SOLAR FLUX ENHANCER

Abstract: A solar energy concentrator is disclosed. The solar energy concentration comprises: (i) a primary reflector surface (3) for concentrating solar energy towards a focal plane; (ii) a solar energy receiver (5) for receiving reflected solar energy positioned behind the focal plane; (iii) a solar energy flux modifier (7) having a front aperture (9), internally mirrored walls (11) that diverge from the front aperture (13), and a larger rear aperture, the flux modifier being adapted to receive concentrated solar energy from the primary reflector surface via the front aperture and to re-direct solar energy via the mirrored walls and the larger rear aperture to the receiver.

Description: -- - 1 - SOLAR FLUX ENHANCER The present invention relates to a solar energy concentrator assembly. There is a large and expanding demand for electric power. For the long term, it is desirable to have a generating system which is ecologically sound, inexhaustible, and economically attractive. Presently, the generation of electricity from solar energy using photovoltaic cells satisfies the first two criteria well. However, because of the high cost of photovoltaic cells, the economies are only attractive for certain applications. In order to reduce the cost of solar electric power generation it has been proposed that concentrating mirrors be used to intercept a relatively large area of solar energy and focus this energy onto an array of photovoltaic cells that has a relatively small total surface area. ith the cost of mirrors being an order of magnitude less than photovoltaic cells, the indications are that this approach may reduce the cost of solar generated electric power (SGEP) by 2 to 10 times and thus satisfy the third criteria (of economy) for the optimal solar power station. Recognised methods of solar concentration include the use of parabolic concentrators such as parabolic troughs or dishes which can produce average concentrations of 50 and several thousand, respectively, and peak concentrations of two or more times the average concentration, with a single stage. With reference to Figure 1, typically, a conventional solar energy concentrator assembly, as shown in vertical section, comprises a parabolic dish 3 which is arranged to reflect and concentrate solar radiation onto an array of photovoltaic cells 5 located at the focal plane of the parabolic dish 3. As is shown in Figure 1, the focal plane is a plane that is perpendicular to the central axis X of the parabolic dish 3 and contains the focal point on the central axis X. A consequence of the use of concentrated solar energy is that it is necessary to use specially designed and fabricated photovoltaic cells (pv cells) with the complexity and cost of the pv cells increasing with the level of concentration and power delivery required. In order to gain a useful voltage from pv cells, it is necessary to connect the pv cells in series into an array (pv array) . In practice it is found that the distribution of the focussed solar energy on a pv array plays a significant role in determining the electrical output of the system. As shown in Figure 2, at the focal plane the intensity of the beam from a parabolic concentrator that is incident on a "target" surface of a pv array varies with distance from the central axis of the concentrator and takes the form of a gaussian distribution. With reference to Figure 2, for a pv array with a radius or half distance X, while most of the concentrated energy is intercepted the variation in intensity is large, with the outer edges receiving a flux of perhaps one tenth (1/10) of that at the axis (point "0"). Generally, the power output of a series connected pv array is limited by the pv cell in the area of lowest solar flux. As a consequence, the variation of beam intensity shown in Figure 2 may result in the power output of the entire pv array being less than half of that in situations where the same amount of flux was evenly distributed from the axis to X. In addition, the flux which is incident on the focal plane target at a distance greater than X misses the target altogether and is wasted. In an attempt to deal with flux variation it has been proposed recently to construct a stepped faceted hemispherical pv array. Whilst some success with this approach has been reported, the great structural complexity of such a pv array would be prohibitive in production. In an attempt to recover the lost flux it is known to use obtuse scavenger reflectors. Typical scavenger mirrors are identified by the numeral 7 in Figure 1. While the use of scavenger mirrors will intercept the escaping rays the net effect on the electrical output may even be negative for the following reasons. (1) The scavenger mirrors increase the shading of the primary mirror and thus reduce the flux that is incident on the target surface. (2) The rays which are intercepted by the scavenger mirrors are reflected onto the cover glass on the pv array at a very low angle (<30°) with the result that most of the "captured" flux is reflected from the pv array and wasted. (3) The flux distribution due to the addition of the scavenged rays is not significantly improved and may even be made worse. An object of the present invention is to provide a solar energy flux modifier assembly which alleviates the disadvantages of the prior art arrangements described in the preceding paragraphs and enhances the suitability of the solar energy for application to a series connected pv array. According to the present invention there is provided a solar energy concentrator assembly which comprises: (i) a primary reflector surface for concentrating solar energy towards a focal plane, as described herein; (ii) a solar energy receiver for receiving reflected solar energy positioned behind the focal plane; (iii) a solar energy flux modifier having a front aperture, internally mirrored walls that diverge from the front aperture, and a larger rear aperture, the flux modifier being adapted to receive concentrated solar energy from the primary reflector surface via the front aperture and to re¬ direct solar energy via the mirrored walls and the larger rear aperture to the receiver. The term "focal plane" as described herein is understood to mean the plane that is perpendicular to the central axis of the primary reflector surface and contains the focal point on the central axis of the primary reflector surface. In a conventional solar energy concentrator assembly (as shown in Figure 1), typically, the solar energy receiver is positioned at the focal plane of the primary reflector surface. The present invention is an alternative approach in which the solar energy receiver is placed behind the focal plane. At this location the beam rays from the primary reflector surface are divergent and a significant proportion are reflected by the internally mirrored surface of the solar energy flux modifier onto the solar energy receiver. It is preferred that the flux modifier be positioned between the solar energy receiver and the focal plane of the primary reflector surface. It is preferred that the solar energy receiver be a pv array. It is preferred that the solar energy receiver be a receiver that requires an even flux distribution over the target surface of the receiver for optimum performance. The solar energy receiver may be of any suitable shape. The solar energy flux modifier may be of any suitable construction. It is preferred that the solar flux modifier be frusto-conical or truncated pyramidal in shape when the primary reflector surface is adapted to focus solar energy onto a focal point. It is preferred that the solar flux modifier comprise a pair of divergent planar members when the primary reflector surface is adapted to focus solar energy onto a focal line. In this case the front aperture of the solar flux modifier is defined by the gap between the front edges of the planar members. In a situation where the solar energy receiver requires an even flux distribution for optimum performance, it is preferred that the solar flux modifier be formed to redirect solar energy that is incident on the mirrored walls so that there is a uniform flux distribution over the solar energy receiver. For a concentrator assembly in which the primary reflector surface is parabolic and the solar energy receiver is a flat square pv receiver, it is preferred that the solar energy flux modifier be an internally mirrored truncated pyramid positioned between the focal plane of the primary reflector surface and the solar energy receiver. Such an arrangement and its effects is illustrated in Figures 3 and . Figure 3 is a vertical section through a preferred embodiment of a solar energy concentrator assembly of the present invention. The concentrator assembly comprises a primary reflector in the form of a parabolic dish 3 and a solar energy receiver in the form of a flat square pv array 5 positioned behind the focal plane of the reflecting surface of the parabolic dish 3. The concentrator assembly further comprises a solar energy flux modifier, generally identified by the numeral 7, which is in the form of a truncated pyramid and comprises, a front aperture 9, side walls 11 that diverge from the front aperture 9 and are internally mirrored, and a larger rear aperture 13. It can readily be appreciated from the line marked with the numeral 15 that the flux modifier 7 is able to direct rays of solar radiation onto the pv array 5 that otherwise would be outside the usual target area of the pv array. Figure 4 provides an indication of the performance of the concentrator assembly shown in Figure 3. The figure is a plot of the variation of the flux intensity at the pv array 5 with distance from the central axis X - X of the parabolic dish 3. The dotted line in the figure is a plot for the arrangement shown in Figure 3 without the flux modifier 7. The solid line is the plot for the arrangement shown in Figure 3 with the flux modifier 7 and illustrates that the flux modifier makes it possible to achieve an even flux distribution, albeit at lower peak flux intensity, over the area of the pv array. In summary, the overall effect of the above described solar energy flux modifier of the present invention is to both increase the amount of flux reaching the target surface and to improve the distribution of that flux. This is achieved by the concomitant reduction in peak intensity at the central axis of the concentrator assembly with the movement of the target surface behind the focal plane of the concentrator and the reflection of the 11 off-target" rays back onto the outer edges of the target surface. Figures 5 and 6 illustrate the results of computer modelling work carried out by the applicant on a conventional paraboloidal dish-based solar energy concentrator assembly without a solar flux modifier (Figure 5) and a solar energy concentrator assembly of the present invention with a solar flux modifier (Figure 6) . It is clear from the Figures that the invention eliminates entirely the peaked solar energy distribution of Figure 5 and increases overall the amount of flux reaching the target surface. In addition, experimental work of the applicant has shown that improvements in pv array output of up to 120% have been achieved using this solar energy concentrator assembly of the invention. The invention has the following features. (1) The invention does not increase shading of the primary reflector surface and thus full reflected beam power is maintained. (2) The invention straightens the scavenged rays so that they approach the solar energy receiver more directly and thus reduce the chance of re- reflection. (3) The invention improves the flux distribution. The scavenged rays are directed to the outer edge of the solar energy receiver and the flux in this area is built-up to match the intensity at the centre and thus an even flux distribution is produced across the entire receiver surface area. (4) The apparent size of the solar energy receiver is also significantly increased to a radius Y as shown in Figure 3. This gives rise to the following further significant advantages. (i) The apparent increase in size of the receiver make the chances of hitting the receiver high and it is possible to have a receiver efficiency of approaching 100% for a system with accurate optics. (ii) Minimum requirements for optical accuracy may be relaxed and an acceptable receiver efficiency still be achieved. (iii) Tracking accuracy can be reduced, with good receiver efficiency still being achieved. (iv) The optical accuracy of the primary concentrator may be reduced. (5) The averaging effect of the invention requires less expensive pv cells and heat sink since the peak flux to be absorbed is considerably reduced. (6) The temperature of the pv array is more even with the positive effect of reducing peak thermal stress and negative temperature coefficients of voltage. (7) The shape of the target pv array (parallel to and behind the focal plane) is also governed by the flux modifier. The correct choice of flux modifier allows the pv array to be of a simple shape, eg a flat square. Many modifications may be made to the preferred embodiment of the invention shown in Figures 3 and 4 without departing from the spirit and scope of the present invention.

Claims: ClaATM.S ; 1. A solar energy concentrator which comprises: (i) a primary reflector surface for concentrating solar energy towards a focal plane, as defined herein; (ii) a solar energy receiver for receiving reflected solar energy positioned behind the focal plane; (iii) a solar energy flux modifier having a front aperture, internally mirrored walls that diverge from the front aperture, and a larger rear aperture, the flux modifier being adapted to receive concentrated solar energy from the primary reflector surface via the front aperture and to re¬ direct solar energy via the mirrored walls and the larger rear aperture to the receiver. 2. The assembly defined in claim 1, wherein the flux modifier is positioned between the solar energy receiver and the focal plane of the primary reflector surface. 3. The assembly defined in claim 1 or claim 2 wherein the solar energy receiver is a pv array. 4. The assembly defined in any one of the preceding claims wherein the solar energy flux modifier is formed to re-direct solar energy that is incident on the mirrored walls so that there is a uniform flux distribution over the solar energy receiver. 5. The assembly defined in any one of the preceding claims wherein the solar energy flux modifier is frusto-conical or truncated pyramidal shape when the primary reflector surface is adapted to focus solar energy onto a focal point. 6. The assembly defined in any one of claims 1 to 4 wherein the solar energy flux modifier comprises a pair of divergent planar members when the primary reflector surface is adapted to focus solar energy onto a focal line.

Inventors: LASICH JOHN BEAVIS, LASICH, JOHN, BEAVIS
Applicants: LASICH JOHN BEAVIS, SOLAR RAPS PTY LTD, LASICH, JOHN, BEAVIS, SOLAR RAPS PTY. LTD.
Application year: 1996
Main classification: F24J2
IPC codes: F24J2, H01L31