FR2983571A1 - HELIOSTAT HAS TWO MIRRORS - Google Patents

Abstract

In order to reduce the cost of heliostats which are used in solar concentrating devices, the present invention describes a two-mirror heliostat (1A, 1B) disposed at the end of a main axis of rotation 6 which is oriented North / South and parallel to the axis of rotation of the Earth. This heliostat reflects solar radiation to two concentrators that may be Fresnel lenses (11A) or fixed mirrors (11B). A plurality of heliostats according to this invention can then be rotated simultaneously with simplified mechanics and a sun tracking method using a bi-directional optical sensor and control logic that does not require any calculation or memorization of the solar positions.

Description

The present invention relates to solar concentrators and more particularly to those using heliostats for concentrating solar radiation on a fixed target, the latter being for example a thermal sensor for the production of heat energy and / or a photovoltaic sensor for the production of electrical energy and / or a sensor with chemical reactions for the production of hydrogen.

STATE OF THE ART Most heliostatic solar collectors use mirrors that rotate about two axes of rotation according to the changing position of the sun in order to redirect solar radiation permanently towards a fixed target. The combination of all solar rays from heliostats to a single target causes a concentration of solar energy on this target. The target is very often in height we speak of solar tower. This solar concentration is used for different applications such as raising the temperature of a liquid and possibly boiling it to produce distilled water or operating a heat engine, an air motor, an engine Stirling type, a thermodynamic generator, or illuminate a photovoltaic solar panel to generate electrical power. The cost of installing a solar tower is often proportional to the cost of heliostats because they are numerous, from a few tens to hundreds or even thousands, and these heliostats contain by necessity two motors for the pivoting of the mirror to follow the course of the sun along two axes of rotation, one to follow its diurnal motion and the other to follow its seasonal movement. A solar field containing N heliostats will then require 2 x N motors. In order to reduce the cost of a heliostatic field, it is theoretically advantageous to reduce the number of motors by positioning these heliostats relative to one another so that their respective axes of rotation are all parallel and that their respective axes are relatively stable. 8 3 5 7 1 2 rotational speeds are all identical. Two motors, one for each axis of rotation, will then be sufficient to rotate all the heliostats. But the technical solution to this problem of reducing the number of engines has never been found in a complete way. Only a few attempts have made it possible to reduce the number of motors in a heliostatic field, such as patent US20050034752A1 which describes the coupling of the reflectors along their vertical axis to halve the number of motors, or the patent US20060060188A1 which describes a network of reflectors that can move towards a target by means of a plate whose displacement rotates an axis integral with the mirrors. But this last device requires a particular dimensioning and positioning of all the components, especially according to a surface of Nicomedes Conchoid type of revolution difficult to achieve. US4261335A1 uses heliostats groups controlled by a single motor, but the mirrors are concave with a specific curvature difficult to achieve. Also known is a particular device described in US5787878A1 which allows to set in motion, with a single motor, a plurality of heliostats, which reduces the number of engines and therefore the overall cost of an installation. But the mechanical part associated with these heliostats is still complex and the power of the engine that is needed must be important. OBJECT OF THE INVENTION The main object of the invention is to overcome the drawbacks of known devices, and to describe a heliostat which utilizes the particular properties of the axes of rotation which are parallel to the axis of rotation of the Earth. will allow in particular to couple them together using a simplified mechanical device and thus reduce the overall cost of the installation compared to solar concentrators known in the state of the art. Another object of the invention is to make it possible, thanks to this innovative heliostat, to use at the same time several types of solar concentration, which will make it possible to respond in a simple manner to different needs.

SUMMARY OF THE INVENTION The present invention describes a particular positioning and movement of the heliostats with respect to the target, with the characteristic that only one or two motors will be sufficient to properly orient a whole row of heliostats and to concentrate the solar radiation reflected by each of them towards the target. This reduction in the number of engines will then cause the reduction of the cost of the heliostats, proportionally to their number, and therefore result in a reduction in the overall cost of the solar concentrator. To arrive at this innovative result, the present invention solves a mechanical and optical problem still unsolved to date and which finds a solution in a configuration where all the respective axes of rotation of the heliostats are parallel to each other. The rotation of all these axes can then be performed globally by a simplified mechanical control. The invention therefore relates to a heliostat comprising a first axis of rotation, a first mirror and a second mirror, a second axis of rotation and a third axis of rotation, characterized in that the first axis of rotation is parallel to the axis of rotation of the Earth, that the first mirror pivots about the second axis of rotation which is integral and perpendicular to the first axis of rotation, that the second mirror pivots about the third axis of rotation which is integral and perpendicular to the first axis of rotation , that the second and third axes of rotation are parallel to each other, and that the two mirrors are oriented around the three axes of rotation so that the incident solar radiation which is reflected on the two mirrors are oriented along the first axis of rotation on the one hand towards the South and on the other hand towards the North. Advantageously, a Fresnel lens or a secondary mirror is positioned in the path of the rays reflected by the first mirror and / or by the second mirror, which makes it possible to concentrate the radiation reflected by the two mirrors towards one or more targets.

These targets may consist of one for several elements taken together comprising a photovoltaic panel composed for example of crystalline and / or amorphous silicon or photosensitive multilayers, a Stirling motor, a thermal sensor with or without a circuit of a liquid. coolant, a chemical reactor, a hydrogen catalyst, a seawater evaporator, and a solar cooker. The use of a Fresnel lens makes it possible to focus the light in a point or rectilinear manner, that is to say that the focal length of the lens constitutes a point or a line. The fixed secondary mirror may be planar or concave.

According to the invention, the first axis of rotation of the heliostat is composed of two hollow tubes, both of which are arranged in the same extension, each hollow tube being provided with an opening in the form of an elongated slot practiced along its axis, and each of these hollow tubes being integral at one of its ends with a second respective axis of rotation, and being integral with the other of its ends respectively with a first and a second gear wheel perpendicular to the axis of the two hollow tubes. The interior of the two hollow tubes is traversed by a threaded rod coaxial with the two hollow tubes and which comprises at its center a third gear wheel perpendicular to the threaded rod and on either side of which are positioned two nuts adapted to move along the threaded rod, said nuts each being provided with a flange which extends through the opening in the hollow tubes so as to prevent the rotation of the nuts and cause them to move along the threaded rod when the latter is rotated relative to the hollow tubes, said flanges being each connected to the back of one of the two mirrors by a rigid link so that the displacement of the nuts along the threaded rod causes the rotation of the mirrors around their second axis of rotation. According to an alternative embodiment of the rotational actuation of the hollow tubes, the two integral toothed wheels of the hollow tubes are connected by a coupling device which allows them to be rotated identically by means of a first worm, and the toothed wheel which is integral with the threaded rod is itself rotated by a second worm. The first and second worm are rotated by a selective gear which consists of a motor driven main gear driven by a motor and two driven secondary gear wheels each of which is secured to one of the two worm. The main driving gear wheel is adapted to move either to couple to the first worm, or to couple to the second worm, or to couple to the two worm at the same time. The main gear of the selective gearing enables rotation of the first and / or second secondary gearwheel so that the difference in the number of rotational speeds of a secondary gear with respect to the other secondary gear causes either advance or recoil or immobility of the nuts on the threaded rod. Of course, the main gear of the selective gear is rotated by a motor coupled to the selective gear. This motor can be of different types, for example a DC or AC electric motor or a stepper motor, or a motor using potential energy, gravitational or mechanical.

In another embodiment, the worms are not coupled to a selective gear, but are each controlled by an independent motor. In another embodiment of the actuation of the mirrors around their secondary axis, the second and third rotation axes integral in rotation with their respective mirror are rotated by means of a motor or electromagnet positioned directly at the rods so as to lengthen or shorten their lengths, said motor or electromagnet being powered by a battery or a super capacitor electrically charged by a photovoltaic cell, and the starting instructions of the motor or electromagnets being transmitted by remote remotes that use airwaves. According to a particularly advantageous embodiment of the heliostat according to the invention, and independently of the variant chosen for driving the mirrors around their two axes of rotation, the heliostat comprises a bidirectional optical sensor capable of measuring a the amount of light received from the sun and the amount of light reflected to a target and control logic able to act on the positioning of each mirror around each axis of rotation in order to optimize the amount of light received from the sun by the bidirectional optical sensor and the amount of light reflected by the mirror towards the target. Such a bidirectional sensor can be fixed on the moving element of one of the mirrors.

According to the invention, the bidirectional optical sensor comprises: a first planar photovoltaic cell, active on its two faces, and positioned on the heliostat so that the plane which includes said first cell also includes the first axis of rotation; a second planar photovoltaic cell, active on its two faces, and positioned on the heliostat so that the plane which includes said second cell is both parallel to the first axis of rotation and perpendicular to the plane of said first cell. Ideally, the two photovoltaic cells are arranged on the bidirectional optical sensor so that the first photovoltaic cell can receive only the light reflected by a mirror and not the direct light of the sun, and so that the second photovoltaic cell can receive only the direct sunlight and not the light reflected by a mirror. The first and second photovoltaic cells of the bidirectional optical sensor are integral with the first axis of rotation of the equipped mirror, the position of the first cell being adjusted so that the brightness on each of its two faces is identical when the mirror carrying the bidirectional optical sensor is positioned so that its axis perpendicular to the plane of the mirror is aligned with the sun, and the position of the second cell is set so that the brightness on each of its two faces is identical when the mirror carrying the bidirectional optical sensor is positioned around its second axis of rotation so that the rays reflected by the mirror are parallel to the first axis of rotation and directed towards the target. Adjusting the position of the first and second photovoltaic cells of the bidirectional optical sensor is performed by said control logic controlling the movement of the motors operating the mirrors around their first and second axes of rotation. The bidirectional optical sensor is electrically connected to a comparator electronic circuit which compares the brightness received on each of the two faces of the two cells and which transmits the result of the comparison to the control logic connected to the motors actuating the mirrors. The two mirrors are driven around their first common axis of rotation from East to West, this training being performed at a substantially constant speed of one revolution per day in normal operation, and at a speed slightly greater or slightly less than this speed. during a set period of time, under the control of said control logic. To optimize the positioning of the mirrors according to the radiation received from the sun and the radiation reflected towards the target, the control logic is configured to accelerate or slow down the rotation of the mirrors around each axis of rotation for a set period of time, until the positioning of the bidirectional optical sensor, and therefore the positioning of the mirrors which are integrally related to it, is optimized.

In an advantageous embodiment, this control logic is configured to execute a program comprising the following steps: substantially adjusting the position of the mirror carrying the bidirectional optical sensor along its two axes of rotation so that the solar radiation received by said mirror be reflected in the extension of the first axis of rotation and towards the target; using the bidirectional optical sensor, measure the brightness of the sun and repeat this measurement until the measured brightness is greater than a predetermined threshold of minimum brightness; when the measured brightness is greater than the minimum brightness threshold, testing the orientation of said mirror according to the signal delivered by the bidirectional optical sensor; if the signal delivered by the bidirectional optical sensor indicates a positioning error of said mirror along the first axis of rotation or the second axis of rotation, temporarily modify the speed of rotation of said mirror about said axis of rotation, then repeat the test of orienting said mirror until the mirror orientation test indicates the absence of a positioning error of the mirror around each of its axes of rotation.

The invention provides that this program may be executed, but not necessarily, only after a stability test of solar irradiance, indicating that the brightness received by the mirror is greater than the threshold of minimum brightness for a period of time greater than a predetermined time delay.

The invention also relates to a device or a field of heliostats, comprising a plurality of heliostats as described above, and characterized in that said heliostats are aligned along an East / West axis and that the three axes of rotation different heliostats are respectively parallel to each other and in that at least one mechanical connection connects the respective axes to each other so that all the rotations of the mirrors are identical and at the same time. In order to exploit the invention for applications in solar concentration, at least four particular devices are described below: 1 - the two-way heliostat according to the invention redirects the solar radiation to a Fresnel lens arranged in the extension the first axis of rotation, either on the South side or on the North side, or a Fresnel lens arranged on each side, and so as to concentrate the solar energy on one or two targets. 2 - the two-way heliostat according to the invention redirects the solar radiation to a fixed secondary mirror disposed in the extension of the first axis of rotation, either on the south side or on the north side, or a secondary mirror disposed on each side, and so that this or these secondary mirrors redirect solar radiation received from the heliostat to one or two targets. 3 - the two-way heliostat according to the invention redirects solar radiation, from one side to a Fresnel lens and the other side to a fixed secondary mirror. 4 - In order to create on a target a solar concentration effect with fixed secondary mirrors, a plurality of heliostats are aligned, preferably along an East / West axis, and so that all the corresponding rotation axes of the different heliostats are respectively parallel to each other. All the first axes or main axes are connected together by a first mechanical collective training device that allows their simultaneous rotation and identical. All second and third axes are also provided with a second mechanical collective training device that allows their simultaneous rotation and identical. The concentration effect is obtained by the superposition on the target of all the solar radiation reflected by the secondary mirrors. Thanks to the characteristic of the heliostat to be able to direct the solar radiation in two directions, the plurality of heliostats can use at the same time secondary mirrors and Fresnel lenses to meet two different applications, for example the high concentration solar on small surfaces for multi-junction photovoltaic applications, and the average solar concentration on large areas for heating in the home.

In a particular embodiment, the secondary mirrors are concave, which allows a pre-focusing of the beams of light that are directed towards the target. DETAILED DESCRIPTION OF THE INVENTION The invention is now described in more detail with the aid of the description of the indexed FIGS. 1 to 8. - Figure 1 is a block diagram in elevation and in section of a heliostat according to the invention. FIG. 2A illustrates a particular application of the heliostat of FIG. 1, with a secondary mirror and a Fresnel lens. - Figure 2B illustrates a variant of the heliostat of Figure 1, associated with two Fresnel lenses and a target. FIG. 3 illustrates a solar field comprising a row of heliostats according to FIGS. 1 and 2 and controlled by a single motor. FIG. 4 is a diagram in elevation and in section of the mechanical part of rotation of the two mirrors of the heliostat of FIG. 1. FIGS. 5A, 5B and 5C illustrate the three possible positions of the clutch selector FIG. 6 is an elevation and sectional diagram of a two-mirror heliostat according to FIG. 1, to which an optical solar position detector has been added. FIG. 7 is a perspective diagram of the solar position detector used on the heliostat of FIG. 6. FIG. 8 represents a flowchart of an algorithm implemented by the logic of FIG. position control control of the heliostat of FIG. 6. Referring to FIG. 1, which represents a heliostat which redirects the solar radiation 4A and 4B along the axis of rotation of the Earth 7 by means of two plane mirrors 1A and 1B and three rotational axes 6, 2A and 2B. The first axis of rotation 6 is oriented North / South and inclined at an angle h relative to the horizontal. This angle h equals the latitude of the geographic location of the facility. More precisely, this axis of rotation 6 is parallel to the axis of rotation 10 of the earth 7 and is thus directed substantially towards the pole star 10. The second axis of rotation 2A and the third axis of rotation 2B are perpendicular to the first axis rotation 6, and integral with the latter. These second and third rotational axes 2A, 2B are preferably parallel to each other and arranged at each end of the first axis of rotation 6. These second and third axes of rotation allow each of them to rotate a respective plane mirror 1A and 1B , with a rotation of plus or minus 12 degrees of angle. The first axis of rotation 6, it allows to rotate these two mirrors lA and 1B following a complete turn, or 360 degrees. The first mirror 1A and the second mirror 1B are adjusted so that the incident solar rays 4A and 4B are reflected in the same direction as the first axis of rotation 6. The first mirror 1A redirects the solar radiation 4A in the direction opposite to that of the polar star 10, so to the south, while the second mirror 1B redirects the solar radiation 4B to the polar star 10 so to the north. It can then be shown that the reflected solar beams 5A, 5B maintain their orientation 7 during the daily movement of the sun simply by rotating the first axis of rotation 6 with a constant speed of one revolution per day, which corresponds to the speed of rotation. apparent rotation of the sun around the Earth. To correct the seasonal shift of the sun, which is 47 ° per year, that is to say, plus or minus 23.5 ° above and below its position at the equinoxes, the second and third axis of rotation should rotate the first and second mirrors at an angle of plus or minus 11.75 ° (half of 23.5 °) around an initial position that has been set for the equinoxes. The rotation is 11.75 ° instead of 23.5 ° because indeed the total deflection of a ray by reflection on a mirror is twice its angle of incidence measured relative to the perpendicular of the mirror. The rotational speed of the second and third axes is not constant, it follows a sinusoidal progression which is known to those skilled in the art. The three axes of rotation 6, 2A, 2B are rotated to follow the sun by means of a driving device comprising toothed wheels 8A, 8B, 9 and worms, pulleys or threaded rods, as well as one or two wired or remotely controlled motors. A detailed description of this drive device is given in connection with FIG. 4. According to a particular embodiment of the invention corresponding to FIG. 2A, the heliostat according to the invention redirects the solar radiation 4A towards a lens of FIG. Fresnel 11A positioned in the extension 7 of the first axis of rotation 6, either on the south side or on the north side, or a Fresnel lens arranged on each side (not shown), or again towards one or more Fresnel lenses arranged between the two mirrors (variant shown in Figure 2B), and so as to concentrate the solar energy on a target 12, 12A. According to another particular embodiment, the heliostat according to the invention redirects the solar radiation 4B to a fixed secondary mirror 11B disposed in the extension 7 of the first axis of rotation 6, either on the South side, or on the North side, or on a fixed secondary mirror disposed on each side (not shown) and so that this or these secondary mirrors 11B redirect solar radiation 5B received from the heliostat to a target 12B.

According to another particular embodiment, the heliostat according to the invention redirects the solar radiation, from one side to a Fresnel lens 11A and the other side to a fixed secondary mirror 11B. As shown in FIG. 3, in order to create on a target 12B a solar concentration effect with fixed secondary mirrors 11Bx, a plurality of heliostats H1, H2, H3 ... H6 are aligned, preferably along an East / West axis, and so that all the respective axes of rotation of the mirrors are parallel to each other. Thus, yous the first axes 6 are connected together by a mechanical collective drive device 15 which allows their simultaneous rotation and identical. All second and third axes are also provided with a mechanical collective drive device 14 which allows their simultaneous rotation and identical. The concentration effect is obtained by the superposition on the target 12B of all the solar radiation reflected 13x by the secondary mirrors 11Bx. Thanks to the characteristic of the heliostat to be able to direct the solar radiation in both directions (North and South), the plurality of heliostats can use at the same time secondary mirrors 11Bx and Fresnel lenses 11Ax to meet two needs different, for example the high solar concentration on small areas 12A for multi-junction photovoltaic and the average solar concentration on large areas 12B for heating in the habitat. In another particular embodiment, the secondary mirrors 11Bx are concave, which allows a pre-focusing of the light beams 13x which are directed towards the target 12B.

We will now describe in more detail in relation to Figure 4, a device for driving mirrors around their different axes of rotation, in particular the secondary axes 2A, 2B. The first axis of rotation 6 or main axis consists of two hollow tubes 6A 6B, one and the other arranged in the same extension of the axis Earth - polar star 7, and each of them 6A, 6B presents an opening 16A, 16B in the form of a longitudinal slot 16A, 16B made along its axis, and each of these tubes 6A, 6B is respectively secured at one of its ends with a second axis of rotation 2A and a third axis of rotation 2B , and at the other of its ends with a first toothed wheel 8A and a second 8B gearwheel perpendicular to their axis 7. The interior of the two tubes 6A, 6B is traversed by a single threaded rod 19 which comprises at its center a third gearwheel 9 perpendicular and on either side of which is screwed on the threaded rod 19, a nut 17A, 17B, a portion 18A, 18B, preferably flat, extends in the form of a flange through the opening 16A, 16B made in the hollow tube 6A, 6B. This makes it possible to prohibit the rotation of the nut 17A, 17B with respect to the corresponding hollow tube 6A, 6B, and to cause the displacement of each nut along the threaded rod 19 when the latter is rotating relative to the hollow tubes. . The flange 18A, 18B of the nut 17A, 17B is connected to the back of the corresponding mirror 1A, 1B by a rigid link 3A, 3B so that the displacement of the nut 17A, 17B sliding along the slot 16A, 16B causes the mirror 1A, 1B to pivot about its axis of rotation 2A, 2B. The two toothed wheels 8A, 8B integral with the hollow tubes 6A, 6B are connected by a coupling device 20 which allows their identical rotation by a first worm 15. The toothed wheel of the middle 9 which is secured the threaded rod 19 is itself rotated by a second worm screw 14. The two toothed wheels 8A, 8B integral with the hollow tubes 6A, 6B and the third toothed wheel 9 integral with the threaded rod 19 have their axes of rotation. concentric rotation and coinciding with the axis 7, so that when their rotation speeds are identical the nuts 17A, 17B remain fixed and the mirrors 1A, 1B rotate only about their first axis of rotation 6 which is the one which is parallel to the axis of rotation of the Earth 7 and which corresponds to the hourly movement of the sun. When the third gear 9 rotates faster or slower than the other two gears 8A, 8B, the nuts 17A, 17B move respectively in one direction or the other and cause the mirrors 1A, 1B to pivot in one direction or in the other following respectively their second 2A and their third axis of rotation 2B, this to follow the movement of the sun in its movement in height, that is to say, its seasonal movement. The first toothed wheel 8A and the second toothed wheel 8B on the one hand, and the third toothed wheel 9 on the other hand, are independently rotated by means of the first worm 15 and the second worm 14. two worms 14 are rotated for example by a selective gear 21 (FIGS. 5A, 5B, 5C). This selective gear 21 consists of a main gearwheel 22 actuated by a motor (not shown) and two gearwheels R1 R2, each of them being integral with one of the two worms 14, 15 so that said main wheel 22 can move either to couple to the first worm 15, or to couple to the second worm 14, or to couple to the two worm 14 at the same time. The driving main ground wheel 22 as well as the two other gear wheels R1, R2 of the gearwheel always rotate in the same direction but a control logic orders the coupling and therefore the rotation of a wheel R1 and / or the other wheel R2 so that the first axis of rotation 6 follows well the continuous displacement of the sun during the day, and so that the difference in the number of revolutions of a wheel R1 relative to to the other R2 provokes either the advance or the retreat of the nuts 17A, 17B and therefore the tracking of the sun in its height.

The difference in the number of laps traveled between the two wheels R1, R2 is either positive or negative, that is to say that one of the two wheels can get ahead of the other which then leads the advance or the retreat of the nuts 17A, 17B on the threaded rod 19. The nuts 17A 17B can also be held stationary simply by requiring the two wheels R1, R2 to rotate at the same speed.

The motor used to drive the main gear 22 of the selective gear 21 is an electric motor, DC or AC motor or stepper motor, or a motor using potential, gravitational or mechanical energy. In the particular configuration shown in FIG. 3 comprising a plurality of aligned heliostats, a single first worm 15 and a single second worm 14 of great length are necessary, and they connect all the heliostats H1, H2 ... H6 so as to rotate all the mirrors 1Ax, 1Bx by the single control of the selective gear 21. Thus a single motor allows the simultaneous rotation and in an identical manner of all heliostats. The heliostats thus redirect the solar radiation along the axis of rotation of the Earth 7, in both directions (North and South) with a constant rotation speed equal to one turn per day, ie one turn in 23h56mn while the second and third axes of rotation 2A, 2B are used to follow the sun in its seasonal movement, 12 degrees above and 12 degrees below the celestial equator, which corresponds to an average rotation of 4 degrees per month. Thus, the selective gearing 21 operates most of the time with equal rotational speeds for the two wheels R1, R2 (FIG. 5C) and only during very short adjustment periods with different speeds (FIGS. 5A and 5B). This characteristic makes it possible to significantly reduce the operating mode changes of the selective gearing 21 and thus reduce the amount of wear and energy expended for this operation.

A variant (not illustrated) embodiment is to actuate the second and third axes of rotation 2A, 2B of each heliostat locally, without remote mechanical transmission so without second worm 14 and without central gear 9, but by positioning a motor or an electromagnet directly at the level of the 3A, 3B bielletes so as to lengthen or shorten their length, which will cause the desired tilting of the mirrors 1A, 1B around the second and third axes of rotation 2A, 2B. This is made possible for example by storing the electricity supplied by a photovoltaic cell in a battery or a super capacitor attached to each heliostat, then using this electrical energy to actuate the engine of a cylinder coupled to each rod 3A, 3B. As the need to cause this movement is very punctual (1 degree per week), the motor may also be made up of two electromagnets which, under the effect of a few electrical impulses, will move the cylinder either in one direction or in the other direction. Instructions for starting the engine or electromagnets can be preferably remote remotes that will use for example radio waves. In another particular embodiment, the selective gearing 21 is replaced by two motors, each of them being coupled to one of the two worms 14, 15 and the rotation speeds are adapted to allow the tracking of the run of the sun.

FIG. 6 shows the location of the optical solar position detector 23 which makes it possible to orient accurately the heliostat according to the invention without the need for a computer or electronics which would contain or memorize solar positions. This solar position optical detector 23 is fixed on the first axis of rotation 6 and is placed in the path of the reflected rays, for example on the path 5A South side. FIG. 7 shows in detail an embodiment of the solar position detector 23. This is a bi-derectional optical sensor comprising a first plane photovoltaic cell C1, active on its two faces, and positioned so that the plane which includes the cell C1 also includes the first axis of rotation 6, and having a second photovoltaic cell C2, active on both its faces, and positioned so that the plane which includes the cell C2 is both parallel to the first axis of rotation 6 and perpendicular to the plane of the cell C1. The cells C1 and C2 measure the brightness or quantity of light that is received on each of their two faces. Cells Cl and C2 being integral with the first axis of rotation 6 they rotate with the latter by following the sun in its clockwise movement from East to West. The cell C1 is set so that the brightness on each of its two faces is identical when the mirror 1A is correctly positioned around its first axis of rotation 6 and faces the sun. The cell C2 is set so that the brightness on each of its two faces is identical when the mirror LA is positioned correctly around its second axis of rotation 6 and the reflected rays of the sun 5A are parallel to the first axis of rotation 6. The bidirectional optical sensor 23 is electrically connected to an electronic circuit (not illustrated) which it supplies with the photovoltaic current generated by the two cells and which continuously compares the brightness on each of the two faces CiA, C1B and C2A, C2B of the two C1, C2 cells. A variation of brightness of a face with respect to its opposite face then informs on the nature of the offset which has occurred with respect to the ideal position of the mirror 1A, this information is then transmitted to a logic circuit which will correct this offset. by controlling the speed of rotation of the first axis 6 and / or by controlling the inclination of the mirror 1A with respect to its second axis of rotation 2A. The corrections made to the positioning of the first mirror 1A are automatically reflected in the second mirror 1B by the set of mechanical transmissions described in connection with FIG. 5. The logic that is used to decide the positional corrections to be made to the heliostat is shown schematically in FIG. the flowchart of the algorithm of FIG. 8. After the mirrors 1A, 1B have been adjusted along their three axes of rotation 6.2A, 2B so that the solar radiation is well reflected in the extension 7 of the first axis of rotation 6, one imposes on the first axis 6 a rotation of East to West of a turn in 23H56mn in order to follow the sun in its hourly movement. Then we measure the brightness L of the sun. If this brightness is below a minimum threshold LS then the measurement is repeated until the brightness of the sun exceeds this value LS. This LS value is given for example by a photovoltaic cell or by a pyranometer. This LS value can be expressed in Lux, or in number of milliamperes produced by a photovoltaic cell. As soon as the brightness L of the sun is greater than or equal to LS, then a timer T is triggered while continuing the measurements of L. If a period of time TS, for example of 2 minutes, flows with a luminosity L> LS, this means that the brightness of the sun is sufficiently stable to allow comparative measurements on each of the faces of the two cells C1 and C2. If the brightness is different between each face of the cell C1 (CiA and C1B) it means that there has been a drift in the tracking of the sun in its hourly movement, so a set point is given by the logic circuit is to increase the rotational speed of the first axis of rotation 6 if CiA is greater than C1B, or to decrease the rotational speed of the first axis of rotation 6 if CiA is less than C1B. A reiteration of the comparative measurement is started until the values of CiA and C1B are equal. A measurement of the brightness of the two faces of cell C2 (C2A and C2B) is then similarly initiated. If the brightness is different between C2A and C2B, this means that there has been a drift in the tracking of the sun in its seasonal movement, so a set point is given by the logic circuit to increase the inclination of the mirror lA following its second axis of rotation 2A if the brightness on C2A is greater than that on C2B, or to decrease the inclination of the mirror 1A according to its second axis of rotation 2A if the brightness on C2A is lower than that on C2B.

As soon as the brightness values on C2A and C2B are equal, the clock T is reset (T = 0) and a new measurement loop is started on the brightness L. This basic logic can be supplemented by additional tests. to further secure this correction protocol, such as a test on the number of corrections made per minute. If this number is too large it means that there is an abnormal instability of the system, so an alarm can be generated. The correction which is made on the first mirror 1A by the solar position detector 23 could be made in the same way for the second mirror 1B, which would require a second solar position detector placed on the second mirror 1B. North side of the first axis of rotation 6. But it can be shown that an angular correction on the first mirror 1A requires the same angular correction on the second mirror 1B for reasons of symmetry. Thus along the threaded rod 19, the displacement of the first nut 17A in one direction will require the displacement of the second nut 17B also in the same direction. What is already provided in the drive device 21 described above. A concrete example of embodiment is now described. A solar concentrator located at 35 ° N latitude consists of a row of ten heliostats aligned in an East / West direction. Each heliostat is composed of two rectangular flat mirrors A1, A2 1 x 1.50 m each fixed at the end of a first axis of rotation 6 oriented North / South and inclined by 35 ° with respect to the horizon 15 North this axis 6 is integral at its ends with a second axis of rotation 2A and a third axis of rotation 2B which are perpendicular thereto. The first axis of rotation 6 is fixed to the ground at a height sufficient for the mirrors A1, A2 can make a complete turn without encountering obstacles or touching the ground. The second axis of rotation 2A and the third axis of rotation 2B can rotate the two mirrors 1A, 1B by plus or minus 12 °. A rectangular plane mirror 11B, rectangular 1 x 1.20 m is attached to the end of a vertical mast so that its center is on the extension of the first axis of rotation 6. This secondary mirror 11B is oriented definitively so the solar radiation 5B it receives from the second mirror 1B is directed to a target 12B. Target 12B is a 1 x 1 m square solar photovoltaic panel made of crystalline silicon cells. The target 12B is positioned south of the heliostats and at a height of about 3 m so as to minimize reflection angles of the reflected solar radiation 13 and thus minimize the size of said mirrors 11B. The other mirror 1A reorients the solar radiation 4A towards a square Fresnel lens of 1 meter of sides 30 placed on the extension of the first axis 6. This Fresnel lens allows solar concentration on a small multi-layer photovoltaic cell thin 3 cm of sides which therefore receives concentrated solar radiation by a factor of 500.

The first axis of rotation 6 is 30 mm in diameter and contains a threaded rod 19 and two nuts 17A, 17B which move along the rod 19 to rotate the two mirrors along their second 2A and third 2B axes of rotation. A mechanism contains 3 toothed wheels of 100 teeth integral with the three axes of rotation. Two worm 14,15 coupled to a selective gear 21 allow the rotation of each axis to follow the sun in its diurnal motion and seasonal movement. A single rotary electric motor of 10 watts of power is sufficient to control the two mirrors 1A, 1B of the heliostat. The control logic receives information from a bidirectional optical sensor 23 which provides information on the exact relative position of the sun as soon as the brightness corresponds to a clear sky and that this brightness is stable for at least 2 minutes. If necessary a position correction is made by rotating the worms 14 and 15 with different speeds. This heliostat is duplicated 30 times and the 30 heliostats are aligned along an East / West axis so that all the respective axes of rotation are parallel to each other. The motor is a 300 Watts motor. Thirty small photovoltaic cells 12A under concentration of a factor 500 will produce about 200 Watts Crete each, a total of 6 kWp. All thirty secondary mirrors will redirect the light on the photovoltaic panel of a square meter that will produce about 3 kWp peak power. This solar electric generator with a power of about 9 kWp will ultimately be controlled by a simple mechanics and a single motor. BENEFITS OF THE INVENTION Finally, the invention makes it possible to control a plurality of heliostats with a single motor and a simplified mechanism. This invention is therefore particularly suitable for producing solar concentration at a lower cost.

Claims (26)

REVENDICATIONS1. Heliostat comprising a first axis of rotation (6), a first mirror (1A) and a second mirror (1B), a second axis of rotation (2A) and a third axis of rotation (2B), characterized in that the first axis of rotation (6) is parallel to the axis of rotation of the Earth, that the first mirror (1A) pivots about the second axis of rotation (2A) which is integral and perpendicular to the first axis of rotation (6), that the second mirror (1B) pivots around the third axis of rotation (2B) which is integral and perpendicular to the first axis of rotation (6), that the second (2A) and third (2B) axes of rotation are parallel to each other, and the two mirrors (1A, 1B) are oriented around the three axes of rotation (6,2A, 2B) so that the incident solar radiation (4A, 4B) which is reflected (5A 5B) on the two mirrors (1A, 1B ) are oriented along the first axis of rotation (7.6) on the one hand towards the South (5A) and on the other hand towards the North (5B). 2. Heliostat according to claim 1, characterized in that a Fresnel lens (11A) or a secondary mirror (11B) is positioned in the path of the reflected rays (5A, 5B) by the first mirror (1A) and / or by the second mirror (1B). 3. Heliostat according to any one of the preceding claims, characterized in that the rays reflected by the mirrors (1A, 1B) are oriented towards a target (12; 12A, 12B). 4. Heliostat according to claim 3, characterized in that said target is constituted by one for several elements taken from the set comprising a photovoltaic panel, a Stirling engine, a thermal sensor with or without a heat transfer liquid circuit, a chemical reactor, a hydrogen catalyst, a seawater evaporator, and a solar cooker. 5. Heliostat according to claim 2, characterized in that the Fresnel lens (11A) concentrates the light in a point or rectilinear manner, and / or that the secondary mirror (11B) is concave. . 6. Heliostat according to any one of claims 1 to 5, characterized in that the first axis of rotation (6) is composed of two hollow tubes (6A, 6B) one 5 and the other arranged in the same extension (7), each hollow tube (6A, 6B) being provided with an elongated slot-shaped opening (16A, 16B) along its axis, and each of said hollow tubes (6A, 6B) being integral with a of its ends of a respective second axis of rotation (2A, 2B), and being secured to the other of its ends respectively by a first (8A) and a second (8B) 10 toothed wheel perpendicular to the axis (7) of the hollow tubes (6A, 6B). 7. Heliostat according to claim 6, characterized in that the interior of the two hollow tubes (6A, 6B) is traversed by a threaded rod (19) coaxial with the two hollow tubes and which comprises at its center a third gear ( 9) 15 perpendicular to the threaded rod and on both sides of which are positioned two nuts (17A, 17B) adapted to move along the threaded rod, said nuts being each provided with a flange (18A, 18B) ) which extends through the opening (16A, 16B) made in the hollow tubes (6A 6B) so as to prevent the rotation of the nuts (17A, 17B) and cause them to move along the rod threaded (19) when the latter is rotated relative to the hollow tubes (6A, 6B), said flanges (18A, 18B) being each connected to the back of one of the two mirrors (1A, 1B) by a rigid link (3A , 3B) so that the displacement of the nuts (17A, 17B) along the threaded rod (19) causes the pivoting mirrors (1A, 1B) about their second axis of rotation (2A, 2B). 25 8. Heliostat according to claim 7, characterized in that the two toothed wheels (8A, 8B) integral with the hollow tubes (6A, 6B) are connected by a coupling device (20) which allows their rotation in the identical manner. by a first worm (15), and the toothed wheel (9) which is integral with the threaded rod (19) is itself rotated by a second worm (14). 9. Heliostat according to claim 8, characterized in that the first (15) and the second (14) worm are rotated by a selective gear (21) which consists of a main gear wheel (22) driven by a motor and two driven secondary gear wheels (R1, R2) each of which is secured to one of the two worms (14, 15). 10. Heliostat according to claim 9, characterized in that said main driving gear (22) is adapted to move either to couple with the first worm (15), or to couple with the second worm (14). ), or be coupled to the two worm (14,15) at the same time. Heliostat according to Claim 10, characterized in that the main gear (22) of the selective gear (21) enables the rotation of the first and / or second secondary gear (R1, R2) so as to the difference in the number of rotational speeds of a secondary wheel (R1) relative to the other secondary wheel (R2) causes either the advance or the recoil or the immobility of the nuts (17A, 17B) on the threaded rod (19). 12. Heliostat according to claim 11, characterized in that the main gear (22) of the selective gear (21) is rotated by a motor coupled to the selective gear. 13. Heliostat according to claim 12, characterized in that the motor used to actuate the main gear (22) of the selective gear (21) is a DC or AC electric motor or a stepper motor, or a engine using potential, gravitational or mechanical energy. 14. Heliostat according to claim 8, characterized in that the worm (14,15) are each controlled by an independent motor. 15. Heliostat according to any one of claims 1 to 7, characterized in that the second (2A) and third (2B) rotational axis integral in rotation with their mirror (1A, 1B) are rotated by an engine or an electromagnet positioned at the links (3A, 3B) so as to lengthen or shorten their lengths, said motor or electromagnet being powered by a battery or a super capacitor electrically charged by a photovoltaic cell, and start-up instructions for the motor or electromagnets being transmitted by remotes remotely using radio waves. 16. Heliostat according to any one of the preceding claims, characterized in that it comprises a bidirectional optical sensor (23) capable of measuring firstly the amount of light received from the sun and secondly the quantity of light reflected to a target (12A, 12B), and a control logic adapted to act on the positioning of each mirror (1A, 1B) around each axis of rotation (6,2A, 2B) to optimize on the one hand the amount of light received from the sun by the bidirectional optical sensor (9) and on the other hand the amount of light reflected by the mirror (1A, 1B) towards the target (12A, 12B). 17. Heliostat according to claim 16, characterized in that the bidirectional optical sensor (23) comprises: a first planar photovoltaic cell (C1), active on both its faces, and positioned on the heliostat so that the plane which includes said first cell (C1) also includes the first axis of rotation (6); a second plane photovoltaic cell (C2), active on both its faces, and positioned on the heliostat so that the plane which includes said second cell (C2) is at the same time parallel to the first axis of rotation (6) and perpendicular in the plane of said first cell (C1). 18. Heliostat according to claim 17, characterized in that the two photovoltaic cells (C1, C2) are arranged on the bidirectional optical sensor (23) so that the first photovoltaic cell (C1) can receive only the light reflected by a mirror (1A, 1B) and not direct sunlight, and so that the second photovoltaic cell (C2) can receive only the direct light of the sun and not the light reflected by a mirror (1A, 1B). 19. Heliostat according to claim 17 or claim 18, characterized in that said first and second photovoltaic cells (C1, C2) of the bidirectional optical sensor (23) are integral with the first axis of rotation (6), the position of the first cell (C1) being set so that the brightness on each of its two faces is identical when the mirror (1A, 1B) carrying the bidirectional optical sensor (23) is positioned so that its axis perpendicular to the plane of the mirror is aligned with the sun , and the position of the second cell (C2) being adjusted so that the brightness on each of its two faces is identical when the mirror (1A, 1B) carrying the bidirectional optical sensor (23) is positioned around its second axis of rotation ( 2A, 2B) so that the reflected rays (13) from the mirror (1A, 1B) are parallel to the first axis of rotation (6) and directed towards the target (12A, 12B). 20. Heliostat according to claim 19, characterized in that the adjustment of the position of the first cell and the second photovoltaic cell (C1, C2) is performed by said control logic slaving the movement of the motors operating the mirrors around their first and second axes of rotation (6,2A, 2B). 21. Heliostat according to claim 19, characterized in that the bidirectional optical sensor (23) is electrically connected to a comparator electronic circuit which compares the brightness received on each of the two faces of the two cells (C1, C2) and which transmits the result comparison with the control logic connected to the motors actuating the mirrors (1A, 1B). 22. Heliostat according to any one of claims 16 to 21, characterized in that the mirrors (1A, 1B) are driven around their first axis of rotation (6) from East to West, this training being done at substantially speed. constant one revolution per day in normal operation, and at a speed slightly greater or slightly less than this speed for a set period of time, under the control of said control logic. 23. Heliostat according to any one of claims 16 to 22, characterized in that to optimize the positioning of the mirrors (1A, 1B) as a function of the radiation received from the sun and the radiation reflected towards the target (12A, 12B), the control logic is configured to accelerate or slow the rotation of the mirrors around each axis of rotation for a set period of time, until the positioning of the bidirectional optical sensor (23) is optimized. 24. Heliostat according to claim 23, characterized in that said control logic is configured to execute a program comprising the following steps: substantially adjusting the position of the mirror (1A, 1B) carrying the bidirectional optical sensor (23) according to its two axes of rotation (6 and 2A or 2B) so that the solar radiation received by said mirror is reflected in the extension (7) of the first axis of rotation (6) and towards the target (12A, 12B); - Using the bidirectional optical sensor (23), measure the brightness (L) of the sun and repeat this measurement until the measured brightness is greater than a predetermined threshold of minimum brightness (LS); when the measured brightness (L) is greater than the minimum brightness threshold (LS), testing the orientation of said mirror (1A, 1B) according to the signal delivered by the bidirectional optical sensor (23); - if the signal delivered by the bidirectional optical sensor (23) indicates a positioning error of said mirror (1A, 1B) along the first axis of rotation (6) or the second axis of rotation (2A, 2B), temporarily changing the speed rotating said mirror about said axis of rotation (6,2A, 2B), and then reiterating the orientation test of said mirror (1A, 1B) until the mirror orientation test indicates the absence of a mirror mirror positioning error around each of its axes of rotation (6, 2A, 2B). 25. Heliostat according to claim 24, characterized in that said program is executed only after a stability test of the solar irradiance, indicating that the brightness (L) received by the mirror (1A, 1B) is greater than threshold (LS) of minimum brightness for a period of time greater than a predetermined delay (Ts). 26. Device comprising a plurality of heliostats (H1, H2, H3, ... H6) according to any one of the preceding claims, characterized in that said heliostats are aligned along an East / West axis and that the three axes of rotation (6,2A, 2B) of the different heliostats are respectively parallel to each other and in that at least one mechanical connection connects the respective axes to each other so that all the rotations of the mirrors are identical and in same time. 10