WO2009093128A2

WO2009093128A2 - Photovoltaic receiver for a photovoltaic generation system and photovoltaic generation system thereof

Info

Publication number: WO2009093128A2
Application number: PCT/IB2009/000110
Authority: WO
Inventors: Andrea Antonini
Original assignee: Cpower S.R.L.
Priority date: 2008-01-23
Filing date: 2009-01-22
Publication date: 2009-07-30
Also published as: WO2009093128A3; ITBO20080039A1

Abstract

In a photovoltaic generation system (1) provided with a concentrator (2) of sunlight having an optical axis (3) and a curved reflecting surface (2a) to concentrate, according to a concentrated beam (6, 7) of rays focusing in a focus (F) on the optical axis (3), the sunlight that strikes, according to an incident beam (13, 14) of rays parallel to the optical axis (3), on the reflecting surface (2a), a photovoltaic receiver (8) has a first photovoltaic converter (9) positioned between the concentrator (2) and the focus (F) so that to intercept the incident beam (4, 5) thus producing a shadow volume (7a) in the concentrated beam (6, 7), a second photovoltaic converter (15) positioned within the shadow volume (7a), and a glass plate (10a) interposed between the two converters (9, 15) and covered with a dichroic coating (10b) to separate the concentrated beam (6, 7) in a first and second beam (11, 12, 13, 14) having a different sunlight spectrum and so that the first beam (11, 12) is transmitted to the first converter (9) and the second beam (13, 14) is reflected towards the second converter (15).

Description

PHOTOVOLTAIC RECEIVER FOR A PHOTOVOLTAIC GENERATION SYSTEM AND PHOTOVOLTAIC GENERATION SYSTEM THEREOF

TECHNICAL FIELD The present invention relates to a photovoltaic receiver for a photovoltaic generation system and to a photovoltaic generation system provided with this receiver.

BACKGROUND ART As is known, one of the simplest photovoltaic generation systems is of the type comprising a photovoltaic receiver consisting, for instance, of a small photovoltaic cell panel, and a concentrator consisting of a curved reflecting surface disc, for instance paraboloidal, to concentrate a great amount of sunlight on the photovoltaic cell panel. The cost of the photovoltaic cell panel is mainly due to the cost of the semiconductor the single photovoltaic cells are made of . The use of the concentrator serves to reduce the number of photovoltaic cells employed in the panel and therefore to reduce the cost thereof, depending on the

^■ desired electric power to be generated. However, the photovoltaic cell panels made of the same semiconductor have a limited electrical efficiency if exposed to the whole sunlight spectrum. As a matter a fact, only photons having an energy at least equivalent to the width of the so-called energy band gap of the semiconductor provide a useful energy contribution equivalent to the band gap itself, whereas excess energy and energy carried by the other photons is dissipated as heat.

'Patent application WO-2006./108806 discloses a photovoltaic generation system comprising a dichroic reflecting surface concentrator that allows to separate sunlight in multiple beams for different wavelength bands of sunlight, each beam being oriented according to a respective optical direction and being concentrated on a respective focus lying on the optical direction, and a photovoltaic receiver provided with a plurality of photovoltaic converters , each of which is arranged near one of the focuses and consists of a photovoltaic cell panel made of a semiconductor that maximises the conversion efficiency for photons having wavelengths corresponding to the concentrated beam on that focus . This solution is in any case rather expensive as extensive areas of dichroic material are required to cover the surface of the concentrator. DISCLOSURE OF INVENTION

It is the object of the present invention to provide a photovoltaic receiver for a photovoltaic generation system provided with a curved reflecting surface concentrator, and a photovoltaic generation system employing this photovoltaic receiver, which do not have the above disclosed drawbacks and are at the same time easy and cost-effective to make. According to the present invention a photovoltaic receiver for a photovoltaic generation system and a photovoltaic generation system according to the appended claims are provided. BRIEF DESCRIPTION ^"θF THE DRAWINGS

The present invention will now be described with reference to the accompanying drawings, which set forth non-limitative embodiments thereof, in which:

- Figure 1 shows a photovoltaic generation system comprising the photovoltaic receiver obtained according to an embodiment of the present invention according to a simplified longitudinal section view;

- Figure 2 shows a photovoltaic generation system comprising the ^' photovoltaic receiver obtained according to a further embodiment of the present invention according to a simplified longitudinal section view;

- Figure 3 shows the photovoltaic generation system of Figure 1 according to an axonometric view;

Figures 4 and 5 show two variants of the photovoltaic generation system of Figure 3 ; and

- Figures from 6a to 7b show the distribution of the flow of sunlight on the photovoltaic receiver for different embodiments of the photovoltaic generation system shown in Figures 1 to 4. ' BEST MODE FOR CARRYING OUT THE INVENTION

In Figure 1, numeral 1 generically indicates a photovoltaic generation system comprising a sunlight concentrator 2, which consists of a disc having a curved reflecting surface 2a and an optical axis 3 passing through an optical centre C lying on reflecting surface 2a and through a focal zone, in which the rays of sunlight that strike parallelly to optical axis 3 on reflecting surface 2a are ideally focused. In particular, the focal zone consists of an area (not shown) defined on a plane (not shown) perpendicular to optical axis 3 and centred in a point F of optical axis 3, which is hereinafter designated as focus. Furthermore, numerals 4 and 5 indicate the traces of some of the incident rays which define an incident beam of sunlight, and numerals 6 and 7 indicate the traces of corresponding reflected rays converging in the focal zone, which define a concentrated beam of sunlight. Photovoltaic generation system 1 comprises a photovoltaic receiver 8 arranged between optical centre C and focus F to receive concentrated beam 6, 1 of. sunlight and convert the received sunlight into electric energy. Receiver 8 comprises a first photovoltaic converter 9 consisting of a substantially square (Figure 3) photovoltaic cell panel of the known type, which is arranged so that optical axis 3 centrally and perpendicularly passes therethrough at a first point P of optical axis 3 between optical centre C and focus F and at a short distance from focus F. Due to its arrangement, converter 9 intercepts incident beam 4, 5 therefore projecting a shadow 9a on reflecting surface 2a having the same shape as converter 9 itself and centred on optical axis 3. Accordingly, a shadow volume substantially having the shape of a truncated pyramid coaxial to optical axis 3 is produced within concentrated beam 6, 7, which is reflected by the portion of reflecting surface 2a which is not shaded. In Figure 1, this shadow volume is represented by the space between the traces of rays 7 and is indicated by 7a.

Receiver 8 further comprises a solar spectrum separator 10 comprising a plate 10a of non-diffusive material transparent to sunlight, for instance glass, plate 10a being positioned in front of converter 9 transversally to optical axis 3 to intercept concentrated beam 6, 7, and a thin dichroic coating 10b of the known type that covers a surface of plate 10a facing concentrator 2. Dichroic coating 10b is adapted to separate concentrated beam 6, 7 in two beams having a different sunlight spectrum. A first one of the two beams, symbolically represented by the traces of rays indicated by 11 and 12, is transmitted to converter 9 through plate 10a, and the second one of the two beams, symbolically represented by the traces of rays indicated by 13 and 14, is reflected to focus on a virtual focal zone located within shadow volume 7a and centred in a point VF, which is hereinafter designated as virtual focus and generally lies on a plane (not shown) perpendicular to optical axis 3 at a point of optical axis 3 between optical centre C and point P. In particular, the virtual focal zone consists of an area (not shown) defined on the plane virtual focus VF lies on.

In particular, according to the embodiment shown in_. Figure 1, plate 10a has flat and parallel _. surfaces and is arranged perpendicularly to optical axis 3 in such a way that virtual focus VF lies on optical axis 3. Therefore, virtual focus VR is specular to focus F with respect to point P. In other terms, the distance of virtual focus VF from optical centre C is substantially equivalent to the difference between twice the distance of point P from optical centre C and the distance of focus F from optical centre C (focal distance) . Hereinafter, for the sake of simplicity and clarity, the first beam is designated as transmitted beam 11, 12 and the second beam is designated as reflected beam 13, 14.

Plate 10a having flat and parallel surfaces results in plate 10a being easy and cost-effective to make and having an invariant optical behaviour with respect to possible translations along a plane parallel to said surfaces. This optical behaviour ensures a good tolerance to positioning errors of plate 10a with respect to optical axis 3.

Figure 1 shows plate 10a at a certain distance from converter 9 so as to show transmitted beam 11, 12. Actually, plate 10a is preferably rested on converter 9 so that to be substantially positioned at point P of optical axis 3. Indeed, plate 10a is preferably a glass sheet already incorporated in converter 9 in order to protect the corresponding photovoltaic cells from atmospheric agents. The gap between plate 10b and photovoltaic cells is typically filled' with an optical coupling medium having a refractive index higher than the refractive index of air.

Receiver 8 further comprises a second photovoltaic converter 15 consisting of another panel with one or more photovoltaic cells of the known type and arranged within shadow volume 7a of concentrated beam 6, 7. This panel is substantially square (Figure 3) and is arranged so that optical axis 3 passes therethrough centrally and perpendicularly substantially at virtual focus VF. In other terms, the point of optical axis 3, in which converter 15 is positioned, lies in a neighbourhood of virtual focus VF such that reflected beam 13, 14 is totally received by converter 15 itself, i.e. so that a cross section of reflected beam 13, 14 defined on the photovoltaic cell panel of converter 15 is totally contained within the area of the panel itself .

With reference to Figure 3 , which shows photovoltaic generation system 1 according to an axonometric view, reflecting surface 2a of concentrator 2 has a parabolic curvature of the known type obtained by the rotation, about optical axis 3, of a generatrix 16 consisting of a parabola branch described by a function of the type

Y = a-x², (1) and has a substantially square front section so as to be compatible with the shape of the photovoltaic cell panels that form converters 9 and 15.

Photovoltaic- cells of converter 9 are made of a first semiconductor with an energy band gap having width BGl and photovoltaic cells of converter 15 are made of a second semiconductor different from the first semiconductor and having an energy band gap with a width BG2 greater than BGl. The first semiconductor is of the lower-cost type, for instance silicon (Si) , and the second semiconductor is of the type appropriate to operate at higher light concentrations, and therefore of the more expensive type, for instance gallium arsenide (GaAs) , indium gallium phosphide (InGaP) , or other semiconductors formed by elements of group III and V elements of the periodic table.

Dichroic coating 10b defines a cut wavelength having a value WLcut such that transmitted beam 11, 12 comprises the radiation of sunlight having wavelengths greater than or equal to cut wavelength WLcut itself, and reflected beam 13, 14 comprises the radiation of sunlight having wavelengths smaller that cut wavelength WLcut. In particular, cut wavelength WLcut is inversely proportional, by means of the known Planck's formula, to energy band gap BG2 so that the solar radiation carried by photons having an energy at least equivalent to BG2 corresponds to wavelengths smaller than cut wavelength WLcut and the solar radiation carried by photons having an energy lower than BG2 corresponds to wavelengths greater than the cut wavelength.

The operation of receiver 8 and of photovoltaic generation system 1 incorporating it is apparent from the above. It should in any case be noted that receiver 8 actually operates with a two level concentration of sunlight for two respective parts of the solar spectrum. This two level concentration requires a reduced number of photovoltaic cells of the more expensive type, i.e. made of the second semiconductor and therefore appropriate for reflected beam 13, 14 which has a higher concentration of transmitted beam 11, 12. This leads to the double advantage of allowing to obtain a small-sized converter 15, so as to prevent converter 15 itself from being hit by concentrated beam 6, 7 from concentrator 2, and of reducing the overall cost of photovoltaic generation system 1.

Furthermore, above disclosed receiver 8 maintains a high photovoltaic conversion efficiency even in the presence of a non ideal spectrum separation. As a matter a fact, should photons corresponding to wavelengths smaller than cut wavelength WLcut, and therefore having an energy higher than GB2, be transmitted to converter 9 instead of being reflected towards converter 15, they would anyway be converted into electric energy, even though with a lower efficiency with respect to that which would be obtained by converter 15. According to a further embodiment (not shown, but very similar to that shown in Figure 1) , the photovoltaic cell panels that form both concentrator 9 and 15 are rectangular or hexagonal and accordingly concentrator 2 has a rectangular or, respectively, hexagonal front section.

According to a third embodiment (not shown) of the present invention, the surface of plate 10a covered by dichroic coating 10b has a hemispherical curvature so that virtual focus VF is closer to optical centre C with respect to the previously disclosed embodiments, i.e. so that the distance of virtual focus VF from optical centre C is smaller than the difference between twice the distance of point P from optical centre C and the distance of focus F from optical centre C. This solution allows to have more space, transversally to optical axis

3, for arranging converter 15, the size of converter 9 being the same, or allows to use a smaller-sized converter 9, the size of converter 15 being the same, without concentrated beam 6, 7 from concentrator 2 being intercepted by converter 15 and/or without incident beam

4, 5 being intercepted by converter 9. Having a greater space for converter 15 provides for a greater constrictive freedom therefore, especially with reference to heat dissipation, whereas a smaller-sized converter 9 projects a smaller shadow 9a on reflecting surface 2a. According to a fourth embodiment of the present invention shown in Figure 2 , in which corresponding elements are indicated by the same numerals or abbreviations as in Figure 1, converter 15 is arranged at a second point Q of optical axis 3 , between optical centre C and virtual focus VF. Receiver 8 comprises a light collector 17 of the known type, which consists of a solid body made of transparent material having a refractive index higher than that of air and having a longitudinal optical axis (not shown) , an inlet window 18 and an outlet window 19 for sunlight defined on respective planes (not shown) transversal to this longitudinal optical axis, and is adapted to convey sunlight from inlet window 18 to outlet window 19 by means of total inner reflection. Collector 17 is longitudinally sized so as to be arranged with outlet window 19 at point Q and inlet window 18 substantially at virtual focus VF, i.e. in such a way that virtual focus VF falls within inlet opening 18, or in any case that a cross section of reflected beam 13, 14 defined on the plane of inlet window 18 lies therein. Thereby, collector 17 allows to convey the sunlight of reflected beam 13, 14 up to converter 15 avoiding that, by propagating from virtual focus VF to point Q, reflected beam 13 , 14 widens again from virtual focus VF to the extent that it may no longer be totally received by converter 15. Furthermore, collector 17 is preferably arranged with its longitudinal optical axis parallel to optical axis 3 of concentrator 2.

This solution has many advantages. First of all, distancing converter 15 from virtual focus VF, and therefore approaching it to optical centre C, allows to have more transversal space for converter 15, the size of converter 9 being the same, or allows to use smaller- sized converter 9, the size of converter 15 being the same, similarly to the solution employing plate 10a having a hemispherical curvature. Furthermore, this solution ensures, with respect to the other previously described solutions, a more uniform distribution of sunlight on converter 15, improving the conversion efficiency, independently of small transversal misalignment errors of collector 17 with respect to optical axis 3, provided that these misalignment errors maintain reflected beam 13, 14 within inlet opening 18. Finally, it allows to collect all of reflected beam 13, 14 even for small longitudinal positioning errors of collector 17 with respect to virtual focus VF.

Hemispherical curvature plate 10a may be used in combination with collector 17 so as to combine the effects thereof and approach converter 15 nearer to optical centre C. According to a fifth embodiment of the present invention, generatrix 16 of reflecting surface 2a of concentrator 2 consists of a curve ^' described by a polynomial function of degree in the range between 2.1 and 1.9, but different from 2, and in particular by a function of the type y = aoc^b, (2) in which a is a proportionality constant and b- is a real number different from 2 and in the range between 1.9 and 2.1. For the sake of simplicity, this curvature will hereinafter be designated as pseudoparabolic curvature .

The pseudoparabolic curvature of concentrator 2 allows to increase the uniformity of distribution of the flow of the solar radiation on both converter 9 and 15 with respect to the parabolic curvature . In this respect, Figures 6a and βb show the distribution of the flow of solar radiation on converter 9 and, respectively, on converter 15 obtained by the parabolic curvature, and Figures 7a and 7b show the distribution of the flow of solar radiation on converter 9 and, respectively, on converter 15 obtained by the pseudoparabolic curvature. As may be noted in Figures 6a-7b, there is a certain improvement in the distribution uniformity on converter 9 and an even more apparent improvement on converter 15.

According to a sixth embodiment of the present invention shown in Figure 4, in which corresponding elements are indicated by the same numerals or abbreviations as in Figures 1 and 3 , concentrator 2 has a through-opening 20 centred on optical axis 3. In particular, opening 20 substantially has the same shape as shadow 9a which is projected by converter 9 on reflecting surface 2a and is sized so as to be totally- covered by shadow 9a. In other terms, opening 20 has an edge 21 which remains totally within shadow 9a.'. Opening 20 may for instance be obtained by cutting out a corresponding central portion of concentrator 2 already profiled with the desired curvature. The purpose of opening 20 is to make concentrator 2 lighter and less sensitive to wind, the amount of received and converted sunlight being the same .

According to a further embodiment of the present invention shown in Figure 5, which only shows a portion of reflecting surface 2a around opening 20, edge 21 of opening 20 is defined by a vertex-free line (Figure 5) or by a generic curved line and the pseudoparabolic curvature of reflecting surface 2a is generated by a movement, along edge 21, of a generatrix 16 formed by a branch of the curve described by function (2) which lies on a determined plane (not shown) . In particular, the generation movement consists in a movement of the plane of generatrix 16 such that an extreme point G of generatrix 16 moves along edge 21 and the line that defines edge 21 always passes through the plane perpendicularly at the extreme point G. The main advantage of above disclosed photovoltaic receiver 8 and therefore of photovoltaic generation system 1 employing this receiver 8, is to obtain a high energy conversion efficiency, especially in the version comprising collector 17, in relation to the cost of the components employed. As a matter a fact, converter 15 which operates at higher concentrations employs a reduced number of photovoltaic cells, the high cost of which further increases the overall cost of system 1.

Furthermore, photovoltaic generation system 1 in the version comprising pseudoparabolic surface concentrator 2 further improves the conversion efficiency as it allows a better distribution uniformity of the light radiation flow on both converter 9 and 15.

Claims

1. A photovoltaic receiver for a photovoltaic generation system (1) comprising a sunlight concentrator

(2) , ^' which has an optical axis (3) and a curved reflecting surface (2a) to concentrate, according to a concentrated beam (6, 7) of rays ideally focusing in a focal zone through which the optical axis (3) passes, the sunlight that strikes, according to an incident beam

(4, 5) of rays parallel to the optical axis (3) , on the reflecting surface (2a) ; the photovoltaic receiver (8) comprising a first photovoltaic converter (9) , which is adapted to be arranged at a first point (P) of the optical axis (3) between the concentrator (2) and the focal zone, and being characterised in that the first converter (9) is adapted to be arranged in such a way as to intercept the incident beam (5, 4) so that to produce a shadow volume (7a) in the concentrated beam (6, 7) , and in that it comprises a second photovoltaic converter (15) adapted to be arranged within the shadow volume (7a) , and solar spectrum separating means (10) interposed between the first (9) and the second (15) converter in order to separate the concentrated beam ( 6 , 7) of sunlight in a first and second beam (11, 12, 13, 14) having a different sunlight spectrum and so that the first beam (11, 12) is transmitted to the first converter (9) and the second beam (13, 14) is reflected towards the second converter (15) .

2. The photovoltaic receiver according to claim 1, wherein said solar spectrum separating means (10) are adapted to concentrate _.said second beam (13, 14) in a •virtual focal zone located within said shadow volume (7a) and centred in a virtual focus (VF) ; said second converter (15) being arranged in a neighbourhood of the virtual focus (VF) such that said second beam (13, 14) is totally received by the second converter (15) .

3. The photovoltaic receiver according to claim 1, wherein said solar spectrum separating means (10) are adapted to concentrate said second beam (13, 14) in a virtual focal zone located within said shadow volume

(7a) and centred in a virtual focus (VF) ; said second converter (15) being arranged at a second point (Q) of said optical axis (3) between said concentrator (2) and a plane perpendicular to the optical axis (3) the virtual focus (VF) lies on; the photovoltaic receiver

(8) comprising a light collector (17) arranged between the plane the virtual focus (VF) lies on and the- second point (Q) of the optical axis (3) to convey the sunlight of the second beam (13, 14) from the virtual focus (VF) up to the second converter (15) so that the second beam (13, 14) is totally received by the second converter (15) .

4. The photovoltaic receiver according to claim 3, wherein said collector (17) consists of a solid body, which is made of a transparent material having a refractive index greater than the refractive index of air, has a longitudinal axis, an inlet window (18) and an outlet window (19) for sunlight defined on respective planes transversal to the longitudinal axis and is adapted to convey sunlight from the inlet window (18) to the outlet window (19) ; the collector (17) being longitudinally sized so as to be arranged with the outlet window (19) at said second point (Q) and with the inlet window (18) substantially at said virtual focus

(VF) in such a way that a cross section of said second beam (13, 14) defined on the plane of the inlet window (18) lies therein.

5. The photovoltaic receiver according to claim 4 , wherein said collector (17) is arranged with said longitudinal axis coaxial to said optical axis (3) .

6. The photovoltaic receiver according to one of claims 2 to 5 , wherein said virtual focus (VF) lies on said optical axis (3) .

7. The photovoltaic receiver according to one of the preceding claims, wherein said solar spectrum separating means (10) comprise a plate (10a) of material transparent to sunlight arranged transversalIy to said optical axis (3) and a dichroic coating (10b) , which covers a surface of the plate (10a) and is adapted to separate said concentrated beam (6, 7) of sunlight in said first and second beam (11, 12, 13, 14) .

8. The photovoltaic receiver according to claim 7, wherein said plate (10a) rests on said first converter (9) so that to be positioned substantially at said first point (P) of said optical axis (3) .

9. The photovoltaic receiver according to claim 8 , wherein said surface of said plate (10a) is flat.

10. The photovoltaic receiver according to claim 8, wherein said plate (10a) is arranged with said flat surface in a position perpendicular to said optical axis (3) so that said second beam (13, 14) is concentrated in a virtual focus (VF) lying on the optical axis (3) in a position specular to said focus (F) with respect to said first point (P) of the optical axis (3) .

11. The photovoltaic receiver according to claim 8 , wherein said surface of said plate (10a) has a hemispherical curvature .

12. The photovoltaic receiver according to claim 11, wherein said plate (10a) is arranged in such a way that said second beam (13, 14) is concentrated in a virtual focus (VF) lying on said optical axis (3) .

13. The photovoltaic receiver according to one of claims 7 to 12, wherein said dichroic coating (10b) defines a cut wavelength (WLcut) such that said first beam (11, 12) comprises a sunlight radiation ^' having wavelengths greater than the cut wavelength (WLcut) and said second beam (13, 14) comprises a sunlight radiation having wavelengths smaller that the cut wavelength (WLcut) .

14. The photovoltaic receiver according to claim 13, wherein said first converter (9) comprises at least one first photovoltaic cell, which has an energy band gap having a first width (BGl) , and said second converter (15) comprises at least one second photovoltaic cell, which has an energy band gap having a second width (BG2) greater than the first width (BGl) ; said cut wavelength (WLcut) being inversely proportional to the second width (BG2) of energy band gap.

15. A photovoltaic generation system comprising a sunlight concentrator (2), which has an optical axis (3) and a curved reflecting surface (2a) to concentrate, according to a concentrated beam (6, 7) of rays ideally focusing in a focal zone through which the optical axis (3) passes, the sunlight that strikes, according to an incident beam (4, 5) of rays parallel to the optical axis (3) , on the reflecting surface (2a) , and a photovoltaic receiver (8) arranged between the focal zone and the concentrator (2) in such a way as to receive the concentrated beam (6, 7) of sunlight and adapted to convert received sunlight into electrical power,- and being characterised in that said photovoltaic receiver (8) is of the type claimed in one of claims 1 to 14.

16. The photovoltaic generation system according to claim 15, wherein said concentrator (2) has a through- opening (20) centred on said optical axis (3) ; the opening (20) having substantially the shape of a shadow (9a) projected on said reflecting surface (2a) by said first converter (9) intercepting said incident beam (4, 5) and being sized so as to be completely covered by the shadow (9a) itself.

17. The photovoltaic generation system according to claim 15 or 16, wherein said reflecting surface (2a) of said concentrator (2) has a curvature generated by rotation about said optical axis (3) of a generatrix

(16) comprising at least one branch of a curve described by a polynomial function of degree in the range between

2.1 and 1.9, but different from 2.

18. The photovoltaic generation system according to claim 16, wherein said reflecting surface (2a) of said concentrator (2) has a curvature generated by a movement, along an edge (21) of said opening (20) of said concentrator (2) , of a generatrix (16) comprising a branch of a curve described by a polynomial function of degree in the range between 2.1 and 1.9, but different from 2.

19. The photovoltaic generation system according to claim 18, wherein said edge (21) of said opening (20) is defined along a vertex-free line and said generatrix

(16) lies on a determined plane; said movement, along the edge (21) , of said generatrix (16) consists in a movement of said determined plane of the generatrix (16) such that an extreme point (G) of the generatrix (16) moves along the edge (21) and the line defining the edge (21) always passes through the plane of the generatrix (16) perpendicularly at the extreme point (G) .