WO2005119136A1

WO2005119136A1 - Line focus heliostat and operating method thereof

Info

Publication number: WO2005119136A1
Application number: PCT/ES2005/000297
Authority: WO
Inventors: Rafael Monterreal Espinosa
Original assignee: Centro De Investigaciones Energeticas, Medioambientales Y Tecnologicas (Ciemat)
Priority date: 2004-05-26
Filing date: 2005-05-25
Publication date: 2005-12-15
Also published as: ES2244339B1; ES2244339A1

Abstract

The invention relates to a line focus heliostat and to the operating method thereof. The line focus heliostat comprises a cylindrical or cylindrical-parabolic reflective surface (1) which can rotate around a primary axis (3) that is solidly connected to a mobile support (2) which, in turn, can rotate around a secondary axis (5) that is perpendicular to the primary axis (3). According to the invention, the secondary axis (5) is oriented in a particular manner such as to be perpendicular to a main image plane (10) which contains a fixed linear target (11) and the primary axis (3). The inventive structure and arrangement enable the line focus to be confined to the linear target (11), without the reflection of the light outside of the optical axis shifting same over the course of the day. The movements are obtained using a primary actuating means (4) and a secondary actuating means (6) which are controlled by a control device (8). In the specific case of a vertical linear target (11), the main image plane (10) is also vertical and, as a result, the secondary axis (5) is horizontal.

Description

LINEAR FOCUS HELIOSTAT AND OPERATING METHOD

OBJECT AND FIELD OF APPLICATION The present invention relates to the realization of a new heliostat of cylindrical optics and pseudo-horizontal mount, whose task is to distribute and stabilize the sunlight reflected on its surface along a defined direction on a certain target throughout the day. It is this direction of concentration of solar radiation that results from projecting the central generatrix of the cylindrical optics of the heliostat on the surface of the target. The field of application of the invention would be the central receiver thermo-solar plants whose design demands that the distribution of solar energy reflected by the heliostat field follow a preferred direction on said receiver. Since the image of the Sun given by a conventional heliostat on a target is circular, this directional distribution of energy has traditionally been achieved in conventional thermo-solar power plants by operating techniques based on targeting strategies on groups of heliostats. The thermo-solar power plants of the tower type, or central receiver, base their operation strategy on the contribution of heat to a certain conventional thermodynamic cycle, by concentrating solar radiation by a high number of heliostats. Among the functional characteristics of heliostats is their ability to concentrate solar radiation in the receiver or boiler, for which they are provided with a reflective surface of spherical geometry and mechanical mount of the type called horizontal. Except for some very basic requirements about the optical quality of its reflective surface, its pointing capacity or its land occupation factor, the rest of the design parameters of a heliostat, such as geometry, size, control, or type of mechanism of operation, they have traditionally been imposed by essentially economic criteria. It is striking that the design of the heliostat has not come so far, in any of its parameters, conditioned by the type of solar receiver on which it must distribute the radiant energy it reflects from the Sun. However, the large solar receptors they have a geometry in which at least one of its dimensions is much larger than the others, such as tubular or volumetric cylindrical receivers. In all these projects the fact that the image of the Sun generated by spherical heliostats is much smaller than the dimensions of the solar receiver itself has been verified, which is a serious inconvenience, since the complete irradiation of this requires pointing the heliostats to the along its entire surface with some specific criteria or strategy, which are entrusted to the point control system of the heliostat field. Since it is no longer aimed at a single place on the receiver, the control algorithms basically have to strategically disperse the radiation that has previously been concentrated by each heliostat. This essentially contradictory fact and of questionable success, since it jeopardizes the performance and the integrity of the solar receiver itself, has led to reflect on the advisability of reviewing the heliostat design parameters, so that the irradiance distribution given by each heliostat on the solar receiver it adapts in origin to the geometric shape of the latter and, consequently, the point control strategies are minimized or even suppressed. The revision of these design parameters involves the optics, the pointing mechanism and the orientation of its location within the field of heliostats. As an alternative to the conventional design, the linear focus and pseudo-horizontal mount heliostat is proposed, object of the present invention. Throughout the document the following terms will be used, with the meaning described: - Solar energy: radiant energy that comes from the Sun and that reaches the Earth's surface with a characteristic intensity and spectral composition. - Heliostat: Mirror of great focal length, equipped with movement in two axes and whose mission is to reflect, concentrate and maintain static the image of the Sun in a certain place throughout the day. - Cylindrical optics: Reflective surface of revolution obtained by rotating a segment of arbitrary length around an axis parallel to it, called the axis of revolution of cylindrical optics. - Cylindrical heliostat: heliostat provided with cylindrical optics. - Geometric axis of a cylindrical heliostat: It is the axis of revolution of its cylindrical optics. - Optical axis of a cylindrical heliostat: virtual straight line that passes through the centers of the cylindrical optics, white and Sun, assumptions aligned, and cuts orthogonally to the geometric axis of the heliostat. - Central plane of a cylindrical heliostat: plane containing the geometric and optical axes. - Central generator of a cylindrical heliostat: Of all the generatrices of the cylindrical heliostat, that which is contained in its central plane. - Main incident ray: the one that comes from the center of the solar disk and cuts at the midpoint of the central generatrix of the heliostat optics. - Reflected main beam: the one that comes from the midpoint of the central generatrix of the heliostat optic and cuts at the midpoint of the target.

- Reflection plane: The one that contains the main incident beam and the main reflected beam. - Main plane object of a cylindrical heliostat: Plane containing the central generatrix and the main incident beam.

- Main plane image of a cylindrical heliostat: Plane containing the central generatrix and the reflected main beam.

- Focal line of a cylindrical heliostat: Geometric place of intersection with the central plane of the solar rays reflected on its optical surface, assuming the incidence paths of said rays parallel to the optical axis. In incidence not parallel to the optical axis, the focal line moves to the main image plane and modifies its genuine characteristics due to astigmatism, becoming a pseudo-focal line. - Primary axis of a cylindrical heliostat: Axis of rotation of the heliostat coinciding with its central generatrix.

- Secondary axis of a cylindrical heliostat: Axis of rotation of the heliostat that is orthogonal to the primary axis.

- Horizontal mount: Mechanical device of orientation in two axes of a heliostat with respect to a topocentric system of horizontal coordinates, called azimuth and height. The fundamental plane is the horizon of the observer and the fundamental point is the true North. The orientation of the heliostat, depending on the diurnal evolution of the Sun in this same coordinate system, is achieved by azimuthal turns (horizon arches from the fundamental point) and height or zeniths (orthogonal arcs to the horizon plane towards the zenith of the observer). The mechanical axis of azimuthal rotation is orthogonal to the plane of the horizon and of fixed orientation. On the contrary, the axis of zenith rotation is parallel to the plane of the horizon and of variable orientation, due to the existence of a mechanical ligation between both movements, which causes the "dragging" of the zenith axis each time the azimuthal rotation occurs.

- Pseudo-horizontal mount: Variant of the horizontal mount, whereby the orientation of the heliostat is achieved through turns around the primary and secondary axes of the heliostat, orthogonal to each other but arbitrarily oriented with respect to the plane of the horizon, and assuming the position of the Sun given in horizontal coordinates. The position of the mechanical axis of rotation The zenith is fixed and its orientation no longer refers to the plane of the horizon, but to the main image plane of the heliostat, with respect to which it must necessarily be orthogonal. The mechanical axis of azimuthal rotation must be orthogonal to the aforementioned zenith axis and, therefore, is fully contained in the main image plane of the heliostat, its orientation being variable in said plane due to the existence of a mechanical bond between both movements, which causes the "drag" of the azimuthal axis every time the zenith turn occurs.

- Facets: Individual specular elements of which the reflective surface of some heliostats is composed.

- Alignment or edging of a heliostat: Action to orient the facets in such a way that the intersection of all the optical axes of these is on the optical axis of the heliostat, at a distance to the vertex of this double of its focal. The alignment gives focal to the heliostat, supposedly faceted its reflective surface.

- Declination: Variation of the height of the Sun over the celestial equator when the earth, throughout the year, travels its trajectory (the ecliptic) around the Sun.

- Heliostat field: Also called primary concentrator, it is a set of heliostats arranged in a limited terrain and whose mission is the contribution of radiant energy to a target or receiver.

- Solar receiver: Device that intercepts and absorbs the solar radiation provided by a field of heliostats, in order to transfer it through a heat exchanger to the power block of the plant.

- Linear target: Solar receiver that must receive radiant energy along a line (or at least one of its dimensions is much larger than the others).

- Central receiver thermo-solar plant: electric power production plant that bases its operation strategy on the heat contribution to a certain conventional thermodynamic cycle, by concentrating solar radiation by a high number of heliostats on a single receiver .

- Aiming strategy: Operation procedure of a thermo-solar plant that consists of defining a set of coordinates on the receiver to which each of the heliostats of the field must aim to achieve the distribution of energy required on it. - Dynamic aiming strategy: It is an aiming strategy in which the coordinates on the receiver change over time, following certain control criteria. - Confined focal line: Condition determined by the optics and orientation mechanism of the heliostat with respect to the apparent movement of the Sun, whereby the orientation of the main image plane of the heliostat remains invariant and, consequently, the orientation of its focal line is stabilized on the target That is to say, orienting the optics to follow the Sun in its daytime movement it can be achieved that the radiation reflected by the heliostat, grouped around its focal line, is permanently distributed on a linear target. BACKGROUND OF THE INVENTION The first heliostats considered as industrial elements were developed at the beginning of the eighties for the experimental central solar thermal solar plants, with the purpose of testing the viability of solar thermal energy in the production processes of Industrial scale electricity. Table 1 summarizes the projects carried out due to the international initiative:

Table 1. International projects of central solar thermal-solar plants Once the demonstration projects were completed, most of these plants were closed. In Europe, only the heliostat fields corresponding to the CRS and CESA-1 plants continued in service, thanks to a collaboration agreement between the German and Spanish governments, constituting the Almeríé Solar Platform (PSA). In the USA the Solar One plant was remodeled and, with the same field of heliostats, the Solar Two plant was put into operation, which has been operating until the beginning of 1999. The PSA continues to operate these heliostat fields today thanks to a great diversity of projects that have been carried out in recent years. The objective of these projects has been the development and evaluation of new solar components in this technology, mainly heliostats and solar receivers. Table 2 summarizes the prototypes of heliostats currently developed and their optical and mechanical characteristics.

Table 2. Heliostat model and its optical and mechanical characteristics As shown in Table 2, all described heliostats have had spherical optics, that is, any beam of rays parallel to the optical axis converges once reflected in a single point, called focus or focal point. Its objective is, therefore, the reproduction of the image of the Sun in a certain target, in contrast to the heliostat of cylindrical optics, object of the invention, whose purpose is not the reproduction of the image of the Sun, but the distribution of its radiant energy along a preferred direction. Thus the concept of focal point disappears, replacing it with that of a focal line; that is, any beam of rays parallel to the optical axis converges once reflected along a line, called the focal line. But the confined focal line cylindrical heliostat needs two additional requirements, which are immobility and specific axis orientation. secondary. Said axis, as such, does not exist in any of the reviewed heliostats, since all are spherical. However, Table 2 shows that only the HELLAS spherical heliostat has a fixed zenith mechanical axis with a non-specific orientation for each heliostat, but a general East-West for all heliostats in the field; Since the secondary axis as such does not exist, its immobility responds only to mechanical criteria, without impact on the optical performance, which is the object of the present invention. The problems posed by the use of spherical heliostats are perfectly reflected in US 3,892,433 by Martin Marietta, in which an installation for producing electrical energy using solar energy concentrated by a field of spherical heliostats on a circular target is described. In order to follow the sun during its daytime variation of height and azimuth, the reflective surface of each heliostat must be able to pivot around a horizontal axis, which is in turn orientable around a vertical axis. As shown in Figure 4 of said document, such a solution introduces various astigmatism errors due to the incidence of light outside the optical axis of heliostats which causes, on the one hand, a dispersion of energy as a result of the focus does not match the plane of the receiver, and on the other hand, a selection of the image throughout the day. It is understood that this selection will be especially harmful if it is intended to move from the concept of focal point to that of focal line, replacing spherical reflectors with cylindrical reflectors. Proposals for the use of cylindrical heliostats with single-axis orientation movement in solar plants with linear type receivers, both small scale (US 3,861, 379) and large scale (US 4,141, 626), are also known. Thus, the linear focus concentrating device described in Anderson's US Pat. No. 3,861, 379, comprises a plurality of rectangular flat reflectors arranged at different angles of inclination to concentrate sunlight on a domestic hot water collector located above and a short distance from the reflectors. The angle of incidence of the reflectors varies simultaneously during the day so that the reflected image of the sun affects the collector at all times. Since high performance is not pursued, the inclination of the device relative to the plane of the sun's apparent movement is fixed. That is to say, the variation in the decline of the sun during the year is not taken into account, with at most two positions, winter and summer, between which it is manually switched. As a consequence, the linear focus moves longitudinally over the receiver throughout the year, although without serious consequences given the small focal length. Such a solution can only work. for low power devices and very short focal length, such as domestic hot water applications. It has also been proposed to correct astigmatism errors through the use of variable geometry optics such as that described in US Pat. No. 4,141,626 of FMC Corporation in which the curvature of the reflective surface of the heliostat is modified during the day. This modification will be maximum during the summer solstice and minimum in the winter solstice, due to the extreme positions (maximum and minimum, respectively) of the azimuth and height of the Sun on the horizon. The cited document uses a certain type of heliostat with a deformable cylindrical reflective surface, which points to a horizontal target, oriented in an East-West direction, the primary axes of said heliostats being parallel thereto, while the secondary axis as such does not exist, and The deformation of the image by astigmatism is compensated by an action on the cylindrical reflective surface, varying its focal length. Such a solution eliminates the dispersion of the image, considerably improving the focus and, given the orientation of the primary axis of the heliostats parallel to the target, suppresses the picking, but cannot prevent the linear focus from moving longitudinally over the target along of the day This displacement is very important when working with high focal lengths, so that the length of the target is limited, the number of hours of operation of the device is considerably reduced. Note that if we provide this device with a second vertical axis to follow the sun's hourly movement, as recommended by Martin Marietta, we would achieve the longitudinal immobilization of the linear focus on the target throughout the day but, in return, would occur a selection of the linear focus, since the parallelism of the primary axis of the heliostat with the target is no longer maintained. In certain cases, the use of cylindrical heliostats with associated linear moving target, and two-axis orientation movement for sun tracking is proposed. Thus, US 5,253,637 of Maíden poses the problem of fixed reflector and white solar collectors, which have high losses and a severe limitation of the number of hours of use. The proposed solution consists of coupling a linear receiver on a horizontal primary axis cylindrical reflector that can rotate with respect to a vertical secondary axis. In this way, the linear focus does not show any color since it is a mobile target device attached to the reflector. While it is true that in Maiden there is a main image plane (the primary axis and the linear target are parallel by construction), this is a mobile plane, necessarily inclined, and therefore can never be perpendicular to the vertical secondary axis. Maiden solves the problem of In exchange for making the linear target mobile, which implies that it can only be associated with a single reflector, with the consequent limitation of the power absorbed by it. Finally, in GB 2 063 465 of Jubb, the problem posed is that of the low performance of photoelectric cells as an electrical power generating device. The proposed solution consists in dividing the light spectrum by means of a diffraction groove, making each wavelength segment affect different groups of photoelectric cells specially adapted to each wavelength. As in the previous document, it is a cylindrical reflector assembly with associated moving target, in horizontal arrangement. In conclusion, none of the previously known heliostats have the requisites required for a high performance cylindrical and simplicity of construction and operation such as that which is the object of the invention. The problem posed is that of the selection of the focal line on a fixed target independent of the heliostat, and could be solved by a three-axis drive, so that we would need two axes to follow the sun in its diurnal variation in height and azimuth, and a third axis to correct the selection. Such a solution is expensive and complex to realize. Accordingly, it is an objective of the present invention to have a heliostat so constructed and operated that it is capable of following the sun in its daytime movement, maintaining a linear image without a blank on a fixed linear target located at a considerable distance. It is another objective of the present invention to have a heliostat that, fulfilling the above conditions, can be operated by means of its operation in two axes.

DESCRIPTION OF THE INVENTION To achieve the proposed objective, the main image plane of the heliostat object of the invention must remain motionless in space and contain the linear blank, which can be achieved by combining the following elements: 1. A reflective surface of cylindrical or cylinder optics - Parabolic, interchangeably, that it must be able to rotate around a primary axis coinciding with the central generatrix of the reflective surface. 2. A structure consisting of a mobile support capable of rotating with respect to a secondary axis perpendicular to the primary axis, the primary axis being integral with the mobile support. 3. Specific orientation of the secondary axis, perpendicular to a main image plane, motionless in space, which at all times contains the linear target and the primary axis. 4. A primary drive to rotate the reflective surface around the primary axis and a secondary drive to rotate the movable support around the secondary axis, both governed by a control device. As relevant aspects of the embodiment of the invention, we will focus especially on the geometry of the reflective surface, its diurnal orientation, the structure that relates the primary axis and the secondary axis, and the confinement of the focal line. Geometry of the reflective surface. The reflective surface of the heliostat object of the invention has cylindrical or parabolic trough geometry instead of the conventional spherical, which concentrates the solar radiation not on a focal point but on a focal line, of length consistent with the preferred direction dimension of distribution of the intended energy over a certain target. This surface can be of a piece or faceted, in which case the facets are also cylindrical or parabolic trough and with foreseeable focal length that is predictably identical to that of the surrounding surface resulting from aligning them cylindrically. The central generatrix of this reflective surface (faceted or not) must be coincident with the primary axis of the heliostat and, therefore, orthogonal to its secondary axis. For practical purposes it can be admitted that the central generatrix and the primary axis are not exactly coincident if they are parallel and very close and in this sense the word "coincident" will be used in this document. The purpose of this geometry is to replace the focal point of conventional heliostats with a focal line, parallel by construction to the central generatrix of the cylinder, and whose length, at normal incidence, coincides with its dimension. The result is a distribution of the radiant energy of the Sun reflected by the heliostat along a preferred direction over the target. Daytime orientation of the reflective surface to achieve the aim. Like any heliostat, the confined focal line cylindrical needs to be oriented in space in order for the rays from the Sun and reflected on its surface to hit the target. This is achieved through the intervention of two drive mechanisms, which rotate said surface a certain angle around the primary and secondary axes, after calculating the solar vector and direction of the beam reflected by a control device. Structure of the set. The special arrangement of the primary and secondary axes just described constitutes a radical change with respect to the arrangement used in conventional heliostats. In these, the primary axis of azimuthal orientation of the reflective surface is vertical and remains motionless in space. The reason is fundamentally economic, since it is very favorable to support said primary axis on the heliostat pedestal itself, which holds it to the ground. Said pedestal is usually constituted by a vertical metal tube, which can accommodate inside the rotating bearings of the primary shaft, which is thus hidden in the pedestal itself. Such an arrangement is absolutely inadequate to meet the condition of confinement on the target of the focal line. The proposed solution is to relate directly to the pedestal not the primary axis but the secondary axis, with which the primary axis of azimuthal movement is freely dragged during the zenithal movement around the secondary axis, which now remains in a fixed orientation in space . Focal line confinement. The confinement (immobility) of the focal line of the cylindrical surface described above is achieved by immobilizing the orientation of the secondary axis of the heliostat in its systematic procedure of aiming at a target during the day. This immobilization of the secondary axis also immobilizes the main image plane and, therefore, its intersection with the surface of the target, which is the geometric place where irradiance is distributed (focal line). Since the position of the focal line on the target depends on the intersection of the main image plane with the target plane, and that the orientation of said main image plane depends on the orientation of the secondary axis of the heliostat, it follows that the orientation from it it can be modified at will by simply orienting (and immobilizing) said secondary axis. In summary, the confinement of the focal line (i.e. immobility and orientation of the secondary axis) by the previous means will enable the focal line of the reflective surface to be oriented according to the direction to which the concentrated light is intended to be distributed by this on a receiving element (linear white) and, what is more important, that it keeps it static in that direction, without the reflection of the light outside the optical axis being able to pick it since, at all times, the focal line lies on the main image plane, motionless, being "confined" throughout the day. The cylindrical heliostat object of the invention clearly exceeds the conventional spherical heliostat in those applications in which the radiant energy of the Sun reflected on a given receiver needs to be specifically concentrated along a preferred line or direction. In this case the shape of the flow distribution over the receiver is no longer circular or elliptical Gaussian, but will have a Gaussian profile in one direction and straight in the orthogonal direction, which coincides with that of the heliostat focal line. The advantages that derive from this fact are basically: 1. Individual irradiance profile by default of each heliostat on the target coinciding with the design for the entire field, which avoids the construction of the latter by means of a composition of individual Gaussian irradiance profiles (aiming strategy). 2. Global distribution of more stable irradiance on the receiver, since there is no stinging (turning on the plane of the target of each of the individual irradiance distributions) throughout the day, due to the immobility and orientation of the secondary geometric axis of heliostats. 3. Adjustable irradiance profile, since the mechanism allows the secondary geometric axis of the heliostat to be oriented in any direction. 4. Damping of the classic irradiance peaks due to various causes during the operation, such as spurious blurring of heliostats (breakdowns, loss of control), presence of clouds partially shading the heliostat field, loss of communications, etc. This is because the default profile of each heliostat does not support the presence of abrupt concentration peaks. 5. Absence of dynamic aiming strategies, which will allow a relaxed and safer operation of the plant, since each heliostat will aim throughout the routine operation to a single place. 6. Simplification of the alignment procedure or edging of the faceted heliostat since there are no two directions of alignment but one. This reduces the set-up time of the heliostat in the field. 7. Relief of stresses on the reflective surface of the heliostat, since, being cylindrical, we eliminate one of the curvatures. This fact will foreseeably extend the average life of said surface.

BRIEF DESCRIPTION OF THE DRAWINGS To complete the above description and in order to help a better understanding of the features of the invention, a detailed description of a preferred embodiment will be made based on a set of drawings that are attached hereto descriptive, and where the following is merely for guidance and non-limiting purposes: Figure 1 shows a central solar thermal receiver plant where the heliostat of the invention can be used. Figure 2 represents a detail of the receiver showing the circular image of the sun produced by a conventional heliostat with a spherical reflective surface. Figure 3 represents a detail of the receiver showing the elongated image of the sun produced by a cylindrical reflective surface at noon. Figure 4a shows the image of the sun produced by a cylindrical reflective surface in conventional assembly, in the morning. Figure 4b shows the image of the sun produced by a cylindrical reflective surface in conventional assembly, at noon. Figure 4c shows the elongated image of the sun produced by a cylindrical reflective surface in conventional assembly, in the afternoon. Figure 5a shows the image of the sun produced by a cylindrical reflective surface in the assembly of the invention, in the morning. Figure 5b shows the image of the sun produced by a cylindrical reflective surface in the assembly of the invention, at noon. Figure 5c shows the image of the sun produced by a cylindrical reflective surface in the assembly of the invention, in the afternoon. Figure 6 shows a rear perspective view of the "horizontal" mount of a heliostat with cylindrical reflective surface. Figure 7 shows a rear perspective view of a "pseudo-horizontal" mount of a variant of the heliostat of the invention with cylindrical reflective surface. Figure 8 shows an elevation of the heliostat object of the invention, in general configuration. Figure 9 shows an elevation of the heliostat object of the invention in a preferred embodiment in which the linear blank is arranged vertically and the mobile support is of the fork type. Figure 10 shows an elevation of the heliostat object of the invention in a preferred embodiment in which the linear blank is arranged vertically and the mobile support is straight, giving rise to a configuration of the reflective surface in "clapper". Figure 1 1 shows a side view of the heliostat of Figure 9. Figure 12 shows a plan view of the heliostat of Figure 9. Figure 13 shows, schematically, the positions of the earth relative to the sun during the solstices. Figure 14 shows, schematically, one of the devices known in the prior art and its arrangement in relation to the apparent path of the sun with respect to the ground plane for solstices and equinoxes, Figure 15 shows, schematically in perspective, spatial geometry on which the invention is based. Figure 16 is an elevation of Figure 15. Figure 17a shows, schematically in perspective, the spatial geometry of a conventional mounting heliostat. Figure 17b shows, schematically in perspective, the spatial geometry of a heliostat with the assembly of the invention. Figure 18a shows, schematically in perspective, the azimuthal movement of a conventional mounting heliostat. Figure 18b shows, schematically in perspective, the azimuthal movement of a heliostat with the assembly of the invention. Figure 19a shows, schematically in perspective, the overhead movement of a conventional mounting heliostat. Figure 19b shows, schematically in perspective, the zenith motion of a heliostat with the assembly of the invention. It is noted that Figures 1 to 7 correspond to the field of application of the invention, prior art and necessity of the invention, Figures 8 to 12 correspond to the structural description of the invention, and Figures 13 to 19 correspond to the explanation of the mode of operation of the invention. In these figures the numerical references correspond to the following parts and elements. 1. Reflective surface. 2. Mobile support. 3. Primary axis. 4. Primary drive. 5. Secondary axis. 6. Secondary drive. 7. Pedestal. 8. Control device. 9. Zenith axis. 10. Main image plane. 11. Linear white. 12. Sun. 13. Earth. 14. North - south axis of rotation of the earth. 15. Ecliptic. 16. Ground. 17. Apparent plane in the equinoxes. 18. Apparent plane in the summer solstice. 19. Apparent plane in the winter solstice. 20. Main object plane. 21. Bisector plane. 22. Main beam incident. 23. Main beam reflected. 24. Plane of reflection. 25. Optical axis. 26. Tower. 27. Flat reflectors. 28. Collector. DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT Figure 1 shows a thermo-solar central receiver plant, where a detail of the area of the tower where the target is located is shown. In a conventional installation, with spherical reflective surface heliostats, the image projected by each of them on the target is circular, as seen in Figure 2. If, as is the case, a dimension of this is considerably larger than The transversal dimension will be necessary to use a targeting strategy of the different heliostats to cover the target. Alternatively, and as seen in Figure 3, the use of a cylindrical reflective surface allows the entire blank to be covered with a single heliostat. In this way all the heliostats in the field point to the same place, considerably simplifying the operation of the plant. However, if we simply equip a conventional mounting heliostat with a cylindrical reflective surface, we will only obtain a vertical image at noon as shown in Figure 4b. In the morning the image will appear in one direction (figure 4a), while in the afternoon the image will choose in the opposite direction (figure 4c). Since the objective is that the image does not choose throughout the day, as shown in Figures 5a, 5b and 5c, we must modify the assembly of the reflective surface. Figure 6 shows the conventional assembly of a heliostat that has been equipped with a cylindrical reflective surface. The image projected on the target corresponds to that represented in figures 4a, 4b and 4c. Note how the primary axis (3) is inserted into the pedestal (7), while the secondary axis (5) is "dragged" along the primary axis (3) itself. We will call such a configuration "7-3-5" according to the numerical references used. The proposed solution, to obtain an image without staining, passes, as a precondition, by immobilizing the secondary axis as explained above. Figure 7 shows the new assembly used in the heliostat of the invention, in which the secondary axis (5) is directly related to the pedestal (7), while the primary axis (3) is "dragged" by the own secondary axis (5). We will call such a configuration "7-5-3" to distinguish it from the conventional "7-3-5" configuration, highlighting that it has a radically different functional behavior. Later we will justify the need for this election to achieve the proposed objective. Although the assembly of Figure 7 is functionally correct, it is not very practical from a constructive point of view. In addition, the mismatch between the primary axis (3) and the central generatrix of the reflective cylindrical surface may introduce certain focus aberrations. Finally, as can be seen in Figure 7, interference from the pedestal itself (7) could occur in the movement of the reflective surface. A preferred embodiment from the type of constructive view is shown in Figure 8, in which no restriction on the orientation of the target has been introduced. The heliostat object of the invention comprises a reflective surface (1), cylindrical or parabolic trough, capable of rotating by means of a primary drive (4) around a primary geometric axis (3) integral with a movable support (2) which, at in turn, it is capable of rotating around a secondary geometric axis (5) perpendicular to the primary geometric axis (3) and that forms an angle of inclination α with the zenith axis (9), by means of a secondary drive (6). Both drives (4) (6) are governed by a control device (8). The assembly is supported by a pedestal (7) whose design must be such that it allows the movement of the reflective surface (1) and the mobile support (2) without interfering with the pedestal itself (7). The reflective surface (1) is preferably cylindrical or cylindrical-parabolic. In a particular embodiment, which we will call zenith mounting, the linear target (11) is arranged vertically on a tower (26) so that the focal line, when coinciding with it, will also be vertical, which leads to a arrangement as shown in figures 9, 10, 11, 12, 15 and 16, in which the secondary axis (5) is horizontal and the primary axis (3) is contained in the so-called main image plane (10), vertical . Figure 10 shows a variant of the heliostat of the invention with a single support point of the reflective surface (1), to constitute a "clapper" type configuration as opposed to the "fork" type configuration shown in Figure 9. A a very simple solution, however, has greater demands on the single point of attachment of the primary axis (3) to the mobile support (2). Referring now to Figures 12, 15 and 16 we can see how the primary axis (3) is contained in the main image plane (10), which in turn contains the heliostat focal line, and the linear target (11) . For the primary axis (3) to be contained at all times in the main image plane (10), the secondary axis (5) must be perpendicular to the main image plane (10), and therefore horizontal, so that it is defined during the initial assembly of the heliostat, its orientation not depending on the relative position of the heliostat with respect to the target. The initial assembly of a heliostat like the one described is very simple, it is enough in a first phase to have the primary axis (3) vertical so that it is contained in a vertical plane that passes through the linear target (11) to, in a second phase orient the secondary axis (5) perpendicular to said plane. In order to explain the method of operation of such a heliostat and referring to Figures 13 and 14, we will make a brief introduction to the apparent movement of the sun with respect to the earth, which in any case will be well known by the person skilled in the art. Figure 13 shows, very schematically, the position of the earth (13) in its movement around the sun (12) when it travels the ecliptic (15). Due to the inclination of the axis of north-south rotation of the earth (14), with respect to the plane of the ecliptic (15) at an angle of 23 degrees 27 minutes, the sun's rays (12) affect a point on the surface terrestrial with a variable angle of incidence throughout the year. This angle of incidence will be maximum in the summer solstice (position A) and minimum in the winter solstice (position B). The position of the earth (13) for the equinoxes in this figure would correspond to an axis perpendicular to the plane of the same over the position in which the sun has been represented (12). Figure 14 shows, very schematically, the apparent movement of the sun with respect to the ground (16), the apparent plane having been represented in the equinoxes (17), the apparent plane in the summer solstice (18) and the apparent plane in the winter solstice (19). The observer O will see the ortho and sunset of the sun diverted to the north (N) in the summer solstice (V) and diverted to the south (S) in the winter solstice (I), while in the equinoxes the sun it leaves by the east and it is put by the west with an apparent route of 180 degrees. Also shown in Figure 14 is the Anderson solar concentrator as described in US 3,861,379. If the set of flat reflectors (27) is arranged perpendicular and centered with respect to the apparent plane in the equinoxes (17), the image of the sun converted into a line will be projected onto the collector (28) only in the equinoxes, suffering for others days of the year a longitudinal displacement on it that will be maximum in solstices. This penalty is the inevitable result of single-axis control, and in the case of the Anderson concentrator it is admissible due to the small focal length "d". Referring now to Figures 11, 12, 15 and 16, for a heliostat such as that described in the present invention, the method of operation resides, basically, in governing the two drives (4) (6) in such a way as to ensure that the Radiant energy reflected by the heliostat affects the linear target (11) at all times throughout each day and for every day of the year. According to the law of specular reflection, the incident main beam (22) and the reflected main beam (23) are in the same reflection plane (24), and the optical axis (25) of the heliostat must also be on the bisector line of both main rays, this bisector is defined as the intersection of the reflection plane (24) with the bisector plane (21) of the main object plane (20) and the main image plane (10). This requires: • A movement of the primary drive (4) to bring the optical axis (25) of the reflective surface (1) a lateral angle γ on the bisector plane (21). See figure 12. • A movement of the secondary drive (6) for, taking into account that the positions of the sun and the target are predefined, vary the elevation angle β of the mobile support (2), in order to take the optical axis ( 25) of the reflective surface (1) on the reflection plane (24). See figure 11. Next, we will analyze, in a simplified example, the step-by-step operation of a conventional heliostat and the heliostat of the invention highlighting the reasons that lead to the special structure and arrangement of the latter. Figure 17a schematically shows a heliostat of cylindrical optics and conventional horizontal mount, of the type shown in Figure 6, in the rest position, with the vertical primary axis, oriented its reflective surface towards the South which is where it is also assumed that the white (the orientation of said surface is fixed by the direction and direction of the normal unit vector ñ passing through its center). The central generatrix g of said cylindrical surface is contained in the plane π of the meridian of the place and is, by construction, normal to the plane of the local astronomical horizon. According to Figure 17a, if we projected said central generatrix in the South direction on a plane π 'normal to the horizon, the resulting line would remain contained in the meridian of the place. Let's see how the orientation movement of the heliostat affects this projection of its central generatrix and its consequences from an optical point of view. Due to the nature of its mechanical mount, the heliostat is oriented in the Euclidean space by means of two rotation mechanisms: a) the azimuth rotation mechanism Ω _acim , which runs around a vertical axis to the horizon plane, called the primary axis (3), and which normally coincides with the axis of the heliostat pedestal; and b) the zenith mechanism Ω _cβnι that runs around an axis parallel to the plane of the horizon, called the secondary axis (5) and normally coincides with the horizontal support arm of the heliostat. Suppose that the Sun is now, in the morning, somewhere on the horizon plane, so that the heliostat - obeying the laws of Geometric Optics - needs to orient itself in this coordinate system to reflect and maintain the image of the Sun on a fixed solar receiver, with an elongated and vertical shape, located on top of a tower throughout the day. As stated above, the heliostat must necessarily reach the orientation for the correct aim at the fixed target by executing two well-defined turns. The first of these, according to Figure 18a, is the azimuthal rotation Ω _acim around the primary axis, whose angle of rotation _assumes a magnitude w _acιm . It also follows from the figure that the orientation of the primary axis is not altered with the rotation but, due to a mechanical bond between them, as a consequence of this first rotation the secondary axis leaves its initial orientation and rotates exactly an angle of magnitude w _βcιm . In the new orientation, the family of solar rays reflected in the central generatrix of the reflective surface of the heliostat will project on the base of the tower, still with all its rays contained in the planq π of the meridian of the place. Since the solar receiver is on top of the tower, the heliostat should now execute the second turn that is still missing to raise the image to the desired point. Figure 19a shows the zenith rotation Ω _cβn of magnitude w _Cθn that must be executed around the secondary axis to reach the objective. The fundamental question is now: in what position is the central generatrix g of the heliostat found after this second turn? When the orientation of the secondary axis has been altered due to the azimuthal rotation, and now the reflective surface is rotated around it, the central generatrix g bends over the plane of the horizon while also leaving the plane of the meridian of the place. From an optical point of view, the family of solar rays reflected in the central generatrix of the cylindrical surface will be projected on the solar receiver at the top of the tower, but its rays are no longer fully contained in the plane of the meridian of the place . In summary, the heliostat aiming procedure has had as a geometric consequence the removal of both the central generatrix and its projection on the target from the meridian plane and, consequently for optics, that of selecting the focal line on said target, thus losing the requirement of verticality That is, the linear image given by the heliostat adapts to the solar receiver in terms of shape (elongated), but not in terms of to the orientation requirement (vertical). The consequences on the distribution of solar irradiance on the linear receiver are shown in Figure 4 through the analysis of the energy isoflow lines. Figure 17b schematically shows a particular case of cylindrical optics heliostat and pseudo-horizontal mount, similar to that of Figure 10, in rest position, with the vertical primary axis, oriented its reflective surface towards the South (the orientation of said surface it is fixed by the direction and sense of the normal unit vector ñ that passes through its center). The central generatrix g of said cylindrical surface is contained in the plane π of the meridian of the place and is, by construction, normal to the plane of the local astronomical horizon. According to Figure 17b, if we projected this central generatrix in the South direction on a plane TΓ 'normal to the horizon, the resulting line would remain contained in the meridian of the place. Let's see again how the orientation movement of the heliostat object of the invention affects this projection of its central generatrix and its consequences from an optical point of view. Due to the nature of its pseudo-horizontal mechanical mount, the heliostat is oriented in the Euclidean space by means of two rotation mechanisms: a) the azimuthal rotation mechanism Ω _ac ¡ _m . which runs around an axis orthogonal to the plane of the horizon, called the primary axis (3) and whose orientation coincides with that of the central generatrix of the heliostat; and b) the zenith mechanism Ω _cβn , which runs around an axis parallel to the horizon plane, called the secondary axis (5) and which coincides in this case with the horizontal support arm of the heliostat. Suppose the Sun is now somewhere on the horizon plane, so that the heliostat - obeying the laws of Geometric Optics - needs to be oriented in this coordinate system to reflect and maintain the image of the Sun on a fixed solar receiver. , with an elongated and vertical shape, located on top of a tower. As with the conventional model, the proposed heliostat must necessarily achieve the proper orientation by executing two well-defined turns. The first of them, according to figure 18b, is the azimuth turn

whose angle of rotation we assume a magnitude w _az . It also follows from the figure that the orientation of the primary axis is not altered with the rotation; also, due to the special mechanical relationship between both axes, the secondary axis does not alter its initial orientation either as a result of the first Ω _ac i _{m turn} - The family of solar rays reflected in the central generatrix of the reflective surface of the heliostat is going to project on the base of the tower, but with all its rays still contained in the plane π of the meridian of the place. Since the solar receiver is on top of the tower, the heliostat should now execute the spin that is still missing to raise the image to the desired point. Figure 19b It shows the zenith rotation Ω _cβn of magnitude w _Cθn that must be executed around the secondary axis to reach the objective. The essential question is now: in what position is the central generatrix of the heliostat after this second turn Ω _cβn ? Since the orientation of the secondary axis has not been altered, and the cylindrical surface is now rotated around it, the central generatrix tilts with respect to the horizon, but remains fully contained in the plane π of the meridian of the place. From an optical point of view, the result is that the family of solar rays reflected in the central generatrix of the cylindrical reflective surface will be projected on the solar receiver at the top of the tower, but with all its rays contained in the plane of the meridian of the place. In summary, the geometric result of the heliostat aiming procedure has been to confine the central generatrix of the reflective cylindrical surface and its projection on the target on the meridian of the site. consequently, the focal line of the optical system remains vertical over it for any solar position. That is, the image given by the heliostat, adapts to the solar receiver both in shape (elongated) and orientation (vertical) for any time of the day. The consequences on the distribution of solar irradiance on the linear receiver, for an arbitrary position of the Sun, are shown in Figures 5a, 5b and 5c through the analysis of the energy isoflow lines. A series of alternative embodiments that allow adapting the design to the specific technical and economic conditions of each specific embodiment will be evident to a person skilled in the art. Thus, for example, in the description of a preferred embodiment, shown in Figure 9, the mobile support (2) is in the form of a "fork" but it is evident that the same practical result can be obtained with any other mounting of axle fasteners primary and secondary, including the reflecting surface hung in a "clapper" type arrangement, such as that shown in figure 10. Similarly, the expert knows the relative movement of the earth and the sun throughout the year, so which may determine the performance of the control device (8) to ensure that the reflected image of the sun strikes at all times on the linear target (11), throughout each day and for every day of the year.

Claims

1. Linear focus heliostat of the type used to form heliostat fields in central receiver solar thermal plants consisting of a linear target (1 1) fixed on a tower (26), characterized by understanding; a reflective surface (1), cylindrical or parabolic trough, capable of rotating relative to a primary axis (3), coinciding with the central generatrix of the reflecting surface (1). a mobile support (2), capable of rotating with respect to a secondary axis (5) perpendicular to the primary axis (3), the primary axis (3) being integral with the mobile support (2), and the secondary axis (5) being arranged perpendicular to a main image plane (10), motionless in space, containing at all times the linear target (11) and the primary axis (3), a primary drive (4) to rotate the reflective surface (1) around the primary shaft (3), a secondary drive (6) to rotate the mobile support (2) around the secondary shaft (5), a control device (8) to govern the primary drive (4) and the secondary drive (6 ).

2. Linear focus heliostat according to claim 1, characterized in that the linear blank (1 1) is vertical.

3. Method of operation of the heliostat of claims 1 to 2 characterized by comprising the following maneuvers; movement of the primary drive (4) for, taking into account that the positions of the sun (12) and the linear target (11) are predefined, bring the optical axis (25) of the reflecting surface (1) a lateral angle γ on a bisector plane (21) of the main image plane (10) and of the main object plane (20) defined as those containing the primary axis (3) and the reflected main beam (23) or the incident main beam (22) respectively, movement of the secondary drive (6) for, taking into account that the positions of the sun (12) and the linear target (11) are predefined, vary the elevation angle β of the mobile support (2), in order to take the optical axis (25) of the reflective surface (1) on the reflection plane (24), defined as the plane containing the main reflected ray (23) and the main incident ray (22);

in such a way that it is ensured that the reflected radiant energy affects the linear target (11) at all times, throughout each day and for every day of the year.