WO2008107921A1

WO2008107921A1 - Adjustable photovoltaic prism lit up from inside

Info

Publication number: WO2008107921A1
Application number: PCT/IT2007/000303
Authority: WO
Inventors: Daniele Piazza; Vincenzo Varazi
Original assignee: Daniele Piazza; Vincenzo Varazi
Priority date: 2007-03-06
Filing date: 2007-04-24
Publication date: 2008-09-12
Also published as: ITMI20070447A1

Abstract

The invention concerns a photovoltaic prism obtained by assembling single photovoltaic modules of crystalline silicon joined together by their longer sides, so that the active surface lies inside the prism. The base of the prism, opposite to the aperture for entry of sunlight, is closed by a flat bottom, the colour of which is essentially white, that reflects the light entering the prism towards its internal walls so as to exploit the phenomenon of luminous diffusion. Lateral slits are cut in the lower perimeter to allow rainwater to be drained off. Orientation of the photovoltaic prism is controlled by a sun follower to ensure that the axis of the prism is kept constantly parallel to the sun's rays. Several photovoltaic prisms, preferably hexagonal in shape, can be assembled to form a honeycomb configuration oriented as above by a sin le sun follower.

Description

^"

Adjustable photovoltaic prism lit up from inside

Field of application of the invention

The present invention relates to the field of photovoltaic applications, and in particular to an adjustable, photovoltaic prism with internal illumination. Review of the known art

Photovoltaic means for producing energy offer the possibility of exploiting a renewable source of electricity, alternative to the use of fossil fuels but as yet uncompetitive as regards its cost per kilowatt hour. Certain situations already exist, however, in which the advantages of photovoltaic generators of electricity compared with conventional types may already be appreciated; such as their use in supplying radios operating in inaccessible places, in the sea on light buoys, along pipe lines for operating control equipment, but also in many other contexts where ^problems would arise over operating autogenous generators or for replacing electric batteries. The material most frequently used for making photovoltaic cells is wafer silicon in single- crystalline, polycrystalline or amorphous forms. . A photovoltaic or solar panel comprises an integral and coplanar assembly of photovoltaic modules fitted together in various ways, in series and/or parallel, and each consisting of a number of series-connected cells. Rows of panels set up in an area constitute a photovoltaic field for the ~-oduction of electricity. The sizes of photovoltaic modules of crystalline silicon already on the market vary from 0.5 to 1.5 m² and these supply energy at an almost constant potential difference usually 12 or 24V, though the potential difference produced is usually at least 4V higher than these levels so that a storage battery can be charged and connected to a suitable charge regulator. The efficiency of a photovoltaic cell is established by estimating the ratio between the energy it produces and the radiant energy over its entire surface. Typical values for cells made of crystalline silicon are about 15% (much higher ones can be obtained by more expensive cells such as those optimized for space probes). Radiant power from the sun reaching cell surfaces my be estimated starting from the 1,367 Watts that pass through the section of a square metre placed on the earth's orbit perpendicular to the solar rays. Reductions to this value must be made according to the latitude of the site where the cells are installed, according to atmospheric attenuation and to how light intensity varies during the 24 hours and from one season to another. At European latitudes average radiation, estimated as above, is about 200 W/m².

In the most common commercial modules used under standard conditions of insolation, power at 24V is about 150 Wp (peak Watt) generally reached by using 72 series-connected photovoltaic cells. The surface required for these modules is about 7.5 m²/kWp, meaning that an area of about 7.5 square metres of photovoltaic surface is needed to generate 1,000 Watt of peak power. The size of the surface so covered is one of the main drawbacks to the adoption of photovoltaic technology used on the ground. A partial remedy, but applicable only to the smaller panels, is to be found by keeping the photovoltaic surface constantly perpendicular to the direction of sun rays,, in this way increasing cell efficiency. A number of structures for this purpose are available on the market; these consist of mechanical supports for the panels moved by the methods used for equatorial or high-azimuthal installations for photographic astronomical telescopes engaged in following stars. These structures enable the panel to be partially rotated around two axes one orthogonal to the other. A microprocessor has to be specially adjusted, in accordance with the geographical coordinates for the site where the installation is set up, as well as date and time, previously stored, so that the actuators can maintain continuous (and cyclic, during daylight) control over the angle of elevation, above the horizon normal to the photovoltaic surface and to the azimuth. More simple automated types are also known that adjust tne angle of elevation only.

As stated above, the use of a sun follower is not worthwhile at the present time beyond a certain size of roofing, mainly because of the cost of the mechanical parts which must be strong enough to withstand the bending moment exerted by the panels as a whole, stresses that increase with the size of the roofed area; alternatively several of these structures must be installed. At present it is considered preferable to have fixed panels, placing them in the most suitable position according to latitude, as is done in the case of solar fields or on the roofs of houses or factories. In this way it has been found that average loss of efficiency amounts to about 20% of peak level, and that no significant increase is achieved by orienting the panels around the perpendicular. Loss of efficiency can be countered by increasing the panel-covered surface. From the. above it will be. clear that at present it is not worthwhile to install a sun follower where photovoltaic panels are used for domestic purposes. Purpose of the invention

The purpose of this invention is to overcome the present limits affecting the use of photovoltaic panels, namely the large area that they must cover in order to produce a significant amount of electric power. For photovoltaic power stations a sufficient area can be allocated but this is not possible in the case of having to provide power for just one or more houses, especially if the demand arises in a residential area. Where the need is felt in some inaccessible spot the space available for solar panels could be entirely inadequate.

Summary of the invention

To satisfy the above requirements, the subject of the present invention is a photovoltaic generator of electricity, as will be described in Claim 1. The main features of this generator include the following: at least one predominantly hollow body placed along a certain axis, open at a first end to permit entry of sun rays; - means for reflecting sunlight placed at a second end of the hollow body opposite to the first end; photovoltaic transducer means iόr lining tne internal surface of the hollow body; - mechanical means for supporting the above component parts, ■ these comprising rotation actuators for keeping said axis constantly parallel to the direction from which the sun rays come.

Further characteristics of the present invention considered as innovative are described in the dependent claims. In a preferred form of its realization, the means for reflecting sunlight consist of a flat base essentially white. or off-white in colour. The reflecting base is preferably made of a waterproof and hail-resisting material, holes for draining off rainwater being cut along the lower edge of the hollow body in a peripheral strip devoid of photovoltaic material. ^■ According to another aspect of the invention, the photovoltaic modules are assembled to form polygonal prisms having N sides, preferably N > 4, formed of single modules or rectangular photovoltaic panels joined by their longer sides. For this invention- the hexagonal prism is decidedly preferable. . Though without placing any limitation on materials, a crystalline silicon is the one preferred for the panels used in the invention.

To avoid a reduction of efficiency in optical-electrical conversion, the reflecting means may consist of a pyramid having N faces, each face lying opposite a corresponding face of the polygonal prism with N faces. An alternative to the pyramid consists in the use of a reflecting spherical dome. In another form of realizing the invention, the hollow body is a cylinder and the photovoltaic material lining the walls consists of flexible, electrically connected cells.

If the user needs more electric power, several prisms or cylinders can be assembled to form a structure that can be oriented by a single sun follower. The hexagonal prism can be advantageously used to form a "honeycomb" structure that entirely covers the area available. As in the case of photovoltaic panels with a fixed orientation, the present invention expressly forgoes any attempt to achieve the peak power that would be theoretically possible; contrary to the known art, however, the aim here is not to avoid using sun followers but rather to make a decided reduction in the horizontal space encumbered. For this reason the invention resorts to a prevailingly vertical, disposition of the modules or panels, together with the use of a sun follower essential for ensuring that the maximum amount of light rays reach the cavity of the photovoltaic prism perpendicular to the reflecting bottom and parallel to the photovoltaic surface of the modules (this latter feature constituting a real novelty). It is a surprising fact that, after many experiments with prototypes using various types of reflecting elements, it has been found that a flat reflecting bottom of a pale or mainly white colour produces the greatest amount of energy at ^• the terminals. To explain the theory underlying this result is no easy matter. However, in attempting to do so, it may be said that reflection of sunlight from the pale or white bottom mainly occurs by diffusion in all directions of the rays touching the microscopic sharp points on. the surface which is not perfectly smooth, contrary to what happens in mirrors where the law of Snell applies. Thanks to this diffusion and assuming the direction of reflection as a random variable, a calculation may be made of the average percentage of light rays, entering through the aperture of a hexagonal prism, that are reflected from the pale or white bottom towards the photovoltaic surface according to those directions exploitable for purposes of optical- electrical conversion. It is obvious that these calculations are approximate only, since a real statistical distribution of the diffused directions is not yet known; on this basis, however, it might be possible to assume that there is an even distribution that does not favour any particular direction. Using 1 to 3 as a ratio between the smaller and the larger side of the photovoltaic module, a calculation may be made of the cone aperture with the apex at a central point of the white bottom tangential to the uppermost aperture. This cone includes that fraction of radiation which on an average is re-radiated outwards and therefore lost. A quick calculation shows that the aperture of ^' the cone lies at about 16°, so that the remaining 164° (out of 180°) would be free and available for photovoltaic purposes. In order to be consistent with what has been said above about compensating for lack of incidence perpendicular to the surface of the modules, an empirical reduction of 20% could be made thus leaving about 131° available to exploit for greater efficiency. This means that about 73% of peak power flow that enters parallel to the walls of the module is still utilizable for photovoltaic purposes. A further aid to a more efficient exploitation of the light rays entering the module is obtained by closing the aperture of the photovoltaic prism using a lid made of material fully transparent to light, preferably of plastic, with an index of refraction greater than that of air, its thickness being treated so as to produce a lens slightly diverging from incident radiation towards the upper part of the prism. What has been said fully justifies the measures taken and explains the advantages obtained in terms of horizontal space saved (photovoltaic surface used being equal).

This result makes possible a fair comparison between areas covered respectively by the prism-shaped photovoltaic modules, singly o grouped in a "honeycomb" structure as proposed by the present invention, and a completely flat arrangement as envisaged by the known art. In practice it is found that the invented hexagonal prism, and consequently the "honeycomb" group structure, covers a surface about one quarter the size of a fiat configuration using the same modules. To give an example, irϊ order to satisfy the requirements of a single domestic user (3 kW), instead of the 25 m² required for the flat configuration, the present invention would use a honeycomb structure consisting of seven prisms, the base of which occupies a surface area of only 6 m².

Photovoltaic covered surfaces being equal, the new honeycomb structure is more compact than a flat structure, loads being closer to the centre of gravity thus reducing the bending moment, the sun follower being therefore much less expensive so that its installation becomes worthwhile.

Short description of the figures Further purposes and advantages of the present invention will be made clearer by the following detailed description of an example of its realization and by a study of the attached drawings provided for explanatory purposes only, in which: Figure IA shows an axonometric view of a single photovoltaic prism (or tower) of the present invention;

Figure IB shows a configuration of several prismatic elements as in Figure

1, grouped to form a "honeycomb" type of structure;

Figures 2 A and 2B show two perspective views from above of the photovoltaic prism in Figure 1, one according to a preferred form of realizing the invention (Figure 2A) and another giving lesser output (Figure

2B).

Figure 3 shows a perspective view of the photovoltaic prism in Figure 1, complete and mounted on a supporting base; , Figure 4 shows a perspective of the honeycomb configuration in Figure 1 mounted on a supporting base and operated by a sun follower device.

Detailed description of some preferred forms of realizing the invention

With reference to Figure IA, an element 1 can be seen in the form of a hexagonal prism consisting of six photovoltaic modules 2, assembled so that the surfaces of the photovoltaic cells face towards the inside of the prism 1.

To complete the invention the photovoltaic prism 1 must be oriented by means of a sun follower. Figure IB shows a group of seven prisms of type

1 assembled so as to form a honeycomb type of structure 3 then to be oriented by a sun follower. The modules 1 are chosen from among those normally available on the market, such as modules of crystalline silicon functioning at 12 or 24 V with a photovoltaic surface area of 30 x 90 cm.

With reference to Figure 2A, a prism 1 can be seen, with its aperture for • entry of sunrays facing upward in order to show a flat bottom 4 that fits onto the hexagonal base. The bottom 4 may be made of several kinds of materials as long as its colour is white or whitish and as long as it is waterproof and resistant to hail. Plastic materials are the cheapest and the most suitable for this purpose. However, especially where the site is particularly sunny, the photovoltaic modules must be protected from overheating that could be caused by the hot air coming off the flat bottom. For the bottom, therefore, a good heat-conducting material is preferable, one able to disperse heat quickly without allowing the temperature to rise above that at which the modules can function most efficiently. Use might be made of a thin sheet of aluminium with its upper surface painted white, its underneath possibly being fitted with vertical heat-exchanging fins. The modules 2 are those made of crystalline silicon (polycrystalline is less expensive), available on the market in the desired sizes. If considered advisable in the future, the prism 1 could be replaced by a hollow cylinder clad with flexible photovoltaic material, in which case the bottom 4 would be circular. Figure 2B shows an alternative in which the reflecting bottom consisting of a hexagonal pyramid 5, the triangular faces of which consist of mirrors each facing towards the corresponding face of the hexagonal prism L^' It has been found, however; that the reflecting base 5 is less effective than a reflecting white one 4, for conveying light towards the photovoltaic modules 2.

In Figure 3 two electric wires, 7 and 8, can be seen leaving their respective boxes, 9 and 10, placed close to the upper rim of two adjacent modules 2 of the photovoltaic prism 1. Generally speaking, each of the six modules of type 2 has its own contact box from which its own wire emerges. The six wires of type 7 and 8 lead to a second box 11 fixed to a supporting base 6 on which rests the bottom 4 of the prism 1. Inside the box 11, the single wires are connected in parallel to a larger cable 12 that comes out from the box and continues as far as the user equipment. Cuts, 13, 14 and 15, are made along the lower edge of the wall of each module, this being done in a part which has no photovoltaic cells. The main purpose of these cuts is to permit discharge of rainwater that collects in the bottom 4, but also to help hot air to escape outside the prism. The prism 1 is formed by joining the sides of single modules 2 using ordinary mechanical . methods for the purpose. With reference to Figure 4, a view is given of the honeycomb configuration 3 of photovoltaic prisms 1 (Figure IB) showing their lower ends fixed to an amply sized base 6 connected to a sun follower mechanism 23 the purpose of which is to maintain the axis PS of the prisms 1 constantly oriented towards the sun. In the honeycomb configuration 3, the functions of slits 13, 14, etc. are maintained as far as concerns drainage of rain water and circulation of heated air as in any case cavities among the assembled prisms are created by the ribbing on the longer sides and by the upper outwardly- projecting rims. The sun follower 23 consists of a base 24 that includes a first motor 25a for diurnal rotation of a shaft 25 along a vertical axis. Al. The base of a strongly built fork 26 is fixed to the end of the shaft 25. Hinged between the arms of the fork 26 is a second shaft 27 lying along a horizontal axis A2, rotation of which is controlled by a second motor 28. Motor 25a is a DC brushless motor coupled to a speed reducer with a high ratio of reduction. If required, the motor 28 can also be DC brushless or else of the step-by-step type. Shaft 27 can be kept perfectly horizontal by means of a spirit level placed on the shaft itself and by adjusting screws on the surface supporting the base 24. Perfect orthogonality between the axes Al and A2 is ensured by manufacturing tolerances. The shaft 27 carries two arms, 29 and 30, placed V- wise and joined to the rear face of the flat support 6 under the group of prisms 3. The support 6 is square in shape and has slots 6a, aligned on the surface of the supporting base outside the group of prisms and cut through it. The purpose of these slots is to lighten the support 6 and also make it more rigid and therefore less likely to warp under the effects of bending and torsion moments. .Said slots 6a also serve to reduce the surface that absorbs sunrays and assists the passage of hot air thus helping to lower the heat level. A sheet of reflecting metal on the upper face of the support 6 would also reduce heating. Support 6 could also be limited to the base only of the group of prisms 6, or else could be made in the form of a framework. The electric cable 12 that collects the current generated by the entire photovoltaic group 3 is connected to a box 31 fixed to one arm of the fork 26. A second cable 32 emerges from the box 31 and enters the. base 24; from said base, electrical connection continues along a cable 33 towards a cabinet 34 for housing control equipment, and from there a cable 35 takes the current to the user's equipment (not shown). The following parts are housed in the cabinet 34: a storage battery to receive the charge generated by the prism group 3, a charge regulator for controlling the battery, an inverter for continuous transformation of DC current in 24V, a microprocessor specially programmed for controlling rotations of the two motors in the sun follower 23, and for supervising the entire apparatus and operation of the interface towards the operator. For this purpose the cabinet contains a display, a keyboard with light-touch keys-, a voltmeter and an ammeter. All the parts seen in the Figure are thoroughly waterproof. As regards freedom of movement for the prism group 3, the cable 12 must be long enough to allow partial rotation (max. ±90°) in both directions around the A2 horizontal axis; the same applies to the cable 32 that must ensure partial rotation (max. ±90°) in both directions around the vertical- axis Al. Length of the connection between the box 31 and the motor 28 (inside the- fork 26) remains constant.

Calculations of the two rotations are widely known in the art of sun

. followers but, to complete the description, these are given in the Appendix entitled: HOW TO CONTROL THE SUN FOLLOWER. The formulae can easily be calculated by the microprocessor once having found the elevation of the sun above the horizon (around axis A2). The geographical coordinates of the place where the photovoltaic prisms are installed must be previously set, also the date and starting time in hours, minutes and seconds.

After this, the azimuthal motor 25a will rotate around axis Al at the constant angular speed making one complete turn every 24 hours while,, at previously set intervals of time, the microprocessor will calculate the angular adjustment to apply to the angle of elevation by means of the motor

28. For photovoltaic applications there is no need for the same degree of. precision as that required for the photographic telescopes used in astronomy, so that adjustment of elevation can be made at time intervals of about one or more minutes; this makes the technical requirements of the motor 28 less stringent (a correction every 60 seconds involves an error of 0.25° at the most which a motor that is not excessively costly can manage). The error is negligible compared with the lack ^■ of precision in local insolation calculations caused in establishing statistical variations. Apart from being useless, it is obviously counterproductive to keep the sun follower active when there is no light so that, by using the formulae given below, the - processor must calculate the moments of dawn and sundown for each- period of 24 hours and turn the sun follower on and off accordingly. Before each fresh diurnal switch-on, rotations in the opposite directions must be made to bring the prism group 3 back to the "dawn" position; since the return angles are less than 180°, there is no pull on cables 12 and 32. As stated in the introduction, the entry aperture to each photovoltaic prism 1 making up the prism group 3 can, if desired, be closed by a lid of absolutely transparent plastic material, with an index of refraction greater than that of the ^"air, its thickness being so made as to obtain a slightly diverging lens. This increases the amount of light reaching the upper part of the panels that form the prism 1, compared with the lower part, in order to compensate for the lesser amount of light in the upper part due to its greater distance from the white bottom 4 of the prism. The plastic cover is similar to a flat-concave lens, and is placed so that its slightly concave surface lies towards the inside of the prism 1. From the description here given of how a preferred example has been realized, it is clear that a technician expert in the field could make a number of alterations to it without departing from the sphere of the invention, as will appear from the following claims. APPENDIX

How to control the sun follower . The angle of azimuth and the angle of solar elevation are two angles used for orienting the photovoltaic modules during the hours of insolation^' between dawn and sundown. These angles are calculated on the basis of Local Solar Time (LST), which differs from Local Time (LT) after allowing for the eccentricity of the earth's orbit and adoption of summer time. From ancient days, time was measured by observing the movement of the sun that makes a complete rotation around its own axis in about 24 hours (23 h, 56' and 4") which explains why the terrestrial globe has been fictionally subdivided into 24 meridian circles that subtend angles of 15° with a time equivalent of 1 hour each. For any particular "time zone" use is made ^' of Local Standard Time Meridian (LSTM) calculated as follows: LSTM = 15° x ATQ_MT , in which ΔT Q_MT is the difference in hours between LT and the starting point conventionally set at the Greenwich meridian. LST may be calculated by making two corrections to LT combined in the term Time Correction (TC), namely: LST -LT + TC. An initial correction is calculated by an empirical time equation known as Equation of Time (EoT), expressed in minutes, that corrects the eccentricity of the earth's orbit and the inclination of the earth's axis as follows:

EoT = 9, 87 si n(25)- 7, 53 COs(J?) -L 5 Sm(B) wherein: is

expressed in degrees and d is the number of days from the start of the year (January 1^st). The second time correction (expressed in minutes) allows for the variation in LST within a given time zone due to variations in longitude within the time zone itself. This second correction incorporates the first in the expression: TC = 4 (LSTM - Longitude) + EoT. Factor 4 allows for the fact that the Earth turns by 1° every four minutes. The Hour Angle (HRA) converts LST into the number of degrees covered by the sun in is path across the sky. By definition HRA = 0° at the solar midday (the meridian) and increases by 15° each hour. HRA may be expressed thus: HRA = 15°(LST - 12). In the morning the antemeridian (AM) Hour Angle is negative and in the postmeridian (PM) it is positive.

Declination of the sun is the angle between the equatorial plane and a line drawn between the centre of the earth and that of the sun. This angle varies according to the seasons due to the inclination of 23.45° of the Earth's axis of rotation with respect to the orbital plane followed annually by the Earth around the sun. The declination δ may be calculated by the equation:

δ is zero at the

equinoxes (March 22nd and September 22nd), and is positive in the northern hemisphere during the summer and negative during the winter; it reaches its maximum of 23.45° at the summer solstice (June 22nd) and a minimum of -23.45° at the winter solstice (December 22nd). The α angle of elevation is the angular height of the sun in the sky measured from the horizon. The α angle is equal to 0° at dawn and to 90° when .the sun is perpendicular to the ground, something only found at the equator at midday at the equinoxes. The Zenith angle is complementary to the angle ^' of elevation (90° - elevation). The a(t) angle varies during the day partly depending on the latitude of the site concerned and on the day of the year. The elevation may be calculated by the following formula: a (t) = sin^"1 [sin (<5) sin (φ) -f cos (δ) cos (φ) cos (HRAyj , in which ø is the latitude of the place concerned. As the equation shows, the angle of elevation α (f) is not a linear function of the time so that control of motor 28 is fully justified; A preferred control strategy adopts a step-by-step motor, namely an actuator making it possible to vary the angle of the rotor little by little and with a satisfactory degree of accuracy. Alternatively, using a DC motor, this would have to be driven by variable flow current like the function that links it to α(f), similarly to an arc of an ellipse. To bring the curve into line, the current must be adjusted by the microprocessor at fairly frequent time intervals. The moments of dawn and sunset at the horizon are calculated by zeroing the previous equation for elevation to obtain the following:

Claims

1. Photovoltaic electric generator comprising:

- at least one hollow body (1) of a predominantly lengthened form placed along an axis (PS), open at a first end to permit entry of sunlight; - means for reflecting sunlight (4, 5) placed at a second end of the hollow body (1) opposite to the first end;

- photovoltaic transducer means (2) for lining the inner surface of the hollow body;

- mechanical supporting means (23) for sustaining all the above components, comprising rotation actuator means (25, 25a; 27, 28) for keeping said axis (PS) constantly parallel to the direction from which the sunlight comes.

2. The generator in claim 1, wherein said means reflecting sunlight consist of a flat bottom (4) the colour of which is substantially white.

3. The generator in claim 2, wherein said fiat bottom (4) is made of material both waterproof and resistant to hail.

4. _. The generator in claims 2 and 3, wherein said flat bottom (4) consists of a thin sheet of aluminium fitted with fins set substantially perpendicular to the surface opposite that, receiving the sunlight.

5. The generator as in any of the claims from 1 to 4, wherein said long hollow body (1) is a prism, formed of single photovoltaic -rectangular modules or panels (2) joined together by their longer sides. '

6. The generator as in claim 5, wherein said prism (1) presents a cross section the form of which is that of a polygon of N sides, N preferably being equal to six.

7. The generator as in claim 6, wherein said reflecting means (5) consist of a pyramid with N faces reflecting the light, each face placed opposite to a face of the polygonal prism.

8. The generator as in claim 6, wherein said reflecting means (5) consist of a spherical reflecting dome-shaped element.

9. The generator as in any of the claims from 5 to 8, wherein several prisms (1) are assembled together to form an orientable structure (3).

10. The generator as in claim 9, wherein the prisms (1) are hexagonal and the structure, when assembled, presents the form of a "honeycomb" (3).

11. The generator as in any of the claims from 1 to 4, wherein said long hollow body (1) is a cylinder, and said photovoltaic material consists of flexible, electrically interconnected cells.

12. The generator as in claim 11, wherein several cylinders are assembled together to form an orientable structure.

13. The generator as in any one claim from 1 to 12, wherein said long hollow body (1) includes a cover made of transparent material, preferably plastic, possessing an index of refraction greater than that of air, said cover being flat and concave, concavity facing towards the inside of the hollow body (1) so as to form a slightly diverging lens.

14. The generator as in any one claim from 1 to 12, wherein said long hollow body (1) has slits (13-16) cut in it, in an area peripheral to the second end and devoid of photovoltaic material, to allow rainwater to be drained off.

15. The generator as in claim 9 or 12, wherein a flat support (6) is included, connected by one orientable side to said structure (3), and is joined by connecting elements (29, 30) to the other side to said rotation actuator means (27).

16. The generator as in claim 15, wherein said flat support (6) has slots (6a) cut through it, outside the lower perimeter of the structure.

17. The generator as in claim 15, wherein said flat support (6) is limited to the area only underneath said structure (3) and individually orientable.

18. The generator as in claim 5, wherein said flat support (6) consists of a frame.