EP2318774A2

EP2318774A2 - Solar concentrating collector of the cpc type with an improved absorbing cavity, without thermal shorts and optical losses

Info

Publication number: EP2318774A2
Application number: EP09788435A
Authority: EP
Inventors: Manuel Pedro Ivens Collares Pereira; Rodolfo Manuel Leal Torres Branco
Original assignee: Ao Sol - Energias Renovaveis Sa
Current assignee: Ao Sol - Energias Renovaveis Sa
Priority date: 2008-07-17
Filing date: 2009-07-17
Publication date: 2011-05-11
Also published as: PT104133A; WO2010008311A3; WO2010008311A2

Abstract

Low concentration CPC type concentrating collector, without optical losses and without thermal shorts which would be responsible for substantial efficiency losses, comprising at least one absorber (1) one absorber tube (10), concentrating reflectors (2) and at least one bottom set (3), said absorber (1), concentrating reflectors (2) and bottom set (3) being superimposed but without touching each other, and having a concentration value in the range of about 1 to 3. The absorber (1) has a surface selected from the group comprising an inverted V shape, any open polygonal line and/or curve and the like and combinations thereof. The invention further contemplates an absorbing grid formed by said concentrating collectors, which allows the overcoming of the thermal and optical positioning difficulties due to higher temperatures, since it comprises a system of expansion guides (13) allowing for its expansion in the longitudinal direction.

Description

"SOLAR CONCENTRATING COLLECTOR OF THE CPC TYPE WITH AN IMPROVED ABSORBING CAVITY, WITHOUT THERMAL SHORTS AND

OPTICAL LOSSES"

FIELD OF THE INVENTION

The present invention relates in general to a new configuration for a solar concentrator of the CPC type, with the elimination of thermal shorts and of optical losses.

BACKGROUND OF THE INVENTION

Portuguese Patent PT 102938 entitled "Colector solar de baixa concentracao, ideal, do tipo CPC", European Patent EP 0678714 Bl entitled "Solar energy collector of the non-evacuated compound parabolic concentrator type", Portuguese Patent PT 80405 entitled "1.2 X CPC with tubular receiver with headers ideally illuminated" of 1987, Patent Application US-A-3957031, Patent Application US-A-4003638, Patent Application US-A-4230095 and the document of Ari Rabl. from 1985, entitled "Active Solar Collectors and Their Applications" New York: Oxford University Press, Inc. exhibit concentrating collectors of the CPC type, comprising anidolic optics, (also referred as being ideal, since they perform in the limits established by first principles in Physics or belong to the general category of non-imaging systems in English literature). They have been proposed with large acceptance angles for stationary or quasi-stationary (seasonal tilt adjustment).

One recurring concern is to reduce or even eliminate the optical losses that result from the existence of a certain gap between absorber and the mirrors reflecting towards it the incoming solar radiation.

This gap is a necessity in case the collector is of the atmospheric type (in opposition to those that use vacuum for all or for part of their configuration), since, if the absorber touches the mirrors or other surfaces that come close to it, there is thermal short circuit from which unwarranted thermal losses result, which may even create a survival problem for the whole collector.

However, even in the case of collectors using vacuum, optical losses must be controlled (as it is well known from Patent applications US-A-4003638 and US-A-4230095), since in many configurations using glass vacuum tubes, glass thickness and absorber inside them, create gaps for the incident radiation to be lost and never intersecting the absorber.

The portuguese Patent PT 102938 entitled "Colector solar de baixa concentracao, ideal, do tipo CPC" and the European Patent EP 0678714 Bl "Solar energy collector of the non-evacuated compound parabolic concentrator type" disclose a configuration conceived for a large gap, but in such a way that optical losses are zero.

Referring to Fig. 1, the absorber 1 (an inverted V defined by portions AC and CB) may be illuminated on its two faces 1' and 1", as can be seen by what happens to the two parallel rays 4 and 4' that are incident on the concentrator, one 4, on directly incident on face 1 ' of the absorber and the other 4 ' , after two reflections on the down face 1 " of the absorber

This concentrator concentrates onto the absorber ACB with the shape of an inverted V the radiation incident on aperture EF and making angles θ comprised between the values ±θ, angles that the direction EF and FC make with the normal to the aperture EF which is also the symmetry axis of the ensemble.

In fact, the concentrating optics is conceived for a virtual ACB absorber with a triangular shape. However the face AB is virtual, and all radiation crossing it ends up, without losses, on the down surface of the inverted V absorber, allowing for the reflecting mirror to start on the virtual absorber segment AB, without touching its active surfaces and establishing the desired large gap, thus avoiding the thermal short between the absorber and the mirror 2. From the optical point of view, this solution is entirely equivalent to the one presented in Fig. 2, in which the absorber 1, still in the shape of an inverted V, extends itself to the points G and H (the same signaled in Fig. 1 and which result from the intersection AC and CB segments extensions with the reflector surface). By design, AG=BH=AB/2, since the reflector extending from G to M and from this point H, and which is designated as involute, is constituted by two arcs of circumference with the sane radius AB/2 and centered in A and B, respectively.

With reference to Fig. 2, it should be noted that absorber 1 is only active on its face 1 ' , directly exposed to the incident radiation. In this case the same two parallel rays 4 and 4' as in Fig 1 are seen being absorbed, respectively, directly and after one reflection on reflector 2.

From the point of view of a manufacturer of solar collectors, this type of absorber

1, also commonly named by the word fin, only active on one side 1', is available from a large number of manufacturers/suppliers, being a standard product of flat plate solar collector absorbing 1 fins industry. On the other hand, at present, there are practically no suppliers of fins 1 coated on both sides 1' and 1", as in the case of Fig. 1.

A severe limitation of the configuration in Fig.2 is that it promotes the contact between reflector 2 and absorber 1, causing thermal short circuits and thus thermal losses.

Obvious solutions as those shown in Fig. 3(a) and 3(b), in which the absorber 1 is shorter, i.e. , extending only to G' and H' respectively, or in which the reflector 2 is shorter, i.e. , extending only from E to E' and from F to F', are not interesting, since in spite the fact that they resolve the thermal short problem, they do not resolve the optical loss problem, as incident radiation may escape between G and G', H and H', in Fig. 3(a) and between E' and G, F' and H, in Fig. 3(b). Thus there has been a strong incentive for the development of a new configuration, which keeping its advantages might be able to solve the problems of the configurations of the prior art.

Surprisingly the solar collector of the present invention solves the optical and thermal losses problems without losing the advantages of the solar collectors of prior art.

The present invention proposes preferred embodiments that are related with

1) the necessity to guarantee and preserve the configuration of an absorbing cavity

(absorber 1 plus surrounding surfaces) at whatever temperature it might be at, i.e. , accommodating the tendency for geometry change which results from absorber 1 expansion with temperature,

2) the exploration of constructive solutions resulting from the new absorbing cavity, in order to contemplate ways to achieve thermal losses reduction for the whole, through the use of insulating and transparent materials.

SUMMARY OF THE INVENTION

The present invention discloses a low concentration CPC type concentrating collector, comprising at least an absorber, at least an absorber tube and concentrating reflectors, the said concentrator being characterized in that further comprises a bottom set, in that said absorber, concentrating reflectors and bottom set are superimposed, without contacting each other, and in that having concentration values in the range of about 1 to 3, wherein said bottom set has at least two side walls and a bottom wall, with each side wall making an angle (γ) of at least 90° with said bottom wall and said absorber has a surface selected from the group comprising an inverted V shape, any profile with the shape of a line or a similar polygonal and open curve and combinations thereof. In a preferred embodiment of the present invention the low concentration CPC type concentrating collector has a surface with an open polygonal line shape with at least three straight segments.

In another preferred embodiment, the low concentration CPC type concentrating collector of the present invention has a reflecting bottom set.

In another preferred embodiment the low concentration CPC type concentrating collector of the present invention has a bottom set comprising a thermal insulation.

Still in another preferred embodiment the low concentration CPC type concentrating collector of the present invention, has thermal insulation having a reflector totally or partially covering its upper face.

A preferred embodiment of the low concentration CPC type concentrating collector of the present invention shows an asymmetrical geometry.

Another preferred embodiment of the low concentration CPC type concentrating collector of the present invention has a concentration value of 1.5.

In a preferred embodiment of the present invention, an absorber grid made of concentrators of the invention, comprising at least two of said concentrating collectors, one box comprising at least with one cover, at least, one insulation and one back part, one fluid inlet and one fluid outlet one header tube, one collector tube and expansion guides, is characterized by said header, said fluid inlet and said fluid outlet being installed in a top of a same side of said box; said collector tube and said expansion guides being installed in a top at the opposite side of the box, and by said header tube being blocked in its middle point.

Still in another further embodiment of the present invention, said cover comprises a transparent material with a polygonal structure of the honeycomb type or the like, a single thin film, a double film or the like and combinations thereof. Other aspects and advantages of the invention will be apparent from the following detailed description and from the accompanying drawings which illustrate, by way of example, the features of the invention.

BRIEF DESCRIPTION OF THE FIGURES

Fig. 1 shows in a schematic view a prior art solar collector of the CPC type.

Fig.2 shows in a schematic view another prior art collector of the CPC type, equivalent to the one of Fig.1 but without involute.

Fig. 3(a) shows, in a schematic view, CPC type collectors of the prior art, having an inverted V shape absorber 1, which absorber 1 is truncated, not extending up to reflector 2.

Fig. 3(b) shows a schematic view of another prior art collector equivalent to the one of Fig. 3 (a) but in which the reflector 2 is truncated, not extending up to absorber 1.

Fig. 4 shows in a schematic view, an embodiment of a absorbing cavity of the present invention, comprising reflectors 2, an inverted V shape absorber, a tubular section 10 and a bottom set 3, constituted by 5 walls.

Fig. 5(a) shows another embodiment of the present invention comprising thermal insulation 6 and having an upper side that may be reflecting or not.

Fig 5(b) shows another embodiment of the present invention, in which the absorber 1 is a surface with an open polygonal line shape with 4 straight segments. Fig 6(a) shows a perspective view of an embodiment of the present invention illustrating an absorber grid constituted by several concentrating collectors of the present invention, placed side by side and contained inside a box 7, with a transparent glass cover 8 and thermal insulation 14 on a back side 9.

Fig. 6(b) shows a schematic detail of the absorbing grid of Fig. 6(a) which, for drawing clarity sake, only shows the header tube 11 and the collector tube 11'. It further shows a system of guides 13, fixed to a side wall of the box 7.

Fig. 7 shows another embodiment of the present invention in which the geometry of the concentrating collector is not symmetrical.

Fig. 8 further shows another embodiment of the present invention in which the concentrating collector is truncated and comprises a transparent insulation 15 placed in between reflectors 2 and cover 8.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a new configuration of a solar collector of the

CPC type, enabling the development of an anidolic type optics (ideal or non imaging optics) preserving characteristics of the prior art which allowed for lossless optical performance, and simultaneously avoiding thermal shorts which would result in substantial efficiency losses. The configuration developed further shows features that allow it to preserve this behavior even at high temperatures, in spite of the expansion that results from them. The concentrating collector of the present inventions is susceptible of presenting larger concentration values and better control thermal losses.

Fig. 4 shows an embodiment of the present invention in which the absorber used for the definition of anidolic reflectors EE' and FF' is the segment E'CF' in inverted V shape, having an opening angle 2α represented in dashed line to underline its virtual nature. This figure further shows a real inverted V absorber 1 with slightly larger dimensions, with an angle 2β slightly smaller (L e., β < α), at least one absorber tube 10 and concentrating reflectors 2. On the base of this collector there is a bottom set 3, comprising at least two side walls 3' and one bottom wall 3" which correspond respectively to the straight segments IJ, LM and JL. The referred bottom set 3 may be reflecting totally or partially. Side segments IJ and LM make an angle γ with segment JL. Usually this angle Y will be larger than 90°. Points I and M are defined such that the absorber 1, concentrating reflectors 2 and bottom set 3, never touch each other and even leave considerable distance between each other from the point of view of thermal shorts. The bottom set 3 may be reflecting and, in that case, constitutes a reflecting/irradiative cavity, practically without optical losses.

The bottom set may also be formed in a non-reflecting and insulating material, like cardboard or the like.

Tubular section 10, shown in Fig. 4 is an absorber tube 10 in which a fluid is circulated to extract the energy absorbed by the referred absorber 1 (or fin 1).

Referring to Fig. 5(a) another embodiment of the present invention is shown. This figure shows that the volume between the bottom set 3 and the absorber 1 may be filled with thermal insulation 6, of the rock wool type or the like. The referred thermal insulation 6 may (or may not) present its upper horizontal face 5 as a reflecting one.

In Fig. 5(b) another embodiment of the present invention is shown in which the absorber 1 is an open polygonal line defined by 4 straight segments, i.e. , it presents itself slightly bent in points C and C", with the reflectors EE' and FF' configured to preserve the anidolic and ideal characteristics of the collector. This configuration of absorber 1, with four segments, generates more mechanical rigidity in the direction perpendicular to the one being represented.

As to the concentration value previously referred, it is defined as the ratio between the entrance aperture EF and the length of the active side of the absorber 1. It is also related to the half acceptance angle θ (see Fig.4) by the equation C= l/sin(θ) for symmetrical configurations. It should be noted that the concentrating collector of the invention may be configured in an asymmetric fashion.

In reference to Fig. 7, a preferred embodiment of the present invention, the new concentrating collector shows an asymmetrical geometry, in which the full acceptance angle is the sum of two angles, θi and Θ2, (wherein θi is different from Θ2).

In the case of the present invention, the concentration values are in the range of 1 to 3, usually less than 2 for totally stationary collectors as described in the document by Ari Rabl, 1985, entitled "Active Solar Collectors and Their Applications", New York: Oxford University Press, Inc.

In a preferred embodiment of the present invention the concentrating collector has a concentration of 1.5.

In Fig. 6 (a) a perspective view is shown, of another embodiment of the present invention, in which several concentrating collectors of the invention, having absorbers 1 of inverted V shape, are placed side by side, so as to form an absorbing grid. The concentrating collectors of the invention are contained in a box 7 comprising a transparent glass, thermal insulation 14 and a back part 9.

In Fig. 6(b) there is shown a schematic detail of the embodiment of Fig. 6(a) which represents said absorbing grid. For the sake of drawing clarity only absorber tubes 10 are shown connected to two tubes 11 and 11', header and collector, with expansion guides 13 also being shown. For design purposes, said header tube 11, said fluid inlet R and said fluid outlet S are installed at one top of the same side of said box 7, said collector tube and said expansion guides being installed on the top of the opposite side of the box 7.

This figure underlines in a schematic way the intake of fluid by the absorber grid from the outside, from the side of the header tube 11 which extends into the box 7 in the R point, and delivers heated fluid to the outside, on the side of the same header tube 11 which exits the box at S point. This header tube 11 is blocked in the middle in a way such that it forces the fluid to flow along the absorber tubes 10, which tubes extend before that point, between R and N, up to the collector tube 11 'and forced to return through the remaining absorber tubes 10, again up to the header tube 11.

This configuration of the absorber grid of the present invention is different from the conventional ones, which usually use two or four grid inlets/outlets, in the former case in diagonal and in the latter one in four symmetrical points.

The main reason for this configuration is intimately related to the optical solution in figures 4 and 5. This because the temperature at which the absorber grid will be when exposed to the sun will expand the tubes 10, and thus a conventional configuration, with 4 outlet/inlet points or just two diagonal ones, would lead to their arching changing the optically and thermally correct position, represented in Fig. 4 and 5. For this reason the present invention also proposes a system of expansion guides, fixed to the side walls of box 7, schematically represented by P and Q in Fig. 6(b) and in detail 12. Thus the grid may expand in the longitudinal direction, without ever arching and get away from its ideal optical position.

The solution for the absorbing cavity of the present invention, as for instance identified in Fig. 4, allows various types of design solutions that distance themselves from those above cited in the Portuguese Patent PT 102938, entitled "Colector solar de baixa concentracao, ideal, do tipo CPC", European Patent EP 0678714 Bl, entitled "Solar energy collector of the non-evacuated compound parabolic concentrator type" and Portuguese Patent PT 80405, 1987, entitled "1.2 X CPC with tubular receiver with headers ideally illuminated" and which used the injection of polyurethane foam to fill all of the volume between the reflector and the bottom of the collector box.

Thus the present invention allows for a larger flexibility in volume filling with thermal insulation; in particular with the possibility of discard, totally or partially, the use of expanded polyurethane foam. As polyurethane foam is a material that has obvious life limitations with respect to temperatures above 100 ⁰C, the solutions of prior art were mainly directed to lower concentration values.

As above-mentioned, with the present invention higher concentrations may be achieved and thermal losses reduction mechanisms may be considered by natural convection control inside the concentrator.

On the other hand the new configuration of the absorbing cavity is also compatible with truncated CPC optical configurations, L e. , wherein the reflector 2 does not reach the surface EF (Fig. 7) which would be the entrance aperture for the solar radiation of the non-truncated concentrator, as in patents PT 102938, EP 0678714 Bl and

PT 80405.

The referred truncation, together with the absorbing cavity of the present invention allows:

1) a small penalization of the effective concentration by the removal of reflector portions close to cover 8 (Fig. 8) with obvious savings in terms of materials consumption, and

2) for the removal of said portions to enable the proposal of a new solution for convective losses reduction, through the creation of a gap between the top of the reflectors 2 (points V and Z in Fig. 8) and the lower face of cover 8 (points T and U). This gap enables the simple placement (L e. , with no attaching or support points other than the points V and Z itself) of a transparent insulation 15 allowing for a reduction of the thermal losses of the ensemble, with a thickness equivalent to distance TV or UZ. Thus, cover 8 may be designed to comprise a transparent material of a polygonal honeycomb type structure or a single or double thin film and combinations thereof. The skilled in the art will appreciate that the present invention may be subject to several modifications and changes without departing from the scope of the accompanying claims.

Claims

1. A low concentration CPC type concentrating collector, comprising at least one absorber (1), at least an absorber tube (10) and concentrating reflectors (2),

characterized by

further comprising at least a bottom set (3),

said absorber(l), concentrating reflectors (2) and bottom set (3) being superimposed without touching each other and

having concentration values in the range of about 1 to 3, wherein

said bottom set (3) comprises at least two side walls (3 ') and one bottom wall (3"), each of said walls making an angle (γ) of at least 90° with said bottom wall (3") and

said absorber (1) has a surface selected from the group comprising an inverted

V shape and any other profile with the shape of a line and/or open polygonal curve and the like and combinations thereof.

2. Low concentration CPC type concentrating collector according to claim 1, characterized by said absorber (1) having a surface with an open polygonal line shape comprising at least three straight segments.

3. Low concentration CPC type concentrating collector according to claim 1, characterized by said bottom set (3) being reflecting.

4. Low concentration CPC type concentrating collector according to claim 1, characterized by the bottom set (3) comprising thermal insulation (6).

5. Low concentration CPC type concentrating collector according to claims 1 and 4, characterized by said thermal insulation (6) being partially or totally reflecting on the upper surface (5) thereof.

6. Low concentration CPC type concentrating collector according to any of preceding claims, characterized by having an asymmetric geometry.

7. Low concentration CPC type concentrating collector according to any of preceding claims, characterized by having a concentration value of 1.5.

8. An absorbing grid of concentrating collectors as defined in the preceding claims, comprising at least two of said concentrators, one box (7) comprising at least one cover (8), at least one insulation (14) and one back part (9), one fluid inlet (R) and one fluid outlet (S), one header tube (11) one collector tube (H ') and expansion guides (13),

characterized by

said header tube (11), said fluid inlet (R) and said fluid outlet (S) being installed in a top of a same side of the box (7); said collector tube (11 ') and said expansion guides (13) being installed in a top of the opposite side of the box (7), and

said header tube (11) being blocked in its middle point (N).

9. Absorbing grid of concentrating collectors according to claim 8, characterized by the respective cover (8) comprising a transparent material (15) having a polygonal structure of the honeycomb type or the like, a single thin film, a double film or the like and combinations thereof.