GB2073870A - Solar collector - Google Patents

Abstract

A solar collector has a reflector 1 whose reflecting surface is formed as a surface of revolution formed by straight-line generatrices. This reflecting surface is centred on an axis A with the generatrices intersecting the axis at a common point 9. An elongate tubular light- absorbing body 2 is provided at the axis and fluid such as water, is circulated through this body. In a modification, the reflector is in an inverted disposition with its apex upwardly directed, and a tubular light-absorbing body surrounds the reflector. <IMAGE>

Description

SPECIFICATION Solar collector The present invention relates to a solar collector. More particularly the invention concerns a solar collector of the type used to heat a liquid such as water.

Two main types of solar collectors are already known. One such type is the so-called collector panel that is installed to lie substantially at right angles to the impinging light rays. A liquid may flow through tubing in such a panel, or some kind of transducer may be provided in the panel to transform the light energy directly into another form of energy, normally electrical energy. The disadvantage of such system is that the amount of energy collected per unit of surface area is relatively low and is difficult to exploit efficiently.

The other main type of collector is the concentrating type. Typically such collector has a parabolic reflector whose reflecting surface is centred on an axis and formed by a plurality of parabolic generatrices. With this arrangement, all light impinging axially on the reflecting surface is directed inwardly to a point at which a light-absorbing or light-converting body is provided. Such a system therefore generates, at the body, considerable energy which manifests itself as the occurrence of an extremely high temperature at the body. This temperature frequently reaches a level of as high as 3300"C. Working with such high temperatures is a problem, normally requiring sophisticated technology and expensive materials.

It is therefore an object of the present invention to provide a solar collector which is of the concentrating type but which does not operate at such high temperatures so that the practical difficulties associated therewith are minimised.

This object is attained according to the instant invention in a solar collector having a reflector formed as a surface of revolution formed in turn by straight-line generatrices and centred on an axis with the generatrices intersecting the axis at a common point. Thus the reflecting surface is conical or frustoconical. The collector has a light-absorbing body, which is elongate, at its axis. In a collector wherein the surface is the outer surface of a cone or frustum the body is a tube surrounding this surface, and in a system wherein the surface is the inner surface of a cone the body lies on the axis inside the cone or cone frustum. In both cases the arrangement is such that fluid may be circulated through the body to carry off the collected heat.

The advantage of the collector of the invention is that although the solar energy is collected and concentrated to a significant extent, it is nonetheless not so very concentrated as to make operation of the device difficult. In addition, in the simplest system with an internally-mirrored conical or frustoconical collector centred on an axis pointed at the sun and having a central axially-extending tube constituting the body, it is possible simply to circulate water or other fluid through this tube in an extremely simple manner for heat exchange. What is more, the making of a frustoconical or conical reflector is a relatively simple task, easily performed by warping a planar reflective surface. This can be contrasted with the much more difficult job of manufacturing a parabolic reflector.

According to further features of this invention the body is a tube extending along the axis and having a round, polygonal, oval or other cross-section. It may alternatively be of cylindrical section, shaped as a cone, a cone frustum, or a pyramid. The body has a lightabsorbing outer surface, normally matt black, and a heat emitting inner surface, normally also matt black. It is possible to use different materials for light absorption and light emission; thus, the tube may be of a laminate, with one layer constituting the outer surface and another constituting the inner surface.

The reflector is normally made of polished metal such as aluminium, having a mirrored inner surface. It is possible to make the reflector in the collector according to this invention simply by warping a planar flexible metallic sheet and riveting the edges together. This construction procedure is enormously simpler than that employed for standard parabolic reflectors well known in the art. The angle of conicity of the conical or frusto-conical reflector is preferably between 20 and 175 and it is possible for the distance between the reflecting surface and the axis to vary somewhat, as well as to have the conical angle vary and to mount the light-absorbing body somewhat eccentrically or at an angle inside the reflector.

According to further features of this invention the reflector has several frusto-conical reflective bands alternating with frusto-conical nonreflecting bands or regions. Then the lightabsorbing body has regions that do not absorb light aligned with the non-reflecting regions and regions that do not absorb light and heat aligned with the reflective regions.

In use, the axis of the reflector is preferably continuously aligned at the sun. For this, support means may be provided for permitting this alignment and even for tracking the sun.

The invention will be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a largely schematic side view illustrating a preferred embodiment of the collector according to the instant inventton; Figure 2 is a diagrammatic view illustrating operation of the collector of Fig. 1; Figures 3 and 4 are largely schematic views illustrating another collector according to this invention; Figures 5 and 6 are schematic views illustrating yet another collector according to this invention; Fig areas 7 and 8 are schematic views illustrating a further collector according to the present invention; Figure 9 is a largely schematic perspective view illustrating another collector according to this invention; and Figure 10 is a side sectional and partly schematic view of yet another collector according to the invention.

As shown in Figs. 1 and 2 a first embodiment of the solar collector according to this invention has a conical reflector centred on an axis A and mounted on a support 21 for pivoting about a swivel 21 a by means of a cylinder 22 so that the axis A can be substantially aligned with the sun at all times. Provided inside this reflector 1 is a heat-absorbing body in the form of a cylindrical tube 2 having black outer and inner surfaces and aligned directly on the axis A. This tube 2 extends at one end from the apex or point 2 of the reflector 1 to its opposite end 10 which is at the plane of the outer edge of the reflector 1.

As shown in Fig. 2, light-rays R impinging axially on the reflector 1 will be reflected inwardly at the axis A. The angle 3 of conicity of the conical reflector 1 is 90 so that these light rays will impinge substantially radially on the axis A.

Figs. 3 and 4 show a collector comprising a reflector 1 a which is frusto-conical, but still with an angle of conicity substantially equal to 90 . In this arrangement the light-absorbing body 5 does not need, therefore, to extend all the way to the apex of the generatrices forming the reflector 1 a.

In Figs. 5 and 6, the collector has a frustoconical reflector 1 b having an angle 3b of conicity greater than 90 . In this arrangement a light-collecting body 6 on the axis A lies above the plane defined by the rim at the wide end of the collector 1 b, and the light rays fall on this collector body 6 at an angle.

In Figs. 7 and 8 a frusto-conical collector 1 c is shown whose angle 3c of conicity is less than 90 . Thus, the collecting body 8 must be displaced toward and beyond the plane defined by the rim at the narrow end 1 b of the collector and the light rays falling on it will, once again, fall on it at an acute angle.

Fig. 9 shows a collector 1 having a plurality of reflecting zones 11, 12, 13 and 14 alternating with non-reflecting zones. Lightabsorbing tube 15 has regions 16, 17, 18 and 1 9 aligned with the reflecting zones 11 to 14, but otherwise has non-light-absorbing regions so that heat is not lost.

Finally, Fig. 10 shows a collector comprising a reflector 11 8 having an outer lightreflecting surface 11 8' centred on the axis A.

The light collecting body is a cylindrical tube 11 9 surrounding the reflector 118 and centred on the axis A. Liquid can be caused to flow through this outer double-wall tube 11 9 by means of a pump 23 through a heat exchanger 24, the water being collected at a pipe 20 at the bottom of the body 119. In this system, therefore, light is reflected outwardly onto the inner surface of the tubular light-receiving body 11 9. Of course the pump 23 and heat exchanger 24 are provided in all of the arrangements, and the heat exchanger 24 can form part of a heating system, an electricity-generating plant, a desalinating system, or the like.

Claims (16)

1. A solar collector comprising a reflector having a reflecting surface constituting as a surface of revolution formed by straight-line generatrices and with said generatrices intersecting said axis at a common point centred on an axis, whereby light impinging axially on said surface is reflected at said axis, an elongate light-absorbing body at said axis and means for absorbing heat from said body.

2. A solar collector as claimed in claim 1 wherein said body is elongate along said axis.

3. A solar collector as claimed in claim 1 or 2 wherein said body is a tube.

4. A solar collector as claimed in claim 1, 2 or 3 wherein said body is of uniform crosssection along its length.

5. A solar collector as claimed in claim 1, 2 or 3 wherein said body is of increasing cross-section away from said point.

6. A solar collector as claimed in any preceding claim wherein said body has a lightabsorbing outer surface.

7. A solar collector as claimed in any preceding claim, wherein said body has a heat-emitting inner surface.

8. A solar collector as claimed in claims 6 and 7 wherein said body has a wall formed of a pair of layers, one of which has said outer surface and one of which has said inner surface.

9. A solar collector as claimed in any preceding claim wherein said surface of the reflector is mirror-reflective.

10. A solar collector as claimed in any preceding claim wherein said surface of the reflector converges to said point.

11. A solar collector as claimed in any preceding claim, wherein said surface of the reflector is generally conical and has an angle of conicity of between 20 and 175".

1 2. A solar collector as claimed in any preceding claim wherein said surface of the reflector has reflecting and non-reflecting regions and said body has a light-absorbing surface only in alignment with said reflecting regions.

1 3. A solar collector as claimed in any preceding claim wherein said body has a black outer surface.

1 4. A solar collector as claimed in any preceding claim further comprising support means for orienting said reflector and body with said axis directed substantially at and said point directed away from the sun.

1 5. A solar collector as claimed in claim 1 4 wherein said support means includes means for tracking the sun.

16. A solar collector as claimed in claim 14 or 1 5 wherein said reflector is mounted upon a swivel.

1 7. A solar collector as claimed in any preceding claim wherein the means for absorbing heat from the body comprises means for circulating a fluid through said body.

1 8. A solar collector substantially as hereinbefore described with reference to and as illustrated in Figs. 1 and 2, in Figs. 3 and 4, in Figs. 5 and 6, in Figs. 7 and 8, in Fig. 9 or in Fig. 10 of the accompanying drawings.