Abstract:
A central solar receiver comprises an axisymmetric housing having front and rear ends and having an aperture at the front end, an elongated tubular window mounted in the aperture co-axially with the housing, a volumetric solar absorber disposed within the housing and extending around and along the elongated window for absorbing solar radiation that has passed therethrough; and working fluid ingress and egress formed in the housing so as to, respectively, inject thereto, and withdraw therefrom, a pressurized working fluid in a manner enabling the working fluid interaction with the volumetric absorber. The window has an open front end which is secured to the housing at the front end thereof, and a closed rear end which is disposed adjacent the rear end of the housing and is free of any securing thereto.

Description:
FIELD OF THE INVENTION 
     This invention relates to a central solar receiver, and, particularly, to a window for use in such a receiver for admitting and passing thereto concentrated solar radiation. 
     BACKGROUND OF THE INVENTION 
     The present invention is particularly directed to a central solar receiver which comprises a housing with a solar absorber adapted to absorb concentrated solar radiation at high temperatures, commonly—above 500° C., and a working fluid circulating therein in indirect or direct heat exchange relationship with the solar absorber. The housing of such a solar receiver is formed with an aperture located in the focal region of a solar radiation concentration system, and holding a window adapted to admit highly concentrated solar radiation from the concentration system and to pass it towards the solar absorber. 
     The solar absorber used in central solar receivers of the above kind is often a volumetric solar absorber that is fabricated in the form of a three-dimensional matrix enabling the working fluid to flow therethrough, thereby transferring the heat to the working fluid. Such solar receivers are described, for example, in IL 97091 and U.S. Pat. No. 5,323,764. 
     The working fluid used in central solar absorbers normally either serves as heat carrier fluid or else is designed to perform a heat induced, possibly catalyzed, endothermic chemical reaction between components of the working fluid. 
     For various industrial applications such as the operation of gas turbines for electricity generation or the performance of endothermic reactions of the kind specified, it is necessary to enable the working fluid to circulate through the system at an elevated pressure of at least about 2 atmospheres. At such a pressure, the density of the circulating gaseous working fluid is higher than it would have been if it were in the non-pressurized state and, consequently, pressure losses during circulation are lower. 
     One of the most critical problems associated with a pressurized central solar receiver having a window, is the mechanical strength of the window. Materials that have required optical and thermal properties, tend to be brittle, which means that while they can withstand large compression stresses, they tend to crack or shatter under even relatively small tension stresses. Stresses in the window are created by the gas pressure in the interior of the receiver and also by uneven thermal expansion of the window and other receiver components that are in contact therewith, as they heat up during operation. 
     U.S. Pat. No. 5,323,764 and U.S. Pat. No. 5,421,322 disclose a central solar receiver with a window having a frusto-conical shape having a front large-diameter and rear small-diameter open ends both formed with cylindrical front and rear rims at which the window is secured to the housing at its front and rear ends. The front end of the housing is formed with an annular positioning groove and the front rim of the window is received therewithin and secured there by means of an O-ring. The rear rim of the window is secured within a metal block which inter alia includes thermal expansion absorbing bellows and a reflector adapted to protect the block and also the bellows from concentrated solar radiation entering the window. 
     It is an object of the present invention to provide a new window for use in a central solar receiver and a new central solar receiver using the same. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, there is provided a central solar receiver comprising: 
     an axisymmetric housing having front and rear ends and having an aperture at the front end; 
     an elongated tubular window mounted in said aperture co-axially with the housing, the window having an open front end which is secured to the said housing at the front end thereof, and a closed rear end which is disposed adjacent the rear end of the housing and is free of any securing thereto, the window having a first surface facing incident concentrated solar radiation and a second surface facing the interior of the housing; 
     a volumetric solar absorber disposed within said housing and extending around and along said elongated window for absorbing solar radiation that has passed therethrough; and 
     working fluid ingress and egress formed in said housing so as to, respectively, inject thereto, and withdraw therefrom, a pressurized working fluid in a manner enabling the working fluid interaction with said volumetric absorber. 
     In accordance with another aspect of the present invention, there is provided a window for use in a central solar receiver of the above kind, for admitting and passing into the receiver incident highly concentrated solar radiation, the window having an elongated tubular shape with an open front end adapted for being secured in said aperture of the receiver, and a closed rear end free of any securing means. 
     The design of the window of the present invention and the manner of its mounting in the central solar receiver, whereby the window is secured only at its front end, simplify the receiver&#39;s design and prevent the window from having undue constraints, which renders it suitable for use at high temperatures and elevated pressures. 
     The window is preferably of a frusto-conical shape, with its open front end being a large diameter end and its closed rear end being a small diameter end. However, the window may have any other elongated axisymmetric shape, e.g. a cylindrical shape, in which case its front end may be formed with a rim having a diameter greater than that of the remainder of the window. 
     Preferably, the window&#39;s front end is secured to the housing by elastic mounting devices such as, e.g. spring-loaded clamps, that permanently exert on the window&#39;s body axial force that keeps the window in place and prevents its motion under forces acting thereon during thermal and pressure cycles when the receiver is operated. 
     Preferably, the rear closed small-diameter end of the window is in the form of a cap-like concave surface continuously merging with the window&#39;s conical surface. This shape of the closed end, and the frusto-conical shape of the window&#39;s body, ensure that any pressure exerted on the window from within the housing produces force components acting perpendicular and along the window&#39;s surface, which only results in compression stresses rather than tension stresses that may cause the window to crack. 
     Preferably, the working fluid ingress is located adjacent the rear end of the housing co-axially therewith so as to face said closed end of the window. It is still more preferable that an additional solar absorber is mounted in the housing between the working fluid ingress and the closed end of the window so as to absorb concentrated solar radiation that has passed through the closed end. Thereby, the working fluid ingress is protected from direct penetration thereto of concentrated solar radiation and is pre-heated, while entering the receiver, before its interaction with said volumetric solar absorber. The additional solar absorber may also be a volumetric solar absorber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In order to understand the invention and to see how it may be carried out in practice, a preferred embodiment will now be described, by way of non-limiting example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional schematic view of a central solar receiver according to the present invention; 
     FIG. 2 is an enlarged view of the area A of the receiver shown in FIG. 1; and 
     FIGS. 3A,  3 B and  3 C are alternative designs of the central solar receiver shown in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A central solar receiver of the present invention, shown in FIG. 1, is designed to be associated with a solar radiation concentration system (not shown), and it comprises a metal housing  1  having a longitudinal axis X, a front end  3  with an aperture  4  therein facing concentrated solar radiation, a rear end  6  and outer side walls  8 . The receiver further comprises an elongated tubular window  10  mounted in the aperture  4  co-axially with the housing  1 , the window being adapted for the admission of highly concentrated solar radiation from the solar radiation concentration system, and being capable of withstanding high temperatures. For this purpose, the window may be made of any conventional material having desired optical and thermal properties such as, for example, fused quartz. 
     The window  10  has an open front large-diameter end  11 , a rear closed small-diameter end  12  and a frusto-conical elongated window body  13  axially extending therebetween, with a first surface  16  facing incident concentrated solar radiation and a second surface  18  facing the interior of the housing  1 . The window body  13  has a circular cross-sectional shape but this shape may be of any other desired type. Also, the window body may be cylindrical, with the front large-diameter end being in the form of a rim, as shown in FIG.  3 B. If desired, the window  10  may be of a double-pane type with a gap between inner and outer panes, as shown in FIG.  3 C. 
     The window  10  is held in the housing  1  solely by means of its large-diameter end  11  being secured in the aperture  4  at the front end  3  of the housing  1 , as schematically shown in more detail in FIG.  2 . The front end  3  of the housing is formed with an annular groove  20  having a groove bottom with an annular seal  22  and a groove side wall  24 . The large diameter end  11  of the window is secured in the annular groove  20  by means of a plurality of adjustable spring-loaded mounting devices  30  (one of which is shown in FIG.  2 ), located along the circumference of the aperture  4  of the housing. Each mounting device  30  comprises a mushroom-like clamp  40  having an eccentric head  41  and a leg  42  with a skirt  43 . The head  41  of the clamp  40  has a conical circumference to suit the profile of the frusto-conical body  13  of the window  10 , and it is lined by a soft ceramic rope  44  adapted to contact the window without damaging it. The skirt  43  of the clamp  40  is held between a circular first spring  45  and a flange  46  pressed by a second spring  48 . Setting of the clamp  40  is accomplished by turning it around its axis until the rope  44  of the eccentric head  41  abuts the second surface  18  of the window&#39;s body  13 , exerting thereby a proper pressure on the window  10 , and the springs  45  and  48  are designed so as to keep this pressure permanent. Due to the conical shape of the window&#39;s body  13 , this pressure produces an axial force pushing the window towards the seal  22  at the bottom of the annular groove  20  to keep the window in place and prevent its motion under forces acting thereon during thermal and pressure cycles when the receiver is operated. 
     The rear closed small-diameter end  12  of the window  10  is in the form of a cap-like surface smoothly merging with the frusto-conical body  13  of the window. Since the small-diameter end  12  is free of any securing means, its cap-like surface may have any desired shape, e.g. it may be concave, spherical or non-spherical, or rather it may even be flat, as shown in FIG.  3 A. 
     The receiver further comprises a first, tubular volumetric absorber  50  extending around and facing the frusto-conical body  13  of the window  10 , and a second, disc-shaped volumetric through-flow type solar absorber  52 , disposed at the rear of the housing  1  and facing the closed end  12  of the window  10 . The absorbers  50  and  52  define with the second surface  18  of the window  10  a receiver chamber  53 . 
     The first volumetric absorber  50  has a frusto-conical base  54  with an array of spike-like absorber members  56  spaced from each other and projecting from one face of the base  54  towards the window  10 . However, the absorber  50  may be of any other suitable design. For example, it may not be frusto-conical but rather may have polyhedral-prismatic, cylindrical, paraboidal, ellipsoidal, or the like shape. The spike-like configuration of the absorber members  56  is not critical and any other suitable configuration may also be used such as, for example rods, hollow cylindrical tubes, frustum, flat panels and the like. 
     The second volumetric absorber  52  may have a design similar to that as described above with respect to the absorber  50 , with a difference that a base body of the absorber  52  should be perforated or designed otherwise to pass therethrough a working fluid. It may also be of any other suitable type such as, for example, a honeycomb type, grid type, wire-mesh type, foam type etc. The parallel disc shape of volumetric solar absorber  52  is not critical and any other suitable shape may also be used. For example, the absorber  52  may be dome shaped, may have the form of a disc with non-parallel faces, disc with bores and the like. 
     Each absorber  50 ,  52  is made of any suitable heat-resisting material such as a ceramic material, a ceramic-coated metal alloy, silicone carbine, alumina, a special type stainless-steel, a nickel alloy or the like. 
     The receiver further comprises a working fluid ingress duct  58  located at the rear end  6  of the housing co-axially therewith, for the ingress of working fluid into the receiver chamber  53 . The duct  58  has an ingress funnel  59  of a transverse dimension greater than that of the closed end  12  of the window. The second volumetric absorber  52  described above is preferably mounted in the funnel  59 , whereby it is ensured that the duct  58  is protected from concentrated solar radiation penetrating the window at its closed end  12  and that all working fluid is pre-heated to a certain extent by the small absorber  52  before being injected in the receiver chamber  53 . 
     The receiver further comprises a working fluid annular egress chamber  60  surrounding the ingress funnel  59 , with an annular egress orifice  62  located to the back of the large volumetric solar absorber  50 , and a tubular egress vent  64 , for the withdrawal of working fluid from the receiver chamber  53 . 
     The working fluid used in the central solar receiver of the present invention is preferably, a gas such as air, that is adapted to circulate in the receiver chamber  53  at high temperatures (about 500° C. and higher) and at elevated pressures of at least about 2 atmospheres. The working gas may be of any kind suitable to serving as heat carrier for removal of heat generated in the solar absorber. In addition or alternatively it may consist of a mixture of two or more components which, upon contact with the hot solar absorber, are induced to react with each other, i.e. to perform a thermo-chemical process. In the latter case, the projecting members  56  of the volumetric absorber  50  may be coated with a suitable catalyst. 
     It should be noted that the geometry of the ingress and egress of the pressurized working fluid does not need to be as described above but rather may be modified to meet specific design requirements. 
     The housing  1  further comprises an insulating material  65  which fills all its volume between the housing outer walls  8  and the base  54  of the first volumetric absorber  50 , and which also surrounds the ingress duct  58 , the egress vent  64  and the annular egress chamber  60 . 
     In operation, concentrated solar radiation is admitted via the large diameter end  11  of the window  10 . Most of the radiation penetrates through the frusto-conical body  13  of the window and impinges on the absorber members  56  of the first volumetric solar absorber  50 . The incident solar radiation that does not penetrate the window body  13  penetrates through the window&#39;s closed rear end  12  and impinges the second volumetric solar absorber  52 . The solar absorbers  50  and  52  are thereby heated up. 
     Pressurized working fluid is injected into the receiver via the ingress duct  58  and its ingress funnel  59 , wherein it flows through the second volumetric absorber  52  and is thereby heated up to some extent prior to its entering the receiver chamber  53 . After having passed through the second absorber  52 , the working fluid flows towards and along the window&#39;s closed end  12  and further along the frusto-conical body  13  of the window  10 , whereby the window is cooled all along its surface. The same effect may be obtained with a window having a double-pane design as mentioned above, where the working fluid will flow along the gap between the panes, subject to the provision of appropriate fluid passages in the window&#39;s inner pane at the front and rear ends of the window. 
     After having approached the area of the receiver chamber  53  adjacent the front end  3  of the housing, the working fluid is turned back and flows through the volumetric absorber  50  intersecting the array of its members  56 , where it is heated up and/or becomes involved in an endothermic chemical reaction. The hot working fluid and/or the reaction product further enters the annular egress chamber  60  via the egress orifices  62  and is discharged via the egress vent  64 , e.g. for operating electric power generating turbines. 
     In the course of operation, any motion of the window  10  relative to the receiver housing is compensated by the spring-loaded mounting devices  30 . Any thermal expansion of any receiver components, whether symmetric or asymmetric, is not transmitted to the window  10  due to its being set at the large diameter end  11  only. 
     It should be understood that the above-described embodiment is only one example of a central solar receiver and a window used therein according to the present invention, and that the scope of the present invention as defined in the claims fully encompasses other embodiments which may become obvious to those skilled in the art.