GB2483093A

GB2483093A - Solar concentrator with orthogonal linear reflectors

Info

Publication number: GB2483093A
Application number: GB1014249.5A
Authority: GB
Inventors: Peter Ernest Lockley
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-08-26
Filing date: 2010-08-26
Publication date: 2012-02-29
Also published as: GB201014249D0

Abstract

A solar energy concentrator has a secondary linear reflective element 2 receiving sunlight from a primary linear reflective element 1, with the foci lines of the reflective elements orthogonal or perpendicular. The elements 1,2 can be parabolic cylindrical mirrors (fig 4), or the second element 2 could be a Fresnel mirror (fig 3). Secondary element 2 is described as having a greater focal length than primary element 1, and a solar cell 3 can be further from secondary element 2 than its focal point (fig 2). The elements can be off-axis or asymmetric (fig 5). Solar cell 3 can be photo-voltaic or for solar heat, and the concentrator could be retrofitted to existing generators.

Description

OPTICAL SYSTEM FOR SOLAR POWERED APPARATUS

The present invention relates to optical systems for use with solar powered apparatus such as photo-voltaic power generators or solar furnaces.

Photo-voltaic solar power generators fall into two classes; those in which incident solar radiation is not concentrated and those in which incident solar radiation is concentrated by an optical system. The present invention relates to specifically to the latter class of solar power generators, but also is applicable to solar powered furnaces.

Photo-voltaic solar power generators require incident solar radiation to be concentrated, but not sharply focussed as the resulting temperatures would be so high as to damage existing photo-voltaic devices. It is known to provide solar radiation concentrators which include two optical elements, a primary focussing reflector and a secondary optical element which can take the form of a conventional mirror or a lens or reflector configured in Fresnel form. Examples of such optical systems are shown in patent specifications US 4,131,485, US application US2009/027425 Al, US 4,307,936 and AU502077B B2.

Patent specification US 4,131,485 discloses a three element optical concentrating system which has a concave primary reflective element, a convex reflective element co-axial with the first element but inclined to the optical axis of the first reflective element such as to direct solar radiation into a trough at the bottom of which is a photo-voltaic element. The walls of the trough also are reflective and act to collect off axis rays from the second element and direct them towards the photo-voltaic element.

Although it is stated that the reflective elements may be of Fresnel type, only conventional mirrors are described and illustrated. Most of the embodiments described have circular symmetry, but one is of linear form.

US published Patent Application US2009/0272425 Al discloses another three element optical system, which is arranged so as to concentrate solar radiation upon a thermal cycle engine, which then drives an electrical generator. In this invention, the first optical element is a concave mirror which directs solar radiation towards a co-axial annular convex mirror and thence to a co-axial thermal cycle engine receiver.

Solar radiation is refracted through the central aperture in the annular convex mirror towards the thermal cycle engine receiver by a Fresnel lens mounted in front of the annular mirror.

US patent specification US 4,307,936 discloses a three optical element system for collecting solar energy. Again a convex primary mirror is used, together with a face plate which is made up of parallel optical fibres which act to collect incident solar radiation over a range of angles of incidence and direct them onto the primary mirror.

One embodiment of the invention however, uses a Fresnel lens instead of the primary and secondary mirrors.

Australian patent specification AU502077B B2 discloses a solar furnace which utilises an optical system, which consists of an annular primary concave mirror which directs solar radiation to a secondary concave mirror facing the primary mirror and co-axial with it. The secondary mirror directs solar radiation through a Fresnel lens mounted in the centre of the primary mirror and thence to a furnace positioned behind the optical system.

The second embodiment of the invention disclosed in US patent 4,131,485 is the only one of the embodiments of the inventions disclosed in the prior art which is linear in form and this is described only cursorily, so that apart from the general statement in the specification that the mirrors used in the invention may be of the Fresnel type, there is no disclosure of their actual use. In figure 3 of the above specification, which is the relevant figure, the secondary mirror (4) is shown as being continuous so, that its optical axis is parallel with those of the other mirrors (2&6) of the invention disclosed in the above specification. Also, the manner of construction of the apparatus is such that all the optical components are in a fixed relation to one another.

It is an object of the present invention to provide an improved optical solar rádiation concentrator for use with solar powered apparatus According to the present invention there is provided an optical solar radiation concentrator for use with solar powered apparatus comprising a first reflective element adapted to produce a linear focus, a second reflective element also adapted to produce a linear focus and so arranged as to receive solar radiation reflected from the first reflective element wherein the linear foci of the first and second reflective elements are orthogonal to each other.

Preferably the arrangement is such that the focal length of the first reflective element is shorter than that of the second reflective element and there is included means for positioning a solar energy convertor cell in a position between the focal planes of the first and second reflective elements.

The second reflective element and the means for positioning a solar energy convertor at a position between the focal planes of the reflective elements may be positioned so as to be off-axis with respect to the first reflective element.

At least the second reflective element may be a mirror of the Fresnel type.

The invention will now be described, by way of example only, with reference to the accompanying drawings, in which, Figure 1 is a perspective view of a solar energy convertor apparatus embodying the invention, Figure 2 is an elevation of the embodiment of the invention shown in Figure 1 showing the ray paths in the focal plane of a first reflective element incorporated in the apparatus of solar radiation incident upon the apparatus, Figure 3 is a plan of the of the embodiment of the invention shown in Figure 1, showing the ray paths in a focal plane orthogonal to that of to the first reflective element produced by a second reflective element of the Fresnel type positioned with its optical axis orthogonal to that of the first reflective element and Figure 4 is a plan of a second embodiment of the invention in which both reflective elements of the apparatus are mirrors of cylindrical section and Figure 5 is an elevation of another embodiment of the invention in which the second reflective element and the means for positioning a solar energy convertor at a position between the focal planes of the first and second reflective elements are so positioned as to be off-axis relative to the first reflective element.

Referring to the drawings, a solar energy convertor apparatus embodying the invention consists of a primary reflective element 1 in the form of a cylindrical mirror of parabolic cross-section, a secondary, linearly extending reflective element 2 positioned with its longitudinal axis parallel with that of the reflective element 1, a multi-junction photo-electric solar energy convertor cell 3 and stays 4 and 5 for maintaining the secondary reflective element 2 and solar energy convertor cell 3, respectively, in their operative positions with respect to the primary reflective element 1. The secondary reflective element 2 in the embodiment of the invention being described is a mirror of the Fresnel type, but it can be of conventional form, as js is described later. The secondary reflective element 2 has reflecting surfaces 6, which are arranged to produce a linear focus like that of the primary reflective element 1, but which is perpendicular to that of the primary reflective element 1. The focal length of the secondary reflective element 2 is greater than that of the primary reflective element 1. The secondary reflective element 2 is positioned within the focal length of the primary reflective element 1.

The primary reflective element 1 concentrates solar radiation normal to its longitudinal axis and directs the concentrated solar radiation towards the secondary reflective element 2, which may be a single unit or a bank of separate units positioned within the focal length of the primary reflective element 1.

The secondary reflective element 2 concentrates solar radiation in a plane normal to that of the primary reflective element 1 and the combined effect of the primary and secondary reflective elements 1 and 2, respectively, and the position of the solar energy convertor cell 3 is to produce incident upon the solar energy convertor cell 3 an area of concentrated solar radiation.

The size and shape of the area of concentrated solar radiation produced by the combined actions of the primary and secondary reflective elements 1 and 2, respectively, can be varied by altering the aperture of the primary reflective element 1, the length of the secondary reflective element 2, or both and by varying the position of the solar energy convertor cell 3 between the focal planes of the primary and secondary reflective elements 1 and 2, respectively. Normally, changes would be made to the secondary reflective element 2 rather than to the primary reflective element 1. If so desired, the secondary reflective element can be asymmetric in form.

In the vertical plane, as shown in figure 2, solar radiation is brought to a focus and then de-focused before being incident upon the solar energy convertor cell 3. This feature enables the solar energy convertor cell 3 to be placed further from the primary reflector I than otherwise would be the case. On the other hand, in the horizontal plane as shown in figure 3, no such focusing and de-focusing is required and the solar energy convertor cell 3 is positioned before the focal plane of the secondary reflective element 3. As the angles of incidence on the solar energy convertor cell 3 of rays of solar radiation reflected from the outer facets of the secondary reflective element 2 are greater than those of rays of solar radiation reflected from inner facets of the secondary reflective element 2 its length is not restricted. Again, this enables a great deal of flexibility in the placing of the secondary reflective element.

Figure 4 shows a plan of a second embodiment of the invention in which the secondary reflective element 2 is section of a conventional mirror of cylindrical form.

Optically, the arrangement is as before, solar radiation incident upon the primary reflective element 1 is focused and then de-focused in the vertical plane before being incident upon the solar energy convertor cell 3, but not in the horizontal plane.

Figure 5 shows another embodiment of the invention in which the secondary reflective element 2 and the solar energy convertor cell 3 are positioned off-axis with respect to the primary reflective element 1. Optically this embodiment of the invention functions as before and the secondary reflective element 2 may be either a Fresnel type or a conventional mirror.

The solar energy convertor cell 3 may be a photo-voltaic electricity generator or a solar powered heating device.

The solar radiation concentrator of the present invention can be retro-fitted to existing solar energy power generators which employ cylindrical mirrors as solar radiation concentrators.

Claims

CLAIMS1. A solar energy concentrator for use with solar powered apparatus, comprising a first reflective element adapted to produce a linear focus, a second reflective element also adapted to produce a linear focus and so arranged as to receive solar radiation reflected from the first reflective element, wherein the linear foci of the reflective elements are orthogonal to each other.
2. A solar energy concentrator according to claim I wherein the focal length of the first reflective element is less than that of the second reflective element and there is included means for positioning a solar energy convertor cell between the foci of the first and second reflective elements.
3. A solar energy concentrator according to claim I or claim 2 wherein the second reflective element is a mirror of the Fresnel type.
4. A solar energy concentrator according to claim 1 or claim 2 wherein the second reflective element is a cylindrical mirror of parabolic cross-section.
5. A solar energy concentrator according to any of claims 2 to 4 wherein the solar energy convertor cell is a multi-junction photovoltaic electricity generator.
6. A solar energy concentrator substantially as herein before described and with reference to the accompanying drawings