Abstract:
Insulating element for expansion compensation device and method for manufacture thereof, of the type used in solar energy collector absorber tubes, being formed by a single piece in the form of a ring and with a bellows-like end portion, which is filled with rock wool or other equivalent insulating material, so as to create a hot air chamber which minimizes heat losses.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention describes an insulating element that allows reducing thermal losses in the expansion compensation device of tube-shaped solar energy receivers. 
       BACKGROUND OF THE INVENTION 
       [0002]    The general principle of solar thermal technology is based on the concept of the concentration of solar radiation to produce steam generally, which is then used in standard power plants. 
         [0003]    One of the greatest challenges in the development of solar thermo power plants is to increase the capture of solar energy, which has a relatively low density. There are two types of solar concentrators: line-focus concentrators and point-focus concentrators. The line-focus concentration is easier to install, since it has fewer degrees of freedom, but it has a lower concentration factor and therefore it can reach lower temperatures than point-focus concentration technology. 
         [0004]    That is why the aim is to continue progressing in the development of the receiver tubes used in line-focus concentrators, to increase the capture of solar energy and to reduce thermal losses, such that the overall performance of the concentration plant increases. 
         [0005]    The invention that concerns us, revolves around one of the elements that makes part of said receiver tubes, specifically it deals with an insulating element for an expansion compensation device, which is explained below. 
         [0006]    In general, a receiver tube consists of two concentric tubes between which the vacuum is produced. The inner tube, through which the fluid that is heated flows, is made of metal and the outer tube is made of glass, usually borosilicate. 
         [0007]    The expansion compensation device is placed between both tubes, in such a way that it allows the movement in the longitudinal direction of the tubes and ensures the vacuum, absorbing any strain that the existing differences between the metal and the glass expansion coefficients might create. 
         [0008]    However, this element needs to be insulated at its extremity in order to avoid losses that can take place due to natural convection. This insulating element is the one being developed in this invention. 
         [0009]    In the state of the art are known several solutions for this element, but the element that provides better results is the one disclosed by SCHOTT in the patent U.S. Pat. No. 7,013,887. In said document the heat losses at the end portion of the absorber tube are reduced by means of the double vacuum insulation, resulting from the element connecting to the metal tube, which already provides an insulation and, a second insulation formed between the folding bellows of the expansion compensation device and the glass tube, given that the free space left is very narrow so there is no air exchange. Besides, at the end of the absorber tube (metal tube) is placed a protector tube ( 20 ) whose axial length matches the length of the bellows, so it can be placed in either, the glass tube zone or the glass-to-metal transition zone, thus preventing that the bellows or expansion compensation devices are affected by the direct sun radiation. 
         [0010]    Basically, the invention deals with an element that allows insulating this zone to decrease thermal losses in addition to avoiding any direct heating due to the incidence of the concentrated light from the mirrors upon the glass-to metal seals. 
         [0011]    With this insulation system, it has been observed that when it comes to introducing improvements in the expansion compensation device increasing the solar collector performance, no element addresses the issue of thermal losses at the tubes extremities. 
         [0012]    Because all the foregoing, the object of the present invention is to provide an insulating element that decreases thermal losses at the extremities of the absorber tube to improve the system effectiveness. 
         [0013]    That is why, unlike in the known state of the art, our protector element protects from thermal losses a part, which is not the part mentioned in the prior patent (it protects the cover and part of the metal absorber tube, according to proposal 1 or the cover and the vessel according to proposal 2) and to this end it uses a bellow system, which allows to absorb the relative movement between its component parts, something that would be unfeasible with the elements known up to now. 
       DESCRIPTION OF THE INVENTION 
       [0014]    The invention consists of a piece which allows to insulate and to decrease thermal losses in the expansion compensation device, following the relative movements that may take place between the assembly formed by the borosilicate tube, the Kovar ring or glass-to-metal transition element, the expansion compensator, the vessel (end of the metal absorber tube), the cover and the metal receiver tube. 
         [0015]    The piece consists of a bellow-shaped element manufactured by hydro conformation, which houses inside an insulating material like rock wool, which will copy the geometry of the expansion compensation device. 
         [0016]    In this way an air chamber is generated which allows decreasing the losses at the extremities of the receiver tubes. 
         [0017]    There are two ways of placing the insulating element of the invention: either completely enveloping the compensating device, thus achieving a greater reduction of thermal losses, or stretching it until the middle portion of the vessel (end of the metal absorber tube), solution that is adaptable to the currently available commercial supports. 
         [0018]    The choice of a bellow-type geometry allows to have an element that follows the relative movements of the compensating of device without having to introduce any load, which, even though it is not a problem given the low rigidity values of the expansion compensation device, it could become one if it didn&#39;t allow relative movements between the borosilicate tube and the metal absorber tube. 
         [0019]    This system, unlike the known state of the art, drastically decreases the losses due to natural convection providing a complete insulation to the extremity of the absorber tube, in such a way that if this insulating element was employed in the currently existing tube developments, thermal losses would decrease by 0.02% per tube and since a solar plant of 50 MWe has approximately 90,000 tubes, the thermal losses reduction in the entire plant would be extremely high. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0020]    To complete the description that is being made herein and in order to facilitate a better comprehension of the invention, a set of drawings are attached hereto, where by way of illustration and by no way of limitation, the following has been represented: 
           [0021]      FIG. 1 : it shows an overview of the extremity of the absorber tube. 
           [0022]      FIG. 2 : it shows an overview, according to proposal 1 
           [0023]      FIG. 3 : it shows an overview, according to proposal 2 
           [0024]      FIG. 4 : it shows an elevation view of the insulating element 
           [0025]      FIG. 5 : it shows a plan view of the insulating element 
           [0026]      FIGS. 6 ,  7 : show the extremity of the absorber tube over the assembly supporting elements 
       
    
    
     PREFERRED EMBODIMENT OF THE INVENTION 
       [0027]    To achieve a better comprehension of the invention, below is going to be described, with the help of the figures, the insulating element according to a preferred embodiment. 
         [0028]    In  FIGS. 1 ,  2 , and  3  are presented the overviews of the extremity of the absorber tube. 
         [0029]    In  FIG. 1 , it can be observed the assembly of the elements placed at the extremity of the absorber tube and the location of the insulating element ( 1 ) between them. 
         [0030]    The insulating element ( 1 ) is bellow-shaped and it is filled with an element like rock wool ( 2 ) or similar to achieve the intended insulation since, thanks to this insulation a hot air chamber is created, thus obtaining a thermal loss reduction. 
         [0031]    This element ( 1 ) covers the extremity of the expansion compensation device ( 3 ) and its external portion overlaps the glass tube ( 5 ). 
         [0032]    It can be arranged according to two possible proposals:
       Proposal 1 ( FIGS. 1 and 2 ): the insulating element envelopes completely the upper portion of the expansion compensation device (between them is the getter ( 6 ), but it lacks of any interest for the invention being disclosed herein)   Proposal 2 ( FIG. 3 ): the insulating element ( 1 ) reaches until the middle portion of the vessel, meaning by vessel the extremity or final portion of the metal absorber tube ( 4 ), through which the heat-transfer fluid flows.       
 
         [0035]    The differences between both proposals are based on the fact that proposal 1 achieves a greater thermal insulation than proposal 2; however the latter achieves a complete adaptation to the current commercially available supports. 
         [0036]    In  FIGS. 4 and 5 , an elevation and a plan view of the claimed insulating element ( 1 ) are exhibited. It can be observed that it is a bellow-type ring, wherein one of its bases has a shorter diameter than the other has, so it can be adapted to the site where is going to be placed, either according to proposal 1 or 2. 
         [0037]    In  FIGS. 6 and 7  are exhibited the end extremity of the tube placed over the supporting elements ( 7 ). For absorber tubes of approximately 4 meters, these elements are placed at both extremities of the tube, although a pair of additional bearings is required along the tube to avoid any eventual breaking. As it was previously mentioned, proposal 2, that is to say, the embodiment wherein the insulating element ( 1 ) reaches until the middle portion of the vessel, although with a lower thermal insulation, achieves a complete adaptation to the current commercially available supports ( 7 ). 
         [0038]    The piece can be manufactured separately and independently from the cover, so it can be adapted to already existing tubes or it could be manufactured in a specific way, adapted to the expansion compensation device in which is going to be installed. 
         [0039]    The manufacturing process consists in manufacturing the piece by means of a hydro conformation technique. Basically, it involves a process to conform a material (generally a metal) through the action of a fluid under high pressure. The most common application consists in the conformation of a steel tube against the walls of a bellow-shaped matrix, by introducing a fluid under high pressure. The pieces thus obtained are welded by micro-plasma onto the cover or casing. This system is specially designed to be applied in the absorber tubes of solar collectors, but it cannot be disregarded its extension to other fields of the industry requiring similar characteristics.