Abstract:
A device is provided for the air-based solar thermal generation of process heat in order to assist in drying processes. The device may add the solar-thermally generated hot air as a function of the relative temperature of the solar-thermally generated hot air as a partial flow of the process air generated by other systems.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a national stage application under 35 U.S.C. 371 of international application number PCT/DE2011/001114 filed May 26, 2011 (WO 2012/025073 A2) and also claims priority to German application number 10 2010 022 517.7 filed Jun. 2, 2010, all of which are hereby incorporated by reference in their entirety. 
     SUMMARY 
     The invention relates to a method and a device for the solar thermal generation of process heat in the temperature range of from 300° C. to 700° C. 
     Many solar thermal plants are known, which for the most part operate with water or thermal oil as a fluidic transfer medium for the captured solar energy. In this case, the solar radiation is concentrated onto a usually linear radiation center in the form of a steel tube through which a fluid flows. The steel tube may be enclosed by a vacuum-tight transparent glass tube. Another design consists of a thermally insulated, so-called receiver tube in an elongated box which has a transparent glass pane on the front side and, on the inside, a linear mirror which directs the incident radiation onto the rear side of the steel tube. 
     If only air is envisioned as the transfer medium, however, simple flat collectors are mostly used for the low-temperature range (about 100° C.) and so-called heliostats are mostly used for the high-temperature range. These are usually a multiplicity of mirrors which concentrate the solar radiation onto a receiver operating with air on a tower. The air flowing through such an air receiver reaches temperatures of 1200° C. 
     The following documents, inter alia, are known from the patent literature in relation to the prior art. DE 100 65 485 A1 describes a central solar receiver having an axisymmetric housing and having, on the front side facing toward the sun, an opening which is closed by a window. The object of this document is to provide a new window for use in a central solar receiver, and a new type of central solar receiver in which this window is used. To this end, a volumetric solar absorber is integrated into the housing in such a way that the pressurized working fluid can interact with the volumetric absorber. The working fluid used in this central solar receiver is preferably a gas, for example air, which is suitable for circulating at high temperatures (about 500° C. or more) and at increased pressures of at least about two atmospheres in the receiver chamber. Operation with air at a relatively low pressure is not provided for. 
     DE 102 57 458 A1 discloses a solar thermal receiver unit comprising a shaped ceramic body, and a method for the production of corresponding ceramic bodies. The object of this laid-open specification is to provide a shaped ceramic body as an absorber body which has an increased lifetime, is easy to produce on an industrial scale and is effective in operation. In order to achieve this object, it is essentially claimed that, in an edge region of the shaped body, the length of the channels which permit a medium to flow through the shaped body decreases continuously or in stages from the middle to the edge. 
     The special configuration of an absorber body, however, merely represents the solution to a partial problem which relates to the technical solar generation of process heat. 
     DE 199 52 174 A1 relates to a receiver for concentrated solar radiation for carrying out high-temperature processes. In the processes considered in this case, the use of falling-film absorbers for melting salt is considered, the energy resulting from the salt melting being used to generate water vapor. Environmentally unfriendly vapors are thereby formed. In this case, it is therefore necessary to operate with closed receivers. The configuration of such a receiver is described in this document. Although the melting of salt entails temperatures in the range of from 500° C. to 600° C., which would be of interest for use as usable process heat, the heating of air is not a primary consideration in this case. 
     It is therefore an object of the method according to the invention, and the corresponding device, respectively to provide a method and a device for the air-based solar thermal generation of process heat, with which an arbitrarily connectable, environmentally friendly and economical source for the generation of process heat is provided. 
     The device according to the invention will be described in more detail below. In detail: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an outline overview of the heat generation according to the invention. 
         FIG. 2 a    shows a perspective representation of a heat exchanger; 
         FIG. 2 b    shows a representation of special radiation-absorbing surfaces of heat exchangers; 
         FIG. 3  shows a representation of an air receiver field; 
         FIG. 4  shows a cross section through a heat exchanger; 
         FIG. 5  shows a cross section of a fan drive; 
         FIG. 6  shows exemplary schematic coupling of the process heat generated according to the invention to a dryer field; 
         FIG. 7  shows an arrangement of air receivers having a different radiation concentration; and 
         FIG. 8  shows an arrangement for the combination of air receivers. 
     
    
    
     DETAILED DESCRIPTION 
     The outline overview of the heat generation according to the invention, as represented in  FIG. 1 , shows a Fresnel mirror collector field  2  from which concentrated sunrays strike the heat-absorbing surface of a heat exchanger  3  in an air receiver  1 . 
       FIG. 2 a    shows the perspective representation of a heat exchanger  3  corresponding to the representation in  FIG. 1 . 
       FIG. 2 b    shows the representation of special radiation-absorbing surfaces of heat exchangers  3  as respectively different configurations of these surfaces. A receiver surface, which owing to its special curvature is particularly suitable for use in trough collectors, is represented here by  28 . In the example  29  shown underneath, the dashed representation is intended to show that this surface is configured to be highly porous in order to offer minimal reflection for a special application of the sunrays. The last example  30  shown here is designed particularly for use in a Fresnel mirror collector. The purpose of the “serrated” structure which can be seen in this case is that each subsurface placed at an angle to one another is respectively assigned to one mirror of the Fresnel mirror collector. 
       FIG. 3  shows the representation of an air receiver field. The box-shaped container in the perspective representation represents an air receiver  1  in which a fan is denoted by  4 . The other fan indicated there does not have a reference. For reasons of clarity, the heat exchangers are not indicated. They are located in the space which is not occupied by the fans  4 . 
       FIG. 4  shows a cross section through an air receiver  1 . In the middle, there is the actual heat exchanger  3 , here merely represented by shading with a straight radiation-absorbing surface. Furthermore, the free space between the dentate configurations of this heat exchanger  3  is arranged for the purpose, on the one hand, of allowing the air flowing through to contact with the heat exchanger  3  heated by the sunrays over a large area, and on the other hand to impede this air as little as possible when it flows through. The free space  5  is delimited by a high temperature-stable insulating material  6 . There are suitable industrial products for this purpose, but sand packing between two walls of metal plate is substantially more economical in this case. This insulating material is followed by a layer of a further thermally stable insulating material  7  and an insulating material  8 . The outer dimensions of this heat exchanger  3  correspond to the cross section shown in  FIG. 2  for the air receiver  1 . The heat exchangers  3  are arranged behind one another in an air receiver  1 . They may have a groove on one side and on the other side a spring having dimensions corresponding thereto, so as to be connected to one another essentially in a plug-in fashion. In this way, the mechanical strength of the air receiver  1  is increased and the heat transfer resistance between two successive heat exchangers  3  is reduced. 
     Above all, A1 2 0 3  (aluminum oxide) is suitable as the material for a heat exchanger. Graphite is also to be mentioned for this purpose. It has a very good thermal conductivity and can be used for temperatures greatly in excess of 1000° C. The ceramic ribs of a heat exchanger  3  also serve overall as a short-term heat store when, for example, the insolation is temporarily blocked. Another possibility for storing heat is provided by the proposed sand packing in the region of the layer  6  of the high temperature-stable insulating material. 
     In order to reduce the frictional resistance of the surfaces over which turbulent flow takes place inside an air receiver  1 , the surfaces with which the air flow comes in contact may be configured with a sharkskin structure  31 . The so-called riblets applied in this case consist of fine ribs which have a very sharp rib tip. In  FIG. 2B , a thermally conductive “tip” of the heat exchanger  3  is therefore also denoted by  31 . 
       FIG. 5  represents the cross section of a fan drive in an air receiver  1 . There is no heat exchanger  3  in this region. In the cross section represented, only the insulation is shown as an outer boundary for the air receiver  1 . The fan rotor wheel  16  is driven via a drive belt  14  by means of a Stirling engine  9 . The sealing bearing  19  supports the drive shaft  12  of this engine. The hot cylinder  10  of the Stirling engine  9  lies in the hot air flow inside the air receiver  1 , and the cold cylinder  11  is exposed to the cold external air. The fan rotor wheel  16  is mounted with its drive shaft  13  in two bearings  15 . These bearings are configured to be thermally stable, for example as carbon bearings. The drive belt  14  transmits the drive energy of the Stirling engine  9  from the drive shaft  12  by means of a belt pulley onto the drive shaft  13  of the fan, likewise by means of a belt pulley. An insulation penetration  18  provides the freedom of movement of the drive belt  14 . The arrow direction indicates the direction of the air flow  17 . 
       FIG. 6  represents the exemplary schematic coupling of the process heat generated according to the invention to a dryer field. This dryer field is a part of a much more extensive plant for drying gypsum plates. It comprises the fresh air supply  24  for the gas burner  25  as well as the connection  23  for the moisture-laden discharge air of a dryer field and the circulation fan  26  of a dryer field. The line denoted by  21  stands by way of example for the supply of a partial flow of heated air from the plant according to the invention to said plant for drying gypsum plates. 
     The fresh air supply for a solar field is denoted by  22  in this figure, and a fan of such an air receiver is denoted by  27 . In this case, air is not only to be regarded as pure air, but also moisture-laden air (for example 50 to 300 g/kg dry air ). The purpose of the plant is to heat the air (partial flow of the circulation air ( FIG. 6 ) and fresh air FIG.  6 , reference  22  into ranges of from 500° C. to 700° C. The initial temperature is about 30° C. (fresh air) or about 150 to 200° C. recycled air from the dryer. At the start of the heating process, the air temperature reached is still low. This means that the concentration factor (the area of the mirror field in relation to the absorption area on the air receiver  1 ) is still small. With an increasing air temperature, however, the concentration factor must increase so that high temperatures can be achieved at the air receiver. This means that, for a constant mirror area, the air receiver must be smaller in order to reach the desired higher temperatures. It is therefore expedient to arrange the air receivers behind one another in staged sizes. 
       FIG. 7  represents such an arrangement of air receivers having a different radiation concentration. The references  1 A,  1 B and  1 C correspond to different stages of the radiation concentration. The individual air receivers  1  have the same structure, and merely their geometrical dimensions vary in terms of width and height.  1 A denotes for example a receiver for a low radiation concentration (50 times),  1 B denotes for example a radiation concentration of 80 times intensity and  1 C of 110 times intensity.  20  respectively denotes a solar field without the representation of a mirror system. The upper part of  FIG. 7  shows three receivers in a linear arrangement, and the lower part of  FIG. 7  shows three receivers in an angled form and irradiation by two solar fields. 
       FIG. 8  shows an arrangement for the combination of a multiplicity of small air receivers, which are denoted in a bent form by  32  and which comprise at least one fan  27 . Here, the hot air generated is combined in a main collection tube  33 . The air feeds of the individual air receivers  32  lead at an angle into the main tube in order to utilize the upthrust forces of the rising air. In another variant (not additionally represented here), one of the mutually opposite air receivers  32  shown in pairs still enters the main collection tube  33  at a slight angle, but below the central transverse axis thereof. In addition, two mutually opposite air receivers  32  may be offset in terms of their tangential entry points relative to the longitudinal axis of the main collection tube  33 . In this way, an additional rotation in the direction of the receiver  32  flanged-on above is imparted to the air flowing in from the two receivers  32 , which accelerates the dynamics of the flowing hot air. 
     The control of the inclination of the mirrors of the collector field and the regulation of the air flows  20  require a special control program. 
     As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this application. This description is not intended to limit the scope of this application in that the invention is susceptible to modification, variation and change, without departing from the spirit of this application, as defined in the following claims. 
     LIST OF REFERENCES 
     
         
         
           
               1  air receiver ( 1 A,  1 B,  1 C) 
               2  Fresnel mirror collector field 
               3  heat exchanger 
               4  fan 
               5  free space for the through-flow of air 
               6  high temperature-stable insulating material 
               7  thermally stable insulating material 
               8  insulating material 
               9  Stirling engine 
               10  hot cylinder 
               11  cold cylinder 
               12  drive shaft with belt pulley 
               13  drive shaft for fan 
               14  drive belt 
               15  bearing 
               16  fan rotor wheel 
               17  air flow 
               18  insulation penetration 
               19  sealing bearing 
               20  solar field, without mirror system 
               21  line for a partial flow of heated air 
               22  fresh air supply for a solar field 
               23  moisture-laden discharge air of a dryer field 
               24  fresh air supply for the gas burner of a dryer field 
               25  gas burner of a dryer field 
               26  circulation air fan of a dryer field 
               27  fan of an air receiver 
               28  radiation-absorbing surface of a heat exchanger 
               29  radiation-absorbing surface of a heat exchanger 
               30  radiation-absorbing surface of a heat exchanger 
               31  coating with artificial sharkskin 
               32  bent air receiver 
               33  main collection tube