US20080142000A1

US20080142000A1 - Optic spacing nubs

Info

Publication number: US20080142000A1
Application number: US11/640,052
Authority: US
Inventors: Michael Milbourne
Original assignee: Sol Focus Inc
Current assignee: Sol Focus Inc
Priority date: 2006-12-15
Filing date: 2006-12-15
Publication date: 2008-06-19
Also published as: WO2008076781A1

Abstract

A solar energy system, including a front panel and at least two mirrors, is provided. The mirrors are used to focus light onto a photoconductive cell. In the preferred embodiment, three or more nubs are an integral part of at least one of the mirrors. When the system is assembled, these nubs are configured between the panel and a mirror to provide a substantially uniform gap for an adhesive. The mirror is secured to the panel by the adhesive. Thus, the nubs assist with desired attachment and alignment of a mirror to the panel in the solar energy system.

Description

BACKGROUND OF THE INVENTION

It is generally appreciated that one of the many known technologies for generating electrical power involves the harvesting of solar radiation and its conversion into direct current (DC) electricity. Solar power generation has already proven to be a very effective and “environmentally friendly” energy option, and further advances related to this technology continue to increase the appeal of such power generation systems. In addition to achieving a design that is efficient in both performance and size, it is also desirable to provide power units and corresponding solar systems that are characterized by reduced cost and increased levels of mechanical robustness.
Solar concentrators are solar energy generators which increase the efficiency of conversion of solar energy to DC electricity. Solar concentrators which are known in the art utilize parabolic mirrors and Fresnel lenses for focusing the incoming solar energy, and heliostats for tracking the sun's movements in order to maximize light exposure. A new type of solar concentrator, disclosed in U.S. patent application Ser. No. 11/138,666, entitled, “Concentrator Solar Photovoltaic Array with Compact Tailored Imaging Power Units” utilizes a front panel for allowing solar energy to enter the assembly, with a primary mirror and a secondary mirror to reflect and focus solar energy onto a solar cell. A back panel and housing enclose the assembly and provide structural integrity. The surface area of the solar cell in such a system is much smaller than what is required for non-concentrating systems, for example less than 1% of the entry window surface area. Such a system has a high efficiency in converting solar energy to electricity due to the focused intensity of sunlight, and also reduces cost due to the decreased amount of costly photovoltaic cells required. Because the receiving area of the solar cell is so small relative to that of the power unit, the ability of the mirrors to accurately focus the sun's rays onto the solar cell is important to achieving the desired efficiency of such a solar concentrating system.
In this type of solar concentrator panel, one of the key factors in mirror alignment is the process by which a mirror is adhered to the front or back panel. Uncontrolled adhesive application may result in variations in adhesive thickness across the bonding area of the mirror, which in turn may affect the alignment of the mirror as well as the bond strength which is important for withstanding high temperature conditions in the solar power assembly. In another instance, the proper amount of adhesive may be applied, but pressing the mirror and panel together in an uncontrolled manner may cause the adhesive to be exuded beyond the desired bond area and into the clear aperture of the system. Difficulty in attaining consistent adhesive application can decrease manufacturability and thus the commercial feasibility of such a design.
One solution to this problem of mirror alignment and attachment is using spacers to set the distance between the mirror and panel to which it is to be bonded. U.S. Pat. No. 5,433,911 entitled “Precisely Aligning and Bonding a Glass Cover Plate Over an Image Sensor” discloses an electronics package which includes a spacer plate, a glass cover plate, an image sensor, and a carrier. In order to achieve the tight tolerances for spacing and parallelism which are required to align the various planar components in this assembly, precision ground and lapped spacers are used. While the spacers result in the desired alignment and spacing between the plates and image sensor, the precision to which they must be made and the accuracy with which they are mounted increase the labor and cost of the assembly. The fact that the spacers are separate components also adds complexity to the manufacturing process.
Spacer particles are another approach to setting uniform distances between surfaces. U.S. Pat. No. 7,102,602 entitled “Doubly Curved Optical Device for Eyewear and Method for Making the Same” discloses a pair of substrates sealed together by a fluid material with spacers disbursed therein. The substrates thus have a uniform controlled distance there between due to the presence of the spacers. The spacers may be placed between the substrates prior to application of the fluid, or they may be mixed into the fluid material first and then applied to the unopposed substrates. While spacer particles are useful in setting the gap of a critical dimension, an assembly with more than one critical dimension would require specifically-sized spacer particles for each application. Such a situation raises the likelihood for potential manufacturing errors should one spacer size be mistakenly used in place of another size. Furthermore, each batch of adhesive would require verification of the proper ball diameter, and the step of mixing spacers into the fluid or adhesive adds labor to the manufacturing process.
Thus it is desirable to facilitate reliable alignment and attachment of the mirrors in a solar energy system in a manner which enhances manufacturability and therefore reduces overall cost and improves mechanical robustness.

SUMMARY OF THE INVENTION

The present invention is a solar energy system, including a front panel and at least one mirror. In the preferred embodiment, three or more nubs are an integral part of the mounting surface of the mirror. When the system is assembled, these nubs are configured between the panel and the mirror and provide a substantially uniform gap for an adhesive. The mirror is secured to the panel by the adhesive. Thus, the nubs assist with desired attachment and alignment of the mirror to the panel in the solar energy system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 provides a perspective illustration of an exemplary embodiment of the solar power unit;

FIG. 2 shows a cross-sectional view of the assembly of FIG. 1, with additional housing components;

FIG. 3 illustrates a side view of a secondary mirror with nubs mounted onto a front panel;

FIG. 4 provides a plan view of one embodiment of nubs on a secondary mirror;

FIG. 5A gives a perspective view of an alternative embodiment of a secondary mirror;

FIG. 5B is a cross-sectional view of the embodiment of FIG. 5A;

FIG. 6 shows a perspective view of an exemplary primary mirror with nubs on its perimeter;

FIG. 7 shows an exploded perspective view of an embodiment of the assembly process for aligning and attaching a secondary mirror onto a front panel; and

FIG. 8 is a simplified flowchart illustrating basic steps in the fabrication process.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Reference now will be made in detail to embodiments of the disclosed invention, one or more examples of which are illustrated in the accompanying drawings. Each example is provided by way of explanation of the present technology, not limitation of the present technology. In fact, it will be apparent to those skilled in the art that modifications and variations can be made in the present technology without departing from the spirit and scope thereof. For instance, features illustrated or described as part of one embodiment may be used on another embodiment to yield a still further embodiment. Thus, it is intended that the present subject matter covers such modifications and variations as come within the scope of the appended claims and their equivalents.
The alignment and attachment means described in this disclosure are based on a solar power unit design incorporating optically aligned primary and secondary mirrors. The solar power unit design is described in detail in related, co-pending patent applications as follows: (1) “Concentrator Solar Photovoltaic Array with Compact Tailored Imaging Power Units;” Ser. No. 11/138,666; filed May 26, 2005; and (2) “Optical System Using Tailored Imaging Designs;” Ser. No. 11/351,314; filed Feb. 9, 2006, which claims priority from U.S. provisional patent application 60/651,856 filed Feb. 10, 2005; all of which are hereby incorporated by reference as set forth in full in this application for all purposes.
Note that variations on the design described in the priority applications may be achieved by modifing specific steps and/or items described herein while still remaining within the scope of the invention as claimed.
With reference to FIG. 1, a simplified perspective view of an exemplary solar power unit 100 is shown. The main optical elements of the power unit 100 are a protective front panel 110, a primary mirror 120, a secondary mirror 130, and a receiver assembly 140. Note that for commercial application, the single power unit 100 would typically be replicated to form an array of adjoining power units as part of a complete solar panel. Protective front panel 110 is a substantially planar surface, such as a window or other transparent covering, which provides structural integrity for a power unit and protection for other components thereof In a preferred embodiment, front panel 110 is composed of glass; however, any type of transparent or transmissive planar sheet, such as polycarbonate, may be suitable for use in the solar power unit. Sunlight enters the solar unit 100 through front panel 110 and reflects off of primary mirror 120 to secondary mirror 130, where it is further reflected and focused onto receiver assembly 140. In the preferred embodiment, receiver assembly 140 houses an optical rod and a photovoltaic cell where the intensified sunlight is converted into electrical energy.
In reference still to FIG. 1, primary mirror 120 and secondary mirror 130 are substantially co-planar, at least a portion of both mirrors being in contact with front panel 110. In the depicted configuration, primary mirror 120 is generally circular such that the entire perimeter 160 of primary mirror 120 is contact with front panel 110. Primary mirror 120 is preferably a second surface mirror using, for example, silver, and slump-formed from soda-lime glass. In one exemplary embodiment, primary mirror 120 may have a diameter of approximately 280 mm and a depth of approximately 70 mm. Secondary mirror 130 is also generally circular, and is typically a first surface mirror using silver and a passivation layer formed on a substrate of soda-lime glass. In a preferred embodiment, secondary mirror 130 may have a diameter of approximately 50 mm. Nubs 150, to be described in further detail in reference to later figures, are present on the surface of secondary mirror 130 which is facing panel 110.
Turning now to FIG. 2, a cross-sectional view of a solar power unit 200 is shown. The same elements given in FIG. 1 of a front panel 210, primary mirror 220, secondary mirror 230 with nubs 250, and receiver assembly 240 are shown. In the view provided in FIG. 2, however, the additional components of a housing 260 and back panel 270 are illustrated in basic form. Housing 260 may be built from more than one piece of material, such as but not limited to stamped metal or polyethylene terephthalate (PET) and is designed to accommodate the total number of power units provided in a given solar energy system. The housing contains a lip 262 that allows the front panel 210 to be mounted, preferably with a rubber gasket (not shown) to seal the edges of panel 210. Back panel 270, which may also be referred to as a base plate, serves as a heat dissipation element for the solar unit and may be formed of phosphor-bronze or an aluminum alloy. Housing 260 and back panel 270 may be secured to the solar energy system by bolts, screws, or similar means (not shown) well-known in the art.
FIG. 3 provides a closer view of secondary mirror 330 and front panel 310. Nubs 350 are shown as projections from mounting surface 340 of secondary mirror 330. Nubs 350 can be separate pieces from the secondary mirror 330, or are preferably integrally fabricated as part of secondary mirror 330. By having nubs 350 integral to secondary mirror 330, any manufacturing tolerances resulting from either fabricating separate nub components or adhering nubs 350 to surface 340 are eliminated. Integral nubs fabrication could entail the nubs 350 being molded into the shape of the mirror 330 during the mirror pressing process. Alternatively, nubs 350 could be separate components that are insert-molded into the mirror during the pressing process. The height of nubs 350 are substantially equal, which advantageously sets a substantially uniform gap between mounting surface 340 of secondary mirror 330, and bottom surface 360 of front panel 310. This uniform gap thereby substantially aligns secondary mirror 330 in parallel to front panel 310. In a typical embodiment, the distance between mounting surface 340 of secondary mirror 330 and back surface 360 of the front panel 310 is 50 microns to 2.0 mm. Secondary mirror is secured to front panel 310 by adhesive 320, which fills the space between surfaces 340 and 360. In a preferred embodiment, silicone adhesive is used; however, any adhesive (epoxies, RTV, acrylics, etc.) which is appropriate for the substrates and operating conditions of this assembly may be utilized.
FIG. 4 next illustrates a plan view of the mounting surface of secondary mirror 410. In this embodiment, three nubs 420 are shown to be equally distributed near the circumference 430 of the mirror 410. The presence of three nubs 420 establishes the planar stability of secondary mirror 410. Alternatively, more than three nubs may be used for aiding the visual inspection that nubs 420 are contacting the front panel (surface 360 of FIG. 3), or for mechanical redundancy should any of the nubs 410 be damaged during the manufacturing process. While the placement of nubs 410 near circumference 430 as shown is desirable for increasing planar stability, the nubs may be placed in other configurations away from the circumference. For example, a nub positioned in the center of the secondary mirror 410 could be used to help center the secondary mirror onto the front panel. In another instance, the placement of nubs can assist in outlining the zones in which adhesive is to be dispensed.
Still referring to FIG. 4, nubs 420 are shown to be circular. However, other shapes may be used, such as rectilinear footprints, or even a hemispherical nub wherein the contact surface with the front panel would be a point. The specific cross-sectional area of nubs 420 chosen would be determined by the level of visual inspection desired as well as by the manufacturing limitations of the process by which the secondary mirror is fabricated. Also to be taken into account is that the shape and size of the nubs should not be conducive to damaging the panel, which may be glass, against which they are being placed. Furthermore, the impact of the total surface area occupied by the nubs would need to be considered so as not to impact the bond strength of this joint.
FIG. 5A depicts an alternative embodiment of the secondary mirror 510. While previous embodiments have shown secondary mirror 510 to be a solid entity, FIG. 5 shows secondary mirror 510 in the case where it is hollow. Moreover, an alternative nubs embodiment consisting of four nubs being present and equally distributed around the mounting surface 530 of secondary mirror 510 is given. In this exemplary embodiment depicted in cross-section in FIG. 5B, nubs 520 are integral to secondary mirror 510. That is, nubs 520 are formed during the fabrication of secondary mirror 510. The nubs 520 are shown to be cylindrical in nature, but as previously described, they may take the form of rectilinear or other shapes as desired to facilitate fabrication of the secondary mirror, or to aid in the process of assembling the solar power unit. Edges 540 of nubs 520, in this embodiment as well as others described in this disclosure, are preferably filleted to prevent damage to the front panel when the mirror 510 is placed in contact with the front panel.
FIG. 6 illustrates a further embodiment of primary mirror 610. In this embodiment, primary mirror 610 is shaped such that there are truncated sections 620 of the curved primary mirror 610. The truncated sections advantageously allow adjacent power units to fit tightly together in a solar array, thus maximizing the number of power units which can be packed into a solar energy system. For example, in the depicted configuration where there are four truncated sections, adjacent power units would fit together to form an orthogonal grid. The peaks of the truncated sections terminate in flat mounting tabs 630, upon which nubs 640 are placed or formed. In the truncated design, only the mounting tabs 630 with the nubs 640, rather than the entire perimeter of primary mirror 610, are in contact with the front panel. The heights of nubs 640 are substantially equal, thus setting a substantially uniform gap for adhesive to be applied, and thus substantially aligning the primary mirror to the front panel.
Returning to the secondary mirror, FIG. 7 gives an exploded view of a template tool being used in the manufacturing process to secure secondary mirror 710 to front panel 720. Template tool 730 includes a precision cutout 740 for centering secondary mirror 710 over the transparent front panel 720. It should be appreciated that for manufacturing an array of solar power units, the template tool 730 would incorporate multiple cutouts 740 for the multiple mirrors in the array. Template tool 730 could also be a part of a larger tool which includes additional functionality. While cutout 740 provides for proper planar positioning along the face of front panel 720, nubs 750 ensure that the mounting surface 760 of secondary mirror 710 is aligned substantially parallel to front panel 720. That is, nubs provide alignment in the axis perpendicular to the front panel. Placement of the mirror 710 onto the front panel 720 could be achieved by automated machinery, in which case the fixed spacing provided by the nubs would have further importance in manufacturing reliability.
FIG. 8 is a simplified flowchart illustrating the basic steps in securing a mirror to the front panel. In FIG. 8, flowchart 800 is entered at step 810. Step 820 is first performed to position the template tool over the front panel. This can be accomplished by registering the template tool with the front panel using visual, mechanical, or other means well-known in the art (pin registration, magnetic or other sensing, etc.). Next, if the nubs are not integral to the mirror, step 830 is performed to fix the nubs onto the mirror. In step 840, adhesive is dispensed onto either the front panel or mirror. Typically, the adhesive is dispensed in a discrete location or locations such as lines or dots. In step 850, the mirror is placed on the front panel through the aforementioned cutout on the template tool. Step 860 is then performed to distribute the adhesive over the mounting surface of the secondary mirror. For example, in the case of a solid secondary mirror (FIG. 4), the adhesive could be first applied in two lines forming an “X” in step 840. Then, rotating the mirror in step 860 would distribute the adhesive across the circular mounting surface. In the case of a hollow secondary mirror (FIG. 5A), the adhesive could be dispensed in dots between the nubs. Rotation of the mirror in step 860 would then distribute the adhesive around the perimeter of the mirror's mounting surface. In an alternative embodiment, after the adhesive is applied, compression may be used to distribute the adhesive between the mounting surface of the mirror and the front panel. This pressure could be applied through the mirror, through the front panel or from both sides.
Still referring to FIG. 8, step 870 provides verification of proper adhesion. In the preferred embodiment, after the adhesive has been distributed the operator would verify in step 870 whether the nubs are in full contact with the front panel. Verification methods could include a qualitative visual check, or quantitative means such as measurement of the mirror height before and after bonding. Failure for all nubs to be in contact would imply that a uniform adhesive gap has not been achieved, and that the mirror is misaligned. In this case, step 880 calls for adjusting mirror placement. Adjustment could involve such measures as applying more pressure to the mirror or removing excess adhesive which may have seeped under the nubs. Once it is verified that nubs are properly in contact with the front panel, the subassembly is complete.
Although embodiments of the invention have been discussed primarily with respect to specific embodiments thereof, other variations are possible. Lenses or other optical devices might be used in place of, or in addition to, the primary and secondary mirrors or other components presented herein. For example, a Fresnel type of lens could be used to focus light on the primary optical element, or to focus light at an intermediary phase after processing by a primary optical element. Beyond solar energy systems, nubs may be used to align a lens in an optical assembly, or to provide spacing with respect to mating components.
It may be possible to use non-planar materials and surfaces with the techniques disclosed herein. Other embodiments can use optical or other components for focusing any type of electromagnetic energy such as infrared, ultraviolet, radio-frequency, etc. There may be other applications for the fabrication method and apparatus disclosed herein, such as in the fields of light emission or sourcing technology (e.g., fluorescent lighting using a trough design, incandescent, halogen, spotlight, etc.) where the light source is put in the position of the photovoltaic cell. In general, any type of suitable cell, such as a photovoltaic cell, concentrator cell or solar cell can be used. In other applications it may be possible to use other energy such as any source of photons, electrons or other dispersed energy that can be concentrated.
Steps may be performed by hardware or software, as desired. Note that steps can be added to, taken from or modified from the steps in this specification without deviating from the scope of the invention. In general, any flowcharts presented are only intended to indicate one possible sequence of basic operations to achieve a function, and many variations are possible.
While the specification has been described in detail with respect to specific embodiments of the invention, it will be appreciated that those skilled in the art, upon attaining an understanding of the foregoing, may readily conceive of alterations to, variations of, and equivalents to these embodiments. These and other modifications and variations to the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention, which is more particularly set forth in the appended claims. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention.

Claims

1. A solar power unit, comprising:

a substantially planar surface;

a primary mirror radially symmetric about a first axis, said primary mirror having a perimeter wherein at least a portion of said perimeter is attached to said planar surface;

a secondary mirror radially symmetric about a second axis, said secondary mirror having a mounting surface wherein at least a portion of said mounting surface is attached to said planar surface;

three nubs on said mounting surface, said nubs having nub heights, wherein said nub heights are substantially equal; and

an adhesive substance;

wherein said secondary mirror is secured to said planar surface by said adhesive substance, and wherein said nubs provide a substantially uniform gap between said mounting surface and said planar surface for said adhesive substance.

2. The solar power unit of claim 1, wherein said nubs are integral to said secondary mirror.

3. The solar power unit of claim 1, wherein said primary mirror and said secondary mirror focus light on a photovoltaic cell.

4. The solar power unit of claim 1, wherein said nub heights determines the bond thickness of said adhesive.

5. The solar power unit of claim 1, wherein said first axis is coaxial to said second axis.

6. A method of attaching and aligning a mirror with integral nubs to a panel of a solar power unit, comprising:

dispensing an adhesive onto said panel;

positioning said mirror over said panel;

placing said mirror with said integral nubs in contact with said adhesive;

distributing said adhesive; and

confirming contact of said nubs with said panel;

wherein said integral nubs have nub heights, and wherein said nub heights provide a substantially uniform gap in which to distribute said adhesive substance.

7. The method of claim 6, wherein said mirror is a curved primary mirror with a perimeter, wherein said nubs are configured on said perimeter of said primary mirror, and wherein said nub heights are substantially equal.

8. The method of claim 7, wherein four nubs are configured on said perimeter of said curved primary mirror.

9. The method of claim 6, wherein said mirror is a curved secondary mirror with a mounting surface, wherein said nubs are configured on said mounting surface of said secondary mirror, and wherein said nub heights are substantially equal.

10. The method of claim 9, wherein three nubs are configured on said mounting surface of said curved secondary mirror.

11. The method of claim 6, wherein said distributing of said adhesive comprises rotating said mirror.

12. The method of claim 6, wherein said distributing of said adhesive comprises compression.

13. A solar energy system, comprising:

a panel;

a mirror having a mounting surface;

three nubs with nub heights, wherein said nubs are integral to said mounting surface of said mirror; and

an adhesive substance, wherein said mirror is secured to said panel by said adhesive substance, and wherein said nub heights provide a substantially uniform gap between said panel and said mounting surface of said mirror.

14. The solar energy system of claim 13, wherein said nub heights determine the bond thickness of said adhesive substance.

15. The solar energy system of claim 13, wherein said mirror is a curved primary mirror with a perimeter, wherein said nubs heights are substantially equal, and wherein said nubs are configured on said perimeter of said primary mirror.

16. The solar energy system of claim 13, wherein said mirror is a curved secondary mirror with a mounting surface, wherein said nub heights are substantially equal, and wherein said nubs are configured on said mounting surface of said secondary mirror.

17. The solar energy system of claim 13, wherein said mirror directs light to a photovoltaic cell.

18. The solar energy system of claim 13, wherein said mirror is symmetric about a first axis; and

further comprising a secondary mirror symmetric about a second axis substantially coaxial to said first axis, said secondary mirror having a mounting surface wherein at least a portion of said mounting surface is attached to said panel.

19. A solar energy system comprising;

a panel;

a pressed optic having a mounting surface;

multiple nubs with nub heights, wherein said nubs are integrally formed on said mounting surface;

an adhesive substance, wherein said pressed optic is secured to said panel by said adhesive substance, and wherein said nub heights establish a spacing between said mounting surface and said panel.

20. The solar energy system of claim 19, wherein said pressed optic is a mirror.

21. The solar energy system of claim 19, wherein said pressed optic is a lens.

22. The solar energy system of claim 19, wherein said nubs assist in alignment of said pressed optic with said panel.