SOLAR CONCENTRATOR

Abstract

A collector includes a convergent lens (2) having a focal distance f and an image focal plane (PFI). The convergent lens (2) defines one of the walls of a casing (1) defined by two pairs of side walls (4a, 4c), a bottom wall (3) and a front wall defined by the lens (2), the side and bottom walls on the inside of the casing being reflective, and the depth p of the casing being lower than the focal distance f of the lens so that after multiple reflections, the ray beam (R1, R2) thus reflected is concentrated on a final image focus (I′) located inside the casing, the collector including a mobile receptor (6a) held inside the concentrated beam or in a position at least intersecting the beam by elements controlling the movement of the collector (6a) with that of the beam.

Description

The subject of the present invention is a solar concentrator of the type comprising, as collector, a convergent lens having, in a manner known per se, a focal distance and an image focal plane on which are concentrated, along a line, called “primary image focus”, the beam of the solar rays that said lens receives, said concentrated beam moving with the trajectory of the sun. Such a lens is said to be “linear” in that its focus is a line.

To take account of the variation of the direction of the solar rays in the course of the day and in the course of seasons, the known solar concentrators either use costly parabolic mirrors or are included in pivoting and motorized equipment that is complicated and fragile.

The present invention proposes to provide a simple and effective solution to remedy these drawbacks.

To this end, the present invention provides a solar concentrator of the abovementioned type, in which said convergent lens forms one of the walls of a chamber defined by:

two pairs of side walls, a bottom wall and a front wall formed by said lens, the side walls of each pair being parallel to each other, and each pair of side walls being perpendicular to the other pair,

the side and bottom walls, inside the chamber, being reflective,

the depth p between the front wall and the bottom wall being less than the focal distance f of the lens,

such that, after multiple reflections, the duly reflected beam of rays is concentrated on a line called “final image focus” symmetrical to said primary image focus relative to said bottom wall and belonging to a “near image focal plane” itself symmetrical to said image focal plane relative to said bottom wall, but located inside said chamber,

said concentrator enclosing a mobile receiver maintained within said concentrated beam, or in a position at least secant to said beam, by means servo-controlling the movement of said receiver to the movement of said beam.

In a preferred embodiment, the front and bottom walls of the chamber are perpendicular to the side walls; in other words, the chamber takes the form of a rectangular parallelepiped.

In this particular case, the depth p of the chamber satisfies the relation:

p=0.5*(f+e+b)

where:

• • e is the penetration thickness of the lens in the chamber, and
  • b is the distance between the lens and the near image focal plane or useful operating distance,

said concentrator enclosing a mobile receiver maintained within said concentrated beam, or in a position at least secant to said beam, by means servo-controlling the movement of said receiver to the movement of said beam.

Thus, the structure of the concentrator according to the invention makes it possible to follow the trajectory of the sun, by servo-controlling on this trajectory, not the orientation of the chamber, but the position of the receiver in the chamber. It follows from this, on the one hand, that the servo-control means can be considerably lighter than if the entire chamber had to be moved and, on the other hand, that the mobile element (the receiver) is protected from the external medium by the chamber.

In practice, the receiver is fitted to move, in the near image focal plane of said lens or in a plane parallel to said near image focal plane.

The servo-control means that can be used are within the area of competence of those skilled in the art. They can in particular apply similar principles to those implemented in the known concentrators. It will be understood that, in the absence of solar rays or when there is insufficient radiation, the receiver can remain temporarily immobile in the chamber and be repositioned relative to the concentrated beam when the radiation has returned to a sufficient level.

In one possible embodiment, to control the speed and the direction of movement of said receiver, the latter is provided with a photon flux meter adapted to send signals to driving means to which the receiver is subjected.

For the position of the receiver to be optimal, that is, for it to receive all the concentrated beam, the center of the receiver must be located within an area that affects an extent ranging from +k to −k either side of said near image focal plane, median to said area, k satisfying the relation:

$k=\frac{r}{\sin \left[A\tan \left(\frac{d}{f}\right)\right]}$

where:

• • r is the radius of the transverse section of the receiver if this section is circular or of the circle inscribed in the section of the receiver if this section is not circular, given that the expression “center of the receiver” is understood to mean the straight line parallel to the final image focus and which passes through the center of said circle;
  • sin[A tan] stands for sinus[arc tangent];
  • d is the distance between the optical axis of the lens and the edge of the lens, taken in the plane containing said optical axis and which is perpendicular to the bottom of the chamber and orthogonal to the final image focus.

However, it is possible to place the center of the receiver outside this optimal area while still obtaining an acceptable result, for example an economically acceptable result, if the loss of performance is offset by a significant reduction in the cost of the concentrator.

The convergent lens can take various forms, provided that it concentrates the solar rays along a line. Thus, the convergent lens can be flat-convex, biconvex or convergent meniscus.

Preferably, and without being exclusive, the convergent lens will be a Fresnel lens in the interests of reducing the footprint and weight of the lens. A Fresnel lens also has the advantage of being less absorbent to the rays that pass through it than other lenses.

In the case of a flat-convex Fresnel lens, that is, a lens that has a flat face and a sawtooth face, said lens will, preferably, be fitted so that its flat face faces the outside of said chamber.

This orientation has the advantage of placing the face of the lens that is most likely to trap dirt inside the chamber, the flat, outer face obviously being easier to clean. For the same reason, if the Fresnel lens is biconvex, that is, a lens having a smooth convex face and a sawtooth face, the lens will, preferably, be fitted so that its convex face faces the outside of said chamber.

In another embodiment, the lens can be a convergent meniscus lens, that is, a lens that has a convex face and a concave face; such a lens will necessarily be fitted so that its convex face faces the outside of said chamber.

The receiver is advantageously a heat pipe covered with a material, the heat absorption coefficient of which is greater than the heat emission coefficient.

In a particular embodiment of the invention, more particularly intended for solar power plants, the heat pipe takes the form of a pipe, possibly flexible, included in a pipe under vacuum, to limit the thermal loss.

The heat pipe is advantageously connected to an extraction exchanger fed with a heat-carrying fluid to exploit the heat obtained, for example to heat water or another fluid, to heat a device or to generate solar cold.

In another embodiment, the receiver is an extraction exchanger fed with a heat-carrying fluid.

In yet another embodiment, the receiver can be a photovoltaic cell receiver.

In a preferred embodiment, the receiver can occupy two positions, namely a service position in which it receives a certain thermal energy and a retracted position in which it receives a lesser thermal energy than in the service position, retracting means being able to move the receiver from its service position to its retracted position, where there is risk of overheating, for example, in the case where the circulation of heat-carrying fluid is no longer in the extraction exchanger.

The receiver can be connected to a Stirling engine, that is, an engine that exploits a temperature difference between a hot source and a cold source, notably for the purpose of producing electricity.

Preferably, the surfaces of the lens are treated to reduce their potential degradation over time, degradations that can consist, mainly on the outer side, in dirt, and on the inner side, in the deposition of metal particles ejected from the reflective surfaces. Such a treatment can consist of a non-stick surface treatment increasing the wettability and obtained by applying thin layers consisting of compounds based on SiOx (SiO₂ etc.) and/or of coatings that make it possible to reduce the attachment of different pollutants, such as TiO₂-type photocatalytic compounds.

It can also be a protection against ageing of the lens material, obtained by deposition on the outer surface of the lens of conventional anti-glare-treatment optical layers. Such an anti-glare treatment also has the advantage of reducing the reflection, by the lens, of the rays that it receives on certain incidences.

The same applies to the reflective walls of the chamber which will advantageously be treated to reduce their potential degradation over time.

Concerning the reflective walls, as a variant, they can be made of reflective panels that can be removed for the purposes of cleaning, replacement or complete flat-packing of the chamber for transport or moving purposes.

The invention will be better understood from reading the following description, given with reference to the appended drawings in which:

FIG. 1 is a cut-away perspective diagrammatic view of an embodiment of a chamber according to the invention,

FIGS. 2a, 2b and 2c illustrate various types of lenses that can be used according to the invention with identification of the thickness e;

FIGS. 3a and 3b are diagrams of one embodiment of the chamber according to the invention, illustrating the effect of the useful distance b on the depth of the chamber;

FIGS. 4a and 4b are diagrams of an embodiment of a chamber according to the invention, seen in cross section in a plane perpendicular to the general direction of the lens, and illustrating the path of the solar rays along two different incidences;

FIGS. 5a and 5b are diagrams illustrating the parameter k and the optimal positioning area of the receiver, FIG. 5b being a larger-scale view of the area of the final image focus of FIG. 5a, and

FIG. 6 is a diagram illustrating one embodiment of a driving mechanism for the heat pipe.

As can be seen in FIG. 1, the chamber 1, in this embodiment of the invention, is of rectangular parallelepipedal form, consisting of a front wall comprising a linear convergent lens 2, a rear or bottom wall 3 and side walls 4a-d. The internal faces of the side walls 4a-d and bottom wall 3 of the chamber 1 are reflective, either coated with a reflective film or lined with a removable reflective wall.

The side wall 4b includes a slot such as 5, in which a heat pipe 6 can slide in a plane parallel to the general plane of the lens 2, the heat pipe being supported, opposite the slot, by appropriate means (not represented) allowing this sliding movement.

The heat pipe 6 is clad with a material having a thermal dissipation coefficient less than its thermal absorption coefficient to limit the losses, as much as possible. An extraction exchanger 7, fed with fluid that is cold on 7a and leaves hot on 7b, evacuates the heat from the heat pipe for appropriate exploitation.

The chamber 1 is prolonged by a housing 8 (the start of which is shown by broken lines in FIG. 1) for the extraction exchanger 7 and a driving mechanism not represented in FIG. 1 (see FIG. 6). The housing 8 can have the same rectangular section as the chamber 1 and be closely connected thereto to avoid any ingress of rainwater or dust. It can advantageously be opaque to slow down the ageing of the flexible pipes 9a and 9b (FIG. 6).

The lens 2 of the chamber 1 is struck by the solar rays along an incidence that varies with the time of day, the season, and so on, and two such different incidences are illustrated by the rays R and R′.

To return to the optical plane, FIG. 4a, which represents the chamber 1 without the heat pipe 6 or the extraction exchanger 7 for clarity of representation, it can be seen that the lens 2 comprises a flat-convex Fresnel lens 2, the flat face of which faces the outside of the chamber. The thickness of the lens is exaggerated in the figure also in the interests of clarity. The lens 2 has an optical axis AA, a focal distance f greater than the depth p of the chamber 1 and an image focal plane PFI which is beyond the bottom 3 of said chamber 1.

As indicated above, the depth p of the chamber must satisfy the relation:

p=0.5*(f+e+b)

This relation is explained by reference to FIGS. 2a-2c and 3a-3b.

FIGS. 2a, 2b and 2c respectively show

• • a flat-convex lens 2a, in this case a Fresnel lens,
  • a biconvex lens 2b, and
  • a meniscus lens 2c,

forming one of the walls of a chamber, of which the beginning of the side walls 4a and 4c can be seen.

In the case of the Fresnel lens 2a, the flat face of the lens coincides with the plane FF passing through the adjacent edge of the side walls 4a-4d, and the penetration thickness e is the distance between this plane FF and the plane TT tangential to the most prominent part of the lens inside the chamber.

In the case of the biconvex lens 2b, one of the convex faces of the lens projects outside the chamber 1 and the other convex face projects towards the interior of the chamber. The penetration thickness e is the distance between the median plane of the lens, a plane that coincides with the plane FF, passing through the adjacent edge of the side walls 4a-4d, and the plane TT tangential to the most prominent part of the lens inside the chamber.

In the case of the meniscus lens 2c, no part of the lens penetrates into the chamber (apart from the lens fixing), so the thickness e is more or less zero.

As can be seen from FIGS. 3a and 3b, where the lens has been diagrammatically represented in the form of a simple rectangle designated by 2a-c, to show that it can be any one of the lens types 2a, 2b or 2c illustrated in FIGS. 2a to 2c, the parameters needed to determine the depth of the chamber are indicated.

In FIGS. 3a and 3b, it can be seen that the lens 2a-c has a thickness e and a focal distance f, a distance that determines the image focal plane PFI.

In the case of FIG. 3a, a useful distance b1 is provided, a distance that needs to allow for the accommodation of the receiver, in other words the heat pipe 6 in the embodiment of FIG. 1, and its support opposite the slot 5, taking into account the heat sensitivity of the lens, therefore of the material of the lens.

Initially, for simplicity, it will be assumed that the receiver 6 is in the PFIR plane located at e+b1 of the plane FF, which is a particular case, as will be seen in light of FIGS. 5a and 5b. The bottom 3 of the chamber must also be equidistant from the PFIR plane and the PFI plane.

In the case where b=b₁ (FIG. 3a), the distance between PFIR and PFI is 2*x₁.

In the case where b=b₂ with b₂>b₁ (FIG. 3b), the distance between PFIR and PFI is 2*x₂.

The choice of b is within the scope of those skilled in the art. It depends on the footprint of the receiver 6 and means that are associated with it, as well as the lens material.

As an example, for a flat-convex Fresnel lens of 50 cm×100 cm, made of glass with a refractive index n=1.5, a focal distance f of 80 cm and a thickness e=1.5 cm, observing a useful distance b=15 cm, the depth p of the chamber 1 will be equal to the product 0.5(f+e+b)=0.5*(80+1, 5+15)=48.25 cm. Obviously, these values are given only to enable the reader to clearly understand the principle of the invention. In practice, these values can be other values and the depth of the chamber can be even smaller relative to the focal distance.

To return to FIG. 4a, in the absence of the bottom of the chamber, solar rays striking the lens 2 parallel to the ray R would be concentrated on a primary image focus, in the image focal plane PFI. However, the reflective side walls, such as 4a, and the reflective bottom 3 of the chamber stop the rays R and reflect them until they are concentrated on a final image focus, in a near image focal plane PFIR parallel to the image focal plane PFI, but inside the chamber 1. In FIG. 4a, this final image focus is seen in cross section, therefore in the form of a dot I.

FIG. 4b is similar to FIG. 4a, except that it illustrates another direction of impact of the rays, such as R′, on the lens 2. As can be seen, after multiple reflections, these rays R′ are concentrated on a final image focus, also located in the plane PFIR, and this final image focus is seen in cross section in FIG. 4b, therefore in the form of a dot I′.

Thus, the final image focus of the rays R and that of the rays R′ are located in the same plane PFIR, but on two different lines or, to express it differently, the linear final image focus moves translation-wise in the plane PFIR following the trajectory of the sun.

The heat pipe 6 which, in the particular case considered, is positioned in the plane PFIR, moves to follow this translation movement of the linear final image focus. To this end, motor means are provided, servo-controlling the movement of the heat pipe to the trajectory of the sun, or more precisely to the trajectory of the beam of rays concentrated towards the final image focus. This servo-control takes into account the place where the chamber is installed, the season, the time of day, and so on.

As indicated above, the heat pipe can, furthermore, be subject to retracting means adapted to move it, if necessary, out of its service position, to avoid overheating. To this end, the retracting means move the heat pipe from its service position where it receives a certain thermal energy to a retracted position where it receives a lesser thermal energy than in the service position.

As can be seen from the example with figures indicated above, the invention considerably reduces the distance needed between the lens and the heat pipe. Without the invention, in the example given, this distance would be f−e=80−1.5=78.5 cm, whereas according to the invention and still in the example concerned, it is only 48.25 cm.

Referring to FIG. 5a, the chamber 1 is shown with its lens 2 and its bottom 3. Also to be seen in this figure is a receiver 6a which can be presented in the form of an appliance of circular section of radius r (see FIG. 5b), but which is not necessarily circular. If the receiver is not of circular section, the circle inscribed in the non-circular section is taken into account. Also identified in this figure is the distance d used in calculating the value k.

R₁ and R₂ indicate solar rays forming the outer limits of the beam of rays striking the lens 2 with a zero incidence. The beam bounded by R₁ and R₂ converges towards the plane PFI but is stopped and reflected by the bottom 3 to converge in a concentrated beam towards the plane PFIR that it crosses along a line seen in cross section at I″, corresponding to the final image focus, to diverge beyond the plane PFIR. It will be understood that the concentrated beam thus delimits, either side of the final image focus I″, two crossed planes with an angle between them of α.

As long as these planes are tangential to the receiver (position 6a-FIGS. 5a and 5b) or secant to the receiver (position 6b-FIG. 5b), the receiver receives all the concentrated beam. However, if the receiver is indeed located between these planes, without these planes being secant or tangential to it (position 6c-FIG. 5d), a part of the concentrated beam, namely the part that is between, respectively, the plane of the rays R₁ and R₂ and the tangents T₁ and T₂ to the receiver 6c, does not strike the receiver.

Thus, as can be seen in FIG. 5b, for the receiver to occupy an optimum position, the line passing through the center of the receiver and parallel to the final image focus I″ (line Ca for the receiver in position 6a, line Cb for the receiver in position 6b) must be located in an area of extent E ranging from +k to −k, either side of the plane PFIR, k necessarily satisfying the relation:

$k=\frac{r}{\sin \left[A\tan \left(\frac{d}{f}\right)\right]}$

where r, d and f are as defined above, the center of the receiver possibly being able to coincide with said final image focus I″ (abovementioned particular case).

A position of the receiver such as 6c, where the line Cc is outside the area of extent E, is not however a situation beyond the scope of the invention; this position can be acceptable, even though the receiver does not receive all of the concentrated beam, for example if it is less costly to position the receiver at 6c than at 6a or 6b.

Actually, the positions 6a, 6b and 6c could equally be on the other side of the plane PFIR.

FIG. 5b shows, furthermore, for the receiver 6a, a service position (in this case, within the concentrated beam and tangential to the planes bounding this beam) and a retracted position, illustrated at 6a′ where the receiver is totally outside the concentrated beam. The position 6c could also be considered to form the retracted position of the receiver 6a.

Referring to FIG. 6, the references E1 and E8 respectively designate the internal space of the chamber 1 and the internal space of the housing 8, separated by the partition 4b, slotted at 5. The heat pipe 6 and the exchanger 7 with its supply of cold fluid on 7a and its discharge of hot fluid on 7b are located therein, as represented in FIG. 1. More specifically, this supply and this discharge take place via flexible pipes, respectively 9a and 9b, connected to nozzles, respectively 10a and 10b, themselves in fluid communication with the inside of the exchanger 7. Flexible pipes 9a and 9b are used, obviously, to enable the heat pipe 6 to move.

To obtain this movement, the heat pipe 6 is connected, via a collar 11 provided with a fork 12a, 12b, to the rotation axis of a gear 13 which meshes with a rack 14, the gear 13 itself being driven rotation-wise by a motor 15.

A photon flux meter is diagrammatically represented as 16, making it possible to send signals to said motor means to control the direction and the speed of rotation of the gear 13, and therefore of the heat pipe 6.

Obviously, the invention is not limited to the embodiments described and represented. Thus, the lens and the bottom of the chamber are not necessarily perpendicular to the side walls of said chamber, and not necessarily parallel to each other. Instead of incorporating a heat pipe, the chamber could contain an extraction exchanger fed with heat-carrying fluid or a linear volume clad with photovoltaic cells, both mobile as described for the heat pipe. Moreover, it is possible to juxtapose several lenses, each forming a face of a “sub-chamber”, the duly juxtaposed sub-chambers having components in common, notably a common driving mechanism, to limit the quantity of constituent materials used and to reduce the number of servo-controls for moving the receiver.

Claims

1-18. (canceled)

19. A solar concentrator of the type comprising, as collector, a convergent lens having a focal distance and an image focal plane, on which are concentrated, along a line, called “primary image focus”, the beam of the solar rays that said lens receives, said concentrated beam moving with the trajectory of the sun, wherein said convergent lens forms one of the walls of a chamber defined by:

two pairs of side walls, a bottom wall and a front wall formed by said lens, the side walls of each pair being parallel to each other, and each pair of side walls being perpendicular to the other pair,

the side and bottom walls, inside the chamber, being reflective,

the depth between the front wall and the bottom wall being less than the focal distance of the lens,

such that, after multiple reflections, the duly reflected beam of rays is concentrated on a line called “final image focus” symmetrical to said primary image focus relative to said bottom wall and belonging to a “near image focal plane” itself symmetrical to said image focal plane relative to said bottom wall, but located inside said chamber,

said concentrator enclosing a mobile receiver maintained within said concentrated beam, or in a position at least secant to said beam, by means servo-controlling the movement of said receiver to the movement of said beam.

20. The concentrator as claimed in claim 19, wherein said chamber is rectangular parallelepiped and wherein the depth p of the chamber, for this purpose, satisfies the relation where:

p=0.5*(f+e+b)

e is the penetration thickness of the lens in the chamber, and

b is the distance between the lens and the near image focal plane or useful operating distance.

21. The solar concentrator as claimed in claim 19, wherein the center of said receiver is located within an area that affects an extent ranging from +k to −k either side of said near image focal plane, median to said area, k satisfying the relation: k = r sin  [ A   tan  ( d f ) ] where:

r is the radius of the transverse section of the receiver if this section is circular or of the circle inscribed in the section of the receiver if this section is not circular, given that the expression “center of the receiver” is understood to mean the straight line parallel to the final image focus and which passes through the center of said circle;

sin[A tan] stands for sinus[arc tangent];

d is the distance between the optical axis of the lens and the edge of the lens, taken in the plane containing said optical axis and which is perpendicular to the bottom of the chamber and orthogonal to the final image focus.

22. The concentrator as claimed in claim 19, wherein said convergent lens is flat-convex, biconvex or convergent meniscus.

23. The concentrator as claimed in claim 19, wherein said convergent lens is a Fresnel lens.

24. The concentrator as claimed in claim 19, wherein said convergent lens is a flat-convex Fresnel lens fitted so that its flat face faces the outside of said chamber.

25. The concentrator as claimed in claim 19, wherein said convergent lens is a biconvex Fresnel lens, fitted so that its convex smooth face faces the outside of said chamber.

26. The concentrator as claimed in claim 19, wherein said receiver is a heat pipe covered with a material, the heat absorption coefficient of which is greater than the heat emission coefficient.

27. The concentrator as claimed in claim 19, wherein said receiver is a heat pipe covered with a material, the heat absorption coefficient of which is greater than the heat emission coefficient, said heat pipe taking the form of a pipe included in a pipe under vacuum.

28. The concentrator as claimed in claim 19, wherein said receiver is a heat pipe covered with a material, the heat absorption coefficient of which is greater than the heat emission coefficient, said receiver being connected to an extraction exchanger fed with a heat-carrying fluid.

29. The concentrator as claimed in claim 19, wherein the receiver is a photovoltaic cell receiver.

30. The concentrator as claimed in claim 19, wherein said receiver is an extraction exchanger fed with heat-carrying fluid.

31. The concentrator as claimed in claim 19, wherein said receiver can occupy two positions, namely a service position in which it receives a certain thermal energy and a retracted position in which It receives a lesser thermal energy than in the service position.

32. The concentrator as claimed in claim 19, wherein the receiver is connected to a Stirling engine.

33. The concentrator as claimed in claim 19, wherein the surfaces of the lens and/or the reflective walls of the chamber are treated to reduce the potential degradation of their material over time.

34. The concentrator as claimed in claim 19, wherein the outer surface of the lens includes an anti-glare treatment.

35. The concentrator as claimed in claim 19, wherein the reflective walls are made of removable reflective panels.

36. The concentrator as claimed in claim 19, wherein, to control the speed and the direction of movement of the receiver, the latter is provided with a photon flux meter adapted to send signals to driving means to which said receiver is subjected.

37. The solar concentrator as claimed in claim 20, wherein the center of said receiver is located within an area that affects an extent ranging from +k to −k either side of said near image focal plane, median to said area, k satisfying the relation: k = r sin  [ A   tan  ( d f ) ] where:

r is the radius of the transverse section of the receiver if this section is circular or of the circle inscribed in the section of the receiver if this section is not circular, given that the expression “center of the receiver” is understood to mean the straight line parallel to the final image focus and which passes through the center of said circle;

sin[A tan] stands for sinus[arc tangent];

d is the distance between the optical axis of the lens and the edge of the lens, taken in the plane containing said optical axis and which is perpendicular to the bottom of the chamber and orthogonal to the final image focus.