The present invention is comprised in the technical field of renewable energies, more specifically in the field relating to both solar thermal and thermoelectric energy as well as photovoltaic solar energy.
The most widely available thermal solar panels on the market today are two-dimensional planar structures in which solar radiation is concentrated in the fluid carrying pipes by means of metal fins covered with radiation absorbing paint. Heat dissipation is prevented by means of insulation with rock wool or similar elements, but there are still convection losses that cannot be prevented in this concept. The entire system is comprised within an aluminum frame, and the front surface is sheet glass. The entire assembly is heavy, weighing over 30 kg for a 2 m2 panel.
These panels, called planar collectors, are relatively inexpensive and highly efficient for warm climates and moderate increases in heat-carrying fluid temperature, up to 50° C., which limits their application both to regions with said climate and to low fluid heating ranges. If the panel is to be placed in cooler areas or if the fluid is to be heated to higher temperatures (over 100° C. and up to 150° C.), two other concepts are needed. On one hand, the so-called vacuum tube collectors are needed. In said collectors, the pipe to be heated is introduced in a glass tube where the vacuum is formed, minimizing heat losses due to convection. On the other hand, the so-called compound parabolic collectors, or CPC, are needed, and they concentrate light on the pipes by means of pseudoparabolic mirrors. In addition to being heavy, both concepts have the major drawback of their price, because they require complicated technology and/or materials increasing the price to two or even three times that of the planar collector.
Therefore, it is suitable to develop a product in this field which is highly efficient in different regions and temperature ranges, from 50 to 150° C., while at the same time is much lighter and has a price that is comparable to or less than the price for a planar collector.
As regards photovoltaic solar modules, the most widely available photovoltaic solar modules on the market are planar modules with a glass front, an aluminum frame and virtually the entire surface covered with photovoltaic solar cells. This structure is also heavy, weighing about 20 kg for a conventional 250 W module. Given that solar cells represent by far the most significant part of the cost, there has been a decades-long effort to reduce their surface by replacing them with concentrator elements which are theoretically less expensive and can direct all the light received on them. However, photovoltaic solar concentration systems of many different kinds have failed to successfully penetrate the market up until now. The main reasons are the price as well as the highly complicated final structure of the complete system which requires solar tracking. Furthermore, the concentrations achieved, greater than 20 times the sun, or 20×, and up to 1,000× in high concentration systems usually add another problem: the solar cell heats up excessively, and an active or passive cooling system must be considered. This adds complexity and cost to these systems.
Holography as an optical technology has many advantages with respect to other optical concentrator systems (lenses or mirrors, for example): it is much more versatile and less expensive than optical concentrator systems. It also eliminates the need for solar tracking when used at a low concentration, whereby reducing system complexity.
There have been earlier attempts to use holography in solar panels. U.S. Pat. No. 4,863,224, granted to Afian et al., for example, uses a hologram and a prism or plate. However, this solar concentrator must be aligned with the sun and it does not have any passive tracking capacity. Another invention which also has this drawback is U.S. Pat. No. 5,268,985 granted to Ando et al. Said invention comprises a hologram and a total reflection surface, but in addition to requiring tracking, it is constructed for capturing monochromatic light and wastes most of the solar spectrum. U.S. Pat. No. 5,877,874 and U.S. Pat. No. 6,274,860, granted to Rosenberg, discloses a holographic planar concentrator in which at least one multiplexed holographic film, achieving high spectral and angular bandwidths, concentrates the light on solar cells placed in the same plane. This invention has the drawback of having excessive spectral losses and the need for using bifacial cells, as well as the need for placing the entire photovoltaic solar system in a planar location with the ground painted white to reflect the albedo. Patent US20080257400, granted to Mignon and Han, also discloses a holographic planar concentrator but with two different surfaces, in which there are multiplexed transmission and reflection holograms, with the solar cells perpendicular to said collector surfaces. In addition to the losses due to various reflections and transmissions in the various holograms, the main drawback of this design is the difficulty in building it, which can prevent manufacturing it at competitive costs. Finally, patent US20120125403, granted to Orlandi, proposes applying holographic films directly on conventional photovoltaic modules, such that any radiation striking from different angles is used as radiation perpendicular to the plane of the module. Although this concept is highly marketable due to scarce interference in the original design, it does not reduce the weight or the manufacturing cost of current modules.
None of the aforementioned inventions aims to reduce panel weight, an important factor for both the cost and mounting difficulty (which also has a bearing on the cost of solar energy as an overall concept). The present invention uses plastic materials that are widely available on the market for constructing the panels. Furthermore, it combines not only one or two, but up to three optical elements for concentration purposes, which significantly increases solar spectrum collection, and it does all this at an industrial production cost which is even less than current conventional panels.
The study of the state of the art shows that the main problem involved in implementing the holography in both thermal and photovoltaic solar applications is collecting as much of the solar spectrum as possible. This refers both to the variation in the angles of incidence throughout the different seasons of the year and the wide range of energetically significant wavelengths which must be collected.
In terms of wavelengths, in order to collect a significant part of the solar spectrum, the hologram must be capable of collecting at least the region between 500 nanometers (nm) and 1,100 nm. This portion contains 70% of all the energy of the solar spectrum. Yet even more ideally, the hologram must be capable of collecting between 400 nm and 1,200 nm, i.e., 80% of the total spectrum. However, current holograms, particularly reflection holograms, are capable of collecting for each diffraction grating a maximum of 300 nm, and this is by means of special processes. Therefore, at least two superposed, i.e., multiplexed, diffraction gratings will be necessary for capturing the required minimum of 70%.
However, those wavelengths must be collected throughout the year, from morning to night. Generally, the annual variation of the angles of incidence of sunlight is kept at about 60° in a wide range of terrestrial latitudes. As seen in
On the other hand, in a planar configuration such as that of
It is obvious that a planar solar panel configuration, particularly a planar capture by the hologram, as presented in most of the solutions mentioned in the state of the art, is insufficient and will always lead to limited performances.
For this reason, the present invention proposes as a solution a three-dimensional structure repeated several times, the 3D unitary structure of which can be observed in a front section view in
A system in which the radiation receivers (6) or (8) can be substantially reduced is thus obtained. In other words, the distance between pipes in a thermal solar panel and the distance between branches of solar cells in a photovoltaic solar module can be greater. It must be pointed out that the 3D unitary structure is asymmetrical because the angles of incidence of solar radiation (2) and (3) are different in winter and summer if the panel is tilted at latitude.
As seen in
Due to the inability to capture the entire angular variation, the present invention incorporates not only reflection holograms (9) as a concentrating optical element (see
The 3D unitary structure of the panel is defined as follows (see
Therefore, the three optical elements are combined and work in the following manner to capture the entire 150° of variation in the angles of incidence:
Therefore, it is assured that in the medium (11) all the radiation returned either as a result of being diffracted from the hologram (9) or reflected from the reflective surface (10) does not leave the medium, since it strikes its inner surface with an angle greater than the critical angle. The radiation is therefore returned through total internal reflection (TIR) to within the medium (11), where either the hologram (9) or the reflective surface (10) will work again successively until reaching the radiation receiver (6) (pipes for a thermal solar panel) or (8) (photovoltaic solar cells for a photovoltaic solar module). The TIR has 100% efficiency, so there are no losses in it. As regards the hologram (9) or the highly reflective surface (10), efficiencies exceed 95% and even 98%, whereby minimizing losses in each diffraction or reflection. Furthermore, the 3D unitary structure is designed so that the maximum number of diffractions and/or reflections until reaching the radiation receiver (6) or (8) is not more than three, so losses are even lower.
To better explain these effects,
In
In
In
In this manner, the mentioned 3D unitary structure thus captures radiation during every season of the year and very efficiently directs it to the radiation receiver (6) or (8). A thermal solar panel or a photovoltaic solar module having a power that is equivalent to those available on the market today (see
Both the base (12) made of an environmentally resistant polymeric material resistant and the medium (11) made of an environmentally resistant optical polymeric material (silicone or polyurethane, for example) can be extruded by means of plastic molding. They assure rigidity, thereby making a frame unnecessary, as well as a significant weight reduction. On the other hand, since the base (12) is made by extrusion from a mold, it can include in the same extrusion all the anchoring elements necessary for fixing the panels to the mounting structures of any photovoltaic solar system. It can also include, for example, in the case of a thermal solar panel, the openings or cavities necessary for housing at the ends of the panel the collector pipes (13) having a larger diameter (see
It must be mentioned that there is a fundamental difference between a thermal solar panel and a photovoltaic solar module affecting the present design: in a thermal solar panel, it is of interest to retain heat inside the structure to minimize losses and assure heating of the heat-carrying fluid (referring to losses due to conduction, since losses due to convection are insignificant as the pipes are completely imbued in a solid medium). In a photovoltaic solar module, however, as much heat as possible should be dissipated since the efficiency of the solar cells decreases with the temperature thereof.
In the present design, this difference is resolved by choosing different plastic materials both for the plastic base (12) and for the medium (11), which are in any case environmentally resistant. Specifically, for a thermal solar panel, plastic materials with very low thermal conductivity K, for example around 0.02-0.03 W·m−1·K−1, are of interest. For a photovoltaic solar module, the reverse is applicable. Therefore, for photovoltaic solar modules, the plastic materials making up both the plastic base (12) and the medium (11) must have thermal conductivity greater than 0.05 W·m−1·K−1, for example, and even greater than 0.07 W·m−1·K−1.
In a preferred but non-exclusive configuration, both the thermal solar panel and the photovoltaic solar panel will consist of eight 3D unitary structures as described in
Since the plastic base (12) can be made in a mold, it can include all the necessary elements, including anchors for the mounting system or openings for versatile connection of the photovoltaic solar cells, both in series and in parallel. Likewise, for the case of a thermal solar panel, said plastic base (12) can be made with the necessary extensions for resistant to the elements taking in the collector pipes (13) (see
In the case of a thermal solar panel, the radiation receivers are pipes (6). In the described embodiment, they can be copper pipes having an outer diameter of 8 mm. The collector pipes (13) have a larger diameter, for example, 18 mm. Since there is a total number of eight pipes (6), the fluid heating capacity achieved is similar to that of a conventional planar collector. However, the efficiency thereof will be improved for heating fluids at high temperatures because sealing with the medium (11) minimizes losses due to convection. Furthermore, construction with materials having low thermal conductivity also significantly reduces losses due to conduction.
The photovoltaic solar module in this embodiment can consist of an array of 120 cells of 31×125 mm, attached in eight branches of 15 cells each. The complete module will therefore have dimensions of about 1,800×1,000×80 mm. If conventional cells having 17% efficiency are used, this configuration obtains a module having a rated power of about 250 W. To obtain the same electrical parameters as a conventional photovoltaic module of the same power, the connection must be made with four branches in parallel, connected in series with the next four branches.
Number | Date | Country | Kind |
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P201331199 | Aug 2013 | ES | national |
Filing Document | Filing Date | Country | Kind |
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PCT/ES2014/070630 | 8/1/2014 | WO | 00 |