Abstract:
An absorber pipe for solar collectors is provided. The absorber pipe includes a metal pipe for and a cladding pipe surrounding the metal pipe to form an annular space that can be evacuated. The absorber pipe can include a wall extending between the cladding pipe and the metal pipe for sealing the annular space and a retaining device for a getter material or a container filled with getter material or inert gas. The retaining device has a receiving section for receiving the getter material or the container. The retaining device is fastened to the wall. The absorber pipe can alternately include a getter material disposed in the annular space for binding free hydrogen present in the annular space and a reflector disposed in the annular space for reflecting radiation. The reflector has a housing with a support section for fastening and protecting the getter material from the radiation.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a U.S. National Stage Entry under 35 U.S.C. §371 of PCT/EP2010/064498, filed on Sep. 29, 2010, which claims the benefit of German Application No. 10 2009 045 100.5, filed on Sep. 29, 2009. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an absorber pipe, in particular for solar collectors in solar thermal power plants, having at least one collector mirror, comprising a metal pipe for conducting and heating a heat transfer medium; a cladding pipe surrounding the metal pipe for forming an annular space that can be evacuated; a wall extending between the cladding pipe and the metal pipe for sealing the annular space; and a retaining device for a getter material or a container filled with getter material and/or protective gas, having a receiving section for receiving the getter material or the container. In addition, the invention relates to an absorber pipe, in particular for solar collectors in solar thermal power plants, having at least one collector mirror, comprising a metal pipe for conducting and heating a heat transfer medium; a cladding pipe surrounding the metal pipe for forming an annular space that can be evacuated; and a getter material disposed in the annular space for binding free hydrogen present in the annular space. 
         [0004]    2. Description of Related Art 
         [0005]    Solar collectors, for example, can be equipped with a parabolic mirror, also called a collector mirror, and are used in so-called parabolic trough power plants. In known parabolic trough power plants, for example, a thermal oil that can be heated up to approximately 400° C. by means of solar rays reflected from the parabolic mirrors and focused onto the absorber pipe is used as the heat transfer medium. The absorber pipe is thus usually composed of a metal pipe, which has a radiation-absorbing layer and a cladding pipe typically made of glass, which surrounds the metal tube. The heated heat transfer medium is conducted through the metal pipe and, for example, is introduced into a device for producing steam, by which the heat energy is converted into electrical energy in a thermal process. The metal pipe and the cladding pipe run parallel and concentrically to one another. An annular space, which is sealed axially by a wall that is usually composed of metal, is formed between the metal pipe and the cladding pipe. The individual absorber pipes are welded together approximately at 4 m or longer lengths and are formed into solar field loops with a total length of up to 800 m. Absorber pipes of this type are known, for example, from DE 102 31 467 B4. 
         [0006]    Commonly used heat transfer media, and thermal oils in particular, with increasing aging, release hydrogen, which is dissolved, for example, in the thermal oil. The quantity of dissolved hydrogen depends on the thermal oil used and on the operating conditions of the oil circuit. 
         [0007]    The decomposition rate and thus the formation rate of hydrogen increases with increasing temperature. The decomposition of the thermal oil can be accelerated additionally by contaminants, for example by water, which gains access to the oil circuit by leakages in the heat exchanger. As a consequence of permeation through the metal pipe, the hydrogen being released gains access to the evacuated annular space, the permeation rate through the metal pipe also increasing with increasing operating temperature of the metal pipe. As a consequence of this, the pressure in the annular space also increases, which has as a consequence an increase in heat conduction through the annular space, which in turn leads to heat losses and to a lower efficiency of the absorber pipe or the solar collector. As a final effect, the service life of the absorber pipe is reduced, since after a certain time, a sufficient heat output can no longer be generated in order to be able to effectively conduct the thermal process. 
         [0008]    In order to at least reduce the pressure increase in the annular space and thus to prolong the service life of the absorber pipe, the free hydrogen that has entered the annular space can be bound by getter materials. The absorption capacity of the getter materials is limited, however. After reaching the maximum loading capacity or after saturation of the getter material, the pressure increases in the total annular gap, dependent on the hydrogen partial pressure of the gas phase, until it is in equilibrium with the partial pressure of the free hydrogen that has dissolved out of the thermal oil. Previously, equilibrium pressures of several millibars (mbars) could be detected by means of field measurements. Due to the free hydrogen, increased heat conduction arises in the annular gap with the above-named disadvantageous consequences for the efficiency of the solar collector. 
         [0009]    Absorber pipes, which are provided with getter materials in the annular space, are known, for example, from WO 2004/063640 A1. A retaining device for getter material, in which the getter material is stored in a trough-shaped track or loop, is described herein. The loop is attached via feet to the metal pipe. The feet are welded to the metal pipe, so that leakage can readily occur here, whereupon the heat transfer medium can enter the annular space and the vacuum in the annular space can be lost. In addition, it is a disadvantage in this retaining device that the strong temperature differences occurring during operation between the metal pipe and the carrier device and thus different length expansions must be considered, in order to prevent a buckling or a tearing off of the loop, which requires an increased expenditure for construction. 
         [0010]    Further, the loop is found in a region that can be subject to direct solar radiation. In particular, rays that come from the mirror and miss or only brush against the metal pipe (defocused radiation) can lead to a heating of the loop and thus of the getter material. This is disadvantageous because the absorption capacity of the getter material for free hydrogen decreases with increasing temperature of the getter material, so that hydrogen that is already bound to the getter material is again released, whereby the pressure in the annular space and thus the heat conduction through the annular space again increase. Since the loop is joined via the feet directly to the metal pipe, a heat transfer, in particular a conductive heat transport, to the getter material occurs over it, which contributes to its heating. 
         [0011]    As already mentioned initially, absorber pipes of this kind usually have walls with which the annular space is sealed. For this purpose, they extend between the metal pipe and the cladding pipe. Since the metal pipe and the cladding pipe are composed of different materials and are heated very differently during operation of the absorber pipe, they expand very differently, particularly in the axial direction. The wall comprises an expansion-equilibrating unit, with which the different thermal expansions can be equilibrated. Expansion-equilibrating units are manufactured at least partially of metal, so that they are impermeable to solar radiation. Consequently, the heat transfer medium in the region that is surrounded by expansion-equilibrating units is not heated, so that the efficiency of the absorber pipe deteriorates, the larger the region surrounded by expansion-equilibrating units. 
         [0012]    On the other hand, the getter material can be advantageously disposed in the expansion-equilibrating units. Since, as described above, they are impermeable to solar radiation, the solar rays cannot reach the getter material or at least reach it only to a reduced extent and correspondingly do not heat it or heat it less strongly. Consequently, the absorption capacity of the getter material for free hydrogen is not reduced by solar radiation or at least is reduced less strongly in comparison to direct irradiation. A corresponding arrangement of the getter material is known from DE 10 2005 022 183 B3. 
         [0013]    In order to increase the efficiency of the absorber pipe, however, one attempts to design expansion-equilibrating units as small as possible, in order to minimize the region of the absorber pipe surrounded by them. In this connection, one speaks of an enlargement of the aperture area of the absorber pipe, whereby the aperture area denotes the region of the absorber pipe that is accessible in an unhindered manner to solar radiation. Together with minimizing the region that is surrounded by expansion-equilibrating units, the space that is available for arranging the getter material in the expansion-equilibrating units is also minimized. Thus, a situation may occur, in which sufficient getter material can no longer be disposed in the expansion-equilibrating units, so that the quantity of hydrogen released during operation of the absorber pipe can no longer be adsorbed to the required extent. The absorption capacity for free hydrogen is proportional to the quantity of getter material utilized. Consequently, in the case of absorber pipes with maximized aperture area, the absorption capacity of the getter material is exhausted prematurely and the efficiency of the absorber pipe decreases prematurely, so that it needs to be changed prematurely for a new absorber pipe, which negatively influences the economic balance. 
         [0014]    Absorber pipes currently available on the market are provided with an expansion-equilibrating unit, which either extends into the annular space between the absorber pipe and the cladding pipe (DE 102 31 467 B4) or which joins the absorber pipe and the cladding pipe on the outside with one another (DE 60 223711 T2). With a temperature increase of the absorber pipe, the expansion-equilibrating unit extending into the annular space is thus compressed, whereby the aperture of the absorber pipe increases under the temperature conditions during operation. 
         [0015]    The wall with which the annular space is sealed is composed of metal, at least in sections, so that a glass-metal connection must be provided at the end of the cladding pipe. Since metal and glass directly transition into one another in the glass-metal connection, the different length expansions due to a temperature change are particularly critical here. As a consequence of the different length expansion, damage occurs frequently at the glass-metal connection, which leads to a loss of the vacuum in the annular space. This results in a clear reduction in the efficiency of the solar collector, which then can no longer be operated economically. 
         [0016]    The expansion-equilibrating unit extending toward the annular space screens the half of the glass-metal connection turned away from the collector from defocused, concentrated radiation. The compression of the expansion-equilibrating unit that accompanies higher temperatures can lead to the circumstance that the glass-metal connection is subjected to defocused radiation, particularly in the case of an axially shortened configuration of the expansion-equilibrating unit. 
         [0017]    In the case of the outer-lying expansion-equilibrating unit, the latter offers no protection for the glass-metal connection. Therefore, a shield is provided elsewhere for the protection of the glass-metal connection (DE 60 223 711 T2). 
         [0018]    The defocused radiation contributes to the heating of the glass-metal connection, but not to the heating of the thermal oil, so that it provides no contribution to the generation of electrical energy. Thus, the efficiency of the solar collector decreases with an increasing fraction of defocused radiation. Secondary mirrors, which are disposed in the annular space in the half of the absorber pipe turned away from the collector mirror in order to increase the efficiency of the solar collector, are known from U.S. Pat. No. 4,432,345 and U.S. Pat. No. 4,273,104. 
         [0019]    The problem of the present invention is thus to at least reduce the above-discussed disadvantages of known retaining devices of the prior art and to further develop the absorber pipe so that the heating of the getter material is at least reduced and a simple manufacture and assembly of the absorber pipe is made possible, whereby the retaining device can be supplied with both getter material as well as with a container that is filled with getter material and/or protective gas, and the getter material will be arranged as desired. 
         [0020]    In addition, the problem of the present invention is to respond to the disadvantages of known absorber pipe designs, in particular the reduction in the capacity of the getter materials for free hydrogen and the heating of the glass-metal connection due to defocused radiation and the thus accompanying loss of defocused radiation. 
       SUMMARY OF THE INVENTION 
       [0021]    The problem is solved in that the retaining device is attached to the wall. No direct heat conduction occurs between the metal pipe and the retaining device. Heat conduction only occurs via additional components, to which the retaining device is attached. The longer the path of heat conduction is, the smaller the heat transfer will be, so that the heating of the getter material is reduced. The wall is thermally largely decoupled from the metal pipe, so that it is barely heated during operation of the absorber pipe. Due to the fact that the retaining device is attached to the wall, no heat or only a small amount of heat can enter into the getter material, so that this material is also not heated during operation or is heated only to a small extent. 
         [0022]    Further, the different length expansions as a consequence of the heating of the metal pipe and the retaining device need not be considered. Since the retaining device is not attached to the metal pipe, it can expand independently from the metal pipe, without damage occurring. 
         [0023]    The wall of the absorber pipe preferably has an outer ring, a transition element and/or a connection element, whereby the retaining device is attached to the outer ring, to the transition element or to the connection element. Outer rings, transition elements and connection elements are typical components of an expansion-equilibrating unit, with which the different expansions of the metal pipe and the cladding pipe will be equilibrated during operation of the absorber and simultaneously, the annular space will be sealed. 
         [0024]    The wall preferably comprises an expansion bellows, the retaining device being attached to the expansion bellows. Many expansion-equilibrating units also comprise an expansion bellows that equilibrates the axial displacements as a consequence of different expansions of the metal pipe and the cladding pipe. According to the invention, it is possible to arrange the retaining device annularly around the metal pipe without needing to take additional fastening measures. In this way, the retaining device can be fixed to or suspended from the expansion bellows by fastening means. Since the expansion bellows is usually manufactured from a light-impermeable material such as metal, in this arrangement, it protects the getter material from solar radiation at least on one side, which also leads to a reduction in the heating of the getter material. 
         [0025]    According to the invention, the retaining device has a first region and a second region and the absorber pipe has one half facing the collector mirror and one half turned away from the mirror. In this case, the getter material or a first container filled with getter material is arranged in the first region, and a second container filled with protective gas is disposed in the second region, whereby the first region is found in the half turned away from the collector mirror and the second region is found in the half facing the collector mirror. 
         [0026]    The half turned away from the collector mirror is shaded by the metal pipe, so that the first region is not subjected to the focused solar radiation. Subsequently the getter material is not heated or is heated only slightly, for which reason its absorption capacity for free hydrogen is not reduced. 
         [0027]    The protective gas, which is found in the second container, is not particularly temperature-sensitive. The second container is configured so that it can be opened by an external action, for example, a heating effect, so that the protective gas flows out and is distributed in the annular space. Protective gases, e.g., carbon dioxide or inert gases, have a very small heat conductivity, so that in spite of a relatively high hydrogen concentration, they reduce heat conduction through the annular space, which in turn limits the heat losses of the absorber pipe. 
         [0028]    Preferably, the retaining device comprises a receiving section for receiving the getter material or the container, the receiving section being configured annularly, and a radiation protection shield against solar radiation and heat radiation. By means of the annular configuration, the getter material can be disposed as desired around the metal tube in the annular space of the absorber pipe. The receiving section and thus the retaining device can be designed in one piece, which facilitates assembly in the annular space. The radiation protection shield protects the getter material from solar radiation that either directly enters the absorber pipe from the sun or is reflected by the collector mirror to the absorber pipe. Further, the heat radiation, which does not emanate directly from the sun, but rather from the hot metal pipe, for example, is also prevented from heating the getter material. 
         [0029]    In this way, the heating of the getter material and the thus-accompanying reduction in the absorption capacity of the getter material for free hydrogen are reduced. The retaining device can also have several radiation protection shields that are disposed, for example, distanced radially outward from one another, when viewed from the longitudinal axis of the absorber pipe. Each time depending on the position of the getter material in the retaining device, the first radiation protection shield sometimes takes over a variable portion of bearing the getter material, and the second radiation protection shield also sometimes takes it over. Further, a separate radiation protection shield can be provided, which is disposed inside the retaining device, when viewed radially outward from the longitudinal axis of the absorber pipe, and has no bearing function. It can be accommodated thermally decoupled on the retaining device or on the metal pipe or on the wall and repulses solar radiation before it can reach the retaining device. 
         [0030]    The retaining device preferably comprises a highly reflecting metal and/or the retaining device has a reflecting layer for reflecting solar radiation. In this way, the radiation that strikes the reflecting layer of the retaining device cannot absorb it or absorbs it only to a very small extent, for which reason the retaining device and thus also the getter material can be heated less intensely. In addition, the reflected radiation can be conducted to the metal pipe, where it can contribute to the heating of the heat transfer medium, so that this radiation is not lost. 
         [0031]    The retaining device preferably has a cladding for the protection of the getter material from solar radiation. The cladding can be configured, for example, as a wire mesh. In this case, it does not assume an isolating function, but reduces the amount of solar radiation penetrating the getter material, e.g., by shading it. Therefore, the cladding is at least partially constructed of light-impermeable material. In order to not make it difficult for free hydrogen to access the getter material, however, the cladding has small holes that can be laser-cut, for example. 
         [0032]    In a preferred embodiment, the cladding comprises a reflecting section for reflecting solar radiation. Solar radiation that strikes the cladding does not heat the cladding or only heats it to a very small extent and is reflected back, for example, to the metal pipe, where it contributes to heating the heat transfer medium. The solar radiation is thus utilized more effectively. 
         [0033]    In an advantageous enhancement of the retaining device, which has a first end and a second end, a joining element is provided for joining the first and second ends. In this enhancement, the retaining device is flexible due to the use of a first spring. The retaining device can be closed into a torus-like unit with the joining element. 
         [0034]    The absorber pipe preferably comprises a reflector disposed in the annular space for reflecting radiation, in particular solar radiation, into the metal pipe. The reflector can be designed as a stand-alone component and can be configured so that a particularly large portion of the defocused radiation is reflected to the metal pipe. In addition, specific optical properties, for example, a specific curvature of the reflector, can be considered in order to provide a bundling of the radiation, which cannot be provided for the reflecting layer of the retaining device or can be provided only at great expense. 
         [0035]    In an advantageous configuration, the absorber pipe comprises a metal wall running between the cladding pipe and the metal pipe, at least in sections, in order to seal the annular space, whereby the wall transitions into the cladding pipe via a glass-metal connection and the reflector or the retaining device are disposed so that they protect the glass-metal connection from radiation. The glass-metal connection is particularly sensitive to fluctuations in temperature, which can lead to a failure of the glass-metal connection. A failure is followed by a loss of the vacuum in the annular space, which causes a significant reduction in the efficiency of the solar collector. The reflector and the retaining device are disposed so that they shade the glass-metal device and thus reduce heating due to defocused radiation. This leads to a reduced load of the glass-metal connection, so that it can remain in service longer. 
         [0036]    In addition, the problem is solved by an absorber pipe of the type named initially, which comprises a reflector disposed in the annular space for reflecting radiation, in particular solar radiation, into the metal pipe, whereby the reflector has a housing with a storage section for storing and for protecting getter material from radiation. A reflector, for example, may be composed of a plate-shaped piece of sheet metal, without having a housing. Only if the reflector is configured in such a way that it provides a section that is enclosed by a wall, at least partially, and can be closed off and in which an object, for example, the getter material, can be stored and protected, will it by definition comprise a housing, 
         [0037]    In the ideal case, the collector mirror is configured so that it reflects the total radiation, in particular, the solar radiation, onto the metal pipe, which can contribute to the heating of the heat transfer medium therein. Based on manufacturing imprecisions or on mechanical effects occurring during operation of the solar collector, such as wind and hail, however, it may happen that a part of the radiation reflected by the collector mirror misses the metal pipe and cannot contribute to heating the heat transfer medium. This part of the radiation (defocused radiation) therefore remains unutilized, which reduces the efficiency of the absorber pipe and thus the efficiency of the solar collector. According to the invention, by means of the reflector disposed in the annular space, the part of the radiation that misses the metal pipe after reflection by means of the collector mirror is reflected into the metal pipe. This part of the radiation can now contribute to heating the heat transfer medium and is not lost unutilized. Imprecisions in the manufacture of the collector mirror or disturbances occurring during operation of the solar collector, according to the invention, do not lead to a reduction in the efficiency of the absorber pipe, or at least only lead to a smaller reduction in this efficiency. 
         [0038]    The storage section of the housing of the reflector serves for receiving the getter material, which is simultaneously protected from radiation. No other structural units are provided for the getter material, which leads to simplifying the structure and thus to a cost-effective manufacture of the absorber pipe. 
         [0039]    In another enhancement, the absorber pipe according to the invention comprises a metal wall running between the cladding pipe and the metal pipe, at least in sections, in order to seal the annular space, whereby the wall transitions into the cladding pipe via a glass-metal connection and the reflector is disposed so that it protects the glass-metal connection from radiation. According to the invention, the reflector is disposed so that the glass-metal connection is protected from defocused radiation and is thus heated less strongly by this radiation. As mentioned previously, heating and temperature fluctuations of the glass-metal connection that are too strong are frequently the cause of its failure, which has as a consequence a loss of the vacuum in the annular space. The protection of the glass-metal connection with the arrangement of the reflector according to the invention causes the functionality and performance capacity of the solar collector to be maintained. 
         [0040]    The reflector preferably comprises a reflecting layer introduced or installed on the housing. For example, it can be provided as a reflecting foil or film, which is introduced onto the housing. A corresponding coating of the housing is also conceivable. The reflecting layer can thus be introduced even during the manufacture of the housing; expensive fastening measures are not necessary, and further, the use of separate fastening means can be omitted. The use of a highly reflecting material is also conceivable for the housing. 
         [0041]    In addition, the reflector comprises a polished surface. This polished surface can be a part of the housing surface. In this embodiment, additional reflecting components may also be omitted, which leads to simplifying the production of the reflector. 
         [0042]    In an advantageous configuration of the invention, the storage section comprises one or more cavities, into which the getter material can be introduced. By this measure, the position of the getter material in the housing and thus its position in relation to the reflector can be established in a structurally simple manner. The cavity can be produced in the form of a groove or fold that is milled, punched or formed by bending. The number and size of the cavities can be adapted to the required quantity of getter material. The slipping of the getter material, in particular during the assembly of the absorber pipe or the maintenance of the solar collector can be prevented by means of these cavities. 
         [0043]    More advantageously, the cavity can be closed off by a closure or seal. This closure can be designed as a net or grid, for example. Care should be taken that the closure limits as little as possible the accessibility of the getter material to free hydrogen found in the annular space. The closure prevents the getter material from spilling out of the cavity. The getter material is usually supplied and used in the form of pressed, cylindrical pieces, also called pills. Alternatively, the getter material may also be pressed into other shapes, so that the shape of the reflector can also be considered in the selection of the shape of the pressed getter material. Hydrides form on the getter material when free hydrogen is added, and these can cause a particulation of the pills due to an increase in volume. The particles can then spread in an uncontrolled manner in the annular space and are heated by the radiation. This leads to local temperature increases (hot spots) therein, which adversely affects the service life and the efficiency of the absorber pipe. The cladding pipe manufactured from glass is particularly damaged by the hot spots. This can be prevented by providing the closure. 
         [0044]    In an advantageous configuration of the device according to the invention, the housing is fastened to the wall. In this configuration, it is not necessary to fasten the housing to the metal pipe or to the cladding pipe. Fastening it to the metal pipe is thus particularly a disadvantage, since it is strongly heated during operation, whereby, first of all, thermal expansions would have to be considered for the fastening, and this requires an increased expenditure for construction. As long as no corresponding equilibration units are provided for equilibrating the different lengthwise expansions, there is always the danger of buckling or tearing off of the housing when it is fastened to the metal pipe. 
         [0045]    The wall seals the annular space relative to the environment. As already presented above, the wall is constructed of metal, at least in sections. Since metals usually have a good heating capacity, when the housing is attached to the wall, heat from the housing can be discharged to the environment over the wall. The heating of the getter material is consequently reduced. 
         [0046]    In an advantageous enhancement of the invention, in which the wall comprises a connection element and an expansion bellows, the housing is fastened to the connection element or to the expansion bellows. Expansion bellows are typical components of an expansion-equilibrating unit, with which the different expansions of the metal pipe and the cladding pipe will be equilibrated during operation of the absorber and simultaneously, the annular space will be sealed. These bellows and the connection element are usually placed at least partially in heat-conducting contact to the environment of the absorber pipe. Consequently, they discharge at least a certain amount of heat to the environment. The discharged quantity of heat can no longer enter into the getter material and heat it. 
         [0047]    In a preferred configuration of the absorber pipe according to the invention, in which the expansion bellows has an inner end and an outer end, the housing is fastened to the inner end. The inner end points in the direction of the annular space or is found in the annular space. As presented initially, one attempts to maximize the aperture area of the absorber pipe. The expansion bellows in this case play an important role inasmuch as they help determine the axial extent of the expansion-equilibrating units. In maximizing the aperture area, one attempts to configure the expansion bellows as short as possible. Consequently, the number of folds in the expansion bellows used is limited to the necessary minimum. By means of the fastening of the housing to the inner end of the expansion bellows according to the invention, the size or the number of folds of the expansion bellows plays no role in the accommodation of the getter material. According to the invention, it is assured independently from the axial extension of the expansion-equilibrating unit, in particular the expansion bellows, that sufficient getter material can always be disposed in the annular space, since the reflector is disposed axially inwardly of the expansion-equilibrating unit and is thus independent of the axial extension. 
         [0048]    In a preferred configuration of the present invention, in which the wall comprises an outer ring and a connection element, the housing is attached to the outer ring or to the connection element. Outer rings are typical components of an expansion-equilibrating unit, with which the different expansions of the metal pipe and the cladding pipe will be equilibrated during operation of the absorber and simultaneously, the annular space will be sealed. These rings are usually placed at least partially in heat-conducting contact with the environment of the absorber pipe. Consequently, they discharge at least a certain amount of heat to the environment. The discharged quantity of heat can no longer heat the getter material. 
         [0049]    In a preferred enhancement of the invention, in which the absorber pipe has one half facing the collector mirror and one half turned away from it, the reflector is disposed in the half turned away from the collector mirror. The radiation reflected from the collector mirror crosses through the half facing it and strikes the metal pipe. The metal pipe brings about a shading that is largely free of radiation in the half turned away from the collector mirror. Correspondingly, the heating up of the getter material disposed in the housing is also reduced, if it is disposed in the half that is turned away. 
         [0050]    In the half turned away from the collector mirror, the reflector does not bring about any shading of the radiation, so that the aperture area on the principally relevant half facing the collector mirror is not reduced. In addition, the reflector can reflect defocused radiation back onto the metal pipe. With this arrangement of the reflector, the absorber pipe can be operated with an increased efficiency. 
         [0051]    The reflector preferably comprises one or more planar reflecting sections. The reflector can be constructed in a particularly simple manner by means of the planar sections, without reducing its efficiency in a noteworthy manner. In fact, the reflector may also comprise curved sections, but on the one hand, these are more difficult to manufacture, and, on the other hand, must be incorporated more precisely, so that the reflected radiation will actually be reflected into the metal pipe. This planar design of the reflector does not require such a precise installation site. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0052]    The invention will now be explained in detail based on preferred examples of embodiment with reference to the appended figures. Herein: 
           [0053]      FIG. 1  shows a schematic representation of a solar collector, 
           [0054]      FIG. 2  shows a half-section through a first example of embodiment of an absorber pipe having a first example of embodiment of a retaining device according to the invention, 
           [0055]      FIG. 3  shows a sectional view of the first example of embodiment of the retaining device according to the invention shown in  FIG. 2 , in enlarged form, 
           [0056]      FIG. 4  shows a half-section through the absorber pipe according to the first example of embodiment having a retaining device according to a second example of embodiment, 
           [0057]      FIG. 5  shows a sectional view of the second example of embodiment of the retaining device according to the invention shown in  FIG. 4 , in enlarged form, 
           [0058]      FIG. 6  shows a half-section through the second embodiment of the absorber pipe having a third example of embodiment of the retaining device according to the invention, 
           [0059]      FIG. 7  shows a partial sectional view of the third example of embodiment of the retaining device according to the invention shown in  FIG. 6 , along the longitudinal axis, in enlarged form, 
           [0060]      FIG. 8  shows a top view onto the third example of embodiment of the retaining device according to the invention shown in  FIGS. 6 and 7 , 
           [0061]      FIG. 9  shows a top view onto a fourth example of embodiment of the retaining device according to the invention, 
           [0062]      FIG. 10  shows a sectional view through the second example of embodiment of the absorber pipe having the fourth example of embodiment of the retaining device according to the invention, 
           [0063]      FIG. 11  shows a half-section through a third example of embodiment of the absorber pipe having the first example of embodiment of the retaining device according to the invention, 
           [0064]      FIG. 12  shows a half-section through a fourth example of embodiment of the absorber pipe having the second example of embodiment of the retaining device according to the invention, 
           [0065]      FIG. 13  shows a half-section through the second embodiment of the absorber pipe having a first example of embodiment of a reflector according to the invention, 
           [0066]      FIG. 14  shows a not-to-scale sectional view through another example of embodiment of the absorber pipe, which is largely identical to the example of embodiment shown in  FIG. 13 , except for the dimensions, along the sectional plane A-A defined in  FIG. 13 , including the collector mirror, for illustration of the beam path, 
           [0067]      FIG. 15  shows a sectional view through an absorber pipe having the first example of embodiment of the reflector according to the invention, 
           [0068]      FIG. 16  shows a half-section through an absorber pipe having a second example of embodiment of the reflector according to the invention, and 
           [0069]      FIG. 17  shows a not-to-scale sectional view through another example of embodiment of the absorber pipe, which is largely identical to the example of embodiment shown in  FIG. 16 , except for the dimensions, along the sectional plane B-B defined in  FIG. 16 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0070]    A solar collector  10  of the known type is shown in  FIG. 1 . Solar collector  10  comprises a collector mirror  12 , which reflects solar irradiation  14  and directs reflected solar irradiation  16  onto an absorber pipe  18 . Collector mirror  12  is configured in trough shape, so that it brings about a focusing of the reflected solar radiation along a focal line that runs through the longitudinal axis  20  of absorber pipe  18 . Absorber pipe  18  has a metal pipe  22  and a cladding pipe  24 . Metal pipe  22  is coated with a radiation-absorbing layer (not shown) and a heat transfer medium flows through it. Cladding pipe  24  surrounds metal pipe  22 , so that an annular space  26  is formed between metal pipe  22  and cladding pipe  24 . Cladding pipe  24  is typically composed of glass. Based on the trough-shaped configuration of collector mirror  12 , absorber pipe  18  can be divided into one half  28  facing collector mirror  12  and one half  30  turned away from it. 
         [0071]    The flow direction of the heat transfer medium is indicated by arrows P. When it flows through metal pipe  22 , the heat transfer medium will be heated by reflected solar radiation  16 . The temperature that can be reached amounts to approximately 400° C. The heated heat transfer medium is introduced into a process that is not shown in more detail here, in which electrical energy is obtained. Half  30  of absorber pipe  18 , which is turned away from collector mirror  12 , is cooled by mixed convection, thus by natural convection and by forced convection due to wind, for example, which leads to heat losses and thus adversely affects the heating process of the heat transfer medium. Thus, one attempts to reduce the heat conduction from metal pipe  22  outwardly as much as possible, this conduction being effected by means of the annular space  26  formed with cladding pipe  24 . The space can either be evacuated or filled with a protective gas. A combination of the two measures is also possible. Both measures cause a reduction in the heat conduction through annular space  26 , whereby heat losses are limited. 
         [0072]      FIG. 2  shows a first absorber pipe  18  having a first example of embodiment of a retaining device  32   1  according to the invention, in a half-sectional view. Annular space  26 , in the direction of longitudinal axis  20 , is sealed by a wall  34  that comprises a transition element  36  fastened to cladding pipe  24 , an outer ring  38  and a connection element  40 , in the example of embodiment shown. A glass-metal connection  37  is provided at the transition of transition element  36  into cladding pipe  24 . In order to equilibrate the axial displacements of cladding pipe  24  relative to metal pipe  22 , which are caused by the different expansions during the operation of absorber pipe  18 , an expansion bellows  41 , which is correspondingly compressed or expanded, is disposed between outer ring  38  and connection element  40 . Outer ring  38  may also be applied onto connection element  40 , it being axially displaceable, of course, on connection element  40 , so that it can transfer the expansions onto expansion bellows  41 . Expansion bellows  41  has an inner end  42  pointing toward annular space  26  and an outer end  43  pointing away from annular space  26 . In this example of embodiment, connection element  40  is connected to outer end  43 , and outer ring  38  is connected to inner end  42  of expansion bellows  41 . 
         [0073]    Retaining device  32   1  is fastened to connection element  40  proceeding radially from longitudinal axis  20  inside expansion bellows  41  in this example, but can also be attached to transition element  36  or to outer ring  38 . Expansion bellows  41  is usually manufactured from light-impermeable material such as metal. The arrangement of retaining device  32   1  thus utilizes the shading effect of expansion bellows  41 , so that retaining device  32   1  is protected from solar radiation at least on one side, which reduces heating. In each case, retaining device  32   1  is disposed in annular space  26  without the existence of a direct contact to metal pipe  22 . Thus heat cannot be transported conductively in a direct way from metal pipe  22  into retaining device  32   1 , so that the heating of retaining device  32   1  will also be reduced thereby. 
         [0074]    The example of embodiment of retaining device  32   1 , which is shown in  FIG. 2 , is isolated from absorber pipe  18  and shown enlarged in  FIG. 3 . In this example of embodiment, retaining device  32   1  has a receiving section  44 , which can receive a getter material  46  or a container  48  filled with getter material  46 . Getter material  46  in powder form can be filled into container  48 . Alternatively, getter material  46  can be pressed into portions, usually of cylindrical shape. In this case, container  48  can be omitted. 
         [0075]    Both the getter material  46  pressed into portions as well as container  48  can be placed on spacer elements  50 . These spacer elements  50  serve for the purpose of preventing heat conduction to getter material  46 . Receiving section  44  has a boundary section  54 , which prevents getter material  46  or container  48  filled with getter material from slipping under receiving section  44 . 
         [0076]    In this embodiment, retaining device  32   1  is closed and configured annularly, so that it can completely enclose the metal pipe. In this case, another receiving section  44  can be provided outside receiving section  44 , when viewed radially, and this can then prevent getter material  46  from falling out (not shown). 
         [0077]    In addition, retaining device  32 , as shown in  FIG. 2 , is fastened to connection element  40 . In turn, the latter is in contact with metal pipe  22  and surrounds it in a gas-tight manner, for which special seals are provided, which are not shown here. The seals are usually composed of a poorly heat-conducting material, so that connection element  40  is largely thermally decoupled from metal pipe  22 . In order to reduce heat conduction as much as possible, however, one attempts to keep the contact surface between receiving section  44  and connection element  40  as small as possible. This can be done, for example, by joining retaining device  32   1  pointwise to connection element  40 . It is thus achieved that only a small heat conduction can result from connection element  40  to getter material  46  or to container  48  filled with getter material  46 . 
         [0078]    Further, retaining device  32   1  comprises a reflecting layer  60 , which points toward metal pipe  22  and is fastened to receiving section  44 . Reflecting layer  60  deflects solar rays that have missed or just brushed against metal pipe  22  and fall onto reflecting layer  60 , back to metal pipe  22 . In this way, it is prevented, on the one hand, that retaining device  32   1  absorbs solar rays, which could lead to a heating of getter material  46 , and, on the other hand, the reflected rays in metal pipe  22  can contribute to the heating of the heat transfer medium. Alternatively, receiving section  44  can be formed wholly or partially as a highly reflecting metal  60 . 
         [0079]    A second example of embodiment of a retaining device  32   2  is shown in  FIG. 4 . It largely corresponds to the first example of embodiment of retaining device  32   1 , but, of course, here it is not attached to connection element  40 , but rather is attached to the expansion bellows with fastening means  61 . These fastening means  61  may be configured as a part of receiving section  44 . Fastening means  61 , for example, can be introduced into a fold of expansion bellows  41 . This then offers a constructively simple solution, if retaining device  32   2  surrounds metal pipe  22  by 180° or more. In addition, retaining device  32   2  according to the second embodiment is designed longer than retaining device  32   1  according to the first embodiment. In all, five containers  48  can be received by retaining device  32   2 . In addition, with the lengthened version, it is possible to shade glass-metal connection  37  and thus to protect it from heating. 
         [0080]    Since retaining device  32   2  is not placed in contact with connection element  40 , heat cannot conductively enter into retaining device  32   2  from connection element  40  and thus to getter material  46 . Here, it is also valid that retaining device  32   2  is not placed in direct contact with metal pipe  22 , so that heat cannot be transported conductively from metal pipe  22  directly into retaining device  32   2 . The lengthwise expansion of metal pipe  22  does not influence retaining device  32   2 . 
         [0081]    Retaining device  32   2  is shown enlarged in  FIG. 5 . One sees that getter material  46  is surrounded by a cladding  62 , which holds getter material  46  in retaining device  32   2 . This cladding  62  can be formed as a metal mesh or a cloth sock. In order to assure that free hydrogen has access to getter material  46 , cladding  62  has perforations  64 . 
         [0082]    A second absorber pipe  18  having a third example of embodiment of retaining device  32   3  according to the invention is shown in  FIG. 6 . Of course, here retaining device  32   3  is wrapped around expansion bellows  41 . For this purpose, in contrast to the example shown in  FIG. 2 , connection element  40  is connected to the inner end, and outer ring  38  is connected to the outer end of expansion bellows  41 . The axial extent of wall  34  is reduced thereby, so that a larger section of metal pipe  22  can be subjected to solar radiation, which increases the efficiency of absorber pipe  18 . In addition, glass-metal connection  37  is shaded by connection element  40  and by expansion bellows  41  and is protected from defocused radiation. 
         [0083]    The third example of embodiment of retaining device  32   3  is shown in  FIG. 7  in a partial sectional view along a longitudinal axis  66  (see  FIG. 8 ). Receiving section  44  and fastening means  50  are combined here and configured as a first spring  76  with windings  77 . Container  48  or getter material  46  is disposed in the space enclosed by windings  77  and is held in place by these windings. Longitudinal axis  66  of retaining device  32   3  can be bent by using first spring  76 . 
         [0084]    Cladding  62  in this example of embodiment is designed as a wire mesh  68 , which is pulled over first spring  76 . Wire mesh  68  protects getter material  46  by shading it against solar rays, but simultaneously guarantees that free hydrogen can easily reach getter material  46 . Wire mesh  68  does not reduce heat conduction to getter material  46 . 
         [0085]      FIG. 8  shows a top view of the third example of embodiment of retaining device  32   3  according to the invention. Retaining device  32   3  has a first end  70  and a second end  72 , which are joined to a connection element  73 , so that first spring  76  is bent. Connection element  73  has a prestressing element  74 , which exercises a prestressing force when it is extended. Prestressing element  74  is designed here as a third spring  79 . The length of retaining device  32   3  or of connection element  73  in this case is selected so that the first and second ends  70 ,  72  are pulled apart from one another during assembly, for example when it is fitted around expansion bellows  41 , as is shown in  FIG. 6 , so that prestressing element  74  is extended and generates a prestressing force. A part of this prestressing force produces a frictional force between retaining device  32   3  and expansion bellows  41 , so that retaining device  32   3  is established in its position. Reflecting sections  78 , which reflect solar rays and reduce the heating of getter material  46 , are disposed on wire mesh  68 . 
         [0086]    A fourth example of embodiment of a retaining device  32   4  according to the invention is shown in  FIG. 9 . It is essentially constructed as the example of embodiment shown in  FIGS. 7 and 8 . Here, retaining device  32   4  is divided into a first region  80  and into a second region  82 . Getter material  46  or one or more first containers  84  filled with getter material  46  are found in first region  80 , while one or more second containers  86  filled with protective gas are disposed in second region  82 . 
         [0087]    An absorber pipe  18  having a fourth example of embodiment of retaining device  32   4  according to the invention is shown in  FIG. 10 . Retaining device  32   4  is disposed so that first region  80  is found in the half  30  turned away from collector mirror  12  and second region  82  is found in the half  28  of absorber pipe  18  facing collector mirror  12 . Concentrated solar radiation coming from collector mirror  12  does not strike half  30  which is turned away. Consequently, the getter material  46  according to the invention found in the turned-away half  30  is not heated by solar radiation, so that its absorption capacity for free hydrogen is not reduced. In this way, for example, the arrangement of second container  86  in the half  82  facing collector mirror  12  is not to be construed that it must be completely disposed in half  82 . It can also be disposed at least partially in the turned-away half  80 . 
         [0088]    Absorber pipe  18   3  according to a third embodiment, which largely corresponds to absorber pipe  18   1  shown in  FIG. 2  and has a retaining device  32   1  according to the first example of embodiment, is shown in  FIG. 11 . Here, wall  34  is constructed somewhat differently. In this example of embodiment, wall  34  has no outer ring  38 . Rather, expansion bellows  41  is directly joined with transition element  36 . Cladding pipe  24  and expansion bellows  41  are dimensioned such that transition element  36  has a constant diameter. Retaining device  32   1  is disposed so that it shades glass-metal connection  37 . 
         [0089]    Absorber pipe  18   4 , which largely corresponds to absorber pipe  18   2  shown in  FIG. 4  and has a retaining device  32   2  according to the second example of embodiment, is shown in  FIG. 12 . Here also, however, wall  34  is constructed somewhat differently. Here, expansion bellows  41  is joined directly with cladding pipe  24  via transition element  36 , without disposing an outer ring in between. In contrast to the example of embodiment shown in  FIG. 11 , the diameter of transition element  36  changes, so that the diameter of cladding pipe  24  and expansion bellows  41  need not be adapted to one another, since diameter differences can be equilibrated with transition element  36 . 
         [0090]    Absorber pipe  18   2  according to the second example of embodiment is shown in  FIG. 13  in a half-sectional view. A reflector  94   1  with a housing  90  is fastened to the inner end  42  of expansion bellows  41  and to connection element  40  in the example shown. Reflector  94   1  reflects the reflected radiation  16  striking it from collector mirror  12  (see  FIG. 14 ) to metal pipe  22 . Reflector  94   1  comprises a reflecting layer  96  applied on housing  90 . Reflector  94   1  is concavely curved. The reflection of radiation  16  through reflector  94   1  is indicated by arrows P 2 . 
         [0091]    Housing  90  has a storage section  92 , into which getter material  46  can be introduced. Storage section  92  comprises a cavity  102  and an opening  100 , through which getter material  46  can be introduced into cavity  102 . Opening  100  of cavity  102  is closed off with a closure  104 , which can be formed as a grid, for example. 
         [0092]    The second example of embodiment of absorber pipe  18   2  is shown in  FIG. 14  based on a sectional view along sectional plane A-A defined in FIG.  13 —but not to scale. In addition, collector mirror  12  is shown. Concavely curved reflector  94   1  can be well seen, wherein the curvature can run elliptically or parabolically or otherwise, and the curvature of  102  filled with getter material  46  can also run similarly. In addition, opening  100  can be seen, through which getter material  46  can be introduced into cavity  102 . Housing  90  with reflector  94   1  and getter material  46  are exclusively disposed in half  30  of absorber pipe  18   2  turned away from collector mirror  12 . Half  28  facing collector mirror  12  and half  30  of absorber pipe  18   2  turned away from it are well recognizable. 
         [0093]    Arrows P 3  to P 6  are drawn to illustrate the beam paths of solar rays  14 . The rays that run along arrows P 4  and P 5  strike collector mirror  12  and are reflected from it directly into metal pipe  22 , where they contribute to heating the heat transfer medium. The rays that run along arrows P 3  and P 6  also strike collector mirror  12 . These are not reflected into metal pipe  22  from collector mirror  12 , but rather miss it (defocused radiation), for example, as a consequence of manufacturing imprecisions of collector mirror  12 . Normally, these would pass through cladding pipe  24  on half  30  turned away from collector mirror  12  and could not contribute to heating the heat transfer medium. 
         [0094]    According to the invention, however, these rays strike reflector  94   1 , which is configured so that it reflects the rays back into metal pipe  22 , so that they can contribute to heating the heat transfer medium and do not remain unutilized. Reflector  94   1  and getter material  46  are thus positioned with respect to one another so that getter material  46  cannot be heated by the defocused radiation. According to the invention, on the one hand, it is achieved that rays that miss metal pipe  22  are reflected back through reflector  94   1  into metal pipe  22  and thus do not remain unutilized, and, on the other hand, that getter material  46  is not heated by these rays, which would reduce its absorption capacity for free hydrogen. 
         [0095]    A fifth example of embodiment of an absorber pipe  18   5  according to the invention is shown in  FIG. 15 . In contrast to the first example of embodiment, here housing  90  and reflector  94   1  surround metal pipe  22  completely; thus they pass through both halves  28 ,  30  of absorber pipe  18   5 , the half facing collector mirror  12  and the half turned away from the mirror. Getter material  46 , of course, is disposed only in half  30  of absorber pipe  18   5  that is turned away from the mirror. Further, in this example of embodiment, the number of folds of expansion bellows  41  is reduced to the absolutely required minimum. According to the invention, the arrangement of getter material  46  is independent of the axial expansion of expansion bellows  41 , so that sufficient getter material  46  can always be accommodated in housing  90 . 
         [0096]    Transition element  36  forms glass-metal connection  37  at its transition into cladding pipe  24 . In the dimensioning of housing  90  and its arrangement inside annular space  26 , taking into consideration the axial extension of expansion bellows  41 , in this example of embodiment, care is to be taken that glass-metal connection  37  is shaded as much as possible. Glass-metal connection  37  is sensitive to thermal expansions for which reason a shading increases the reliability of glass-metal connection  37 . 
         [0097]    The reflection of radiation  16  through reflector  94   1  is indicated by arrow P 7 . 
         [0098]    The absorber pipe shown in  FIG. 16  has a second example of embodiment of reflector  94   2 , which is constructed from several planar sections  106 . Planar sections  106  can be designed as reflecting layer  96  of housing  90  or as separate components. Reflecting layer  96  can be designed as a polished surface  110 , which is also reflecting. Reflector  94  is fastened to a bracket  108 , which extends from outer ring  38  to reflector  94  without touching expansion bellows  41 . Reflector  94  is disposed so that it shades glass-metal connection  37 . Defocused radiation  16 , whose course is indicated by arrow P 7 , is prevented from striking glass-metal connection  37  by means of reflector  94 . In addition, reflector  94  provides for the defocused radiation to be again deflected back to metal pipe  22  and to contribute to heating the thermal oil. 
         [0099]    Storage section  92 , in which getter material  46  is found, is disposed in housing  90  of reflector  94   2 . Storage section  92  in turn is designed as cavity  102 , which can be closed off with closure  104 . 
         [0100]    Absorber pipe  18   5 , which largely corresponds to the one in  FIG. 16  except for the dimensions, is shown in  FIG. 17 . Absorber pipe  18   5  is shown based on sectional plane B-B defined in  FIG. 16 . It can be seen that reflector  94   2  is disposed in half  30  of absorber pipe  18  turned away from collector mirror  12 . 
       LIST OF REFERENCE CHARACTERS 
       [0000]    
       
           10  Solar collector 
           12  Collector mirror 
           14  Solar irradiation 
           16  Reflected solar irradiation 
           18   1 - 18   6  Absorber pipe 
           20  Longitudinal axis of the absorber pipe 
           22  Metal pipe 
           24  Cladding pipe 
           26  Annular space 
           28  Half of the absorber pipe facing the collector mirror 
           30  Half of the absorber pipe turned away from the collector mirror 
           32   1 - 32   4  Retaining device 
           34  Wall 
           36  Transition element 
           37  Glass-metal connection 
           38  Outer ring 
           40  Connection element 
           41  Expansion bellows 
           42  Inner end 
           43  Outer end 
           44  Receiving section 
           46  Getter material 
           48  Container 
           50  Spacer element 
           54  Boundary section 
           60  Reflecting layer 
           61  Fastening means 
           62  Cladding 
           63  Radiation protection shield 
           64  Perforations 
           66  Longitudinal axis of the retaining device 
           68  Wire mesh 
           70  First end 
           72  Second end 
           73  Connection element 
           74  Prestressing element 
           76  First spring 
           77  Windings 
           78  Reflecting section 
           79  Third spring 
           80  First region 
           82  Second region 
           84  First container 
           86  Second container 
           90  Housing 
           92  Storage section 
           94  Reflector 
           96  Reflecting layer 
           100  Opening 
           102  Cavity 
           104  Closure 
           106  Planar section 
           108  Bracket 
           110  Polished surface 
         P Flow direction of heat transfer medium