ES2772937B2 - PHOTOVOLTAIC RECEIVER FOR CONCENTRATED SOLAR RADIATION BY REFLECTION IN PARALLEL TO THE SUNLIGHT - Google Patents

Description

DESCRIPTION

PHOTOVOLTAIC RECEIVER FOR CONCENTRATED SOLAR RADIATION

BY REFLECTION IN PARALLEL TO THE SUNLIGHT

TECHNICAL SECTOR

The invention belongs to the field of renewable energy engineering, and in particular to that of photovoltaic energy. The innovation is associated with a peculiar way of reflecting direct solar radiation, which allows the photodiodes of the receiver to be always attacked perpendicular to them.

TECHNICAL PROBLEM TO BE SOLVED AND BACKGROUND OF THE INVENTION

The problem consists of arranging the photodiodes in a suitable way to receive, with the best possible geometry, the radiation reflected by mirrors, capable of reorienting the direct radiation of the sun, towards a given focal line. By "best" it is to be understood a configuration of affordable cost, and that allows the natural evacuation of the heat contributed by the incident radiation, without the photodiodes reaching unacceptably high temperatures. This limitation attends to two phenomenologies to take into account: the reduction of photovoltaic efficiency when the temperature increases; and the fusion of some materials used in the photovoltaic panel, such as electrical insulators.

The problem, then, is to ensure the natural cooling of the photodiodes by passive means, while taking advantage of all the radiation arriving at the concentrator, and reflected by it towards the receiver.

Regarding precedents, it must be taken into account that this invention applies to a very particular conformation of the reflected radiation, and that is that the reflected radiation (any ray of it) falls perpendicularly on the receiver (that is, each one of the reflected rays has its trajectory contained in a plane that is perpendicular to the focal line of the receiver, and this plane also contains the virtual line that goes from the Sun to the point of the mirror where the reflection occurs.

The most direct precedent to achieve this form of reflected radiation is found in the invention ES 2713799 B2; which deals with the concentrator itself, not the receiving part, which is the characteristic of the novelty presented here. But without this precedent, this innovation would not make sense, as it lacks the apparatus on which to base it.

Also as a precedent that enables the application of this invention, mention must be made of patent ES 2345427 B2, which presents the intrinsic characteristics of reflection by Fresnel mirrors, with a focal axis parallel to the axis of rotation of the concentrating mirrors. The patent includes the theorem that in these mirrors, as they rotate, as the Sun moves, to keep always focused, on the focal axis, the ray reflected from any point of the axis of rotation of a mirror, the property is that the rays reflected from any specified point of the mirror are all parallel to each other. This geometric property serves to limit the maximum lateral displacement of the reflected rays, which involves limiting the width of the mirror.

Finally, to rotate the mirrors in the most efficient and cheap way possible, it is possible to use what is prescribed in the invention ES 2512190 B2, which shows a system of "push-pull" bars, articulated at the ends of a virtual diameter of the circumference that the mirror would describe in its rotation; and that they would make all the mirrors rotate at the same speed, which is another important property of this mode of reflection. Indeed, all mirrors must rotate half the rotation experienced by the figure of the Sun, in its ascent and descent within the plane perpendicular to the focal axis, however, the mirrors do not carry the same phase, which is precisely determined for a position of the Sun that is taken as reference.

On the other hand, precedents typical of the photodiode part do not exist as such, although the "Euclides Project" can be cited, with a photovoltaic longitudinal receiver on each side of the focal axis, the reflected and concentrated radiation from a longitudinal mirror with parabolic straight section. Information about the project can be found at https://www.iter.es/proyectoeuclides/. One of the essential difficulties of this project is that with this type of concentrating geometry, temperatures can be reached considerably high temperatures of 400 ° C, for example. This is unacceptable for the photodiode, which has to be thoroughly cooled, so that it does not lose much performance, and so that all its components resist. But this cooling is not simple or easy to do by natural means.

In fact, the vast majority of photodiodes that work with concentrated radiation do so by refraction, that is, with lenses. This serves to concentrate the entire lens area into a very small focal area. In this way, the energy reaching the photodiode being very limited, it can be eliminated by fins. Examples of this can be found in documents ES 2477191 T3, ES 2538815 T3 and ES 2654300 T3.

Nor can multilayer photodiodes be considered precedents, such as the one specified in JP 4170004 B2, since these layers refer to various superimposed pn junctions, which is not what is proposed in this invention, in whose structure there are different layers, not superimposed, but with a certain unfolding geometric. In turn, this is not related to the multiple space devices used in photovoltaic tomography, as can be seen in US 6115448 A.

EXPLANATION OF THE INVENTION

The invention corresponds to a photovoltaic receiver of solar radiation, concentrated in a very particular way, and the reflected radiation always goes in a direction perpendicular to the focal axis of the receiver, which in turn is perpendicular to the direction of direct solar radiation, which is reflected by a set of mirrors, equipped with axes of rotation arranged parallel to each other, and parallel to the focal axis of the receiver , which comprises a virtual volume of reflected radiation, which is made up of as many individual volumes as there are mirrors, covering each volume individual a zone comprised from the reflective surface of the mirror to the focal axis; and each individual virtual volume is divided in turn into a series of consecutive portions, forming virtual wedges of reflected radiation of a thickness selected to fit a photodiode, the width of which is equal to said thickness of the virtual radiation wedge; For the use of which by capturing photons, the receiver is located close to the focal axis, said receiver comprising:

- a set of photodiodes, each one of them with rectangular geometry on its active face, adapted to the dimensions of each individual virtual wedge, its active face oriented towards the origin of the radiation reflected by the concentrating mirrors; while its opposite face, corresponding to the rear face, is covered by a plate of highly heat conductive material, with conductivity greater than 10 W / m ° C, which is pressed towards the body of the photodiode by means of a strap, with an elongated configuration in a direction perpendicular to the surface of the plate, and each photodiode having a perimeter frame;

- Said strips comprising the plate, and also comprise at least two pins for connection to the plate, configured to perform functions of pressure of the strip and in addition to cooling, for which they are made of a material with conductivity greater than 10 W / m ° C , said pins having a geometry provided with planes perpendicular to the rear face of the photodiode, and the thermal contact at the junction between the plate and the pins being stimulated thanks to a curved part of said templates, further configured to produce pressure tightening between the base of the plate and the body of the photodiode; - for which, the end of the strap remote from the plate is pressed by a retainer that is firmly attached to a support structure; - Said supporting structure comprising longitudinal beams, parallel to the focal axis, and cross members perpendicular to said longitudinal beams, forming a criss-cross that establishes a set of rectangular-shaped frames, which define nests in which the photodiodes support, as they have their frame perimeter coincident with the corresponding rectangle of the frame;

- The support structure has a three-dimensional configuration in which, in each individual virtual radiation wedge, and in proximity to the focal axis, the photodiode nests are arranged, at a decreasing distance towards the focal axis, establishing different levels, where a number of nests per wedge, or levels, is chosen arbitrarily, of which only one is occupied per wedge, and the prescription of not occupying with photodiodes two nests of the same level, in contiguous wedges.

In particular, for the individual volume of radiation of a given mirror, the nests in which the photodiodes are housed form a ladder structure that goes up and down between the lowest and highest levels, as they go from one wedge to the next. contiguous, and in each step not only is raised or lowered a level, but the nest of the photodiode moves to one side, parallel to the focal axis of the receiver, and the displacement with respect to the previous level occupied by a photodiode, is equal to the width of the photodiode, which in turn is equal to the virtual thickness of the wedge.

EXPLANATION OF THE FIGURES

The figures, in general, are not to scale, since the relative sizes of the elements are very different, and not all the elements would be appreciated; but they are representative of the invention and its principles of operation.

Figure 1 shows a general diagram of the solar device as a whole, that is, not only the part of the receiver, where the invention is located, but also the part where the reflection that concentrates the radiation occurs.

Figure 2 shows the straight section of the receiver area, in a plane perpendicular to the focal axis, and therefore parallel to the direction of solar radiation. It includes radiation filters, which are not specific to the invention, but can improve its performance.

Figure 3 shows the plan section of the set, in which only one mirror has been represented, and where it can be seen that the reflected radiation is parallel to the incident, and can be grouped by corridors or contiguous strips, which is what has called virtual radiation wedges.

Figure 4 shows a possible model of the location of the photodiodes in their nests, differentiating between the 4 levels of said nests that are in each wedge.

Figure 5 shows a photodiode stack with the straps at the rear. Each photodiode corresponds to a wedge, and a level, different from the others in the stack.

Figure 6 shows, in horizontal perspective, a strap coupled to the rear of a photodiode.

Figure 7 corresponds to a photodiode and its seat in the support structure, where the strap has been omitted, with the exception of the plate.

Figure 8 shows a receiver module, with 7 radiation wedges and 4 photodiode levels.

Figure 9 corresponds to an aggregation of wedges greater than in the previous figure.

To facilitate understanding of the figures of the invention, and its modes of implementation, the relevant elements thereof are listed below:

1. Mirrors parallel to the focal axis, and always focused on it in their reflected radiation.

2. Axis, horizontal, of rotation of the mirrors for their focus. The rotation is determined because the normal to the mirror at its central point, which is the axis, is the bisector of the angle formed by the incident sun ray, and the visual from the central point to the focal axis.

3. Support bench for the axles (2) and the mirrors (1).

4. Rotating platform with vertical rotation axis, on which the benches (3) and other structural elements sit.

5. Wheels to enable the platform to rotate.

6. Chassis for setting the wheel hubs and supporting the platform and other elements of the rotating assembly.

7. Floor, or raceway.

8. Rotation (virtual) axis. The mechanism of the turns is not shown.

9. Pillar, supported on the platform (4) or on the chassis (6), which supports the receiver (11). The two pillars (9) plus the collector support structure (15) form a rigid gantry

10. Direction of direct solar radiation.

11. Solar receiver, composed of the various elements of the invention

12. Sloped side rails to stiffen the receiver structure

13. Stringers at the rear of the stiffening structure.

14. Filters (optional) that do not allow solar infrared radiation to pass through, so as not to thermally overload the photodiodes. In figure 2 a distinction is made between them, according to how they occupy a place to filter the radiation reflected by each mirror, in which the filter 141 filters what is reflected by the mirror closest to the pillars (9) that support the receiver: 142 to what is reflected by the 2nd; 143 to what is reflected by the 3rd; and the 144 to what is reflected by the 4th.

15. Support structure for the photodiodes of the receiver (11).

16. Buckled temples, from strap (28)

17. Closing plate for the back of the photodiode, which constitutes a base of the strap on which the pins are attached.

18. Side tabs for collecting the photodiode, which are an extension of the plate (17).

19. Semiconductor photodiode where the photovoltaic effect is generated

20. Perimeter frame of the photodiode

21. Back layer of the photodiode, electrical connections and others.

22. Delfleje seals

23. Longitudinal stringers of the structure (15)

24. Radial frame cross members (15)

25. Nests of the photodiodes, in which the perimeter frame (20) of each photodiode sits, in the structure (15).

26. Virtual wedges of reflected radiation, forming consecutive portions.

27. Focal axis of the receiver.

28. Straps fitted to the rear parts of the photodiodes, in turn held under pressure. by structure 15.

In addition, the following labels are used with intrinsic internal information, such as those referring to the position of the photodiodes, which consist of at least 3 digits, and specifically 3 in the figures shown in this document, which correspond to:

- the first digit is the order number of the mirror it affects, counted from the closest, horizontally, to the virtual vertical plane that contains the focal axis;

- the second digit is the order number of the nest levels, starting with the one furthest from the focal axis;

- the third is the order number of the virtual radiation wedge, starting from the left of the device in plan, seen from the zenith, and with the incoming solar radiation from the top of figure 3.

Generically the positions of the photodiodes are designated with the label 100; and the position of the strips that go to the rear faces are designated 100F. As examples in the figures are:

111. Position of the photodiode that receives the radiation reflected from the first mirror, and is located at the nest level closest to said mirror, and is also in the first radiation wedge. It is associated with the 111F to indicate its strap.

222. Position of the photodiode affected by the radiation reflected by the second mirror, which is located in the second nest level with respect to the mirror, and is also in the second virtual radiation wedge. It has on its back the strap labeled 222F.

333. Position of the photodiode affected by the radiation reflected by the third mirror, and which is located in the third nest level with respect to the mirror, and is also in the third radiation wedge. It has on its back the strap fin labeled 333F.

444. Position of the photodiode affected by the radiation reflected by the fourth mirror that is located in the fourth nest level with respect to the mirror, and is also in the fourth radiation wedge. It has on its back the strap fin labeled 444F.

The generic label 100F is the position of the strips that are on the back of each photodiode, and are numbered parallel to these (100), as seen above.

The position of the virtual radiation wedges also carry special numbering, with a 4-digit label: the first two digits are number 26, generic for wedges; the third corresponds to the order number of the mirror, and the fourth, to the order number of contiguity of the wedges, starting from the left, as in the case of photodiodes.

2611. Position of the wedge closest to the virtual plane of the portal of the focal axis, and first from the left.

2621. Position of the second wedge closest to the virtual plane of the portal of the focal axis, and first from the left.

2631. Position of the third wedge closest to the virtual plane of the portal of the focal axis, and first from the left.

2641. Position of the fourth wedge closest to the virtual plane of the portal of the focal axis, and first from the left.

In addition, the labels are used identifying the angle of each family of wedges belonging to the same individual volume of reflected radiation, which consist of the letter A followed by the order number of the mirror to which they are associated. Specifically, the following are used:

A1: angle, on the focal axis, of the individual volume associated with the first mirror. A2: angle, on the focal axis, of the individual volume associated with the second mirror. A3: angle, on the focal axis, of the individual volume associated with the third mirror. A4: angle, on the focal axis, of the individual volume associated with the fourth mirror. And the levels of the nests also have mixed labels, which are identified by the letter N followed by the order number of the level.

MODE OF EMBODIMENT OF THE INVENTION

The invention is assembled as an integral structure to the set of mirrors, and therefore must be firmly anchored on the rotating platform (4) or on its chassis (6). The structure is essentially formed from its constituent parts, which are the stringers (23) and cross members (24), made of suitable material, such as construction steel, and the assembly must leave the nests (25) of the photodiodes (19), as well as the retainers (22) for fixing and tightening the strips that also act as fins must be mounted.

The material of these should not be steel, but another metal with higher thermal conductivity, such as aluminum or copper (although this is much heavier).

The fundamental condition that is sought in this assembly is to provide a natural way of evacuation of the heat that the radiation deposits on the photodiode.

The problem is that the concentration of the radiation sought with the reflection focused towards a point of the focal axis (27), is very useful to reduce the amount of photodiode used, which is usually the most expensive factor of the photovoltaic cost, but its performance is It reduces a lot when the photodiode gets very hot, and there even comes a time when the deterioration of its connectors makes its operation unviable.

The concentration factor can be estimated as the distance from the focal axis to the center of the mirror divided by the distance from the axis to the center of the photodiode, already installed in its nest (25). This factor changes slightly as the sun it moves on its path, because the effective width of the mirror varies, but the maximum value of the concentration hardly varies, and is of the order of the estimate given. For example, if the axis-mirror distance is 5 meters, and there is 25 cm from the axis to the photodiode, the concentration will be 20. That means that if the direct radiation has an intensity of 1 kW / m2, and by effect of The cosine of the angle of reflection is reduced to 850 W / m2, radiation of intensity 17 kW / m2 will impact the photodiode. If filters are interposed (to stop long wavelength radiation, which does not produce photovoltaic effect) the intensity could be reduced to about 10 kW / m2, but the system would become more expensive, inducing turbulence in the air that would scatter part of focused radiation, and there would also be a greater shadow effect.

The application of the invention without other accessories would imply radiation of the said intensity, 17 kW / m2 from each photodiode, of which a fraction is converted into electrical energy, say 25% (as it is a high-performance photodiode, and therefore expensive, and hence the need to reduce the amount of photodiode to be required, to 1/17 of what would be needed in an assembly without concentration. The remaining 75% would be heat to evacuate, that is, about 13 kW / m2 .

If the width of the photodiode is A (m) per unit length, 13A kW / m would have to be evacuated.

The low weight of the structure allows it to be manufactured from various materials, but with adequate form rigidity, and sufficient resistance to temperature, which becomes the most difficult requirement.

A detailed analysis would imply having to design the strap, but for a conceptual estimation, it is enough to adopt representative values of the various coefficients. The problem is to determine what restrictions can be imposed on the width A, so that a very high value is not reached in the temperature of the photodiode, or more precisely in its back face, which is where the most temperature-sensitive part is. This would be the temperature of the plate that constitutes the base of the strap, and it could be requested, by design, that it was not higher than a value such that AT would be above the value of the temperature of the air in the atmosphere, with which there would be a natural convection film coefficient, h. If we call the effectiveness of the fin f , we could write the following thermal balance (for a double strap, of longitudinal extension L)

4fLATh = IA

where I is the intensity absorbed as heat. The effectiveness of the strap can be expressed as

f = (1 / mL) Th (mL)

where m is an inverse length whose value depends on h, on the conductivity of the strip and on its geometry. For a mL = 2 value, the effectiveness is 48%, which seems adequate. If it is lowered to mL = 1, the effectiveness goes up to 76%. With the first of these values, we find that L = 0.2 m for m = 10. Add to the hypotheses values such as h = 4 W / m2 ° C and AT = 125 ° C, and with the thermal balance condition given above, a width A of the photodiode of 1.5 cm would be obtained. Although it seems very narrow, it is perfectly commensurate with the usual widths of solar panels.

The particular embodiment described above is susceptible to detail modifications provided they do not alter the fundamental principle and the essence of the invention.

Claims (1)

r e iv in d ic a tio n s 1 - Photovoltaic receiver for concentrated solar radiation by reflection in parallel to sunlight, in which the reflected radiation always goes in a direction perpendicular to the focal axis of the receiver, which in turn is perpendicular to the direction of direct solar radiation, which It is reflected by a set of mirrors, equipped with axes of rotation arranged parallel to each other, and parallel to the focal axis of the receiver characterized by comprising a virtual volume of reflected radiation, which is made up of as many individual volumes as there are mirrors, covering each volume individual an area comprised from the reflective surface of the mirror (1), to the focal axis (27); and each individual virtual volume is divided in turn into a series of consecutive portions, forming virtual wedges of reflected radiation (26), of a thickness selected to fit a photodiode (19), whose width is equal to said thickness of the virtual radiation wedge; For the use of which by capturing photons, the receiver is located close to the focal axis, said receiver comprising: - a set of photodiodes (19), each of them with a rectangular geometry on its active face, adapted to the dimensions of each individual virtual wedge, its active face oriented towards the origin of the radiation reflected by the concentrating mirrors (1) ; while its opposite face, corresponding to the rear face, is covered by a plate (17) of highly heat conductive material, with conductivity greater than 10 W / m ° C, which is pressed towards the body of the photodiode by means of a strap ( 28), with an elongated configuration perpendicular to the surface of the plate, and each photodiode having a perimeter frame (20); - Said strips comprising the plate, and also comprise at least two pins (16) for connection to the plate, configured to carry out pressure functions of the strip and in addition to cooling, for which they are made of a material with conductivity greater than 10 W / m ° C, said pins having a geometry provided with planes perpendicular to the rear face of the photodiode, and thermal contact being stimulated at the junction between the plate and the pins thanks to a curved part of said templates, configured, in addition, to produce pressure tightening between the base of the plate, and the body of the photodiode; - for which, the end of the strap remote from the plate is pressed by a retainer (22) that is firmly attached to a support structure (15); - Said support structure comprising longitudinal beams (23), parallel to the focal axis (27), and cross members (24) perpendicular to said longitudinal beams, forming a criss-cross that establishes a set of rectangular-shaped frames, which define nests (25 ) on which the photodiodes rest, since their perimeter frame (20) coincides with the corresponding rectangle of the frame; - the support structure (15) having a three-dimensional configuration in which, in each individual virtual radiation wedge (26), and in proximity to the focal axis (27), the photodiode nests (25) are arranged, at a decreasing distance towards the focal axis, establishing different levels, where a number of nests per wedge, or levels, is arbitrarily chosen, of which only one per wedge is occupied, and the prescription of not occupying two nests with photodiodes (19) is fulfilled (25 ) of the same level, in contiguous wedges. 2 - Photovoltaic receiver for concentrated solar radiation by reflection in parallel to sunlight, according to claim one, characterized in that for the individual volume of radiation of a certain mirror (1), the nests (25) in which the photodiodes are housed , form a staircase structure that goes up and down between the lowest and highest levels, as you go from one wedge (26) to the next, and on each step you not only go up or down one level, but the The nest (25) of the photodiode moves to one side, parallel to the focal axis of the receiver (27), and the displacement with respect to the previous level occupied by a photodiode, is equal to the width of the photodiode (19), which in turn is equal to the virtual thickness of the wedge (26).