Patent Publication Number: US-2012031095-A1

Title: Absorber pipe for the trough collector of a solar power plant

Description:
The present invention relates to an absorber pipe for a solar power station according to claim  1  and a method for its manufacture according to claim  12 . 
     Solar thermal power stations have already been producing for some time power on an industrial scale at prices, which—compared with photovoltaic technology—are closer to today&#39;s customary commercial prices for power generated in a conventional manner. 
     In solar thermal power stations the radiation from the sun is reflected by means of collectors with the aid of a concentrator and systematically focused onto a location at which high temperatures arise as a result. The concentrated heat can be led away and used for the operation of thermal power machines such as turbines, which in turn drive the generators that generate the electrical power. 
     Today there are three basic forms of solar thermal power stations in use: dish-Sterling systems, solar tower power station systems, and parabolic trough systems. 
     Parabolic trough power stations feature a large number of collectors, which have long concentrators with a small lateral dimension, and thus possess not a focal point, but rather a focal line; this fundamentally differentiates this design from that of the dish-Sterling and solar tower power stations. Today these line concentrators feature lengths from 20 m up to 150 m, while the widths can be as much as 5 m or 10 m, or more. Along the focal line runs an absorber pipe for the concentrated heat (as a rule up to about 400° C.); the pipe transports this heat to the power station. A fluid such as, for example, thermo oil or superheated stream comes into consideration as the transport medium; this circulates in the absorber pipework. 
     Although a trough collector is preferably designed as a parabolic trough collector, trough collectors with spherical or only approximately parabolic designs of concentrators are often used, since an exact parabolic concentrator with the dimensions cited above can only be manufactured with great effort that is not really justified economically. 
     The 9 SEGS trough power stations in southern California together produce a power output of approximately 350 MW, and an additional power station in Nevada should be connected to the network at around the present time and deliver more than 60 MW. A further example of a trough power station is the Andasol 1 in Andalusia currently on trial, with a concentrator surface area of 510,000 square metres and a power output of 50 MW, with the temperature in the absorber pipework at approximately 400° C. The pipework system for the circulation of the heat-transporting fluid can in such power stations reach a length of up to 100 km or more if the design concepts for future large facilities are implemented. The costs for Andasol 1 total  300 million. 
     It can be estimated that roughly 40% or more of the total costs for a solar power station fall upon the collectors and the pipework system for the heat-transporting fluid, and that the efficiency of the power station is decisively determined by the quality of the absorber pipework. 
     Conventional concentrators permit a concentration ratio in the range from 30 to 80, which leads to the desired high temperatures in the heat-transporting medium. Unfortunately this results in turn in a significant level of heat radiation from the absorber pipework that can reach 100 W/m, which for a pipework length of the order of the 100 km cited above significantly impairs the efficiency of the power station. 
     Accordingly the absorber pipework is increasingly being built in a more complex manner in order to avoid these energy losses. Thus widely used conventional absorber pipework is designed from glass and a metal pipe, with a vacuum present between glass and metal pipe. The metal pipe guides the heat-transporting medium in its interior, and on its outer surface is provided with a coating that absorbs the inward radiated light in the visible spectrum but features a low outward radiation rate for wavelengths in the infrared range. The encasing glass tube protects the metal pipe from cooling by wind and acts as an additional barrier for the outward radiation of heat. What is disadvantageous here is that the encasing glass wall both partially absorbs but also reflects the incident concentrated solar radiation, with the result that a coating is applied to the glass to reduce the reflection. 
     In order to reduce the laborious cleaning effort required for such absorber pipework, and also to protect the glass from mechanical damage, the absorber pipework can also be fitted with an encompassing mechanically protective tube, which, while it does have to be provided with an opening for the incident solar radiation, otherwise protects the absorber pipework in a very reliable manner. 
     Such structures are complex and accordingly expensive both in manufacture and also in maintenance. it is therefore the object of the present invention to provide absorber pipework of the type cited that can be used in a more cost-effective manner and with the highest possible temperatures of the heat-transporting fluid. 
     US PS 1 644 473 now shows an externally insulated absorber pipe with an absorber cavity extending lengthwise through the pipe internally, into which concentrated radiation enters via a similarly lengthwise running slot on the absorber pipe. 
     This allows the external face of the absorber pipe to be insulated effectively and at low cost in a simple manner, and thus to hold the heat losses at a low level compared with today&#39;s widely-used, complicated and maintenance-intensive designs. Moreover such a design is robust and simple to manufacture. 
     Furthermore in the document cited means are disclosed whereby the radiation that has entered through the slot into the absorber cavity is distributed by means of reflection over as much as possible of the total wall region of the absorber cavity, and thereby accordingly increases the absorbing wall surface at the expense of the slot opening. These means consist in the first instance of two deflecting mirrors positioned opposite to the slot opening, a collecting lens then preferably being arranged in the slot, which lens directs the collected incident radiation onto the deflecting mirrors. The radiation is then distributed by the mirrors over the wall surface. In another form of embodiment the absorbing wall of the absorber cavity is fitted with alternating peaks and troughs, on which the incident radiation is scattered by means of reflection and is thus similarly distributed over the whole wall surface. 
     A heat-transporting fluid flows around the absorbing wall of the absorber cavity and carries the heat away. 
     Absorber pipework of the type cited is now also to be improved beyond the object as set. 
     This object as set is achieved by means of an absorber pipe with the features of claim  1 . A preferred form of embodiment of an externally insulated absorber pipe has the features of claim  3 . 
     As a result of the means for reduction of the radiation emitted from the absorbing surface reducing the radiation emitted with increasing temperature of the absorbing surface to an increasing extent, or vice versa, reducing the radiation emitted less at a location of comparatively low temperature, the effort required to manufacture an absorber pipe can be reduced. The technical effort required to reduce the emitted radiation also climbs steeply with the operating temperature of the absorbing surface; this is of particular consequence if the temperature of the heat-transporting fluid increases above today&#39;s usual 400° C. to increase the efficiency of the power station and is to be provided for use on an industrial scale. According to the invention complex means for the reduction of the emitted radiation are concentrated at the exit side of the absorber pipe, i.e. in the region with high operating temperatures of the absorbing surface, and simple (or no) measures are provided for reduction of the emitted radiation at the entry side. 
     In the case of a conventional absorber pipe these can be assembled in the form of a kit of various modules, which are shielded in various ways against the emission of radiation. It is conceivable to have an entry side first section without any shielding, a middle section with some first, beneficial shielding, and a third exit side section with more complex, accordingly more effective, but also expensive and maintenance-intensive shielding. Such an arrangement noticeably reduces the costs of a collector field for a solar power station on an industrial scale. 
     For a preferred form of embodiment of an externally insulated absorber pipe designed according to the invention, there ensues: 
     As a result of the emergence of the radiation emitted from the wall of the absorber cavity being impeded, the efficiency of the absorber pipe increases; in that this takes place only in zones with a high operating temperature, the structure of the absorber pipe is simplified; despite the increased efficiency the pipe can still be manufactured comparatively cost effectively. The temperature of the wall of the absorber cavity basically increases linearly from the entry point for the heat-transporting fluid up to the exit, while the emission of the radiation increases exponentially with increasing temperature. In the entry region of the absorber pipe the radiation emission is therefore of little significance, but in its exit region it is of great significance. 
     Beyond the object as set the preferred form of embodiment of the present invention is particularly suitable for trough collectors with a spherically curved concentrator. Such concentrators do not generate a focal line, but rather a focal line region, which as such presupposes a comparatively wide thermal opening. Particularly in the case in which high temperatures are to be achieved in the wall of the absorber cavity for improved efficiency, a wide thermal opening is critical for a high efficiency on account of the radiation losses. According to the invention the radiation losses are now reduced where they occur, while where the radiation losses are low, the simple cost-effective structure with a wide thermal opening can be retained unmodified. 
     Thus there results in turn a relevant reduction of the manufacture, installation and maintenance costs of a solar power station with use of the absorber pipe according to the invention. 
     The features of preferred forms of embodiment are described in the dependent claims. 
     Further advantages of the absorber pipe according to the invention are described in more detail in conjunction with a preferred form of embodiment, as represented with the aid of the figures. 
    
    
     
       IN THE FIGURES 
         FIG. 1  shows schematically a trough collector with an absorber pipe according to the prior art, 
         FIG. 2  shows a cross-section through an externally insulated absorber pipe with an internal cavity, 
         FIG. 3  shows a view of the absorber pipe according to the invention, 
         FIG. 4  shows a representation of the flux distribution of the concentrated radiation in the thermal opening, and 
         FIGS. 5   a  to  5   d  show the flux in the four different sections of the absorber pipe of  FIG. 2 , and 
         FIG. 6  shows a partial section through the absorber pipe designed according to the invention with an optical element. 
     
    
    
       FIG. 1  represents a trough collector  1  of the type that finds application in its thousands, in the SEGS solar power stations, for example. A trough-shaped concentrator  2 , in cross-section approximated as well as possible to a parabola, and designed as a mirror, rests on suitably designed struts  3 . Solar radiation  4  is reflected from the mirror of the concentrator  2  and deflected onto an absorber pipe  5 ; the latter is sited at the location of the focal line  7  of the mirror. In the case where the curvature of the mirror is only approximate parabolic, in particular in the case where the curvature is spherical, a focal line region is formed instead of a focal line  7 , with the result that the exterior of the absorber pipe receives incident radiation and is heated up over the whole of its cross-sectional dimension. 
     The absorber pipe  5  is suspended on suitable supports  6  at the location of the focal line or focal line region. Depending on the design the mirror is supported on the struts  3  such that it can pivot so that the mirror can track the seasonal (or even the daily) position of the sun. 
     In the absorber pipe  5  supplied fluid collects the heat introduced into the pipe by the concentrated solar radiation and transports this via a suitable, conventional pipework system (not represented in any further detail so as to simplify the figure) to the thermal machinery of the power station where the electrical power is generated. 
     Such trough collectors  1  are of known art in all details of the design to the person skilled in the art in a wide variety of forms of embodiment. Likewise the person skilled in the art is familiar with the suitable pipework runs that guide the heat-transporting fluid to and from the trough collector in question of a solar power station. As a rule, but not necessarily, the heat-transporting fluid is located in a circuit. 
     A wide variety of fluids are used for the heat transport; in particular fluids such as oil that possess a high thermal capacity are preferred. Hardly used at all—and definitely not for solar power generation on an industrial scale—are water or air, the latter because as a result of its comparatively low thermal capacity relative to its volume large volumes must be moved through the pipework system of the power station, which creates its own problems. 
     However, the use of oil or water, for example, is also not without its problems. In order to use the thermal capacity of the oil in an optimal manner, and to maintain the efficiency of the power station as high as possible, the oil is heated to a high temperature. A suitable circuit then runs, for example, at 390° C. and a pressure of 10 bar. In addition to the high costs of such an oil a further disadvantage is that the oil breaks down as soon as the temperature increases to 400° C., and thus complex temperature regulation is required. A water circuit can, for example, be operated at 300° C. and a pressure of 200 bar. While it is true that no denaturation of the water is to be feared at temperature peaks, the high pressures create design problems in the construction of the absorber pipework, while the thermal capacity is not as good as that of oil. Also the corrosive effect of the water, not least with the phase change from water to steam, is not to be underestimated. 
       FIG. 2  shows in cross-section an externally insulated absorber pipe  10  in a form of embodiment preferred for the application of the present invention. A thermal opening  14  here designed as a slot  11  with edges  22 ,  23 , running lengthwise along the absorber pipe  10  allows the passage of concentrated solar radiation through into the interior of the pipe  10 , as represented in the figure in the example of a solar ray  4 . 
     An absorber cavity  12  runs lengthwise in the interior of the absorber pipe  10  up to the absorbing wall  13 , preferably designed as a thin-walled hollow profile with an essentially constant wall thickness. 
     A jacket  18  encases the absorber cavity  12  essentially concentrically, and such that a cavity  19  annular in cross-section is formed between the jacket and the absorbing wall  13 ; the cavity runs lengthwise through the absorber pipe  10 . 
     The heat-transporting fluid (in the present case, for example, a gas) circulates through this annular cavity  19 , which lies in an outer region of the absorber pipe  10 , as is indicated by the double arrow  20  showing the possible directions of circulation. 
     In the form of embodiment shown in the figure the absorbing wall  13  is designed as a waveform profile in cross-section. As a result an incident concentrated solar ray  4 , insofar as it is not absorbed by the absorbing wall  13 , is multiply reflected (and in the process is each time partially absorbed) and thus the incident radiation is scattered, as represented in the example by its reflected components  4 ′ to  4 ′″. In this manner the energy introduced by the ray  4  is distributed over the whole region of the absorbing wall  13 , with the result that the latter is distributed by the concentrated radiation  4  over its periphery and is thereby heated very evenly. 
     Under operational conditions the heat-transporting fluid flows continuously from the entry side of the absorber pipe to its exit side, the absorbing wall  13  being cooled most strongly at entry; correspondingly the operating temperature of the absorbing wall  13  is a minimum at entry, and then increases evenly up to the exit side, where it is a maximum. 
     The heat-transporting fluid enters the absorber pipe  10 , for example, with a temperature of e.g. 60° C., is heated up while passing through the latter and leaves with an exit temperature, which in the application of the present invention, e.g. in the case of air (or also other media), can lie at 650° C. The absorbing wall  13  is therefore most strongly cooled at entry and most weakly cooled at exit; in the present example its temperature T AW  at entry is 150° C., then increases linearly over its length and at exit is ultimately 650° C. ( FIG. 3 ). 
     The jacket  18  features an insulating layer that impedes the transfer of heat from the absorber pipe  10  to its surroundings. Since this insulation does not have to be transparent for incident radiation, as is the case in a widely-used design in accordance with the prior art, it can simply (and thus also cost-effectively) and at the same time effectively, be executed e.g. in rock wool. 
     Overall the result is a robust and cost-effective design that can even be manufactured on-site during the construction of a solar power station, for example, in the desert with limited access. Simple transport and simple on-site installation, combined with a robust design, are features that are not to be underestimated in a technology, which in the nature of things also has to be used in sparsely populated regions that have little or no infrastructure. 
       FIG. 3  shows a view of the absorber pipe  10  of  FIG. 2 , looking onto its thermal opening  14 . The entry-side connection  20  for heat-transporting fluid is schematically represented, while the exit of the absorber pipe  10  is designated as  21 . 
     As mentioned with reference to  FIG. 2 , the absorbing wall  13  heats up in the form of embodiment here preferred from 150° C. at the entry side up to 650° C. at the exit side, see the representation of the operating temperature distribution T AW  of the absorbing wall  13  over the length l of the absorber pipe  10 . Here it is to be noted that for an improved efficiency, in particular of the industrial power generating solar power stations, what is viewed today as a high concentration of solar radiation, in the present example 80 times (according to the invention even more), i.e. 80 suns, is desirable, as is also as high as possible a temperature of the heat-transporting fluid (and thus also of the absorbing wall  13 ) and therefore these should be aimed for. 
     Under operational conditions, i.e. at the operating temperature, the absorbing wall  13  now for its part radiates thermal radiation outwards, as is described below. This radiation is emitted outwards over the surface area of the thermal opening  14 , thereby reducing the efficiency of the absorber pipe  10 . 
     According to the Stefan/Boltzmann Law thermal radiation, essentially infrared radiation  24 , is fundamentally emitted from any body, with the emission increasing with the fourth power of the temperature of the body. The emitted radiation W is given by W=σT 4  W/m 2  and in the present case, with a temperature of the absorbing wall  13  of 650° C., corresponds to 40,000 W/m 2 . Starting from the premise that the energy radiated from the sun onto the earth&#39;s surface corresponds to a flux of 1,000 W/m 2 , it follows that this loss is equivalent to 40 suns. If ultimately the collector now achieves an 80 times concentration, this means an average flux of 80,000 W/m 2  (80 suns) of concentrated radiation  4  through the thermal opening  14  into the absorber cavity  12 . At an absorbing wall  13  temperature level of 650° C. there now necessarily ensues at the same time a loss of 40 suns out of the opening  14 , which corresponds to 50% of the concentrated radiation. 
     According to the invention means are now provided on the absorber pipe  10 , which as a function of the operating temperature of the absorbing wall  13 , rising over the length of the thermal opening, reduce the emergence of radiation  24  emitted outwards through the thermal opening. In  FIG. 3  the thermal opening  14  is to this end subdivided over its length into four sections  26  to  29 , which in each case have the following means: 
     In the first section  26  no such means are yet provided, thanks to the still low temperature of the absorbing wall  13 ; the thermal opening  14  has its full width b v , not a reduced width. In the second section  27  these means have a thermal opening with a reduced width b red 27 , in the third section  28  the thermal opening  14  is provided with a covering  30 , which is transparent for radiation in the visible spectrum and is non-transparent, or of reduced transparency, for radiation essentially in the infrared range. Finally in the fourth region  29  an optical element  31  is arranged on the thermal opening  14  of reduced width b red 29 ; this is designed to guide also such concentrated radiation  4  that is incident outside the thermal opening  14  of reduced width b red 29  by diffraction of the radiation path through the thermal opening  14  ( FIG. 6 ). The optical element is preferably further designed such that the radiation  4  that is captured is incident in a width that corresponds to that of the thermal opening of non-reduced width b v . 
     A covering of the thermal opening  14  in sections  26  and  27  can be dispensed with if the opening is directed downwards, since the hot air in the absorber cavity  12  does not flow out by means of convection, so that no heat loss takes place. 
       FIG. 4  now shows a general representation of the distribution K of the flux of the concentrated radiation  4  in the region and over the width of the thermal opening  14 . In particular if the collector  2  ( FIG. 1 ) is not curved parabolically, but spherically, a focal line region arises instead of a focal line; this in turn leads to a distribution K of the concentrated radiation  4  as represented in the figure. The largest proportion of the radiation is concentrated in a central region of the thermal opening  14 , marked by the vertical axis F of the diagram; the peak value, in our example 160,000 W/m 2 , is however limited to a very narrow region. This leads to the width b of the thermal opening  14  being designed to be as large as possible in order to capture the total concentrated radiation  4 . An average value D of concentrated radiation  4  of 80,000 W/m 2  then ensues, and this enters through the thermal opening  14  into the absorber cavity  13  since the hatched regions in the figure are of equal area. In other words, by means of the concentrator  2  an 80 times concentration (or 80 suns) is achieved. 
     At this point it should be noted that the solar radiation incident onto the concentrator  2  ( FIG. 1 ) is usually assumed to be parallel. The Sun&#39;s cone angle is approximately 0.5°, and this can be taken into account in the dimensioning of the width b of the thermal opening  14  and the flux of the concentrated radiation  4 . 
       FIGS. 5   a  to  5   d  now show four diagrams  26 * to  29 *, corresponding in each case to the diagram of  FIG. 4 , and corresponding to the conditions in the sections  26  to  29  of the absorber pipe  10  ( FIG. 3 ), while the flux W of the radiation  24  emitted from the absorbing wall  13  is also plotted. Since the absorbing wall  13  is heated essentially uniformly, the distribution W of the flux of radiation  24  is a horizontal straight line; the emitted radiation  24  exits over the whole width b of the thermal opening with an essentially uniform intensity. 
     If the direction of the concentrated radiation  4  is taken to be positive (into the pipe  10 ), the direction of the emitted radiation  24  is negative (out of the pipe  10 ). Accordingly the flux W should be indicated in the negative region of the vertical axis of the diagrams. To simplify the presentation, however, (and to show the intersection points of the distribution K with the flux W), W is plotted as a positive value. 
     Assuming a flux W=40,000 W/m 2  at 650° C., the following data apply: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Section of the 
                 Operating temperature of 
                 Flux W of the radiation 24 
               
               
                 absorber pipe 10 
                 the absorbing wall 13 
                 emitted from the wall 13 
               
               
                   
               
             
            
               
                 26 
                 150° C. 
                   133 W/m 2   
               
               
                 27 
                 275° C. 
                  5,700 W/m 2   
               
               
                 28 
                 400° C. 
                 17,000 W/m 2   
               
               
                 29 
                 650° C. 
                 40,000 W/m 2   
               
               
                   
               
            
           
         
       
     
     In section  26  the flux W 26  is insignificant. The width b of the thermal opening  14  is therefore not reduced, and is determined as the full width b v  of the distribution K of the concentrated radiation  4 . The conditions of  FIG. 4  apply; the average flux D 26  through the opening  14  amounts to 80,000 W/m 2  or 80 suns. 
     In section  27  the flux W 27  is already significant. Accordingly the width of the thermal opening is here reduced according to the invention to the width b red 27 , such that within the width b red 27  the sum of the fluxes K+W (concentrated radiation  4  and emitted radiation  24 ) is at least zero at each point (which outside b red 27  would no longer be the case). Over each point of the width b red 27  more radiation enters in total than exits. Thus over the total width b red 27  a solely positive introduction of energy into the absorber chamber  12  ensues, in spite of the thermal emission W caused by the radiation  24 . The average flux D 27  (see once again the hatched regions) amounts to more than 80,000 W/m 2  or 80 suns, so that in spite of the reduced width b red 27  the introduction of energy through the opening  14  is optimal. 
     In section  28  the flux W 28  is considerable. Here the additional effort of providing a covering  30  for the thermal opening  14  is worthwhile; this covering is transparent for radiation  4  essentially in the visible spectrum, and for radiation  24  essentially in the infrared range it is non-transparent or of reduced transparency. Accordingly the flux emitted from the absorbing wall  13  W 28  is reduced to the flux W 28′  that actually exits through the opening  14 ; here the latter is crucial for the dimensioning of the width b red 28 , which in turn is dimensioned such that the sum of the flux F and the emitted radiation W is always at least zero???. Thus an optimised introduction of energy into the absorber chamber  12  also ensues in section  28 . 
     In section  29  the flux W 29  is of critical importance. Here the additional effort of providing an optical element  31  on the thermal opening  14  is worthwhile; by diffraction of the radiation path the optical element guides the incident concentrated radiation  4  through the thermal opening  14 . This has the result that the distribution of the concentrated radiation  4 , after passing through the optical element  31 , is modified compared with those in  FIGS. 4 ,  5   a  to  5   c . The distribution is now approximately uniform; by means of the optical element  31  the radiation  4  that is preferably captured is that incident in the region of the opening  14  over the non-reduced width b v . This means that the quantity of energy that enters, now as before corresponds to the full power output of the concentrator  2  ( FIG. 1 ), while the heat loss through the emitted radiation W, corresponding to the reduced width b red 29 , is massively reduced. The optical element  31  thus additionally concentrates the radiation  4  concentrated by the concentrator  2 , the distribution of the flux F 29  being advantageously modified compared with those of  FIG. 4  and  FIGS. 5   a  to  5   c  as per the curve plotted in the figure. 
     To a first approximation the width b red 29  can basically be reduced to approximately 70 of the full width b v . By the use of such an optical element  31  the advantage moreover ensues that an increased quantity of concentrated radiation  4  enters through the opening  14 ; this comes from the non-parallel solar radiation (cone angle of the solar radiation of approx. 0.5°, see above), and from solar radiation scattered at the concentrator  2  ( FIG. 1 ). A diffractive index of 1.5 (glass) allows the width b red 29  to be further reduced, ultimately to approx. 50% of the full width b v , while nevertheless energy corresponding to a concentration of 80 suns (parallel radiation) is received by the pipe  10 . As a result, with an essentially unmodified high introduction of energy, corresponding to that in  FIG. 5   a , the energy loss W 29  can therefore be reduced by half. In section  29 , therefore, despite the high temperature of the absorbing wall  13 , the loss no longer amounts to 50% (corresponding to 40,000 W/m 2 ) of the concentrated radiation  4  made available from the concentrator  2  ( FIG. 1 ), but only 25%. 
       FIG. 6  shows a cross-section through a part of the absorber pipe  10  in section  29  at the location of the thermal opening  14 . The absorbing wall  13 , jacket  18 , annular cavity  19  and optical element  31  are represented. A concentrated solar ray  4  impinges onto the optical element  31  and is diffracted towards the perpendicular  40 , so that it passes as a ray  4 * through the optical element  31  and as a ray  4 ** reaches the absorbing wall  13 , where it is scattered into the absorber cavity  12 . From the figure it can be seen that, as stated with regard to  FIG. 5   d , concentrated radiation is captured over the total width b v  and passes into the absorber chamber  12  via the width b red 29 . With a suitable design of optical element  31  this is true also for the non-parallel rays  4  of the sun. The shape of the optical element  31  can be graphically designed by the person skilled in the art and manufactured correspondingly. According to the invention the element that is then difficult to manufacture is arranged only in that section where the losses as a result of the emitted radiation  24  would otherwise be too high. 
     The example represented in  FIGS. 4 and 5  relates to a preferred form of embodiment, depending on local conditions the person skilled in the art will suitably design and adapt the concentration factor of the concentrator  2  ( FIG. 1 ), that is to say, the distribution of the flux of the concentrated radiation  4  in the region of the thermal opening (and also the latter itself). Thus the means for the reduction of the emitted radiation  24  (here the reduced width of the opening, the covering  30  and the optical element  31 ) can be suitably combined with one another, or other such means can also be provided. Likewise, for example, the width of the opening  14 , instead of exhibiting a stepwise variation between the sections  26 ,  27 ,  28  and  29 , can be continuously adapted to the rise in the operating temperature of the absorbing wall  13 . Moreover the means according to the invention can be used at even higher operating temperatures than 650° C. 
     As a result it is possible to design an absorber pipe for higher and maximum temperatures of the heat-transporting fluid, without the effort required for this becoming prohibitive, since the means appropriate in each case are only provided at the efficiency-sensitive sections.