GB2328034A

GB2328034A - Solar energy collecting device

Info

Publication number: GB2328034A
Application number: GB9716360A
Authority: GB
Inventors: Peter Reginald Hall
Original assignee: SOLARGEN ENERGY CORP
Current assignee: SOLARGEN ENERGY CORP
Priority date: 1997-08-02
Filing date: 1997-08-02
Publication date: 1999-02-10
Also published as: GB9716360D0

Abstract

A solar energy collecting device comprises a concave part-spherical primary mirror (1) for collecting solar energy, a secondary concave bell-shaped mirror (2) directed towards the primary mirror for receiving solar rays reflected from the latter, and a tertiary convex mirror (3) positioned between the primary and secondary mirrors at a relatively short distance from the latter. The secondary mirror has a central aperture (8, Fig.4) at its pole through which the rays pass onto an absorber (10, Fig.4). The arrangement and shape of the mirrors is such that the inner rays received by the primary mirror towards the centre thereof are reflected and passed directly through said aperture onto said absorber, while the remaining outer rays towards the outside of the primary mirror are reflected by the secondary and tertiary mirrors before reaching the absorber.

Description

Title: Solar Energy Collecting Devices Field of Invention This invention relates to a solar energy collecting device particularly for concentrating solar energy to heat an absorber medium for converting the energy, eg to electrical power.

Backaround to the Invention Various design configurations of mirrors have been proposed for collecting a beam of solar energy and concentrating the beam on to a collection means, for example a heat exchanger or a photovoltaic cell array.

In one type of design a concave part-spherical primary mirror focuses the beam towards a secondary mirror which in turn focuses the beam onto an absorber and power train located beneath the secondary mirror. Such a design requires a relatively large secondary mirror, creating a significantly large unused area (ie. a shadow) in the centre of the primary mirror, which results in low inherent efficiencies of the system.

A further disadvantage is that the absorber and power train have to be located some distance from the secondary mirror, which requires the power train to be suspended by a substantial structure. Since in a commercially sized installation the secondary mirror is itself of a significant weight (of the order of several tons) and must also be capable of withstanding high wind forces, the resulting structures become excessively large and heavy.

It is an object of the present invention to provide an improved solar energy collecting device.

Summary of the Invention According to the present invention there is provided a solar energy collecting device comprising a concave part-spherical primary mirror for collecting solar energy, a secondary concave bell-shaped mirror directed towards the primary mirror for receiving solar rays reflected from the latter, and a tertiary mirror positioned between the primary and secondary mirrors at a relatively short distance from the latter, the secondary mirror having a central aperture at its pole through which the rays pass onto a collection means, wherein the arrangement and shape of the mirrors is such that the inner rays received by the primary mirror towards the centre thereof are reflected and passed directly through said aperture onto said absorber, while the remaining outer rays towards the outside of the primary mirror are reflected respectively by the secondary and tertiary mirrors before passing through said aperture onto said collection means.

Preferably the secondary mirror has a cusped, odd-aspheric concave reflecting surface.

The tertiary mirror may similarly have an odd-aspheric reflecting surface, but of convex shape.

The mathematical pole of the secondary mirror is preferably located at a position beyond the point of intersection of the innermost rays which are reflected by the secondary mirror.

The ratio of the radius of curvature of the primary mirror to the radius of its aperture may be in the range 0.55 to 0.65.

Said ratio may be in the range 0.57 to 0.60.

Although the primary, secondary and tertiary mirrors preferably each have a continuous reflecting surface, at least the primary mirror may alternatively be formed of a large number of facets b each of which has a substantially flat reflecting surface.

Brief Description of the Drawings An example of a solar energy collecting device in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows the overall system with primary, secondary and tertiary mirrors; Figure 2 shows the path followed by the suns outer rays, ie in the outer zone of the primary mirror; Figure 3 shows the path followed by the inner rays, ie in the inner zone of the primary mirror; Figure 4 is an enlargement of Figure 2 showing the reflection of the outer rays by the secondary and tertiary mirrors; Figure 5 is an enlargement of Figure 3 showing the direct path of the inner rays; Figure 6 is similar to Figure 4 and shows the offset rays arriving from the edge of the suns disk; and Figure 7 is a combination of Figures 4 and 5 showing how the rays from the inner and outer zones fit together.

Detailed description Referring first to Figure 1 the arrangement shown comprises a primary concave mirror 1, a secondary concave bell-shaped mirror 2 and a tertiary convex mirror 3.

The diameter of the aperture of the primary mirror 1 is typically 36m with a radius of curvature typically of 30m.

The secondary mirror 2 is of an ogive or cusped shape having a bell-shaped concave surface, the surface having an oddaspheric shape. The diameter of the secondary mirror is typically Sm.

The tertiary mirror, seen better in Figure 4, has a convex, odd-aspheric shape. Typically the diameter of the tertiary mirror is between im and l.5m.

Figures 2 and 4 show the paths of the suns rays which fall on to the primary mirror within an outer zone, eg between the diameters of 36m and 23m. In being reflected and passing between the primary and secondary mirrors the rays intersect one another along a caustic surface 4. The rays are then reflected off the secondary mirror on to the tertiary mirror 3. The latter focuses the rays generally towards the pole of the secondary mirror, in which there is formed a circular aperture 8, typically of lm diameter. Behind the aperture is located an absorber area 10, which may comprise a heat exchanger or a photovoltaic cell array.

The innermost of these outer zone rays, which are reflected by the secondary mirror around an annulus just outside the edge of the aperture 8, intersect one another at a local focus point, shown generally at reference 6, positioned a short distance in front of the pole of the secondary mirror.

Referring now to Figures 3 and 5, there are shown the inner rays falling within an inner zone on to the primary mirror 1, eg between the diameters of 5m and 23m. As best seen in Figure 5, the rays are reflected and focused into a broadly convergent beam which passes directly through the aperture 8, so that it impinges on the absorber 10 without being reflected by the secondary mirror 2.

As the secondary and tertiary mirrors are spaced approximately 4m apart, it will be apparent that the distance travelled by the rays in the inner zone is approximately 8m shorter than that travelled by the rays in the outer zone, since the latter are reflected by the secondary and tertiary mirrors. The rays received by the absorber 10 in this arrangement are therefore incoherent, and would therefore not be suitable for optical or radio purposes, for example for astronomy or radio astronomy.

Figure 6 shows the rays in the outer zone which arrive from the edge of the sun disk. Since these rays are approximately 0.25 off the central axis of the system, the beam of rays reflected by the tertiary mirror 3 are deflected or offset to one side of the absorber 10.

Figure 7 is an enlargement of Figure 1, showing the paths of the rays from the inner and outer zones when combined together.

As best seen in Figures 3 and 5, the innermost rays, ie at Sm diameter, pass the edges of the tertiary mirror 3 without obstruction.

With the arrangement described, ie with a secondary mirror of Sm diameter and an aperture of 36m for the primary mirror, it is apparent that an inherent efficiency of 98.1% is available.

Referring again to Figure 4, this shows that a large part of the area of the secondary mirror is devoted to just the outermost zone of the beam. In a modified arrangement, the diameter of the secondary could be reduced to approximately 4.07m, which would only reduce the beam diameter to 34m.

The efficiency of the system, relative to the diameter of the secondary mirror, has been found to be critically dependent on the distanceoby which the secondary mirror is positioned from the primary mirror. The higher the secondary mirror is positioned, the smaller is the area which is imaged directly onto the absorber 10, and the larger is the aperture area which has to be handled by the assembly of the secondary and tertiary mirrors.

Although it has been found that excellent results can be obtained in some circumstances with tertiary mirrors having a cusp (odd-aspheric), good designs have also previously been achieved with tertiary mirrors having no cusp (even-aspheric) shapes. The optimum design solution appears sensitively to depend on the distance between the primary and secondary mirrors.

Claims

1. A solar energy collecting device comprising a concave part-spherical primary mirror for collecting solar energy, a secondary concave bell-shaped mirror directed towards the primary mirror for receiving solar rays reflected from the latter, and a tertiary mirror positioned between the primary and secondary mirrors at a relatively short distance from the latter, the secondary mirror having a central aperture at its pole through which the rays pass onto a collection means, wherein the arrangement and shape of the mirrors is such that the inner rays received by the primary mirror towards the centre thereof are reflected and passed directly through said aperture onto said absorber, while the remaining outer rays towards the outside of the primary mirror are reflected respectively by the secondary and tertiary mirrors before passing through said aperture onto said collection means.

2. A device according to claim 1 in which the secondary mirror has a cusped, odd-aspheric concave reflecting surface.

3. A device according to claim 1 or claim 2 in which the tertiary mirror has an odd-aspheric reflecting surface.

4. A device according to any one of claimsa 1 to 3 in which the tertiary mirror is of convex shape.

5. A device according to any one of claims 1 to 4 in which the mathematical pole of the secondary mirror is located at a position beyond the point of intersection of the innermost rays which are reflected by the secondary mirror.

6. A device according to any one of claims 1 to 5 in which the ratio of the radius of curvature of the primary mirror to the diameter of its aperture is in the range 0.55 to 0.65.

7. A device according to claim 6 in which said ratio is in the range 0.57 to 0.60.

8. A device according to any preceding claim in which each of the mirrors has a continuous reflecting surface.

9. A device according to any one of claims 1 to 7 in which at least the primary is formed of a large number of facets each of which has a substantially flat reflecting surface.

10. A solar energy collection device substantially as herein described with reference to the accompanying drawings.