WO2010043743A2

WO2010043743A2 - Improved solar receiver for parabolic-trough collectors

Info

Publication number: WO2010043743A2
Application number: PCT/ES2009/070429
Authority: WO
Inventors: Jorge Insa Tello; Jesús SANTOS EGEA; Jorge Sampedro Feito; Francisco Javier Gala Lupiani
Original assignee: Iberdrola Ingeniería Y Construcción, S. A. U.
Priority date: 2008-10-14
Filing date: 2009-10-09
Publication date: 2010-04-22
Also published as: WO2010043743A3; ES2375006B1; ES2375006A1; AR073827A1

Abstract

Improved solar receiver for parabolic-trough collectors, comprising an inner metal tube (2), with selective coating, glass protective cover (3), and thermal-expansion compensator (4) with a vacuum in the space (6) between them, which is closed by the compensator (4), comprising three elements: one welded to the metal tube (2), another glued to the protective cover (3) using adhesive (5) and a bellows means. To create the vacuum there is a valve (7) installed in the protective cover (3) to connect a vacuum pump.

Description

PERFECTED SOLAR RECEIVER FOR CYLINDER-PARABOLIC COLLECTORS

D E S C R I P C I Ó N

OBJECT OF THE INVENTION

The invention, as stated in the present specification, refers to a solar receiver perfected for parabolic trough collectors, which provides several advantages and innovative characteristics, apart from others inherent to its configuration and organization, and which will be described in detail later, which represent a remarkable improvement compared to what is already known in this field for the same purpose.

More particularly, the object of the invention is focused on a solar receiver of the type constituted by an absorber tube, whose purpose is to transform the solar irradiation into thermal power by heating a thermal or caloric fluid that runs through it, being carried out said transformation by the concentrated incidence of solar irradiation in each parabolic trough collector properly disposed in front of the tube; in which said tube has the particularity of having a series of improvements that substantially improve its effectiveness.

Specifically, the new absorber tube has, among other novel aspects, a vacuum valve that allows the vacuum between the outer surrounding cover and the inner metal tube once again, once put into operation, with a closure between the cover and the metallic tube of adhesive type that advantageously allows, in case of failure, to be easily replaced with the manifold mounted, without the complete replacement of the absorber tube being necessary.

FIELD OF APPLICATION The field of application of the present invention is within the industry sector dedicated to the manufacture of parabolic trough collectors for solar thermal plants.

BACKGROUND OF THE INVENTION

As is known, thermo-solar plants with parabolic trough collector technology are based on the concentration of solar energy on a receiver or absorber tube by means of a collector mirror on a thermal, calorific or heat-carrying fluid, from which it is gradually extracted. energy to move a turbine and a generator.

The absorber tube generally comprises the following elements:

An inner metal tube, through which the thermal fluid circulates, being generally made of steel due to the range of pressures, temperatures and mechanical stresses to which it is subjected under operating conditions.

- A selective coating of the metal tube, since the characteristics of the The surface of said metal tube depends largely on the efficiency with which the solar irradiation reflected by the parabolic profile of the collector is transformed into thermal power in the form of thermal fluid. Therefore, the surface of the metal tube must have a high absorbency in the solar radiation spectrum and a reduced thermal emissivity at the operating temperature, a function that said coating fulfills.

- A protective cover, whose purpose is to protect the compounds of the selective coating of the metal tube from atmospheric agents (mainly environmental humidity and oxygen) by which it presents a low permeability to the gases of the atmosphere, allowing as much as possible the passage of the solar radiation, so it has a high transmissivity to solar radiation (wavelengths less than 2.5 μm, and reduce, as far as possible, the thermal losses of the metal tube to the environment, so it has a reduced Infrared radiation transmissivity, the boron silicate glass of low alkali content, commonly called "Pyrex" being the most appropriate and generally used material.

An anti-reflective coating of the protective cover, to improve the optical qualities of the glass, applied to both its outer and inner surface, which, given the working conditions to which it is exposed, has a high adhesion to the glass.

- A thermal expansion compensator, since the expansion of the glass tube or protective cover and the metal tube is different in Operating conditions. This difference is absorbed by this component to minimize mechanical stresses in both elements.

- Creation and conservation of vacuum in the annular space between the metal tube and the protective glass cover. To eliminate losses by natural convection between the metal tube and the glass tube, optionally, a certain degree of vacuum can be created in the space between the two tubes. To maintain this degree of vacuum, a device, called a "setter", is conventionally installed, capable of absorbing the hydrogen molecules resulting from the thermal degradation of the oil that migrate from inside the metal tube to the annular space. The support of the "getters" usually has a special design, since they must be placed in the coldest part of the metal tube.

- Close the protective cover with the metal tube, which must be carried out in such a way as to maintain the previously described vacuum existing in the annular space of the absorber tube, making said space completely waterproof. This closure is generally carried out by glass-metal welding or by using an elastomer or the like.

At present, and as a reference to the state of the art, it should be noted that, on the part of the applicant, there is knowledge of the existence of various types of receivers of the type that concerns here, among which the described ones can be highlighted as more relevant in the following patents:

ES2125828, which discloses a system of Slot solar collectors in which the absorber is composed of a conduit tube through which the fluid that carries the heat circulates and an outer envelope tube so that there is an annular space between them. Since the tubes are of different material and the absorbers work at high temperatures, there are differences in their behavior and also in irradiation in certain areas. As a solution it is proposed to fill this annular space with a medium that thermally joins both zones.

ES2259254, which discloses a receiver for solar collectors consisting of two tubes, the interior is the absorption tube, and the exterior, glass, is the envelope tube. This wrapping tube comprises a structure that focuses on the absorption tube to improve system performance, it being possible to place this structure on the inner or outer side of the tube. As geometry options, a plurality of prisms or a saw-shaped blade are proposed.

ES 2251320, which discloses a concave parabolic manifold whose absorber comprises expansion compensators to dampen the different radiation behaviors of the absorber tube and the surrounding surrounding tube. It also includes a reflector collar that reflects solar radiation in the absorber's active zone to improve system efficiency.

WO2007076578, which discloses a system of parabolic trough collectors with an absorber formed from a metal tube through which the fluid circulates surrounded by a glass tube. It also includes a helmet-shaped vacuum element that can be placed on the outside of the glass tube or on the inside, by means of supports. This helmet is made of thermally insulating material such as fiberglass or rock, polyurethane, etc. and it has a film that reflects the radiation emitted by the absorber tube.

US5460163, which discloses a collector for solar systems whose absorber has the characteristic of acting as a vaporization chamber. It is formed by two concentric tubes and in the annular chamber existing between both steam is formed which is condensed on the outer surface of the inner tube and evacuated into a structure located in the inner zone of the outer tube.

US2004055593, which discloses an absorber for parabolic trough collectors formed by an outer glass tube inside which the vacuum has been achieved and in which the absorber tube itself is located. This element is located eccentrically with respect to the outer tube, and is formed by an absorption tube through which the working fluid circulates, with absorption sheets welded inside and with a curved shape to direct the sun's rays towards the tube. In turn, the absorber is surrounded by a reflection channel that redirects the radiation reflected by the absorber back towards it. In this channel there is a hole in the focal line of the collector through which the sun's rays penetrate.

DE10033240, which discloses an absorber tube for solar energy collector systems, formed by an outer tube and an inner tube through which the working fluid circulates in which, as in the patent anterior, the inner tube is offset with respect to the axis of the outer tube. The proposed configuration indicates that the inner surface of the outer tube is partially occupied by mirrors, and that the absorber tube is away from the axis in the opposite direction to said mirrors. The configuration is completed with a sheet that acts as a flat absorber.

DE102006056536, which discloses an absorber, and a method to produce it, for parabolic collectors in which there is a reflective protective layer that, in this case, reflects the waves in the infrared range. This protective layer is configured from two layers, and is composed of gold, silver, platinum or copper.

US2008087277, which discloses a system of parabolic trough solar collectors in which the absorber is formed by a central metal tube, surrounded by a tube of transparent material, the space between the two tubes being empty. To compensate for differences in expansion of the two tubes due to the high working temperatures of the absorber, compensators are proposed in the form of bellows or membrane, which are attached only to one of the tubes, using an intermediate piece as a union with the other tube .

US2007034204, which discloses an absorber tube for use in parabolic collectors consisting of a metal central tube, a glass tube surrounding it and a bellows structure as an expansion compensation element that allows the relative movement of both tubes. There is a connection element of the internal part of the bellows element with the central tube and that extends into the annular space between them. Between the compensating element and the outer tube there is an absorbent filler element.

Thus, it is found that in conventional tubes, generally, the closure of the protective cover with the inner metal tube is carried out by means of a glass-metal weld, and that the vacuum in the space annuls both elements, when they exist, is maintained by the installation of "getters".

This welding has the following drawbacks:

- In addition to being complex to perform, it can only be done in a very specialized workshop.

It is an industrial process with a very low performance, since only six out of every ten welds that are performed are satisfactory.

- It is the most common point of rupture of absorbent tubes during operation.

In conventional tubes, once this welding is broken, air enters the space between the metal tube and the glass cover, the selective surface degrading rapidly and increasing thermal losses by convection. This causes that the replacement of the absorber tube by a new one is necessary, since it has no repair once installed in the manifold.

Changing an absorber tube in a manifold is a process that requires labor, apart from rendering the complete collector loop unusable for approximately two days, since it is required to drain the entire thermal fluid loop, with the associated loss of thermal power.

On the other hand, the absorber tubes are gradually emptying due to microscopic imperfections of their elements, which causes that after a while there is air inside the tube, despite the failure of glass-metal welding. In this case, in the known absorbers, the tube must also be changed, since in them the replacement of the vacuum is not economically viable again, a new glass tube, a new "getters", two additional glass-metal welds are required to be carried out in a specialized workshop, etc.

It is, therefore, an essential objective of the present invention to create an improved absorber tube that solves in a practical and economic way the aforementioned drawbacks presented by those currently known in the market, by incorporating a closure between the protective glass cover and the tube metallic interior that being equally tight is easy to carry out, as well as the incorporation of a system that allows to maintain the vacuum in the space between both elements quickly and simply.

EXPLANATION OF THE INVENTION

Thus, solar receiver perfected for parabolic trough collectors that the present invention proposes is configured, as a remarkable novelty within its field of application, since, to In accordance with its creation, it is achieved, in a restrictive way, to satisfactorily solve the aforementioned objectives as suitable, being the characterizing details that make it possible and that distinguish it, adequately included in the final claims that accompany this specification.

Specifically, the solar receiver perfected for parabolic trough collectors that the invention advocates is configured in a conventional manner from essentially from an inner metal tube, constituting the absorber tube itself, through which the thermal fluid circulates, and which, preferably, is made of steel.

In order for the photothermal conversion to be efficient, said metal tube has, on the outside, a selective coating with a high absorption capacity in the solar radiation spectrum, and a reduced thermal emissivity at the operating temperature, it being provided for that coating is chemically stable in the presence of air and at a temperature of up to 450 ⁰ C.

Following the invention, a protective cover consisting of a glass tube is contemplated on the metal tube, whose purpose is essentially to protect the compounds of the selective coating provided on the surface of the metal tube, allowing a high transmissivity of solar radiation and a reduced transmissivity to infrared radiation to minimize thermal losses to the environment. In addition, to improve the optical qualities of the glass, an anti-reflective treatment is applied to the outer and inner surface of the said protective cover, which will be of high adhesion, given the working conditions to which it is exposed.

The solar receiver also has a conventional thermal expansion compensator designed to absorb the difference in expansion lengths that, under operating conditions, have the protective glass cover and the inner metal tube, which is composed of three elements: one rigidly attached to the metal tube, another rigidly attached to the protective cover and another constituted by a bellows, which is what really absorbs the aforementioned difference in lengths.

The closure of the protective cover with the metal tube, already in a characterizing way, is carried out having as an intermediate element the thermal expansion compensator, which is rigidly attached to the metal tube by welding, while joining the protective cover by means of an adhesive applied by a method specially developed for this purpose.

On the other hand, to eliminate thermal losses by natural convection between the metal tube and the protective glass cover, vacuum is created in the annular space between the two elements. Said vacuum is created through a valve installed in the protective cover. To this end, a vacuum pump is connected to said valve, when initially the absorber tube or solar receiver is put into operation, allowing said operation to be repeated from time to time, as appropriate, without the need to disassemble the solar receiver.

The described solar receiver perfected for parabolic trough collectors represents, therefore, an innovative structure with structural and constitutive characteristics unknown until now for this purpose, reasons that together with its practical utility, provide it with sufficient foundation to obtain the privilege of exclusivity that is request.

DESCRIPTION OF THE DRAWINGS To complement the description that is being made and in order to help a better understanding of the characteristics of the invention, this descriptive report is attached, as an integral part thereof, of a set of drawings, in those that with illustrative and non-limiting nature have represented the following:

Figure number 1.- Shows an elevational view of an embodiment of the improved solar receiver for parabolic trough collectors object of the invention, which shows the main parts of which it consists, as well as the configuration and arrangement of the same.

PREFERRED EMBODIMENT OF THE INVENTION

In view of the described figure 1 and only, and according to the numbering adopted therein, it can be seen how the solar receiver (1) recommended is essentially configured from a metal tube (2) internal, constituent of the absorber tube itself, through which the thermal fluid circulates, and which, preferably, is made of steel, with an external diameter that covers a range of between 70mm and 105mm.

Said metal tube (2) has, externally, a selective coating that is chemically stable in the presence of air and at a temperature of up to 450 ⁰ C.

Arranged on the metal tube (2) a protective cover (3) consisting of a glass tube is contemplated, whose purpose is essentially to protect the compounds of the selective coating provided on the surface of the metal tube (2), allowing a high transmissivity of solar radiation and reduced transmissivity to infrared radiation to minimize thermal losses to the environment.

This protective cover (3) preferably has a diameter between 125mm and 187.5mm.

To improve the optical qualities of the glass, a high adhesion anti-reflective treatment is applied to the outer and inner surface of said protective cover (3).

The solar receiver (1) also has a thermal expansion compensator (4) designed to absorb the difference in expansion lengths that the protective glass cover (3) and the inner metal tube (2) have, said being compensator (4) composed of three elements: one rigidly attached to the metal tube (2), another rigidly attached to the protective cover (3) and another constituted by a bellows, and that is what really absorbs the aforementioned difference in lengths.

It is important to note that the closure of the protective cover (3) with the metal tube (2), and already in a characterizing way, is carried out having as an intermediate element the described compensator (4) of thermal expansion, which, is attached to the tube metal (2) by welding, while joining the protective cover (3) by means of an adhesive (5).

On the other hand, to eliminate thermal losses by natural convection between the metal tube (2) and the protective glass cover (3), vacuum is created in the annular space (6) between both elements, counting, for the creation of said vacuum with a valve (7) installed in the protective cover

(3) . In said valve (7) a vacuum pump is connected, when initially the absorbent tube or solar receiver (1) is put into operation, and from time to time, as appropriate.

Describing sufficiently the nature of the present invention, as well as the way of putting it into practice, it is not considered necessary to extend its explanation so that any expert in the field understands its scope and the advantages that derive from it, stating that, within its essentiality, it may be implemented in other embodiments that differ in detail from that indicated by way of example, and to which it will also achieve the protection that is sought provided that it is not altered, changed or modified its fundamental principle .

Claims

1.- PERFECTED SOLAR RECEIVER FOR CYLINDER-PARABOLIC COLLECTORS, of the type consisting of an inner metal tube (2), constituting the absorber tube itself, through which the thermal fluid circulates, with selective coating, a protective cover (3) of glass, with anti-reflective treatment of high adhesion, and a thermal expansion compensator (4) that closes tightly joining both elements, there is a void in the annular space (6) between the metal tube (2) and the protective cover (3 ), characterized in that the closing of the protective glass cover (3) with the metal tube (2), which is carried out by means of the thermal expansion compensator (4), is achieved so that said compensator (4) , composed of three elements: one rigidly attached to the metal tube (2), another rigidly attached to the protective cover (3) and another constituted by a bellows that absorbs said length difference , is attached to the metal tube (2) by welding, and to the protective cover (3) by means of an adhesive (5).

2.- PERFECTED SOLAR RECEIVER FOR

CYLINDER-PARABOLIC COLLECTORS, according to claim 1, characterized in that, to create the void in the annular space (6) between the protective glass cover (3) and the inner metal tube (2), there is a valve (7) installed in the protective cover (3) which is suitable for connecting a vacuum pump.

3.- PERFECTED SOLAR RECEIVER FOR CYLINDER-PARABOLIC COLLECTORS, according to claim 1, characterized in that the Selective coating that the metal tube (2) has externally is chemically stable in the presence of air and at a temperature of up to 450 ⁰ C.

4.- PERFECTED SOLAR RECEIVER FOR

CYLINDER-PARABOLIC COLLECTORS, according to claim 1, characterized in that the inner metal tube (2) has an external diameter that covers a range of between 70mm and 105mm.

5.- PERFECT SOLAR RECEPTOR OPENS CYLINDER-PARABOLIC COLLECTORS, according to claim 1, characterized in that the protective glass cover (3) has an external diameter that covers a range of between 125mm and 187.5mm.