WO2015104729A1

WO2015104729A1 - Solar concentrator and method for optimizing the irradiance of such solar concentrator

Info

Publication number: WO2015104729A1
Application number: PCT/IT2015/000002
Authority: WO
Inventors: Bruno MARANO; Giuseppe Consentino; Alessandra GIANNUZZI; Emiliano DIOLAITI; Matteo LOMBINI; Adriano Giuseppe DE ROSA; Laura Schreiber; Italo FOPPIANI; Giovanni BREGOLI
Original assignee: Alma Mater Studiorum - Universitá Di Bologna; Istituto Nazionla Di Astrofisica
Priority date: 2014-01-13
Filing date: 2015-01-12
Publication date: 2015-07-16

Abstract

A solar concentrator (1) is described, composed of a system of mirrors (2) and of at least one receiver (3), defined for a certain concentration ratio (X). Effective shapes of reflecting surfaces of such system of mirrors (2) and shape, sizes and electric connection of such receiver (3) are determined depending on a uniform distribution of irradiance.

Description

SOLAR CONCENTRATOR AND METHOD FOR OPTIMIZING THE IRRADIANCE OF SUCH SOLAR CONCENTRATOR

The present invention refers to a solar concentrator and to a method for optimizing the irradiance of such solar concentrator.

Systems with photovoltaic concentration (CPV) in recent years have been subjected to an increasing interest by researchers and market, due to the engineering of more and more efficient photovoltaic devices (PV) .

Both for high-concentration systems (HCPV) and for low-concentration systems (LCPV) , the installed capacity has strongly increased at world level in these latest years.

A clear advantage introduced by concentration in photovoltaic applications (PV) is that the sun light, focused through optical elements such as lenses or mirrors, allows reducing the area of used photovoltaic material with the same collected power .

This concept can bring about a relevant reduction of the cost of photovoltaic systems (PV) and therefore of produced energy, since optical components are cheaper than high-efficiency photovoltaic cells. Price and performances make such cells unsuitable and costly if used in applications which do not provide for the concentration.

Systems based on the use of receivers with dense array have been recently studied and tested as possible solution to lower the system cost per watt of produced power. In this type of devices, sun light is conveyed by a big-sized reflecting optical element (dish) , on the order of meters or tenths of meters. With respect to lenses, mirrors have the strong advantage of not suffering from chromatic aberration and therefore of focusing all wavelengths in the same spot. Generally, in systems equipped with dense-array receivers, mirrors are shaped as rotation paraboloids. From the few examples of big mirrors (dishes) in photovoltaic mode (PV) present on the market, manufacturers tend to approximate the paraboloid surface with a mosaic of plane mirrors assembled on the same structure. This due to such operating sizes as to prevent the construction of monolithic mirrors and the extreme simplification introduced by the use of plane surfaces .

The element to which a radiation is addressed is composed of a group of high-efficiency cells, densely packaged one beside the other to form a single receiver (PV) . The receiver is integral to the optics, which globally needs a sun following along two directions. The system is devised to be used in high-concentration mode, namely to amplify the direct component of the sun flow up to many hundreds of times its value on the earth surface.

From the optical point of view, the problems connected to systems with photovoltaic concentration depend on:

- a circular image produced by an imaging mirror, like a paraboloid coherent with the intrinsic shape of the sun;

an irradiance distribution curve similar to a Gaussian curve projected on the plane of the receiver.

Dense array receivers are composed of blocks of photovoltaic cells, with a rec angular/square shape, connected in series and in parallel.

With respect to the circular image produced by a monolithic or segmented, parabolic or spherical mirror, the rectangular/square shape of a receiver implies a decrease of optical efficiency, both in case of a bigger receiver having some totally or partially obscured cells, and in case of a smaller receiver using only part of the focused light.

Identical cells, under the same irradiance and temperature conditions, produce the same amount of current and voltage, with an increase of such amount which is respectively proportional to the number of used parallel and series connections.

Cells with different characteristics, or identical cells under different lighting and temperature conditions, produce energy losses.

It is known that serially connected identical cells subjected to non-uniform irradiance produce different currents in direct proportion to the concentration factor. The more scarcely lighted cell produces less current and moreover dissipates the excess current produced by the other cells, with the danger of overheating and breaking the cell itself.

Current in cells in series therefore must have as much as possible the same value to avoid decreasing the total connection efficiency, in addition to a physical damage of the cells. A common way to prevent such electric behavior is suitably installing bypass diodes in parallel with any string of serially connected cells. However, this method prevents the total breakage of the cells, but does not guarantee the complete recovery of lost efficiency.

In general, a uniform flow and transforming the image shape are theoretically feasible by exclusively designing a primary optics and/or adding a secondary optical element (SO) capable of modifying the irradiance given by the primary optics. The presence of a secondary optical element (SO) also allows increasing the "acceptance angle" of the optical system with the advantage of reducing the aligning tolerances and the "optical following". In systems with single cell, the use of a refractive secondary optical element (SO) currently appears as the preferred solution. Refractive optics however introduce radiation losses by absorption and add mechanical problems related to stability, fastening and positioning.

A possible strategy to increase optical efficiency and maximizing conversion efficiency is operating exclusively on the primary optics without the addition of further optical components and without conceiving complex receivers or with different types of cells. In this case, the decrease of the "acceptance angle" introduced by the optical system with respect to an hypothetical system with secondary optics (SO) must be compensated with more accurate follower systems.

Applications of solar concentrators are known in the art, comprising a concentrator of the non- i aging dish concentrator type and a photovoltaic receiver capable of producing a uniform solar flow.

Document US2009015785 discloses a system of lenses equipped with an electro-active element, a sensor to detect a change of environmental light, a controller connected to said sensor and a plurality of electrodes operating on the surface of a lens and arranged on concentric rings electrically connected to the controller. The controller applies a voltage to the plurality of electrodes when the sensor detects a change in ambient light. The application of voltage generates a variation of the refraction index of the electro-active element to correct a spherical aberration due to the detected variation in ambient light. The spherical aberration produced by the system of lenses are mathematically described by using a series of Zernike polynomials .

The correction of spherical aberrations of this type of system of lenses however is not enough to make the irradiance of a solar concentrator uniform.

Document US2013319506 discloses a solar concentrator comprising: a non- imaging dish concentrator composed of a plurality of optics; a photovoltaic receiver configured to be able to equalize the size of the image formed by overlapping the sun reflections created by the plurality of optics.

This type of solar concentrator however is not satisfactory and has the following problems: the solar concentrator is a mirror formed of hundreds of small plane mirrors which can be oriented, which have no focal power. Therefore, the images of every square mirror are almost square due to the projection effect of square mirrors; every mirror must be individually oriented and calibrated; plane mirrors reflect the sun rays in bigger and bigger areas depending on their position upon increasing their distance from the center of the plurality of optics, due to the projection effects and the solar divergence. This reduces the chance of choosing both the concentration factor, and the size of the convolute image, once having fixed the individual size of the mirrors, the total size of the plurality of mirrors and the distance of the mirrors from the receiver.

Object of the present invention is solving the above prior art problems by developing optics which alone can solve the problems related to systems with photovoltaic concentration (CPV) and the related method to design them.

A further object is providing a solar concentrator capable of correcting aberrations and, at the same time, of suitably degrading the sun image to transform it into a set of square/rectangular points (spot) with a distribution of irradiance which is as much as possible uniform.

A further object is defining an optimum compromise between the characteristics of the solar pseudo- image and the performances of the dense array receivers.

A further object is inducing modifications to the distribution of brilliance which optimize the performances of the adopted receiver, thereby introducing optic aberrations. A further object is correcting the optic aberrations caused by the misalignment between sun and receiver, in order to increase its concentration ratio.

A further object is providing a method for optimizing the irradiance of such solar concentrator.

The above and other objects and advantages of the invention, as will appear from the following description, are obtained with a solar concentrator as claimed in claim 1, and with a method for optimizing the irradiance of such solar concentrator as claimed in claim 15. Preferred embodiments and non-trivial variations of the present invention are the subject matter of the dependent claims.

It is intended that the enclosed claims are an integral part of the present description.

The present invention will be better described by some preferred embodiments, provided as a non- limiting example, with reference to the enclosed drawings, in which:

- Figures 1, 2 and 3 are an axonometric, a plan and a side view of a diagram of a solar concentrator of an embodiment according to the present invention;

Figure 4 is a geometric diagram of the solar concentrator of Figures 1 , 2 , 3 ;

Figure 5 is an image of the distribution shape of the set of points [spots) of homogeneous light of an embodiment according to the present invention;

Figures 6 and 7 are a set of modules of cells and an electric connection diagram of a receiver of an embodiment according to the present invention;

Figures 8 and 9 are an image and a graph of the irradiance corresponding to position parameters and optic variables of an embodiment according to the present invention;

Figures 10, 11, 12 and 13 are an axonometric, a front and a side view of a handling system of the alt-azimuth type of an embodiment according to the present invention;

Figures 14, 15 and 16 are a front, a side and a rear view of a system of mirrors of an embodiment according to the present invention; Figure 17 is an enlarged view of a portion of the system of mirrors of Figure 15; - Figure 18 is a side view of an actuator for recording the shape of the surface of a mirror of an embodiment according to the present invention;

- Figures 19 and 20 are a diagram of a method to test the shape of the mirrors and to align the system to the sun in an embodiment according to the present invention;

- Figure 21 is a mask sized according to the receiver of an embodiment according to the present invention.

With reference to Figures 1 to 21, a preferred embodiment of the present invention is shown and described. It will be immediately obvious that numerous variations and modifications can be made to the described apparatus and method (for example related to shape, sizes, arrangements, various colors and parts with equivalent functionality) without departing from the scope of the invention as appears rom the enclosed claims.

The operation of the solar concentrator will now be described, together with the related method for optimizing the irradiance of such solar concentrator according to the present invention.

With reference to Figures 1, 2, 3, a solar concentrator 1 is composed of a system of mirrors 2 and of a receiver 3 having an optical axis z parallel to a beam of rays 4 coming from the center of the sun.

The receiver 3 can be of the photovoltaic or thermal types .

The present invention refers to a photovoltaic receiver 3 with dense array.

Therefore, both a conventional photovoltaic receiver, and a thermal receiver are compatible with the scope of the present invention.

The solar concentrator 1 is of the type capable of following the sun during the day, through a handling system of the alt-azimuth or polar types. Taking into account that, from an optical point of view, the position of the sun is at infinite, the position of the rotation axes of the structure with respect to an optical focus of the solar concentrator 1 is not particularly important .

The system of mirrors 2 is arranged according to a hexa-polar grid, in order to guarantee the maximum filling of the entry pupil of the radiation by means of a beehive scheme.

The system of mirrors 2, composed of mirrors of the same diameter, comprises a central mirror 21 and mirrors 22 placed in a ring centered on axis z.

A variation (not shown) to said configuration takes into account a upper filling which can be obtained with hexagonal mirrors, but with higher construction costs. However, other arrangements which do not specifically comply with an hexa-polar symmetry, also fall within the scope of the present invention, as well as systems which apply the same concepts to mirrors with different openings, such as circular, square, hexagonal, etc. At the same time, diagrams which do not provide for the central mirror, as well as diagrams with hexa-polar symmetry with a greater number of concentric rings, fall within the scope of the present invention.

With reference to Figure 4, the solar concentrator 1 is composed of the mirrors 22 with the same circular opening with diameter D at distances from the center d which are slightly greater than diameter D. The difference between D and d can be changed depending on the fact that one has to prevent or not the shading of the mirrors 22 by the central mirror 21.

Taking into account the plane mirrors, the mirrors 22 are slanted so that the rays 4 coming from the sun center are all exactly reflected into the central point of a plane 31 of the receiver 3 placed at a distance h from the vertex of the central mirror 21. the hexa-polar grid, if d is known, after having chosen the position of the mirrors 21, 22, chosen the optical condition, namely where the image created on the receiver 3 has to be centered, it is possible to obtain the geometric laws which determine the inclination in many cases which are compatible with the scope of the present invention.

Considering the central mirror 21 and two mirrors 22 of the ring, respectively designated with 22-2, along direction Y+ and 22-3, along direction Y-, so that their centers all lie in the same plane Y-Z, the inclination angle of the mirrors 22-2, 22-3, is unique, with the same value in modulus and lies in the same plane. It depends on mentioned parameters as follows:

tan ¹ \ - seano(d)

where \V«* // iiss angle β. The distance modulus allows distinguishing between symmetrical mirrors with respect to the central one. For all mirrors on rings whose centers to not lie in plane Y-Z, the inclination angles are two and can be deduced from the angle a of the mirror 22-2, according to the following:

a₄Jt = -|an^T(-lX(sina■ cos φ) /cosa ) |■ segno yis) ¾y = Icos^T(-l)(cos a / cos

J ) | - segnoixis^") where ¾.y*) are the coordinates of the centers of the mirror considered in the chosen reference system. Angle ^ay lies in plane Y-Z like a, while ¾ in plane X-Z. In the hexa-polar grid, knowing d, it is immediate to obtain the coordinates for each mirror of the first ring according to trigonometric rules, since everyone is placed at the vertexes of a hexagon, namely with an angle of 60° with respect to the adjacent ones. Actually, after having chosen the position of the mirrors and the optical condition, namely where the image created on the receiver has to be centered, it is possible to obtain the geometric laws which determine the inclination in many cases which are compatible with the scope of the present invention, For example, the central rays could not be made convergent, but they could be arranged on a preset grid on the plane of the receiver, or make them converge in blocks, also in case of multiple receivers, or still such varying convergence spot could be chosen and traced as parameters in an optimization method which will be described below.

Through a computation, a shape of the reflecting surface is determined for each of the mirrors 21, 22, arranged according to the geometric diagram of Figure 4, depending on the desired mean concentration ratio.

The concentration ratio is defined as the total area of the mirrors perpendicular to the sun direction divided by the total area on which light is focused:

X = ^Ain/_A

Then, an actual shape of said surface is determined, depending on irradiance uniformity conditions and shape of a desired image.

A squared image 5 with a homogeneous irradiance at the center and degrading towards the edges is a case obtained for a certain concentration ratio (Figure 5) .

The analytic tool used for determining said actual shape are the so-called "Zernike polynomials", of which there are several mathematical formulations and standardizations. As an example, the "Zernike polynomials" are treated with reference to:

] if m≠0

ZodA.j = nZ ()V¾siii(77.0)

Z_j = _>/π Ϊ/?·(ρ) if = 0 where: P is the standardized radial coordinate, namely the distance of a spot of the surface from the optical axis of the single mirror,

0 is the azimuth coordinate, w is the radial degree and "i the azimuth frequency. If a spherical or parabolic mirror is considered, namely with a conical section, and possible deformations present thereon, such surface can be well approximated by the following: z = ====== + AiZiip,60 where is the number of the polynomial of the series, Ai is the coefficient associated with the i-th polynomial, ^r is always the radial coordinate ma in the chosen measuring unit, P and are the already defined polar coordinates, ^c is the curvature and * the taper constant. The first addendum represents an ideal mirror, for example spherical for k=Q , while the second addendum represents the deformations according to the above defined polynomials ^zi . In general, as many the deformations present and on different scales are, as higher the number of polynomials will be, necessary to appropriately describe the surface .

It is certainly possible to locate a greater number of coefficients with respect to those specifically determined: however, the choice of adopting the smallest possible number thereof in order to limit the active deformation points of the surfaces both during constructions and when calibrating, is a way to maximally simplify the construction.

Also the receiver 3 is designed with the purpose of minimizing the effects of the lack of irradiance uniformity, depending on the desired mean concentration ratio.

An example of shape, sizes and electric connection of the receiver 3 is shown in Figures 6, 7, in which the modules 32 are composed of cells 33 electrically connected in series. The electric connection between the modules 32 comprises central modules 321 connected in series to groups of mutually adjacent modules and side modules 322 connected in series always to groups of modules arranged according to concentric frames A-F from outside towards inside. All resulting groups are mutually connected in parallel.

A method for optimizing the irradiance of the photovoltaic concentrator 1 is implemented to maximize the efficiency of the receiver 3 with respect to its rated value, according to the following steps: defining suitable deformations of said mirrors 21, 22; analytically/numerically reconstructing an image produced by said deformed mirrors 21, 22; determining the efficiency of the receiver 3 related to the image produced by the mirrors with the introduced deformations; modifying the deformations of said mirrors 21, 22 depending on the maximization of efficiency of the receiver 3.

Specifically, the method for optimizing the irradiance of the solar concentrator 1 is numerical and is based on the fact that the Zernike polynomials can be used to model the deformations on the real surface of the mirrors . The method starts from the definition of an initial value of said coefficients associated to some Zernike polynomials and the iterative modification of said coefficients, within a certain predefined interval, till values are obtained for which the optics generate a homogeneous and rectangular distribution within a certain distance from the center and degrading towards the edge, similar to the squared image 5.

Through said series of Zernike polynomials, the deformations of the mirrors 21, 22 are located, useful as regards the request of uniformity and shape of the area to be obtained.

After having located the deformations of the mirrors 21, 22, the difference is minimized between the rated efficiency and the efficiency obtained with the deformed optics of the implemented system, depending on the receiver 3.

After having defined the optics 2 and the receiver 3 through the analytical formulas, after having set positional parameters and optical variables and having optimized the values for the searched coefficients, image and irradiance are obtained, respectively shown in Figures 8, 9.

The process or any other concentration has to be simulated remains similar, even if the number of used modules 321, 322 and the number of cells 33 therein are different, or for a different electrical diagram of the receiver 3.

The results obtained by optimizing the irradiance of the solar concentrator 1 are further validated by implementing the computation of the tolerances related to all optical and geometric parameters of the solar concentrator 1, including for every mirror the Zernike coefficients within an established radial degree, according to the following steps: rated image and efficiency are computed with the values returned from the optimization depending on the considered receiver 3; for every parameter a range is defined within which its rated value can change; a threshold value is chosen for the accepted efficiency degrading of the system; the parameters are modified, one at a time, within the above range weighing the tolerance threshold with the total number of parameters, thereby supposing that they can be not independent and the degrading occurring for many parameters simultaneously; the image is produced, associated with the new set of parameters and after having iteratively computed the related efficiency till the difference between the rated value and the computed value is lower than the chosen threshold value .

As regards the optics design, the system of mirrors 2 is approximated in the optimization procedures with a grid of points. The sizes of the collecting area are about 50.00 m2, in line with the sizes of industrially manufactured pointing and tracking systems. The main dimensional parameters being considered are: diameter D of single mirrors: 2.6 m; mirror opening: circular; distance d between vertex of the central mirror 21 and vertex of a mirror 22 on the ring: 2.68 m; distance h between the vertex of the central mirror 21 and the center of the receiver 3: 4.8 m.

The plane 31 of the receiver 3 is simulated with an array of pixels and the source is modeled to reproduce the angular size of the sun equal to 0.53°. The number of the sub-openings of the mirrors, of the pixels and of the directions to be considered within the solar cone are chosen in order to prevent sampling errors. In the computations, a perfect solar following condition is assumed.

As regards the modeling of the receiver 3, the cells 33 taken as reference are manufactured by AZUR SPACE. The characteristics of the considered cells are: construction material: GalnP/GaAs/Ge on a Ge substrate; chip size: 5.59 mm x 6.39 mm = 35.25 mm2; active cell area: 5.5 mm x 5.5 mm = 30.25 mm2.

The array of cells 33 is designed in the simplest possible way, using modules with the same number/type of cells mutually in series and afterwards connected in series/parallel, following a homogeneous and rectangular distribution of light within a certain distance from the center and degrading towards the edge of the squared image 5.

According to a configuration seen, for example, for the concentration 500, the receiver 3 is composed of 56 rectangular modules, each one of 36 cells in series, for a total of 2016 cells. A central area of 32 modules plus 4 lateral areas composed of 6 modules each. The electric connection between the modules takes into account the 32 central modules connected in series to group of 4 mutually adjacent ones and the side modules connected in series always at groups of 4, but according to concentric frames A-F from outside towards inside (Figure 7) . For example, all external modules A of the lateral area are mutually connected in series and so on. The 14 resulting groups are then mutually connected in parallel.

Improvements can be applied to this scheme, above all related to the loss of light at the edges, in which there are no cells, with the purpose of performing a further simplification.

With reference to Figures 10, 11, 12, 13, a handling system of the alt-azimuth type comprises a mobile structure 11 connected to a mobile structure 12 supporting the system of mirrors 2 and the receiver 3.

The mobile structure 11 moves along the azimuth by rotating with respect to an axis 13.

The mobile structure 12 moves along the altitude by rotating with respect to an axis 14 by means of a pair of hinges 15 connected to the structure 11.

The low rotation speed necessary for the movements with respect to axes 13, 14 is preferably obtained though epicyclical wheels (not shown) capable of developing high torques with low rotation speeds of the outlet shaft, keeping the electric motors with optimum rotation speeds, above 1500 revolutions/minute, with low rated inlet torque. With reference to Figure 14, the mirrors 22 can be assimilated, with suitable deformations, to identical mirrors regarding diameter and curvature radius . From an analysis of the Zernike polynomials used in the particular embodiment described, it is assumed that a number of check points 23 reasonable to obtain the required deformation is angularly of one point every 10° and, radially, of at least three circumferences 24, 25, 26.

The mirrors 21, 22, of a monolithic construction, can be obtained starting from the stamping of sheets. A suitable material, both due to its light-weight and workability and due to its easy passivation to atmospheric agents, is aluminum. However, also a plastic material, malleable and compatible with the deposition of a layer of high reflectivity material and with the established tolerances, is an alternative solution.

The surface of the mirror, after the mechanical working of stamping and preparing the fastening points, is further worked to obtain the necessary optical characteristics.

The required reflectivity can be obtained both through surface lapping, and through electrodeposition of a surface layer of nickel. Alternatively, after the mechanical workings of stamping and preparing the fastening points, the surface is deformed through adjustable systems 17.

With reference to Figures 15, 16, 17, 18, a carrier structure 16 of the mirror 21, 22, is aluminum welded to keep, in addition to lightweight, a construction simplicity. In fact, by so doing, ribs bent or obtained through mechanical working are present on the mirrors.

The carrier structure 16 is a trellis, in order to obtain a rigid, and at the same time light-weight, structure. The choice of a trellis- type structure allows making the whole system composed of the structures 11, 12, 16, adopting as a maximum three or four sizes of a unified tube, concentrating the majority of workings in cutting and welding. The mirrors in fact are assembled on the adjustable systems 17, of the mechanical type, with thicknesses or with registration screws, or of the electric type, thereby limiting the number of mechanical components to be made with high accuracy.

The receiver 3 is composed of a group of high efficiency cells 33, densely packaged one beside the other, to form a single receiver (PV) . The receiver 3 is integral with the optics 2.

The photovoltaic cells 33 are connected in series and in parallel, have rectangular shapes and therefore the receiver 3 mirrors this geometry.

A support 18 of the photovoltaic cells 33 is made with aluminum tubes to reduce as much as possible the shade generated on the central mirror 21 (Figure 13) . In order to limit the masses, the single support 18 is made by axially welding tubes of a diameter decreasing towards the photovoltaic cells 33. The minimum tube diameter is constrained, in addition to the mechanical characteristics, also by the need of using the tube both for the passage of ducts for the cooling liquid of the cells 33, and for the passage of the electric cables for electricity produced by the receiver. Since the motion of the axes of the concentrator is of the alternate rotary motion with amplitude lower than the round angle, it is not necessary to device particular systems for passing the above cables. The choice of adopting a refrigerating fluid to remove heat present in the system of cells is linked both the smaller mass assembled projecting on the structure, and to the chance of using this part of collected energy in co-generative systems which can be installed also separately from the solar concentrator 1.

A procedure for testing the shape of the mirrors 21, 22 and for aligning the system 1 to the sun is divided into two steps:

calibrated registration of the shape of every single mirror 21, 22 fastened onto its own carrier structure 16 by acting on the adjustable systems 17;

alignment of the solar concentrator 1 to the sun.

With reference to Figures 19, 20, the regulation through the adjustable systems 17 is performed with the help of a spot -like source PS, a ray separator BS, a wave- front sensor SH of the Shack-Hartmann type with a certain number of sub- openings and a camera C. The regulation procedure can be schematized as follows:

- the spot-like source PS is placed in the exact center of curvature of the mirror 22;

- the rays incident on the mirror are reflected backwards towards the ray separator BS deviating the rays respectively towards the sensor SH and the camera C;

- the image is shot by the camera C and analyzed to locate the shape of the wave- front and, through this, the map of the surface of the mirror 22;

- the adjustable systems 17 placed on the back of the mirror are iteratively modulated till the measured surface for the mirror 22 corresponds to the rated shape within the computed tolerances.

To accelerate the alignment procedure, an iteration array is built, by recording the response of the wave-front sensor to the specific movement of every single adjustable system 17. Such array is afterwards inverted and used to convert the value of the residuals, between the measured surface and the rated surface to reach, in modulation signals of the adjustable systems 17.

The step of aligning the solar concentrator 1 to the sun is implemented in the field, when the sun or the full moon is present, by means of a mask 6 in agreement with the receiver 3 as shown in Figure 21. Such mask 6 must be made of a material resisting to temperatures of the order of some hundreds of grades. On the mask 6, concentric frames of small holes 61 transmit a small part of the light incident thereon to diodes placed in the back or other optical-electronic devices for measuring the luminous intensity (not shown) . This type of device allows sampling the distribution of light on the illuminated surface of the receiver 3. The procedure in detail is composed of the following steps:

- the mirrors 21, 22, placed on the respective carrier structures 16, are assembled on the mobile structure 12 ;

the mask 6 is placed in front of the receiver

3;

- n-1 mirrors are darkened through removable covers ;

the concentrator 1 starts following the sun/moon, in order to obtain on the mask 6 the image produced by the only mirror which is not darkened;

the distribution of the radiation is measured by the diodes.

At that time, if the distribution does not correspond to the foreseen one, the mirror is iteratively moved and inclined till the searched distribution is reached. Again, to accelerate such procedure, an interaction arrays is built, which records the response of the diodes with respect to the degrees of freedom to be aligned. This array is inverted and translated into signals to be applied to the movements of the mirror.

Claims

Solar concentrator (1) , composed of a system of mirrors (2) and of at least one receiver (3) , said system of mirrors (2) being adapted to reflect towards said receiver (3) at least one deformed solar image having a uniform irradiance, actual shapes of reflecting surfaces of said system of mirrors (2) and shape, sizes and electric connection of said receiver (3) being determined depending on such uniform distribution of irradiance, characterized in that said solar concentrator

(1) is adapted to be aligned with the sun through a mask (6) associated with said receiver (3) and equipped with an array of small holes (61) and respective diodes or other optical-electronic devices for measuring a luminous intensity.

Solar concentrator (1) according to claim 1, characterized in that said system of mirrors

(2) is composed of at least one mirror (21) . Solar concentrator (1) according to claim 1, characterized in that said system of mirrors (2) is arranged according to an hexa-polar grid composed of a mirror (21) centered on an axis (z) and mirrors (22) placed in at least one ring centered on said axis (z) .

Solar concentrator (1) according to claim 1, characterized in that said receiver (3) is composed of modules (32) which are electrically connected and composed of cells (33) .

Solar concentrator (1) according to claim 4, characterized in that said cells (33) are electrically connected in series, the electric connection between said modules (32) comprising central modules (321) connected in series to groups of mutually adjacent modules and lateral modules (322) connected in series always in groups of modules arranged along concentric frames (A-F) from outside towards inside, said central and lateral groups being mutually connected in parallel.

Solar concentrator (1) according to claim 2 or 3, characterized in that the reflecting surfaces of said mirrors (21, 22) are mechanically worked.

Solar concentrator (1) according to claim 2 or 3, characterized in that the reflecting surfaces of said mirrors (21, 22) are mechanically deformed by means of adjustable systems (17) operating in control points (23) arranged on the surface of said mirrors (21, 22) .

8. Solar concentrator (1) according to claim 7, characterized in that said control points (23) are equally distributed on the surface of said mirrors (21, 22) on one or more concentric circumferences (24, 25, 26).

9. Solar concentrator (1) according to claim 7, characterized in that a carrier structure (16) of each one of said mirrors (21, 22) supports said adjustable systems (17) .

10. Solar concentrator (1) according to any one of the previous claims, characterized in that a movement system of the alt-azimuth type comprises a mobile structure (11) rotating with respect to an axis (13) and a mobile structure (12) rotating with respect to an axis (14) by means of a pair of hinges (15) connected to the structure (11) supporting the system of mirrors (2) and the receiver (3) .

11. Solar concentrator (1) according to claim 9, characterized in that said adjustable systems (17) are adapted to perform the calibrated adjustment of the shape of every single mirror (21, 22) fastened onto its own carrier structure (16) , said adjustable systems (17) being iteratively modulated till the measured surface is equal to the rated surface within the computed tolerances.

12. Solar concentrator (1) according to any one of the previous claims, characterized in that said receiver (3) is photovoltaic.

13. Solar concentrator (1) according to any one of the previous claims, characterized in that said receiver (3) is photovoltaic with dense array.

14. Solar concentrator (1) according to any one of the previous claims 1 to 11, characterized in that said receiver (3) is thermal.

15. Method for optimizing the irradiance of a solar concentrator (1) according to any one of the previous claims, the method comprising the steps of: defining deformations of said mirrors (21, 22) ; analytically/numerically reconstructing an image produced by said deformed mirrors (21, 22) ; determining the efficiency of the receiver (3) related to an image produced by said mirrors (21, 22) with the inserted deformations; modifying the deformations of said mirrors (21, 22) depending on maximizing the efficiency of the receiver (3); defining a starting value of some coefficients associated with Zernike polynomials; iteratively modifying said coefficients, within a certain predefined range, till values are obtained for which the optics generate a homogeneous light distribution inside said image (5) , characterized by implementing a computation of tolerances related to all optical and geometrical parameters, including for every mirror the Zernike coefficients within an established radial degree, according to the following steps: computing rated image and efficiency with values returned by optimizing depending on the receiver (3) being taken into account; for every parameter, defining a range within which its rated value can change; choosing a threshold value for degrading the accepted system efficiency; modifying the parameters, one at a time, within the above range, weighing the tolerance threshold with the total number of parameters, thereby supposing that they can be not independent and that degrading can occur for many parameters simul aneously; producing the image associated with the new set of parameters and iteratively computing the related efficiency till the difference between rated value and computed value is lower than the chosen threshold value.

Method according to claim 15, characterized in that said image (5) is squared within a certain distance from its center and decreasing towards its edges .