GB2510505A - Solar reflectors having topographical features - Google Patents

Abstract

A solar reflector comprises a light scattering reflecting surface (52, figure 4) having topographical features 38 that cause sunlight to be scattered with substantially uniform luminous intensity within a defined limit of angular divergence. Preferably the reflecting surface comprises a mosaic of embossed topographical features having a shallow spherical form, which more preferably repeat in a hexagonal pattern (figure 3a). The reflecting surface may be applied to the surface of a tensile fabric having a core of high strength fabric (31) coated with plastic (32, 33) on at least one side. The tensile fabric preferably has keder cord edging (45) and may be tensioned in two directions by a frame (54, figure 4) to lie in a flat plane.

Description

Booster reflector for fixed P-V solar arrays

Background to the invention

The invention relates to solar energy technology in which fixed (non-tracking) solar photo-voltaic arrays convert sunlight directly to electricity, and in which non-concentrating reflectors reflect additional sunlight onto the arrays to boost the sunlight received directly. The invention provides improvements in the design of such reflectors using known principles of tensile fabric architectural design, and shows how these principles may be adapted to the construction of reflectors at low cost. The invention accommodates residual optical imperfections of fabric reflectors by providing surfaces which scatter light in a specific efficient and controlled manner. In this way additional and uniform illumination is reflected onto a solar array, matching the characteristics of photo-voltaic solar cells.

The problem Although sunlight is free, photo-voltaic solar cells are expensive. Since reflectors can be far cheaper per area than solar cells, it would appear to be economically attractive to place them to reflect additional sunlight onto arrays of solar cells as this would boost the electrical power and energy generated at small additional capital cost. However for fixed arrays of solar cells directly exposure to the sun, there is an inconvenient technical problem that makes reflectors impractical or less economic. The problem, which is solved by this invention, arises from a conflict between an electrical and an optical principle.

Individual solar cells which act as electrical current sources are unable to conduct any more electrical current than is generated within them by incident sunlight. For practical reasons arrays have large numbers of solar cells that are connected electrically in a series string. As a consequence an array can only generate as much current as is generated by its least well illuminated cell. Thus if additional reflected light is to be used effectively it must be spread very uniformly over the entire array. Unfortunately the large size of arrays and the consequently necessary large size of reflectors dictate that reflected light has to be projected a considerable distance to illuminate much of the array. It has proved difficult to make large rigid reflectors with sufficient optically accuracy to ensure the required.uniform illumination. Slight bulges, convexities, wrinkles or misatignments of the reflector surface, are optically magnified by the projected distance to cause localised attenuations or voids in the light projected onto the array Thus a large reflector with a mirror-like surface must have high optical accuracy to effectively boost the power output of an array of solar cells. Due to the cost this entails, reflectors have become uneconomic as the cost of solar cells has reduced with time.

Prior art

Reflectors used to boost the power generated by fixed (non-tracking) arrays of solar cells have been described with both specular and diffusing reflctin surfaces. 3 Wenzel in US4933020 describes an arrangement with specular reflector arranged geometrically in a similar manner to the present invention, but this arrangement has been adopted very little for the economic reasons described above. Pursuing the same purpose, Kurram K. Nawab in WO 2010/060003 Al teaches methods based on aluminium composites to produce and employ reflecting metal sheets free from waviness. The Goldensun Company of Slovakia, employ elaborate and expensive support structures of concrete and steel to ensure the accuracy of mirror-like reflectors (SKSS91 Jan Zupa.8 Nov 2008), and also accurately position individual mirrors using feedback control sensing solar position (Jan Zupa SK5784 6 July 2006) Light scattering or diffusive reflective surfaces are well known in low concentration solar energy technology. See Maria Brogren Optical efficiency of low-concentrating solar energy systems1 page 124, and S Ebert in DEI.02006043635. D Meyer in WO 2010/096833 and WO 2012/021650, describes small scale structures intended for rooftop installation, with solar modules closely coupled to reflectors to reduce the distance light must be projected. Diffusive reflectors of non-specific character are described as achieved through "anisotropic texturing" or "stippling" which "substantially avoid the concentration of reflected light rays onto individual PV cells", an advantage which is considerably refined and improved in the present invention. Despite the claimed advantage, most embodiments in this disclosure depend on special "illumination agnostic" solar modules unaffected by concentration and non-uniformity in illumination. Suitable "Illumination agnostic" solar modules are described in US 20090183760 using electronic "control modules" to reconfigure solar cells in series or parallel according to the non-uniformity of illumination. Such modules in redudng the sensitivity to non-uniform illumination, would be an alternative solution to the problem addressed by the present invention, but being more complex than the mass produced conventional solar module would be more expensive.

The use of fabric to support a semi-diffuse reflective surface for use with solar systems is described by S Ebert in DE102006043635. The fabric is deployed from a roller by a hinge or telescopic mechanism only when weather and solar conditions are suitable. In US 12/062410 Bronstein employs tension to correct the form of an arcuate (not fiat) fabric reflecting surface, to a more ideal form to concentrate solar radiation onto a track-able (not fixed) energy absorbing surface that is not directly illuminated.

In summary the cited prior art appear to anticipate the problem, but not the solution provided by the present invention of a fabric surface of specific reflection characteristics.

Summary of the invention

The present invention aims to provide a reflector system that is substantially flat and able to provide additional illumination uniformly over an array of solar cells which may be of large size, and is also of low installed cost relative to the array. This way the capital cost of a solar power plant may be reduced relative to the energy produced. To meet that aim, the invention provides a membrane of reflective fabric tensioned within a frame or structure, the fabric having a semi-diffusive reflecting surface, scattering sunlight in a manner optimised to compensate for small but unavoidable departures of the fabric membrane from perfect flatness, such as wrinkles.

It is known that a scattering or diffusive reflecting surface reduces the variation in intensity of the illumination of a solar array caused by imperfections in the reflecting surface. However it can be shown and demonstrated experimentally that when the reflected sunlight is not merely randomly diffused, but is diffused with uniform luminous intensity throughout a conical solid angle centred about direction of sunlight specularly reflected, then the intensity of light illuminating the array surface can be not only more uniform but also maximised. Figure le in which reflection according to this invention is compared with random diffusion of figure id will provide an intuitive appreciation of why this is so, resulting as it does from the minimisation of reflected light 9 that fails to illuminate the array 2 and so is wasted. The surface form detailed in this specification ensures that this favourable and efficient mode of diffusion is achieved.

Tensile fabric architecture consists of a web of plastic coated fabric held semi-rigidly in tension applied by a metal structure. Light in weight, easily assembled, and cheap per unit area, these structures offer large surfaces free of discontinuities. This disclosure shows tensile fabric methodologies adapted to provide a surface as flat as possible, that in combination with a light scattering reflecting surface, provides uniform illumination over solar arrays.

The disclosure includes means to incline the reflector units to the most effective angle, and to assemble long evenly illuminating reflectors.

The drawings la. Perspective sketch of part of a typical solar farm of prior art.

lb. Cross section showing a flat reflector positioned between the reflectors.

lc. As figure lb showing exaggerated the effect of deviation from flatness.

id. As figure ic showing the remedial effect of a light scattering or diffusive reflection surface.

le. As figure ic showing advantage of this invention in reducing wasted sunlight.

2a. Diagram showing reflection of sunlight from a spherical surface.

2b. Isometric illustration of a surface according to this invention showing hexagonal mosaic of topographical features.

2c. Section through a surface according to this invention with sharp blend between adjacent topographical features.

2d. Section through a surface according to this invention with adjacent features blended with constant curvature.

3a. Reflective tensile fabric with two alternative keder edgings.

3b. Isometric view of a topological feature illustrating spherical surface and blending of adjacent features.

3c. A further alternative keder edging.

4. Perspective view of a large array with reflector according to this invention.

4a. Section through beam 56 showing retention of keder edge 5. Perspective view of a domestic array with reflector according to this invention. -Sa. Section through tube 76 showing retention of keder edge.

Sb. Section through tube 76 showing alternative attachment of web to tube.

Description of the invention

The optical principles involved in the problem and of its solution, are described first so that the purpose of each part of the invention may be easily understood. Then two examples are described of the practical application, one industrial and the other domestic.

Optical principles Figure la shows a small part of a typical solar farm of prior art. Arrays of solar panels 1 are arranged to stretch in an east-west direction to receive direct sunlight through most of the day. The arrays are separated by spaces 3 sufficiently wide to avoid shadowing of one row by another when the sun has a low ascension.

Figure lb shows in cross-section how a flat reflector 4 can be positioned within the space 3. Reflector 4 can be according to prior art or according to this invention. Reflector 4 is angled to reflect further sunlight S onto array 1.

Reflector 4 may be longer than the array 1 in order to fully illuminate array 1 early and late in the day. Reflector 4 if perfectly flat and perfectly reflecting would increase the illumination of array 1 uniformly by 30% for the ideal geometry shown, producing an equal percentage increase in electrical power output.

Figures lb to le are intended to convey an approximate and intuitive idea of the principles involved in the invention. Sunlight is depicted in these figures conventionally as light rays.

Figure lc shows in exaggeration how a deviation from flatness at reference 6 of reflector 4 can cause a void 7 in the additional illumination provided by reflector 3 on array 1. It is explained above under the heading "Problem" how this void 7 would severely reduce any increase in electrical output of array 1 that could be produced by reflector 4.

Figure id illustrates how diffused reflected light 8 from a reflector 9 of prior art can fill-in the void 7 of figure ic and so restore the additional electrical output that would have been produced in with a perfect reflector. (as figure lb.) However a significant proportion of rays which are diffused at large angles to the normal direction of reflection are seen at 10 to be wasted as they do not illuminate the array 1.

Figure le shows a how a diffusing reflector 11 according to this invention, in which the diffusion of reflected rays 12 away from the normal specular direction of reflection is limited to a small and defined solid angle of divergence, minimises the wasted illumination 13, thereby increasing the intensity of illumination of array 1 to the greatest extent and hence maximising the electrical output power. It can further be shown that a reflector which in addition diffuses the incident sunlight with uniform intensity throughout the limited angular range of divergence can provide the maximum uniformity of illumination for a given distortion of the reflector while at the same time minimising wasted light. To meet these desirable aims the invention provides a surface which scatters sunlight with substantially uniform luminous intensity, limited to a conical solid angle of divergence centred about the path of sunlight specularly reflected from the plane of the reflector and isotropic in the plane of the reflector. How such a reflecting surface may be achieved is now described.

Figure 2a which is on a much smaller scale than the previous figures illustrates how a small area of a reflecting surface having a circular boundary 21 and a form that is part of a sphere (shown in a conventional way at 20), has the geometrical and optical property of reflecting collimated light such as sunlight 22 into a conical beam 23 with a uniform distribution of luminous intensity within that cone. This is because a sphere has the property ofa uniform distribution of solid angles of its surface. Further it may be appreciated from physical laws of reflection, that as a result of boundary 21 being circular, reflected sunlight is limited in its divergence to an angle 25 from the direction of a ray 24 specularly reflected from the plane of the surface. The defined angle 25 is twice the angle of the slope 28 of the reflecting surface at its circular boundary 21 shown in figure 2d.

Thus a mosaic of such features together tiling a flat surface, provides the advantageous diffusive reflecting properties of substantially uniform luminous intensity and limited angular divergence. For the mosaic surface to be entirely of this spherical form, the boundaries 26 between adjacent features 29 would be necessarily of hexagonal shown in figure 2b rather than circular. Also a sharp discontinuity of surface slope would occur at the hexagonal boundaries 26 as is shown in section in figure 2c, but such a sharp boundary cannot easily and durably be formed on the tensile fabric of this invention. These two problems are avoided by blending the surface between adjacent features as is illustrated in section in figure 2d. If the blending surface 40 has a true radius 27 in passing from one spherical form to the next, or stated more precisely has constant curvature in the direction of the blend, then the beneficial property of light being reflected with uniform intensity throughout a conical solid angle with limited divergence, is also retained by this non-spherical part of the reflecting surface. That this is so may be understood by recognising that such radiused blends approximate to sections of ring toruses of various sizes. The torus is a solid figure which shares with a sphere the property of uniform distribution of solid angles of Its surface. Such radiused blends allow the transition between spherical surface and blend surface to be circular. This is illustrated at 42 in the isometric view of figure 3b, which also shows hatched at 40 the extent of the blending surface surrounding the transition.

Thus the surface topography described of this invention contributes at all points to reflective properties that can maximise the output power of an illuminated solar array.

Detail description

To provide a durable, weather tolerant solar reflector at low cost, the reflecting surface, is applied to a web of fabric tensioned within a supporting structure, an existing technology known as tensile fabric architecture. Tensile fabric is produced in commodity quantities by coating both sides of a high strength fabric such as polyester, with a plastic such as plasticised poly vinyl chloride (PVC). Tensioned within a surrounding metal structure, the fabric is used to construct temporary and permanent buildings such as halls, marquees, and canopies, making this technology eminently suitable for solar reflectors. The following description of the invention refers to tensile fabric, but the invention is not restricted to any particular combinations of materials, although PVC coated polyester fabric has been found particularly suitable.

The invention has a reflective layer applied or laminated to the tensile fabric.

This may be adhesively attached but alternatively could be applied by other means such as metal evaporation. The reflective layer must necessarily be of high reflectivity, and highly durable. Specifically it must be able to retain its reflectivity substantially unchanged over a period of years despite exposure to all weather, to sunshine including ultra-violet radiation, and to being cleaned.

Suitable specular reflective layers are commercially available in the form of self adhesive films for reflective solar power concentration. Suitable films have one or more layers of specular reflective metal alloy, plus further layers to optimise the optical properties. These are applied to plastic film, and covered by an outer protective layer, transparent to visible light but providing protection from ultra-violet radiation. Such specular reflective films are described in US patents 6989924, 7612937, and other patents, where they are described as mirror films or mirrors, but this invention is not restricted to any particular reflective layer technology.

The reflective tensile fabric, of this invention laminated or otherwise, has a surface topography embossed or impressed on its surface with the form and properties described under the heading "Optical principles" above. A particular example or embodiment of the invention will now be described.

Referring to figure 3a, a web or membrane of tensile fabric 30 is formed of pblyester fabric 31, having a coating 32 of PVC on one side and a further coating 33 of PVC on the reflective side. A reflective film 34 is adhesively laminated on to coating 33. Reflective film 34 comprises a polymeric substrate layer 35, with a transparently protected specular reflective layer on the upper surface 36. Reflective tensile fabric 37 thus formed, is embossed so that reflective layer 36 is raised into a mosaic of topographical features 38. The mosaic of features 38 repeats on a hexagonal grid indicated at 26. Each topographical feature 38 comprises a low hump of shallow spherical form. The surface of adjacent features are blended smoothly together through surfaces having constant curvature through the blend. Figure 3b provides an alternative illustration of the spherical surface of a feature 38 surrounded by a smoothly blending surface shown hatched 40.

The means by which the topographical forms 38 are created can be as is illustrated at 41 by deformation of the entire thickness of the reflective tensile fabric 38. Such deformation applied to the reflective tensile fabric laminate by compression between engraved dies or rollers is permanently retained under tension by the materials described. However alternative examples of the invention are possible in which the deformation of the under layer 31 and the fabric 32 are less than the deformation of the reflective layer or minimal.

The scale of the hexagonal mosaic grid 26 as measured for instance by the length of one of its sides is not critical to the operation of the invention provided that it is several times the wavelength of light, say 5 microns or more. More critical however is the slope angle of the surface form at the commencement of the blends with respect to the average plane of the reflective fabric shown at 27 in figure.2d. As explained under the heading "principles" this angle determines the limit on divergence of the reflected light and hence the increase in electrical power than may be obtained for a given reflector and solar array geometry. In practice this constraint leads to a slope angle 28 of about one degree and to a hexagon size in the range of 5 to 20 millimetres.

The reflective fabric so far described, may be produced in large webs, several metres wide and hundreds of metres in length, all of its area having the surface form as described. Figure 3a shows how the edges of reflective fabric are 44 are prepared for attachment to a supporting structure in accordance with common practice with tensile fabric architecture. A thick cord 45 known as Keder cord, is enclosed by a polymeric fabric tape 46 and is welded to the edges of reflector fabric 37 by induction heating under pressure. The resultant thickened edge is known as keder edging, and is housed within a slot running along the length of a supporting member. For the reader's information, keder edging is commonly employed to attach yacht sails to masts and spars Figure 3a shows two examples of how the welding may be accomplished. At reference 47 keder tape 46 is welded to the top surface of reflective film 33 and to the under surface 31 of tensile fabric 30. For some varieties of reflective film it is necessary to remove the metallic reflective layer by abrasion in order that it is not destroyed by induced electric currents during welding. In an alternative construction of keder welding at reference 48, a margin of the reflective film has been removed allowing keder tape 46 to be welded to both top and bottom surfaces of tensile fabric 30. Figure 3c shows a third alternative construction of reflective tensile fabric and welded keder edging. Keder cord 45 is attached to tensile fabric 29 by enclosure within fabric tape 46 welded directly to both sides of tensile fabric 30. Pre-embossed reflective film 49 is then laminated to tensile fabric 30 and covering the welded tape.

Figure 4 shows the invention applied to an industrial scale solar array 51. A web 52 of reflective tensile fabric according to this invention, has a reflective surface 53 and is tensioned within a quadrilateral shaped frame 54. Frame 54 is formed of longitudinal supporting beams 55 and 56 and transverse supporting beams 57 and 58. Beams 55 to 58 are formed of hollow aluminium extrusions, shown at 64 in the sectional drawing of figure 4a, and have a continuous slot 65 to accept and retain under tension the Keder edge 66 of web 52. Frame 54 is shown as being square, but need not necessarily be so.

In figure 4, frame 54 has at each corner a knuckle 59 having spurs which enter the respective tubes 55 to 58, and wedges one of which is illustrated at 60, by which the size of the frame may be expanded to apply tension in both orthogonal directions to web 52. Expandable struts 61, extending between longitudinal beams 55 and 56 and beneath web 52, are adjusted to provide uniform transverse tension of web 52 along its length. Frame 54 is pivotally attached to the ground at a plurality of co-axial locations 62 along longitudinal beam 55 adjacent to array 51.. Electrically operated struts 63 support beam 56, at several points along its length, by which means the tilt of frame 54 and hence of reflective surface 53 may be adjusted to ensure the flatness of surface 53 and to best illuminate array 51 for the time of year.

Figure 5 shows a second example of the invention applied to a domestic solar array 71. A web 72 of reflective tensile fabric according to this invention, has a reflecting surface 73 and is tensioned within a quadrilateral shaped frame 74. Frame 74 is formed of longitudinal steel tubes 75 and 76 and transverse tubes 77 and 78. Tubes 75 to 78, shown at 85 in the sectional drawing of figure 5a, have a longitudinal cut 86 along their length to accept and retain under tension the keder edge 87 of web 72.

In figure 5 frame 74 has at each corner a knuckle 79 having spurs which enter the respective tubes 75 to 78, and wedges one of which is illustrated at 80, by which the size of the frame may be expanded to apply tension in both orthogonal directions to web 52. Expandable struts 81, extending between longitudinal beams 75 and 76 and beneath web 72, are adjusted to provide uniform transverse tension of web 72 along its length. Frame 74 is pivotally attached to the ground at a plurality of co-axial locations 82 along longitudinal beam 75 adjacent to array 71. Struts 83 adjustable in length support and secure beam 76 from the ground, and by means of which the tilt of frame 74 and hence of reflective surface 73 may be adjusted to ensure the flatness of surface 73 and to best illuminate array 71 for the time of year.

Figure Sb shows in section an alternative means of attaching web 72 to tubes to 78. Web 72 is wrapped around the respective tube 88 and is welded to itself at 89 to form a pocket which extends along the full side of the web. This means of attachment does not require tubes 75 to 78 to be cut along their length.

Summary

The invention relates to solar reflectors which reflect sunlight onto fixed (not track-able) arrays of photo-voltaic solar cells in order to boost the sunlight received directly by the cells. The invention comprises improvements in the optical design enabling such reflectors to be constructed at low capital cost through the use of known principles of tensile fabric architectural design.

Examples of the construction of tensile fabric reflectors are described.

Semi-diffusive reflector surfaces are described which scatter light more efficiently than prior art reflector surfaces, evening out irregularities in reflected light intensity caused by the small imperfections of such membrane reflector surfaces, while minimising the loss of scattered light. In this way additional illumination of a solar array is provided matching the characteristics of photo-voltaic solar cells.

While the invention has been shown and described with reference to the above examples, it will be apparent that changes in form and connection and detail may be made without departing from the principles and scope of the invention as defined in the claims.

Claims (11)

1. Claims 1. A solar reflector with a light scattering reflecting surface having topographical features which cause sunlight to be scattered with substantially uniform luminous intensity within a defined limit of angular divergence.
2. 2. A solar reflector as in claim 1 in which the light scattering reflecting surface is a specular reflector at a small scale with a mosaic of embossed topographical features at a larger scale, the features having a shallow spherical form which may be either raised or depressed.
3. 3. A solar reflector as in claim 2 in which the mosaic of topographical features repeats in a hexagonal pattern.
4. 4. A solar reflector as in claim 2 in which the surface form of adjacent topographical features blend smoothly together through surfaces having constant surface curvature through the blend.
5. 5. A solar reflector as in claim 1 n which the light scattering reflecting surface is applied to the surface of a web of tensile fabric, the tensile fabric having a core of high strength fabric coated with plastic on at least one side.
6. 6. A solar reflector as in claim 5 in which the light scattering reflective surface is applied to the tensile fabric by lamination, the composite laminate being embossed through its thickness.
7. 7. A solar reflector as in claim 5 in which in which the light scattering reflective surface is the surface of reflective film pre-embossed with topographical features and adhesively laminated to tensile fabric.
8. 8. A solar reflector according to claim 5 in which the tensile fabric with applied light scattering reflecting surface has keder edging.
9. 9. A solar reflector as in claim S in which the tensile fabric is supported and is tensioned in two directions by a frame to lie in a flat plane.
10. 10. A solar reflector as in claim 5 in which the web of tensile fabric with light scattering reflective surface has keder cord edging and is supported and tensioned in longitudinal and transverse directions by a frame, the frame having members with slots along their length, to retain the keder cord edging.
11. 11. A solar reflector according to claim 12, in which the web of tensile fabric with light scattering reflective surface is tensioned by expansion of struts extending between longitudinal members of the supporting frame.