WO2011128468A1

WO2011128468A1 - System for connecting a metal tube and a glass-encased tube in a solar radiation-absorbing tube, and absorbing tube provided with the connection system

Info

Publication number: WO2011128468A1
Application number: PCT/ES2010/070233
Authority: WO
Inventors: Mario ALGUACIL ALGARRADA; Enmanuel OZAEZ MUÑOZ
Original assignee: Albiasa Gestion Industrial, S.L.
Priority date: 2010-04-16
Filing date: 2010-04-16
Publication date: 2011-10-20

Abstract

The invention relates to a system for connecting a metal tube and a glass-encased tube in a solar radiation-absorbing tube, comprising an annular vacuum chamber (4), an outer flange (6) having a first inner annular part (6a) that encircles an end thickening (3b) of an end part (3a) of the glass tube (3), and having a second inner annular part (6b) connected to a metal sleeve (7) that encircles the end part (2a) of the metal tube (2), an inner flange (8) connected to the outer flange (6) and having a first inner annular part (8a) that encircles a second perimeter area of an end thickening (3b) and having a second inner annular part (8b) that inserts into a perimeter slot (3d) in the end thickening (3b) of the glass tube (3), a resilient heat-resistant O-ring (9) between the second inner annular part (6b) of the outer flange (6) and a front part (3e) of the end thickening (3b).

Description

JOINT SYSTEM OF A METAL TUBE AND A CIRCUANTING GLASS TUBE IN A SOLAR RADIATION ABSORBING TUBE AND TUBE

PROVIDED ABSORBER OF THE UNION SYSTEM

TECHNICAL FIELD OF THE INVENTION

The present invention falls within the technical field of parabolic trough collectors ("CCP") and, particularly, in the field of solar radiation absorber tubes arranged in the focal lines of such collectors, which receive and convert solar radiation. in thermal energy that is transferred to a calorific fluid that flows through these tubes.

BACKGROUND OF THE INVENTION

In thermo-solar plants, parabolic trough collectors form a circuit in which solar radiation heats a heat fluid flowing through the circuit, usually a thermal oil, at nominal temperatures of 290-39G-C, without prejudice to power there are other temperature ranges between room temperature and 560 ^e C, to convert solar energy into thermal energy that is then transformed into electricity. Parabolic cylinder mirrors are used to collect solar radiation. At the focus of the parabola is an absorber tube that receives the concentrated rays of the sun, and that is the element in charge of converting the concentrated solar energy into thermal energy transferred to the heat-carrying fluid.

The absorber tube is one of the fundamental elements of parabolic trough collectors since its overall performance depends largely on it. It basically comprises two concentric tubes, namely an inner metal tube, for example steel, and an outer glass tube, for example borosilicate. Usually, the metal tube has a selective coating, such as a cermet composite material that provides high absorptivity (approx. 94%) in the solar radiation range and low emissivity in the infrared spectrum (approx. 15% ) that provide high thermal efficiency. The main problem of most conventional selective coatings is that they degrade in contact with air and / or other gases when they are operating at high temperatures. This degradation leads to the irreversible loss of the properties of absorptivity and emissivity, so it is necessary to have a high vacuum in the annular chamber located between the inner metal tube and the outer glass tube. Given the high temperatures of Conveyor fluid flowing through the interior of the metal tube during the operation of the collectors, and given that the glass tube is designed to never reach more than 100 ⁵ C, and given the thermal expansion coefficients other than metal and glass, There are different expansion variations in glass tube and metal tube. This constitutes a traditional problem of the absorber tubes that base the glass-metal union in a welding of both materials, being able to cause breakage due to thermal fatigue and causing the corresponding loss of vacuum.

On the other hand, at both ends of the absorber tube, there are metal bellows attached, on one end, to the outside of the glass tube and, on the other, to the metal tube. Inside the metal bellows that is arranged around the metal tube, an annular space communicated with the chamber is defined so that it is also in a high vacuum state.

Patent application DE-10231467-A1 describes an absorber tube that has an inner metal tube surrounded by an outer glass tube and an expansion compensation element comprising a metal bellows partially arranged in the annular chamber between the outer and inner tubes and attached to the outer tube by means of a connecting element with a conical or cylindrical section that functions as a longitudinally movable compensation joint. The longitudinal displaceability of this joint calls into question the effectiveness and durability of such compensation joint.

Patent application WO 2009/121987-A1 refers to a tight mechanical connection between a glass tube and a metal tube by means of a flexible HELICOFLEX type mechanical seal located between a glass flange and a set of metal parts to keep it tight the annular chamber between both tubes. The genericity of what is described in this document does not allow to elucidate a concrete structure of such mechanical union.

Patent application ES-2316321 -A1 describes a selective solar absorbent coating applicable to the inner metal tube of the absorber tube of a parabolic trough solar collector composed of layers of dielectric, reflective, absorbent and anti-reflective materials, no bond structure being described between a glass outer tube and metal inner tube.

Patent application ES-328313-A1 describes an absorber tube in which the inner and outer surfaces of the inner metal tube are coated with respective barrier layers substantially impermeable to hydrogen and containing chromium oxide. This document also does not describe any connection structure between an outer glass tube and the inner metal tube of the absorber tube.

US-4151828-A, US-4231353-A and patent application CN-292441 describe various types of joint joints between the inner and outer tubes of absorber tubes in solar thermal collectors.

None of the documents of the state of the art describe a joining system of a metal tube and a surrounding glass tube in a solar radiation absorber tube that allows to maintain effective forming while prolonging a high vacuum in the annular chamber between the inner metal tube and the outer glass tube of the absorber tube, which constitutes a serious inconvenience since the loss of the vacuum in the annular chamber leads to the degradation previously detailed and, therefore, to the need to replace the damaged absorber tubes .

DESCRIPTION OF THE INVENTION

The present invention aims at overcoming the drawbacks of the state of the art detailed above, on the one hand, by means of a joining system of a metal tube and a surrounding glass tube in a solar radiation absorber tube, where the metal tube and The glass tube is concentrically arranged in such a way that an annular vacuum chamber is defined between them, and which comprises a compensation element for differences in expansion between the metal tube and the glass tube, disposed at an end part of the tube of glass and in an end zone of the metal tube, the system comprising an outer flange with a first internal annular part dimensioned to embrace a first perimeter zone of an extreme thickening of the end part of the glass tube, and with a second internal annular part attached to a metal bushing that hugs the end zone of the metal tube and that presents an annular bending that hugs an edge rear thickening end of the glass tube;

an inner flange, attached to the outer flange with a first inner annular part sized to embrace a second perimeter zone of the extreme thickening adjacent to said first perimeter zone, and with a second inner annular part sized to enter a perimeter groove in the extreme thickening of the glass tube;

an elastic heat-resistant O-ring, between the second inner annular part of the outer flange and a front part of the extreme thickening. According to the invention, the second inner annular part of the outer flange can be welded to said metal bushing, while the outer flange can be welded to the inner flange, conveniently, when the outer flange and the inner flange are joined by welding , these comprise adjacent edges with individual chamfered inlets that form an annular groove whose purpose is to increase the surface area for the weld contribution material.

The o-ring may be a metal seal comprising an internal stainless steel structure and a copper jacket, available under pressure between said second internal annular part and the front part of the end greasing of the glass tube. Alternatively, the O-ring can be a doped indium metal gasket, welded to said second internal annular part and to the end of the glass tube's extreme greasing part, since this material has the characteristic that it can weld materials of coefficients of different thermal expansion or expansion and provides the necessary flexibility and improves resistance to thermal fatigue. These types of joints ensure the tightness of the vacuum chamber while being able to absorb the differences in thermal expansion between the metal tube and the glass tube.

In a preferred embodiment of the invention, the compensation element is a metal bellows that surrounds the end section of the metal tube and comprises a first end attached to the second inner annular part of the outer flange and a second end attached to a closing flange. that hugs the metal tube. This metal bellows is a bellows of the external type that works on traction, and can be connected by its second end to the closing flange by fusion welding, without material input. On the other hand, the closing flange can be attached to the metal tube by means of a continuous welding bead. The metal bellows may be surrounded by an external protective tube attached to the second end of the metal bellows.

The invention also relates to a solar radiation absorber tube, comprising a metal tube and a glass tube concentrically arranged such that an annular vacuum chamber is defined therebetween, and comprising at least one compensation element of Differences in expansion between the metal tube and the glass tube, disposed in an extreme part of the glass tube and in an extreme area of the metal tube, the absorber tube being characterized in that it comprises at least one of its ends a joint system such as which is defined in any one of the preceding claims. In this absorber tube the outer face of the metal tube may be covered with a coating that does not degrade in the absence of vacuum at a temperature below 550 ⁹ C, while the inner face of the glass tube may be coated with an anti-reflective coating.

It must be taken into account that, with this selective coating, the absorber tubes are practically not degraded when they are in contact with the air and, therefore, it is not necessary to maintain a high level of vacuum, as in conventional absorber tubes. This leads to the fact that it can be operated at vacuum levels not so high, without degrading the thermal properties of the absorber tubes. This also implies that the absorber tubes if they are in contact with air, can be repaired in-situ.

In accordance with the foregoing description, the present invention overcomes the drawbacks of the state of the art by means of a simple and efficient joining system.

BRIEF DESCRIPTION OF THE FIGURES

Aspects and embodiments of the invention are described below on the basis of drawings, in which

Figure 1 is a schematic side elevational view of an absorber tube comprising an embodiment of the joint system according to the present invention;

Figure 2 is a horizontally sectioned perspective view of a front end of the tube shown in Figure 1;

Figure 3 is a vertically sectioned perspective view of a front end of the tube shown in Figure 1.

In these figures numerical references appear that identify the following elements:

1 absorber tube

2 metal tube

2 ci end section of the metal tube

2b outer face of metal tube

3 glass tube

3H end of the glass tube

3b extreme thickening of the glass tube

3c rear edge of glass tube

3d perimeter groove glass tube 3e front glass tube

3f inner face of glass tube

4 annular vacuum chamber

5 expansion compensation element

5th first of the compensation element

5b second end of the compensation element

6 outer flange

6th first internal annular part

6b second internal annular part

6c edge

6d chamfered edge

7 metal bushing

7th ring bending

8 inner flange

8a first inner annular part

8b second inner annular part

8c edge

8d chamfered incoming

9 o-ring

10 closing flange

1 1 external protective tube

12 olive extraction

13 vacuum indicator

MODES OF CARRYING OUT THE INVENTION

Figures 1 to 3 illustrate an embodiment of an absorber tube -1 - of solar radiation, with a metal tube -2- and a glass tube -3- of surrounding borosilicate, arranged concentrically so that between them it is defined an annular vacuum chamber -4-. At the ends of the absorber tube -1 - metal bellows -5- are arranged that compensate for the differences in expansion between the metal tube -2- and the glass tube -3-, arranged respectively at an end -3a- of the tube of glass -3- and in an extreme area -2a- of the metal tube -2. The outer face -2b- of the metal tube -2- is covered with a coating that does not degrade in the absence of vacuum at a temperature below 400-C. On the other hand, the inner face -3f- of the glass tube -3- is covered with a coating anti-reflection. The end part -3a- of the glass tube embraced by the flanges -6, 8- is polished to ensure tightness. The glass tube -3- is provided with an extraction olive 12- itself conventional, through which the vacuum chamber -4- can be evacuated. Inside the glass tube -3- there is also a vacuum indicator -13- ("getter") if conventional.

With these selective coatings, a greater durability of these absorber tubes is obtained because they do not degrade in such a marked way when in contact with the air, as in the case of conventional absorber tubes, which allows them to be repaired on-site '.

The joining system comprises an outer flange -6- with a first internal annular part -6a- sized to embrace a first perimeter zone of an extreme greasing -3b- of the end part -3a- of the glass tube -3-, and with a second internal annular part -6b- welded to a metal bushing -7- that hugs the end zone -2a- of the metal tube -2- and that has an annular bending -7a- that hugs a rear edge -3c- of the greasing end -3b- of the glass tube -3-. To the outer flange -6-, an inner flange is welded with a first inner annular part -8a- sized to embrace a second perimeter zone of the extreme greasing -3b- adjacent to said first perimeter zone, and with a second inner annular part - 8b- sized to enter a perimeter groove - 3d- in the extreme greasing -3b- of the glass tube -3-. The outer flange -6- and the inner flange -8- comprise adjacent edges -6c, 8c- with individual chamfered recesses -5d, 7d- that form an annular groove that serves to increase the surface for the weld supply material. The flanges -6, 8- and the metal bushing -7- can be made of AICE 316 type steel. The metal bushing -7- has the function of protecting the gasket -9- and the areas near it from radiation coming from the tube metallic and concentrated solar radiation from the parabolic mirror.

Between the second internal annular part -6b- of the outer flange -6- and a front part -3e- of the end greasing -3b- of the glass tube an elastic metal o-ring -9- is clamped under pressure, which resists higher temperatures at 400 ^S C and comprising an internal stainless steel structure and a copper jacket. Metal gaskets of this type are marketed by the American firm GARLOCK SEALING TECHNOLOGIES.

As can be seen, the metal bellows -5- comprises a first end - 5a- welded to the second internal annular part -6b- of the outer flange -6- and a second end -5b- welded by fusion, without material input, to a closing flange -10- and that embraces the metal tube The closing flange -10-, which can also be made of AICE 316 steel, closes the vacuum assembly. The metal bellows -5- is an external bellows that works on traction. The closing flange -10- is connected to the metal tube -2- by means of a continuous welding bead. In addition, the metal bellows -5- is surrounded by a protective tube -1 1 - external attached to the second end -5b- of the metal bellows -5-. The function of the protective tube -1 1 -, which can be made of a stainless steel plate of a very reduced thickness, is to prevent concentrated sunlight from the parabolic mirror from reaching the metal bellows area -5-, and prevent dirt entry in the bellows area -5-. At its end, the protective tube -1 1 - can be provided with small ventilation holes (not shown in the figures) that favor the circulation of air to cool the area where the metal bellows is located -5-.

The metal bellows -5- is designed to absorb the differences in the dilations between the glass tube -3- and the metal tube -2-, when the metal tube - 2- passes, when the absorber tube is put into operation -1 - , from room temperature to about 400 ⁹ C. Likewise, the metal bellows -5- is designed to support, in conjunction with the bellows at the other end of the absorber tube -1 -, the total weight of the glass tube - 3-.

This union described is favored by the polishing treatment that is subjected to the outer face of the glass tube, to obtain an effective seal in the union with the flanges and the joint.

Claims

1. Connection system of a metal tube and a surrounding glass tube in a solar radiation absorber tube, where the metal tube (2) and the glass tube (3) are concentrically arranged in such a way that between them is defined an annular vacuum chamber (4), and comprising a compensation element (5) for differences in expansion between the metal tube (2) and the glass tube (3), disposed at an end part (3a) of the tube glass (3) and in an extreme area (2a) of the metal tube (2), characterized comprises

an outer flange (6) with a first internal annular part (6a) sized to embrace a first perimeter zone of an extreme thickening (3b) of the end part (3a) of the glass tube (3). and with a second internal annular part (6b) attached to a metal bushing (7) that hugs the end zone (2a) of the metal tube (2) and which has an annular bending (7a) that hugs a rear edge (3c) of the extreme thickening (3b) of the glass tube (3);

an inner flange (8), attached to the outer flange (6) with a first inner annular part (8a) sized to embrace a second perimeter zone of the extreme thickening (3b) adjacent to said first perimeter zone, and with a second annular part interior (8b) sized to enter a perimeter groove (3d) in the extreme thickening (3b) of the glass tube (3);

an elastic heat-resistant O-ring (9), between the second inner ring part (6b) of the outer flange (6) and a front part (3e) of the extreme thickening (3b).

2. Joint system according to claim 1, characterized in that said second internal annular part (6b) of the outer flange (6) is welded to said metal bushing (7).

3. Joint system according to claim 1 or 2, characterized in that the outer flange (8) is welded to the inner flange (8).

4. Joint system according to claim 3, characterized in that the outer flange (6) and the inner flange (8) comprise adjacent edges (6c, 8c) with both chamfered inlets (5d, 7d) that form an annular groove.

5. Union system, according to one of the preceding claims, characterized in that the O-ring (9) is a metal seal comprising an internal stainless steel structure and a copper jacket, available under pressure between said second internal annular part (6b ) and said front part (3e) of the extreme greasing (3b) of the glass tube (3).

6. Joint system according to one of claims 1 to 4, characterized in that the o-ring (9) is a doped indium metal seal, welded to said second internal annular part (6b) and to said front part (3e) of the extreme greasing (3b) of the glass tube (3).

7. Joint system according to one of the preceding claims, characterized in that the compensation element (5) is a metal bellows that surrounds the end section (2a) of the metal tube (2) and comprises a first end (5a) attached to the second inner annular part (6b) of the outer flange (6) and a second end (5b) attached to a closing flange (10) that embraces the metal tube (1), the metal bellows (5) being a bellows of exterior type that works on traction.

System according to claim 7, characterized in that the second end (5b) of the metal bellows is connected to the closing flange (10) by welding or by means of screws.

9. System according to claim 7 or 8, characterized in that the closing flange (10) is connected to the metal tube (2) by welding.

10. The joining system according to one of the preceding claims, characterized in that the metal bellows (5) is surrounded by an outer protective tube (1 1) attached to the second end (5b) of the metal bellows (5).

eleven . Bonding system according to one of the preferred claims, characterized in that the polishing treatment is subjected to the outer face of the glass tube (3).

12. Solar radiation absorber tube, comprising a metal tube (2) and a glass tube (3) concentrically arranged such that between them defines an annular vacuum chamber (4), and comprising at least one compensation element (5) for differences in expansion between the metal tube (2) and the glass tube (3), arranged at an end part (3a) of the glass tube (3) and in an end zone (2a) of the metal tube (2), characterized in that it comprises at least one of its ends a joining system such as that defined in any one of the preceding claims.

13. Solar radiation absorber tube according to claim 12, characterized in that the outer face (2b) of the metal tube (2) is covered with a coating that does not degrade in the absence of vacuum at a temperature below 400 ^B C.

14. Solar radiation absorber tube according to claim 12 or 13, characterized in that the inner face (3f) of the glass tube (3) is coated with an anti-reflective coating.