IT201800007965A1 - Solar concentrator - Google Patents

Description

Title: "Solar concentrator"

Description

The object of the present invention is a solar concentrator for illuminating an algal suspension which comprises:

- at least one optical element (1) adapted to receive solar radiation;

- at least one optical fiber (3), having an input face (11) and an output face (12);

- at least one spectrally selective optical filter (9), interposed along the path between said solar radiation and said input face (11) of said at least one optical fiber (3);

characterized in that said optical filter (9) transmits only the spectral portion which is most active for photosynthetic purposes, reflecting and / or absorbing the spectral portion which is scarcely useful for photosynthetic purposes.

State of the art

Algae crops need a certain amount of light radiation to promote the photosynthesis process and metabolize CO2 by releasing O2. Algal proliferation is favored by particular feeding and lighting regimes, proliferation that allows the exploitation for the extraction of chemical substances to be used, for example, for the synthesis of fuels and fertilizers.

The light radiation with which algal growth is promoted is commonly provided by the sun. The algal culture can be exposed directly to solar radiation or indirectly. In the latter case, the light radiation is concentrated through optical devices, mirrors or lenses, and possibly conveyed through optical fibers or light tubes to the culture medium.

The use of indirect lighting has several advantages, including the ability to provide an optimal level of light radiation intensity for algal growth, without having to filter or absorb part of the excess light radiation.

Indirect lighting methods have the flexibility of being able to be designed in such a way as to convey the luminous flux necessary to ensure the highest rate of algal proliferation towards the culture medium.

WO2015192159A1 discloses an optical element for concentrating the sunlight which is a parquet of Fresnel lenses, where coupling elements with optical fibers are placed on the focal plane of these lenses. These optical fibers carry the focused light from the Fresnel lenses towards the culture medium, typically placed in a remote container maintained under controlled temperature conditions.

Optical fibers exploit the principle of total internal reflection to trap light rays and guide them along the path of the fiber itself. Only the portion of light that hits the input surface of the optical fiber with an angle lower than a critical angle called the "acceptance angle" is refracted by the optical material of the fiber itself and undergoes the phenomenon of total internal reflection. The light radiation that enters the fiber with an angle greater than the acceptance angle is rapidly diffused or absorbed by the cladding of the fiber itself and is not guided along the optical path.

Only the light radiation within the acceptance angle is effectively guided by the optical fiber towards the culture medium and, due to the effect of the refraction phenomenon that occurs at the interface between the output surface of the fiber and the external medium, diverges in a beam known as a "cone of illumination", whose amplitude is greater than the angular acceptance. The amplitude of the illumination cone is typically less than 40 ° and typically about 35 °. The focusing of solar radiation within optical fibers allows an efficient transport of concentrated light radiation (at high intensity) for its subsequent diffusion inside the photobioreactor.

The optical fibers used to transport solar radiation to the growing medium constitute an important cost item for the entire system. The increase in the solar concentration factor allows to transport a greater radiant flux for each fiber and therefore constitutes a preferential solution to reduce the overall cost of the system. Typical concentration factors range from 100x to 1000x, but in some cases they can be as high as 2500x.

The main limit to the use of very high concentration optical systems is the optical and thermal stability of the optical fibers. The absorption of light radiation by the material itself or by any impurities present within the core of the fiber can cause localized heating which leads to a change in the optical and mechanical characteristics of the fiber itself. The material typically used in optical fibers is PMMA, whose softening temperature is about 84 ° C. It is clear that the higher the concentration factor, the higher the probability that the absorption of a portion of the spectrum of the incident radiation causes the fiber to overheat. As an alternative to optical fibers with a plastic core, it is possible to use optical fibers with a fused silica core. This material is characterized by excellent transparency but has an intrinsic cost significantly higher than that of PMMA and for this reason the fused silica optical fibers are not normally used for lighting systems such as the one object of the present invention.

The use of optical fibers with a plastic core therefore appears to be the only economically viable solution for the construction of concentrated solar systems for the cultivation of algae.

There is a strong need for a system capable of overcoming the limits imposed by the material used for optical fibers which prevent the exploitation of highly concentrated optical systems, thus imposing a constraint on the number of fibers and therefore on the overall cost of the system.

Detailed description of the invention

The object of the present invention is a solar concentrator which makes use of optical devices to concentrate the solar radiation on a plurality of optical fibers having the input surface substantially placed on the focal plane of said optical devices. These optical fibers are suitable for transporting concentrated solar radiation to a reactor inside which there is an algal culture in suspension.

Description of the figures:

FIGURE 1: Diagram of two types of optical fiber solar concentrator known to those skilled in the art (comparative purpose).

FIGURE 2: Diagram of an embodiment of the optical fiber solar concentrator according to the present invention.

FIGURE 3: Detail of the structure that supports the optical fiber supports on the focal plane of the lenses.

FIGURE 4: Detail of the optical fiber support and the filter.

FIGURE 5: Graph of the power spectral density of direct solar emission.

FIGURE 6: Graph of the photosynthetic activity of chlorophyll A.

FIGURE 7: Graph of the photon flux of the solar source as a function of length, compared with the flux of photons active for photosynthetic purposes.

FIGURE 8: Graph of the transmittance of a filter for the rejection of light radiation not very useful for photosynthetic purposes.

FIGURE 9: Diagram of a further embodiment according to the present invention, in which the interference filter is placed in contact with the front glass of the concentrator.

FIGURE 10: Front section of an embodiment of the solar concentrator object of the present invention in which the following have been highlighted: a foreground (100), of the arrangement of the lenses (1), a second plane (101), of the arrangement of the spectrally selective (9), a third plane (102) which is the focal plane.

FIGURE 11: Front section of an embodiment of the solar concentrator object of the present invention in which the following have been highlighted: a first plane (100), of the arrangement of the lenses (1), a second plane (101), of the arrangement of the spectrally selective (9), a third plane (102) which is the focal plane.

The solar concentrator described in this patent application differs from those known to the person skilled in the art for a series of construction details which give a surprising advantage, details better highlighted by the detailed description of the following images.

Figure 1, for comparison purposes, shows a typical concentrator. It is composed of optical focusing elements, lenses (1) that focus the incident solar radiation (7) in a series of focal regions (4) near which optical fibers (3) are positioned which have the function of transporting the concentrated solar radiation to the algal solution. The optical fibers (3) are kept in position by a perforated support (2), kept by mechanical means (5) at a distance such as to allow the formation of the focus of the lenses (1) on said optical fibers (3). The entire optical system, consisting of lenses, optical fibers, mechanical supports, spectrally selective filters and secondary homogenizers is enclosed within a closed container (15) which has the purpose of protecting said optical components from atmospheric agents. Said closed container is preferably made of metallic material. Said container is preferably in the shape of a parallelepiped and has six faces. The upper face (16) faces the solar source and is substantially transparent, for example it is a front glass (6), preferably said front glass is a sheet of tempered glass.

In a preferred form, said optical focusing elements (1) are Fresnel lenses made of silicone on glass, directly deposited on said front glass. it passes through a series of interfaces between air and optical medium before reaching the algal solution. These interfaces are: (i) Air - front glass (6); (ii) front glass (6) - air; (iii) air - lens (1); (iv) lens (1) - air; (v) air - support (2); (vi) support (2) - optical fiber (interface that affects little, where the refractive index of said support and the optical fiber is almost equal); (vii) optical fiber - air. At each interface a portion of the intensity estimated to be about 4% is reflected. Furthermore, the high power density incident on the input face of the fibers (3) is such as to cause yellowing and possibly fusion, precluding the possibility of using very high optical concentration factors.

Figure 2 schematically shows the solar concentrator object of the present invention.

Said solar concentrator (10) includes:

at least one optical element (1) adapted to receive solar radiation;

at least one optical fiber (3), having an input face (11) and an output face (12);

at least one spectrally selective optical filter (9), interposed along the path between said solar radiation and said input face (11) of said at least one optical fiber (3). In an embodiment, of which an example is schematized in Figure 10, said optical filter (9) is interposed along the path between said at least one optical element (1) and said input face (11) of said at least one optical fiber (3). In a further embodiment, of which an example is schematized in Figure 11, said optical filter (9) is interposed along the path between said solar radiation and said optical element (1).

Said spectrally selective optical filter (9) transmits only the spectral portion which is most active for photosynthetic purposes, reflecting and / or absorbing the spectral portion which is scarcely useful for photosynthetic purposes.

In a preferred form, said spectrally selective optical filter (9) transmits the spectral portion in the region between 400 and 700 nm. Even more preferably, in the region from 400 to 460 nm and / or from 610 to 700 nm.

Each optical fiber (3) is kept in position by a mechanical support (8), where said mechanical support (8) is integral and coaxial with respect to said optical fiber (3). As schematized in figure 10 and in figure 11, each optical element (1) defines an optical axis (99) and the optical fiber (3) which corresponds to said optical element (1) and the mechanical support (8) that supports said fiber optics (3) have the same axis of symmetry (99).

Said solar concentrator comprises a first plane (100), a second plane (101) and a third plane (102), focal plane, said planes being substantially parallel to each other.

A plurality of said substantially identical optical elements (1) are arranged on said first plane (100), perpendicular to the optical axis (99) of each of said optical elements (1); said spectrally selective optical filter (9) is arranged on said second plane (101); a plurality of said optical fibers (3) are arranged so as to have said inlet face (11) substantially positioned on said third plane (102).

In the embodiment schematized in Figure 10, where said second plane (101) is distant from said first plane (100) by a distance equal to the focal distance of said optical elements (1) and said planes are arranged as follows, from the outside towards the interior of said container (15): first floor (100), second floor (101), third floor (103).

In a further embodiment, schematized in figure 11, said second plane (101) is outside of said container (15), and said planes are in the following sequence, from the outside towards the inside of said container ( 15): second floor (101), first floor (100), third floor (102).

Said mechanical support (8) is inserted in a structure (2) with which it is in thermal contact. Said structure (2) keeps each mechanical support (8) in such a position as to be aligned and coaxial with the optical axis (99) of each optical element (1).

Advantageously, said mechanical support (8) is made of metal, so as to increase the thermal dissipation capacity of the optical fiber (3).

In the solar concentrator (10) according to the present invention, the incident solar radiation (7) is focused by said at least one optical element (1) in proximity to the inlet surface of said at least one optical fiber (3).

In a further embodiment, said optical element (1) is a lens and said spectrally selective optical filter (9) is a film placed in direct contact with the front glass (6) on which it can possibly be deposited with the SOG (Silicon On Glass) the lens (1). Said film is preferably an adhesive polymeric dichroic filter, glued to the inlet surface of the panel (front glass) and therefore is in direct contact with the flat surface of the SOG lens.

In this embodiment, since said optical filter (9) is placed on the inlet surface of the light radiation, it is hit by direct, non-concentrated light. This means that said optical filter (9), being hit by a light radiation with a reduced power density, can be advantageously made by sputtering or other known technique on a filmpolymer.

In a further embodiment, said optical elements (1) which are lenses are made of an optical material having a spectral transmittance curve which allows the transmission and focusing of the light radiation which is most useful for photosynthetic purposes and reflects or absorbs the spectral portion scarcely useful for photosynthetic purposes. By way of example, said material is selected among the plastics commonly used for injection molding such as Poly-methyl-methacrylate PMMA, polycarbonate PC, polystyrene PS, suitably added with selective spectral absorption dyes, so as to avoid the unwanted portion of the solar spectrum reaches the fibers.

Said structure (2), in the embodiment shown in figure 3, is advantageously provided with shaped holes (10), where said mechanical supports (8) are inserted. Said shaped holes (10) preferably have a shape that has a series of teeth (14), so as to increase the mechanical interference with the supports (8), where the insertion of said mechanical supports (8) leads to a deformation of said series of teeth (14) obtained in the structure of the holes themselves (10).

Said structure (2) is advantageously made by laser cutting a metal plate. In order to lighten the structure (2), said structure (2) is also provided with openings (11) of arbitrary shape, preferably not intersecting. These openings (11) leave conductive paths that allow the heat collected by the mechanical support (8) to be dissipated to the outside, so as to avoid overheating.

In one embodiment, said structure (2) is in the same metal with which said closed container (15) containing the solar concentrator is made so as to ensure that there are no mechanical deformation effects induced by the differential thermal expansion of the components.

Figure 4 shows a detail of the mechanical support (8) coaxial and integral with the optical fiber (3). Said mechanical support (8) comprises an upper face (81) and an upper face (82), where said upper face (81) is exposed towards said optical element (1) and said optical fiber (82) exits from said lower face (82) 3). In one embodiment, said upper face (81) is mirror polished and has a shaped opening (12). In this embodiment, said mechanical support (3) also performs the function of secondary concentrator or homogenizer, or said upper face (82) in this embodiment, thanks to the shaped opening and the fact that it is mirror polished, reflects inside the optical fiber (3) the light radiation possibly focused outside it. Advantageously, the section of the shaped opening (12) is tapered towards the inside of said mechanical support (8), in order to create a further optical concentration effect. In a preferred form, said shaped opening (12) has a truncated cone, truncated pyramid or compound parabolic concentrator (Winston cone). The mechanical support (8) can also be advantageously provided with a seat (13) in which said spectrally selective optical filter (9) is housed which transmits to the optical fibers (3) only the spectral portion most useful for the photosynthetic action and reflects or it absorbs the spectral portion scarcely useful for photosynthetic purposes.

An interferential optical filter uses a series of superimposed thin films, each made of a different material so as to have a different refractive index, such as to transmit some spectral bands and reflect others. Typically, the thickness of each layer is a few nm and the overall thickness of the filter rarely exceeds one micron. The individual layers are sequentially deposited on a rigid substrate, typically by evaporation under vacuum. For the purpose of the present invention, the spectrally selective optical filter (9) is made on a rigid support which is borosilicate glass, quartz or other transparent material which also has excellent resistance to high temperatures. Said filter is of the long pass type, that is, it transmits the longest wavelengths, or short passes, it transmits the shortest wavelengths or band pass, or it transmits only a defined band.

Alternatively, the optical filter (9) is made of a material capable of selectively absorbing the light radiation in the spectral region which is scarcely useful for photosynthetic purposes. Optical filters with these characteristics have proved to be particularly advantageous when applied in the embodiment of figures 2, 4, 9 and 10.

A further embodiment is schematized in figure 9. Said optical element (1) is a lens, made in direct contact with said front glass (6) which constitutes the upper surface (16) of the container (15). Using technologies known to those skilled in the art, it is possible to produce refractive optical components in silicone directly deposited on tempered glass. This reduces by two units the number of optical interfaces that the light radiation has to pass through before reaching the fibers (3). Advantageously, the spectrally selective filter (9) is coupled to the front glass (6) by means of an optically transparent double-sided adhesive film (17). The spectrally selective optical filter (9) is preferably covered with further transparent layers resistant to scratches or having a chemical composition such as to minimize the contamination phenomena of the surface of the filter itself.

The photosynthetic action does not exploit the entire solar spectrum but only some specific bands. Figure 5 shows the graph of the power spectral density of direct solar emission. The graph extends from about 300 nm to more than 1000 nm, the photosynthetically active region (PAR) is limited to the 400-700 nm band. Even within the PAR band, there are spectral regions that are scarcely useful for photosynthetic purposes and therefore reference is usually made to a curve that describes the photosynthetic efficacy as a function of the wavelength (action spectrum).

Figure 6 shows the graph of the photosynthetic activity (action spectrum) of chlorophyll A, often present in microalgae. As can be seen from the graph, there are two distinct absorption bands, one in the blue region (400-460 nm) and the other in the red region (640-700 nm).

Figure 7 shows the graph of the photon flux of the solar source as a function of the wavelength (solid line), compared with the flux of active photons for photosynthetic purposes in the case of type A chlorophyll (dashed line). From this graph it is evident that the portion of photons from 460 nm to 640 nm, which constitute the majority of the photons of natural solar radiation, are scarcely used for photosynthetic purposes. This portion of photons between 460 nm to 640 nm contributes significantly to unwanted heating of the optical fibers. The solution according to the present invention, by removing this spectral component, allows a significant reduction in heating with a completely negligible impact on the photosynthetic action obtained, with the same light radiation incident on the system.

Figure 8 shows the transmittance spectrum of a band-pass interference filter for the rejection of light radiation which is scarcely useful for photosynthetic purposes, in the case of type A Chlorophyll, applicable to the concentrator according to the present invention. The spectral band that is rejected goes from 460 to 620 nm while the complementary radiation is transmitted towards the algal suspension.

A further object of the present invention is a system for the cultivation of algae which comprises a solar concentrator according to the present invention. In a preferred embodiment, said system comprises an algal suspension in which light tubes are immersed, illuminated by the optical fibers (3) of the solar concentrator according to the present invention.

One of the advantages related to the invention object of this patent application is to significantly increase the flow of photons useful for photosynthetic action compared to what is known to the skilled in the art. The effect of this increase is the possibility of reducing the number of optical fibers required for each concentrated solar panel and thus reducing the overall cost of the system without affecting the photosynthetic activity

The solution according to the present invention also contributes to improving the optical efficiency which, for a concentrating solar system such as the one described, is a dimensionless parameter that depends on the ratio between the output power of the fibers and that incident on the exposed surface of the system itself. . The higher the optical efficiency of each concentrated solar panel, the lower the number of panels required for each plant, due to the increased radiant intensity transported to the algal culture. The optical efficiency is influenced by several factors, including the transmittance of the optical fibers, the transmittance of the material of which the optical element (1) and the front glass (6) are made, and the number of interfaces that the light radiation it must cross before reaching the exit surface of the fibers.

Surprisingly, the solution described here shows an increased optical efficiency compared to that obtained with the systems of the state of the art, where the solution of the present invention has allowed to intervene on the number of interfaces. The state of the art solutions described the use of anti-reflective treatments, so as to reduce the impact at the interface from 4% to about 0.5%. This solution, widely practiced in photographic and ophthalmic lenses, however, has an extremely high cost and is not feasible on large surfaces. The solution proposed here, in the embodiment that couples the components, i.e. where the lens is deposited directly on the glass, the interfaces between optical material and air are limited to two, with a significant improvement in efficiency, where the interface between the material of the lens and that of the glass leads to a negligible decrease in efficiency, since the two materials lens and glass have a very similar refractive index.

Surprisingly, a reduction of at least 8% in the reflection losses was observed by applying the solution object of the present invention compared to the solutions known to the skilled in the art and described, for example, in the patent WO2015192159A1.

Indeed, the same authors of the present invention have shown that the optimum light intensity level for algae proliferation is typically between 10 µmol / m <2> s and 250 µmol / m <2> s, and preferably between 50 µmol / m <2> s and 150 µmol / m <2> s. The direct light radiation typically has an intensity of 2500 µmol / m <2> s, therefore much higher than the optimal light intensity. The solution according to the present invention advantageously allows to operate with a light intensity that falls within the optimal range for algal growth.

Claims (10)

CLAIMS 1. A solar concentrator (10) to illuminate an algal suspension which includes: - at least one optical element (1) adapted to receive solar radiation; - at least one optical fiber (3), having an input face (11) and an output face (12); - at least one spectrally selective optical filter (9), interposed along the path between said solar radiation and said input face (11) of said at least one optical fiber (3); characterized in that said optical filter (9) transmits only the spectral portion which is most active for photosynthetic purposes, reflecting and / or absorbing the spectral portion which is scarcely useful for photosynthetic purposes. 2. Solar concentrator according to claim 1, wherein said spectrally selective optical filter (9) transmits the spectral portion in the region between 400 and 700 nm, preferably, in the region between 400 and 460 nm and / or between 610 and 700 nm. 3. Solar concentrator according to claim 1, where said at least one optical fiber (3) is held in position by a mechanical support (8), where said mechanical support (8) is integral and coaxial with respect to said optical fiber (3), said mechanical support (8) being inserted in a structure (2) with which it is in thermal contact. 4. Solar concentrator according to claim 3, where said at least one optical element (1) defines an optical axis (99) and said optical fiber (3) which corresponds to said optical element (1) and said mechanical support (8) which supports said optical fiber (3) have the same symmetry axis (99). 5. Solar concentrator according to one of the preceding claims which comprises a first plane (100), a second plane (101) and a third plane (102), focal plane, substantially parallel to each other, where: - a plurality of said substantially identical optical elements (1) are arranged on said first plane (100), perpendicular to the optical axis (99) of each of said optical elements (1); - said spectrally selective optical filter (9) is arranged on said second plane (101); -a plurality of said optical fibers (3) are arranged so as to have said inlet face (11) substantially positioned on said third plane (102). 6. Solar concentrator according to claim 5 wherein said second plane (101) is interposed between said first plane (100) and said third plane (102), focal plane. 7. Solar concentrator according to claim 5 wherein said first plane (100) is interposed between said second plane (101) and said third plane (102), focal plane. 8. Solar concentrator according to one of the preceding claims, in which said optical elements (1) are Fresnel lenses made of silicone on glass directly deposited on said front glass (6). An algae growing system comprising a solar concentrator as described in any one of claims 1 to 8. 10. System for the cultivation of algae according to claim 9 in which the optical fibers (3) are conveyed to one or more light diffusing tubes immersed within an algal suspension.