PHOTOVOLTAIC SYSTEM WITH DOUBLE-REFLECTION SOLAR CONCENTRATOR
TECHNICAL FIELD
The present invention relates to a photovoltaic system comprising at least one double-reflection solar concentrator .
BACKGROUND ART
Photovoltaic systems are known, which employ a solar concentrator of double-reflection type, i.e. comprising a curved primary reflector for collecting and concentrating sunlight towards a focal area, and a flat secondary reflector arranged in a point of the optical path of the light concentrated before the focal area, for reflecting at least part of the sunlight concentrated by the primary reflector towards a different focal area, and photovoltaic converting means arranged so as to receive the concentrated sunlight.
For example, patent application WO 2009/093128 describes a photovoltaic system having a double- reflector solar concentrator, which comprises a parabolic primary reflector and a flat dichroic secondary reflector arranged aligned with the primary reflector to receive the light reflected and concentrated by the primary reflector and reflect back only a portion of the sunlight spectrum received, a first photovoltaic converter coupled with the secondary reflector for receiving the part of light not reflected by the secondary reflector and a second photovoltaic converter for receiving the light reflected by the secondary reflector. A second embodiment discloses an optical guide arranged with the input on the focus of the secondary reflector and the output on the second photovoltaic converter to homogenize the light on the second converter.
Double-reflection solar concentrators of the above- described type have various drawbacks. For example, such concentrators are very sensitive to pointing or alignment errors of the primary reflector with respect to the incidence direction of the sunlight. Moreover, photovoltaic systems of the above-described type, i.e. of the type comprising a double-reflection concentrator, a spectral separator and a double photovoltaic converter, have a spatial configuration which does not allow effective passive cooling of the photovoltaic converters without decreasing optical effectiveness. Indeed, there is a need to use some dissipators which inevitably intercept the incident light beam and/or the concentrated light beam, in order to effectively cool the photovoltaic converters.
DISCLOSURE OF INVENTION
The object of the present invention is to provide a photovoltaic system comprising a double-reflection solar concentrator, which is free from the above-described drawbacks and at the same time is easy and affordable to make .
In accordance with the present invention, a photovoltaic system is provided as defined in the accompanying claims .
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will now be described with reference to the accompanying drawings, which show a non-limiting embodiment thereof, in which:
- figure 1 shows, in a cross-section view parallel to the incidence direction of the sunlight, the photovoltaic system made according to the dictates of the present invention;
- figure 2 shows, in a side view and in greater detail, a solar receiving assembly of the photovoltaic system in figure 1;
- figure 3 shows, in a front top view, the solar receiving assembly of the photovoltaic system in figure 1; and
- figure 4 shows, in a cross-section view parallel to the incidence direction of the sunlight, a further embodiment of invention.
BEST MODE FOR CARRYING OUT THE INVENTION
Numeral 1 in figure 1 generically indicates a photovoltaic system as a whole comprising one or more photovoltaic concentration and conversion units 2 arranged adjacent to one another to collect and concentrate sunlight. In particular, figure 1 shows two photovoltaic concentration and conversion units 2.
With reference to figure 1, each photovoltaic concentration and conversion unit 2 comprises a curved primary reflector 3 for collecting and concentrating sunlight and a flat secondary reflector 4 for reflecting at least part of the sunlight reflected and partially concentrated by the primary reflector 3. The incident sunlight on the primary reflector 3 is indicated with numeral 5 and is oriented according to an incidence direction D. The sunlight reflected and partially concentrated by the primary reflector 3 is indicated with numeral 6 . In this document, partially concentrated sunlight means the sunlight not yet completely concentrated, i.e. before a focal area. The primary reflector 3 is suited to concentrate the sunlight towards a hypothetical focal area thereof, not shown and hereinafter called primary focal area, in which the concentration of the light would be maximum. The secondary reflector 4 is arranged between the primary reflector 3 and the primary focal area, i.e. in such a position to intercept the sunlight concentrated by the primary reflector 3 and prevent the formation of the primary focal area. Each photovoltaic concentration and conversion unit 2 has a plane of symmetry parallel to the incidence direction D. Figure 1 shows the photovoltaic system 1 according to a cross-section view along such plane of symmetry.
The secondary reflector 4 is a dichroic reflector capable of separating the concentrated sunlight 6 into two portions of sunlight spectrum such that a first portion of spectrum crosses the secondary reflector 4 and the second portion of light is reflected by the secondary reflector 4. Furthermore, each photovoltaic concentration and conversion unit 2 comprises a first photovoltaic converter 7, which consists of a photovoltaic cell which is sensitive to the first portion of spectrum and is arranged immediately behind the second reflector 3 to receive the first portion of spectrum, and a second photovoltaic converter 8, which consists of a photovoltaic cell which is sensitive to the second portion of spectrum and is arranged to receive the second portion of spectrum.
The primary reflector 3 consists of a parabolic mirror having a relatively short focal distance, i.e. such that the maximum angle that any one concentrated sunlight beam 6 forms with the respective incident light beam 5 is greater than or equal to 90°. The maximum angle is formed on the edge of the primary reflector 3 the farthest from the secondary reflector 4. The short focal distance of the primary reflector 3 allows the secondary reflector 4 to be positioned within the maximum volume of the primary reflector 3. Furthermore, the primary reflector 3 is an asymmetrical reflector, in the sense that it does not have a symmetrical shape with respect to any plane it is crossed by, except the aforesaid plane of symmetry, in such a way that the primary focal area is located outside the incident sunlight beam 5. This allows the secondary reflector 4 to be arranged outside the incident sunlight beam 5 so as to not generate shadows on the primary reflector 3. The primary reflector 3 has a curve which is defined by a portion of a rotation parabolic surface, such a portion of surface having, according to a view in the incidence direction D, a quadrangular perimeter, and in particular a rectangular perimeter.
With reference to figures 1 and 2, each photovoltaic concentration and conversion unit 2 comprises a respective aspheric lens 9, which is arranged between the primary reflector 3 and the secondary reflector 4, has a curved front surface 10 (figure 2) having an input portion 11 which defines a refracting interface for receiving the concentrated sunlight 6, and has a flat rear surface 12 which is optically coupled with the secondary reflector 4 to define a reflective interface, and a light guide 13, which has an input section 14 (figure 2 ) which is optically coupled with the front surface 10 of lens 9 and an output section 15 (figure 2 ) which is optically coupled with the photovoltaic converter 8 . The input portion 11 of lens 9 is shaped in such a way that the sunlight which has entered lens 9 is distributed on the rear surface 12 , and hence on the secondary reflector 4 , and after having been reflected by the secondary reflector 4 and having crossed lens 9 again from the rear surface 12 to the front surface 10 , is concentrated in a substantially circular-shaped focal area F (figure 2 ) substantially located in the input section 14 of the light guide 13. The input portion 11 is convex and hence lens 9 is substantially a flat-convex lens. Therefore, the secondary reflector 4 , the photovoltaic converters 7 and 8 , the aspheric lens 9 and the light guide 13 form a single solar receiving assembly.
With particular reference to figures 2 and 3 , which show the solar receiving assembly in greater detail according to a side view and a top front view, respectively, the input section 14 of the light guide 13 is optically coupled with a peripheral portion 18 of the front surface 10 of lens 9 . Lens 9 has a symmetrical structure with respect to the plane of symmetry (which is shown and indicated with S in figure 3 ) of the photovoltaic concentration and conversion unit 2 . The peripheral portion 18 extends along the entire width of lens 9 and the latter is cut laterally parallel to the plane of symmetry S . The peripheral portion 18 is peripheral in the sense that it is in a decentralized position in the front surface 10 , as it is apparent in figures 2 and 3 . Moreover, the peripheral portion 18 is separated by the input portion 11 so as to receive a negligible quantity of the concentrated sunlight 6 . Thereby, the body of the light guide 13 negligibly interferes with the concentrated sunlight 6 coming from the primary reflector 3 .
The light guide 13 consists of an elongated body made of transparent material. The main function of the light guide 13 is to sufficiently separate the photovoltaic converter 8 from the secondary reflector 4 to prevent the photovoltaic converter 8 from intercepting the sunlight coming from the primary reflector 3 . The sunlight propagates inside the light guide 13 by total internal reflection along the side walls thereof 19 , as in a common optical fibre. In use, the sunlight beams enter the input section 14 substantially concentrated in the focal area F, initially diverge from the latter and propagate along the light guide 13 thus undergoing, along the side walls 19 up to reaching the output section 15 , a variable number of total reflections, which are substantially uniformly distributed in the area of the output section 15 . Therefore, the light guide 13 serves the further function of homogenizing the light in the output section 15 thus allowing the efficiency of the photovoltaic converter 8 to be increased.
The secondary reflector 4 lies on a plane crossed askew by the incidence direction D of the sunlight. The light guide 13 extends along a longitudinal axis 20 thereof which lies on the plane of symmetry P and which crosses the output section 15 perpendicularly and the secondary reflector 4 askew. The incidence direction D is substantially to be parallel to axis 20 . In other words, the photovoltaic system 1 is perfectly aligned with the incident sunlight 5 when the latter is oriented parallel to axis 20 . Axis 20 may therefore be considered as the optical axis of the photovoltaic system 1 .
In use, the sunlight entering lens 9 from the input portion 11 undergoes a refraction which allows the effects of any alignment errors of the photovoltaic system 1 to be decreased. An alignment error exits when the incident sunlight 5 is not oriented parallel to axis 20 . Alignment errors determine a shift of the focal area F. As long as the focal area F remains inside the input section 14 , the sunlight beams remain inside the light guide 13 and hence the quantity of light which leaves the output section 15 remains the same. The presence of lens 9 decreases the shifts of the focal area F, the entity of alignment errors being the same, and hence allows increased alignment errors to be tolerated. Moreover, the flat surface of the primary reflector 4 contributes to improving the tolerance to the pointing errors of the primary reflector 3 with respect to a curved reflecting surface. Given a certain tilt of the secondary reflector 4 with respect to the average incidence direction of the sunlight on the secondary reflector 4 itself, and given a certain input section 14 of the light guide 13 , the shaping of the input portion 11 of lens 9 is determined by successive approximations with the purpose of maximizing the intensity of the sunlight in the output section 15 .
The light guide 13 has a circular or square-shaped cross section. Advantageously, the input section 14 of the light guide 13 has a larger area with respect to the one of the output section 15 . In other words, the light guide 13 has a truncated-conical or truncated-pyramid shape which widens towards the input section 14. This allows the tolerance to alignment errors to be increased. However, the length of the light guide 13 being the same, the input section 14 must not be too much wider than the output section 15 to keep the length of the optical paths of the light inside the light guide 13 , and hence the optical losses, within acceptable limits. Moreover, a very large output section 15 involves using a photovoltaic converter 8 which is just as large, and hence particularly costly. Thus, the main role of lens 9 in decreasing the effects of alignment errors is understood. The particular shaping of lens 9 and the particular reciprocal arrangement between lens 9 , optical guide 13 and secondary reflector 4 allows alignment errors up to 2 ° to be tolerated by the photovoltaic system for geometric concentration factors approximately between 600 and 700 . The geometric concentration factor is defined as the ratio between the area of the front section, perpendicular to the incidence direction D, of the primary reflector 3 , and the area of the output section 15 of the light guide 13 .
With reference to figure 2 , the rear surface 12 of lens 9 is optically and mechanically coupled with a front face 16 of the secondary reflector 4 . The photovoltaic converter 7 is optically and mechanically coupled with a rear face 17 of the secondary reflector 4 . Figure 2 shows, by way of example, certain beams of the concentrated sunlight 6 and the respective refracted beams in the interface of the input portion 11 , which are reflected by the secondary reflector 4 and conveyed by the light guide 13. All the beams shown lie on a single plane parallel to the incidence direction D (figure 1 ) . The secondary reflector 4 is made, for example, by means of a plate of material which is transparent to the sunlight, whose front face 16 is covered with a thin dichroic coating. Preferably, the front face 16 is completely encapsulated in lens 9 at precisely the rear surface 12 of the lens. The encapsulation of the front face 16 in lens 9 provides an excellent optical and mechanical coupling between the secondary reflector 4 and lens 9 and also significantly slows down the ageing of the dichroic coating as the latter is not exposed to weather conditions. The photovoltaic converter 7 comprises a substrate 7a and a layer which is sensitive to light 7b. The sensitive layer 7b is optically coupled with the rear face 17 of the secondary reflector 4 by means of a layer of silicon-based transparent material 25. Substrate 7a is mechanically coupled with the secondary reflector 4 by means of small rods 26, and, optically, to the rear face 17 of the secondary reflector 4 by means of an adhesive which is transparent to the sunlight.
The light guide 13 is rigidly coupled with the peripheral portion 18 of the front surface 10 of lens 9 at the input section 14. Advantageously, the guide light 13 and lens 9 are made of one single piece of material which is transparent to sunlight. This allows the optical losses to be eliminated in the passage from lens 9 to the light guide 13. Moreover, the photovoltaic converter 7 is rigidly coupled, by means of the rods 26 , to the secondary reflector 4 , which is mechanically coupled with lens 9 at the rear surface 12 , and the photovoltaic converter 8 is rigidly coupled with the light guide 13 at the output section 15 . Therefore, the solar receiving assembly, which consists of the group of components 4 , 7 , 8 , 9 and 13 , has a rigid and compact structure. The photovoltaic concentration and conversion units 2 are arranged adjacent to each other in such a way that the solar receiving assembly of each photovoltaic concentration and conversion unit 2 is completely arranged, with respect to the incidence direction D of the sunlight, behind the primary reflector 3 of an adjacent photovoltaic concentration and conversion unit 2 . Such an arrangement is made possible by means of the compact structure of the solar receiving assembly and by the use of asymmetrical primary reflectors 3 and with reduced focal distance.
Moreover, the photovoltaic converters 7 and 8 are rigidly connected to each other by means of a single support element 21 . The particular structure and the particular arrangement of the solar receiving assembly with respect to the primary reflector 3 result for the support element 21 to have the possibility of being shaped and positioned to not interfere with the concentrated sunlight 6 coming from the primary reflector 3. In the example shown in figures 1 and 2, the support element 21 is U-shaped and is positioned with the middle part 21a under the peripheral portion 18 of lens 9. Alternatively, the support element 21 may be positioned with the middle part 21a beside the peripheral portion 18.
Again with reference to figure 1, the photovoltaic system 1 comprises a support frame 22 designed to enable the dissipation of heat. For example, the support frame 22 may be equipped with cooling fins, known per se and hence not shown. The photovoltaic concentration and conversion units 2 are fitted adjacent to each other along a fitting side 22a of the support frame 22. When present, the cooling fins are arranged, for example, on side 22b of the support frame 22 opposite to the fitting side 22a. In greater detail, the primary reflector 3 for each photovoltaic concentration and conversion unit 2 is fitted, with its back, on the fitting side 22a by means of a respective support element 23, and the solar receiving assembly consisting of the components 4, 7, 8, 9 and 13 is also fitted on the fitting side 22a at a respective seat 24 by means of the support element 21. This particular and simple fitting is made possible due to the use of asymmetrical primary reflectors 3 and with short focal distance and due to the compact structure of the solar receiving assembly. Moreover, the heat accumulated by the photovoltaic converters 7 and 8 is dissipated through the common support element 21 and the support frame 22 . For such purpose, the support element 21 must be made in material having high heat conductivity, for example metal.
A first further non-illustrated embodiment of the present invention differs from the one shown in figures 1 to 3 in that the secondary reflector 4 consists of a simple flat mirror for totally reflecting the sunlight concentrated by the primary reflector 3 and in that the photovoltaic converter 7 is no longer present.
A second further embodiment of the present invention differs from the one shown in figures 1 to 3 in that the reflectors 3 and 4 , the aspheric lens 9 , the light guide 13 and the converters 7 and 8 extend for a same length along an axis, hereinafter called axis of extrusion, which is perpendicular to the incidence direction D, and respective curved lines or shapes along such an axis of extrusion are generated by translation. In other words : the primary reflector 3 and the inlet portion 11 of lens 9 have respective curvatures generated by translation of respective curved lines along such an axis of extrusion; the secondary reflector 4 extends parallel to the axis of extrusion; and the light guide 13 consists of a plate of transparent material which extends in the two directions defined by axis 20 and by the axis of extrusion. Accordingly, the focal area is substantially a linear shape which extends along the axis of extrusion and the input 14 and output 15 sections of the light guide 13 are rectangular-shaped with the larger sides parallel to the axis of extrusion. The generating curve of the curvature of the primary reflector 3 consists, for example, of a section of parabola. Therefore, figure 1 depicts the second further embodiment of the photovoltaic system according to a sectional view along any one plane parallel to the incidence direction D.
A third further embodiment of the present invention differs from the one shown in figures 1 to 3 in that lens 9 is a spherical lens.
A fourth further embodiment of the present invention shown in figure 4, in which only one photovoltaic concentration and conversion unit 2 is shown and in which the corresponding elements are indicated by the same numbers and acronyms used in figures 1 and 2, differs from the one shown in figures 1 to 3 in the following main aspects.
The primary reflector, which is indicated with numeral 30 in figure 4, consists of a parabolic mirror which is symmetrical with respect to the optical axis 31 thereof which is parallel to the incidence direction D for reflecting and concentrating the sunlight towards a primary focal area which is substantially centered on the optical axis 31 . The primary reflector 30 does not have a short focal distance, i.e. such that the maximum angle that any one of the concentrated sunlight beams 6 forms with the respective incident light beam 5 is less than 90 ° . Advantageously, such a maximum angle is between 30 ° and 60 ° . The secondary reflector 4 , which is arranged between the primary reflector 3 and the primary focal area and therefore generates a shadow on the primary reflector 3 , is perpendicular to the optical axis 31 . The lens, which is coupled with the secondary reflector 4 , indicated with numeral 32 , is a spherical lens having a curved front surface 33 and a flat rear surface 34 . The front surface 33 has the shape of a spherical cap with centre substantially located in the focus of the primary reflector 30 . The front surface 33 has a ring-shaped input portion 35 for the entering of the concentrated sunlight 6 coming from the primary reflector 30 , and a middle portion 36 which is optically coupled with the input section 14 of the light guide 13 . The light guide 13 is coaxial with the optical axis 30 and has the output section 15 arranged in proximity of the reflective surface of the primary reflector 30 at a middle hole 37 of the primary reflector 30 to facilitate the fitting, by means of a support element 38 , of the solar receiving assembly 4 , 7 , 8 , 13 and 32 on the fitting side 22a of the support frame 22. The primary- reflector 30 is fitted, with its back on the fitting side 22a by means of one or more support elements 23.
The study and experimental work which resulted in this invention benefited from financing of the Seventh Framework Programme of the European Community (7th PQ/2007-2013) within the scope of Grant Agreement no.
[213514] .