compound semiconductor photovoltaic cell can increase the cell's efficiency to over 37%. Aspects of a solar cell system include the specification of the number of cells used to make up an array, and the shape, aspect ratio, and configuration of the array.
One aspect of a solar cell system is the physical structure of the semiconductor material layers constituting the solar cell. Solar cells are often fabricated in vertical, multijunction structures to utilize materials with different bandgaps and convert as much of the solar spectrum as possible. One type of multijunction structure is the triple junction solar cell structure consisting of a germanium bottom cell, a gallium arsenide (GaAs) middle cell, and an indium gallium phosphide (InGaP) top cell.
In the design of both silicon and III-V compound semiconductor solar cells, one electrical contact is typically placed on a light absorbing or front side of the solar cell and a second contact is placed on the back side of the cell. A photoactive semiconductor is disposed on a light-absorbing side of the substrate and includes one or more p-n junctions, which creates electron flow as light is absorbed within the cell. Grid lines extend over the top surface of the cell to capture this electron flow which then connect into the front contact or bonding pad.
The individual solar cells are typically disposed in horizontal arrays, with the individual solar cells connected together in electrical series. The shape and structure of an array, as well as the number of cells it contains, and the sequence of electrical connections between cells are determined in part by the desired output voltage and current of the system.
Another aspect of terrestrial solar power systems is the use of light beam concentrators (such as lenses and mirrors) to focus the incoming sunrays onto the surface of a solar cell or solar cell array. The geometric design of such systems also requires an appropriate solar tracking mechanism, which allows the plane of the solar cells to continuously face the sun as the sun traverses the sky during the day, thereby optimizing the amount of sunlight impinging upon the cell.
Accurate solar tracking is advantageous because the amount of power generated by a given solar cell is related to the amount of sunlight that impinges on it. In an array, therefore, it is advantageous to optimize the amount of sunlight that impinges on each constituent solar cell. For example, misalignment of about one degree can appreciably reduce efficiency. Because arrays are often mounted outdoors and are large, heavy structures, this presents challenges. Even moderate wind can cause bending and the array can bend under its own weight. These problems are usually most pronounced in regions near the perimeter of the array. As a result, the solar cells that are disposed in the regions where bending occurs can become misaligned with the sun, compromising power generation.

SUMMARY

The invention relates to a concentrator photovoltaic solar cell array system for producing energy from the sun using one or more sun-tracking solar cell arrays.
In some implementations, the system includes a central support mountable on a surface and a solar cell array including triple junction III-V semiconductor compound solar cell receivers and a support frame coupled to the solar cell array and carried by, and rotatable with respect to, the central support about an axis orthogonal to the central longitudinal axis. The support frame can include (i) a first frame assembly coupled to the solar cell array and (ii) a second frame assembly coupled to the first frame assembly (e.g., including a truss) arranged to increase the rigidity of the first frame assembly. The system also has an actuator for rotating the central support and the support frame so that the solar cell array is maintained substantially orthogonal to the rays from the sun as the sun traverses the sky. The actuator also can pivot the support frame so as to adjust its angle with respect to the earth's surface.
Some implementations provide one or more of the following advantages. For example, the system can provide an improved solar cell array utilizing a III-V compound semiconductor multijunction solar cells for terrestrial power applications. Some implementations provide a solar cell array for producing approximately 25 kW peak DC power on full illumination. Some implementations provide the second frame assembly aligned along the greatest perpendicular dimension (e.g., along the horizontal axis) of the solar cell array. Some implementations provide a truss coupled to the first frame assembly including a lower chord, an upper chord substantially parallel to the lower chord, two or more substantially parallel brace chords coupled to the upper and lower chords, and at least one diagonal chord disposed between two brace chords and coupled to the upper and lower chords. Some implementations provide a truss coupled to the first frame assembly comprising a lower chord, an upper chord substantially parallel to the lower chord, and at least one diagonal chord coupled to the lower chord and upper chord. Some implementations provide a plurality of series-connected receivers each with a III-V semiconductor solar cell in a Fresnel lens based solar concentrator subarray for terrestrial power applications. Some implementations provide a lower chord including at least a portion of the first frame assembly. Some implementations provide a first frame assembly including a generally rectangular frame member comprising upper and lower parallel members oriented in a direction substantially parallel to the surface to which the center support is mountable, wherein the upper chord is coupled to the lower parallel member by at least one truss support member. Some implementations provide brace chords arranged substantially orthogonal to a plane defined by the solar cell array. Some implementations provide that the direction of the perpendicular distance from the lower chord to the upper chord is substantially orthogonal to a plane defined by the solar cell array. Some implementations provide that the width of the lower chord is substantially the same as the width of the solar cell array, wherein the width of the solar cell array is measured in a direction substantially parallel to the surface to which the central support is mountable. Some implementations provide that the width of the first frame assembly and the width of solar cell array are substantially the same, wherein the width of the solar cell array is measured in a direction substantially parallel to the surface to which the central support is mountable. Some implementations provide that the truss is arranged in a direction orthogonal to a plane defined by the first frame assembly. Some implementations provide a solar cell array including a plurality of solar cell modules, each module including a plurality of Fresnel lenses wherein each Fresnel lens is disposed over a single solar cell for concentrating by a factor in excess of 500× the incoming sunlight onto the solar cell and producing in excess of 13 watts of DC power at AM 1.5 solar irradiation per solar cell with conversion efficiency in excess of 37%. Some implementations provide a solar cell array including a plurality of solar cells and a corresponding plurality of Fresnel lenses each of which is disposed over a single solar cell for concentrating by a factor in excess of 500× the incoming sunlight onto the solar cell and producing in excess of 13 watts of DC power at AM 1.5 solar irradiation per solar cell with conversion efficiency in excess of 37%. Some implementations provide a truss mounted above the vertical center (i.e., above the center of its height) of the solar cell array. Some implementations provide a central support constituted by a first member provided with means for mounting the central support on the surface, and a second member rotatably supported by, and extending upwardly from, the first member. Some implementations provide the advantage that the support frame is mounted on a cross member which is rotatably mounted with respect to the second member of the central support about an axis orthogonal to the central longitudinal axis. Some implementations provide a first frame assembly including a generally rectangular frame member having a plurality of parallel support struts that are parallel to the shorter sides of the rectangular frame member. Some implementations provide a first frame assembly further includes a plurality of oblique support struts. Some implementations provide that the truss prevents a deflection greater than 1 degree near the perimeter of the solar cell array. Some implementations provide an array of III-V semiconductor solar cell concentrator modules with a solar tracker for terrestrial power applications. Some implementations provide a terrestrial solar power system constituted by a plurality of solar cell arrays each mounted on a post to track the sun, wherein each array is sized and spaced apart from each other over the ground so as to maximize the number of cells that can be implemented over a given ground area. Some implementations provide a solar cell array system in which a single solar tracking tower produces 25 kW of peak DC power for terrestrial power applications.
Other features and advantages will be readily apparent from the detailed description, accompanying drawings and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a perspective view of an implementation of a terrestrial solar cell system.

FIG. 1B is a second perspective view of the implementation of FIG. 1A.

FIG. 1C is a perspective view of an implementation of a terrestrial solar cell system.

FIG. 1D is a perspective view of an implementation of a support frame for use with the terrestrial solar cell system of FIG. 1C.

FIG. 1E is a simplified side view of an implementation of a terrestrial solar cell system.

FIG. 1F is a side view of an implementation of a terrestrial solar cell system.

FIG. 2 is a perspective view of the solar cell system implementation of FIG. 1A viewed from the opposite side thereof.

FIG. 3 is a perspective view of a portion of an implementation of a solar cell subarray utilized in a terrestrial solar cell system.

FIG. 4 is a perspective view of an implementation of a solar cell receiver utilized in a solar cell subarray.

FIG. 5 is a top plan view of a single solar cell subarray.

Additional advantages and features will become apparent to those skilled in the art from this disclosure, including the following detailed description. While the invention is described below with reference to implementations thereof, it should be understood that the invention is not limited to those implementations. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications and implementations in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of utility.

DETAILED DESCRIPTION

Overview

A terrestrial solar power system converts sunlight into electrical energy utilizing, e.g., multiple mounted arrays spaced in a grid over the ground. The array of solar cells has a particular optical size and aspect ratio (e.g., between 1:3 and 1:5), and is mounted for unitary movement on a cross-arm of a vertical support that tracks the sun. The array can include subarrays, sections, modules and/or panels.
The solar tracking mechanism allows the plane of the solar cells to continuously face the sun as the sun traverses the sky during the day, thereby optimizing the amount of sunlight impinging upon the cells. The amount of power generated by the array is directly related to the amount of sunlight impinging upon the constituent solar cells. Since a given array can comprise many (e.g., a thousand or more) solar cells, it is advantageous to maintain the solar alignment of the entire array. This, however, is difficult in practice because it is not uncommon for an array to be upwards of 18 meters wide (about 59 feet) and 7.5 meters high (about 25 feet). Given the size of the array, solar cells near the perimeter may become misaligned due to bending or flexing of the array. Bending or flexing can arise, e.g., as a result of wind or the weight of the array causing the structure to bend. Since misalignment as little as one degree or less is detrimental in some implementations, it is desirable to minimize bending or flexing of the array.

Implementations of a Terrestrial Solar Cell System

An implementation of a terrestrial solar cell system is illustrated in FIG. 1A. In general terms, the system comprises three major components. The first major component is the central support (11 a and 11 b). The central support is mounted to a surface and is capable of rotating about its longitudinal axis. Depending on the implementation, the surface can be, e.g., the ground or a concrete foundation formed in the ground. Disposed on or adjacent to the surface is a drive mechanism 100 (e.g., a gearbox) that couples to the central support. The drive mechanism 100 enables the inner member 11 b to rotate relative to the outer member 11 a, e.g., for moving the solar cell array such that it tracks the sun.
The second major component is the support frame 15. The support frame 15 couples to the central support and is adapted to support a solar cell array (e.g., array 10). The third major component is the solar cell array 10. The solar cell array 10 includes multiple subarrays or panels 16 and is coupled to, and supported by, the support frame 15. The solar cell array 10 converts sunlight into electricity, and normally is kept facing the sunlight by the rotation of the central support. In this implementation, each of the solar cell subarrays 16 is divided into thirteen sections 17. Each section 17 includes a 2×7 panel of concentrating lenses (e.g., item 320 of FIG. 3) each lens disposed over a single receiver (e.g., item 19 b of FIGS. 3 and 4). The receiver, a printed circuit or subassembly, includes a single III-V compound semiconductor solar cell together with additional circuitry such as a bypass diode (not shown). In some implementations, each section 17 is a module, e.g., a discrete assembly. In some implementations, the sections 17 are separated from each other by perforated dividers.
In the illustrated implementation, the central support includes an outer member 11 a and an inner member 11 b. The outer member 11 a is connectable to a support mounted on the surface by bolts. The inner member 11 b is rotatably mounted within the member 11 a and supports a cross member 14 which is connected to a support frame 15. The support frame 15 also is supported on the inner member 11 b by a pair of inclined arms 14 a which extend respectively from two of the support struts 150 b (visible in FIG. 1B) to the base of the inner member 11 b. The inclined arms 14 a are coupled to each other by a cross-member 14 b (see also FIG. 1B) that increases their structural integrity. The mounting of the support frame 15 in this manner ensures that it is fixed to the inner member 11 b of the central support in such a manner that it is rotatable about its central longitudinal axis through members 11 a and 11 b.
The support frame 15 has a rectangular frame 15 a and a truss 15 b. The rectangular frame 15 a includes two shorter members (see items 15 a 3 and 15 a 4 of FIG. 1B) that are oriented in a direction parallel to the height (see dimension “C” of FIG. 1B) of the solar cell array 10 and two longer members (see items 15 a 1 and 15 a 2 of FIG. 1B) that are oriented in a direction parallel to the width (see dimension “A” of FIG. 1B) of the solar cell array 10. In this implementation, the width of the rectangular frame 15 a is approximately equal to the width of the solar cell array 10. Although this configuration can result in improved rigidity (e.g., less bending of the solar cell array 10 near its perimeter), it is not required. For example, to reduce material cost, the width of the rectangular frame 15 a can be reduced.
The truss 15 b is coupled to the rectangular frame 15 a in a manner that increases the rigidity of the rectangular frame 15 a, and thus, the rigidity of the solar cell array 10. The truss, therefore, improves alignment of the constituent solar cells (particularly those near the perimeter) such that power generation is substantially improved. The truss 15 b can function to prevent deflection greater than 1 degree near the perimeter of the solar cell array 10. In some implementations, the truss 15 b is aligned with
In this implementation, the truss 15 b includes a lower truss chord 152 d, an upper truss chord 152 c, parallel truss brace chords 152 b and diagonal truss chords 152 a. The parallel truss brace chords 152 b and diagonal truss chords 152 a are coupled between the upper and lower truss chords 152 c and 152 d. The parallel truss brace chords 152 b are oriented substantially parallel to one another and perpendicular to the upper and lower truss chords 152 c and 152 d. The particular configuration of chords 152 a-d can vary with the implementation. For example, truss 15 b may include no diagonal truss chords (e.g., a Vierendeel truss), no parallel truss brace chords (e.g., a lattice truss), or the relative orientation of the diagonal truss chords can vary (e.g., a Pratt truss or a Howe truss).
In this implementation, the truss 15 b is coupled to the rectangular frame 15 a by truss support members 151 a. Also, in this implementation the rectangular frame 15 a and truss 15 b are integrated, i.e., the lower truss chord 152 d comprises one of the longer members of the rectangular frame 15 a. In this implementation, the width of the truss 15 b (e.g., the width of the lower chord 152 d) is approximately equal to the width of the solar cell array 10 and the rectangular frame 15 a. Although this configuration can result in improved rigidity (e.g., less bending of the solar cell array 10 near its perimeter), it is not required. For example, to reduce material cost, the width of the truss 15 b can be reduced.
In this implementation, the truss 15 b is arranged such that the direction of its height (i.e., the perpendicular direction between the lower truss chord 152 d and the upper truss chord 152 c) is substantially orthogonal to the plane defined by the height and width of the solar cell array 10. Although this configuration can result in improved rigidity, it is not required. For example, to accommodate packaging requirements, the truss 15 b can be coupled such that the direction of its height is not substantially orthogonal to the plane defined by the height and width of the solar cell array 10.
In the illustrated implementation, the truss 15 b is not disposed in the vertical center (i.e., along dimension “C” of FIG. 1B) of the solar cell array 10. The inventors discovered that placing the truss 15 b above the vertical centerline of the solar cell array 10 can result in improved maneuverability with respect to the center support. As a result, the central support can move the solar cell array 10 to track sunlight without interference by the presence of the truss 15 b.
Although the illustrated implementation utilizes a truss 15 b to increase the rigidity of the rectangular frame 15 a, other structures are possible. For example, a solid plate can be used. In another example, a solid plate having one or more cutouts can be used. Moreover, a very simple truss can be used that omits chords 152 a and 152 b in favor of simply coupling upper truss chord 152 c to the lower truss chord 152 d. Such a truss can include one or more additional members that are oriented parallel to the upper truss chord 152 c.
FIG. 1B is a rear-facing view of the terrestrial solar cell system of FIG. 1A, with the solar cell array 10 oriented orthogonally to the surface to which the central support is mounted (e.g., the ground). As illustrated, the truss 15 b aligned along the greatest perpendicular dimension (i.e., along dimension “A”) of the array 10. This is advantageous because the array is generally more prone to bending along a longer axis than along a shorter axis (e.g., along dimension “C”). In this implementation, dimension “A”, the width of the solar cell array 10, is approximately 18.1 meters (approximately 59.4 feet), dimension “B”, the width of subarray 16, is approximately 1.8 meters (approximately 5.9 feet) and dimension “C”, the height of the solar cell array 16, is approximately 7.5 meters (approximately 24.6 feet). Such an implementation has a solar collecting area of approximately 98.95 square meters (approximately 1065.1 square feet) and weighs approximately 10,191 kilograms (approximately 10.03 tons). If constructed in a manner consistent with this disclosure, such an implementation can have a wind survival rating of 145 kilometers/hour (approximately 90.1 miles/hour).
In FIG. 1B, the view of the truss 15 b is largely obscured because it is arranged orthogonally to the plane defined by the height and width of the solar cell array. However, this view illustrates truss support members 151 a, which couple the truss 15 b to the rectangular frame 15 a. In particular, the truss support members 151 couple to a long member 15 a 1 or 15 a 2 of the rectangular frame 15 a (in this implementation, the lower long member 15 a 2) and the upper truss chord 152 c (see FIG. 1A). In this implementation, four truss support members 151 a are shown arranged diagonally. While arranging the truss support members 151 a diagonally offers the advantage of resisting tension and compression, it is not necessary. Also, more or fewer truss support members 151 a can be employed depending on the implementation.
This view also reveals additional features of the rectangular frame 15 a. To improve the structural integrity of the rectangular frame, several cross members 150 a couple the upper long member 15 a 1 to the lower longer member 15 a 2. The cross members 150 a are complemented by parallel members 150 b (which, in this implementation, are oriented substantially parallel to the shorter members 15 a 3 and 15 a 4). Two of the parallel members 150 b serve the additional purpose of providing a mounting point to which the cross member 14 couples.
This view again illustrates that the width of the rectangular frame 15 a is approximately the same width as the solar cell array 10 (i.e., it is about 18.1 meters wide). This view also illustrates that the location of the truss 15 b is above the centerline of dimension C.
FIG. 1C illustrates an implementation of a terrestrial solar cell system with the plane defined by the height and width of the solar cell array 10 oriented parallel to the surface to which the central support is mounted (e.g., the ground). This implementation utilizes a truss 15 b′ having a configuration slightly different than that of 15 b. This truss 15 b′ omits parallel truss brace chords 152 b in favor of using all diagonal truss chords 152 a. FIG. 1D illustrates a perspective view of a support frame 15 comprising truss 15 b′.
FIG. 1E is simplified view of a terrestrial solar cell system, viewed from a direction orthogonal to the plane defined by the height and width of the solar cell array 10. As illustrated, the truss (15 b or 15 b′ depending on the implementation) is disposed above the centerline of dimension C. Also, the truss (15 b or 15 b′) in this implementation is oriented at a right angle (θ) relative to the solar cell array 10.
FIG. 1F is a side view of an implementation of a terrestrial solar cell system, viewed from a direction orthogonal to the plane defined by the height and width of the solar cell array 10. As illustrated, the truss (15 b or 15 b′ depending on the implementation) is disposed above the centerline of dimension C. By locating the truss above the vertical center of the solar cell array, the truss does not obstruct movement of the array relative to the central support (11 a, 11 b). Jackscrew 111 and mating threaded rod 112 together can adjust the angle of the array 10 through at least a portion of the range indicated by path 113. Thus, the jackscrew 111 (e.g., in combination with a drive mechanism such as item 100 of FIG. 1A) enables pivoting the support frame 15, and thus the array 10, so as to adjust its angle with respect to the earth's surface
FIG. 2 is a perspective view of the solar cell system implementation of FIG. 1A viewed from the opposite side thereof. This perspective illustrates the division of each subarray 16 into sections 17. Each section 17 includes a base 18, which provides a structural foundation for each receiver 19 (see FIGS. 3 and 4). In some implementations, there is one base 18 per subarray 16, shared by each constituent section 17. In some implementations, the base 18 is structurally distinct for each section 17.
FIG. 3 is a cutaway view of a solar cell subarray 16 depicting one section 17 on base 18. In this implementation, section 17 includes a sheet 320 including a 2×7 matrix of Fresnel lenses (20 a-20 j are shown), a 2×7 matrix of secondary optical elements (“SOE”, an example of which is shown as item 201) and a 2×7 matrix solar cell receivers 19 (fourteen are shown, including items 19 a-19 j). In some implementations, the sheet 320 is an integral plastic panel and each Fresnel lens (e.g., items 20 a-20 j) is a nine-inch square. In the illustrated implementation, each Fresnel lens (e.g., 20 b) and its associated receiver (e.g., 19 b) and SOE (e.g., 201) are aligned such that the light concentrated by the lens is optimally received by the solar cell of the associated receiver. In the illustrated implementation, section 17 is delineated from the remainder of the base 18 by a divider 301 (which can be perforated). The base 18 also which serves to dissipate heat from the receivers, and more particularly from the individual solar cells.
FIG. 4 illustrates a receiver 19 b in more detail. The receiver 19 b has a plate 203, a printed circuit board (“PCB”) 204, an SOE 201 and a mount 202. The plate 203 couples the receiver 19 b to the base 18 (see FIGS. 2 and 3). In some implementations, the plate 203 is constructed of a material having a high thermal conductivity such that the heat from the PCB 204 (which includes, for example, a solar cell and a bypass diode) is dissipated away efficiently. In some implementations, the plate 203 is made of aluminum. In some implementations, the PCB 204 includes a ceramic board with printed electrical traces.
The mount 202, which is coupled to the plate 203 at two positions, forms a bridge that aligns the SOE 201 with the solar cell of the PCB 204. The SOE 201 gathers the light from its associated lens 20 and focuses it into the solar cell on the PCB 204. In some implementations, each solar cell receiver 19 is provided with a corresponding SOE 201. The SOE 201 includes an optical inlet 201 a and optical outlet (facing the PCB 204) and a body 201 b. The SOE 201 is mounted such that the optical outlet is disposed above the solar cell of the PCB 204. Although it can vary depending on the implementation, the SOE 201 in the illustrated example is mounted such that the optical outlet is about 0.5 millimeters from the solar cell. The SOE 201 (including the body 201 b) can be made of metal, plastic, or glass or other materials.
In some implementations, the SOE 201 has a generally square cross section that tapers from the inlet 201 a to the outlet. The inside surface 201 c of the SOE reflects light downward toward the outlet. The inside surface 201 c is, in some implementations, coated with silver or another material for high reflectivity. In some cases, the reflective coating is protected by a passivation coating such as SiO2 to protect against oxidation, tarnish or corrosion. The path from the optical inlet 201 a to the optical outlet forms a tapered optical channel that catches solar energy from the corresponding lens 20 and guides it to the solar cell. As shown in this implementation, the SOE 201 has four reflective walls. In other implementations, different shapes (e.g., three-sided to form a triangular cross-section) may be employed.
In some cases, the corresponding lens 20 does not focus light onto a spot that is of the dimensions of the solar cell or the solar tracking system may not perfectly point to the sun. In these situations, some light does not reach the solar cell. The reflective surface 201 c directs light to the solar cell 30. The SOE also can homogenize (e.g., mix) light. In some cases, it also has some concentration effect.
In some implementations, the optical inlet 201 a is square-shaped and is about 49.60 mm×49.60 mm, the optical outlet is square-shaped and is about 9.9 mm×9.9 mm and the height of the optical element is about 70.104 mm. These dimensions can vary with the design of the solar cell module, section and/or the receiver. For example, in some implementations the dimensions of the optical outlet are approximately the same as the dimensions of the solar cell. For an SOE having these dimensions, the half inclination angle is 15.8 degrees.
In a particular implementation, as illustrated in the plan view of FIG. 5, the subarray 16 is about 7.5 meters high (y direction) and 1.8 meters wide (x direction) and includes sections 17 each having a 2×7 matrix of Fresnel lenses 20 and receivers 19 (see FIGS. 3 and 4). Each receiver 19 produces over 13 watts of DC power on full AM 1.5 solar irradiation. The receivers are connected by electrical cables in parallel or in series so that the aggregate 182 receivers in an entire subarray 16 can produce in excess of 2500 watts of peak DC power. Each of the subarrays is in turn connected in series, so that a typical array (e.g., item 10) can produce in excess of 25 kW of power.
A motor provides drive to rotate the member 11 b relative to the member 11 a, and another motor provides drive to rotate the cross member 14 (and hence the support frame 15) relative to the central support 11 about its longitudinal axis. Control means are provided (e.g., disposed in drive mechanism 100 of FIG. 1) for controlling rotation of the member 11 b relative to the member 11 a, and for controlling rotation of the cross member 14 (and the support frame 15) about its axis to ensure that the planar exterior surface of each of the sections 17 comprising Fresnel lenses 20 is orthogonal to the sun's rays. In some implementations, the control means is a computer controlled machine, using software that controls the motors in dependence upon the azimuth and elevation of the sun relative to the system. In some implementations, each of the Fresnel lenses 20 concentrates incoming sunlight onto the solar cell in an associated receiver (e.g., item 19 b) by a factor of over 500X, thereby enhancing the conversion of sunlight into electricity with a conversion efficiency of over 37%. In some implementations, the concentration is 520×.
In some implementations, the system is refractive and uses an acrylic Fresnel lens to achieve 520× concentration with an f# of approximately 2. A reflective secondary optical element can be used, as described in connection with FIG. 4. An acceptance angle for an individual cell/optics system is +/−1.0 degrees. The efficiency of the optical system on-sun is 90% with the acceptance angle defined at a point where the system efficiency is reduced by no more than 10% from its maximum. Some implementations, however, may define a different acceptance angle, e.g. +/−0.1 degrees. In some implementations, each solar cell is assembled in a ceramic package that includes a bypass diode and a two spaced-apart connectors. In some implementations, 182 cells are configured in a subarray. The number of cells in a subarray are specified so that at maximum illumination, the voltages added together do not exceed the operational specifications of the inverter.
Additional details of an example of the design of the receiver are described in U.S. patent application Ser. No. 11/849,033 filed Aug. 31, 2007 and herein incorporated by reference.
Additional details of an example of the design of the semiconductor structure of the triple junction III-V compound semiconductor solar cell receiver (e.g., item 19) are described in U.S. application Ser. No. 12/020,283, filed Jan. 25, 2008 herein incorporated by reference.
In the illustrated example, the solar cell is a triple junction device, with the top junction based on InGaP, the middle junction based on GaAs, and the bottom junction based on Ge. Typical band-gaps for the cell are 1.9 eV/1.4 eV/0.7 eV, respectively. Typical cell performance as a function of temperature indicate that Voc changes at a rate of −5.9 mV/C and, with respect to temperature coefficient, the cell efficiency changes by −0.06%/C absolute.
One electrical contact is typically placed on a light absorbing or front side of the solar cell, and a second contact is placed on the back side of the cell. A photoactive semiconductor is disposed on a light-absorbing side of the substrate and includes one or more p-n junctions, which creates electron flow as light is absorbed within the cell. Grid lines extend over the top surface of the cell to capture this electron flow which then connect into the front contact or bonding pad. It is advantageous to maximize the number of grid lines over the top surface of the cell to increase the current capacity without adversely interfering with light transmission into the active semiconductor area.
While implementations have been illustrated and described as embodied in a solar cell array using III-V compound semiconductors, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention. Accordingly, other implementations are within the scope of the claims.

Claims

1. A concentrator photovoltaic solar cell array system for producing energy from the sun using one or more sun-tracking solar cell arrays, the system comprising:

a central support mountable on a surface, and rotatable about its central longitudinal axis;

a substantially planar solar cell array including a plurality of triple junction III-V semiconductor compound solar cell receivers;

a support frame coupled to the solar cell array and carried by and rotatable with respect to the central support about an axis orthogonal to the central longitudinal axis, the support frame comprising:

a first frame assembly coupled to the solar cell array; and

a second frame assembly coupled to the first frame assembly arranged to increase the rigidity of the first frame assembly; and

an actuator for rotating the central support and the support frame so that the solar cell array is maintained substantially orthogonal to the rays from the sun as the sun traverses the sky.

2. A system as claimed in claim 1 wherein the second frame assembly comprises a truss.

3. A system as claimed in claim 1 wherein the second frame assembly comprises two shorter beams and one longer beam, wherein a first end of each shorter beam is coupled to the first frame assembly and a second end of each shorter beam is coupled to the longer beam.

4. A system as claimed in claim 1 wherein the second frame assembly is arranged along the greatest perpendicular dimension of the solar cell array.

5. A system as claimed in claim 2 wherein the truss is arranged along the greatest perpendicular dimension of the solar cell array.

6. A system as claimed in claim 2 wherein the truss comprises a lower chord, an upper chord substantially parallel to the lower chord, two or more substantially parallel brace chords coupled to the upper and lower chords, and at least one diagonal chord disposed between two brace chords and coupled to the upper and lower chords.

7. A system as claimed in claim 2 wherein the truss comprises a lower chord, an upper chord substantially parallel to the lower chord, and at least one diagonal chord coupled to the lower chord and upper chord.

8. A system as claimed in claims 6 or 7 wherein the lower chord comprises at least a portion of the first frame assembly.

9. A system as claimed in claims 6 or 7 wherein the first frame assembly comprises a generally rectangular frame member comprising upper and lower parallel members oriented in a direction substantially parallel to the surface to which the center support is mountable, wherein the upper chord is coupled to the lower parallel member by at least one truss support member.

10. A system as claimed in claim 6 wherein the brace chords are arranged substantially orthogonal to a plane defined by the solar cell array.

11. As system as claimed in claim 7 wherein the direction of the perpendicular distance from the lower chord to the upper chord is substantially orthogonal to a plane defined by the solar cell array.

12. A system as claimed in claims 6 or 7 wherein the width of the lower chord is substantially the same as the width of the solar cell array, wherein the width of the solar cell array is measured in a direction substantially parallel to the surface to which the central support is mountable.

13. A system as claimed in claim 1 wherein the second frame assembly is arranged in a direction orthogonal to a plane defined by the first frame assembly.

14. A system as claimed in claim 2 wherein the truss is arranged in a direction orthogonal to a plane defined by the first frame assembly.

15. A system as claimed in claim 1 wherein the second frame assembly is arranged in a direction orthogonal to a plane defined by the solar cell array.

16. A system as claimed in claim 2 wherein the truss is arranged in a direction orthogonal to a plane defined by the solar cell array.

17. A system as claimed in claim 1 wherein the width of the first frame assembly and the width of solar cell array are substantially the same, wherein the width of the solar cell array is measured in a direction substantially parallel to the surface to which the central support is mountable.

18. A system as claimed in claim 2 wherein the truss is mounted above the vertical center of the solar cell array.

19. A system as claimed in claim 1, wherein the solar cell array comprises a plurality of solar cells and a corresponding plurality of Fresnel lenses each of which is disposed over a single solar cell for concentrating by a factor in excess of 500× the incoming sunlight onto the solar cell and producing in excess of 13 watts of DC power at AM 1.5 solar irradiation per solar cell with conversion efficiency in excess of 37%.

20. A system as claimed in claim 1, wherein the central support is constituted by a first member provided with means for mounting the central support on the surface, and a second member rotatably supported by, and extending upwardly from, the first member.

21. A system as claimed in claim 1, wherein the support frame is mounted on a cross member which is rotatably mounted with respect to the second member of the central support about an axis orthogonal to the central longitudinal axis.

22. A system as claimed in claim 1, wherein the first frame assembly comprises a generally rectangular frame member having a plurality of parallel support struts that are parallel to the shorter sides of the rectangular frame member.

23. A system as claimed in claim 22, wherein the first frame assembly further comprises a plurality of oblique support struts.

24. A system as claimed in claim 1 wherein the solar cell array is arranged to produce approximately 25 kW peak DC power on full illumination.

25. A system as claimed in claim 1 comprising at least one inclined strut coupled to the central support and the first frame assembly.

26. A concentrator photovoltaic solar cell array system for producing energy from the sun using a plurality of sun-tracking solar cell arrays, comprising:

a central support mountable on a surface and rotatable about its central longitudinal axis;

a rectangular solar cell array having an aspect ratio between 1:3 and 1:5, with the longitudinal of the array substantially parallel to the surface for producing in excess of 25 kW peak DC power on full illumination and including a plurality of triple junction III-V semiconductor compound solar cell receivers mounted on the support frame; and

a first frame assembly coupled to the solar cell array; and

means for increasing the rigidity of the first frame assembly; and

27. The system of claim 26 wherein the means for increasing the rigidity of the first frame assembly prevents a deflection greater than 1 degree near the perimeter of the solar cell array.

28. A concentrator photovoltaic solar cell array system for producing energy from the sun using one or more sun-tracking solar cell arrays, the system comprising:

a central support mountable on the ground, and being capable of rotation about its central longitudinal axis;

a rectangular solar cell array for producing in excess of 25 kW peak DC power on full illumination including a plurality of triple junction III-V semiconductor compound solar cell receivers;

a support frame coupled to solar cell array and carried by and rotatable with respect to the central support about an axis orthogonal to the central longitudinal axis, the support frame comprising:

a rectangular frame assembly coupled to the solar cell array comprising first and second long members aligned in a direction substantially parallel to the direction of the largest perpendicular dimension of the solar cell array and comprising first and second short members aligned in a direction substantially perpendicular to the direction of the largest perpendicular dimension of the solar cell array; and

a truss assembly coupled to at least one of the first and second long members, the truss assembly comprising a lower chord, an upper chord substantially parallel to the lower chord, and at least one diagonal chord coupled to the lower chord and upper chord; and

29. The system of claim 28 wherein the lower chord comprises one of the first and second long members.

30. The system of claim 28 wherein the direction of the perpendicular distance from the lower chord to the upper chord is substantially orthogonal to a plane defined by the solar cell array.

31. The system of claim 28 wherein the width of the rectangular frame assembly is approximately the same as the largest perpendicular dimension of the solar cell array.

32. The system of claim 28 wherein the truss assembly is located above the vertical center of the solar cell array.

33. The system of claim 28 wherein the first and second long members and first and second short members are formed as a unitary structure.