EP3899378A1

EP3899378A1 - Solar concentrator

Info

Publication number: EP3899378A1
Application number: EP19820661.7A
Authority: EP
Inventors: Paolo DECARLI; Donato Vincenzi; Michele TONEZZER
Original assignee: Trentino Rainbow Energy Srl
Current assignee: Trentino Rainbow Energy Srl
Priority date: 2018-12-20
Filing date: 2019-12-19
Publication date: 2021-10-27
Also published as: IT201800020437A1; WO2020128955A1

Abstract

Described is a solar concentrator for photovoltaic systems, which is equipped with a front input surface (7), substantially flat and transparent to the solar radiation (100), a concave mirror (1), positioned behind the front input surface (7) for receiving, reflecting and concentrating the solar radiation (100), and a refractive optical element (2) having in turn a refractive surface (23) exposed to the solar radiation reflected by the concave mirror (1), a reflective surface (21) exposed to the solar radiation refracted from the refractive surface (23), and a light guide (22) for guiding the concentrated solar radiation (101), reflected by the reflective surface (21) towards at least one photovoltaic receiver (5); the refractive optical element (2) is made in a single integrated block and comprises mechanical alignment elements (8) between the refractive optical element (2) and the concave mirror (1) to ensure the superposing and the parallelism between the optical axes of the two components.

Description

DESORPTION

SOLAR CONCENTRATOR

Technical field

This invention relates to a solar concentrator and in particular to a solar concentrator with a spectral separation for photovoltaic systems.

The photovoltaic systems are basically formed by an optical unit for the concentration of the rays of sunlight, the so-called solar concentrator, a photovoltaic converter and a solar tracking unit.

Background art

The solar concentrator focusses the solar radiation on the photovoltaic converter. The latter generally comprises one or more photovoltaic cells connected in series to each other.

To optimise their thermal dissipation, the photovoltaic cells are positioned in contact with a rear surface of the converter, which, for this purpose, is made of a material designed to transmit and dissipate the excess heat, for example metal or glass with high emission properties

The simplest architecture of a photovoltaic system, widely adopted in commercial solar panels, comprises a matrix of cells positioned close to the focal distance of as many lenses and having the centre basically aligned with the optical axis of the corresponding lens.

Normally, in order to limit the weight and the use of optical material, refractive type Fresnel lenses are used in this type of photovoltaic system. The lenses are usually made of plastic material (polycarbonate or methacrylate) or, recently, also of silicone material deposited directly on glass

The main drawback of the systems based on Fresnel lenses is linked to the fact that the light radiation passes through the material from which the lens is made of, accelerating the aging and causing an unwanted yellowing of the lens. This yellowing is responsible for a partial and undesired absorption of the luminous radiation which produces a loss of conversion efficiency of the entire system. In order to overcome this problem, solar concentrators have been developed based on optical reflecting components in which the phenomenon of yellowing of the materials is reduced or absent.

More specifically, a prior art architecture of solar concentrator is known, the so-called double reflection type, which comprises a first reflective optical element, the so-called main collector, and a further reflective optical element, the so-called secondary reflector.

The secondary reflector is generally defined by a convex mirror, which is kept in a fixed position relative to the primary collector, for example fixed to the front glass of the solar concentrator which is exposed to the light radiation.

The typical structure of the double reflection solar concentrator is derived from the one used in the telescopes known as “Cassegrain”. Patents US2403860, US3158676, and FR2923302 describe telescopes which use this configuration.

The secondary reflectors are responsible for a shadow cone which is projected on the primary collector, which also constitutes the so-called input pupil of the optical system. For this reason, these optical systems are referred to as obscured input pupils.

Due to the shadow cone projected on the collector, the caustic curve generated in the solar concentrators having obscured input pupils is typically characterised by a region with reduced irradiance.

In order to maximise the conversion efficiency of the cells used for the photovoltaic conversion, the concentration photovoltaic systems with obscured input pupils are generally equipped with light focussing systems. These components are designed to render uniform the irradiance profile on the plane of the photovoltaic cells and eliminate any regions with a reduced or too high an irradiance.

The most widespread solution to render uniform the irradiation profile on the plane of the photovoltaic cells consists in the use of a truncated pyramid made of transparent material, generally glass, having the function of light guide.

!n the concentration photovoltaic systems which operate with focussing systems such as that just described, the focal plane generally corresponds to the input face of the focussing system. This expedient means that the multiple reflections which occur inside the guide due to the“ total internal reflection” cause a focussing of the irradiance profile on the output face of the guide. In order to convert all the radiation transmitted by the light guide and to have a uniform irradiance profile, the photovoltaic device is positioned at the output face of the focussing system. The optical coupling between the two components is generally obtained thanks to a transparent sticker.

The focussing systems with total internal reflection are generally solid glass components since this has greater resistance to ageing induced by the UV radiation and is more stable at high temperature.

A further prior art embodiment of the focussing system consists of a truncated pyramid made using flat reflective sheets, folded so as to make a cavity inside of which the light is reflected several times and guided towards the photovoltaic cells positioned close to the output door of the component.

In some cases, the light guide is made in a conical form and is obtained by cold drawing a sheet of reflective material. Irrespective of the shape it adopts, the function it performs is identical.

All the optical components which constitute a double reflection solar concentrator and equipped with a focussing system require a quite precise relative alignment in order to maximise the conversion efficiency of the system. For this purpose, different solutions have been developed for the positioning of the optical components which range from mechanical references to be applied to the front glass of the concentrator to actual mechanical alignment components.

In the concentration solar systems, the aspects of the simplicity of assembly and the tolerance of the system to any small errors in the positioning of the optical components constitute very important factors since they directly influence the final cost of the system.

In the past, micro-concentrators with double reflection have been proposed which use entirely the phenomenon of the total internal reflection and which are free of focussing system.

The simplified structure of these micro-concentrators is less affected by the problem of the alignment between the various optical components of the system but the impossibility of incorporating a light guide into the structure of these micro-concentrators having the functions of focussing system limits the angular acceptance of the optical system and worsens the performance of the cell. Moreover, this structure is not suitable for making large surfaces since the entire volume of the concentrator is completely filled with dielectric material typically with a density of between 1 and 3 g/cm3.

In this context, the main aim of the invention is to overcome the above- mentioned drawbacks.

Aim of the Invention

The aim of the invention is to provide a solar concentrator for photovoltaic systems which has a high efficiency and such that the individual components can be assembled together with a high level of precision and simplicity.

Brief description of the drawings

Further features and advantages of this invention are more apparent in the detailed description below, with reference to a preferred, non-restricting, embodiment of a solar concentrator for photovoltaic systems as illustrated in the accompanying drawings, in which:

Figure 1 is a schematic perspective view of a solar concentrator according to the invention;

Figure 2 is a schematic side view of the solar concentrator of Figure 1 ;

Figure 3 is a schematic side view of the solar concentrator of the preceding drawings showing the optical paths of the rays of sunlight;

Figure 4 is a perspective view of an optical component of the solar concentrator of the preceding drawings; and

Figure 5 is a schematic side view of a variant embodiment of the solar concentrator of the preceding figures.

With reference to Figure 1 , the numeral 10 denotes in its entirety a solar concentrator for photovoltaic systems according to the invention.

The solar concentrator 10 comprises a front input surface 7, shown in Figure 2.

The surface 7 is substantially flat and is exposed to the solar radiation 100.

Advantageously, the input surface 7 is made of transparent material, preferably glass, and allows protection from the weather conditions and the accumulation of dust for all the optical components the concentrator 10 is made of.

The solar concentrator 10 comprises a primary concave mirror 1 , in particular parabolic and also called reflector, and a refractive optical element 2, both positioned behind the input surface 7 or below the same observing, for example, Figure 2.

The refractive optical element 2 is equipped with a refractive surface 23, a substantially reflective surface 21 and a truncated pyramid-shaped support 22 with a polygonal base having the function of a light guide.

Overall, the refractive optical element 2 is made of substantially transparent material and has a refraction index of between 1 and 2.

The solar radiation 100 reflected by the primary concave mirror 1 is sent towards the refractive optical component 2 and, at the surface 23, undergoes a refraction.

The reflective surface 21 is preferably covered by a metallic layer with a high reflectivity and is shaped in such a way as to advantageously use the phenomenon of total internal reflection. In particular, as better illustrated in Figure 4, the refractive surface 23 is defined by a ball and the reflective surface 21 is defined by an upturned hyperbolic dome.

The concave primary mirror 1 is configured for concentrating the solar radiation towards a focal point or region situated between the substantially reflective surface 21 and the input surface 7.

The rays refracted from the surface 23 are reflected from the substantially reflective surface 21 towards the light guide 22. The concentrated beam of light rays, labelled 101 in Figure 3, does not undergo further refraction and is not therefore subject to losses due to reflection.

Inside the light guide 22 the concentrated beam 101 undergoes a total internal reflection and is guided towards a photovoltaic cell 5, or photovoltaic receiver 5, without significant loss of intensity.

In particular, the reflective surface 21 reflects the light radiation towards the light guide 22 which homogenises the beam and focusses it towards the photovoltaic cell 5.

The photovoltaic cell 5 is in contact with the light guide 22 by means of an optical sticker or other means designed to promote the adhesion of the two objects and the transmission of light.

The photovoltaic cell 5 is, in use, subjected to luminous flows in the order of several tens of W/cm2 and this results in the need to dissipate part of this energy flow in the form of heat. For this reason, the photovoltaic cell 5 is placed in thermal contact with the surface of a rear support 6, which acts as a dissipater and which may consist of a sheet of material with high emission or a finned heat dissipater.

The concentrator 10 comprises mechanical means 8 for optical alignment between the concave primary mirror 1 and the refractive optical element 2 in such a way as to guarantee an effective alignment of the optical axis of the two components. This solution allows the optical efficiency of the system to be increased and at the same time simplifies the assembly operations. In other words, the solar concentrator 10 comprises, in an example embodiment:

a front input surface 7, which is substantially flat and is transparent to solar radiation 100;

- a concave mirror 1 , inlet positioned behind the front input surface

7 for receiving, reflecting and concentrating the solar radiation 100; and a refractive optical element 2 in turn comprising:

a refractive surface 23 exposed to the solar radiation reflected from the concave mirror 1 ;

- a reflective surface 21 exposed to the solar radiation reflected from the refractive surface 23; and

a light guide 22 having the function of guiding the concentrated solar radiation 101 , reflected from the reflective surface 21 , towards at least one photovoltaic receiver 5.

Advantageously, in this structure the refractive optical element 2 is made in a single integrated block, that is to say, in the form of a single-block, and it comprises mechanical alignment means 8 between the refractive optical element 2 and the concave mirror 1 to ensure the superposing and the parallelism between the optical axes of the two components.

Preferably, the mechanical alignment means 8 between the refractive optical element 2 and the concave mirror 1 also guarantee the correct positioning of the latter on the surface of the rear support 6.

Preferably, the refractive optical element 2 is made by injection moulding of plastic material.

Preferably, the primary mirror 1 is made by injection moulding of plastic material.

Preferably, both the refractive optical element 2 and the primary mirror 1 are made by injection moulding of plastic material.

Preferably, the above-mentioned mechanical alignment means 8 are made in an integrated form in the moulding of the refractive optical element 2 and/or of the primary mirror 1. In one embodiment, the refractive optical element 2, the primary mirror 1 and the mechanical alignment means 8 are made in an integrated form, that is to say, in the form of a single-block, by injection moulding of plastic material.

In all the cases, the primary mirror 1 and the refractive optical element 2 are preferably fixed as one with the surface of the rear support 6.

The rear support 6 acts preferably as a support and heat dissipater for the photovoltaic receiver 5 and may be equipped with mechanical references to guarantee the correct relative positioning of the primary mirror 1 , refractive optical element 2 and photovoltaic receiver 5.

During operation, the primary parabolic mirror acts as a solar collector, whilst the integrated refractive optical element acts as a secondary reflector, as a light guide and focussing element, reflecting the radiation focussed by the primary mirror towards the photovoltaic receiver and keeping it confined inside it thanks to the total internal reflection phenomenon. The integrated refractive optical element, which is advantageously self-supporting and self-aligning and is placed in direct contact with the photovoltaic receiver, illuminates the latter with uniform density.

In this context, the integrated refractive optical element advantageously has a triple function: constituting the secondary reflector, the light guide and the centring element between the primary mirror and the photovoltaic receiver. One of the main problems of the double reflection optical systems consists in the alignment between the primary focussing optical system, the secondary optical system and the photovoltaic receiver. The use of a single integrated optical element which makes it possible to perform these three functions, that is to say, a secondary reflector, light guide and centring element, thus constitutes an element with obvious technological advantages.

The solar concentrator described above therefore guarantees a high level of alignment precision, as well as a high level of assembly simplicity, of the various components of the system, in particular of the primary mirror, the refractive optica! element in its entirety and the photovoltaic receiver.

Figure 5 shows a further embodiment of the solar concentrator according to the invention.

!n this embodiment, the final portion of the light guide 22 is finished at 45° and is covered by a spectral selectivity film which allows transmission of a portion of the spectrum of the light radiation to a first photovoltaic receiver 5 and the portion complementary to it to a second photovoltaic receiver 55. The choice of the cut-off wavelength of the spectra! selectivity film makes it possible to optimise the superposing between the spectra! density of power incident on each cell and the spectral response of the cell.

In other words, the light guide 22 is equipped in its final part with a system for spectra! separation of the solar radiation based on dichroic film. This spectral separation system is positioned and shaped for transmitting a first spectral portion of the concentrated solar radiation 101 to a first photovoltaic receiver 5 and for reflecting a second spectra! portion, complementary to the first spectra! portion, towards a second photovoltaic receiver 55.

This expedient makes it possible to maximise the conversion efficiency and to render this photovoltaic system more tolerant to the phenomenon of daily and seasonal variation of the incident radiation spectrum ( spectral mismatch).

Claims

1. A solar concentrator for photovoltaic systems comprising:

a front input surface (7), which is substantially flat and is transparent to solar radiation (100);

- a concave mirror (1 ), inlet positioned behind the front input surface

(7) for receiving, reflecting and concentrating the solar radiation (100); and a refractive optical element (2) in turn comprising:

a refractive surface (23) exposed to the solar radiation reflected from the concave mirror (1 );

- a reflective surface (21 ) exposed to the solar radiation reflected from the refractive surface (23); and

a light guide (22) having the function of guiding the concentrated solar radiation (101 ), reflected from the reflective surface (21 ), towards at least one photovoltaic receiver (5);

characterised in that the refractive optical element (2) is made in a single integrated block and that it comprises mechanical alignment means (8) between the refractive optical element (2) and the concave mirror (1 ) to ensure the superposing and the parallelism between the optical axes of the two components.

2. The solar concentrator according to claim 1 , characterised in that the refractive optical element (2) is made by injection moulding of plastic material.

3. The solar concentrator according to claim 1 or 2, characterised in that the primary mirror (1 ) is made by injection moulding of plastic material.

4. The solar concentrator according to claim 2 or 3, characterised in that the mechanical alignment means (8) are made in an integrated manner in the moulding of the optical refractive element (2) and/or of the primary mirror (1 ).

5. The solar concentrator according to any one of claims 1 to 4, characterised in that the primary mirror (1 ) and refractive optical element (2) are fixed as one to a surface of a rear support (6), which acts as a support and heat dissipater for the photovoltaic receiver (5) and is provided with mechanical references to ensure the correct relative positioning of the primary mirror (1 ), refractive optical element (2) and photovoltaic receiver (5).

6. The solar concentrator according to any one of claims 1 to 5, characterised in that the light guide (22) is equipped in its final part with a system for spectral separation of the solar radiation based on dichroic film; the spectral separation system being positioned and shaped for transmitting a first spectra! portion of the concentrated solar radiation (101 ) to a first photovoltaic receiver (5) and to reflect a second spectral portion, complementary to the first spectral portion, towards a second photovoltaic receiver (55).

7. A photovoltaic system, incorporating inside at least one solar concentrator according to any one of claims 1 to 6.