US20080087277A1

US20080087277A1 - Collector for a Solar Thermal Power Station

Info

Publication number: US20080087277A1
Application number: US11/661,795
Authority: US
Inventors: Holger Schweyher
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-09-06
Filing date: 2004-09-06
Publication date: 2008-04-17
Also published as: AU2004323129A1; CN101065621A; ES2313084T3; WO2006027003A1; CN100567846C; EP1787067B1; DE502004007755D1; EP1787067A1

Abstract

A parabolic trough collector has a supporting structure (5) configured as a dual-shell torsion box (9), which increases the rigidity of the collector.

Description

This application is the national stage of PCT/EP2004/009918 filed on Sep. 6, 2004.

BACKGROUND OF THE INVENTION

The invention concerns an absorber tube in accordance with the preamble of the independent claim.
The currently available absorber tubes consist of a central tube in which a heat carrier, such as e.g. thermo oil or water/water vapor, circulates, and an enveloping tube of glass surrounding the central tube. In conventional absorber tubes, sections of a length of approximately 4 m are each combined into one segment. The central tube and the envelope tube are connected to each other at the front end and rear end of the segment in a gas-tight fashion, e.g. by soldering. Flanges are provided in order to compensate for the diameter differences between the central tube and the envelope tube. The maximum length of these segments is limited to approximately 4 m due to the different thermal expansion of central tube and envelope tube and deflection of the central tube. A plurality of such conventional absorber tube segments can be connected in series up to a length of approximately 200 m, due to thermal length changes. Between the series-connected segments, there are short connecting pieces between the segments of the absorber tubes, which contribute only little to coupling energy into the heat carrier in the central tube. This means that, with increasing effective length of the absorber tube segments, the efficiency of the absorber tube increases, thereby increasing the economic efficiency of the parabolic trough power station.
It is the underlying purpose of the present invention to provide a concentrated collector for a solar thermal power station, which can bear high mechanical loads, has a long service life and good efficiency, and is inexpensive to run and maintain.

SUMMARY OF THE INVENTION

This object is achieved in accordance with the invention by an absorber tube for concentrators with linearly focussing focal line, in particular, for parabolic trough collectors and Fresnell lens collectors with at least one central tube and a transparent envelope tube, wherein the at least one central tube is surrounded by the envelope tube in a gas-tight fashion, and with means for balancing axial length changes of the central tube and the envelope tube, wherein the means for compensating axial length changes comprises a compensator, in particular, an expansion bellows and/or a roll membrane, and with an axially displaceable sealing flange, and a fixed flange which is connected to the control tube in a gas-tight fashion, wherein the corrugated tube connects the sealing flange and the fixed flange in a gas-tight fashion, the sealing flange having a receptacle for the envelope tube.
In particularly advantageous embodiments of the inventive absorber tube, the sealing flange is borne on the at least one central tube and/or the receptacle for the envelope tube is designed as a shoulder or a groove in an end face of the sealing flange.
This easily creates a vacuum space between the envelope tube and the central tube, wherein the thermal expansions between the central tube and the envelope tube can be easily compensated for. The inventive absorber tube is not limited to an overall length of ≦(smaller or equal to) 4 m but it is possible to bridge a length of up to 8 m between the fixed flange and the sealing flange. This increases the effective length of the inventive absorber tube relative to the overall length of the absorber tube, which increases the efficiency of the inventive absorber tube. The construction costs are moreover reduced which has also a positive effect on the economy of the parabolic trough power system.
It has turned to be advantageous to provide a sealing ring, in particular, an O-ring or a C-ring between the sealing flange and envelope tube. These sealing rings may be produced from metal or a temperature-resistant organic material such as graphite, polytetrafluoroethylene or a combination of these materials. The use of these sealing rings, which are known per se, further improves sealing between the fixed flange and the envelope tube and between the sealing flange and the envelope tube which leads to better maintenance of the vacuum between the envelope tube and the central tube. This increases the thermodynamic efficiency of the absorber tube and reduces the required pumping energy for maintaining the vacuum between the envelope tube and the central tube. The use of sealing rings facilitates assembly and repair of an inventive absorber tube, such that even installed absorber tubes can be repaired directly on site by the power station operating staff.
In order to maintain the vacuum between the central tube and the envelope tube, a connecting sleeve for a vacuum pump may e.g. be provided on the fixed flange. The vacuum pump may be switched on in dependence on the underpressure in the intermediate space between the envelope tube and the central tube, to ensure that a vacuum is always maintained.
In order to reliably hold the absorber tube outside of the focal line of the parabolic trough collector, the compensator may be subdivided by a holder into a first section and a second section, wherein the holder is connected to the first and second sections in a gas-tight fashion, and radially supports the at least one central tube.
With this measure, the weight of the absorber tube can be accepted by a holder without transmitting thermal expansion of the central tube or the envelope tube to the holder. This is possible by disposing the central tube in the holder such that it can be axially displaced and only radial forces can be transmitted from the central tube to the holder. The holder may optionally be connected to a supporting structure of the reflector or be fixed to the foundation. A bearing bushing may be provided between the holder and the central tube in order to reduce friction between the holder and the central tube.
It has also turned out to be advantageous to provide at least one spring element between the fixed flange and the sealing flange, such that the pressure exerted by the flanges and the envelope tube onto the sealing rings is increased, thereby improving the tightness. If desired, the force acting on the sealing rings may be axially limited by a suitable constructive design of the fixed flange or the sealing flange. This ensures that no sealing ring is squeezed or otherwise destroyed.
In order to further increase the length of the continuous absorber tube, the envelope tube may be composed from several sections of a length of e.g. more than 4 m, wherein a centering flange is provided between two neighboring sections, the two sides of which each having one receptacle for a section of the envelope tube. This centering flange can be disposed on the at least one central tube. The inventive design of the envelope tube permits e.g. construction of a continuous central tube of a length of more than 200 m, which consists of several partial sections that are welded together. A holder and/or a centering flange is disposed between the envelope tube segments to ensure that the absorber tube does not sag and that the envelope tube is always centered relative to the absorber tube. This measure considerably further increases the effective length of the absorber tube relative to the overall length, which, as was mentioned above, has a positive effect on the costs of the parabolic trough power station. This embodiment has the further advantage that, when a segment of an envelope tube breaks, repair of the absorber tube is very easy. The gap produced by the broken envelope tube is closed in that the envelope tube segments disposed next to the gap are pushed into the gap until the gap has moved to the end of the central tube. The flange must be removed at that location, a new envelope tube segment is mounted, the flange is reconnected to the central tube and the space between the envelope tube and the central tube is evacuated. This means that a cracked envelope tube can be replaced with the simplest means and with only little downtime. This is possible i.a. in that the centering flanges can be displaced on the central tube.
In advantageous embodiments of the inventive absorber tube, the receptacle for the envelope tube is designed as a shoulder or groove in an end face of the centering flange, and/or a sealing ring, in particular an O-ring or a C-ring, is provided between the centering flange and the envelope tube.
The segments of conventional absorber tubes are limited to a length of four meters due to deflection of the central steel tube, in order to be able to support it at the respective ends. In inventive central tubes of carbon fiber ceramic or carbon fiber carbon (CFC) with gas-tight liner, preferably of steel or a nickel-based alloy, the separations between the supports may be substantially larger. However, deflection of the transparent envelope tube limits the segment length of the absorber tube to approximately eight meters.
The length of the absorber tube thereby corresponds to the length of the collector. The small heat expansion of CFC is thereby advantageous in that only little length changes must be compensated for at the ends. The surrounding glass envelope tube of a length of eight meters is connected at the ends to the respective holders of the absorber tubes in a gas-tight fashion.
The expansion compensation of conventional absorber tubes is disadvantageous, since it contributes only little to heat gain, while causing substantial losses due to radiation and convection. This is a particularly noticeable disadvantage when the solar radiation is weak.
Further advantages and advantageous embodiments can be extracted from the following drawing, its description and the claims.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a perspective view of an embodiment of an inventive parabolic trough collector;
FIG. 2 shows a cross-section through an inventive supporting structure; and
FIG. 3 shows a side view of an inventive parabolic trough collector;
FIG. 4 shows a cross-section through a first embodiment of an inventive absorber tube;
FIG. 5 shows a cross-section through a second embodiment of an inventive absorber tube;
FIG. 6 shows a cross-section through a third embodiment of an inventive absorber tube;
FIG. 7 shows an inventive absorber tube with holder;
FIG. 8 shows an embodiment of an inventive absorber tube with two central tubes;
FIG. 9 shows a section of an inventive absorber tube with two envelope tube segments and an intermediate centering flange;
FIG. 10 a, b show cross-sections through two embodiments of inventive absorber tubes.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a perspective view of an inventive parabolic trough collector. The parabolic trough collector has a reflector 1 which generally consists of glass which is coated with silver and acts as a mirror. The reflector 1 has a parabolic cross-sectional shape. An absorber tube 3 is located in the focal line (not shown) of the reflector 1, in which a heat carrier, such as e.g. thermo oil or water/water vapor circulates. The heat carrier and the structure of the absorber tube 3 are not shown in FIG. 1.
A supporting structure is provided to bear the reflector 1, which consists of an inner shell 5 and an outer shell 7. The inner shell and the outer shell can be produced e.g. from composite materials with a wall thickness of a few millimeters. As is shown in FIG. 1, the inner shell 5 supports the glass reflector 1 over a large area to largely prevent deformation of the reflector 1, which would change its optical behavior, and to also prevent any inadmissible mechanical and thermal strain from acting on the reflector 1 during operation. The support of a large area of the reflector 1 by the inner shell also considerably reduces the danger of glass breaking in a storm.
The inner shell 5 and the outer shell 7 are rigidly connected to each other and form a preferably closed torsion box. The torsion box 9 can be closed at its ends by end pieces 11. This further increases the torsional strength of the torsion box 9.
The torsion box 9 including outer shell 7 is rotatably disposed on supports 13. FIG. 1 does not show the constructive details of this bearing of the torsion box on the supports 13. However, it has turned out to be advantageous to provide a sliding bearing with a metal-plastic pair or a plastic-plastic pair. Alternatively, the bearing may also be provided by rolling bearings or using electromagnetic forces. Recesses 15 are provided in the center and at the ends of the reflector 1 and the inner shell 5. These recesses may be required to mount and operate a rotary drive which rotates the torsion box 9 relative to the supports 13. The rotary drive is not shown in FIG. 1. It is, however, feasible to rotate the torsion box 9 and thereby also readjust the reflector 1 relative to the sun e.g. using a curved toothed rack (not shown) which is connected to the outer shell 7 and a worm gear which is mounted to one or more supports 13 and is electrically driven.
Alternatively, other electric, hydraulic or pneumatic drives may also be provided to turn the torsion box 9 relative to the supports 13.
In order to ensure that the absorber tube 3 is always in the focal line (not shown) of the reflector 1, several holders 17 are provided which are connected at one end to the absorber tube 3 and at the other end to the supporting structure 9. With particular advantage, the holders 17 are connected to ribs (not shown) of the torsion box 9.
FIG. 2 shows a side or cross-sectional view of the inventive parabolic trough collector without supports 13. This illustration shows that the reflector 1 and also the inner shell 5 have a parabolic cross-sectional shape, and the absorber tube 3 is disposed in the focal point of this parabola.
As indicated by the double arrow 19, the cross-sectional shape of the outer shell 7 is that of a circular segment, and the center of this circular segment coincides with the focal point of the reflector 1 and thus also with the location of the absorber tube 3. Therefore, when the torsion box 9 is turned relative to the supports 13 (not shown), the reflector 1 can be simply and precisely readjusted to the sun without the absorber tube 3 leaving the focal line.
FIG. 2 also shows that a longitudinal spar 21 is provided in the torsion box 9 and the holder 17 is disposed in the direct extension of the longitudinal spar 21. This further improves passage of forces from the holder 17 to the torsion box 9.
FIG. 3 shows a side view of the embodiment of FIG. 1. The comparison of the two positions of the reflector 1 in FIGS. 2 and 3 clearly shows the straightforward readjustment of the reflector 1 through turning the torsion box 9 relative to the supports.
FIG. 4 shows a cross-section of a first embodiment of an inventive absorber tube 3. The absorber tube 3 has a central tube 31 which generally consists of metal, in particular steel. The central tube 31 is surrounded on all sides by an envelope tube 33 of a transparent material, in particular, glass. The space between the envelope tube 33 and the central tube 31 is evacuated. A fixed flange 35 is mounted by a welding seam 37 to the central tube 31 in a gas-tight fashion. The fixed flange 35 can, of course, also be connected to the central tube 31 in a different fashion than by welding, in particular, also in a detachable fashion. It is thereby important that no ambient air enter into the space between the envelope tube 33 and the central tube 31 at the connection between the fixed flange 35 and the central tube 31. In the embodiment of FIG. 4, the fixed flange 35 is connected to the holder 17 which permits rigid connection of the absorber tube 3 to the torsion box of the parabolic trough collector or the foundation.
In order to compensate for the different heat expansions of the envelope tube 33 and central tube 31, the envelope tube 33 is not directly connected to the fixed flange 35 but sealingly connected to a sealing flange 39. The sealing flange 39, in turn, is connected to the fixed flange 35 via a compensator 41. The compensator 41 can e.g. be soldered, welded, or glued to the fixed flange 35. The same applies for the connection between the compensator 41 and the sealing flange 39. The sealing flange 39 can be axially displaced on the central tube 31 such that the different heat expansions of envelope tube 33 and central tube 31 can be easily compensated for by the compensator 41. A bearing bushing not shown) may be provided on the sealing flange 39 to reduce friction between the sealing flange 39 and the central tube 31.
A connecting sleeve 43 is provided in the fixed flange 35, to which a vacuum pump (not shown) may be connected. The use of a fixed, installed vacuum pump for one or more collectors is particularly advantageous in order to counteract diffusion of hydrogen, produced in the heat carrier water/vapour, through the tube wall of the central tube in response to discontinuous pumping. This vacuum pump is greatly advantageous compared to conventional absorber tubes with so-called “getters” which absorb hydrogen but become ineffective with time.
In order to ensure pressure compensation between the two sides of the sealing flange 39, a compensation bore 45 is provided in the sealing flange 39. The envelope tube 33 is received in the sealing flange by an annular groove 47. The annular groove 47 is stepped. A first step of the groove 47 receives the envelope tube 33, while a second step of the groove 47 receives a sealing ring (not shown in FIG. 4). The sealing ring, which may be designed as an O-ring or a metallic C-ring, reduces the entry of ambient air into the gap between the envelope tube 33 and the central tube 31 at the connection between the sealing flange 39 and the envelope tube 33.
FIG. 5 shows a second embodiment of an inventive absorber tube. The same components have the same reference numerals, and the description of FIG. 4 applies. Only the differences between the two embodiments are explained below.
In contrast to the first embodiment, the envelope tube 33 of the second embodiment in accordance with FIG. 5 is centered and disposed on the sealing flange via a first shoulder 49. A C-ring 53 is inserted into a second shoulder 51 which joins the first shoulder 49. The C-ring is forced against the sealing flange 39 and the envelope tube 33 by the air pressure acting from the outside onto the C-ring 53, such that the sealing increases with increasing underpressure in the space between the envelope tube 33 and the central tube 31.
In the embodiment of FIG. 5, the compensator 41 is welded to the sealing flange 39, wherein the compensator 41 is surrounded by a supporting ring 55.
FIG. 6 shows a third embodiment of an inventive absorber tube. The essential difference from the above-described embodiments is that a spring element, in the present case, a pressure spring 57, is disposed between the fixed flange 35 and the sealing flange 39, which is guided on a guiding bolt 59. The guiding bolt is connected to the fixed flange 35 via a nut 61 which is screwed onto the guiding bolt 59 and on which one end of the pressure spring 57 is supported. The pretension of the pressure spring 57 can be adjusted by turning the nut 61. This increases the axial pressure between the sealing flange 39 and the envelope tube 33 to increase the tightness of this connection.
In an alternative fashion, the guiding bolt may also be plugged in (not shown).
In the embodiment of FIG. 7, the holder 17 and the fixed flange 35 have separate functions. A loose flange is mounted to the holder 17, which permits axial extension of the central tube 31. In this embodiment, the compensator 41 is divided into a first section 41 a and a second section 41 b. The holder 17 which supports the central tube 31 is disposed between these sections. In this embodiment, the holder 17 also has a compensation bore 45.
In the embodiment of FIG. 8, a second central tube 63 is provided in addition to the central tube 31, which is guided, in a similar fashion as the first central tube 31, through the fixed flange 35, the holder 17 and the sealing flange 39. When the heat expansion of the first central tube 31 and the second central tube 63 are not identical, a second compensator 65 may be provided on the second central tube. In this case, the second central tube 63 is disposed in the fixed flange 35 such that it can be axially displaced. The second central tube 63 serves e.g. as an injection water line for controlling the fresh vapor temperature.
FIG. 9 shows a section of an absorber tube 3, wherein two segments of envelope tubes 33 are connected to each other by a centering flange 67.
The centering flange 67 has the same basic construction as a sealing flange 39 on the side facing the envelope tube 33. The only difference is that the centering flange 67 receives an envelope tube 33 on both sides, while the sealing flange 39 receives an envelope tube 33 on one side and is connected on its other side to the compensator 41. The centering flange 67 is disposed on the central tube 31 such that it can be axially displaced, similar to the sealing flange 39, such that the different heat expansion of envelope tube 33 and central tube 31 can be easily compensated for. This embodiment clearly shows the bearing bushing 69 between the centering flange 67 and the central tube 31.
FIG. 10 a shows a cross-section through an absorber tube 3 with a central tube 31.
The embodiment of FIG. 10 b shows the cross-section through an absorber tube 3 with several central tubes 31. One of these central tubes or an additional tube in the inside of the central tube can be used as a fresh water line in the embodiment of FIG. 10 b.
The embodiments of FIGS. 4 to 9 can be easily adapted to absorber tubes with several central tubes 31.
The embodiments of FIGS. 4, 5, 6 concern gas-tight fixed flanges, while the embodiment of FIG. 7 concerns a gas-tight loose flange. The loose flange permits axial displacement of the central tube through heat expansion. It is preferably installed at the cold end of the collector. To exchange defect glass envelope tubes, this flange is undone and subsequently connected again to the central tube in a gas-tight fashion.
All the features disclosed in the drawing, the description thereof and the claims may be essential to the invention either individually or collectively in arbitrary combination.

Claims

1-39. (canceled)

40. An absorber tube for a concentrator having a linearly focussing focal line for a parabolic trough collector or for a Fresnell lens collector, the absorber tube comprising:

at least one central tube;

a transparent envelope tube surrounding said least one central tube in a gas-tight fashion;

a fixed flange connected to said central tube in a gas-tight fashion;

an axially displaceable sealing flange having a receptacle for said envelope tube; and

a corrugated tube, an expansion bellows, or a roll membrane connected between said sealing flange and said fixed flange in a gas-tight fashion to compensate for axial length changes in said central tube and said envelope tube.

41. The absorber tube of claim 40, wherein said sealing flange is disposed on said at least one central tube.

42. The absorber tube of claim 40, wherein said receptacle for said envelope tube is designed as a shoulder or a groove in an end face of said sealing flange.

43. The absorber tube of claim 40, wherein a sealing ring, an O-ring, or a C-ring, is disposed between said sealing flange and said envelope tube.

44. The absorber tube of claim 43, wherein said sealing ring is produced from metal, temperature-resistant organic materials, graphite, polytetrafluoroethylene, or a combination or partial combination of these materials.

45. The absorber tube of claim 40, wherein said fixed flange has a connecting sleeve for a vacuum pump.

46. The absorber tube of claim 40, wherein said corrugated tube is subdivided by a holder into a first section and a second section, said holder being connected to said first and second sections in a gas-tight fashion, said holder radially supporting said at least one central tube.

47. The absorber tube of claim 46, wherein said holder is mounted to a supporting structure of the concentrator.

48. The absorber tube of claim 46, wherein said holder is at least indirectly connected to underlying ground supporting the concentrator.

49. The absorber tube of claim 46, further comprising a bearing bushing disposed between said holder and said central tube.

50. The absorber tube of claim 40, further comprising a bearing bushing disposed between said sealing flange and said central tube.

51. The absorber tube of claim 40, further comprising at least one spring element disposed between said fixed flange and said sealing flange.

52. The absorber tube of claim 40, wherein said envelope tube has several sections, with one centering flange being provided between each of two neighboring sections, wherein one receptacle for a section of the envelope tube is provided on each of two sides of said centering flange.

53. The absorber tube of claim 52, wherein said centering flange is disposed on said at least one central tube.

54. The absorber tube of claim 52, wherein said receptacle for said envelope tube is a shoulder or a groove in one end face of said centering flange.

55. The absorber tube of claim 52, further comprising a sealing ring an O-ring, or a C-ring, disposed between said centering flange and said envelope tube.

56. The absorber tube of claim 55, wherein said sealing ring is produced from metal, temperature-resistant organic material, graphite, polytetrafluoroethylene, or a combination or partial combination of these materials.

57. The absorber tube of claim 40, wherein said central tube is produced from carbon fiber ceramic or carbon fiber carbon (CFC) with a gas-tight liner, or with a gas-tight liner of steel or a nickel-based alloy.