WO2005124245A2

WO2005124245A2 - Reflecting solar concentrator for the generation of electrical energy

Info

Publication number: WO2005124245A2
Application number: PCT/IB2005/001731
Authority: WO
Inventors: Marco Stefancich
Original assignee: Valsecchi, Alfredo
Priority date: 2004-06-21
Filing date: 2005-06-20
Publication date: 2005-12-29
Also published as: WO2005124245A3; ITMI20041234A1; EP1766298A2; WO2005124245B1

Abstract

The present invention relates to a reflecting solar concentrator for the generation of electrical energy, comprising a generally concave reflector (1), having a plurality of reflecting elements (2) that define a focal area (3) situated before said reflecting elements (2), a photovoltaic receiver (4) having a plurality of photovoltaic cells, which define an active area, which is situated substantially at said focal area (2) and is adapted to receive the sunlight directly from said reflector (1), said photovoltaic receiver (4) being stationary with respect to said reflector (1), said reflecting elements (2) being substantially at the same distance from said focal area(3), characterized in that said reflecting elements (2) have a flat shape.

Description

REFLECTING SOLAR CONCENTRATOR FOR THE GENERATION OF ELECTRICAL ENERGY

This invention relates to a reflecting solar concentrator for the generation of electrical energy, a solar reflector and a method of converting the radiant energy of sunlight into electrical energy. The radiant energy of the sun is known to be convertible into electrical energy by special devices, also known as photovoltaic cells. A photovoltaic cell comprises a wafer of semiconductor material - such as monocrystalline silicon (m-Si) or polycrystalline silicon (p-Si) - which, when exposed to solar radiation, can deliver low-voltage current, typically at about 0.5 Volts. By connecting in series multiple photovoltaic cells, a higher voltage current may be obtained. Suitably connected photovoltaic cells may be arranged on special panels, which are known as photovoltaic panels. Due to their low efficiency, typically of about 12% - and to the high manufacturing costs of photovoltaic cells, photovoltaic panels are not competitive with traditional power generation technologies. Particularly a photovoltaic panel system has a rather poor return on investment. A more cost-effective solution as compared to photovoltaic panels is that of photovoltaic concentration systems. Here, photovoltaic cells may operate with higher luminous intensities than that radiated by the sun to the earth, while maintaining a substantially constant conversion efficiency. For certain types of photovoltaic cells, the proportion between the luminous intensity and the generated electrical energy may be maintained to luminous intensities of above one hundred times that of the sun. By concentrating the sunlight, the surface of photovoltaic cells may be greatly reduced, and the costs of a photovoltaic system may be minimized. For sunlight concentration, it is known to use curved reflecting elements which concentrate the sunlight onto a small photovoltaic receiver. For example, patent application US2001/0036024 discloses a solar concentrator comprising a dish whereto a set of parabolically curved reflecting elements are attached, which concentrate the sunlight onto a photovoltaic receiver. The Web site www.harbornet. com/sunflower discloses a solar concentrator comprising a dish with a set of adjacent mirrors attached thereto, and curved relative to one of the two dish dimensions. The above solar concentrators provide a high concentration but still have certain drawbacks. Particularly, they have poor conversion performances, and the photovoltaic cells of the receiver are often prone to failure or malfunctioning. The patent application EP-A2- 1126529 discloses a solar concentrator comprising a photovoltaic receiver and a few flat mirrors, disposed around the photovoltaic receiver to reflect the sunlight toward the photovoltaic receiver. This apparatus only allows to achieve a low sunlight concentration (a few suns) and does not allow to make use of all the potentialities of photovoltaic receivers. The object of this invention is to obviate prior art drawbacks and particularly the above mentioned drawbacks. Such object is fulfilled by a solar concentrator according to the principle of claim 1, by a reflector according to the principle of claim 13, and by a method of converting solar energy into electrical energy according to the principle of claim 15. Further advantages may be obtained by the additional features of the dependent claims. One embodiment of a reflecting solar concentrator for producing electrical energy as defined in the claims, and of a method of converting the radiant energy of sunlight into electrical energy will be described hereafter with reference to the annexed drawings in which: Figure 1 is a perspective view of a solar concentrator according to the present invention; Figure 2 is a schematic view of a possible solar concentrator carrying system. Referring to the annexed drawings, numeral 1 generally denotes a reflecting solar concentrator for the generation of electrical energy. The concentrator as shown in the figures comprises a generally concave reflector 1, having a plurality of reflecting elements 2 that define a focal area 3 situated before such reflecting elements 2, i.e. in the direction of propagation of the sunlight. A photovoltaic receiver 4 is placed at the focal area 3, and has a plurality of photovoltaic cells (not shown), which are connected in series and define an active area, i.e. an area designed to receive solar energy to be converted into electrical energy. The photovoltaic receiver 4 is stationary with respect to the reflector 1 and receives the sunlight directly from the reflector 1. The reflecting elements 2 of the reflector 1 are substantially at the same distance from the focal area 3, and define a faceted parabolic surface. One feature of the concentrator is that the reflecting elements 2 have a flat shape. This provides a uniform density of the sunlight over the active area of the photovoltaic receiver 4, i.e. over the photovoltaic cells. The uniform solar radiation onto the photovoltaic cells allows to equalize current generation by such photovoltaic cells, and thence to maximize the conversion efficiency of the photovoltaic receiver 4. Such uniform light distribution over the photovoltaic cells 4a reduces the risk of damages caused by overheating of individual photovoltaic cells. Therefore, this affords improved efficiency and reliability as compared with prior art concentrators as described in US 2001/0036024 and in the Web site www.harbornet.com/sunflower, where solar radiation reflected onto the photovoltaic receiver does not have a uniform density. Since the focal area of the reflector 1 is situated before the reflecting elements 2, a high sunlight concentration may be achieved. The concentrator as described above provides a higher sunlight concentration than the apparatus described in EP-A2- 1126529 and allows to generate the same amount of power while minimizing the costs for photovoltaic cells. The reflector preferably comprises more than sixteen, and more preferably more than thirty- six reflecting elements. In the embodiment as shown in the figures, which is particularly advantageous for solar concentrators designed for small families, the reflector comprises one hundred twenty-one flat reflecting elements. Obviously, reflectors with more than one hundred twenty-one reflecting elements may be also provided. However, at the state of the art this does not provide any substantial advantage, as photovoltaic cells become saturated above these concentration values. In the embodiment as shown, the flat reflecting elements 2 have substantially the same shape and size as the active area of the photovoltaic receiver 4. According to a preferred embodiment, the photovoltaic receiver 4 has a substantially square active area and the flat reflecting elements 2 also have a square shape and substantially the same size as the active area of the photovoltaic receiver 4. In one possible embodiment, the flat reflecting elements 2 are attached to a support 5 which may be made of a composite material, such as glass fibers or carbon, of plastic (e.g. ABS) or metal. Preferably, the support 5 is composed of multiple joined pieces (e.g. four pieces), therefore the reflector 1 is also composed of several assembled pieces. This arrangement allows to reduce the costs for the equipment to be used to make the support 5, and thence reduces the manufacturing costs for the reflector 1. The possibility to remove the reflector 1 also allows easier storage and transportation of reflectors. The flat reflecting elements 2 may be obtained, for instance, by attaching a mirror film, manufactured by the US corporation 3M with the trade name VM2002, directly to the surface of the support 5. Otherwise, the flat reflecting elements may be provided in the form of mirrors consisting of a substrate of metallized acrylic resin. Preferably, the reflector 1 has one or more holes or slits for discharging rain water. To further improve the conversion efficiency of the photovoltaic receiver 4, air or liquid means for cooling the photovoltaic cells may be provided. To maintain the conversion efficiency at a high level, the photovoltaic receiver should be maintained at a temperature of about 45 to 55°C. In a first case, the heat exchanger is formed by a pack of metal plate in direct contact with a non-illuminated surface of the photovoltaic receiver. In a second case, the heat exchanger is formed by finned tubes in which a carrier liquid is circulated to extract the heat generated by the photovoltaic receiver. In both cases, a fan may be provided to generate a forced draft passing through the fins of the plate pack. The solar concentrator further comprises carrier and sun-tracking means 7 which allow the reflector 1 to always have the sun on its own main optical axis. The carrying means 7 include a motor-driven supporting and sun-tracking mounting 7. In a first embodiment, the motor-driven mounting 7 of the reflector may be of the altazimuth type, i.e. comprising a first axis of rotation, parallel to the local vertical, and a second axis of rotation, perpendicular to the first axis and parallel to the horizontal plane. This may be a preferred arrangement, if the reflector 1 has a heavy load. Nevertheless, this arrangement has the drawback of requiring frequent corrections on both axes for proper sun tracking. In a second embodiment, the motor-driven mounting 7 may be of the equatorial type, i.e. comprising a first axis of rotation, parallel to the polar axis, and a second axis of rotation, perpendicular to the polar axis and parallel to the equatorial plane. An advantage of this arrangement is that, for daily tracking, one motor, driven at constant speed, is sufficient. The other motion is only requested for slight corrections on a multiple day basis. However, this arrangement may have problems when heavy loads, e.g. a large-size reflector, have to be carried. In a third embodiment, as shown in Figure 2, the motor-driven mounting may have a first axis of rotation 71, parallel to the horizontal plane, and a second axis of rotation 72, orthogonal to the first axis of rotation, which changes its direction as the first axis rotates. In the example as shown the rotation about the first axis 71 is controlled by a first motor 8, and the rotation about the second axis 72 is controlled by a second motor 9, which is integral with the axis of rotation of the first motor, whose axis is orthogonal to that of the first motor. This arrangement allows to adequately support the weight of the reflector 1. Regardless of the type of mounting, the tracking means include a plurality of light detectors 8 (e.g. four photodiodes, each capable of receiving light from 1/4 of the sky vault). The motor drives of the carrier system act in the direction of the photodiode with the highest luminous intensity until reaching a balance condition among the signals received by the various photodiodes. The electronic control equipment may include a two-level comparator which compares the signal from each pair of opposed photodiodes. In a preferred embodiment, the motor-driven mounting 7 is capable of positioning the reflector 1 with a downwardly directed reflecting surface. This arrangement is particularly advantageous when the concentrator is installed in areas subjected to frequent snowfalls and strong winds. When the reflecting surface is directed downwards, the snow collected in the reflector can arged and the resistance to the wind may be reduced.

Claims

1. A reflecting solar concentrator for the generation of electrical energy, comprising a) a generally concave reflector (1), having a plurality of reflecting elements (2) that define a focal area (3) situated before said reflecting elements (2), b) a photovoltaic receiver (4) having a plurality of photovoltaic cells, which define an active area, which is situated substantially at said focal area (2) and is adapted to receive the sunlight directly from said reflector (1), said photovoltaic receiver (4) being stationary with respect to said reflector (1), said reflecting elements (2) being substantially at the same distance from said focal area

(3), characterized in that said reflecting elements (2) have a flat shape.

2. A solar concentrator as claimed in claim 1, wherein said flat reflecting elements (2) have substantially the same shape and size as the active area of said photovoltaic receiver (4).

3. A solar concentrator as claimed in claim 1 or 2, wherein said photovoltaic receiver (4) defines a substantially square active area and wherein said flat reflecting elements (2) have a square shape and substantially the same size as the active area of said photovoltaic receiver (4).

4. A solar concentrator as claimed in any of the preceding claims, wherein said flat reflecting elements (2) are attached to a support (5) which is made of a plurality of joined pieces.

5. A solar concentrator as claimed in any of claims 1 to 4, wherein means (6) are provided for air cooling the photovoltaic receiver (4).

6. A solar concentrator as claimed in any of claims 1 to 4, wherein means (6) are provided for liquid cooling of the photovoltaic receiver (4).

7. A solar concentrator as claimed in any preceding claim, further comprising carrier and sun tracking means (7).

8. A solar concentrator as claimed in claim 7, wherein said carrier means (7) comprise an altazimuth motor-driven mounting.

9. A solar concentrator as claimed in claim 7, wherein said carrier means (7) comprise an equatorial motor-driven mounting.

10. A solar concentrator as claimed in claim 7, wherein said carrier means (7) comprise a motor-driven mounting having a first axis of rotation (71) parallel to the horizontal plane, and a second axis of rotation (72) orthogonal to said first axis of rotation, which changes its direction as the first axis (71) rotates.

11. A solar concentrator as claimed in any of claims 7 to 10, wherein said carrier means allow to direct the reflecting surface of the dish downwards.

12. A solar concentrator as claimed in any preceding claim, wherein said reflector (1) has one or more holes or slits for discharging rain water.

13. A generally concave reflector, having a plurality of reflecting elements (2) that define a focal area (3) situated before said reflecting elements (2).

14. A reflector as claimed in claim 12, wherein one or more holes or slits are provided for discharging rain water.

15. A method of converting the radiant energy of the sunlight into electrical energy, wherein the sunlight is directly reflected onto a photovoltaic receiver (4), in such a manner as to a have a substantially constant illumination density all over the active surface of the photovoltaic receiver and wherein the concentration of the sunlight over the photovoltaic receiver (4) is of more than sixteen suns.