WO2005071325A1

WO2005071325A1 - A receiver for radiation

Info

Publication number: WO2005071325A1
Application number: PCT/AU2005/000064
Authority: WO
Inventors: David Mills; Philipp Schramek; Anne Gerd Imenes; Damien Buie
Original assignee: The University Of Sydney
Priority date: 2004-01-23
Filing date: 2005-01-21
Publication date: 2005-08-04

Abstract

The present invention provides a method of fabricating a receiver for radiation. The method comprises the step of determining a distribution of the radiation from at least one radiation concentrator that in use illuminates a receiving surface of the receiver. The method also comprises the step of selecting a shape for the receiving surface so that in use the receiving surface is more evenly illuminated by the or each radiation concentrator than a flat receiving surface. The present invention also provides a receiver for radiation that is in use substantially evenly illuminated.

Description

A RECEIVER FOR RADIATION

Field of the Invention The present invention broadly relates to a receiver for radiation and a method of fabricating a receiver for radiation. The present invention relates particularly, though not exclusively, to a receiver for collecting sunlight from at least one radiation concentrator such as a reflector dish. Throughout the specification the term "concentrator" is used for any concentrator means including single or multiple concentrating surfaces which in use concentrate radiation to a focal region or point.

Background of the Invention

In many countries, the demand for the generation of electricity from renewable resources is increasing. The generation of electricity from solar radiation, for example by converting the solar radiation into electricity using photovoltaic cells, is of increasing interest as in recent decades solar energy reflector arrays have been developed that concentrate sunlight upon photovoltaic cells. Devices which convert the solar radiation into electricity, such as photovoltaic cells, often operate most efficiently if the radiation is evenly distributed over the surface of the devices . Attempts have been made to provide even illumination and examples include kaleidoscope-like secondary reflectors which transform incoming radiation in such a way that a flat absorbing surface would be uniformly illuminated. However, as the kaleidoscope-like device uses non-ideal reflecting surfaces to change the light ray paths, it will itself absorb radiation which reduces the efficiency of the system.

Summary of the Invention

The present invention provides in a first aspect a method of fabricating a receiver for radiation, the method comprising the steps of: determining a distribution of the radiation from at least one radiation concentrator that in use illuminates a receiving surface of the receiver and selecting a shape for the receiving surface so that in use the receiving surface is more evenly illuminated by the or each radiation concentrator than a flat receiving surface.

The distribution of the radiation typically is associated with direct illumination of the receiving surface by the or each radiation concentrator and the radiation typically is directly received from the sun by the or each radiation concentrator. In a specific embodiment of the present invention the shape of the receiving surface is selected so that in use the receiving surface is substantially evenly illuminated by the or each radiation concentrator.

By shaping the receiving surface in the above-defined manner, there typically is no need for an additional device, such as a kaleidoscope-like device, to evenly illuminate the receiving surface. The efficiency of electricity generation can be improved by selecting a particular profiled shape for the receiving surface that results in more even illumination.

The present invention provides in a second aspect a receiver for radiation fabricated by the above-defined method.

The present invention provides in a third aspect a receiver for radiation comprising: a body having a receiving surface arranged to receive the radiation from a radiation source by at least one radiation concentrator, the receiving surface having a shape that is profiled so that a distribution of radiation originating from the source located at one position relative to the or each radiation concentrator, or an average radiation distribution if the source is located at more than one position during a predetermined time period, results in substantially even illumination of the receiving surface.

For example, the radiation source may be the sun. The receiving surface may have a shape that is profiled so that the receiving surface is substantially evenly illuminated for one position of the sun relative to the or each radiation concentrator. Alternatively, the receiving surface may have a shape that is profiled so that an average radiation distribution for more then one positions of the sun results in substantially even illumination of the receiving surface. The receiver typically is shaped so that a distribution of radiation originating from the source directed directly to the or each concentrator and directed directly from the or each concentrator to the receiving surface results in use in substantially even illumination of the receiving surface. The or each radiation concentrator of the receiver according to the second or third aspect of the invention may be a lens but typically is a reflector such as a sunlight reflector dish or a reflecting heliostat. The receiver according to the second or the third aspect of the present invention may comprise at least one device that transforms solar energy into another form of energy, such as electrical, thermal or chemical energy. For example, the receiver may comprise a photovoltaic device and the receiving surface may comprise a photovoltaically active surface of the or each photovoltaic device. In a specific embodiment of the second or the third aspect of the invention the receiving surface comprises a plurality of surface portions which are shaped and positioned so that the plurality of surface portions together in use are substantially evenly illuminated. Alternatively or additionally the body may comprise a thermal or chemical receiver and at least a region of the receiving surface may be a surface of the thermal or chemical receiver. The or each radiation concentrator of the receiver according to the second or third aspect of the present invention may be a part of an array such as a solar energy reflector array. The radiation concentrators may be arranged to reflect incident solar radiation to the receiver and to be driven to follow relative movement of the sun, in the manner of a heliostat. The receiver may be arranged for positioning over the array of radiation concentrators . The receiving surface may in this case have a dish-like shape, either concave or convex, having a profile that can be approximated by a curve having a central region of lower curvature and edge regions of higher curvature. Alternatively, one concentrator may be coupled to the receiver in a manner such that the one concentrator tracks the movement of the sun together with the receiver. In this case the concentrator may be a paraboloidal dish reflector and the receiving surface may have a curved profile that has a higher curvature in a central region. A movement of the radiation source relative to the radiation concentrators will change the requirements for even illumination of the receiving surface if the receiver according to the second or third aspect of the present invention is arranged to collect radiation from more than one radiation concentrator, but is itself not moved accordingly. In this case the receiving surface of the receiver may be arranged so that the shape of the receiving surface can be changed in a controlled manner. The receiver according to the second or third aspect of the present invention may comprise a drive that is arranged to effect the change in the shape of the receiving surface in a controlled manner and dependent on the position of the radiation source relative to the radiation concentrators . In an alternative embodiment of the receiver according to the second or third aspect of the present invention a sun-tracking reflector dish is arranged to track the movement of the radiation source, such as the relative movement of the sun, and the receiver is arranged to track with the reflector so that the requirements for even illumination on the receiving surface do not change significantly. For example, the reflector may have a concave paraboloidal shape. In this case the receiving surface may also have a curved convex or concave shape that is profiled so that the receiving surface is substantially evenly illuminated, although this curved shape typically is not paraboloidal . The profile of the receiving surface of the receiver according to the second or third aspect of the present invention may be either predominantly or totally convex or predominantly or totally concave relative to the or each radiation concentrator. If the profile of the receiving surface shape is convex, the receiving surface typically is positioned between the or each radiation concentrator and the focal region of the or each radiation concentrator. If the shape is concave, the receiving surface typically is positioned behind the focal region of the or each radiation concentrator.

The invention provides in a fourth aspect a system for receiving sunlight, the system comprising: a radiation receiver having a receiving surface and a solar energy reflector array arranged to reflect sunlight to the receiving surface, the receiving surface having an at least partially non-flat shape which is more evenly illuminated by the solar energy reflector array than a flat receiving surface.

The invention provides in a fifth aspect the system according to the fourth aspect wherein the receiver is according to the second or third aspect of the present invention.

The invention will be more fully understood from the following description of specific embodiments. The description is provided with reference to the accompanying drawings .

Brief Description of the Drawings Figure 1 shows plots representative of the cross- section of a parabolic reflector dish and the cross- sections of a calculated receiving surface according to a first specific embodiment, Figure 2 shows plots representative of a cross- section of receiving surfaces according to a second specific embodiment, Figure 3 shows plots representative of a cross- section of receiving surfaces according to a third specific embodiment and Figure 4 shows plots representative of a perspective view of receiving surfaces according to a fourth specific embodiment .

Detailed Description of Specific Embodiments Specific embodiments concern a receiver for radiation arranged to receive radiation from one or more radiation concentrators. The receiving surface has a non-flat shape that is profiled so that in use the receiving surface is substantially evenly illuminated. Within the radiation envelope between a radiation concentrator or a number of radiation concentrators and the focal position of the concentrated radiation flux at the receiver which receives the radiation solely from said concentrator or concentrators, a contiguous surface may be defined receiving constant illumination over the surface at a value between the value of the incident radiation flux at the concentrator and the maximum concentrated radiation flux at the focus point or points of the radiation concentrator. One may imagine this situation as follows: if one traces a path between the focal point and any point on the concentrator aperture, then somewhere along the path will be a desired illumination level between that at the focal point and that at the concentrator aperture. However, the radiation field inside the radiation envelope will be continuously differentiable. Therefore, a second path can be drawn from the focal point to a position on the concentrator aperture arbitrarily close to the first, and the desired level of illumination can be found along this new path arbitrarily adjacent to the first. In this way a surface of constant illumination can be formed from the entire ensemble of possible ray paths . Similarly, there exists another constant illumination surface beyond the focal point in a direction which points away from the concentrator or concentrators over for which a substantially even illumination also exists. Therefore, for a specific illumination level, a receiver surface can be constructed between the focal point or points and the mirror field or beyond the focal point and away from the mirror field where a substantially even illumination exists. The following describes a method of creating a surface of even illumination in a solar concentrating system. Using a ray-tracing model that simulates the concentrated solar energy conditions about the focal region or regions of a given solar collector device, a detailed model of the concentrated radiation can be gained. This model includes both the spatial and spectral energy distribution of the terrestrial solar radiation, the effect the concentrating optical system has on the solar radiation and the specific optical design of the concentrating system. The ray-tracing program calculates the energy passing though a small volume element about the focus or foci. Given a desired concentrated solar flux value (for example 200 times that of the incident solar energy) , where flux is the energy per unit area of receiver, a surface can be calculated according to that value, producing a surface of even illumination. The surface is defined as the region of the energy intensity map corrected for the solar flux incident on the receiver. For the case of the concentrating system having one focal point the surface will be continuous, for the case of multiple foci, each focal point can be identified with an assemblage of reflectors, and for each such system a surface of constant illumination can be located. Figure 1 shows schematically a plot of a cross- section of a parabolic reflector dish surface 10 and the calculated receiving surface profiles 12 to 15 and 17 to 20 of a receiver and which has a calculated shape that corresponds to substantially even illumination of the receiving surface for sunlight reflected by the reflector. The receiving surfaces 12 to 15 and 17 to 20 are also shown in the insert of Figure 1. Shown are four convex receiving surfaces 12 to 15 and four concave receiving surfaces 17 to 20 which are arranged about focal point 22 (positioned at position 0; 0 of the plot) of the reflector surface 10. The four different concave and convex surfaces correspond to different levels of illumination per radiation surface area. If the receiving surface is positioned between the focal point 22 and the reflector surface 10, the receiving surface has a parabolic shape which is convex. Alternatively, if the focal point 22 is positioned between the receiving surface and the reflector surface 10, the receiving surface has a parabolic concave shape. The surface shapes 12 to 15 and 17 to 20 were calculated using the above procedure supported by suitable computer software including a ray-tracing program. Figure 2 shows plots of receiving surfaces (arbitrary units) for different radiation fields of solar radiation reflectors . In this case the reflectors are arranged in an array of 100% ground coverage and assumed to be arranged to concentrate sun light onto a receiver positioned on a solar tower. The reflectors are arranged to track the relative movement of the sun. The shown receiving surface shapes 30 to 33 have shapes that are calculated for positioning the receiving surface between the reflectors and the focal point of the reflectors . Figure 2 shows plots of 30 to 33 corresponding to 0, 30, 45 and 60° Zenith angle of the sun. For the calculation it was assumed that the reflector field is a continuum field comprising an infinite number of infinitesimal small reflectors . Figure 3 shows plots that are related to those shown in Figure 2, the difference being that the shown plots 40 to 42 were calculated for a multi tower solar array as disclosed in Mills D.R. und Schramek Ph. (1999) . Mul ti Tower Solar Array (MTSA) with ganged heliostats . In: Proceedings of the 9th SolarPACES International Symposium on Solar Thermal Concentrating Technologies, 22.-26. Juni 1998, Font-Romeu, France, Journal de Physique IV, Vol. 9: 83-88. Again, for this calculation it was assumed that the reflector field is a continuum field comprising an infinite number of infinitesimal small reflectors . Figure 4 shows plots (arbitrary units) of the calculated energy per unit volume element for a single solar tower reflector array having a tower carrying a receiver having the receiving surface in the centre of the array. For the calculation of the shapes of the receiving surfaces it was assumed that the sun is positioned in the Zenith, the array has closely packed ideal circular reflectors and the receiver is positioned over the centre of the array at a height above the array that corresponds to one third of the diameter of the array. Figure 4 shows a 3D ray-trace of solar radiation flux per unit volume under the condition that there is no receiver to obstruct the beam. The primary solar beam has an angular spread (which is described in Buie D. , Monger A. J. and Dey C. J. (2003) , "Sunshape distributions for terrestrial solar simulations", Solar Energy 74(2): 113 - 122) which means that each point on the surface receives rays from a restricted area of the concentrator rather than from a particular point. If a receiving surface is placed to correspond to the top to the ^Λpeanut' shaped distribution shown in Figure 4 (as might be done within a volumetric receiver) , it will not obstruct rays passing through the focal region and the distribution of radiation will be substantially uniform over that surface. If, however, a surface is placed along the bottom of the peanut' it will block rays from going to the focus and farther, so that although radiation will be uniform along the lower surface, an upper receiver surface is no longer possible. Such a ^λlower constant illumination surface' may be continued part way up the side of the peanut' until it gets close to the edge of the ray envelope between the focal point and the reflector concentrator array boundary. Somewhere in this region, constant illumination becomes impossible because the ^Λpeanut' shaped distribution increasingly depends upon rays passing through the focal point - which are blocked - rather than those which have not yet passed through the focal point. The allowed perimeter of the constant illumination surface can be determined accurately by a ray-trace that accurately models the input source of radiation. The focal point of the reflector array is in the centre of the structures shown in Figure 5. Shown are three structures that have bottom halves 51, 53, 55 and top halves 50, 52 and 54. The convex exterior of the bottom halves 51, 52 and 53 correspond to three examples of different illumination levels of receiving surface shapes for cases where the receiving surfaces are positioned between the focal point and the array and the interior surface of the upper halves 50, 52 and 54 were calculated for different illumination levels and correspond to the cases of the focal point being positioned between the array and the receiving surface of the receiver. As may be seen from Figures 2 and 3 , the shape of the receiving surfaces associated with even illumination is dependent on the relative positions of the sun. In order to provide for more even illumination of the receiving surface, the receiving surface may be given an "average" profile having a shape that approximates that of a range of ideal profiles for a range of relative sun positions. Alternatively, the receiving surface may be arranged to change its profile dependent on the requirements. For example, selected surface facets may be arranged to move relative to others . In general, the receiving surfaces or facets of the receiving surfaces as shown in Figures 1 to 5 may be associated with the surfaces of photovoltaic cells, thermal or chemical receivers. Although the invention has been described with reference to particular examples, it will be appreciated by those skilled in the art that the invention may be embodied in many other forms. For example, the or each radiation concentrator may be any type of radiation concentrator including Fresnel lenses and other lenses. The or each radiation concentrator may have any suitable shape and a plurality of the radiation concentrators may not be arranged in an array. In addition, the radiation may not be sunlight but may be any other type of comparable electromagnetic radiation. Further, the shape of the receiving surface may be calculated using any suitable method. The receiving surface may also be arranged to reflect radiation in addition to, or alternatively to, transmitting radiation.

Claims

The Claims:

1. A method of fabricating a receiver for radiation, the method comprising the steps of: determining a distribution of the radiation from at least one radiation concentrator that in use illuminates a receiving surface of the receiver and selecting a shape for the receiving surface so that in use the receiving surface is more evenly illuminated by the or each radiation concentrator than a flat receiving surface.

2. The method as claimed in claim 1 wherein the distribution of the radiation is associated with direct illumination of the receiving surface by the or each radiation concentrator and the radiation is directly received from the sun by the or each radiation concentrator .

3. The method as claimed in claim 1 or 2 wherein the shape of the receiving surface is selected so that in use the receiving surface is evenly illuminated by the or each radiation concentrator.

4. A receiver for radiation fabricated by the method as claimed in any one of the preceding claims .

5. A receiver for radiation comprising: a body having a receiving surface arranged to receive the radiation from a radiation source via at least one radiation concentrator, the receiving surface having a shape that is profiled so that a distribution of radiation originating from the source located at one position relative to the or each radiation concentrator, or an average radiation distribution if the source is located at more than one position during a predetermined time period, results in substantially even illumination of the receiving surface.

6. The receiver as claimed in claim 5 being shaped so that a distribution of radiation originating from the source directed directly to the or each concentrator and directed directly from the or each concentrator to the receiving surface results in use in substantially even illumination of the receiving surface.

7. The receiver as claimed in claims 4 to 6 wherein the or each radiation concentrator is a reflector.

8. The receiver as claimed in claims 4 to 6 wherein the or each radiation concentrator is a lens .

9. The receiver as claimed in any one of claims 4 to 8 comprising at least one photovoltaic device and wherein the receiving surface comprises a photovoltaically active surface of the or each photovoltaic device.

10. The receiver as claimed in any one of claims 4 to 9 wherein the receiving surface comprises a plurality of surface portions which are shaped and positioned so that the plurality of surface portions together in use are substantially evenly illuminated.

11. The receiver as claimed in any one of claims 4 to 10 wherein the receiver has a body that comprises a thermal or chemical receiver and at least a region of the receiving surface is a surface of the thermal or chemical receiver.

12. The receiver as claimed in any one of claims 4 to 11 wherein one concentrator is coupled to the receiver in a manner such that the concentrator is arranged to track the movement of the sun together with the receiver and wherein the receiving surface has a dish-like shaped curved profile that can be approximated by a curve having a central region of higher curvature and edge regions of lower curvature.

13. The receiver as claimed in any one of claims 4 to 12 wherein the or each radiation concentrator is arranged to reflect incident solar radiation to the receiver and to be driven to follow relative movement of the sun.

14. The receiver as claimed in claim 13 wherein the or each radiation concentrator is a reflector of a solar energy reflector array.

15. The receiver as claimed in claim 14 wherein the receiver is arranged for positioning over the array of solar energy reflectors .

16. The receiver as claimed in any one of claims 13 to 15 wherein the receiving surface has a dish-like shape having a profile that can be approximated by a curve having a central region of lower curvature and edge regions of higher curvature.

17. The receiver as claimed in any one of claims 13 to 16 wherein the receiving surface is arranged so that the shape of the receiving surface can be changed in a controlled manner.

18. The receiver as claimed in claim 17 comprising a drive that is arranged to effect the change in the shape of the receiving surface in a controlled manner and dependent on the position of the radiation source relative to the or each radiation concentrator.

19. The receiver as claimed in any one of claims 4 to 18 wherein the profile of the receiving surface is convex relative to the or each radiation concentrator.

20. The receiver as claimed in claim 19 wherein the receiving surface is positioned between the or each radiation concentrator and the focal region of the or each radiation concentrator.

21. The receiver as claimed in any one of claims 4 to 18 wherein the profile of the receiving surface is concave relative to the or each radiation concentrator.

22. The receiver as claimed in claim 21 wherein the receiving surface is positioned behind the focal region of the or each radiation concentrator.

23. A system for receiving sunlight, the system comprising: a receiver having a receiving surface and a solar energy reflector array arranged to reflect sunlight to the receiving surface; wherein the receiving surface has an at least partially non-flat shape which is more evenly illuminated by the solar energy reflector array than a flat receiving surface.

24. A system as claimed in claim 23 wherein the receiver is as defined in any one of claims 4 to 22.