GB2493329A

GB2493329A - Curved reflective surface for concentrating EM radiation with obstructing members

Info

Publication number: GB2493329A
Application number: GB1112087.0A
Authority: GB
Inventors: Nicholas Deakin
Original assignee: Athene Works Ltd
Current assignee: Athene Works Ltd
Priority date: 2011-07-14
Filing date: 2011-07-14
Publication date: 2013-02-06
Also published as: GB201112087D0

Abstract

The present invention relates an apparatus for concentrating incident electromagnetic radiation in which obstructing member (4 and 7) project from the reflecting surface perpendicular to the focal plane, in some embodiments the reflective member is a parabolic trough shape or hemispherical bowl shape, the obstructing member may project across or along the apex of the trough or bowl or along the edge of the trough, several obstructing members may extend across the bowl at angles to each other (see figure 7) in other embodiments the obstructing member may be a transparent convex covering the focal surface (see 7 on figure 2), in some embodiments the outer surface of the covering member may be self cleaning, in another embodiment the reflective member may be attached to a framework which defines am opening for receiving at least one ballast receptacle.

Description

APPARATUS

The present invention relates an apparatus for concentrating incident electromagnetic radiation, for example incident electromagnetic radiation with a visible wavelength or a S non-visible wavelength.

Concentrator Photovoltaic (CPV) Panels are known. Typically they comprise a reflector or lens to concentrate electromagnetic radiation into a receiver for generating energy in the form of electricity or heat. Dishes for focussing electromagnetic radiation into a receiver for communications purposes are also known. Examples of receivers include: photovoltaic cell(s) to generate electrical power and antennas. Thermal energy may also be harvested by Thermal Electrical Generators (TEGs) or used directly as heat, for example to power other equipment such as a turbine to produce power.

The use of such concentrators and dishes presents several disadvantages. The reflector is ideally highly reflective to improve efficiency. This can create a highly visible structure, particularly when the concentrator is reflecting visible wavelengths. The shape of the reflector is subject to wind loading, which can impose a significant load. Contaminants may build up on the reflector, reducing efficiency. It may also be difficult to erect a suitable supporting structure for a concentrator or dish. It would be desirable to address one or more of these disadvantages.

To address the disadvantage that the concentrator or dish is highly visible, an aspect of the invention provides an apparatus comprising an obstructing member extending in a direction perpendicular to the focal plane of a reflective focussing surface, The obstructing member has little, if any effect on the focussed reflected light because it is perpendicular to the focal plane. However, to an observer viewing the reflector at an angle to the focal plane, the obstructing member is visible direct and as a reflection in the focussing surface, reducing the visual impact.

According to one aspect of the invention, there is provided an apparatus for concentrating incident electromagnetic radiation, the apparatus comprising: a curved reflective member having a focussing surface for focussing incident electromagnetic radiation onto an area which defines a focal plane; and an obstructing member extending at least partially between the focussing surface and the focal plane in a direction substantially perpendicular to the focal plane.

The curved reflective member preferably has a parabolic curve, although shapes may also be used in other embodiments. In some embodiments, curves of other profiles than parabolas may be used in combination with a corrective secondary reflective surface, such as a corrective mirror.

The obstructing member is preferably thin, for example less than 5mm, more preferably less than 1mm thick. It may be substantially pianar in some embodiments, but may also be curved in other embodiments, provided that curve is in the focal plane, so that the curved obstructing member remains perpendicular to the focal plane.

Preferably, the obstructing member extends from the focussing surface. This maximises the effect of the obstructing member in reducing the visual impact of the focussing surface to an observer.

The obstructing member preferably passes through the apex of the focussing surface. In this context, apex is used to mean the point or line of the focussing surface which is furthest from the focal plane.

The obstructing member is preferably patterned to frirther reduce its visual impact. Any suitable pattern may be used, depending on the location where the apparatus is used. For example, the pattern may be a camouflage pattern, a solid colour or any other pattern. The pattern may be printed or produced electronically, for example by using a electronic bistable display technology such as that conirnercially available from 13 Ink Corporation, US, or any other suitable display technology In some embodiments, the focussing surface is curved in one dimension, defining a trough having a substantially constant cross section. In this case, the obstructing member may then extend along the apex of the trough. Alternatively or in addition, the obstructing member may be positioned along the edge of the trough.

In other embodiments, the focussing surface is curved in two dimensions, defining a bowl.

In that case, the obstructing member may extend across the bowl, through the apex of the bowl. The apparatus may further comprise at least one additional obstructing member extending across the bowl, through the apex of the bowl and positioned at an angle to the first obstructing member. Preferably one additional obstructing member is provided and positioned at 900 relative to the other obstructing member in the focal plane. The obstructing member may also be curved, and positioned around the circumference or edge of the bowl.

To address the disadvantage of increased wind loading because of the shape of the reflector, an aspect of the invention may include a non-planar convex transparent covering member, which defines an aerodynamic profile that reduces the drag compared to reflective member alone, reducing wind loading.

Accordingly, in another aspect of the invention, there is provided an apparatus for concentrating incident electromagnetic radiation, the apparatus comprising: a curved reflective member having a focussing surface which is concave for focussing incident electromagnetic radiation onto an area which defines a focal plane; a non-planar coveting member which is convex and transparent and extends over the focussing surface.

"Transparent" is used to mean that the covering member is transparent to incident electromagnetic radiation focussed by the reflector, but not necessarily transparent to all wavelengths of electromagnetic radiation. For example when used a solar concentrator the covering member may be transparent to optical wavelengths. When used as a communications device, the covering member may be transparent to radio wavelengths.

The reflective member may have a parabolic curve or any other suitable curve which can focus incident electromagnetic radiation. In some embodiments, curves of other profiles than parabolas may be used in combination with a corrective secondary reflective surface, such as a corrective mirror.

The covering member may have any convex shape including curves and other shapes which are composed of two or more facets which together define a convex shape.

Embodiments in which the covering member is faceted include the shapes where the covering member has an inverted "V" shape in cross section.

The combination of the covering member with the reflective member reduces the drag profile of the reflective member alone.

Preferably, the non-planar covering member defines a convex curve and a cross section through the focussing surface and the covering surface in a plane perpendicular to the focal plane defines an intersection of a convex curve with a concave curve having two distinct points. The curve may follow a portion of an ellipse or an arc of a circle Preferably, the angle between a tangent to the focussing surface and a tangent to the covering member at the point of intersection between the focussing surface and covering member is less than 1800.

In one embodiment, a cross section through the focussing surface and the covering surface in the direction perpendicular to the focal plane defines a Vesica Piscis. A Vesica Piscis is the intersection of two equal circles whose centres are offset by a distance equal to the circle radii.

Preferably, the apparatus further comprises a supporting framework for supporting and defining the shape of at least the focussing surface and the covering surface, and wherein the covering surface and the focussing surface are connected to the framework using hook and loop fasteners, for example those commercially available under the trade name Velcro.

Preferably, the covering surface and the reflective surface together define a three dimensional shape with a drag coefficient equal to or less than 1.2 based on a frontal reference area in a plane parallel to the focal plane. In other embodiments the drag coefficient may be less than or equal to 1.0, more preferably less than or equal to 0.5.

To address the disadvantage of contaminants building up on the reflector, another aspect of the invention includes a covering member which is transparent and which comprises a self cleaning material. This reduces the build up of contaminants on the reflector.

Accordingly, another aspect of the invention provides an apparatus for concentrating incident electromagnetic radiation, the apparatus comprising: a curved reflective member having a focussing surface which is concave for focussing incident electromagnetic radiation onto an area which defmes a focal plane; a covering member which is transparent and extends over the focussing surface; wherein the covering member has an external surface which comprises a self cleaning material.

"Transparent" is used to mean that the covering member is transparent to incident electromagnetic radiation focussed by the reflector, but not necessarily transparent to all wavelengths of electromagnetic radiation. For example when used a solar concentrator the covering member may be transparent to optical wavelengths. When used a communications device, the covering member may be transparent to radio wavelengths.

The reflective member may have a parabolic curve or any other suitable curve which can focus incident electromagnetic radiation.

Preferably, the self cleaning material comprises PTFE or ETFE, although any suitable self cleaning material or coating may be used. For example, the self cleaning material may comprise a titanium with a gas barrier coating (such as silicon dioxide) to reduce oxidisation of the titanium coating. An anti-reflective coating may optionally be applied to improve tight transmission through the covering member.

To address the disadvantage that it can be difficult to erect a suitable supporting structure for a concentrator or dish, another aspect of the invention provides a framework for a reflective member that can be placed on the ground. The framework then receives bags that can receive ballast material (such as soil, rocks, or other items from the area that are easily obtainable). This provides a sturdy support that can be erected without requiring groundworks.

Accordingly, another aspect of the invention provides an apparatus for concentrating incident electromagnetic radiation, the apparatus comprising: a curved reflective member having a focussing surface which is concave for focussing incident electromagnetic radiation onto an area which defines a focal plane; and a framework configured to be placed on the ground and for attachment to the reflective member, wherein the supporting framework defines openings for receiving at least one ballast receptacle.

Preferably, the apparatus further comprises at least one ballast receptacle defming an opening in its upper surface and engaged with the supporting framework. The ballast receptacle may be engaged with the supporting framework by attachment, or by other means, such as by the gravitational force of ballast in the receptacle pressing on the supporting framework.

The at least one receptacle preferably comprises a flexible material, which may be a woven or non-woven material or a polymer film.

The features of any of the above described aspects may be combined in any combination in other embodiments. To give one example, the obstructing member may be provided together with a covering member. Any other combination of aspects is also possible.

Any of the above-described aspects may further comprise a collector positioned in the focal point or focal area of the reflective focussing surface for collecting the reflected electromagnetic radiation. The collector may be solar cell, such as a photovoltaic cell, a heat exchanger, a conmiunications antenna, or any other device which requires focussed electromagnetic radiation. In embodiments including a covering member, the collector may be positioned between covering member and focussing surface (i.e. inside the volume defined by the covering member and the reflective member) or outside, further from the focussing surface than the covering member.

All of the above aspects may further include a motor connected to the reflective member which is configured to track the passage of an object of interest with one or two degrees of freedom. For example this allows the tracking of the sun or a communications station across the sky.

Embodiments of the invention will now be defined by way of example only with reference to the accompanying drawings, in which: Figure 1 depicts a perspective view of a solar concentrator according to any embodiment of the invention; Figure 2 depicts the modular nature of the reflective member and covering member of shown in Figure 1; Figure 3 is a diagrammatic representation of a cross section showing incident radiation and the effect of an obstructing member on an observer's view from an oblique angle; Figure 4 is a diagrammatic representation of a cross section of incident wind passing across the apparatus of Figure 1; Figure 4a is a diagranmiatic cross sectional view of a receiver or collector that may be used with the construction of Figure 1; Figure 5 is a diagranunatic representation of an alternative embodiment in cross section showing incident radiation and the effect of an obstructing member on an observer's view from an oblique angle, without a covering member; Figure 6 is a diagrammatic representation of another alternative embodiment in cross section showing incident radiation and the effect of an obstructing member on an observer's view from an oblique angle: an off-centred parabolic minor with receiver module.

Figure 6a is a diagrammatic representation of another alternative embodiment in cross section showing incident radiation and the effect of two obstructing members on an observer's view from an oblique angle: a parabolic mirror with receiver module without a clear covering.

Figure ób is a diagrammatic representation of another alternative embodiment in cross section showing incident radiation and the effect of two obstructing members on an observer's view from an oblique angle: a parabolic mirror with receiver module with a clear covering; Figure 7 depicts a perspective view of parabolic dish with two obstructing members; Figure 7a shows a cross section view of the parabolic dish of Figure 7; Figure 7b is a diagrammatic representation showing the area of the parabolic mirror of Figure 7, that is either obstructed by the obstructing members or a reflection of the obstructing members; and Figure 8 depicts an alternative embodiment of a parabolic dish with a curved, circumferential obstructing member.

Throughout the drawings, like parts are denoted with like reference numbers.

Figure 1 depicts a first embodiment of the invention which is used to collect and focus incident solar radiation onto photovoltaic cells. A heat sink 1 is provided to draw heat away from a receiver 2, in this case, photovoltaic cells. A corrective minor 3 is provided to redirect and stray reflected light from a curved reflective member, in this case a mirror 9, into the receiver 2. The mirror 9 is curved in one dimension, forming a trough. In this embodiment the minor 9 has a parabolic curve, although other forms of curve may be used in alternative embodiments.

To reduce the visual impact of the mirror 9 on an observer, an obstructing member 4 is provided, which is substantially planar. The obstructing member 4 extends perpendicular to the focal plane of the mirror 9 from apex of the trough. It is thin and substantially planar and so does not interrupt the reflection and focussing of incident solar radiation to the receiver 2. The operation of the obstructing member 4 is described in more detail with reference to Figure 3 below. An optional additional obstructing member 21 may be filled to a subframe 19 which supports the minor 9. It is preferred that the obstructing members 4, 21 are printed with a camouflage pattern, such as a disruptive pattern material (DPM), digital pattern material or other camouflage pattern. The pattern may however be any pattern and be printed on both sides.

The apparatus also comprises a covering member 7, which is curved, convex and transparent. In this embodiment the covering member 7 follows a bezier curve in cross-section, although alternative embodiments may use different curve profiles or use a multifaceted convex covering member, such as a covering member which defines an inverted CV" shape in cross section.

Preferably covering member 7 is manufactured from of Polytetrafluoroethylene (PTFE), or Ethylene tetrafluoroethylene (ETFE), or another polymer such as PVC, acrylic or glass.

PTFE or ETFE (more preferred) film is preferred because it provides a very low co-efficient of friction, UV radiation breaks down contaminants and rain dislodges contaminants (such as pollen). The covering member 7 therefore ifinctions as a self cleaning material, which protects the mirror 9 to maintain high optical transmission. The covering member 7 is stitched to a pattern section at the bottom of an envelope (not shown). The covering member 7 may be a sheet but in the preferred embodiment is a film.

In some embodiments an antireflective coating may be applied to the covering member 7 by sputter vapour deposition. In alternative embodiments that do not use PTFE, a titanium and gas barrier coating (such as Silicon dioxide) may be applied by sputter vapour disposition to provide a self cleaning film or sheet.

The mirror 9 and covering member 7 forms parts of an envelope completed by end sections 20. The end sections 20 may be clear and light transmissive for embodiments which provide a motor for tracking about a single axis (typically pitch). In embodiments which provides motors for dual axis (typically pitch and yaw) tracking, the end sections 20 may be printed with a double sided camouflage pattern. The envelope is not required to be airtight. Preferably, the envelope is constructed of polymer films forming the mirror 9, covering member 7 and end sections 20 which are joined by hook and ioop fasteners 6, 22, such as those commercially available under the trade name "Velcro", to the subframe 19. A hooked flap loops over the subframe and is attached to the ioop part 6. A hook and loop seam 22 is provided to allow a minor 9 to be inserted into the envelope.

The minor 9 may be constructed from a silvered / aluminised polymer film, such as Solar Mirror Film, which is commercially available from 3M, or a silvered or aluminised anodised foil sheet, such as Miro or Silver Miro sheet, which is commercially available from Alanod (Germany).

Parabolic supports 8 maintain the shape of the minor 9 as a parabola.

The subframe 19 connects to the supporting frame 15 by a shaft 17. The shaft 17 is connected to a highly geared motor 16 that actuates the parabolic minor 9 into sunlight using a controller (not shown). The geared motor 16 is housed within a sealed container 18 to protect the motor from the elements. In high wind loading, the drive may slip out of the optimal tracking position using a clutch or slip belt drive, this allows the envelope to assume a low drag profile and reduce the overall loading on the apparatus.

As shown in Figure 2, a flap 24 can be attached by hook and loop fastener 23 to restrain the parabolic support 8. Tubular sections 11 are provided at each end of the parabolic support 8. A support rod 10 is inserted into the tubular sections to secure the envelope to the subftame 19. Parabolic sections are fixed at each end to the subframe 19. A top support S is provided on the subframe 19, this allows hook and loop fixings 6 to attach the envelope to the subframe 19.

The end supporting frame sections 15 are bolted to bottom supporting frame sections 13, which may include feet 14. In the preferred embodiment the supporting frame and subframe are constructed of aluminium but any other material may also be used, such as steel. The frame can be powder coated in any suitable colour or pattern. A ballast receptacle 12 is provided that clips onto the bottom frame 13. The ballast receptacle is ideally made from non-woven synthetic fabric, such as polyester, and open at the top. This allows the ballast receptacle 12 to be filled with local materials such as sand, soil or aggregate, to anchor the apparatus.

Figure 3 shows a diagrammatic representation of across section view of the apparatus of Figure 1, to illustrate to working of the obstructing member 4, 15. The parabolic mirror 9 is directed towards an object of interest 25, which may be the sun, aircraft or satellite.

Incoming electromagnetic radiation 26, such as light or radio signals, are reflected from the parabolic mirror 9 into a receiver 2. A corrective mirror 3 may also be employed to reflect stray light into the receiver. A heat sink 1 draws heat away from the receiver and vents the heat to the atmosphere. An observer 30 is at an oblique angle to the apparatus. Line 27 shows the obstructing member 15 being viewed directly; line 28 shows the obstructing member 4 being viewed directly and line 29 shows the reflection of the obstructing member 4 from the minor 9. The observer 30 will therefore see the obstructing members 4,15 arid their reflection. This reduces the visual presence of the apparatus without reducing the optical efficiency of the mirror 9.

The subframe supporting the mirror 9 and other components forming the envelope is preferably attached to the supporting frame off-centre. In high wind loading conditions, the subframe holding the envelope may then slip away from an oriented position into a low drag position, which presents the envelope to oncoming wind with reduced drag, thereby reducing wind loading.

Figure 4 is a diagrammatic representation of wind 31 passing over the envelope of Figure 1. The envelope has a cross section defined by a circle and parabola intersection with two distinct points formed by the combination of the minor 9 and covering member 7. The cross section is preferably similar to a vesica piscis but formed from a circle and a parabola. The construction is an aerodynamic profile and has a much lower drag co-efficient (around 01) compared with a parabolic profile (with drag coefficient typically >1.2) based on a frontal reference area. This shows how the covering member 7 reduces the overall drag on the apparatus. A further benefit of this shape is that it causes wind to pass across the envelope and increase in speed, allowing the receiver 2 and heat sink 1 to be cooled more effectively.

In alternate embodiments, the mirror and covering member may define shapes in cross section which are the intersection of a other curved shapes of the minor with curved or faceted convex covering member. Some examples include a true vesica piscis (in combination with a secondary corrective minor), a parabolic minor together with clear top straight sections forming a faceted covering member, and a minor having a bezier curve (in combination with a secondary corrective mirror) together with clear top straight sections to form a faceted covering member.

Figure 4a shows an alternative embodiment of a receiver for the concentrator apparatus, where the receiver 2a is mounted on either side of the heat sink la and incoming light is reflected via a minor 3 a into the receiver 2a.

Figure 5 shows a simplified cross section of a further embodiment, in which the covering member 7 is not included. In this embodiment the obstructing member 4b extends further than in the embodiment of Figure 1,so the additional obstructing member 15 is also not required.

Figure 6 is a cross section view of another embodiment including an off-centred parabolic mirror 9c and having an obstructing member 4c and a covering member 7c.

Figure 6a is a cross section view of another embodiment including a parabolic minor (trough) 9f and having two obstructing members 4f. The obstructing members 4f are positioned at the edges of the parabolic mirror 9f.

Figure 6b is a cross section view of another embodiment including a parabolic mirror (trough) 9g and having two obstructing member 4g together with a curved, transparent covering member 7g. The obstructing members 4g are positioned at the edges of the parabolic mirror 9g and the covering member 7g extends from the end of the obstructing members furthest from the minor 9g. In this embodiment the cross section is therefore defmed by the combination of the minor 9g, covering member 7g and the obstructing members 4g.

Figure 7 is a perspective view of another embodiment which uses a parabolic dish as a minor 9d. It includes four obstructing members 4d positioned at 90° relative to each other.

Figure 7a is a cross sectional view of the embodiment of Figure 7. Figure 7b is a diagrammatic representation showing the area 4r which is obstructed of the minor 9d in Figure 7 to an observer. The obstruction is either by direct viewing of obstructing members 4d or by viewing their reflection in the minor 9d, Figure 8 is a perspective view of a yet further embodiment where the obstructing member 4e is curved around the circumference of a dish-shaped minor 9d.

From the above described embodiments, it is clear that the obstructing members do not have to be placed at the centre line of the parabola or curve, They may be positioned any where on the mirror providing that they are perpendicular to the focal plane of the mirror.

For example, the obstructing members may be positioned at either end of the minor so there is reduced interference with reflected light from the minor to the collector.

The above described embodiments provide one or more of the following advantages: a self cleaning function to maintain the optical efficiency of the minor to reflect radiation into a receiver; an obstructing member reduces the visual impact of the minor to an observer; ballast receptacles may be filled with locally sourced material to anchor the unit, without requiring ground work for foundations; and a covering member to improve the aerodynamic profile.

Claims

<claim-text>CLAIMS1. An apparatus for concentrating incident electromagnetic radiation, the apparatus comprising: a curved reflective member having a focussing surface for focussing incident electromagnetic radiation onto an area which defines a focal plane; and an obstructing member extending at least partially between the focussing surface and the focal plane in a direction substantially perpendicular to the focal plane.</claim-text> <claim-text>2. An apparatus according to claim 1, wherein the obstructing member extends from the focussing surface.</claim-text> <claim-text>3. An apparatus according to claim 2, wherein the obstructing member passes through the apex of the focussing surface.</claim-text> <claim-text>4. An apparatus according to any one of the preceding claims, wherein the obstructing member is patterned.</claim-text> <claim-text>5. An apparatus according to any one of the preceding claims, wherein the focussing surface is curved in one dimension, defining a trough having a substantially constant cross section.</claim-text> <claim-text>6. An apparatus according to claim 5, wherein the obstructing member also extends along the apex of the trough.</claim-text> <claim-text>7. An apparatus according to claim 5, wherein the obstructing member extends along the edge of the trough.</claim-text> <claim-text>8. An apparatus according to any one of claims ito 4, wherein the focussing surface is curved in two dimensions, defining a bowl.</claim-text> <claim-text>9. An apparatus according to claim 8, wherein the obstructing member extends across the bowl, through the apex of the bowl.</claim-text> <claim-text>10, An apparatus according to claim 8, further comprising at least one additional obstructing member extending across the bowl, through the apex of the bowl and positioned at an angle to the first obstructing member.</claim-text> <claim-text>11. An apparatus according to claim 8, wherein the obstructing member is curved and positioned around the circumference of the bowl.</claim-text> <claim-text>12. An apparatus for concentrating incident electromagnetic radiation, the apparatus comprising: a curved reflective member having a focussing surface which is concave for focussing incident electromagnetic radiation onto an area which defines a focal plane; a non-planar covering member which is convex and transparent and extends over the focussing surface.</claim-text> <claim-text>13. An apparatus according to claim 12, wherein the non-planar covering member defines a convex curve and a cross section through the focussing surface and the covering surface in a plane perpendicular to the focal plane defines an intersection of a convex curve with a concave curve having two distinct points.</claim-text> <claim-text>14. An apparatus according to claim 13, wherein the angle between a tangent to the focussing surface and a tangent to the covering member at the point of intersection between the focussing surface and covering member is less than 180°.</claim-text> <claim-text>15. An apparatus according to claim 13 or 14, wherein a cross section through the focussing surface and the covering surface in the direction perpendicular to the focal plane defines a Vesica Piscis.</claim-text> <claim-text>16. An apparatus according to any one of claims 12 to 15, further comprising a supporting framework for supporting and defining the shape of at least the focussing surface and the covering surface, and wherein the covering surface and the focussing surface are connected to the framework using hook and loop fasteners.</claim-text> <claim-text>17. An apparatus according to any one of claims 12 to 16, wherein the covering surface and the reflective surface together define a three dimensional shape with a drag coefficient equal to or less than 1.2 based on a frontal reference area in a plane parallel to the focal plane.</claim-text> <claim-text>18. An apparatus for concentrating incident electromagnetic radiation, the apparatus comprising: a curved reflective member having a focussing surface which is concave for focussing incident electromagnetic radiation onto an area which defines a focal plane; a covering member which is transparent and extends over the focussing surface; wherein the covering member has an external surface which comprises a self cleaning material.</claim-text> <claim-text>19. An apparatus according to claim 18, wherein the self cleaning material comprises PTFE or ETFE.</claim-text> <claim-text>20. An apparatus for concentrating incident electromagnetic radiation, the apparatus comprising: a curved reflective member having a focussing surface which is concave for focussing incident electromagnetic radiation onto an area which defines a focal plane; and a framework configured to be placed on the ground and for attachment to the reflective member, wherein the supporting framework defines openings for receiving at least one ballast receptacle.</claim-text> <claim-text>21. An apparatus according to claim 20, further comprising at least one ballast receptacle defining an opening in its upper surface and engaged with the supporting framework.</claim-text> <claim-text>22. An apparatus according to claim 21, wherein the at least one receptacle comprises a flexible material.</claim-text> <claim-text>23. An apparatus according to any one of the preceding claims, further comprising a collector positioned in the focal point or focal area of the reflective focussing surface for collecting the reflected electromagnetic radiation.</claim-text>