Heliostato bifacial

Abstract

A bifacial heliostat for a solar thermal power plant, comprising a support structure (1) having a longitudinal axis, a lower end (1.1) for attachment to an anchoring surface (2), and an upper end (1.2) having a panel (3) with a first face (3.1) comprising one or more reflective facets (4) and a second opposite face (3.2), the panel (3) being connected to the upper end (1.2) by means of a rotation mechanism connected in turn to a solar tracking device, capable of allowing movement of the panel (3) according to the solar position and the operating mode, characterized in that it comprises: - one or more photovoltaic cells (5) installed on the second face (3.2) of the panel (3) capable of capturing diffuse solar radiation and / or global solar radiation and / or albedo; - means for connecting the photovoltaic cells (5) to an electrical network; and - means for increasing the albedo of the anchoring surface (2); and wherein the panel rotation mechanism (3) has two degrees of freedom for rotating the panel (3) about the longitudinal axis of the support structure (1) and rotating about an axis perpendicular to the former, so that the panel (3) can have a first position with the first face (3.1) of the panel (3) facing the sun, a second position with the second face (3.2) facing upwards, and a third position with the second face (3.2) facing the sun.

Description

Bifacial heliostat Technical field of the invention The present invention corresponds to the technical field of renewable energy production systems, specifically to a bifacial heliostat for a solar thermal and photovoltaic power plant. Background of the Invention Currently, one method of producing solar thermal energy is through concentrating solar thermal power plants. These plants consist of a field of solar collectors, or heliostats, capable of concentrating incoming solar energy onto a small receiver located at the top of a central tower. A heat exchange system absorbs the concentrated radiation and converts it into thermal energy for steam generation. This steam is used to power a turbine to generate electricity, or alternatively, as heat for industrial processes or the production of fossil fuels. When solar radiation passes through the atmosphere and reaches the Earth's surface, part of the radiation is reflected back into space, another part is absorbed by water vapor and air molecules, another part is scattered by air molecules, water vapor and aerosols, and a final part, called direct radiation, reaches the Earth's surface without undergoing changes in the direction of its path, due to reflection, absorption and scattering. This direct or incident solar radiation is measured as irradiance or energy per unit of time and area (W / m²). On the other hand, the radiation that has been deflected from its path by the phenomenon of scattering is called diffuse radiation, and the sum of both is called global radiation. In the case of concentrating solar thermal plants, only direct solar radiation is concentrated due to the physical laws of specular reflection, where the angle of incidence of the radiation on the heliostat is equal to the angle of reflection from the heliostat to the central receiver of the tower, so only direct solar radiation can be directed to the receiver, for the subsequent production of electrical energy or process heat. In the quest to increase the energy efficiency of these plants, the solar tracking systems of the heliostats have been improved, based on the daily and annual solar ecliptic. The type of reflector materials has also been improved, increasing the amount of reflected solar radiation, as well as the heliostats' pointing towards the solar receiver, using calibration, geometric, and edging methods. However, all these improvements focus solely on the direct component of solar radiation, leaving a large amount of w / m2 of solar radiation existing on the surface of these plants unused, as is the case with diffuse solar radiation. The technology that does take advantage of global solar radiation, without discriminating in direction, is photovoltaic solar energy, which uses solar collectors composed of photovoltaic cells in which solar radiation is transformed into electrical energy by the photoelectric effect, which consists of the emission of electrons by a material when solar radiation hits it. The state of the art has sought to combine both techniques to complement the use of solar radiation and to increase the efficiency of energy production. As an example of the state of the art, reference documents ES2393095, CN110609575, WO2020025107 and CN111596698 can be mentioned. Reference document ES2393095 defines a self-contained solar concentrator with electrical storage via a capacitor. This concentrator comprises a rotating mirrored surface and one or more photovoltaic panels that generate the energy required to power the capacitor. These panels are oriented in the same direction as the concentrator, as they must capture radiation simultaneously with the concentrator when it is active in order to supply it with power. When the concentrator is inactive due to insufficient radiation, the photovoltaic panel ceases to function, and the concentrator disconnects from the capacitor. Thus, the amount of energy supplied by the photovoltaic panels is sufficient for the consumption generated by the concentrator, but it is not possible to consider a greater production beyond self-supply. Reference document CN110609575 describes a heliostat system that is combined with a photovoltaic panel that forms a photovoltaic energy generation subsystem to power the heliostat control subsystem. This subsystem is arranged on the back of the heliostat's mirror body, unlike the first document, and includes a photovoltaic panel that receives light reflected by the blocking of other heliostats, a battery pack connected to the heliostat's control subsystem, and a charge and discharge controller. This increases the system's productivity, but again in limited quantities, which can be allocated to the self-supply of the heliostat system, allowing it to be energy autonomous, thus avoiding having to supply it with energy for solar tracking or other parasitic consumption. For its part, reference document WO2020025107 defines a self-contained facet suitable for use as a reflective element of a solar concentrator, comprising a front layer with a reflective surface of the facet, an intermediate layer with an integrated photovoltaic panel and a back layer for closing, insulating and support. This is a hybrid technology that uses a heliostat along with photovoltaic technology, where both systems are in operation on the same front face of the panel, so that the reflective part of the heliostat reflects infrared radiation and is transparent to visible, ultraviolet and near infrared rays to be used by the second layer of the heliostat, formed by the photovoltaic layer. The problem with this system is that the entire solar spectrum cannot be used in the concentration generated by the heliostat; only a portion of it is utilized. The photovoltaic component uses the portion of the solar spectrum available for photovoltaic energy, specifically between 400 and 1200 nm. Therefore, only the remaining portion of the solar spectrum, from 280 to 400 nm and from 1200 to 2500 nm, is available for concentration. This means that the possibility of concentrating the central part of the spectrum (between 400 and 1200 nm) is being lost, resulting in a significantly lower energy concentration efficiency at the plant's central receiver compared to other systems. Reference document CN111596698 defines a heliostat system comprising a photovoltaic power generation subsystem with a photovoltaic panel connected to a battery pack and located on the panel face opposite the facets. This photovoltaic panel operates only when the direct normal irradiance (DNI) is very low, less than 200 W / m², or when it is very high, greater than 2500 W / m². In these cases, the heliostats do not concentrate the radiation and are oriented towards a point in the vacuum, as the radiation is either too low or too high for the designed receiver to withstand. Thus, the added photovoltaic panels work when the facets cannot because the radiation is too low or too high; therefore, there is an increase in production when the heliostat works in periods that would otherwise be unproductive, but high levels of production are not reached, remaining limited to the battery storage option, although the option of connection to the electrical grid is considered. Therefore, although there has been progress in combining heliostats with photovoltaic panels, the overall performance and productivity are not significantly higher than that obtained without photovoltaic panels. Consequently, their use is limited to powering the heliostat itself, making it energy autonomous and eliminating the need to supply energy for solar tracking or other consumption. To date, no heliostat systems have achieved significant effectiveness and productivity that would allow for uses other than self-supply. It is therefore necessary to find ways to utilize these heliostats to enable greater energy production and efficiency at any given time. Description of the invention The bifacial heliostat for a concentrated solar power (CSP) plant, presented here, comprises a support structure with a longitudinal axis, a lower end for attachment to an anchoring surface, and an upper end with a panel. The panel has a first face with one or more reflective facets and a second, opposite face. The panel is connected to the upper end by a rotation mechanism, which is in turn connected to a solar tracking device. This mechanism allows the panel to be moved according to the sun's position and the heliostat's operating mode. This heliostat comprises one or more photovoltaic cells installed on the second side of the panel capable of capturing diffuse solar radiation and / or global solar radiation and / or albedo, and means of connecting the photovoltaic cells to an electrical network. It also includes means for increasing the albedo of the anchoring surface, which irradiates the photovoltaic cells on the second side. Furthermore, the heliostat's rotation mechanism has two degrees of freedom: one for rotating the panel around the longitudinal axis of the support structure and another around an axis perpendicular to the first. In this way, the panel has a first operating position with the first face of the panel facing the sun, in which the facets are active for capturing direct solar radiation and the photovoltaic cells are active for capturing global and / or diffuse solar radiation, at least reflected by the albedo. These degrees of freedom also allow for a second and third position. Thus, in the second operating position, the panel is horizontal with its second face facing upwards, while in the third position, the second face is oriented towards the sun by the solar tracking device. In both positions, the facets are inactive, and the photovoltaic cells are active, capturing both direct and diffuse solar radiation. The bifacial heliostat proposed here represents a significant improvement over the state of the art. This is because a heliostat is obtained with which it is possible to improve the production, distribution and rational use of energy, increasing the efficiency of the electrical energy production of a solar tower plant. With this bifacial heliostat it is possible to significantly increase energy production by using direct solar radiation on one side of the heliostat for concentration in a receiver, and simultaneously producing electricity directly on the opposite side by harnessing diffuse solar radiation. This enhances the effectiveness of these heliostats, as they allow a solar plant to operate at high performance both on sunny days with high radiation and on cloudy days with high turbidity. When direct radiation is low, it cannot be concentrated on a receiver, so on cloudy days, current heliostats cease operating and supplying the necessary energy to the electrical grid according to demand. With this bifacial heliostat, electricity continues to be produced on cloudy days thanks to photovoltaic production that harnesses diffuse radiation, which is indeed very high on these days. This bifacial heliostat has the great advantage that it is not only capable of producing electricity through photovoltaic cells when there is not enough direct radiation to allow the heliostat to function, but it is also capable of both energy generation systems working simultaneously. That is, while the facets of the heliostat are oriented towards the sun and concentrating the direct solar radiation that reaches them in a receiver, the photovoltaic cells can be taking advantage of diffuse, global radiation and the albedo, generating electrical energy. A very positive effect for electricity generation with this heliostat is the enhancement of diffuse radiation through increased albedo. Thanks to this, photovoltaic cells exhibit high electricity production from diffuse radiation when they are simultaneously generating power from the facets. These photovoltaic cells also receive radiation from the blocks generated in heliostats located further back, thus taking advantage of solar radiation that would otherwise be lost and cannot be concentrated due to this phenomenon. This allows for a significant increase in the heliostat's productivity, making it possible to supply the electricity produced by the photovoltaic part directly to the electrical grid, although the option of storing it in batteries is also possible. It is therefore a very efficient bifacial heliostat that makes use of the global radiation coming from the sun, that is, it takes advantage of both the direct incident radiation and the diffuse radiation, maximizing the latter thanks to the means of increasing the albedo, so the increase in the productivity and performance of the heliostat is remarkable. Brief description of the drawings In order to aid a better understanding of the characteristics of the invention, according to a preferred embodiment thereof, a series of drawings are provided as an integral part of this description, where, for illustrative and non-limiting purposes, the following has been represented: Figures 1.1 and 1.2 show front and back perspective views of a bifacial heliostat in a first operating position, for a preferred embodiment of the invention. Figure 2 shows a perspective view of a bifacial heliostat in a second operating position, for a preferred embodiment of the invention. Figures 3.1 and 3.2 show front and back perspective views of a bifacial heliostat in a third operating position, for a preferred embodiment of the invention. Detailed description of a preferred embodiment of the invention In view of the figures provided, it can be observed how, in a preferred embodiment of the invention, the bifacial heliostat for a solar thermal power plant, proposed herein, comprises a support structure (1) having a longitudinal axis, a lower end (1.1) for fixing to an anchoring surface (2) and an upper end (1.2) having a panel (3) with a first face (3.1) comprising one or more reflective facets (4) and a second opposite face (3.2), the panel (3) being connected to the upper end (1.2) by means of a rotation mechanism connected in turn to a solar tracking device, capable of allowing the movement of the panel (3) according to the solar position and the mode of operation. As shown in Figures 1.2, 2 and 3.2, this heliostat further comprises one or more photovoltaic cells (5) installed on the second face (3.2) of the panel (3) capable of capturing diffuse solar radiation and / or global solar radiation and / or albedo. These photovoltaic cells (5) are connected to an electrical grid by means of connection devices, which allow the energy produced to be supplied to said grid. In other preferred embodiments of the invention, the means of connection to the electrical grid comprise intermediate storage devices, formed by batteries, connected to the photovoltaic cells (5) prior to connection to the electrical grid. Likewise, the heliostat comprises means for increasing the albedo of the anchoring surface (2). In this preferred embodiment of the invention, the albedo-increasing means are formed by highly reflective materials for the anchoring surface (2). In this case, marble dust is used, but in other cases, white gravel, limestone, perlite, boulders, or a material with similar high-reflectivity characteristics may be used. In other embodiments, the means of increasing the albedo are formed by a high diffuse reflectivity paint impregnated on the anchoring surface (2). Furthermore, the panel's rotation mechanism has two degrees of freedom, allowing the panel (3) to rotate around the longitudinal axis of the support structure (1) and around an axis perpendicular to the first. These rotations result in the panel (3) assuming a first operating position, as shown in Figures 1.1 and 1.2. In this first operating position, the first face (3.1) of the panel (3) is oriented towards the sun by means of the solar tracking device, and the facets (4) are active for the capture of direct solar radiation while the photovoltaic cells (5), arranged on the second face (3.2), are active for the capture of at least diffuse solar radiation and that reflected by the albedo. In this preferred embodiment, the photovoltaic cells (5) in the first operating position also capture incident solar radiation from downstream heliostat blocks. Likewise, this heliostat allows a second and third position in which the facets (4) are inactive and the photovoltaic cells (5) are in operation. As can be seen in Figure 2, the second operating position presents the panel (3) in a horizontal position, with the second face (3.2), containing the photovoltaic cells (5), facing upwards. In this position, the photovoltaic cells (5) are active for capturing global solar radiation, that is, both direct and diffuse radiation. This position can occur when, for some reason, such as radiation that is too low for concentration in the receiver, or radiation that is too high and could damage the receiver, or due to maintenance or plant shutdown, the panel (3) is lowered to this horizontal position, and the photovoltaic cells (5) produce electricity from the global radiation that does reach them. Finally, the third position, shown in Figures 3.1 and 3.2, presents the panel (3) with its second face (3.2) oriented towards the sun, using the solar tracking device. In this third position, the photovoltaic cells are active for capturing global solar radiation (direct plus diffuse). This situation occurs on days with high concentrations of particles in the atmosphere, large amounts of water vapor, or cloud cover. These factors make it impossible to concentrate solar radiation, but it is still possible to produce electrical energy thanks to the photovoltaic panels (5), because the levels of direct solar radiation are insufficient, while the levels of diffuse and / or global radiation are high enough to produce photovoltaic solar energy. According to another aspect, in this embodiment, the facets (4) of this heliostat are formed from glass with a reflective surface made of a highly reflective material, which can be aluminum or other highly reflective materials, sufficiently resistant to withstand adverse weather conditions such as wind, rain, and sun exposure. In this case, the structure and frame of the bifacial heliostat are constructed from durable, corrosion-resistant materials, such as galvanized steel or aluminum, depending on the atmospheric specifications of the site of the concentrated solar power plant where the bifacial heliostats are to be installed. The dimensions of this bifacial heliostat can vary depending on the specific design of the solar plant where they are to be installed, being mainly a function of the solar radiation levels of the site and the nominal power desired in the concentrated solar thermal power plant, therefore its power range also depends on this size conditioned by the irradiance characteristics of the site and the nominal power of the plant, being greater for larger surfaces. Each bifacial heliostat placed in the solar field is moved daily and annually using the solar tracking device to keep the facets (4) aligned with the sun by using solar sensors, control algorithms and computer-controlled motors that constantly adjust the position of the heliostat to reflect solar radiation and maximize the amount of solar radiation concentrated at any given moment. In this preferred embodiment, the heliostat's control means comprise software with integrated control algorithms and solar sensors, which monitor and coordinate all device operations. These control means collect data from the solar sensors, calculate the optimal position of the facets (4), and send commands to the motors to make the necessary adjustments. It can also be integrated with other systems of the concentrated solar thermal power plant for coordinated operation. From another perspective, photovoltaic cells (5) are primarily composed of crystalline silicon, which can be either monocrystalline or polycrystalline. Both technologies have similar efficiencies, but monocrystalline cells generally have a more uniform appearance and a higher manufacturing cost. In this preferred embodiment of the invention, each of the photovoltaic cells (5) of this heliostat comprises a protective layer of transparent glass, to protect them from atmospheric elements or corrosion. Furthermore, the heliostat panel (3) comprises a high transparency tempered glass cover on the second face (3.2) of the panel (3), arranged over the photovoltaic cell array (5) that allows sunlight to pass through it and reach the photovoltaic cells (5). The panel (3) also comprises a layer of water-resistant and thermally insulating material, mainly encapsulating polymers, on the back face of the photovoltaic cell assembly (5). This water-resistant layer protects the photovoltaic cells (5) from contact with moisture and prevents heat loss through the back face of said photovoltaic cells (5). The photovoltaic cells (5) comprise metallic contacts on their outer face to collect the generated electricity and direct it towards the connection cables, transporting the electricity from the panel (3) to the distribution system or, where appropriate, to an energy storage system.

Claims

1. Bifacial heliostat, for a solar thermal power plant, comprising a support structure (1) having a longitudinal axis, a lower end (1.1) for fixing to an anchoring surface (2) and an upper end (1.2) having a panel (3) with a first face (3.1) comprising one or more reflective facets (4) and a second opposite face (3.2), the panel (3) being connected to the upper end (1.2) by means of a rotation mechanism connected in turn to a solar tracking device, capable of allowing movement of the panel (3) according to the solar position and the mode of operation, characterized in that it comprises: - one or more photovoltaic cells (5) installed on the second face (3.2) of the panel (3) capable of capturing diffuse solar radiation and / or global solar radiation and / or the albedo; - means for connecting the photovoltaic cells (5) to an electrical network; and - means for increasing the albedo of the anchoring surface (2); and wherein the rotation mechanism of the panel (3) has two degrees of freedom for rotation of the panel (3) about the longitudinal axis of the support structure (1) and rotation about an axis perpendicular to the former, so that the panel (3) can have a first position with the first face (3.1) of the panel (3) facing the sun, a second position with the second face (3.2) facing upwards, and a third position with the second face (3.2) facing the sun.

2. Heliostat according to claim 1, wherein the means for increasing the albedo are formed by highly reflective materials for forming the anchoring surface. 3.A heliostat according to claim 2, wherein the high-reflectivity materials consist of marble dust, white gravel, limestone, perlite, boulders, or a similar high-reflectivity material.

4. A heliostat according to claim 1, wherein the albedo-enhancing means consist of a high-reflectivity paint impregnated onto the anchoring surface.

5. A heliostat according to any of the preceding claims, wherein the grid-connection means comprise intermediate storage devices connected to the photovoltaic cells (5) prior to grid connection.

6. A heliostat according to any of the preceding claims, wherein the control means comprise software with integrated control algorithms and solar sensors. 7.A heliostat according to any of the preceding claims, wherein the facets (4) are formed of glass with a reflective surface made of a highly reflective material.

8. A heliostat according to any of the preceding claims, wherein each of the photovoltaic cells (5) comprises a protective layer of transparent glass.

9. A heliostat according to any of the preceding claims, comprising a highly transparent tempered glass cover on the second face (3.2) of the panel (3), disposed over the photovoltaic cell assembly (5), and a layer of water-resistant and thermally insulating material on the rear face of said photovoltaic cell assembly (5).