FR2927155A1 - SOLAR CAPTOR. - Google Patents

Abstract

The sensor comprises a converging lens (2) having a focal length f and an image focal plane (IFP). The convergent lens (2) constitutes one of the walls of a rectangular parallelepipedic box (1), of depth p <f and whose other walls (3,4a-4c), on the inside of the box, are reflective, so that that after multiple reflections, the ray beam (R1, R2) thus reflected, which moves with the course of the sun, is concentrated on a final image focus (I "), located inside said box , the depth p of the box responding, for this purpose, to the relation p = 0.5 * (f + e + b) where e is the penetration thickness of the lens (2) in the box (1) and b is a useful operating distance, said sensor enclosing a mobile receiver (6a) maintained within said concentrated beam, or in a position at least secant to said beam, by means slaving the displacement of said receiver (6a) to the displacement of said beam.

Description

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

To account for the variation of the direction of the sun's rays during the day and during the seasons, known solar collectors or use expensive parabolic mirrors or are included in pivoting and motorized equipment, complicated and fragile.

The present invention proposes to provide a simple and effective solution to overcome these disadvantages. To this end, the present invention provides a solar collector of the aforementioned type, in which said convergent lens constitutes one of the walls of a rectangular parallelepipedic box, whose depth p is less than the focal length f of the lens and whose other walls, inner side of the box, are reflective, so that at the end of multiple reflections, the ray beam thus reflected is concentrated on a line, called "final image focus", parallel to said primary image focus and belonging to a "close-image focal plane", itself parallel to said image focal plane, but situated inside said box, the depth p of the box responding, for this purpose, to the relation p = 0.5 * (f + e + b) where. E is the thickness of penetration of the lens 35 into the box, and b is the distance between the lens and the focal plane close image or useful distance of operation, said sensor enclosing a mobile receiver held within said concentrated beam , or in a position at least secant said beam, by means slaving the displacement of said receiver to the displacement of said beam. Thus, the structure of the sensor according to the invention makes it possible to follow the course of the sun, by slaving to this race, not the orientation of the box, but the position of the receiver in the box. It follows, on the one hand, that the servo-control means can be considerably lighter than if it were to move the whole box and, on the other hand, that the mobile element (the receiver) is protected from the outside by the box.

In practice, the receiver is mounted movably in the focal plane close to the lens or in a plane parallel to said close-up focal plane. Servo control means that can be used are in the field of skill of the person skilled in the art. In particular, they can apply principles similar to those implemented in the known sensors. It will be understood that in the absence of solar rays or insufficient radiation, the receiver can remain temporarily immobile in the box and come to reposition relative to the concentrated beam when the radiation has recovered to a sufficient level. In order for the receiver position to be optimal, that is, to receive all the concentrated beam, the center of the receiver must be located within an area that ranges from + k to -k on either side of said focal image plane, median to said area, k satisfying the relation r rd)] Atan where. r is the radius of the cross-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, it being understood that by "center of the receiver" is meant the line parallel to the focus image final and passing through the center of the circle; . sin [Atan] means 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 box and orthogonal to the final image focus. However, it is possible to place the center of the receiver outside this optimum zone always 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 sensor. The converging lens can take various forms as long as it concentrates the sun's rays along a line. Thus, the convergent lens may be plano-convex, biconvex or convergent meniscus. Preferably and without being limiting, the convergent lens will be a Fresnel lens, for reasons of reducing the size and weight of the lens. A Fresnel lens also has the advantage of less absorbing rays that pass through it than other lenses. In the case of a plano-convex Fresnel lens, that is to say a lens having a flat face and a sawtooth face, said lens will preferably be mounted so that its face plane is turned towards the outside of said box. This orientation has the advantage of placing inside the box the face of the lens that is most likely to trap dirt, the flat face, outside, obviously being easier to clean. For the same reason, if the Fresnel lens is biconvex, i.e. a lens having a convex smooth face and a sawtooth face, the lens will preferably be mounted so that its convex face is turned towards the outside of said box.

In another embodiment, the lens may be a convergent meniscus lens, i.e. a lens having a convex face and a concave face; such a lens will necessarily be mounted so that its convex face is facing outwardly of said box. The receiver is advantageously a heat pipe covered with a material whose heat absorption coefficient is greater than the emission coefficient of heat.

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

The heat pipe is advantageously connected to an extraction exchanger fed with a heat transfer 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 supplied with a heat transfer fluid. In yet another embodiment, the receiver may be a photovoltaic cell receiver. In a preferred embodiment, the receiver is capable of occupying two positions, namely a service position in which it receives a certain thermal energy and a retracted position in which it receives a lower thermal energy than in the service position. , retractable means being able to move the receiver from its operating position to its retracted position, in case of risk of overheating, for example, in the case where the circulation of heat transfer fluid would no longer be in the extraction exchanger . The receiver may be connected to a Stirling engine, that is, an engine that exploits a temperature difference between a hot source and a cold source, particularly for the purpose of generating electricity. Preferably, the surfaces of the lens are treated so as to reduce their potential deterioration with time, alterations which may consist mainly of dirt on the outer side, and deposition of metal particles on the inner side ejected from the reflective surfaces. Such a treatment may consist of an anti-blocking surface treatment increasing the wettability and obtained by application of thin layers composed of SiOx-based compounds (SiO2 etc.) and / or coatings which make it possible to reduce the adhesion of various pollutants, such as photocatalytic compounds of TiO2 type. It may also be a protection against aging of the lens material obtained by deposition on the outer surface of the lens of conventional optical layers in anti-reflective treatment. Such an antireflection treatment has, moreover, the advantage of reducing the reflection, by the lens, of the rays which it receives according to certain incidences. The same is true of the reflective walls of the box which will advantageously be treated so as to reduce their potential deterioration with time. Regarding the reflective walls, alternatively, they may be made of removable reflective panels for cleaning, replacement or complete flattening of the box for transport or displacement. The invention will be better understood on reading the following description given with reference to the appended drawings, in which: FIG. 1 is a diagram, in perspective, cutaway, of an embodiment of a box according to FIG. 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 the box according to the invention, illustrating the effect of the useful distance b on the depth of the box; FIGS. 4a and 4b are diagrams of one embodiment of a box according to the invention, seen in section in a plane perpendicular to the general direction of the lens, and illustrating the path of the solar rays in two angles. different; and FIGS. 5a and 5b are diagrams illustrating the parameter k and the optimal positioning zone of the receiver, FIG. 5b being a view on a larger scale of the area of the final image focus of FIG. 5a. As can be seen in FIG. 1, the box 1 according to the invention is of rectangular parallelepipedal shape, composed of a front wall consisting of a linear converging lens 2, a rear wall or bottom 3 and side walls 4a. d. The inner faces of the side walls 4a-d and bottom 3 of the box 1 are reflective, either they are coated with a reflective film or they are lined with a removable reflective wall. The side wall 4b has a slot such as 5, in which is slidable a heat pipe 6 in a plane parallel to the general plane of the lens 2, the heat pipe being supported, opposite the slot, by appropriate means ( not shown) allowing this sliding. The heat pipe 6 is sheathed with a material having a heat dissipation coefficient lower than its thermal absorption coefficient to limit the losses as much as possible. An extraction exchanger 7, fed with cold fluid according to 7a hot spring 7b, removes heat from the heat pipe for proper operation. The casing 1 is extended by a housing 8 (primed in dashed lines in FIG. 1) for the extraction exchanger 7. The lens 2 of the casing 1 is struck by the sun's rays at an incidence that varies with the hour of the day, season, etc. and two such different incidences are illustrated by the rays R and R '.

If we come to the optical plane, FIG. 4a, which represents the casing 1 without the heat pipe 6 nor the extraction exchanger 7 for the clarity of the representation, we see that the lens 2 is constituted by a lens of Fresnel 2 convex plane whose flat face is turned towards the outside of the box. The thickness of the lens has been exaggerated in the figure also for the sake of clarity. The lens 2 has an optical axis AA, a focal length f greater than the depth p of the box 1 and an image focal plane PFI which is beyond the bottom 3 of said box 1.

As indicated above, the depth p of the box must correspond to the relation: p = 0.5 * (f + e + b) This relation is explained with reference to FIGS. 2a-2c and 3a-3b.

Figures 2a, 2b and 2c show respectively. a plano-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 box which we see the beginning of the side walls 4a and 4c. In the case of the Fresnel lens 2a, the plane 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 TT plane tangent to the most prominent part of the lens inside the box.

In the case of the biconvex lens 2b, one of the convex faces of the lens protrudes outwardly of the casing 1 and the other convex face protrudes towards the inside of the casing. The penetration thickness e is the distance between the median plane of the lens, which plane merges 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 plane. the lens inside the box.

In the case of the meniscus lens 2c, no part of the lens penetrates into the box (when the lens is attached), so that the thickness e is substantially zero. As is apparent from Figures 3a and 3b, where the lens has been schematized in the form of a simple rectangle designated 2a-c, to show that it may be any of the types of lenses 2a, 2b or 2c illustrated in Figures 2a to 2c, the parameters necessary for the determination of the depth of the box 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, which distance determines the image focal plane PFI. In the case of Figure 3a, there is provided a useful distance b1, distance which must allow to accommodate the receiver, in other words the heat pipe 6 in the embodiment of Figure 1, and its support opposite the slot 5, taking into account the heat sensitivity of the lens, thus the material of the lens. In the first place, for simplicity, it will be considered that the receiver 6 is in the plane PFIR situated at e + b1 of the plane FF, which is a special case, as will be seen with reference to FIGS. 5a and 5b. The bottom 3 of the box must be equidistant from the PFIR plane and the PFI plane. In the case where b = bl (FIG. 3a), the distance between PFIR and PFI is 2 * xl. In the case where b = b2 with b2> b1 (Figure 3b), the distance between PFIR and PFI is 2 * x2.

The choice of b is within the reach of those skilled in the art. I l depends on the size of the receiver 6 and the means associated therewith as well as the material of the lens. By way of example, for a plano-convex Fresnel lens of 50 cm × 100 cm, of glass having a refractive index n = 11.5, a focal length f of 80 cm and a thickness e = 1.5 cm, with respect to a useful distance ID = 15 cm, the depth p of the box 1 will be equal to the product 0.5 (f + e + b) = 0.5 * (80 + 1.5 + 15) = 48.25 cm. It is understood that these values are given only to allow the reader to understand the principle of the invention. In practice, these values can be other and the depth of the box even smaller compared to the focal length.

Returning to FIG. 4a, in the absence of the bottom of the box, solar rays striking the lens 2 parallel to the ray R would focus on a primary image focus in the image focal plane PEI. However, the lateral reflecting walls, such as 4a, and the reflecting bottom 3 of the box stop the R-rays and reflect them until they focus on a final image focal point, in a close-up image plane PFIR parallel to the plane focal image PFI, but inside the box 1. In FIG. 4a, this final image focus is seen in section, thus in the form of a point 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, at the end of multiple reflections, these rays R' concentrate on a final image focus, also located in the plane PFIR, and this focus final image is seen in section in Figure 4b, so in the form of a point I '. Thus, the final image focus of the R-rays and that of the R 'rays are located in the same plane PFIR, but along two different lines or, expressed otherwise, the linear final image focus moves in translation in the plane PFIR as and when measure of the sun's course. The heat pipe 6 which, in the particular case envisaged, is arranged in the plane PFIR, moves to follow this displacement in translation of the final linear image focus. To this end, there are provided motor means that control the movement of the heat pipe to the path of the sun, or more precisely to the path of the beam of concentrated beams towards the final image focus. This enslavement takes into account the location of the caisson, the season, the time of day, etc. As indicated above, the heat pipe may, in addition, be subjected to retractable means adapted to move it, if necessary, out of its operating position, to avoid overheating. To this end, the retracting means will move the heat pipe from its service position where it receives a certain thermal energy to a retracted position where it receives a lower thermal energy than in the service position.

As is apparent from the numerical example indicated above, the invention considerably reduces the distance required between the lens and the heat pipe. Without the invention, in the example given, this distance would be fe = 80-1.5 = 78.5 cm, whereas according to the invention and still in the example in question, it is only 48, 25 cm. If we come to Figure 5a, there is the box 1 with its lens 2 and its bottom 3. It also shows a receiver 6a which has been presented in the form of a circular section device of radius r (see Figure 5b), but not necessarily circular. If the receiver is not circular, consider the circle in the non-circular section. The distance d used in the calculation of the value k is also identified in this figure. At R1 and R2 are indicated solar rays forming the outer boundaries of the beam of rays striking the lens 2 with zero incidence. The beam bounded by R1 and R2 converges towards the PFI plane but is stopped and reflected by the bottom 3 to converge in a concentrated beam towards the plane PFIR that crosses in a line view in section I ", corresponding to the final image focus In order to diverge beyond the plane PFIR, it will be understood that the concentrated beam thus delimits, on either side of the final image center I ", two X-planes forming an angle α between them.

As long as these planes are tangent to the receiver (position 6a - FIGS. 5a and 5b) or secant to the receiver (position 6b - FIG. 5b), the receiver receives the entire concentrated beam. On the other hand, if the receiver is well located between these planes, without these planes being intersecting or tangent (position 6c - figure 5d), a part of the concentrated beam, namely the part which lies between, respectively, the plane of R1 and R2 and the tangents T1 and T2 to the receiver 6c do not hit the receiver.

Thus, as is apparent from FIG. 5b, for the receiver to occupy an optimal position, the line that passes through the center of the receiver and which is 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 situated in a zone of extent E ranging from + k to -k, on either side of the plane: PFIR, k having to satisfy the relation k = r sin Atan where r, d and f are as defined above, the center of the receiver possibly being confused with said final image focal point I "(special case mentioned above). A position of the receiver such as 6c, where the line C is outside the range area E, is however not a case in point outside the scope of the invention: this position may be acceptable, even if the receiver 35 does not receive the entire concentrated beam, for example if it is. less expensive to position the receiver in 6c than in 6a or 6b. Naturally, the positions 6a, 6c and 6c could just as easily be on the other side of the PFIR plane.

FIG. 5b shows, in addition, for the receiver 6a, a service position (in this case, within the concentrated beam and tangent to the planes bounding this beam) and a retracted position, illustrated in 6a ', where the receiver is totally out of the concentrated beam. The position 6c could also be considered to constitute the retracted position of the receiver 6a. It is understood that the invention is not limited to the embodiments described and shown. Thus, instead of incorporating a heat pipe, the box could contain an extraction exchanger supplied with heat transfer fluid or a linear volume coated with photovoltaic cells, both mobile as has been described for the heat pipe. Furthermore, it is possible to juxtapose several lenses each forming a face of a "sub-box", the sub-boxes and juxtaposed having components in common to limit the amount of constituent materials used and to reduce the number of servo for moving the receiver.

Claims (16)

1. A solar collector comprising, as a collector, a converging lens (2; 2a; 2b; 2c) having a focal length f and an image focal plane (PFI), on which, in a line, the image focus is "primary", the beam of solar rays (R; R '; R1r R2) that receives said lens, said concentrated beam moving with the race of the sun, characterized in that said convergent lens (2; 2a; 2b; 2c) constitutes one of the walls of a box (1) rectangular parallelepiped, whose depth p is less than the focal length f of the lens and whose other walls (3,4a-4c), the inner side of the ca: _sson, are reflective beams, so that at the end of multiple reflections, the ray beam thus reflected is focused on a line, called "final image focus" (I; I '; I "), parallel to said primary image focus and belonging to a "close-up focal plane" (PFIR), itself parallel to said image focal plane (IFP), but located at the interior of said box, the depth p of the box responding, for this, to the relationship p = 0.5 * (f + e + b) where. • e is the penetration thickness of the lens (2; 2a; 2b; 2c) in the box (1); and b is the distance between the lens (2; 2a; 2b; 2c) and the close-up focal plane (PFIR) or operating distance of operation, said sensor enclosing a mobile receiver (6; 6a; 6b; 6c) maintained within said concentrated beam, or in a position at least secant said beam, by means slaving the displacement of said receiver (6; 6a; 6b; 6c) to the displacement of said beam. 2. Solar collector according to claim 1, characterized in that the center of said receiver (6; 6a; 6b; 6c) is located within an area which affects an extent (E) ranging from + k to -k from and on the other hand, said near-image local plane (PFIR), median to said area, k satisfying the relation where. . r is the radius of the cross-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, it being understood that by "center of the receiver" is meant the straight line (Ca; Cb Cc) parallel to the final image focus (I; I '; I ") and passing through the center of said circle; • sin [Atan] means sinus [tangent arc]; • d is the distance between the optical axis (A -A ') of the lens and the edge of the lens, taken in the plane containing said optical axis (A-A') and which is perpendicular to the bottom (3) of the box and orthogonal to the final image focus (I; I ' I "). 3. Sensor according to claim 1 or 2, characterized in that said convergent lens is plano-convex (2; 2a), biconvex (2b) or meniscus convergent (2c). 4. Sensor according to any one of claims 1 to 3, characterized in that said convergent lens is a Fresnel lens (2; 2a). 5. Sensor according to claim 3, characterized in that the convergent lens is a plane-convex Fresnel lens (2; 2a) mounted such that its flat face is facing outwardly of said box (1). 6. Sensor according to claim 3, characterized in that the convergent lens is a biconvex Fresnel lens mounted so that its convex smooth face is facing outwardly of said box (1). 7. Sensor according to any one of claims 1 to 6, characterized in that said receiver (6; 6a; 6b; 6c) is a heat pipe covered with a material whose heat absorption coefficient r is greater than the coefficient d emission of heat. 8. Sensor according to claim 7, characterized in that the heat pipe takes the form of a tube included in a vacuum tube. 9. Sensor according to claim 7 or 8, characterized in that the receiver (6; 6a; 6b; 6c)) is connected to an extraction exchanger (7) supplied with a heat transfer fluid. 10 10. Sensor according to any one of claims 1 to 6, characterized in that the receiver is a photovoltaic cell receiver. 11. Sensor according to any one of claims 1 to 6, characterized in that said receiver (6; 6a; 6b; 6c) 15 is an extraction exchanger supplied with heat transfer fluid. 12. Sensor according to any one of claims 1 to 11, characterized in that said receiver is capable of occupying two positions, namely a service position (6a) in which it receives a certain thermal energy and a retracted position. (6a ') in which it receives a lower thermal energy than in the service position. Sensor according to one of claims 1 to 12, characterized in that the receiver (6; 6a; 6b; 6c) is connected to a Stirling motor. 14. A sensor according to any one of claims 1 to 13, characterized in that the surfaces of the lens (2; 2a; 2b; 2c) and / or the reflecting walls (3,4a-4d) of the box are treated. in order to reduce the potential alteration of their material over time. 15. Sensor according to any one of claims 1 to 14, characterized in that the outer surface of the lens (2; 2a; 2b; 2c) comprises an antireflection treatment. 35 16. Sensor according to any one of claims 1 to 15, characterized in that the reflective walls (3,4a-4d) consist of removable reflective panels.