GB2327771A

GB2327771A - Support assemblies for solar reflectors

Info

Publication number: GB2327771A
Application number: GB9715840A
Authority: GB
Inventors: John Harrison
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-07-25
Filing date: 1997-07-25
Publication date: 1999-02-03
Also published as: GB9715840D0

Abstract

Apparatus for reflecting and primarily concentrating solar radiation, the apparatus comprising a mirror (12) being curved in two planes. The mirror (12) being supported by at least one first support member (20), the at least one first support member (20) being supported by at least one second support member (22) which is oriented substantially perpendicular to the first support member (20).

Description

SUPPORT ASSEMBLIES FOR SOLAR REFLECTORS This invention relates to support assemblies for solar reflectors, and in particular to support assemblies for parabolic dish reflectors and Heliostats Solar reflectors used with solar thermal applications include parabolic dishes and Heliostats, which have individual mirrors many of which move independently to track and reflect a solar image into a central receiver. Typically Heliostat and parabolic dishes have focal lengths of 350 metres and 12 metres respectively. This coupled with the demand for high solar concentration ratios e.g., 10,000 for the directly irradiated annular pressurized receiver, require that each mirror be held accurately and consistently at an appropriate orientation and disposition relative the receiver. The support for such reflectors must rigid, in order to prevent the reflector deflecting under its own weight or as a result of wind. Known such Heliostats and individual mirrors forming part of parabolic dishes are typically formed from a laminate of glass having a reflective backing, PVC foam, and sheet steel. With this assembly, the glass tends to be in tension and the steel in compression due to the loading exerted upon it. The PVC foam layer separating the glass and the steel gives the assembly a high moment of inertia thereby keeping the deflections small. However, reflector assemblies constructed out of the above materials tend to be heavy, resulting in a need for a stronger and securely anchored support system. Additionally, because the expansion coefficient of PVC foam is far greater than that of glass, deflections of the mirror may occur as a result of variance in temperature.

The present invention is directed at solar reflector assemblies which are capable of fulfilling broadly the same objectives as those of the prior systems discussed above, but which are significantly lighter, and plate to the mirror 12. The spacing of the second support members 22 is determined by the rigidity of the first support members 20.

The first support members 20 have a parabolic curvature corresponding to the section of dish which they support.

The material from which the first support members 20 are formed must be fairly rigid, perform well under stress and strain, and preferably have a thermal expansion coefficient which is similar to that of glass.

In a preferred embodiment the first support members are formed from vitreous clay which has a thermal expansion coefficient of 7.5 parts per million per degree celsius (ppm/OC) The glass chosen for the mirror 12 typically has a thermal expansion coefficient of about 9 ppm/OC.

Therefore for a one metre length of glass attached to a first support member 20 made from vitreous clay, a difference in length of 0.0755 mm will result for a 500C temperature change. Thus a significantly better result can be achieved by using vitreous clay than by using steel which has a thermal expansion coefficient 12 ppm/OC.

To obtain the maximum stiffness from the minimum material, the support members 20 are preferably formed with an I-shaped cross-section as illustrated in Figure 3. The support members 20 may be manufactured by extruding wet clay in the form of a plastic mass to produce the I-shaped cross-section as shown in Figure 3.

One flange 30 is made thicker than the other 32. The wet clay is then bent over a parabolic form, thick flange down. The extrusion is longer, and the parabolic curve shallower than the desired member length and parabolic shape in order to allow for shrinkage of the clay upon drying and firing. Once the support member 20 has been dried and fired, it has a profile which approximately matches the required parabolic curve. The profile of the support member can then be refined by grinding a portion sub-frame.

An embodiment of the invention will now be described by way of example and with reference to the accompanying schematic drawings wherein: Figure 1 is a perspective view of a parabolic solar reflector dish; Figure 2 is perspective view of a solar reflector support assembly according to the invention; Figure 3 is a perspective view of a reflector support member and; Figure 4 shows a side elevation of the solar reflector support assembly of Figure 2 taken along line A-A; Figure 5 is a perspective view of a solar reflector support assembly according to a second aspect of the invention.

The parabolic solar reflector dish 10 illustrated in Figure 1 is formed from a plurality of mirrors, each mirror 12 being curved in two planes so as to focus solar radiation reflected therefrom onto a receiver. The mirrors are preferably made from glass having a reflective backing. Individual mirrors e.g., mirror 12 must have suitable means of support together with means for attaching the mirror 12 to the solar dish sub-frame 14. Such an assembly is illustrated in Figure 2. A mirror 12 is attached to the curved first support members 20 by an appropriate means, for example glue.

Those members 20 are then attached to and supported by second support members 22 which are curved and are orientated in a direction substantially perpendicular to the first support members 20. The spacing of the first support members 20, is determined by the reflective sheet's rigidity and the effect that this has on the quality of the reflected solar image. A separation between each successive first support members 20 of 10 centimetres to 100 centimetres is typical. However, this distance may be increased by the addition of a backing members 22 bridge the gap between two adjacent arms 16 and are attached thereto as shown in Figure 1. If there are two second support members associated with a mirror, this gives four points of contact which are adjustable in pairs to account for any imperfection in the radial arms 16.

It is of course possible to adapt the system so that the first support members 20 are orientated circumferentially and the second support members 22 are perpendicular to the first support members i.e. their associated axis extend radially. An embodiment according to this second aspect of the invention is illustrated in Figure 5.

of the thick flange 30.

Second support members 22 may be formed from vitreous clay in the same manner as the first support members, or they may be formed from steel or any other appropriate material. It is desirable that they be Ishaped in cross-section to maximise the stiffness to weight ratio although a space-frame or a lattice construction may also be used. These second support members 22 are adapted to contact each first support member 20. This may be achieved by forming the second support members 22 with a curve matching the curvature of the mirror 12 in its second plane. As can be seen from Figure 2 the second support members 22 are attached to the underside of the first support members 20 and are substantially perpendicular to the first support members 20. Joints adapted to allow relative movement between first and second support members are used to attach the second support members 22 to the first support members 20 at the regions where they cross. This helps to take up any difference in thermal expansion.

Alternatively, the second support members 22 may be formed to have a curvature in only one plane and be attached to the first support members as shown in Fig.

4. In this Figure the second support members 22 are disposed at an angle to the first support members 20. In Figure 4, the flange of the second support members 22 is shown to be parallel with the reflector dish's circumference. The gaps between first and second support members can be filled with appropriately shaped wedges 24 and/or joints which allow relative movement between the respective members.

The parabolic dish 10 made in accordance with the invention is supported by a sub frame 14 comprising radial arms 16 which extend beneath the dish 10 from the centre to the outside following the parabolic shape of the dish 10 as shown in Figure 1. The second support formed from vitreous clay.

10. A reflector support assembly according to Claim 3 or Claim 4 wherein the spacing between each support member is within the range of 10 cm to 100 cm.

11. A reflector support assembly according to any preceding Claim wherein the first and/or the second support members are adapted to be connected to first or second support members respective of an adjacent reflector support assembly.

12. A reflector support assembly according to Claim 10 wherein the connection between the first and/or second support members and the first or second support members of an adjacent reflector support assembly is permanent.

13. A plurality of reflector support assemblies according to any preceding Claim, forming part of a parabolic solar dish.

14. A reflector support assembly substantially as described herein with reference to the accompanying schematic drawings.

15. A parabolic solar reflector dish comprising a plurality of reflector support assemblies as claimed in any preceding Claim.

16. A parabolic solar reflector dish according to Claim 14 further comprising a sub-frame having radially extending arms supporting the second support members.

17. A Heliostat comprising at least one reflector support assembly as claimed in any of Claims 1 to 1.

Claims

CLAIMS:

1. A reflector support assembly comprising: a mirror having upper and lower surfaces and being curved in first and second planes; at least one first support member, at least a portion of the at least one first support member contacting a part of the lower surface of the mirror; at least one second support member, and being orientated in a direction substantially perpendicular to the first support member for supporting said first support member(s).

2. A reflector support assembly according to Claim 1 wherein the first support members are curved in the first plane so that the entire length of each first support member contacts the mirror.

3. A reflector support assembly according to any preceding Claim wherein the second support members are adapted to contact and support each first support member

4. A reflector support assembly according to any preceding Claim wherein the first support means comprises at least two support members.

5. A reflector support assembly according to any preceding Claim wherein the second support means comprises at least two support members

6. A reflector support assembly according to any preceding Claim wherein the first supports members have a substantially I-shaped cross-section.

7. A reflector support assembly according to any preceding Claim wherein the second support members have a substantially I-shaped cross-section.

8. A reflector support assembly according to any preceding Claim wherein the second support mean are connected to the first support means using flexible connections allowing relative movement therebetween.

9. A reflector support assembly according to any preceding Claim wherein the first support means are