Background 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.

In another known design of a device for collecting solar energy, a fixed large spherical mirror concentrates light from the sun on to a heat exchanger positioned above the concentrator. The shape of the concentrator is such as to maximise energy collection and it is driven to match the movement of the sun.

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

Summarv of the Invention According to the present invention there is provided a solar energy collecting apparatus comprising a primary mirror for concentrating light rays from the sun, a secondary mirror for receiving rays from the primary mirror and a tertiary mirror adapted to receive rays from the secondary mirror and to beam rays towards a collection means.

In a preferred arrangement the primary mirror is a concave part-spherical mirror, the secondary mirror is a concave bell- shaped mirror directed towards the primary mirror, the tertiary mirror is positioned between the primary and secondary mirrors at a relatively short distance from the latter, and the secondary mirror has a central aperture at its pole through which the rays pass onto the 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 the aperture on to the collection means, while the remaining outer rays towards the outside of the primary mirror are reflected respectively by the secondary and tertiary mirrors before passing through the aperture on to the collection means.

The collection means may then comprise a first heat exchanger containing fluid, and in which fluid passage means communicates between the heat exchanger and a remote location with or without pumping means to enable heated fluid to be conveyed away from the heat exchanger to power a process such as a turbine driven electrical generator, or a heat engine for powering such a generator, or another process involving the application of heat such as heating or boiling water or operating a heat pump such as used in the so-called "Electrolux cycle" for achieving cooling.

In large scale installations where hundreds of kilowatts, or even megawatts of heat energy are collected, eg for electrical power generation, the energy dissipated at the tertiary mirror may become excessively high.

Preferably, therefore, the tertiary mirror includes a second heat exchanger, and means for conveying cooling fluid therethrough so as to cool the tertiary mirror in use.

In an alternative arrangement the secondary mirror is horn- shaped and is drivable to track the sun.

Other features of the invention are defined in the appended claims.

Brief Description of the Drawinas Examples of solar energy collecting devices in accordance with the invention will now be described with reference to the accompanying drawings in which: Figure 1 shows an embodiment of 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 sun's disk; Figure 7 is a combination of Figures 4 and 5 showing how the rays from the inner and outer zones fit together; and Figure 8 is a diagrammatic view of a different embodiment.

Detailed Description of Embodiments 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 odd- aspheric shape. The diameter of the secondary mirror is typically 5m.

The tertiary mirror 3, seen better in Figure 45, has a convex, odd-aspheric shape. Typically the diameter of the tertiary mirror is between im and 1.5m.

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 collects the radiant energy and 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 5m diameter, pass the edges of the tertiary mirror 3 without obstruction.

With the arrangement described, ie with a secondary mirror of 5m 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 distance by 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.

Where the absorber area 10 comprises a heat exchanger containing fluid, the heated fluid will preferably be fed to a remote location to power a generator or heat pump or for heating water.

To avoid the tertiary mirror 3 becoming overheated in large scale installations, cooling fluid may be passed therethrough and fed to a heat exchanger (not shown) which may be either separate from or combined with the main heat exchanger.

In a further modification, not shown, the tertiary mirror 3 is replaced by an absorbing device having a roughened or coated surface so as to absorb radiant energy, which again is utilised in a heat exchanger.

Referring now to Figure 8, there is shown a first fixed spherical or parabolic mirror 11 which concentrates light from the sun onto a secondary mirror 12 above the first mirror. The secondary mirror 12 is drivable to track the movement of the sun, and is horn-shaped to maximise the angle over which light is collected from the first mirror 11.

The secondary mirror 12 reflects light on to a tertiary mirror 14 located above the secondary mirror at the focal point of the concentrated light.

The tertiary mirror 14 beams the collected light energy downwardly to a receiver 16 on the ground where energy conversion and power generation is carried out, as by means of a heat exchanger.