WO2010027289A2

WO2010027289A2 - Cpc type solar collector with evacuated tubes

Info

Publication number: WO2010027289A2
Application number: PCT/PT2009/000047
Authority: WO
Inventors: Manuel Pedro Ivens Collares Pereira; Wildor Maldonado Carbajal; João Correia de OLIVEIRA
Original assignee: Ao Sol - Energias Renovaveis Sa
Current assignee: Ao Sol - Energias Renovaveis Sa
Priority date: 2008-09-02
Filing date: 2009-09-02
Publication date: 2010-03-11
Anticipated expiration: 2011-03-02
Also published as: WO2010027289A3; PT104168A; EP2324300A2

Abstract

Concentrating solar collector of the CPC type with evacuated tubes, which reduces the reflector area to be used, has the ability to adapt to a tracking strategy of the sun's daily motion and facilitates the use in vertical façades of buildings. The new concentrating solar collector is formed by one concentrator (4) and by a new configuration of an absorbing element (8), allowing the use of an absorbing evacuated tube (3), rendering the advantages described above.

Description

DESCRIPTION

^«CPC TYPE SOLAR COLLECTOR WITH EVACUATED TUBES"

FIELD OF THE INVENTION

This invention relates to new concentrator optics of the CPC type for evacuated tubes, which reduces the area of reflectors to be used, has adaptive ability to track the sun's apparent motion and facilitates the use in vertical facades of buildings.

BACKGROUND OF THE INVENTION

The sites http://www.himin.com/english/index.html and http://www.iea-shc.org/, describe solar collectors with evacuated tubes which constitute the largest equipments market for solar thermal applications, below and above 100 ⁰C, specially in China.

Evacuated tubes are placed side by side to yield as large a collection surface as desired, according to each application. Often they are complemented with diffusing and/or reflecting surfaces, placed in a fixed way, behind the tubes, in order to recover some of the incident radiation that, otherwise, would escape between the tubes or to concentrate it.

Sometimes these surfaces have specific optical configurations, using shapes having solar radiation collection properties known as involutes, partially or totally developed, as disclosed in patent US-A-3957031 entitled "Light collectors in cylindrical geometry". In this case they may even become parts of CPC concentrators, as disclosed in Patent US-A-957031 entitled "light collectors in cylindrical geometry", Patent US-A-4003638 entitled "Radiant energy collection", Patent US-A-4230095 entitled "Ideal light concentrators with reflector gaps" and Patent US-6205996 with an optica! treatment of the "gap" question (space between concentrating reflector and absorber) which seeks a reduction to the limit of the optical losses.

When the resulting incident radiation concentration is of the CPC type, also said to be Ideal, Winston type or anidolie, it has the characteristic of corresponding to the maximum concentration that it is possible to achieve for any given acceptance angle of the system.

The formula that describes this fact, for cylindrical geometries (also referred as 2D, since what takes place in the orthogonal direction to the plane is just the continuous reproduction of the figure in the plane) is C = Cm_9x = l/sia(θ), with θ being the half acceptance angle (Fig.l).

If concentration is maximal for a given acceptance angle, the number of tubes to used is minimal, which may be advantageous in terms of the overall system cost and is certainly advantageous in terms of minimization of the thermal losses coefficient, which has a dominant term and proportional to 1/C.

The problem lays in that the involutes (omnipresent when dealing with anidolie optics for tubular absorbers) in addition to the fact that they "consume" lots of reflector in the material realization thereof (see portion 2"in Fig. 1) they have the inconvenience of contributing to the increase of the average number of reflections for the incident radiation making small angles with the normal to the collector entrance aperture, which causes its optical efficiency to have a pronounced minimum at that incidence angle (θ = 0).

The involutes of the type of that from Fig.l further have the inconvenience of substantially increasing the total concentrator height and of not being susceptible to a good adaptation of recent ideas pointing towards the tracking of the sun's apparent motion being achieved by only moving the reflectors, without moving the rest of the collector, as disclosed, for instance, in Portuguese patent application PT 104084, entitled "Colectores solares concentradores quasi-estacionarios com tubos de vacuo ou alhetas e opticas nao estacionarias".

The CPC type concentrator optics of the present invention is able to overcome the above-mentioned problems and disadvantages, aiming to achieve goals comprising distinct improvements over me devices of the prior art.

Thus the concentrator optics of the present invention aims to comprise the following characteristics:

being concentrating and of me CPC type,

being able to substantially reduce the necessary reflector area to be used,

being able to a better adaptation to the tracking of the sun's apparent motion.

Given the increasing interest in the use of concentrating optics in vertical (or near vertical) facades of buildings, since this one will be in the future, one of the most common application of solar collectors for water heating, space heating and cooling, the concentrator optics of the present invention also aims to facilitate such usage, and, for mat effect, being important that the optics to be used have a thickness (height) as small as possible, are easy to mount and eventually have the ability to rotate the reflectors around the tubes. SUMMARY OF THE INVENTION

The present invention relates to a concentrating solar collector of the CPC type, comprising:

one optical axis,

at least one concentrator having

- edges (G, H) defining the entrance aperture (GH) of the collector and

- an acceptance angle for the incident radiation comprising extreme directions according to the angles (+θ) and (-Θ),

at least one absorbing element,

characterized by said absorbing element having an inverted U shape, defined by a semi-circumference and its extension in the descending direction being defined by means of two straight segments (WM, WM'), parallel to each other and parallel to the said optical axis (9).

In a preferred embodiment, the CPC type solar concentrator is characterized by said absorbing element being tubular, of the evacuated type, comprising an inner absorber tube and an outer evacuated tube, concentric with each other, having respectively radii (n) and (r_e) wherein said semi-circumference is the top semi circumference of said inner absorber tube and said straight segments have the length of the radius (r_e) of said outer evacuated tube. In another embodiment, the concentrating solar collector of the CPC type is characterized by said concentrator comprising a straight portion perpendicular to the said optical axis and tangent to said outer tube at a point thereof,

In yet another embodiment, the concentrating solar collector of the CPC type is characterized by not being symmetrical in relation to said optical axis, collecting solar radiation incident along any direction comprised between the extreme directions defined by the angles (θt) and (θa),

In another embodiment, the concentrating solar collector of the CPC type is characterized by the said concentrator being truncated, defining a collector entrance aperture with edges.

Still in another embodiment, the concentrating solar collector of the CPC type is characterized by having concentration values comprised in the range of about 1 to 6.

The present invention also relates to a collecting unit of solar collectors characterized by having at least two of said solar collectors, placed side by side and vertical reflectors, placed transversally to its length.

In an embodiment, the collecting unit is characterized by said reflectors being placed at the edges of the respective solar collectors.

In what follows a detailed description of the present invention is made with reference to preferred embodiments (by way of example) and accompanying figures. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 schematically illustrates a CPC type solar collector of the prior art.

Fig.2 schematically illustrates a CPC type solar collector of the prior art in which the absorber is in vacuum, adopting a position of tube centered inside another transparent glass, with the involute cut short.

Fig. 3 schematically illustrates a CPC type solar collector similar to that in Fig.2 in which the absorber is formed by an arc of circumference extending from E to F and by two straight portions, respectively EA and AF, with a V-shape.

Fig.4 schematically illustrates a solar collector of the present invention.

Fig. 5 schematically illustrates a preferred embodiment of the present invention in which the absorber is a tubular element of the evacuated type.

Fig. 6 schematically illustrates an embodiment of the present invention in which there is no symmetry with respect to optical axis of the system.

Fig.7 schematically illustrates an embodiment of the present invention in which the concentrator (4) is truncated on its edges.

Fig.8 schematically illustrates a collecting unit comprising a set of solar collectors of the present invention placed side by side. DETAILED DESCRIPTION OF THE INVENTION

CPC type concentrators designed for tubular absorbers (see for instance Fig.l) are formed by a combination of curves. One portion (2') of these curves is defined by the fact that extreme incident rays are all reflected tangentialiy to the tubular absorber (3) and another portion (2"), with the name of involute, is a curve which is obtained from a (initial) point on the circumference of the tube (3), such as if a string winded around said tube (3) is unwinded from that tube (3) always under a stretched condition, which represents a material way of obtaining the curve in which each point is on the tangent to the tube (3) at a distance from it, which distance is equal to the arc length from the point of tangency to the initial point.

It should be noted that the concept of involute is susceptible of generalization to non circular absorbers, as it is, in fact, done in the present invention,

Fig, 1 shows an absorber (3) of circular section and radius r and a concentrator, formed by a reflector (2) of the CPC type designed for the half angle θ which is made by the incident solar ray (1') with the optical axis of the system and perpendicular to the entrance aperture GH of the concentrator.

Concentration is defined as:

C = Cm_** = l/sin(θ) = GH/(2w)

Portion (2') of the concentrator's reflector, reflects tangentialiy towards a point on the absorber (3) all solar rays incident on GH and parallel to direction (1') and its symmetrical direction (1") while the portions (2") are exactly the involute which ends at point A, where the optical axis (9) crosses the absorber (3), at the base thereof. Solar ray (1 ') is tangent to the absorber (3) in B' and extends until B", thus defining precisely the point on reflector (2) where the involute (2") starts. The involute has the optical property of reflecting to the absorber (3) without any losses (assuming a reflector with reflectivity equal to 1) all radiation that crosses line B'B".

Actually, the practical situation is the one in Fig.2, where the absorber is now in vacuum, with the shape of a tube (3') concentric with another outer tube (3") of transparent glass, so that vacuum may reduce significantly the system's thermal losses. This configuration of concentric tubes (3', 3") is usually called evacuated type tubular absorber.

To accommodate an evacuated tubular absorber the collectors of prior art consider two known solutions:

-to cut the upper part of the involute, according to Fig, 2 so that the outer tube (3") might be accommodated and

- redesign the optics, according to Fig, 3, such that the absorber is formed by the tube (3') of radius n, augmented by a portion with a V-shape, extending from E to A and from A to F.

The solution of Fig. 3 is more elegant, provides an higher concentration and compensates, with this extra concentration, the optical losses which always occur, in either case, due to radiation traversing the gap between the two tubes (3', 3"),

It should be noted that other solutions exist (as, for instance, the one in US 6205998) but which are not relevant to the line of reasoning of this disclosure.

Referring to Fig,4, the present invention proposes an optical solution that, although designed for an evacuated tubular absorber identical to the above-mentioned one, is defined for a new completely different absorbing element (8), which has the desired advantages over the prior art mentioned above.

Thus and still referring to Fig. 4, the absorber element (8) of the present invention has the shape of an inverted U, defined by the semi-circumference WW and its extension in the descending direction by means of two straight segments (WM, WM'), parallel to each other and parallel to said optical axis (9), The length of said segments corresponds to the length of the desired radius r_e of the outer tube (3") (larger than the radius rt of an absorbing tube 3"), similar to those in Fig, 2 and 3.

This new absorbing element (8) may now be combined with a concentrator (4) of the CPC type. Since Fig. 4 is symmetric in relation to the optical axis (9) of said concentrator (4), only one side, comprising two portions, is following defined:

-portion (4') corresponding to the reflector (which as in Fig. 1, 2, 3 reflects extreme rays (V, 1") tangentially to the absorber) that ends in point B",

-portion (4") having the curvature of the respective involute extending from M to B".

In this case the involute is obtained with a tip of a curve tracer placed initially at M and with an associated string stretched between M and B', unwinding the string until the tip reaches B".

The concentrator is completed by a reflector (4'") corresponding to a straight segment, perpendicular to the optical axis (9) extending from M to A and which, when the invention is applied to evacuated tubes, is able to recover almost all the radiation (heading towards the inner tube) and which, otherwise, would have a tendency to escape. The final concentration is Ideal with respect to the absorber with perimeter p = 3ϊTi+2*WM where WM = n+"gap" ("gap" is the difference between the length of radius re of the outer evacuated tube (3 ' ') and the length of the radius n of the inner absorber tube (3 ')), where n is the radius of the inner absorber tube (3').

The concentration obtained by the solar collector of the present invention is just slightly lower than a prior art one (for instance that of Fig.3), but this new configuration largely compensates said slightly reduction in concentration with the big advantages it provides, such as the reflector area required, the fact that the straight segment MM' is able to support the outer tube (3") around which it is able to rotate and the fact that it allows solutions for the tracking of the above-mentioned apparent sun's daily motion.

It has been found, surprisingly, that this new solar collector fulfills the required goals set out above and presents a new optical configuration which is not, in any way, contained in the configurations of prior art and which cannot be obtained by a person skilled in the art, as a simple extension of the concepts behind it,

Fig. 5 shows a preferred embodiment of the invention in which the absorbing element is an evacuated tubular element.

Fig. 6 shows an embodiment of the present invention in which tiiere is no symmetry with respect to the optical axis (9) of the full system, i.e., in which the angles +θ and -θ are, in this case, the generic angles θi and Θ2 ,

FigJ shows an embodiment of the present invention in which the concentrator (4) is truncated in its edges.

The configuration of the solar collector of the present invention gives a larger flexibility to the design of optics, in particular in situations in which the concentrator (4) is to be placed in building facades, i.e. with an entrance aperture GH vertical » or substantially vertical, with respect to the ground.

Referring to Fig.8 one collector unit is shown to be placed on the roof of a building, a facade, or simply on a terrace or on flat ground, comprising a set of solar collectors (6) of the invention, arranged side by side. Said collector unit heats up a fluid which circulates inside specific tubes (11) internal to the absorbing tubes (3^s) or directly inside them, with or without phase change, for heat extraction.

Said solar collectors (6) of the present invention show at their extremities, or along its length, transversal reflectors (10) for reflection of a longitudinal component (i.e. in the direction of the tubes (3')) of the incident radiation.

The concentration values being in the range of 1 to 6, albeit the fact that above a value of 2 the collector can no longer be fully stationary, becoming quasi-stationary, i.e. it will have more than one tilt per year and/or with the alternative of the reflectors being able to rotate around their respective tubes (3', 3").

Tubes (3 ',3") and their respective concentrators (4) may be installed on a horizontal or tilted plane, oriented East-West, or oriented in a manner orthogonal to this one, i.e., North-South.

It should be noted that the present specification should not be limitative in relation to modifications/variations coming from a person skilled in the art, as long as do not departing from the scope of the accompanying claims.

Claims

1. Concentrating solar collector of the CPC type, comprising:

one optical axis (9),

at least one concentrator (4) having

- edges (G, H) defining the entrance aperture (GH) of the collector and - an acceptance angle for the incident radiation comprising extreme directions according to the angles (+θ) and (-Θ),

at least one absorbing element (8),

characterized by said absorbing element (8) having an inverted U-shape, defined by a semi-circumference (WW), and its extension in the descending direction being defined by means of two straight segments (WM, W'M'), parallel to each other and parallel to the said optical axis (9).

2, Concentrating solar collector of the CPC type according to claim 1, characterized by said absorbing element (8) being tubular of the evacuated type comprising an inner absorber tube (3') and an outer evacuated tube (3"), concentric with each other, having respectively radii (ri) and (ra) wherein said semi-circumference (WW) is the top semi circumference (7) of said inner absorber tube (3') and said straight segments (WM, WM') have the length of the radius (r_e) of said outer evacuated tube (3 ' ').

3, Concentrating solar collector of the CPC type according to claims 1 and 2, characterized by said concentrator (4) comprising a straight portion (4'") perpendicular to said optical axis (9) and tangent to said outer tube (3 ") at a point (A) thereof.

4. Concentrating solar collector of the CPC type according to any of the preceding claims, characterized by not being symmetric in relation to said optical axis (9), collecting solar radiation incident along any direction comprised between the extreme directions defined by the angles (θi) and (θi),

5. Concentrating solar collector of the CPC type according to any of the preceding claims, characterized by said concentrator (4) being truncated, defining a collector entrance aperture (G'H') with edges (G^*, H'),

6. Concentrating solar collector of the CPC type according to any of the preceding claims, characterized by having concentration values comprised in the range of about 1 to 6.

7. Collecting unit of concentrating solar collectors as defined in the preceding claims, characterized by comprising at least two of said concentrating solar collectors placed side by side, and vertical reflectors (10) placed transversely to its length,

8. Collecting unit according to the preceding claim characterized by said reflectors (10) being placed at the edges of the respective solar collectors.