SE2150467A1

SE2150467A1 - A cover device for a receiver fluid tube of a parabolic trough solar collector and such a solar collector

Info

Publication number: SE2150467A1
Application number: SE2150467A
Authority: SE
Inventors: Joakim Byström; Jonatan Mossegård
Original assignee: Absolicon Solar Collector Ab
Priority date: 2021-04-15
Filing date: 2021-04-15
Publication date: 2022-10-16
Also published as: WO2022219203A1

Abstract

A cover device for a receiver fluid tube (8) of a parabolic trough solar collector, comprises at least one elongated cover member (12), wherein said cover member has an axis (A-A) in its elongation. In accordance with the invention, said cover member defines at least one opening (13) extending in the elongation of the cover member (12).The invention also includes a solar collector comprises such a cover device.

Description

Technical Field The present invention relates to a cover device for a receiver ﬂuid tube of a parabolic trough solar collector, comprising an elongated cover means, wherein said cover means has a concentric axis in its elongation.

Background In modern society energy is consumed by people and industries, e.g. for producing various products, for transport and production of food. Energy could be produced in several forms and from different energy sources. For instance, electricity is often produced from hydroelectric power plants, combustion of coal, oil, or gas. Traditionally, heat has been pro- duced from local combustion or district heating power plants.

With an increasing population and demands for services, energy consumption strong- ly increases which significantly negatively affects our environment. Combustion produces large amount of carbon dioxide and other greenhouse gases. Hydroelectric power plants re- quire large territories to be drowned, etc.

In order to reduce our footprint and negative impression on our environment, de- mands have been raised for more clean and environmental friendly energy production. To- day, renewable energy is produced from wind, sun, ocean waves, etc. The sun provides large amounts of energy to our planet in form of radiated sun beams. Solar radiation can be used by solar cells to generate electricity, e.g. in form of solar panels, or by solar collectors to generate thermal heat.

A concentrating solar collector uses mirrors, lenses, or combinations thereof, to focus the solar radiation in form of a point or a line. In trough-formed concentrating solar collec- tors a reﬂector is formed as a curved elongated mirror, which reﬂects the solar radiation on a receiver arranged along a focus-line of the reﬂector. The receiver is commonly a black tube filled with a transport ﬂuid, such as water, glycol, or oil. The tube is heated by the con- centrated solar radiation and the heat is transferred to the transport ﬂuid that is circulated in a system where the heated transport ﬂuid could be used. The heated transport ﬂuid may be used both as process heat in industrial processes as for district heating. 2 The term ”PTC” (Parabolic Trough solar Collector) will be used in this disclosure to denote a concentrating solar collector with a trough-formed reﬂector arranged to concen- trate solar light onto a ﬂuid tube. Typically, PTCs will be pivoted to track the sun during the day and are controlled by a solar tracking arrangement.

A parabolic trough solar collector comprises an elongated reﬂector, which reﬂective surface in a cross-section describes a parabolic curve. The reﬂector focuses direct sunlight on a focus.

An example of such a cover device is known from US 4 l5l 828. The therein de- scribed cover device discloses an evacuated glass tube, encircling the receiver ﬂuid tube in the elongation thereof between a pair of collars for reducing heat losses by convection from the receiver ﬂuid tube. In order to maintain a hard vacuum, a tubular side arm of the glass tube is connected to a vacuum pump.

A tracking arrangement for parabolic trough solar collectors is known from WO20l9/0549l2Al. The therein described solar collector is adapted to pivot each of the parabolic trough solar collectors towards the traveling sun in order to focus the sun rays to- wards the receiver ﬂuid tube. The document also discloses turning the trough upside down e. g. at night in order to reduce heat loss through radiation.

Summary The object of the present invention is to reduce energy losses from the receiver ﬂuid tube. This has been solved by a cover device as initially defined, and further comprising the features that the cover means def1nes an opening extending in the elongation of the cover means. It has also been achieved by a solar collector comprising such a cover device. Here- by, energy losses by convection are reduced.

Preferably, said cover member is provided with a suspension means for said receiver ﬂuid tube, said suspension means being arranged to keep the receiver ﬂuid tube coaxially with said cover member. Hereby is achieved a static relationship between the receiver ﬂuid tube and the cover device.

Suitably, said opening is arranged substantially below a horizontal plane extending through said axis A-A and cover the receiver ﬂuid tube and its closest vicinity. Hereby re- duced energy loss by convection is surprisingly achieved, despite the fact that the cover means is not evacuated.

Preferably, the cross-section of said cover member is substantially annular. Alterna- tively, the cross-section of said cover member is substantially polygonal. Hereby, suitable shapes of the cover device are achieved.

Suitably, said opening comprises an elongated slit defined by elongated edges of the cover member. Hereby, a well-defined opening of the cover member is achieved.

Preferably, said cover member comprises a transparent, heat resisting material. In particular, said cover member comprises any one of glass and a polymer. Hereby is achieved a suitable material for covering the receiver ﬂuid tube.

Suitably, the suspension means of the solar collector comprises a spoke is adapted to keep the receiver ﬂuid tube at a predetermined distance from said parabolic reﬂector. Here- by, the sun rays are focused towards the receiver ﬂuid tube.

Preferably, the parabolic reﬂector of the solar collector is movably arranged in rela- tion to said receiver ﬂuid tube and said cover member. Hereby is achieved a possibility to follow the movement of the sun and in turn to maintain focus of the sun rays towards the receiver ﬂuid tube.

Suitably, transversal dimension of said opening is such that it allows insertion of said receiver ﬂuid tube across said opening. Hereby, assembly of the solar collector is readily achieved.

Preferably, the suspension means is connected to the cover device, such that the opening is maintained static in relation to the horizontal plane through the axis of the re- ceiver ﬂuid tube. Hereby, energy loss due to convention is surprisingly reduced, without need for evacuating the cover member. By eliminating the need for vacuumizing the cover member, a plurality of advantages may be achieved, such as: a cheaper manufacturing and installation process; a less sensitive arrangement as no risk for leakage arise; receiver ﬂuid tubes and cover members could be manufactured and installed separately; etc.

Brief description of drawings The solution will now be described in more detail by means of exemplifying embod- iments and with reference to the accompanying drawings, in which: Figure lA is a perspective schematic view of a solar collector including a receiver ﬂuid tube, a reﬂector, a suspension means and a cover means connected to the receiver ﬂuid tube; Figure lB is a cross-section along lines IB-IB in Fig. lA; Figure 2A illustrates schematically an alternative suspension means and cover means; Figure 2B is a partial side view of the cover means and suspension means shown in Fig. 2A; Figure 3A is a cross-section of the cover means and suspension means along line IIIA - IIIA in Figure 2B; Figure 3B is a cross-section of the cover means and suspension means according to an alter- native embodiment; Figure 3C is a cross-section of the cover means and suspension means according to yet an- other alternative embodiment; Figure 4A is a perspective view of a portion of a further embodiment of the cover means and suspension means including a bearing; Figure 4B is an exploded view of the cover means and the suspension means shown in Fig 4A, and bearing parts; Figure 4C is a cross-section of the cover means and suspension means of Fig 4A along a vertical plane through the axis A-A; Figures 5A - 5B illustrate the upper bearing part in different perspective views; Figures 5C - 5D illustrate the lower bearing part in different perspective views; Figure 5E illustrates an alternative embodiment of the bearing shown in Figs 5A - 5D; Figure 6A illustrates a single part bearing; Figure 6B illustrates a bearing divided in a vertical plane; and Figure 7 is a cross-section of an alternative cover means and suspension means.

Detailed description Figs. lA and lB show a solar collector 2 comprising an elongated parabolic reﬂector 4, i.e. a PTC, provided with gables 6a, 6b at each end. The parabolic reﬂector 4 and the ga- bles 6a, 6b are together rotatably arranged about a static receiver ﬂuid tube 8 and a cover means 10 by means of a suspension means 9. The cover means 10 comprises first and sec- ond cover members l2a,l2b non-rotatably attached to the static receiver ﬂuid tube 8.

The parabolic reﬂector 4 is covered with a coated glass pane 11, for reducing ther- mal losses by convection from the solar collector. Of course, the coated glass pane 11 may also protect the reﬂector 4 and the receiver fluid tube 8 from environmental effects that could decrease performance, e.g, rain, snow, sandstorms, dust, etc.

The first and second cover members l2a, l2b are arranged coaxially with the receiv- er ﬂuid tube 8 along axis A-A. The first and second cover members l2a, l2b are provided with an opening 7 in the form of an elongated slit 13 substantially below a horisontal plane through said axis A-A. According to the embodiment of Figs. lA-lB, the suspension means 9 comprises three spokes 14a, 14b, 14c arranged between the parabolic reﬂector 4 and the receiver ﬂuid tube 8 by means of first, second and third bearings l6a, l6b, l6c, slidingly arranged on said receiver ﬂuid tube 8. As shown in the lower enlargement of Fig. 1A, the bearings l6a, l6b, l6c preferably comprise an annular bearing body 17 including pair of semi-annular bearing parts 18a, 18b of a sliding bearing peripherally encompassed by a bearing lining 20 inside peripheral bearing housing 22. The bearing housing 22 and the bearing lining 20 may be substituted by a combined bearing housing provided with an inner surface acting as bearing lining. The radially inner surface 23 of the bearing parts slidingly bear against the exterior surface of the receiver ﬂuid tube 8.

In case the receiver ﬂuid tube 8 is detachable from the parabolic reﬂector 4, said an- nular body 17 may instead comprise a singular annular bearing part.

In this exemplifying embodiment the parabolic reﬂector 4 is rotatably arranged to pivot about the receiver ﬂuid tube 8, by a mechanism in form of a drum (not referred to) surrounding the receiver ﬂuid tube 8 and a wire encircling the drum. By implementing such a tracking mechanism, the parabolic reﬂector 4 may follow the travelling sun and direct its aperture towards the solar radiation. It is hereby possible to keep focus of the sun rays to- wards the receiver ﬂuid tube 8 during day time hours. Furthermore, by selecting the dimen- sions of the wire and drum, heat loss by radiation may be reduced by turning the parabolic trough substantially upside-down during night hours. It is to be noted that the tracking mechanism may be alternatively implemented within the inventive concept, e. g. by means of a cogwheel encircling the receiver ﬂuid tube 8 and a toothed rack by which the cogwheel is rotated.

It is contemplated that the bearings l6a, l6b, l6c do not only perform a rotatable movement, due to the above described pivotal moment of the reﬂector, but also an axial 6 movement along axis A-A, due to the longitudinal expansion and contraction of the receiver ﬂuid tube 8, because of the varying heat of the focussed sun rays. The first cover member l2a is rigidly attached to the receiver ﬂuid tube 8. For this purpose, as shown in the upper enlargement of Fig. lA, said suspension means 9 comprises a suspension member 30 in- cluding a clamp 32 and a partly annular (i.e. provided with an opening 7) heat insulating ring 34, of only little axial extension at each end of the first cover member l2a. More par- ticularly, the first cover member l2a is attached to the receiver ﬂuid tube 8 between, how- ever not connected to, the bearings l6a and l6b. The cover member l2b is attached to the receiver ﬂuid tube 8 in a corresponding manner between the bearings l6b and l6c. For fa- cilitating understanding the Fig lA, the cover members l2a, l2b have been shown as being separated a distance from the bearings. However, in order to reduce heat losses by convec- tion, it is to be understood that the cover members l2a, l2b are to be arranged as close as possible to the bearings l6a, l6b, l6c.

Fig. 2A and 2B show another embodiment of the solar collector 2. As already men- tioned above in connection with the first embodiment, the first cover member l2a is ar- ranged between but not connected to the first and second bearings l6a, l6b and is fixed to the receiver ﬂuid tube 8 by a suspension means 9 in the form of a suspension member 30. As mentioned in connection with the first embodiment, the cover means 10 is provided with an opening 7 having the shape of an elongated slit 13. The broken line D (Fig 2B) indicates the imaginary perimeter of the cover member l2a.

Also in this embodiment, the suspension member 30 keeps the first cover member l2a in a coaxial inter-relationship with said receiver ﬂuid tube 8. The suspension member 30 of a heat insulating material, comprises a clamp 32 connected the exterior of the cover member l2a and to a rod 36 extending in both directions from said clamp 32 along said axis A-A to fastening means 37, such as clamping rings 38a, 38b, securing the cover means 10 coaxially with the receiver ﬂuid tube 8.

It is contemplated that in case the material of the rod 36 is chosen such that it has substantially the same expansion coefficient as the receiver ﬂuid tube 8, the rod 36 will ex- pand correspondingly to the expansion and contraction of the receiver ﬂuid tube 8 due to the heat of the sun rays, and thus, the cover member l2a will remain substantially static in rela- tion to the receiver ﬂuid tube 8. 7 The cover member 12b is suspended on the receiver ﬂuid tube 8 in a corresponding manner.

Fig 3A is a side view in cross-section along line IIIA-IIIA in Fig. 2B. The cover member 12a is substantially circular, but edges 40a, 40b define them between said open slit 13. The slit 13 is broad enough to allow introduction radially through the slit 13 of the re- ceiver ﬂuid tube 8 and the clamping ring 38a preferably also said clamping ring 38b of the suspension member 30. As already mentioned above, the receiver ﬂuid tube 8 and the cover member 12a are arranged coaxially with axis A-A.

As shown in Fig. 3B, the slit 13 of the cover means 10 may instead be narrow, i.e. to a few degrees of a circle and thus open. In case the cover member is made of a rigid materi- al, this of course requires lengthwise sliding attachment of the cover member 12a onto the clamping ring 38a (and clamping ring 38b) of the suspension member 30. Fig. 3C shows yet another embodiment, according to which the cross-section of the cover means 10 is substan- tially a half circle between edges 40a, 40b, i.e. the slit 13 being substantially 180° of a cir- cle.

In Figs. 3A - 3C, the cross-section of the cover member 12a has been shown as being substantially circular. It should however be noted that it may instead be substantially polyg- onal, such as triangular, square, pentagonal etc., and including a slit below the horizontal plane through axis A-A of the receiver ﬂuid tube.

Common for the embodiments of Figs. 3A - 3C is that the slit is arranged substantial- ly below a horizontal plane through axis A-A. By the cover means 10 shown in f1gs. 3A - 3C, heat losses by convection are reduced to a surprisingly large extent, despite the fact that the cover means is neither evacuated, nor closed, but is instead provided with a slit 13.

However, the inventive concept is not limited to arrange the opening of the cover member as an elongated slit along the entire length of the cover member as described above.

In an alternative exemplifying embodiment, instead a plurality of shorter slits are provided in the cover members such that the rods and clamping rings above are enabled to move in the slits to compensate for expansion of the receiver ﬂuid tube.

In yet another alternative embodiment, no slits are provided in the cover members, but their ends are open.

As it surprisingly has shown that a proper reduction of convection could be achieved also without evacuating the cover member, the cover member could be either provided with 8 a slit along the cover members full length, partial slits by the receiver holders, or openings at its ends. By eliminating the need for vacuumizing the cover member, a plurality of ad- vantages may be achieved, such as: a cheaper manufacturing and installation process; a less sensitive arrangement as no risk for leakage arise; receiver ﬂuid tubes and cover members could be manufactured and installed separately; etc.

Figs. 4A - 4C show an alternative embodiment of a cover means 10, according to which the suspension means 9 integrally comprises the spoke l4. The clamping rings 38a, 38b of the suspension member 30 are connected to the receiver ﬂuid tube 8 indicated with broken lines in Fig. 4A. As described above, the clamping rings 38a, 38b are inter- connected by the rod 36, extending parallel to axis A-A. The bearing l6 comprises an annu- lar bearing body l7, including two semi-annular upper and lower parts l8a, l8b. By upper part is meant substantially above a horizontal plane through the axis A-A. Likewise, by lower part is meant substantially below a horizontal plane through said axis A-A. The lower part l8b comprises a groove 44, adapted to receive rod 36 of the suspension member 30. The groove 44 is arranged parallel to the axis A-A.

As shown in Figs. 4A and 4B, the exterior of the bearing l6 is provided with a plu- rality of radially directed protrusions 46, having a peripheral radially outwardly directed contact surface 48 intended to contact a radial inner surface 52 of a bearing lining 20. The exterior of the bearing l6 is furthermore provided with a pair of annular grooves 62 (see also Figs 5A - 5D), each groove 62 being adapted to receive an O-ring 50, 5l, respectively, in turn adapted to bear against the inner surface of cover members l2a, l2b, respectively.

The protrusions 46 are on each side provided with an axially directed surface 54a, 54b adapted to abut the axial end surface 56 of cover members l2a and l2b, respectively.

The bearing lining 20 is provided about its circumference with an annular peripheral groove 58, adapted to receive annular clutch members 60a, 60b. The clutch members 60a, 60b (see Fig. 4B) are adapted to maintain the two semi-annular upper and lower parts l8a, l8b inside said bearing lining 20, and to maintain the semi-annular bearing parts l8a, l8b on the receiver ﬂuid tube 8, and furthermore to keep the axial end surfaces 56 of the cover members l2a and l2b in abutting relationship with the axially directed surface 54a, 54b of the protrusions 46.

Figs 5A - 5D show the semi-annular bearing parts l8a, l8b in more detail. Both parts l8a, l8b are provided with the above mentioned radial protrusions 46 having axial abutment 9 surfaces 54a, 54b, serving as axial end surface 56 for the cover members l2a and l2b (see Figs 4B - 4C).

The lowermost protrusion 46 of the lower bearing part l8b (see Fig 5C) is provided in opposite directions with an axially directed protrusion 61, provided with circumferential- ly directed abutment surfaces 63a, 63b adapted to abut the edges 40a, 40b of the cover members l2a, l2b (see Figs. 3A - 3C).

The above mentioned groove 44 is provided axially in the lower bearing part l8b, i.e. below a horizontal plane through the axis A-A and is adapted to receive the rod 36 (see Fig. 4B).

Each of the semi-annular bearing parts l8a, l8b is provided with a pair of annular grooves 62 for receiving an O-ring (see f1g. 4B), respectively. The provision of an O-ring in the annular grooves 62, i.e. between the bearing 16 and the cover member l2a, l2b adds to reduced energy loss by convection. Of course, the O-rings also facilitate assembling of the bearing parts l8a, l8b.

Each of the semi-annular bearing parts l8a, l8b is also provided with radially, in- wardly directed protrusions 64. The inwardly directed protrusions 64 are each provided with a sliding surface 66, intended to bear against the exterior surface of the receiver ﬂuid tube 8. The provision of sliding surfaces 66 on protrusions 64, rather than a single inwardly di- rected sliding surface of the bearing l6, adds to reduced energy loss by conduction.

Of course, the bearing lining 20 may be substituted by a bearing housing. On the oth- er hand, depending on the size of the bearing lining 20, a bearing housing may be added between the clutch members and the bearing lining 20. It is contemplated that in case long cover members are provided, a muff may be added on each side of the bearing lining 20 or bearing housing.

When heated by the sun rays, the receiver ﬂuid tube 8 will expand, in particular in its longitudinal direction. By allowing the rod 36 to slide in the groove 44, the expansion due to the heat of focussed solar radiation onto the receiver ﬂuid tube 8 will be absorbed, and will thus not affect the durability of the cover members l2a, l2b, having a much lower coef- ficient of expansion than that of the receiver ﬂuid tube 8. By means of the rod 36, slidable in the groove 44, the position of the cover members l2a, l2b will remain substantially static in relation to the receiver ﬂuid tube 8.

The material of the receiver ﬂuid tube 8 is metal and preferably steel, stainless steel, copper or aluminium.

The material of the bearings 16, 16a, 16b, 16c is chosen in all of the above described embodiments such that it has a low friction in relation to the receiver ﬂuid tube 8 and with- stands high temperatures, such a ceramic material or a high temperature polymeric material, such as a polyaryletherketone (PAEK) material, e.g. polyether ether ketone (PEEK). Such materials also reduce thermal losses of the solar collector due to conduction.

Of course, the semi-annular bearing 18a, 18b may comprise a radial inner surface as described in connection with Figs 1A above. On the other hand, the bearings described in connection with Figs. 1A -lB and Figs 2A - 2B, respectively, may comprise radially di- rected protrusions provided with an inner, radially directed surface intended to bear against the receiver ﬂuid tube 8.

Fig. 5E shows ab alternative embodiment of the bearing 16, comprising a pair of semi-annular bearing parts 18a, 18b divided in a horizontal plane through the axis A-A, in a manner corresponding to that of Figs 5A - 5D, however provided with a pair of axially ex- tending grooves 44a, 44b underneath said horizontal plane. The pair of grooves 44a, 44b are adapted to receive a pair of rods 36 to be connected to the clamping rings 38a, 38b.

Fig 6A shows an alternative embodiment of the bearing 16, comprising a single an- nular body. A trough-hole 68 is provided in a lowermost portion of the annular body rela- tive to a horizontal plane through the axis A-A. The through-hole 68 is adapted to receive the rod 36. Of course, in case the suspension means 9 comprises a pair of rods 36 to be con- nected to the clamping rings 38a, 38b. In such a case, the annular body is provided with two through-holes 68, one for each rod 36.

Fig 6B shows yet another alternative embodiment of the bearing 16, comprising a pair of semi-annular bearing parts 18a, 18b divided in a vertical plane through the axis A-A. Each semi-annular bearing part 18a, 18b is provided with an axially extending groove 69a, 69b of semi-circular cross-section in the lowermost portion of the annular body relative to a horizontal plane through the axis A-A. The grooves 69a, 69b are facing one another, togeth- er forming a circular through-hole adapted to receive the rod 36.

Of course, the semi-annular bearing parts 18a, 18b shown in Fig 6B may be provided with one or two grooves 44 of the kind shown in Figs 5A-5E or with one or two axially ex- tending through holes 68, as described in connection with Fig. 6A. 11 It is to be understood that in the embodiment shown in Figs 5A - 5E, the semi- annular bearing parts l8a, l8b may instead be divided vertically or at any angle.

It is to be understood that the cross-section of the rod 36 may have any shape, such as circular or pentagonal. It is also to be understood that the groove 44, the through hole 68 and the semi-circular grooves 69a, 69b may have any cross-section, respectively that is suit- able for receiving said rod 36.

It is contemplated that the bearing 16 may comprise ball bearings or roller bearings.

The material of the cover members l2a, l2b is preferably transparent and may com- prise glass, or a polymer, such as plexiglass. The cover member 12 may instead comprise a thin film of a polymer, such as a PEAK, e.g. PEEK.

As shown in Fig 7, the cover means 10 may comprise a thin polymer film 70 to be suspended on a suspension means 9 comprising radially directed suspension members 72 attached to the receiver ﬂuid tube 8 by means of a fastening means 74, such as spring clamps, the suspension members 72 extending radially away from the receiver ﬂuid tube 8. The suspension means 9 suspends the film 70 in such a way to provide an elongated open- ing 7 underneath a horizontal plane through the axis A-A of the receiver ﬂuid tube 8.

It is contemplated that the cover member l2, l2a, l2b of the kind shown in Figs. 3A - 3C may be suspended on suspension means of the kind shown in Fig. 7.

It is noted that in all of the above described embodiments, the elongated opening 7 may instead of a slit 13 be made as a plurality of discreet openings, such as circular, oval or elongated holes. The openings need not be arranged in a line, but may be spread randomly over the lower region of the cover member, i.e. underneath a horizontal plane through the axis A-A.

In the embodiments above, it has been described that the cover means 10 comprises two or three cover members l2a, l2b. It should however be understood that the number of cover members may be a single one or more than two, such as three, four etc. Consequently, also the number of spokes 14 may vary depending on the number of cover members and/or the length of each cover members l2.

Reference signs used 2 solar collector 4 parabolic reﬂector 6a,6b 7 8 9 11 12,12a,12b 13 14,14a,14b,14c 16,16a,16b,16c 17 18a,18b 22 23 26 28 32 34 36 37 38a,38b 40a,40b 44 46 48 50,51 54a,54b 12 gab1es opmnng receiver ﬂuid tube suspenüonineans coverrneans glass pane cover member slit spoke bearing annu1ar bearing body semi-annu1ar bearing parts bearing 1ining bearing housing radia1 inner surface toothed rack cogvvheel suspension member c1amp heat insu1ating ring rod fastening means clamping ring edges groove radia11y directed protrusion radia11y directed contact surface O-ring radia1 inner surface axia11y directed surface axia1 end surface annu1ar peripheral groove 60a, 60b 61 62 64 66 72 A-A 13 annular clutch members axially dírected protrusion annular groove inwardly directed protrusion slidíng surface ﬁlm radially directed suspension members axis imagmary diameter

Claims

1. . A cover device for a receiver ﬂuid tube (8) of a parabolic trough solar collector, com- prising at least one elongated cover member (12), Wherein said cover member (12; l2a, l2b) has an axis (A-A) in its elongation, characterized in that the cover member (12; l2a, l2b) def1nes at least one opening (7) extending in the elongation of the cover mem- ber (12, iza, 12b).

2. . A cover device according to claims l, Wherein said cover member (12; l2a, l2b) is pro- vided With a suspension means (30) for said receiver ﬂuid tube (8), said suspension means (30) being arranged to keep the receiver ﬂuid tube (8) coaxially With said cover member (12; l2a, l2b).

3. . A cover device according to claim l or 2, Wherein said opening (7) is arranged substan- tially below a horizontal plane extending through said axis (A-A).

4. . A cover device according to any one of the preceding claims, Wherein the cross-section of said cover member (12; l2a, l2b) is substantially annular.

5. . A cover device according to any one of the preceding claims, Wherein the cross-section of said cover member (12; l2a, l2b) is substantially polygonal.

6. . A cover device according to any one of the preceding claims, Wherein said opening (7) comprises an elongated slit (13) defined by elongated edges (40a, 40b) of the cover member (12; l2a, l2b).

7. . A cover device according to any one of the preceding claims, Wherein said cover mem- ber (12; l2a, l2b) comprises a transparent, heat resisting material.

8. A cover device according to claim 7, Wherein said cover member (12; l2a, 12b) com- prises any one of glass and a polymer.

9. A solar collector comprising a parabolic reﬂector (4) movable about the axis (A-A) of a receiver ﬂuid tube and a cover device according to any one of the preceding claims.

10. A solar collector according to claim 9, Wherein said suspension means comprises a spoke (14) adapted to keep the receiver ﬂuid tube (8) at a predetermined distance from said parabolic reﬂector (4).

11.A solar collector according to claim 9 or 10, Wherein the parabolic reﬂector (4) is mova- bly arranged in relation to said receiver ﬂuid tube (8) and said cover member (12; 12a, 12b).

12. A solar collector according to any one of claims 9 to 11, Wherein the transversal dimen- sion of said opening (13) is such that it allows insertion of said receiver ﬂuid tube (8) across said opening (13).

13. A solar collector according any one of claims 9 to 12, Wherein the suspension means (30) is connected to the cover device, such that the opening (13) is maintained static in relation to the horizontal plane through the axis (A-A) of the receiver ﬂuid tube (8).