Abstract:
A glass structure, such as a mirror facet, having a glass member, a composite structure and a support structure. The composite structure includes a rigid interlayer which is bonded to the glass member and exerts a compressive force thereon to place the glass member in compression. The support structure is used to mount the glass structure and prevents the glass member from collapsing due to the compressive force exerted by the rigid interlayer. The glass structure is particularly well adapted for use in forming heliostats, parabolic dishes, trough concentrators, or other like elements for use in solar power systems, and does not suffer from the limitations or prior forms of such devices.

Description:
TECHNICAL FIELD 
     The present invention relates generally to the construction of mirrors and more particularly relates to a prestressed mirror and a method for fabricating the same. 
     BACKGROUND OF THE INVENTION 
     BACKGROUND ART 
     High concentration solar thermal power systems typically rely on a field of heliostats, or a parabolic dish or trough concentrators to track the sun and reflect solar radiation to a receiver where the solar energy heats a working fluid, such as steam. The working fluid is then employed to provide thermal energy for various industrial and commercial processes or to produce electricity. Similarly, concentrating photovoltaic systems use mirrors of varying types to collect solar energy where it is turned directly into electrical energy. 
     In such systems, it is critical for performance objectives that the mirror facets which make up these systems meet stringent optical performance characteristics such as radius of curvature, reflectivity and surface slope error. It is also critical that these mirror facets be lightweight so as to reduce the cost associated with the drive units that are needed to aim the mirror facets. The mirror facets must also be sufficiently robust to ensure a long life despite their exposure to precipitation, wind and sun. Consequently, these mirror facets must be capable of withstanding sustained winds in excess of 100 m.p.h., temperatures ranging from −40° F. to 130° F., impacts from hail, corrosive elements (e.g., acid rain, salt), humidity changes, etc. Furthermore, as there may be hundreds or even thousands of mirror facets in a system, it is highly desirable that the mirror facet be of highly cost efficient construction. 
     The designs of conventional mirror facets have relied on the thickness of the glass that forms the mirror facet and/or the frame structure of the mirror facet to compensate for the relatively weak tensile properties of glass. This approach has several drawbacks, including losses in reflectivity as a result of the use of relatively thicker glass and a relatively higher weight. Additionally, these mirror facets are not as robust as desired, being highly susceptible to damage during shipping, installation and use. Furthermore, as these mirror facets have relatively weak tensile properties, their exposure to time-varying forces such as wind can cause the propagation of cracks which could permit the reflective finish of the mirror facet to corrode, with the result being impaired performance of the mirror facet. 
     In view of these drawbacks, some conventional mirror facets have obtained additional strength through the use of operations: such as slumping, chemical strengthening, annealing and/or tempering. These processes tend to be relatively expensive, and as such, a substantial cost penalty is incurred if these processes are employed. Furthermore, these mirror facets typically rely on relatively thicker glass and as such are accompanied by drawbacks such as losses in reflectivity and higher weight. 
     SUMMARY OF THE INVENTION 
     It is one object of the present invention to provide a glass structure which is robust yet light in weight and relatively inexpensive to manufacture. 
     It is another object of the present invention to provide a glass structure which is robust yet utilizes a relatively thin glass member. 
     It is yet another object of the present invention to provide a glass structure which employs a structure that applies a compressive force to a glass member to place the glass member in compression as to improve the strength of the glass structure. 
     It is a further object of the present invention to provide a glass structure which employs a relatively lightweight reinforcing member that does not affect the surface slope error of the glass structure. 
     It is yet another object of the present invention to provide a method for forming a glass structure. 
     In one preferred form, the present invention provides a glass structure having a glass member, a composite structure and a support structure. The composite structure includes a rigid interlayer which is bonded to the glass member and exerts a compressive force thereon to place the glass member in compression. The support structure is used to mount the glass structure and prevents the glass member from collapsing due to the compressive force exerted by the rigid interlayer. 
     In another preferred form, the present invention provides a method for forming a glass structure comprising the steps of providing a glass member and securing a rigid interlayer to the glass member such that the rigid interlayer applies a compressive force to the glass member. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims, taken in conjunction with the accompanying drawings, wherein: 
     FIG. 1 is a schematic illustration of a heliostat-type solar power system having a plurality of glass structures each constructed in accordance with a preferred embodiment of the present invention; 
     FIG. 2 is a perspective view of a portion of the glass structure of FIG. 1; 
     FIG. 3 is an enlarged perspective view of the glass structure of FIG. 1; 
     FIG. 4 is a schematic view of the glass structure of FIG. 1 being fabricated on a vacuum tool; 
     FIG. 5 is a schematic illustration similar to that of FIG. 1 but showing a parabolic dish configuration; and 
     FIG. 6 is a schematic illustration similar to that of FIG. 1 but showing a parabolic trough configuration. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIG. 1 of the drawings, an illustrative solar power system is generally indicated by reference numeral  10 . Solar power system  10  is shown to include an elevated receiver  12  and a plurality of heliostats  14 . Each of the heliostats  14  has a base structure  16  and a drive mechanism  18 , as well as a glass structure  20  that is constructed according to a preferred embodiment of the present in invention. Base structure  16  and drive mechanism  18  are conventional in their construction and operation and as such, need not be discussed in detail. Briefly, base structure  16  supports drive mechanism  18  and glass structure  20 . Drive mechanism  18  selectively orients glass structure  20  in a predetermined manner such that incident rays of solar energy  24  are reflected to receiver  12 . Accordingly, drive mechanism  18  operates to change the position (e.g., angularity) of glass structure  20  to track the position of the sun. 
     With additional reference to FIGS. 2 and 3, glass structure  20  is shown to include a glass member  30 , a composite structure  32  and a support structure  34 . Glass member  30  preferably includes a relatively thin glass panel  40  having a thickness of about 0.001 inches to about 0.4 inches. The rear side  42  of glass panel  40  is coated with a reflective material  44 , such as silver, a topcoat  46 , such as copper, and mirror backing paint. 
     Composite structure  32  includes a rigid interlayer  50  that is bonded to and applies a compressive force to glass member  30 . Rigid interlayer. 50  may be formed from a resin such as an unsaturated polyester, a bismaleimide (BMI), an epoxy vinyl ester or another epoxy, which is applied to the rear side of glass member  30  while in a liquid state by any practical means, including brushing, curtain coating, spraying and/or extrusion. The resin is then cured to increase the thickness of the assembly (i.e., the glass member  30  and the rigid interlayer  50 ) to provide increased durability. It should be noted that deformation of the assembly due to stress loads is reduced as the thickness of the assembly is increased, with the deformation being approximately inversely proportional to the cube of the thickness of the assembly. Thus, incorporation of rigid interlayer  50  into glass structure  20  limits the deformation of glass structure  20  even where a relatively thin glass member  30  is used. This permits the thickness of glass member  30  to be reduced so as to improve the reflectivity of glass structure  20 . Besides increasing the thickness and relative stiffness of glass structure  20 , rigid interlayer  50  also applies a compressive force to glass member  30 . In the particular embodiment illustrated, the resin forming rigid interlayer  50  shrinks as it cures, thus applying a compressive force to glass member  30 . Inorganic filler materials, such as calcium carbonate, may be incorporated into the liquid resin to control the shrinkage of the rigid interlayer while it is being cured. 
     In the example provided, composite structure  32  is also shown to include a reinforcing member  60  which further increases the thickness of glass structure  20  and its resistance to deformation. In the particular embodiment shown, reinforcing member  60  is a polymeric matrix composite containing a woven fiberglass mat reinforcement. The fiberglass mat reinforcement is initially saturated in liquid resin and subsequently placed onto rigid interlayer  50 . The resin in preferably the same resin used to form rigid interlayer  50  (i.e., an unsaturated polyester, a bismaleimide (BMI), an epoxy vinyl ester or another epoxy). 
     Support structure  34  is placed onto the fiberglass matting while the resin is still wet. Support structure  34  is adapted for use in mounting glass structure  20  to base structure  16  and spreading loads transmitted between glass structure  20  and base structure  16  over a relatively large area. Support structure  34  may be made from any structural material in any appropriate structural shape. Support structure  34  is preferably fabricated from a high-strength, low-cost and low-weight material such as fiberglass. Alternatively, support structure  34  may be fabricated from a metallic material such as steel or aluminum. In the particular embodiment illustrated, support structure  34  includes a plurality of hat-shaped beam sections  70 , with each section  70  being formed from a continuous strip to include a pair of flanges  72 , a pair of upwardly directed wall members  74  and a generally flat mount  76  as shown best in FIG.  3 . Curing of the resin in reinforcing member  60  bonds the flanges of the support structure  34  to composite structure  32  and causes reinforcing member  60  to apply an additional compressive force to glass member  30 . 
     In FIG. 4, a tool for fabricating glass structure  20  is generally indicated by reference numeral  80 . Tool  80  is shown to have a contoured surface  82  through which a plurality of feed holes  84  have been drilled. The feed holes  84  terminate at a central manifold  86 , which is coupled to a pressure gauge  88  and a shut-off valve  90 . Glass member  30  is initially placed on the contoured surface  82  of tool  80  such that the transparent surface of glass member  30  is in contact with the contoured surface  82 . A vacuum is applied through valve  90  to the central manifold  86 , causing glass member  30  to sealingly contact the contoured surface  82 . This places the front surface of the glass member  30  in compression and the rear surface in tension. Vacuum pressure is maintained through the valve  90  by a conventional vacuum source, such as a vacuum pump, to ensure maintenance of the desired contour during the entire fabrication process. Those skilled in the art will understand that the magnitude of the vacuum may be maintained at a predetermined level throughout the fabrication process or may be varied, depending on a number of factors that are particular to a specific application and need not be detailed herein. Vacuum gauge  88  is used to ensure the proper vacuum level is maintained. 
     Contoured surface  82  is fabricated to a predetermined shape that takes into account the compressive forces that are developed through the curing of resin, as well as the spring-like nature of the components of the glass structure  20  which cause the glass structure  20  to relax somewhat after it is removed from the tool. Resin which forms rigid interlayer  50  is next applied to the rear surface of glass member  30  and cured. As mentioned above, the rear surface of glass member  30  is initially in tension. However, as the resin shrinks when it cures, it generates a compressive force which is applied to the rear surface of glass member  30 . The compressive force is of sufficient magnitude to place all of glass member  30  (i.e., both the front and rear surfaces) in compression. It should be noted that the resin is preferably cured at a temperature that is greater than or equal to the maximum operating temperature of the glass structure  20  (i.e., the curing temperature should meet or exceed the maximum temperature that the glass structure  20  will be exposed to during its operation) so as to prevent the resin from permanently changing dimensionally during the use of the glass structure  20  or decreasing the desired compressive force by expansion of the resin which forms rigid interlayer  50  relative to glass member  30  by shrinkage of the resin which forms rigid interlayer  50  and shrinkage of the composite material  60  relative to the glass member  30 . 
     After rigid interlayer  50  has cured, reinforcing member  60  is applied to rigid interlayer  50 . Support structure  34  is then positioned onto reinforcing member  60  such that the flanges  72  contact the resin. The liquid resin is then cured at an elevated temperature as discussed in the immediately preceding paragraph which details the formation of the rigid interlayer  50 . Support structure  34  is bonded to reinforcing member  60  as the resin forming reinforcing member  60  cures to thereby provide structural support for glass member  30 . Once the curing of the reinforcing member  60  is complete, the vacuum in central manifold  86  is released to permit glass structure  20  to be removed from tool  80 . As mentioned above, support structure  34  provides structural support to glass structure  20  and prevents the residual compressive forces from collapsing the glass member  30 . 
     It is important to note that rigid interlayer  50  provides a uniform and continuous surface for the mounting of glass member  30 . In contrast, if glass member  30  were to be mounted directly to reinforcing member  60 , the small voids between the reinforcing fibers would leave the glass member  30  unsupported in the area of the void, thereby permitting the glass member  30  to dimple in response to the compressive forces that are developed when the resin cures. Accordingly, in a glass structure constructed in this manner (i.e. without rigid interlayer  50 ), the surface of the glass structure obtains an orange peel-like texture which tends to increase the surface slope error of the glass structure, resulting in a substantial decrease in the power delivered to receiver  12 . 
     While the glass structure  20  has been described thus far as being employed in a heliostat, those skilled in the art will appreciate that the invention, in its broader aspects, may be constructed somewhat differently. For example, the glass structure may form a single facet of a relatively large heliostat, or a single facet of a parabolic dish concentrator (FIG. 5) or a parabolic trough concentrator (FIG.  6 ). Accordingly, while the invention has been described in the specification and illustrated in the drawings with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention as defined in the claims. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment illustrated by the drawings and described in the specification as the best mode presently contemplated for carrying out this invention, but that the invention will include any embodiments falling within the foregoing description and the appended claims.