WO2004090437A1

WO2004090437A1 - Solar collector of the cpc type

Info

Publication number: WO2004090437A1
Application number: PCT/PT2004/000006
Authority: WO
Inventors: Manuel Pedro Ivens Collares Pereira; João Vieira Correia OLIVEIRA
Original assignee: COLLARES PEREIRA ENGENHARIA UNIPESSOAL Lda; Jvco-Estudo E Projecto De Engenharia E Ambiente Unipessoal Lda
Current assignee: COLLARES PEREIRA ENGENHARIA UNIPESSOAL Lda; Jvco-Estudo E Projecto De Engenharia E Ambiente Unipessoal Lda
Priority date: 2003-04-09
Filing date: 2004-04-05
Publication date: 2004-10-21
Anticipated expiration: 2005-10-09
Also published as: PT102938A; EP1611401A1; BRPI0409087A

Abstract

The present invention is a solar collector with low concentration, ideal of the CPC type, using ideal non-imaging optics (also said to be anidolic), with a thermal behaviour better than the best in its class and fabrication tolerances which allow for cost reductions without sacrificing that behaviour. This objective is achieved through the choice of a novel configuration for the absorbing element (13), in turn combined with the corresponding ideal (anidolic) optics (17), simultaneously providing a lossless optical solution, in spite of the increase in average distance between the absorbing element (13) and said optics (17), when compared with other and previous solutions.

Description

DESCRIPTION

SOLAR COLLECTOR OF THE CPC TYPE

OBJECTIVE OF THE INVENTION

The objective of the present invention is a new low concentration solar collector, using ideal, non-imaging (or anidolic) type optics of the CPC type (Compound Parabolic Concentrator) for applications up to 160°C, through the heating of a thermal fluid, stationary or seasonally stationary, with a thermal behaviour better than the best in its class and fabrication tolerances allowing for cost reductions which do not compromise its excellent performance. This objective is achieved through an innovative configuration chosen for the absorber (often designated as absorbing fin) , in turn combined with the corresponding ideal optics, simultaneously yielding a solution without optical losses, in spite of the general average increase of the gap between this absorbing element and said optics, when compared with previous solutions. The larger fabrication tolerances and the larger gaps between absorber and optics, also allow for the use of materials like plastic for the box containing absorber and optics, in one of the embodiments of the present invention, with potential consequences in the reduction of production costs.

The box in which absorber and optics are contained can be manufactured in several conventional ways, well known to those skilled in the art, but it does not resort to the use of vacuum, which also contributes to the objective of low fabrication cost and maintenance.

BACKGROUND OF THE INVENTION

CPC type solar collectors, also designated by ideal or Winston type, are specific solutions of non-imaging (anidolic) optics, and have been the object of several patents and patent applications. The following documents deserve specific mention: UDS-A-4 230 095, US-A-3 957 031, US-A-4 003 638 and PT-80 405 and EP 0 678 714.

In all the documents referred above with the exception of the last one, (EP-0 678 714) the inventions pertain to several one to one combinations of absorbing elements of distinct geometries to their corresponding unique ideal solution, but without any one of them solving the very important problem

(important because of fabrication and operational constraints) of allowing for a no optical losses solution, for an absorber placed at a significant distance (gap) from the corresponding ideal optics. However each optical configuration corresponding to each absorber shape was separately and specifically developed as part of the detailed study made in each case, and the differences among each one of them were dully recognized to the point that each one was awarded its own patent.

The applications of this type of optics have been of a varied nature and in particular for the fabrication of solar energy collectors, since it allows for a very convenient way to concentrate the incoming solar radiation in stationary devices, i.e., they do not need to adjust their position in order to compensate to apparent daily motion of the sun in the sky, as it is typical of conventional solar concentrators which use focussing, imaging or non-ideal optics. This results from the fact that ideal optics allows for the development of a concentrator that in spite of having a large acceptance angle, instrumental in providing stationarity, the corresponding concentration is the maximum one achievable for that angle - far superior to the one that would be obtained with focussing optics for the same angle. More concentration means less thermal losses, and higher efficiency is obtained, all else being equal.

However, less thermal losses means that higher temperatures can be reached, which implies a special concern when cheaper materials are chosen for collector fabrication, many of which do not resist to temperatures higher than 100°C. One way to solve this problem is to use optical solutions that accommodate large separations (gaps) between the optics and the absorber (the collector component upon which solar radiation is concentrated and which can reach temperatures above 200°C) .

Until patent EP-0 678 714 was awarded, one of the referred patents - U.S. 4 230 095, was the one that came closer to the solution of the separation (gap) problem, while allowing for solutions where the reflectors constituting the optics could start at a distance from the absorber that, normally, they should touch. However this solution implied optical losses of the incoming radiation, from the fact that there were rays that could now cross (go through) that separation, never reaching the absorber.

For the first time, patent EP 0 678 714 did solve that problem of accommodating a large gap with a lossless ideal solution. It did so for an inverted V absorber configuration, specifying for some variants of that geometry the parabola and circumference arcs that constitute its main object.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The experience with fabrication and operation of solar collectors produced according to that patent, as shown that there exists the need to evolve that type of solution towards a novel configuration, without sacrificing some fundamental aspects of that solution:

- it is an ideal solution - it is a solution for large gaps

However the new configuration is conceived for even larger gaps (30% larger on average), still without optical losses.

In one of the preferred embodiments of the present invention, this innovation is achieved through a new distinct absorber form, which in a first embodiment is, in cross section, a semi circumference (see Fig. 2) and, in another embodiment is an open polygon with four sides (see Figs. 3(a) and 3(b)) for which the unique ideal optics are developed for the first time, really different from the optics defined in EP 0 678 714 Bl. In the first case the optical shape of the reflectors developed for the desired effect is best defined by tangency condition without any specific name, and the rest by parabolic and circumference arcs, yet quite distinct from the ones defined in the above referred document. The present invention innovates with respect to the document EP-0 678 714 Bl in other essential aspects. In fact an important aspect of the operation of solar collectors of this type is the better performance that comes from the use of selective coatings (i.e. with low emissivity in the infrared) coating the absorbing elements.

The fabrication experience already referred above, shows that the choice of producers of selective coatings on both sides/faces of the absorber (as specified in EP 0 678 714) is today practically reduced to one only, while there are several with the capability of coating one side only of the absorber material. It is thus clear that a solution that allows for folding the absorber onto itself, in such a way that an absorber surface coated on only one side shows up as coated on both sides, enhances, the choice of potential suppliers of selective coatings.

In the present invention several possible placements are considered for the tube in which circulates the fluid to be heated and which is an integrant part of the absorber (see Figs. 4(a) and 4(b)). These may be symmetric (a) as in EP 0 678 714 Bl, or asymmetric (b) . This type of solution was not easily achieved with the configuration of EP 0 678 714, since the resulting reduced gap would likely render unacceptable the proximity between mirror and absorber.

Other problems are solved through this invention, for instance:

1) With respect to the fabrication tolerances, practice shows that the optics referred in EP-0 678 714 Bl is still too much sensitive to a deviation from the correct positioning of the absorbing fin, and thus also to deviations resulting from thermal expansion during operation. In the present invention the enlarged gaps allow for better trade-offs for the solution of these problems .

2) With respect to operating temperatures, a larger gap allows for larger temperature differences (gradients) between the absorber and the reflectors. Thus it is possible to relax the stringent conditions associated with the use of cheaper materials for reflector fabrication, with the generic form presented in Fig. 5, essentially in two preferred conforming solutions, supported/formed by expanded polyurethane foam or fixed to plastic pieces (L) . In this last case there will be either just one piece for the whole collector, or two or three pieces at the extremities and at the centre, with just enough width to configure and fix the aluminium sheet type mirrors. These options have clear consequences on the final production costs. In particular the existence of a plastic piece as referred, with the size of the full collector, can result in a direct reflector production technique (like reflecting film deposition or gluing) that also constitutes a claim of the present invention.

It constitutes a further claim of the present invention the fact that the headers tl and t2 feeding the absorbing fins (see Fig.6) may also be placed within a corresponding ideal optics, thus constituting an active solar collection area (Fig.7).

The collector of the present invention may be produced for operating temperatures up to 160°C, clearly beyond the thermal behaviour of corresponding to the operational objectives of the collector described in EP 0 678 714 Bl (120°C) , and therefore, they are a solution in a different category altogether.

A most likely embodiment of the present invention is a solar collector within a rectangular box with about 2m², reduced height, constituted by a series of troughs (5 to 8 are typical values for the number of troughs , depending on the concentration to be achieved) which lay longitudinally, side by side, with two more troughs perpendicular to the first ones, at each end. These troughs are placed within a box, more frequently framed in aluminium or plastic (UV resistant) with a transparent (glass) cover, single or double. In this last case a thin transparent Teflon film (18) or an open honeycomb type transparent insulation material is added to the single glass cover, for control of the internal air convection and consequent heat loss reduction (see Fig.9).

A cross section of the troughs is shown in Figs. 2, 3(a) and 3(b); they are formed by polyurethane injection and respective expansion in mould, or by using plastic pieces with the same form described below.

BRIEF DESCRIPTION OF THE FIGURES

For a better understanding of the present invention several figures illustrating the main features are presented below.

Fig. 1 illustrates in perspective and with a double cut one of the embodiments of the solar collector of the present invention 16, without the transparent cover and without the absorbing grid, for a clear vision of the ensemble box/mirrors, exhibiting the longitudinal troughs J.

Fig. 2 is a cross-section of one trough J of the collector of the present invention, showing the configuration of the trough J characterising the present invention when the absorbing element 13 has, in cross-section, the shape of a D resting on its flat side, constituted, in one of the embodiments of the present invention, by an arc of circumference ABC and a straight segment AEC. In other embodiments of the present invention the arc ABC may not be an arc of circumference, but any other curve, as, for instance, an arc of ellipse. The straight segment AEC will always be virtual from the point of view of the absorbing element 13, which, in cross section , is a line corresponding to the arc ABC. In a preferred embodiment of the present invention the absorbing element 13 is active on both sides, top and bottom, . The ideal optics constituted by the reflecting mirror 17, associated with this absorber 13, is described in the following way (the figure in cross-section has symmetry with respect to the optical axis 00', and thus the detailed description only covers the right side of Fig.2 (likewise for Figs. 3(a) and 3(b)). Its definition starts with the choice of ray G'G, tangent to the absorbing element 13 at G" which defines the acceptance angle (+θ) for the incoming radiation onto the referred optics. Arc 1, extending from E to F is a 1/4 circumference arc with radius EC. The arc designated by 2 is an arc of involute, extending from F to G. Arc 3 belongs to a curve whose definition is the following: all incident rays on it, parallel to GG' are reflected tangent to the absorber ABC . This arc extends from G to H, point at which it intersects the incident ray that reflects into the symmetric direction (-Θ) . Fig. 3(a) and (b) show, in cross section, one embodiment of the present invention in which the symmetry axis is the same 00' and is also designed for the same acceptance half angle θ (direction GG'). The absorbing element 13 has now, in cross section, the form of an open polygon of 4 or 5 sides, 4 when the angle β is 90° (in that case it is an open rectangle) and 5 (AA', A'B, BC, C'C and CA) when β is less than 90°. In Fig. 3 (a) β>θ and in Fig. 3(b) β<θ. As in Fig. 2, the segment AEC is virtual and the absorbing element as an active coating for the absorption of solar radiation which is deposited on both sides, top and bottom.

In particular in Fig. 3 (a) , that reflecting mirror curve, starts first with 1/4 arc of circumference signalled as 4, with a radius EC, extending from E to F. The arc designated by 5, extends from F to G and it is an arc of circumference with radius C'G=C'F extending up to G, point at which it intercepts ray G'G, passing at the tangency point C. Arc 6 extends from G to C", is an arc of parabola with focus on C and axis parallel to G'G, with point C ' the one which corresponds to its intersection with direction BC . Arc 7 extends from C ' to H' and is an arc of parabola with axis parallel to GG' and focus on B, with point H' defined by the intersection of that parabola arc with the direction defined by A'B. Arc 8, extending from H' to H is an arc of parabola whose axis has the same direction as the other ones but with focus on A' , with point H corresponding with its intersection with direction (-Θ) .

In Fig. 3(b) the definition up to point C' is just like the one in Fig. 3 (a) . Arc 9 extending from C ' to G is an arc of circumference of radius BC', with point G resulting from the intersection of that arc with the direction G'G that passes in B. Arc 10 extending from G to H is an arc of parabola with an axis parallel to the direction G'G and focus at B, with point H resulting from the intersection of that arc of parabola with the ray incident with the angle (-Θ) .

Fig. 4(a) shows two versions of the absorber fins 13, in which the tube 14, for the circulation of the fluid to be heated by the collector 16 is in symmetric position with respect to the optical axis. Fig. 4 (b) shows several situations in which tube (14) placement is asymmetric and a situation in which there are two circulation tubes 14. All these possibilities constitute embodiments of the present invention, claimed as such, and they constitute an equal number of ways to facilitate the supply of an absorbing element (fin) with selective coating on both sides, both by direct deposition of the respective materials on either face, or by folding onto itself a plate which is coated on only one side. The double line used in Fig. 4(b) is meant to express that second possibility and constitutes one of objects of the present invention.

Fig.5 is a view with perspective and a cross section of the solar mirror showing a specific way to produce the present invention, through the extrusion or moulding of plastic, or other material in order to get, in whole or in part, one (all) or several (part) of the pieces to support the aluminium mirror sheet. This procedure, an embodiment of the present invention, avoids the need for polyurethane foam injection in order to configure the aluminium sheet to have the ideal corresponding optical shape, simplifying the fabrication procedure and allowing for the use of insulation materials such as rock or glass wool. Fig. 6 shows a way to assemble the absorbing fins 13, described for instance in Figs. 2 and 3, in a tubular grid as risers, with the headers tl and t2 to which are soldered the smaller diameter tubes fixed to the absorbing fins 13. These tubes, at the collector extremities, are themselves integrated in their own ideal, non-imaging asymmetric optics.

Fig. 7 shows in a cross-section, the insertion of an header, for instance tl, at the extremities of the collector, integrated in its own asymmetric ideal or non-imaging optics, conceived for an angle y, with a value between 60 and 90° and with the following geometry: an arc of involute 11 starting at E and conceived for a virtual absorber which includes the straight segment E to E' being tangent to the header and a relevant part of the header (for the definition of arc 11) which extends to point P. Point E lies in a plane perpendicular to the optical axis 00' (and which contains point F referred in Figs. 2 and 3) at a distance from the header tl of the order of magnitude of its respective radius. This arc extends until point J' where it intercepts direction E''J'. Then comes arc 12 described as the one that reflects tangent to the header tl, all incoming rays parallel to the direction described by the angle γ, i.e. parallel to the direction E''J', until point J, determined by the maximum convenient height for the collector. Fig. 7 also shows a rectangular piece with an indent having an arc shape, in order to schematically indicate the connection between the header, for instance tl, with the tube 14 of the absorbing fins in Figs. 4(a) and 4(b).

Fig. 8 signals the truncation points M and M' of present invention collector 16, with the curves described in Fig. 2 and 3. Points M and M' lay on those curves before points H and I, resulting from what is usually described as truncated collector together with the definition of an angle θt which is designated by half truncation angle and such that θ < θt < 90°; in this way there are savings in the use of reflector without too much sacrifice of overall concentration.

Fig.9 shows a partial cross section of collector 16 in which the main glass cover 15, a transparent film 18 (of the transparent Teflon type or as an open transparent honeycomb type structure) are integrated, to control/reduce internal convection within collector 16 and the consequent reduction of thermal losses .

It should be noted that the present invention was described through several illustrative embodiments, by no means limiting or exhausting the scope of the invention, which is defined in the annexed claims.

List of references within the drawings

Claims

1. Low concentration solar collector, stationary or seasonally stationary, using ideal (anidolic) non imaging optics, typical of CPC type solar collectors, with an acceptance angle of 2Θ comprised between 60 and 180°, for applications up to 160°C, characterized to integrate: β Absorbing fins (13) with the geometry of a D with its flat side facing down, constituted by a straight horizontal virtual segment AEC and a semicircular arc

(ABC) or other equivalent curvature as for instance an ellipse

• As set of identical longitudinal troughs (J) , each one of them formed by reflecting surfaces (17) (IEH) whose curvature is symmetrical with respect to the optical axis (00'), of the system and on each side of said optical axis it is constituted by a quarter of a circumference (EF) and an arc of involute (FG) to which an arc (GH) is added, defined by the condition that all incoming radiation parallel to the extreme direction +θ will be reflected tangent to absorbing fin.

2. Collector (16) according to claim 1, characterized by • the fact that the absorbing fins (13) have the shape of an open 4 sides polygon (AA'BC'C), configuring an open rectangle (2β=180°), and

• a symmetric curvature with respect to the optical axis (00' ) of the system troughs (J) , on each side of the optical axis (00' ) of the system, comprising:

- in the case 2β < 2Θ, three arcs of circumference (EF and FC" and C"G) with radii EC, CF and BC" respectively and arc of parabola (GH) with axis parallel to the direction +θ and focus at B - in the case 2β > 2θ, two arcs of circumference (EF and F ' ) with radii respectively EC and FC and three arcs of parabola (GC", C"H', H'H) , all with their respective axis parallel to the direction +θ and foci respectively at C , B and A' .

3. Collector (16) according to claim 1, characterized by absorbing fins (13) with the geometry of an open 5 sides polygon, configuring an open pentagon (2β < 180°) and a symmetrical curvature of the reflecting surfaces (17) according to claim 2.

4. Collector according to claims 2 and 3, characterized by the angle 2β of the top vertex (B) of the open pentagon being less, equal or larger than 2θ.

5. Collector (16) in agreement with the previous claims characterized by absorbing fins (13) having the capacity to absorb the incoming radiation both their respective top and bottom sides, since the horizontal bottom segment (AC) is virtual in all cases.

6. Collector (16) according with the claims above, characterized by exhibiting a truncation angle between θ and 90°, for instance the average value over that interval.

7. Collector according to the previous claims characterized by being formed by a number of troughs, between 1 and 100, preferably between 4 and 12 and even more preferably between 5 and 8, according to the absorbing fin dimensions (13) and the height of the collector box (16) .

8. Collector (16) according to the previous claims, characterized by the absorbing fins (13) being risers in a tubular grid where the two tubes (tl and t2) are the headers.

9. Collector (16) according to the previous claims, characterized by the fact that the headers (tl and t2) are also immersed in an ideal, non imaging optics, asymmetric in relation to the optical axis 00' , and constituted by an arc of involute (EJ' ) corresponding to the virtual absorber

(EE'P) which coincides with part of the header (tl, t2) and the point (E) at which starts the reflecting surface (11) and an arc (12) of a curve defined by a tangency condition to the header (tl, t2) for all rays reflected off that curve and which are incident on it parallel to a direction γ, predefined, and with a value between,,60 and 90°.

10. Collector (16) according to the previous claims, characterized by the fact that each absorbing fins (13) have, fixed to them, one or more tubes for fluid circulation in symmetric or asymmetric situation.

11. Collector (16) according to the previous claims, characterized by each one of the absorbing fins (13) possessing a selective coating on both faces, top and bottom, or being made from a plate coated on only one face but folded onto itself to recreate the effect of a coating on both faces

12. Collector (16) according to the previous claims characterized by the fact that the set formed by the reflectors (17) and the absorbing grid is placed inside a collector box (16), with a transparent cover (15, 18) and thermal insulation obtained through polyurethane expanded foam (19) .

13. Collector (16) according to the claims above, characterized by the reflector support (17) being obtained with plastic or metallic material, previously configured as described in claims 1 and 2, in one full piece or in several separated pieces, and with insulation obtained through the use of materials not necessarily injected.

14. Collector (16) according to the previous claims characterized by having a double cover (18) of the thin film type or of the honeycomb transparent type, placed under the first cover and between that one and the tip of the reflectors.