WO2011127572A1

WO2011127572A1 - Solar concentrators, solar collectors and methods of making same

Info

Publication number: WO2011127572A1
Application number: PCT/CA2011/000407
Authority: WO
Inventors: John Robert Mumford
Original assignee: John Robert Mumford
Priority date: 2010-04-13
Filing date: 2011-04-12
Publication date: 2011-10-20

Abstract

A solar concentrator housing comprises a monolithic hollow member of unitary construction. The monolithic hollow member comprises an optically transmissive portion to admit incoming solar radiation into the hollow member, and at least one reflection portion shaped and positioned to direct incoming solar radiation admitted into the hollow member onto a target region, with the shape and position of the at least one reflection portion defined by an overall shape of the monolithic hollow member. Preferably, the monolithic hollow member is shaped to form a Cassegrain concentrator including both primary and secondary reflector portions.

Description

SOLAR CONCENTRATORS, SOLAR COLLECTORS AND METHODS OF MAKING

SAME

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to United States Provisional Application No.

61/323838 filed on April 13, 2010 and United States Provisional Application No. 61/427177 filed on December 25, 2010, the teachings of each of which are hereby incorporated by reference.

FIELD OF INVENTION

[0002] The present invention relates to solar energy converters that turn electromagnetic radiation from the sun into usable electric energy.

BACKGROUND OF THE INVENTION

[0003] Several types of solar energy collection and conversion devices exist. These devices are used in harnessing solar radiation to produce useful energy, such as by heating of a fluid or generation of electric power. A subset of these devices collects solar radiation onto a semiconductor material where that radiation is converted into electricity by a photovoltaic process.

[0004] Photovoltaic converters can include flat panel photovoltaic devices made of mono- crystalline silicon and polycrystalline silicon and thin flexible film photovoltaic converters made from cadmium/telluride. These types of photovoltaic converters are reasonably inexpensive, durable and easy to manufacture. The semiconductor materials used in these photovoltaic converters are generally capable of converting solar energy into usable electricity at a rate of 10-20% efficiency. Another class of semiconductors comprise those manufactured from Group III-V elements including germanium, gallium arsenide, and gallium indium arsenic phosphide, which can convert solar energy into electricity at significantly higher efficiency. The problem with these group III-V semiconductors is that they are much more expensive to manufacture than those made of mono-crystalline silicon, polycrystalline silicon and cadmium/telluride. [0005] To overcome this limitation, manufacturers have developed solar concentrators comprising various combinations of mirrors and lenses that focus light from a large receiving region onto a small, group III-V semiconductor converter. The objective of these combined solar collection and conversion devices is to achieve an overall improved cost and efficiency by combining large inexpensive solar energy collection with small very efficient solar energy conversion.

[0006] One problem with many known concentrating solar collectors is that they are manufactured from multiple discrete assemblies that have to be carefully assembled and aligned to ensure a good focus of the solar energy onto the solar converter. A further problem is that wear and tear in the field caused by transport, installation, thermal cycling, wind vibration and other issues can cause these alignments to drift over time, resulting in reduced conversion efficiency and possible failure of the solar collector.

[0007] Additionally, these often complex assemblies can have pathways into their structures that allow moisture, dirt and other contaminants to collect which can impede the light gathering and focusing efficiency and thereby degrade the performance of the photovoltaic converter. These contaminant pathways may not be apparent at the time of manufacturing but may develop over time with the degradation of adhesives or de-lamination of film layers. In order to overcome these deficiencies, it is often necessary to implement aggressive and expensive maintenance programs that reduce the appeal of the concentrating solar collection systems by reducing the cost effectiveness of solar energy.

SUMMARY OF THE INVENTION

[0008] The present invention is directed to a solar concentrator housing comprising a monolithic hollow member of unitary construction. The monolithic hollow member comprises an optically transmissive portion to admit incoming solar radiation into the hollow member, and at least one reflection portion shaped and positioned to direct incoming solar radiation admitted into the hollow member onto a target region, with the shape and position of the at least one reflection portion defined by an overall shape of the monolithic hollow member. The housing may comprise at least one additional reflection portion secured to an interior of the housing. Preferably, the monolithic hollow member is shaped to form a Cassegrain concentrator including both primary and secondary reflector portions.

[0009] In one aspect, the present invention is directed to a Cassegrain solar concentrator housing comprising a transparent hollow member. The transparent hollow member comprises a concave primary reflector portion and an optically transmissive planar cover member portion opposed to the concave primary reflector portion. The hollow member is of one-piece, unitary, monolithic construction and has a first seamless transition between the primary reflector portion and the cover member portion. Preferably, the solar concentrator housing further comprises a secondary reflector portion of convex curvature spaced from and opposed to the primary reflector portion and focally aligned with the primary reflector portion. In a particularly preferred implementation, the secondary reflector portion extends from an interior surface of the cover member portion and is integrally formed therewith as part of the hollow member, and the hollow member has a second seamless transition between the cover member portion and the secondary reflector portion.

[0010] In an embodiment, the hollow member further comprises a seamless transition portion between the primary reflector portion and the cover member portion, with the primary reflector portion continuing seamlessly into the transition portion and the transition portion continuing seamlessly into the cover member portion. The seamless transition portion may be substantially cylindrical.

[0011] In an embodiment, the primary reflector portion has a central aperture defined therethrough and a neck portion depends from the central aperture away from the cover member portion.

[0012] A solar concentrator housing as described above may be incorporated into a solar concentrator in which the primary reflector portion has a first reflective surface defining a primary reflector of a Cassegrain optical system and the secondary reflector portion has a second reflective surface defining a secondary reflector of the Cassegrain optical system. The primary reflector and the secondary reflector are arranged relative to one another so that incoming substantially collimated solar radiation passes through the cover member, is reflectively directed by the primary reflector onto the secondary reflector and reflectively directed by the secondary reflector toward a Cassegrain target region. The first reflective surface may be formed by a first reflective coating on an interior surface of the primary reflector portion and the second reflective surface may be formed by a second reflective coating on an interior surface of the secondary reflector portion.

[0013] The above-described solar concentrator may be incorporated into a solar collector, in which at least one solar conversion cell is disposed in the housing, in the Cassegrain target region, so as to receive the directed solar radiation from the secondary reflector. Electrical connectors are in electrical communication with the at least one solar conversion cell and extend from inside the housing to outside the housing through the neck portion, and the housing is hermetically sealed with the solar conversion cell disposed therewithin. In a preferred embodiment, there is an inert environment in the housing, with the inert

environment being selected from the group consisting of vacuum, noble gas, and non- oxidizing non-noble gas.

[0014] In another aspect, the present invention is directed to a method for fabricating a monolithic Cassegrain solar concentrator from glass. The method comprises closing at least two heated mold portions around a hollow generally ellipsoidal pliable glass parison having substantially uniform wall thickness and rotating about a substantially horizontal axis thereof to define a heated mold substantially completely surrounding the parison except for a parison aperture opening into an interior volume of the parison. The mold has a mold cavity whose shape defines a concave primary reflector region of the mold cavity and a cover member region of the mold cavity corresponding, respectively, to a primary reflector and a cover member of the Cassegrain solar concentrator, and has a plurality of vents for venting gas captured in the mold cavity to outside the mold cavity. The method further comprises injecting gas through the parison aperture into the interior volume of the parison to expand the parison so that the closed parison assumes the shape of the mold cavity and becomes a hollow member having a primary reflector portion and a cover member portion. The method additionally comprises opening the mold, removing the hollow member from the mold, and annealing the hollow member.

[0015] In a particular embodiment of the method, the Cassegrain solar concentrator being fabricated is a monolithic Cassegrain solar concentrator. In this embodiment, the shape of the mold cavity omits any secondary reflector region corresponding to a secondary reflector of the Cassegrain solar concentrator so that the cover member region of the mold cavity is substantially flat and the resulting cover member portion of the hollow member is

substantially flat and thereby omits a secondary reflector portion. The method further comprises, after injecting gas through the parison aperture, applying additional heat to an axially central part of the cover member portion so that the axially central part of the cover member portion becomes pliable and inserting a mold element axially through the parison aperture into an interior volume of the hollow member. The mold element has at its tip a secondary reflector region corresponding to a secondary reflector of the Cassegrain solar concentrator. The secondary reflector region comprises a concave surface facing an interior surface of the cover member portion and having a plurality of vacuum apertures defined therein. The method further comprises engaging the secondary reflector region of the mold element with the axially central part of the cover member portion while the additional heat is being applied thereto and applying a vacuum to the plurality of vacuum apertures to form the secondary reflector portion so that the secondary reflector portion is convex and the hollow member assumes a completed shape of the Cassegrain solar concentrator.

[0016] In a particular implementation, a solid rod of glass is welded to the parison prior to closing the mold portions around the parison and is hence welded to the axially central part of the cover member portion when the additional heat is applied to the axially central part of the cover member portion. The solid rod of glass is horizontally coaxial with the parison and rotating synchronously therewith. In this embodiment, the method further comprises moving the rod toward the mold element to move the pliable axially central part of the cover member portion into engagement with the secondary reflector region of the mold element before applying the vacuum to the plurality of vacuum apertures to form the secondary reflector portion. The parison may be formed by heating an open end of a glass tube to become molten and closing the open end of the glass tube to create a closed end, welding the glass rod to the closed end so that the glass rod is coaxial with the glass tube to form a glass punty joint, cooling the glass punty joint below a softening point of the glass, heating a first longitudinal region of the glass tube spaced from the glass punty joint so that the first longitudinal region becomes pliable, moving the glass rod away from the first longitudinal region to neck down the first longitudinal region, cooling the necked down first longitudinal region below the softening point of the glass, heating a second longitudinal region disposed between the necked down first longitudinal region and the closed end to become pliable, and, while the second longitudinal region is pliable, injecting gas into the tube while advancing the glass rod toward the necked down first longitudinal region to expand the pliable second longitudinal region so that the second longitudinal region and the closed end assume a hollow generally ellipsoidal shape and become the parison. Injecting gas into the tube while advancing the glass rod toward the necked down first longitudinal region to expand the pliable second longitudinal region into the hollow generally ellipsoidal parison with substantially uniform wall thickness may comprise alternately increasing gas pressure and incrementally advancing the glass rod toward the necked down first longitudinal region.

[0017] In one embodiment, after annealing the hollow member, the method may further comprise applying a first reflective coating to an exterior surface of the primary reflector portion of the hollow member and applying a second reflective coating to an interior surface of the secondary reflector portion of the hollow member to form the Cassegrain solar concentrator.

[0018] In another embodiment, after annealing the hollow member, the method may further comprise applying a first reflective coating to an interior surface of the primary reflector portion of the hollow member and applying a second reflective coating to an interior surface of the secondary reflector portion of the hollow member to form the Cassegrain solar concentrator.

[0019] In an embodiment of the method, the primary reflector region of the mold cavity has a surface shape that accommodates a wall thickness of the parison during gas injection to form the hollow member so that the interior surface of the primary reflector portion of the hollow member defines a predetermined concave shape. The wall thickness of the parison during gas injection to form the hollow member may vary along a longitudinal axis of the mold.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein:

Figure 1 is a perspective view of a first embodiment of a solar concentrator, according to an aspect of the present invention;

Figure 2 is a side schematic view of the solar concentrator of Figure 1 ;

Figure 3 is a perspective cross-sectional view of the solar concentrator of Figure 1 ;

Figure 4 is a side cross-sectional view of the solar concentrator of Figure 1 ;

Figure 5 is a top view of the solar concentrator of Figure 1 ;

Figure 6 is a bottom view of the solar concentrator of Figure 1 ;

Figure 7 is a perspective view of a first embodiment of a solar collector, incorporating the solar concentrator of Figure 1 , according to an aspect of the present invention;

Figure 8 is a side schematic view of the solar collector of Figure 7;

Figure 9 is a perspective cross-sectional view of the solar collector of Figure 7;

Figure 10 is a side cross-sectional view of the solar collector of Figure 7;

Figure 11 is a top view of the solar collector of Figure 7;

Figure 12 is a bottom view of the solar collector of Figure 7; Figures 13A and 13B are respective side and perspective cross-sectional views of a first version of the solar collector of Figure 7, showing optical paths of solar radiation;

Figures 14A and 14B are respective side and perspective cross-sectional views of a second version of the solar collector of Figure 7, showing optical paths of solar radiation;

Figure 15A is a perspective view showing schematically a first step in a process for making a hollow member for a solar concentrator housing, according to an aspect of the present invention;

Figure 15B is an offset top view showing schematically the first step in the process for making a hollow member;

Figure 16A is a perspective view showing schematically a second step in the process for making a hollow member;

Figure 16B is an offset top view showing schematically the second step in the process for making a hollow member;

Figure 17A is a perspective view showing schematically a third step in the process for making a hollow member;

Figure 17B is an offset top view showing schematically the third step in the process for making a hollow member;

Figure 18A is a perspective view showing schematically a fourth step in the process for making a hollow member;

Figure 18B is a partially exploded offset side view showing schematically the fourth step in the process for making a hollow member;

Figure 19A is a perspective view showing schematically a fifth step in the process for making a hollow member; Figure 19B is a partially exploded offset side view showing schematically the fifth step in the process for making a hollow member;

Figure 20A is a perspective view showing schematically a sixth step in the process for making a hollow member;

Figure 20B is a partially exploded offset side view showing schematically the sixth step in the process for making a hollow member;

Figure 21A is a perspective view showing schematically a seventh step in the process for making a hollow member;

Figure 21B is a partially exploded offset side view showing schematically the seventh step in the process for making a hollow member;

Figure 21C is a perspective view showing schematically an eighth step in the process for making a hollow member;

Figure 21D is a partially exploded offset side view showing schematically the eighth step in the process for making a hollow member;

Figure 22A is a perspective view showing schematically a ninth step in the process for making a hollow member;

Figure 22B is a partially exploded offset side view showing schematically the ninth step in the process for making a hollow member;

Figure 23A is a perspective view showing schematically a tenth step in the process for making a hollow member;

Figure 23B is a partially exploded offset side view showing schematically the tenth step in the process for making a hollow member; Figure 24A is a side view of a mold element for use in the process for making a hollow member;

Figure 24B is an end view of the mold element of Figure 24A;

Figure 24C is a side cross-sectional view of the mold element of Figure 24A;

Figure 25A is a side schematic view of an eleventh step in the process for making a hollow member;

Figure 25B shows a side schematic view of a thirteenth step in the process for making a hollow member;

Figures 26A and 26B are perspective cross-sectional views showing, respectively, first and second stages of a twelfth step in the process for making a hollow member;

Figure 27 is a side schematic view of a partially formed hollow member;

Figure 28 is a perspective view of a completed hollow member;

Figures 29 to 31 are schematic views showing an array of solar collectors carried by a solar tracking system in various orientations;

Figure 32 is a perspective view of a second embodiment of a solar concentrator, according to an aspect of the present invention;

Figure 33 is a side schematic view of the solar concentrator of Figure 32;

Figure 34 is a perspective cross-sectional view of the solar concentrator of Figure 32;

Figure 35 is a side cross-sectional view of the solar concentrator of Figure 32;

Figure 36 is a top view of the solar concentrator of Figure 32;

Figure 37 is a bottom view of the solar concentrator of Figure 32; Figure 38 is a perspective view of a second embodiment of a solar collector, incorporating the solar concentrator of Figure 32, according to an aspect of the present invention;

Figure 39 is a side schematic view of the solar collector of Figure 38;

Figure 40 is a perspective cross-sectional view of the solar collector of Figure 38;

Figure 41 is a side cross-sectional view of the solar collector of Figure 38;

Figure 42 is a top view of the solar collector of Figure 38;

Figure 43 is a bottom view of the solar collector of Figure 38;

Figures 44A and 44B are respective side and perspective cross-sectional views of a first version of the solar collector of Figure 38, showing optical paths of solar radiation;

Figures 45A and 45B are respective side and perspective cross-sectional views of a second version of the solar collector of Figure 38, showing optical paths of solar radiation;

Figure 46 is a perspective view of a lathe assembly for use in the method of forming a hollow member; and

Figure 47 is a flow chart showing a method for fabricating a monolithic Cassegrain solar concentrator from glass.

DETAILED DESCRIPTION

[0021] Aspects of the present invention are directed to solar concentrators and methods of making the same, and to solar collectors each comprising a solar concentrator and one or more solar conversion cells. As used herein, the term "solar conversion cell" refers to any device capable of converting solar radiation into electrical energy. As such, the term "solar conversion cell" includes photovoltaic cells.

[0022] Reference is now made to Figures 1 to 6, which show an exemplary solar concentrator 10 according to an aspect of the present invention, prior to assembly as part of a solar collector. The solar concentrator 10 is a Cassegrain concentrator, and comprises a housing 12 (Figures 7 to 12) together with reflective coatings, as described in greater detail below. The housing 12 comprises a transparent hollow member 14 of one-piece, unitary, monolithic construction, and a plug as described below. The hollow member 14 may be made of any suitable material, and is preferably made from glass having high solar transmissivity and low reflectivity, and may be made from BK7 borosilicate glass or PYREX brand glass offered by Corning Incorporated, having an address at One Riverfront Plaza, Corning, NY 14831 USA, or low lead glass manufactured by Asahi Glass Co., Ltd., having an address at 1-12-1, Yurakucho, Chiyoda-ku, Tokyo 100-8405 Japan. Glass and similar materials are

advantageous because they are durable and rigid, can be molded with a high degree of finish with little or no secondary polishing required, and exhibit low dimensional variation with temperature.

[0023] The hollow member 14 comprises a concave primary reflector portion 16 and an optically transmissive cover member portion 18 opposed to the concave primary reflector portion 16. In the illustrated embodiment, the cover member portion 18 is generally planar to facilitate the installation of a plurality of solar concentrators 10 into arrays, preferably protected by an impact-resistant cover sheet; in other embodiments the cover member portion 18 may have other suitable shapes; for example all or part of the cover member portion 18 may have a curvature.

[0024] The hollow member 14 has a seamless transition between the primary reflector portion 16 and the cover member portion 18. More particularly, in the illustrated embodiment the hollow member 14 comprises a seamless transition portion 20 which is generally cylindrical in shape and extends between the primary reflector portion 16 and the cover member portion 18. The primary reflector portion 16 continues seamlessly into the transition portion 20, and the transition portion 20 continues seamlessly into the cover member portion 18.

[0025] The solar concentrator housing 12 further comprises a secondary reflector portion 22 of convex curvature, relative to the primary reflector portion 16. The secondary reflector portion 22 is spaced from and opposed to the primary reflector portion 16 and is focally aligned with the primary reflector portion 16. The term "locally aligned", as used herein, means that the secondary reflector portion 22 is positioned to have light focused on it by the primary reflector portion 16, as described in greater detail below. In the illustrated embodiment shown in Figures 1 to 6 and 7 to 12, the secondary reflector portion 22 is formed as an inward projection from the cover member portion 18 and is integrally formed therewith as part of the monolithic hollow member 14. As such, the hollow member 14 has a seamless transition 26 between the cover member portion 18 and the secondary reflector portion 22. The indentation 27 in the outer surface of the cover member portion 18 defined by the secondary reflector portion 22 may be filled with a suitable material so that the exterior surface of the cover member portion 18 is smooth. In other embodiments, the secondary reflector portion may be a separate piece which may be secured to the interior surface of the cover member portion by any suitable technique, such as by adhesive or welding.

[0026] The primary reflector portion 16 has a first reflective surface 28 defining a primary reflector 30 of a Cassegrain optical system, and the secondary reflector portion 22 has a second reflective surface 32 defining a secondary reflector 34 of the Cassegrain optical system. In the illustrated embodiment, as shown in Figures 3 and 4, the first reflective surface 28 is formed by a first reflective coating 36 on the interior surface 38 of the primary reflector portion 16, and the second reflective surface 32 is formed by a second reflective coating 40 on the interior surface 42 of the secondary reflector portion 22. The thicknesses of the reflective coatings 36, 40 are exaggerated in the drawings for ease of illustration. The reflective coatings 36, 40 typically comprise at least one base layer, at least one adhesion layer, a reflective layer and at least one protective layer. In a preferred embodiment, the reflective layer of the reflective coatings 36, 40 comprises a microlayer of silver, although other suitable highly reflective materials can also be used. The reflective coatings 36, 40 may be applied by targeted vacuum metal deposition; other techniques may also be used.

[0027] Because the reflective coatings 36, 40 are applied to interior surfaces 38, 42 of the primary reflector portion 16 and the secondary reflector portion 22, respectively, the primary reflector 30 and secondary reflector 34 are first surface mirrors and as such obviate the energy losses that result from the refractive optical effects associated with conventional back surface mirrors. Alternatively, however, reflective coatings may be applied to the exterior surfaces of the primary reflector portion 16 and the secondary reflector 22 rather than the interior surfaces thereof, with suitable sealing to protect the coatings. An anti-reflective coating, for example cerium oxide, may be applied to the exterior surface 44 of the optically transmissive cover member portion 18.

[0028] As will be explained in greater detail in the context of Figures 13A to 14B, the hollow member 14 is shaped to arrange the primary reflector portion 16 and the secondary reflector portion 22, and hence the primary reflector 30 and the secondary reflector 34, in a Cassegrain relationship relative to one another so that incoming substantially collimated solar radiation that passes through the cover member is reflectively directed by the primary reflector 30 onto the secondary reflector 34 and reflectively directed by the secondary reflector 34 toward a Cassegrain target region 46. In the embodiments illustrated in Figures 1 to 6 and 7 to 12, the primary reflector 30 is parabolic and the secondary reflector 34 is hyperbolic; however, other types of Cassegrain concentrator arrangements may also be used, for example both reflectors 30, 34 may be hyperbolic, both reflectors 30, 34 may be parabolic, both reflectors 30, 34 may be aspheric, or any suitable combination thereof. Custom-designed curvatures, with or without rotational symmetry, may also be used.

[0029] As described above, in the preferred embodiment the primary reflector portion 16, the secondary reflector portion 22 and the optically transmissive cover member portion 18 are each defined by the interior shape of the monolithic, rigid, hollow member 14. More particularly, the entire hollow member 14 is formed by a single piece of material, as described in greater detail below. Because the primary reflector portion 16, secondary reflector portion

22 and cover member portion 18 are all integrally formed as part of the single hollow member

14, relative alignment of the primary reflector 30, secondary reflector 34 and optically transmissive cover member portion 18 is an inherent result of the manufacturing process. As such, the primary reflector 30 and the secondary reflector 34 will be automatically aligned and fixed with respect to each other, within manufacturing tolerances. Moreover, because the primary reflector portion 16, secondary reflector portion 22 and cover member portion 18 are all integrally formed as part of the one-piece hollow member 14, any deformation resulting from thermal distortion of the hollow member 14 will be proportional across the entire hollow member 14, so that the optical path of incoming solar radiation suffers limited disruption.

[0030] In the embodiment shown in Figures 1 to 6 and 7 to 12, the primary reflector portion 16 has a central aperture 48 defined therethrough, and the monolithic hollow member 14 includes a cylindrical neck portion 49 depending from the central aperture 48 away from the cover member portion 18. In this embodiment, the secondary reflector portion 22 is in registration with the central aperture 48. The central aperture 48 and neck portion 49 define the Cassegrain target region 46, and the cylindrical neck portion 49 can receive a solar conversion cell, as described in greater detail below.

[0031] Referring now to Figures 7 to 12, an assembled solar collector, comprising a solar concentrator 10 as described above, together with a solar conversion cell 60, is indicated generally by the reference numeral 50. In the illustrated embodiment, the solar conversion cell 60 is a Group III-V semiconductor solar conversion cell. Such solar conversion cells typically comprise a Group III-V semiconductor chip vacuum soldered to an alumina substrate that has been flashed with gold, with mountings to which wire electrodes 66 may be attached. An example of a suitable Group III-V semiconductor solar conversion cell is the Concentrator Triple Junction Receiver Assembly ("CTJ") offered by Emcore, having an address at 10420 Research Rd. SE, Building 2, Albuquerque, NM U.S.A., 87123. While the illustrated embodiments incorporate Group III-V semiconductor solar conversion cell, other suitable types of solar conversion cells, whether now known or hereafter developed, may also be used.

[0032] The solar conversion cell 60 is secured in the neck portion 49 of the hollow member 14 in registration with the central aperture 48 to receive solar radiation from the secondary reflector 34, in particular, the solar conversion cell 60 is carried by a plug 64 sealed in the neck portion 49. The hollow member 14 and plug 64 together form the housing 12 in which the solar conversion cell 60 is disposed. Since the central aperture 48 and neck portion 49 define the Cassegrain target region 46, the solar conversion cell 60 is disposed in the

Cassegrain target region 46 to receive the directed solar radiation from the secondary reflector 34. Electrical connectors in the form of wire electrodes 66 are in electrical communication with the solar conversion cell 60 and extend from inside the housing 12 to outside the housing 12 through the neck portion 49. An inert environment is contained within the housing 12, which is hermetically sealed by the seal between the plug 64 and the hollow member 14. The inert environment 62 may be a vacuum, a noble gas, or a non-oxidizing non-noble gas.

[0033] To provide for dissipation of heat from the solar conversion cell 60, a thermal transfer element 68, such as a heat pipe, which is preferred, or a solid block of suitable material, such as copper, is bonded to the underside of the solar conversion cell, such as by thermal epoxy, vacuum soldering or other suitable technique. To maintain the hermetic seal, the wire electrodes 66 and thermal transfer element 68 are sealingly embedded in the plug 64, which is preferably made from a material which will weld to the material of the hollow member 14 and which has a similar coefficient of thermal expansion.

[0034] Where the plug 64 is made from glass or a similar material, the plug 64 may be formed by placing the wire electrodes 66 and thermal transfer element 68 into a form, filling the form with beads of the plug material, and melting the beads in an oven to form a solid plug 64 of glass (or other material) with the wire electrodes 66 and thermal transfer element 68 hermetically sealed therein.

[0035] After the hollow member 14 has been formed and the reflective coatings 36, 40 have been applied to the primary reflector portion 16 and secondary reflector portion 22, an inert environment 62 is created inside the housing 12 and maintained. The plug 64, with the solar conversion cell 60 secured to the thermal transfer element 68 and the wire electrodes 66 secured to the mountings on the solar conversion cell 60, is then inserted into the neck portion 49 of the hollow member 14 and secured in place with a hermetic seal, for example by way of thermal welding. This hermetic seal will maintain the inert environment 62 inside the housing 12. The hermetically sealed inert environment 62 is advantageous because, where the reflective coatings 36, 40 are disposed on the interior of the housing 12, the inert environment 62 inhibits tarnishing or other deterioration of the reflective coatings 36, 40. The inert environment 62 also protects the solar conversion cell 60 from exposure to oxygen, humidity and detritus. In the preferred embodiment, because the solar radiation is transmitted internally through an inert gas or vacuum environment and reflected off of silver first surface mirrors, losses associated with refraction and transmission are small. When the plug 64 is installed in the housing 12, the solar conversion cell 60 is oriented symmetrically in the solar concentrator 10 at the Cassegrain target region 46.

[0036] As was described above, and as shown in Figures 13A and 14A and 13B and 14B, the solar concentrator 10 comprises a Cassegrain concentrator. Incoming substantially collimated solar radiation SR passes through the optically transmissive cover member portion 18 to the primary reflector 30, is reflectively focused by the primary reflector 30 onto the secondary reflector 34 and reflectively directed by the secondary reflector 34 at the target region 46 and therefore onto the solar conversion cell 60. The primary reflector portion 16 and secondary reflector portion 22 may have any suitable surface shape, such as appropriate parabolic, hyperbolic aspheric or customized surfaces, so that the primary reflector 30 and secondary reflector 34 define a Cassegrain reflector for which the desired concentration of solar radiation onto the solar conversion cell 60 is achieved. Local deviations from mathematically ideal shapes (e.g. hyperbolic, parabolic, aspheric) in the surface shape of the primary reflector portion 16 and secondary reflector portion 22 can be advantageous in achieving improved homogeneity of the concentrated solar radiation over the surface of the solar collector, because the solar collection cells are generally rectangular while the housing may have some defined radius of curvature for at least some portions of its outer perimeter, depending on the manufacturing technique used.

[0037] Figures 13A and 13B show an embodiment of the solar concentrator 50 in which the solar concentrator 10 is configured to direct solar radiation SR at the solar conversion cell 60 by focusing the solar radiation reflected from the secondary reflector 34. Figures 14A and 14B show an embodiment of the solar concentrator 50 in which the solar concentrator 10 is configured to direct solar radiation SR at the solar conversion cell 60 by substantially re- collimating the focused solar radiation SR received from the primary reflector 30 and reflected by the secondary reflector 34. [0038] The use of an embodiment such as that depicted in Figures 14A and 14B, which aims substantially re-collimated solar radiation SR at the solar conversion cell 60, is advantageous where the solar conversion cell 60 is a Group III-V semiconductor solar conversion cell. In particular, indexes of refraction of Group III-V semiconductor solar conversion cell can be quite high; Germanium for example has n = 3.5. Therefore substantial re-collimation of the solar radiation SR aimed at the solar conversion cell 60 can reduce Fresnel losses associated with increasing angles of incidence.

[0039] A plurality of solar collectors 50 can be assembled into an array 180 as shown in Figures 29 through 31, and are formed to have sufficient strength for this purpose. The solar collectors 50 may be secured in a mounting 181 , and the gaps between the solar collectors 50 may be filled with an epoxy or other suitable material to define a flat upper surface, which facilitates cleaning of the exterior surface of the optically transmissive cover members 20. Alternatively, the upper surfaces 44 of the solar collectors 50 may be secured to the underside of an optically transmissive impact-resistant protective sheet 183.

[0040] The wire electrodes 66 from multiple solar collectors 50 can be connected together appropriately for connection to a direct current to alternating current inverter. Similarly, the thermal transfer elements 68 can be attached to a heat transfer and dissipation system such that the solar conversion cell is maintained at an appropriate operating temperature.

[0041] As shown in Figures 29 to 31, one or more arrays 180 of solar collectors 50 can be mounted together on a common frame 182 which itself can be mounted on a solar tracking system 184. Many types of solar tracking systems are known in the art. The solar tracking system 184 adjusts the orientation of the solar collectors 50 throughout the day to maintain the orientation of the solar collectors 50 pointing directly at the sun so that the Cassegrain reflective optics are operable. Wind, vibration, motor and gear backlash, motor step size as well as other factors associated with the solar tracking system 184 may result in slight misalignments of the solar collectors 50 relative to the ideal alignment with the sun. To help compensate for these mechanical misalignments the neck portion 50 of the housing 12 may include optical features, such as Winston cone optics, and suitable reflective coatings, to additionally concentrate resulting stray solar radiation and reflect it back onto the solar conversion cell 60.

[0042] Another aspect of the invention is directed to a method for manufacturing, out of glass, a monolithic Cassegrain solar concentrator housing such as the housing 12, and more particularly to a method for manufacturing a hollow member, such as the hollow member 14, which forms the major part of the housing 12. In describing the method, reference will be made to Figure 47, which shows the method 4700 in flow chart form, and to Figures 15 A through 25B which show the method 4700 schematically.

[0043] As shown in Figures 15A and 15B, a mold 1500 is provided. The mold 1500 comprises three mold portions, namely two side mold portions 1502 and an end mold portion 1504. The mold 1500, when closed, has a mold cavity 1506 (see Figures 2 ID, 22A and 22B) whose shape defines a concave primary reflector region 1508 of the mold cavity 1506 and a cover member region 1510 of the mold cavity 1506 corresponding, respectively, to a primary reflector and a cover member of the Cassegrain solar concentrator. The mold portions 1502, 1504 are movably mounted over a glass-blowing lathe 4600 as shown in Figure 46. Such lathes are known in the art and include those manufactured by Litton Engineering, having an address at P.O. Box 950, 200 Litton Drive, Suite 200, Grass Valley, CA U.S.A. 95945 and Herbert Arnold GmbH & Co. KG, having an address at Weilstr. 6, 35781 Weilburg, Germany. In general these lathes have a headstock 4602 that is fixed with respect to the lathe bed 4604 and a tailstock 4606 that is axially movable with respect to the lathe bed 4604 and hence axially movable relative to the headstock 4602. Each of the mold portions 1502, 1504 has a plurality of vents 151 1 for venting gas captured in the mold cavity 1506 to outside the mold cavity 1506.

[0044] Continuing to refer to Figures 15A and 15B, a hollow cylindrical borosilicate tube 1514 is fixed in a planetary chuck 1512 of the fixed headstock 4602 of the glass blowing lathe 4600 (Figure 46). The end 1514A of the tube 1514 furthest from the mold 1500 is fitted with an appropriate stopper and swivel mechanism 1516 which is connected to a suitable device (not shown) for increasing and decreasing air pressure, including generating a vacuum. In the illustrated embodiment, the tube 1514 has an initial length of approximately 250 mm, with an approximate 40 mm outer diameter and a wall thickness of approximately 5 mm. A solid glass rod 1518 approximately 10 mm in diameter is fixed in a collet chuck 1520 fixed in a planetary chuck 1522 carried by the movable tailstock 4606. The glass tube 1514 and glass rod 1518 are preferably made from the same type of glass.

[0045] The open end 1514B (Figures 15A and 15B) of the tube 1514 closest to the mold 1500 is heated to become molten and then closed to create a generally hemispherical closed end 1514BC, as shown in Figures 16A and 16B. This is done by rotating the tube 1514 and heating an annular ring of glass, drawing it, collapsing that region, removing any excess glass, and then closing and slightly re-inflating the tube 1514 through the stopper and swivel mechanism 1516 to generate a more even wall thickness. This procedure is within the capability of a skilled glassblower.

[0046] The tube 1514 continues to rotate, and the glass rod 1518 is also rotated at a matching speed and in the same direction. Preferably, the glass tube 1514 and the glass rod 1518 both rotate at approximately 50 rpm. The glass rod 1518 is advanced toward the tube 1514 by advancing the movable tailstock 4606 towards the fixed headstock 4602 (Figure 46). The end mold portion 1504 includes a centrally disposed rod aperture 1 24 for the glass rod 1518 and through which the glass rod 1518 extends. As shown in Figures 17A and 17B, the glass rod 1518 is then welded to the closed end 1514BC so that the glass rod 1518 is coaxial with the glass tube 1514, and forms a glass punty joint 1526, which, along with the closed end

1514BC, is then cooled below the softening point of the glass. The glass punty joint 1526 will be used to provide mechanical compression and extension of the glass tube 1514 to form a parison 1534 therefrom, as described below, and assists in keeping the glass tube 1514, and later the parison 1534, centered during rotation.

[0047] Next, as shown in Figures 18A and 18B, a first longitudinal region 1528 of the glass tube 1514 spaced from the glass punty joint 1526 is heated, for example by a burner 1530, so that the first longitudinal region 1528 becomes pliable, and the glass rod 1518 is moved away from the first longitudinal region 1528 to neck down the first longitudinal region 1528, which is then allowed to cool below the softening point of the glass. More particularly, the softened first longitudinal region 1528 of the tube 1514 lengthens while its diameter simultaneously decreases, forming a necked down first longitudinal region 1528. Although shown schematically as a single flame element, in a presently preferred embodiment, the burner 1530 is a ring burner that at least partially surrounds the glass tube 1514 and is spaced

approximately 100 mm therefrom. The tube 1514 and glass rod 1518 continue to rotate during this step.

[0048] As shown in Figures 19A and 19B, after the first longitudinal region 1528 has been necked down and allowed to cool, a second longitudinal region 1532 disposed between the necked down first longitudinal region 1528 and the closed end 1514BC is heated to become pliable. While the second longitudinal region 1532 is pliable, air or another suitable gas is injected into the glass tube 1514 by way of the stopper and swivel mechanism 1516 while advancing the glass rod 1518 toward the necked down first longitudinal region 1528 to expand the pliable second longitudinal region 1532 so that the second longitudinal region 1532 and the closed end 1514BC assume a hollow generally ellipsoidal shape and become a parison 1534. The tube 1514 and glass rod 1518 continue to rotate during this step.

[0049] In a presently preferred embodiment, a 100 mm ribbon burner indicated schematically at 1536 is introduced to selectively heat the second longitudinal region 1532. The softened glass in the heated second longitudinal region 1532 is supported by the cooler necked down first longitudinal region 1528 and the glass punty joint 1526 and glass rod 1518. The interior volume of the tube 1518 is very slightly pressurized via the stopper and swivel mechanism

1516. Once the second longitudinal region 1532 is evenly heated to the softening point the tailstock 4606 is slowly advanced towards the headstock 4602, thereby squeezing the second longitudinal region 1532 against the closed end 1514BC of the tube and the glass rod 1518, while continuing to apply pressure inside the tube 1518. The softened second longitudinal region 1532 expands outward as a result of the internal air or other gas pressure while the continued application of heat causes the wall of the second longitudinal region 1532 to condense and thicken as a result of the surface tension of the glass. Adjusting the rotational speed of the lathe 4600 can also assist in the controlled expansion of the softened second longitudinal region 1532. By careful balancing of the pressure inside the tube 1514, the rotational speed of the lathe 4600, the heat, and the movement of the tailstock carrying the glass rod 1518, the softened second longitudinal region 1532 can be expanded into a substantially ellipsoidal parison 1534 of relatively uniform wall thickness, and preferably into a substantially spherical parison 1534, as shown in Figures 20A and 20B. As used herein, the term "ellipsoidal" encompasses, but is not limited to, spherical shapes. By careful observation of the internal wall thickness during the transition process from the softened second longitudinal region 1532 (Figures 19A and 19B) to the parison 1534 (Figures 20A and 20B), one skilled in the art of glass blowing can generate a consistent, substantially uniform wall thickness by preferentially heating and adjusting the pressure in the tube 1514 while either advancing the tailstock, and hence the glass rod 1518, toward the necked down first longitudinal region, or holding the position of the glass rod 1518 constant. In a preferred embodiment, the second longitudinal region 1532 is formed into the parison 1534 by alternately increasing the pressure in the tube and incrementally advancing the glass rod 1518 toward the necked down first longitudinal region 1528. It is also contemplated that the process can be mechanized by experimentally determining optimal values, at each point in time during the process, for heat, pressure, rotation speed and rate of advance of the glass tube, and programming these values into a control unit for an automated manufacturing system. The tube 1514 and glass rod 1518 continue to rotate during formation of the parison 1534.

[0050] Careful control of the wall thickness results in a symmetrical and balanced parison 1534. Maintaining symmetry and balance is particularly important because the parison 1534 is susceptible to distortions in wall thickness which can prevent proper formation of the hollow member (e.g. hollow member 14).

[0051 ] The parison 1534 is heated to a nearly molten state, and the mold 1500, in particular the mold portions 1502 and 1504, is advanced into alignment with the parison 1534, as shown in Figures 21 A and 2 I B, while the tube 1514, parison 1534 and glass rod 1518 continue to rotate. It should be noted here that the axial position of the parison 1534 remains fixed while the mold portions 1502, 1504 are advanced, to avoid disrupting the shape of the now extremely pliable parison 1534. The mold portions 1502 and 1504 are heated, and are positioned such that the exterior surface of the parison 1534 just contacts the inner surface of the end mold portion 1504, and the end mold portion 1504 may include an annular recess around the rod aperture 1524 to accommodate the glass punty joint 1526. When the parison 1534 is formed using non-automated techniques, there may be minor variations in the shape of the parison 1534, which in turn lead to variances in the mold position for each parison 1534. To compensate for this, once the mold portions 1502 and 1504 have been manually

positioned, the position may be recorded digitally to assist in forming the secondary reflector portion of the housing, as described in greater detail below.

[0052] As shown in Figures 21C and 2 ID, and at step 4702 in Figure 47, the heated mold portions 1502, 1504 are closed around the parison 1534 which, along with the glass rod 1518, continues to rotate about the mutual substantially horizontal axis. When the mold portions 1502, 1504 are closed, the mold 1500 substantially completely surrounds the parison 1534 except for a parison aperture 1534A opening into an interior volume of the parison 1534. In the illustrated embodiment, the parison aperture 1534A is defined by the tube 1514, which extends from the parison 1534.

[0053] Referring now to Figures 22A and 22B, at step 4704 (Figure 47) air or another suitable gas is injected, via the stopper and swivel mechanism 1516 and the tube 1514, through the parison aperture 1534A into the interior volume of the parison 1534 to expand the parison 1534 inside the mold cavity 1506. In particular, the hot and pliable parison 1534 is inflated to contact the interior surface of the mold cavity 1506 and then allowed to cool so that the parison 1534 assumes the shape of the mold cavity 1506 and becomes a rigid hollow member 2214 having a primary reflector portion 2216 and a cover member portion 2218 (see Figure 27). The interior of the parison 1534 is pressurized, and the parison 1534 is expanded, at a controlled rate to evenly blow the still-rotating parison 1534 out into the mold 1500.

[0054] Once the hollow member 2214 has been formed and sufficiently cooled, the mold portions 1502, 1504 are opened and the hollow member 2214 is removed from the mold 1500, as shown in Figure 23 A and at step 4706 in Figure 47. Preferably, the hollow member 2214 is removed from the mold 2214 by moving the mold portions 1502, 1504 away from the hollow member 2214, rather than moving the hollow member 2214, since this maintains axial registration of the hollow member 2214 with the digitally recorded position of the mold 1500. The hollow member 2214 is then preferably annealed, such as by flame annealing, which can be done without axially moving the hollow member 2214. The tube 1514, hollow member 2214 and glass rod 1518 continue to rotate as the mold 1500 is opened and moved.

[0055] As can be seen in the Figures, the shape of the mold cavity 1506, and in particular the shape of the end mold portion 1504, omits any secondary reflector region corresponding to a secondary reflector of the Cassegrain solar concentrator. Specifically, the cover member region 1510 of the mold cavity 1506 is substantially flat and the resulting cover member portion 2218 of the hollow member 2214 is substantially flat, as shown in Figure 27, and thereby omits any secondary reflector portion. A presently preferred method for forming a secondary reflector portion in the hollow member 2214 will now be described.

[0056] As shown in Figure 25A, and at step 4708 in Figure 47, after injecting the air or other gas through the parison aperture 1534A and annealing the hollow member 2214, while the hollow member 2214 is still hot, additional heat is applied to an axially central part 2218 AC of the cover member portion 2218, so that the axially central part 2218 AC of the cover member portion 2218 becomes pliable. At step 4710 (Figure 47), a mold element 2410 is inserted axially through the tube 1514 and neck portion 2249 of the hollow member 2214 into the interior volume of the hollow member 2214, as shown in Figure 23B.

[0057] As best seen in Figures 24A to 24C, the mold element 2410 is a generally cylindrical member whose outer diameter is smaller than the inside diameter of the tube 1514 and the neck portion 2249 of the hollow member 2214 so as to fit therein. The mold element 2410 has at its tip a secondary reflector region 2422 corresponding to a secondary reflector of the Cassegrain solar concentrator. The secondary reflector region 2422 of the mold element 2410 comprises a concave surface 2422S. When the mold element 2410 is slid through the tube 1514 and the neck portion 2249 of the hollow member 2214, the concave surface 2422S faces the interior surface 2218S of the cover member portion 2218. A plurality of vacuum apertures 2412 are defined in the concave surface 2422S and extend axially along the length of the mold element 2410 to communicate with a connector 2414 coupled to a vacuum source. One of the vacuum apertures 2412 is disposed approximately in the center of the concave surface 2422 S and the other vacuum apertures 2412 are disposed at the periphery of the concave surface 2422S.

[0058] Reference is now made to Figures 26A and 26B. At step 4712 (Figure 47), the secondary reflector region 2422 of the mold element 2410 is engaged with the axially central part 2218 AC of the cover member portion 2218 while the additional heat is being applied thereto. Then, negative pressure is applied to the vacuum apertures 2412, via the connector 2414, to pull the pliable axially central part 2218 AC of the cover member portion 2218 against the concave surface 2422S of the mold element 2410. The glass rod 1518 may also be advanced against the axially central part 2218 AC of the cover member portion 2218 to push the axially central part 2218 AC of the cover member portion 2218 toward the concave surface 2422S so that the axially central part 2218AC of the cover member portion 2218 engages the peripheral rim 2422R surrounding the concave surface 2422S. The axially central part 2218 AC of the cover member portion 2218 will assume the shape of the concave surface 2422S to form a convex secondary reflector portion 2222 of the hollow member 2214 so that the hollow member 2214 assumes the shape of a Cassegrain solar concentrator. The digital data regarding mold position, as described above, may be used to axially position the mold element 2410. The tube 1514, hollow member 2214 and glass rod 1518 continue to rotate during formation of the secondary reflector portion 2222.

[0059] The newly formed secondary reflector portion 2222 of the hollow member is then allowed to cool, and the glass rod 1518 can then be removed from the hollow member 2214, as shown in Figure 25B. The mold element 2410 can then be withdrawn, resulting in a fully- formed hollow member 2214 having the shape of a Cassegrain solar concentrator, as shown in Figure 28. The interior surface of the secondary reflector portion 2222 is preferably flame- polished to remove imperfections resulting from contact with the secondary reflector region 2422 of the mold element 2410. [0060] The hollow member 2214 and the still attached tube 1514 are then removed from the lathe 4600. The tube 1514 can be removed from the neck portion 2249 of the hollow member 2214 immediately after removal, but preferably remains attached during subsequent processing steps, as described below. After removal from the lathe 4600, at step 4714 (Figure 47) the hollow member 2214 is annealed (with or without the tube 1514) and then cooled.

[0061] Once the hollow member 2214 has cooled, at step 4716 (Figure 47) a first reflective coating is applied to the primary reflector portion 2216 of the hollow member 2214. The first reflective coating may be applied to the exterior surface of the primary reflector portion 2216 so as to form a back surface mirror, in order to utilize conveyorized coating processes.

Preferably, however, the first reflective coating is applied to the interior surface of the primary reflector portion to form a first surface mirror. Also at step 4716 (Figure 47), a second reflective coating is applied to the interior surface of the secondary reflector portion 2222 of the hollow member 2214. Application of the reflective coatings results in the hollow member 2214 becoming a Cassegrain solar concentrator, such as the Cassegrain solar concentrator 10 as shown in Figures 1 to 6.

[0062] The reflective coatings can be applied to the internal surfaces of the primary reflector portion 2216 and secondary reflector portion 2222 by vacuum metal evaporation. The interior surface 2218S of the cover member portion 2218 is masked, for example by liquid masking, and then the hollow member 2214 is placed inside an evacuated chamber. An e-beam hearth inside the evacuated chamber is inserted through the tube 1514 (if still present) and neck portion 2214 into the internal volume of the hollow member 2214, and used to apply, in succession, one or more base layers, one or more adhesion layers, a reflective layer such as silver, and one or more protective layers. The e-beam hearth is then withdrawn. A sleeve is then inserted into the hollow member 2214 through the tube 1514 (if still present) and neck portion 2214 to mask all but the secondary reflector portion 2222, and the e-beam hearth is then placed in the sleeve to apply a similar reflective coating to the interior surface of the secondary reflector portion 2222. If any reflective coating is inadvertently applied to the interior surface 2218S of the cover member portion 2218, the same may be etched off using an appropriate acid etching and washing technique, since the secondary reflector portion 2222 stands inwardly proud of the interior surface 2218S of the cover member portion 2218 and would be above a thin layer of acid wash coating the interior surface 2218S of the cover member portion 2218.

[0063] The shape of the mold cavity 1506 can be designed such that, with proper control over the shape, size, wall thickness and amount of glass in the parison 1534, the internal surface of the resulting hollow member 2214 will acquire the desired shape of the primary reflector portion 2216. For example, the internal surface of the primary reflector portion 2216 can be a parabolic, hyperbolic, aspheric or customized curve. Correctly shaping the internal surface of the primary reflector portion 2216 will enable the resulting Cassegrain solar concentrator to have first surface mirrors.

[0064] Because the parison 1534 inflates like a balloon, the glass wall of the partially formed hollow member 2214 is generally thicker at the center, where the expanding parison 1534 contacts the surface of the mold 1500 first, and thinner at the edges where the expanding parison 1534 contacts the surface of the mold 1500 later, as shown in Figure 27. Because of this phenomenon, achieving consistent wall thickness and temperature of the parison 1534 prior to blowing is important for controlling the internal surfaces of the resulting hollow member 2214. A statistical profile of the resulting internal surfaces can be developed, which can be used to modify the interior surface of the mold. As more hollow members 2214 are formed, the greater number of measurements will improve the statistical profile, so that the optical performance of the interior surface of the hollow members 2214 can be altered to result in the desired parabolic, hyperbolic, aspheric or customized shape. In other words, the primary reflector region 1508 of the mold cavity 1506 can have a surface shape that accommodates the wall thickness of the parison 1534 during gas injection to form the hollow member 2214 so that the interior surface of the primary reflector portion 2216 of the hollow member 2214 defines a predetermined concave shape.

[0065] By making the walls of the parison 1534 and resulting hollow member 2214 thinner and more consistent, the optical performance difference between a first surface mirror and back surface mirror for the primary reflector will diminish due to decreasing refractive losses, allowing for the use of an external coating on the primary reflector portion 2216 by means of cost-effective conveyorized coating processes.

[0066] It is contemplated that the above-described method for forming a hollow member may be adapted by replacing the glass tube 1514 and glass rod 1518 with a single element that is press-formed from a gob of molten glass using a specialized mold to achieve the desired initial shape.

[0067] As an alternative to the use of the mold element 2410 described above, a secondary reflector portion may be a separate piece secured to the inner surface of the cover member portion, which may be installed through the neck portion 2249. Such a separate piece may have the reflective coating already applied, thereby omitting the need for internal coating of a secondary reflector portion. The neck portion 2249 of the hollow member 2214 can be used to facilitate alignment of the secondary reflector portion when the same is a separate piece. In this embodiment, the hollow member 2214 is preferably fully annealed before the secondary reflector piece is attached.

[0068] Once the primary and secondary reflectors have been formed, for example by one of the techniques described above, an inert environment is created inside the hollow member 2214. At least one solar conversion cell, such as the solar conversion cell 60, is sealed inside the inert environment inside hollow member 2214, in position to receive solar radiation from the secondary reflector, as shown in Figures 7 to 12. For example, a plug such as the plug 64 described above and carrying the solar conversion cell and electrical connections may be slid into the tube 1514 (if still present) and neck portion 2249 and welded into place to form a hermetic seal. The remaining tube 1514, if still present, can then be trimmed, and may be trimmed as part of the process of welding the plug into the neck portion.

[0069] Figures 32 to 37 show an alternate embodiment of a solar concentrator 3210. The solar concentrator 3210 is identical to the solar concentrator 10 shown in Figures 1 to 6, except that it omits any neck portion. As such, identical reference numerals are used to refer to corresponding features, except with the prefix "32". The solar concentrator 3210 may be formed by using the process described above, and trimming away the tube 1514 and neck portion 2249 immediately adjacent the primary reflector portion 2216.

[0070] Similarly, Figures 38 to 43 show a solar collector 3250 which is identical to the solar collector 50 shown in Figures 7 to 12 except for the omission of any neck portion. Again, identical reference numerals are used to refer to corresponding features, except with the prefix "32". Figures 44A and 44B show an embodiment of the solar concentrator 3250 in which the secondary reflector 3234 focuses the solar radiation SR received from the primary reflector 3230 onto the solar cell 3260, and Figures 45A and 45B show an embodiment of the solar concentrator 3250 in which the secondary reflector 3234 substantially re-collimates the solar radiation SR received from the primary reflector.

[0071] Several currently preferred embodiments have been described by way of example. It will be apparent to persons skilled in the art that a number of variations and modifications can be made without departing from the scope of the invention as defined in the claims.

[0072] The above description is intended in an illustrative rather than a restrictive sense. Variations to the exact embodiments described may be apparent to those skilled in the relevant art without departing from the spirit and scope of the claims set out below. It is intended that any such variations be deemed within the scope of this patent.

Claims

WHAT IS CLAIMED IS:

1. A Cassegrain solar concentrator housing, comprising: a transparent hollow member comprising: a concave primary reflector portion; and an optically transmissive planar cover member portion opposed to the concave primary reflector portion; the hollow member being of one-piece, unitary, monolithic construction; the hollow member having a first seamless transition between the primary reflector portion and the cover member portion.

2. The solar concentrator housing of claim 1, further comprising: a secondary reflector portion of convex curvature spaced from and opposed to the primary reflector portion and focally aligned with the primary reflector portion.

3. The solar concentrator housing of claim 2, wherein: the secondary reflector portion extends from an interior surface of the cover member portion and is integrally formed therewith as part of the hollow member; and the hollow member has a second seamless transition between the cover member portion and the secondary reflector portion.

4. The solar concentrator housing of claim 3, wherein: the hollow member further comprises a seamless transition portion between the primary reflector portion and the cover member portion; the primary reflector portion continuing seamlessly into the transition portion; and the transition portion continuing seamlessly into the cover member portion.

5. The solar concentrator housing of claim 4, wherein the seamless transition portion is substantially cylindrical.

6. The solar concentrator housing of claim 1 , 2, 3, 4, 5, or 6, wherein: the primary reflector portion has a central aperture defined therethrough; and a neck portion depends from the central aperture away from the cover member portion.

7. A solar concentrator, comprising: a solar concentrator housing according to claim 2, 3, 4, 5 or 6; the primary reflector portion having a first reflective surface defining a primary reflector of a Cassegrain optical system; the secondary reflector portion having a second reflective surface defining a secondary reflector of the Cassegrain optical system; the primary reflector and the secondary reflector being arranged relative to one another so that incoming substantially collimated solar radiation passes through the cover member, is reflectively directed by the primary reflector onto the secondary reflector and reflectively directed by the secondary reflector toward a Cassegrain target region.

8. A solar concentrator according to claim 7, wherein: the first reflective surface is formed by a first reflective coating on an interior surface of the primary reflector portion; and the second reflective surface is formed by a second reflective coating on an interior surface of the secondary reflector portion.

9. A solar collector, comprising: a solar concentrator according to claim 7 or 8; at least one solar conversion cell disposed in the housing; the at least one solar conversion cell being disposed in the Cassegrain target region to receive the directed solar radiation from the secondary reflector; electrical connectors in electrical communication with the at least one solar conversion cell and extending from inside the housing to outside the housing through the neck portion; and the housing being hermetically sealed with the solar conversion cell disposed therewithin.

10. The solar collector of claim 9, further comprising: an inert environment in the housing; the inert environment being selected from the group consisting of vacuum, noble gas, and non-oxidizing non-noble gas.

1 1. A method for fabricating a monolithic Cassegrain solar concentrator from glass;

comprising: closing at least two heated mold portions around a hollow generally ellipsoidal pliable glass parison having substantially uniform wall thickness and rotating about a substantially horizontal axis thereof to define a heated mold substantially completely surrounding the parison except for a parison aperture opening into an interior volume of the parison; the mold having a mold cavity whose shape defines a concave primary reflector region of the mold cavity and a cover member region of the mold cavity corresponding, respectively, to a primary reflector and a cover member of the Cassegrain solar concentrator; the mold having a plurality of vents for venting gas captured in the mold cavity to outside the mold cavity; and injecting gas through the parison aperture into the interior volume of the parison to expand the parison so that the closed parison assumes the shape of the mold cavity and becomes a hollow member having a primary reflector portion and a cover member portion; opening the mold; removing the hollow member from the mold; and annealing the hollow member.

12. The method of claim 1 1 , wherein: the Cassegrain solar concentrator is a monolithic Cassegrain solar concentrator; the shape of the mold cavity omits any secondary reflector region corresponding to a secondary reflector of the Cassegrain solar concentrator so that the cover member region of the mold cavity is substantially flat and the resulting cover member portion of the hollow member is substantially flat and thereby omits a secondary reflector portion; the method further comprising, after injecting gas through the parison aperture: applying additional heat to an axially central part of the cover member portion so that the axially central part of the cover member portion becomes pliable; inserting a mold element axially through the parison aperture into an interior volume of the hollow member; the mold element having at its tip a secondary reflector region corresponding to a secondary reflector of the Cassegrain solar concentrator, the secondary reflector region comprising a concave surface facing an interior surface of the cover member portion and having a plurality of vacuum apertures defined therein; and engaging the secondary reflector region of the mold element with the axially central part of the cover member portion while the additional heat is being applied thereto and applying a vacuum to the plurality of vacuum apertures to form the secondary reflector portion so that the secondary reflector portion is convex and the hollow member assumes a completed shape of the Cassegrain solar concentrator.

13. The method of claim 12, further comprising, after annealing the hollow member, applying a first reflective coating to an exterior surface of the primary reflector portion of the hollow member and applying a second reflective coating to an interior surface of the secondary reflector portion of the hollow member to form the Cassegrain solar concentrator.

14. The method of claim 12, further comprising, after annealing the hollow member, applying a first reflective coating to an interior surface of the primary reflector portion of the hollow member and applying a second reflective coating to an interior surface of the secondary reflector portion of the hollow member to form the Cassegrain solar concentrator.

15. The method of claim 12, wherein the primary reflector region of the mold cavity has a surface shape that accommodates a wall thickness of the parison during gas injection to form the hollow member so that the interior surface of the primary reflector portion of the hollow member defines a predetermined concave shape.

16. The method of claim 15, wherein the wall thickness of the parison during gas injection to form the hollow member varies along a longitudinal axis of the mold.

17. The method of claim 12, wherein: a solid rod of glass is welded to the parison prior to closing the mold portions around the parison and is hence welded to the axially central part of the cover member portion when the additional heat is applied to the axially central part of the cover member portion; the solid rod of glass being horizontally coaxial with the parison and rotating synchronously therewith; the method further comprising: moving the rod toward the mold element to move the pliable axially central part of the cover member portion into engagement with the secondary reflector region of the mold element before applying the vacuum to the plurality of vacuum apertures to form the secondary reflector portion.

18. The method of claim 17, wherein the parison is formed by: heating an open end of a glass tube to become molten and closing the open end of the glass tube to create a closed end; welding the glass rod to the closed end so that the glass rod is coaxial with the glass tube to form a glass punty joint; cooling the glass punty joint below a softening point of the glass; heating a first longitudinal region of the glass tube spaced from the glass punty joint so that the first longitudinal region becomes pliable; moving the glass rod away from the first longitudinal region to neck down the first

longitudinal region; cooling the necked down first longitudinal region below the softening point of the glass; heating a second longitudinal region disposed between the necked down first longitudinal region and the closed end to become pliable; while the second longitudinal region is pliable, injecting gas into the tube while advancing the glass rod toward the necked down first longitudinal region to expand the pliable second longitudinal region so that the second longitudinal region and the closed end assume a hollow generally ellipsoidal shape and become the parison.

19. The method of claim 18, wherein injecting gas into the tube while advancing the glass rod toward the necked down first longitudinal region to expand the pliable second longitudinal region into the hollow generally ellipsoidal parison with substantially uniform wall thickness comprises alternately increasing gas pressure and incrementally advancing the glass rod toward the necked down first longitudinal region.

20. A solar concentrator housing, comprising: a monolithic hollow member of unitary construction, the monolithic hollow member comprising: an optically transmissive portion to admit incoming solar radiation into the hollow member; and at least one reflection portion shaped and positioned to direct incoming solar radiation admitted into the hollow member onto a target region; wherein the shape and position of the at least one reflection portion are defined by an overall shape of the monolithic hollow member.

21. The solar concentrator housing of claim 20, further comprising at least one additional reflection portion secured to an interior of the housing.