GB2482553A

GB2482553A - Solar collector comprising a reflector

Info

Publication number: GB2482553A
Application number: GB1013275.1A
Authority: GB
Inventors: Simon Boaler
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-08-06
Filing date: 2010-08-06
Publication date: 2012-02-08
Also published as: GB201013275D0

Abstract

A solar collector 10, for heating fluid with electromagnetic radiation, comprises a fluid conveying means for conveying fluid, where the fluid conveying means comprises at least one tube 20, and a reflector 12 arranged to reflect light onto a focal point 30. The fluid conveying means is arranged in association with the reflector such that the focal point lies inside the fluid conveying means, but the fluid conveying means is asymmetrical about the focal point. In a further aspect, a solar collector comprises a reflector arranged to reflect light onto a fluid conveying means, where the fluid conveying means is elongate in cross-section and is arranged in association with the reflector such that the direction of elongation in cross-section extends in a direction away from the reflector. Preferably, the reflector is a Fresnel reflector or a parabolic reflector. The fluid conveying means may comprise a flattened tube arranged in a serpentine layout or a plurality of tubes (40, fig.6) arranged in a stack. Preferably, the reflector includes a protruding ridge 18, formed at the deepest part of the concave cross-section of the reflector, to ensure that solar radiation cannot pass underneath the fluid conveying means without being absorbed.

Description

A Solar Collector The invention relates to a solar collector.

A known solar thermal collector comprises copper tubes on a black steel sheet backed with a layer of insulation and covered by at least one layer of glass. The black steel sheet is arranged so that it absorbs heat from solar radiation and transfers it to the copper tubes. The copper tubes convey water, and so, when the copper tubes are heated, the water inside them is heated.

Another known solar thermal heater comprises a vacuum tube and a focusing mirror. The vacuum tube comprises two concentric glass tubes, the space between the tubes being evacuated and sealed at the end of the tubes. The focusing mirror focuses solar radiation which is incident on the mirror to a focal point which lies within the vacuum tube. The vacuum tube contains fluid in pipes, and the fluid is heated by the focused solar radiation. The vacuum reduces heat loss by convection from the fluid in the tube, However, the seals in the evacuated tubes have to withstand cycling from about -5°C to over 100°C in use. This results in them having a short life time, thus increasing the cost of the heating system and delaying the payback time.

According to a first aspect of the present invention there is provided a solar collector according to claim 1.

According to a second aspect of the present invention, there is provided a solar collector according to claim 3.

In the present invention, as a result of the shape of the fluid conveying means, it is possible for the fluid conveying means to absorb radiation which is reflected by the reflector, but not reflected towards the focal point. Therefore, in diffuse light conditions, for example, cloudy conditions in which light does not enter the reflector from directly above, and also when incident light is not coming from directly above the reflector, a large proportion of radiation hitting the reflector will still be absorbed by the fluid conveying means. This is in contrast to the known system using a reflector, which only works efficiently when light is coming from directly above the reflector. As a result of the increased efficiency, it is not necessary for the conveying means to be a vacuum tube. Therefore, the thermal collector will last for longer before maintenance is needed, and costs of maintenance will be less and the payback time will be shorter. Further, the fact that vacuum tubes are not needed makes it much simpler to provide a serpentine shaped conveying means, which allows a simpler design of thermal collector overall.

In the aspect of the invention in which the fluid conveying means is elongate, for a particular height of fluid conveying means, the efficiency of heating fluid is further increased by the large ratio of surface area to volume.

Preferably, the reflector is elongate and the length of the tube and the direction of elongation of the reflector are substantially parallel. Therefore, the length of the tube that is located inside the reflector can be heated by reflected radiation from the reflector.

Preferably, the reflector is substantially straight in its direction of elongation.

Further, the reflector is preferably substantially concave such that it forms a channel in its direction of elongation. The fluid conveying means is preferably located at least partially within the channel, and more preferably completely within the channel.

The reflector may be a Fresnel reflector. Concave reflectors have the advantage of being simple to manufacture, whilst Fresnel reflectors have the advantage of saving space.

The reflector may be symmetrical, and preferably, where the reflector is elongate, the concave shape of the reflector is parabolic. A parabolic shape enables light entering the reflector directly from above to be focussed on a focal point.

Preferably, the elongate direction of the cross-section of the fluid conveying means is aligned with the axis of symmetry of the reflector.

Where the reflector is symmetrical, the fluid conveying means may be arranged such that the combination of the reflector and the fluid conveying means is symmetrical.

Preferably, the reflector includes a protrusion, arranged beneath the fluid conveying means. When the fluid conveying means is arranged above the general surface of the reflector, the protrusion reduces the amount of radiation which can pass under the fluid conveying means and therefore is not absorbed and preferably eliminates it altogether. Therefore, the protrusion increases absorption efficiency.

Where the fluid conveying means is a single tube with an elongate cross-section, the elongate sides of the fluid conveying means may be straight. Preferably, the fluid conveying means comprises a metal tube, flattened along at least part of its length so that it is elongate in cross-section. The fluid conveying means may be made from copper.

The fluid conveying means may comprise at least two tubes, arranged with their longitudinal axes substantially in parallel. The at least two tubes, therefore, in combination provide a fluid conveying means with an elongate cross-section.

Preferably, adjacent tubes are in contact. Therefore, the efficiency of the fluid conveying means to absorb reflected radiation is increased, as radiation cannot pass through gaps between the tubes. The fluid conveying means preferably comprises three tubes, arranged with their longitudinal axes substantially in parallel. Preferably the tubes are substantially circular in cross-section. Therefore, conventional tubing can be used.

Preferably, the fluid conveying means has a single wall. This reduces the complexity of the fluid conveying means and ensures that seals do not have to be used, for example to maintain a vacuum between double walls. The lack of seals, which may deteriorate rapidly with cycling of temperature, improves the payback time of the collector.

Preferably, the device comprises more than one reflector, the fluid conveying means being arranged in association with each reflector. Therefore, the device can receive radiation from a larger surface area. Preferably, the reflectors are arranged in parallel. Where the reflectors are substantially straight, the fluid conveying means may comprise substantially straight portions, for association with the reflectors, and connecting portions adjoining adjacent straight portions of the fluid conveying means.

Therefore, many reflectors can be used to heat one long serpentine-shaped fluid conveying means, within a compact surface area.

Where the fluid conveying means comprises a tube with an elongate cross-section in at least parts, preferably at least the portions of the fluid conveying means which lie within the reflectors are elongate in cross-section.

The reflectors may be joined, preferably, where the reflectors are elongate, along their edges parallel to the direction of elongation of the reflector. Therefore, the shape of the solar collector can be easily maintained, without the reflectors separating.

Where the reflectors are substantially concave, each reflector may have a down-turned flange along one of its edges parallel to the direction of elongation of the reflector, such that the down-turned flange can overlap the lip of an adjacent reflector without a flange, thus joining the reflectors. A plurality of reflectors can therefore be joined together in a simple manner.

Alternatively, the plurality of reflectors may be made as one piece.

The reflectors may be made by rolling a metal sheet. The or each reflector may be made from aluminium.

A solar collector in two embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a solar collector according to a first embodiment of the present invention, without a double glazing cover, with the front portion of the frame and reflectors removed, to show the serpentine shape of the tube; Fig. 2 is a perspective view of the device of Fig. 1, with a double glazing cover; Fig. 3 is a cross-sectional view of a single reflector of the device of Fig. 1, and the fluid conveying means; Fig. 4 is a cross-sectional view of a portion of the device of Fig. 1, showing several reflectors; Figs. 5a to Sd illustrate how light hitting the reflector and fluid conveying means from a number of different angles is absorbed by the fluid conveying means; and Fig. 6 shows a cross-sectional view of a reflector and a fluid conveying means of a second embodiment of the device of the present invention.

A first embodiment of the invention is shown in Figs. 1 to 5. As shown in Fig. 1, the solar collector 10 in the form of a solar panel comprises an array of reflectors 12. Each reflector 12, as shown in Fig. 3 is generally in the shape of a channel which in cross-section is parabolically concave. Each reflector 12 is formed by rolling a piece of aluminium sheet. Along one side of the channel the top edge 14 of the reflector is arranged to form a downwardly extending flange 16. The reflectors 12 are arranged in an array, such that the flange 16 of one reflector 12 is arranged such that it overlaps the adjacent reflector 12 over its top edge without a flange. In each reflector 12 a protruding ridge 18 is formed at the deepest part of the concave cross-section.

The solar collector 10 further includes a fluid conveying means which is a tube 20. As shown in Fig. 1, the tube extends in straight portions 22 down the middle of each reflector 12. Adjacent straight portions 22 are connected by a connector portion 24. Thus, the tube 20 is serpentine in form. In Fig. 1, two joins 26 can be seen in the tube 20, as the tube 20 comprises several sections joined together. As shown in Figs. 3 and 4, the tube 20 is elongate in cross section. The tube 20 is formed by flattening, or by rolling, a copper tube of circular cross-section. The tube 20 is arranged in association with each reflector 12 such that the dimension which is elongate in cross-section extends in a direction away from the reflector 12. Each straight portion 22 and reflector 12 pair, excluding the flange 16 of the reflector 12, is arranged in a symmetrical manner. Each tube 20 may be 30.5 mm tall and 7.0 mm wide in cross- section. Each reflector 12 may be about 46 mm tall and about 91 mm wide in cross-section. The ridge 18 may be about 2.2 mm tall.

As shown in Figs. 1 and 2, the reflectors 12 and the tube 20 are arranged within a frame 28. A double glazed cover window 13 is attached to the frame 28.

In use, the device 10 is installed, for example, on the roof of a house. As shown in Fig. 5a, when solar radiation is incident on the reflector 12 in a direction parallel to the axis of symmetry of the concave cross section of the reflector 12, the radiation is focussed on a focal point 30, which lies within the tube 20. However, when light enters the reflector 12 from other directions, for example in diffuse light conditions, the elongate cross-sectional shape of the tube 20 allows the tube 20 to receive reflected radiation which is not directed to the focal point 30. The protruding ridge 18, ensures that radiation cannot pass underneath the tube and be reflected out of the reflector 12 without being absorbed by the tube 20.

The second embodiment is shown in Fig. 6. The second embodiment is similar to the first embodiment and the same reference numerals will be used for features which are the same, and only the differences will be described.

The device 10 of the second embodiment differs from the first embodiment in that the fluid conveying means, instead of comprising the flattened tube 20, comprises three tubes 40. The tubes 40 are arranged one on top of the other with their longitudinal axes aligned. Therefore, together they create a fluid conveying means which is elongate in cross-section. Each of the tubes 40 is a conventional circular cross-section copper tube. Each tube 40 may be 11 mm in diameter and the tubes may be arranged with respect to one another at 12 mm centres.

In use, the fluid conveying means 40 acts in the same way as the fluid conveying means 20 in the first embodiment, allowing radiation which is reflected from the reflector 12, but not reflected towards the focal point, to be received by the fluid conveying means 40.

In the above embodiments, the reflector is concave and parabolic, but any suitable shape of reflector could be used, including a Fresnel reflector.

In both embodiments, the fluid conveying means is symmetrical about its direction of elongation, but this does not have to be the case.

In the first embodiment, the entire tube 20 is elongate in cross section, but it will be apparent to the skilled man that does not need to be the case. For example, it is preferable that the entire portion of a tube which is associated with a reflector is elongate in cross section as this maximises efficiency, but the connecting portions do not need to be elongate in cross section.

In both embodiments, the fluid conveying means is elongate in cross section, but in an alternative embodiment it may not be elongate in cross section. For example, the fluid conveying means could be a single tube of circular cross section, non concentric with the focal point axis of the reflector.

In the first embodiment, the tube 20 is a circular tube, flattened to produce a straight-sided elongate cross section. However, the tube could be produced in any suitable way as could be conceived by the skilled man, for example, by extrusion.

Further, the skilled man would appreciate that, the tube, when elongate, does not necessarily have to be straight sided -it could be substantially a quadrilateral, but with curved sides, or it could be oval, for example.

Claims

Claims 1. A solar collector for heating fluid with electromagnetic radiation, the device comprising: -a fluid conveying means for conveying fluid, the fluid conveying means comprising at least one tube, -a reflector arranged to reflect light onto a focal point the fluid conveying means being arranged in association with the reflector such that the focal point lies inside the fluid conveying means, but the fluid conveying means is asymmetrical about the focal point.
2. A solar collector according to claim 1, wherein the fluid conveying means is elongate in cross section and is arranged in association with the reflector such that the direction of elongation in cross section extends in a direction away from the reflector.
3. A solar collector for heating fluid with electromagnetic radiation, the device comprising: -a fluid conveying means for conveying fluid, the fluid conveying means comprising at least one tube, -a reflector arranged to reflect light onto the fluid conveying means the fluid conveying means being elongate in cross section and being arranged in association with the reflector such that the direction of elongation in cross section extends in a direction away from the reflector.
4. A solar collector according to claim 1, claim 2, or claim 3, wherein the reflector is elongate and the longitudinal axis of the tube and the direction of elongation of the reflector are substantially parallel.
5. A solar collector according to claim 4, wherein the reflector is substantially straight in its direction of elongation.
6. A solar collector according to any preceding claim, wherein the reflector is in the form of a channel.
7. A solar collector according to claim 6, wherein the fluid conveying means is located at least partially within the channel.
8. A solar collector according to claim 7, wherein the fluid conveying means is located completely within the channel.
9. A solar collector according to claim 7 or 8, wherein, where the reflector is elongate, the reflector is symmetrical perpendicular to the direction of elongation.
10. A solar collector according to any of claims 6 to 9, wherein the reflector is concave and the concave shape of the reflector is parabolic.
11. A solar collector according to claim 9 or claim 10, wherein the elongate direction of the cross section of the fluid conveying means is aligned with the axis of symmetry of the reflector.
12. A heating device according to claim 11, wherein the fluid conveying means is arranged such that the combination of the reflector and the fluid conveying means therewithin is symmetrical.
13. A solar collector according to any of claims 6 to 12, wherein the fluid conveying means extends away from the reflector to at least 50% of the depth of the reflector channel.
14. A solar collector according to any of claims 6 to 12, wherein, the fluid conveying means extends away from the reflector to at least 65% of the depth of the reflector channel.
15. A solar collector according to any of claims 6 to 12, wherein the fluid conveying means extends away from the reflector to about 70% of the depth of the reflector channel.
16. A solar collector according to any of claims 6 to 15, wherein the ratio of the width of the reflector channel to the height of the reflector channel in cross-section is between 1.75 and 2.25.
17. A solar collector according to any of claims 6 to 15, wherein the ratio of the width of the reflector channel to the height of the reflector channel in cross-section is about 2.
18. A solar collector according to any of claims 6 to 17, wherein the width of the reflector channel in cross-section is 80 to 100 mm.
19. A solar collector according to any of claims 6 to 17, wherein the width of the reflector channel in cross-section is about 90 mm.
20. A solar collector according to any of claims 6 to 19, wherein the depth of the reflector channel is about 35 to 55 mm.
21. A solar collector according to any of claims 6 to 19, wherein the depth of the reflector channel is about 45 mm.
22. A solar collector according to any of claims 1 to 5, wherein the reflector is a Fresnel reflector.
23. A solar collector according to any preceding claim, wherein the ratio of the length of the fluid conveying means in its elongate direction to the cross sectional area of the fluid conveying means is 0.1 to 0.2 mm-'.
24. A solar collector according to any preceding claim, wherein the ratio of the length of the fluid conveying means in its elongate direction to the cross sectional area of the fluid conveying means is 0.15 to 0.165 mm-'.
25. A solar collector according to any preceding claim, wherein the length of the fluid conveying means in its elongate direction in cross-section is 25 to 50 mm.
26. A solar collector as claimed in any preceding claim, wherein the length of the fluid conveying means in its elongate direction in cross-section is 30 to 35 mm.
27. A solar collector as claimed in any preceding claim, wherein the cross sectional area of the fluid conveying means is 150 to 250 mm2.
28. A solar collector according to any preceding claim, wherein the cross-sectional area of the fluid conveying means is 185 to 200 mm2.
29. A solar collection as claimed in any preceding claim, wherein the fluid conveying means extends away from the reflector to a distance of more than twice the distance from the reflector to the focal point of the reflector.
30. A solar collection as claimed in any preceding claim, wherein the fluid conveying means extends away from the reflector to a distance of more than 2.5 times the distance from the reflector to the focal point of the reflector.
31. A solar collection as claimed in any preceding claim, wherein the fluid conveying means extends away from the reflector to a distance of more than 3 times the distance from the reflector to the focal point of the reflector.
32. A solar collector according to any preceding claim, wherein the reflector includes a protrusion, arranged beneath the fluid conveying means, to reduce or preferably prevent radiation from passing under the fluid conveying means.
33. A solar collector according to any preceding claim, wherein the fluid conveying means comprises a single tube.
34. A solar collector according to claim 33, wherein the sides of the tube are substantially flat.
35. A solar collector according to any of claims 1 to 32, wherein the fluid conveying means comprises at least two tubes, arranged with their longitudinal axes substantially in parallel.
36. A solar collector according to claim 35, wherein the fluid conveying means comprises three tubes, arranged with their longitudinal axes substantially in parallel.
37. A solar collector according to claim 35 or claim 36, wherein adjacent tubes are in contact.
38. A solar collector according to any of claims 35, 36 and 37, wherein the tubes are substantially circular in cross-section.
39. A solar collector according to any preceding claim, wherein the fluid conveying means is made of metal.
40. A solar collector according to any preceding claim, wherein the fluid conveying means is made from copper.
41. A solar collector according to any preceding claim, wherein the device comprises more than one reflector, the fluid conveying means being arranged in association with each reflector.
42. A solar collector according to claim 41, wherein the reflectors are arranged in parallel.
43. A solar collector according to claim 41 or claim 42, wherein the fluid conveying means comprises main portions, each main portion being associated with a reflector, and connecting portions, the connecting portions joining adjacent main portions of the fluid conveying means.
44. A solar collector according to any of claims 41, 42 and 43, wherein the reflectors are joined.
45. A solar collector according to claim 44, wherein the reflectors are joined along their edges in parallel.
46. A solar collector according to claim 44 or claim 45, wherein each reflector has a down-turned flange along one of its edges, such that the down-turned flange can overlap part of an adjacent reflector, thus joining the reflectors.
47. A solar collector according to any of claims 41, 42 and 43, wherein the plurality of reflectors is made as one piece.
48. A solar collector according to any of claims 41 to 47, wherein the reflectors are made by rolling a metal sheet.
49. A solar collector according to any preceding claim, wherein the or each reflector is made from aluminium.
50. A solar collector substantially as described herein, and with reference toFigs. lto5or6.