AU3738501A

AU3738501A - High-temperature solar absorber

Info

Publication number: AU3738501A
Application number: AU37385/01A
Authority: AU
Inventors: Bernhard Hoffschmidt; Matthias Krodel; Robert Pitz-Paal; Peter Rietbrock
Original assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV; Kraftanlagen Anlagentechnik Muenchen GmbH
Current assignee: Deutsches Zentrum fuer Luft und Raumfahrt eV; Kraftanlagen Muenchen GmbH
Priority date: 2000-02-19
Filing date: 2001-02-15
Publication date: 2001-08-27
Also published as: ATE331920T1; WO2001061254A1; DE10007648C1; DE50110334D1; EP1255953A1; ZA200206104B; EP1255953B1

Abstract

The invention relates to an absorber module (10) which is comprised of a crucible-shaped absorber wall (11) constructed as a hollow wall and comprising an inner wall (12) and a supporting wall (13). The absorber wall (11) is made of a high-temperature ceramic material. The interior of the hollow wall is provided with a channel structure (15) which is filled with a porous material. A gaseous fluid is guided through the channel structure (15) from an inlet (19) to an outlet (22). Incident solar radiation is absorbed by the absorber wall (11) and is convectively transferred to the fluid. At least one part of the outer surface (A) of the absorber wall is bare and is exposed to incident radiation that enters at an angle with regard to the crucible axis. As a result, not only is radiation that enters the interior of the crucible received, but also radiation that strikes the outer surface (A).

Description

Sg/Dt High-tenperature solar absorber The present invention relates to a high-temperature solar absorber provided to absorb highly concentrated solar radiation for heating a pressurized and par 5 ticularly gaseous fluid. Pressurized solar absorbers for operating a gas turbine exist substantially in two basic variants. One variant is a tube-type receiver wherein a tube is ar ranged for passage of a gaseous fluid therethrough. The tube is on its outer o side subjected to concentrated radiation so that the tube and thus also the fluid flowing therethrough will be heated. This system, however, is useful merely to obtain relatively low average radiation densities. The second variant is a closed volumetric receiver comprising a cup-shaped 15 absorber wall arranged behind a likewise cup-shaped window made of quartz glass. A gas is introduced into the space between the absorber wall and the window, to be pressed through the porous absorber wall while being convec tively heated in the process. The absorption of the radiation and the heat transfer to the gas are performed with high efficiency. In this arrangement, considerably higher radiation densities can be obtained than in the above tube receiver. The most critical component of the closed volumetric receiver, how ever, is the quartz-glass window which has to be cooled during operation. The necessary protection of the flange regions of the window from radiation and overheating requires the provision of a secondary concentrator, provided as a . parabolic reflector arranged before the flange regions, for shading the edge region of the receiver.

2 According to present-day knowledge, the closed volumetric receiver represents the optimum approach with respect to the exergetic effect. However, the in volved technical expenditure is very high and necessitates complex regulating and control mechanisms. The weakest point in this arrangement is the win dow. The quartz glass must be cooled and protected from contamination. In the arrangements practiced as of yet, the above-mentioned secondary concen trator arranged before the flange regions requires a cooling circuit of its own. Moreover, also the restricted acceptable angle is critical. This angle causes still stricter requirements to the geometry of the heliostat field directing the solar O0 energy onto the solar absorber. U.S.-Patent 4,452,232 describes a high-temperature solar absorber wherein the absorber wall is generally cup-shaped, with the bottom of the cup formed as an upward cone. The absorber wall is provided with fluid lines originating at 15 the apex of the cone and extending radially outward from the apex. The fluid lines carry a cooling fluid for heat removal from the absorber wall. Each solar absorber is arranged in an opening of a receiver head arranged on a tower. Directed onto this opening are a large number of reflectors located in a helio stat field. The reflectors are moved to follow the position of the sun. It is im perative that the reflectors be directed onto the respective solar absorber with high accuracy so as to avoid an incidence of radiation on the material of the receiver head which would suffer destruction if exposed to such intensive ra diation. It is an object of the present invention to provide a high-temperature solar absorber having an enlarged absorption area. According to the invention, the above object is achieved by the features indi cated in claim 1. As proposed therein, at least a part of the outer face of the absorber wall directed towards the exterior of the cup is unshielded and is ex posed to radiation incident obliquely to the cup axis. This feature makes it possible that the solar absorber will absorb not only radiation from the cup- 3 shaped inner region but also radiation incident onto the outer face of the ab sorber wall. Both the inner side and the outer side of the absorber are de signed to endure solar radiation of extreme energy. The solar absorber is not arranged in the window of a wall but is kept without an external shielding so S that, on the one hand, the solar radiation can be incident into the interior of the solar absorber while, on the other hand, a part of the radiation can reach the outer face of the absorber. The invention allows for the option to arrange a plurality of solar absorbers in I'D a common surface area, with gaps left between the individual solar absorbers. Although a portion of the incident radiation will enter the gaps, it will nonethe less be received by the solar absorbers. Therefore, a complete area can be utilized for the absorption of solar radiation without unused intervening spaces although, geometrically, the individual solar absorbers do not form a closed 15 surface. In a preferred embodiment of the invention, it is provided that the absorber wall comprises a supporting wall, an inner wall facing towards the interior of the cup, and an outer wall covering at least part of the supporting wall. The 9-o inner wall and the outer wall are each of a lesser thickness than the supporting wall. The inner and outer walls serve as absorption walls. Alternatively, the solar absorber can be of a two-walled design wherein the outer wall forms the supporting wall. M The high-temperature solar absorber can comprise a sole component exclu sively made of a high-temperature ceramic structure, thus reducing the dan ger of breakage and damage. Since only one material is used, this arrange ment can be produced at low cost. Tension in the material caused by different thermal expansion coefficients is avoided. The high-temperature ceramic 3)o structure included in the solar absorber obviates the need for cooling meas ures.

4 The channel structure and/or the outlet can comprise a porous structure. Pref erably, the porous structure is produced integrally with the absorber wall from highly temperature-resistant ceramics, particularly from silicon carbide, e.g. C/SiC. The porous structure preferably comprises foamed ceramics of the 5 same material as that of the absorber wall. Such a porous structure enhances the heat transmission from the absorber wall to the air passing through the channel structure. There is obtained a good heat conduction from the massive wall into the pore structure. The heat transmission from a porous structure into the gas is distinctly better than in case of ribs. 10 The solar absorber can be divided into independent absorber modules adapted to be connected in series and in parallel. The absorber modules can be set to an angular inclination relative to the direction of incidence so that part of the radiation will impinge onto the outer walls. The modules can be arranged at 5 mutual distances to keep them from contacting each other. This provides easy accessibility for maintenance and exchange of individual modules. Preferably, the modules are arranged in a rotationally symmetric configuration while ellip tic or polygonal configurations are possible as well. The term "cup-shaped" as used herein is intended to refer to any convex structure in which incident ra ~O diation is caught as in a trap. The channel structure can include a distributor channel having a plurality of channels originating therefrom, extending around the cup axis internally of the absorber wall. The junction from the distributor channel to the individual ~25 channels can be located at any desired site. Thus, for instance, helical chan nels can extend along on the outer side of the absorber wall and merge into helical channels on the inner side of the absorber wall. At the junction site be tween the distributor channel and the individual channels, the channels are provided with apertures or throttle regions. In this manner, it is accomplished o that all channels will have substantially the same flow resistance and that the air flow will be evenly distributed among all channels.

5 Advantageous embodiments and modifications of the invention are evident from the subclaims and the following description of the drawings. The invention will be described in greater detail hereunder with reference to the accompanying drawings. Fig. 1 is a longitudinal sectional view of a first embodiment of the high-tem perature solar absorber, Fig. 2 is a longitudinal sectional half-view of a second embodiment, Fig. 3 is a longitudinal sectional view of a third embodiment, Fig. 4 is a longitudinal sectional half-view of a fourth embodiment, Fig. 5 is a longitudinal sectional half-view of a fifth embodiment, Fig. 6 is a side view of an absorber field comprising a plurality of absorber modules, Fig. 7 is a front view of the absorber field of Fig. 6 as viewed from the direc tion of the arrow VII, Fig. 8 is a front view of an absorber field comprising hexagonal absorber modules. Fig. 1 shows an absorber module 10 adapted to be combined with a plurality of identical modules to form an absorber field. The absorber module comprises a cup-shaped absorber wall 11 of a double-walled design including an inner 20 wall 12 and a supporting wall 13 parallel thereto. The thickness of the material of inner wall 12 is distinctly smaller than that of supporting wall 13.

6 Inner wall 12 is held at a distance from supporting wall 13 by webs 14 inte grally formed to inner wall 12. The webs 14 define the channels of a channel structure 15 helically surrounding the inner wall 12. Channel structure 15 can comprise a sole channel or multiple channels. A porous structure 16 compris 5 ing a porous or foamed material is arranged to fill the channel structure 15. This open porous structure is suited to have the gas flowing therethrough. In ner wall 12 and supporting wall 13 are impermeable to gas. The inner wall 12 has its front edge 17 bent to the outside to thus merge into an outer wall 12a. Arranged behind the outer wall 12a is a distributor channel 18 surrounding the supporting wall 13 and provided with an inlet 19 for supply of the cold gas. Distributor channel 18 (when seen in longitudinal sectional view) is guided around edge 17 and enters the channel structure 15 to intro duce the cold gas thereinto. On edge 17, the highest radiation density will oc cur. For this reason, edge 17 is especially suited to effect a strong pre-heating of the gas. In tlie front region of the absorber module, i.e. near the edge, the outer face A is formed by outer wall 12a and, beyond the latter, by supporting wall 13. Channel structure 15, which in the present embodiment comprises helical channels, can also be formed as a labyrinth. Extending from the bottom of the cup-shaped absorber wall 11 is a foot 20 of a hollow shape, having the channel structure 15 leading thereinto. Also foot 20 comprises a porous structure 21 of high-temperature ceramics. Foot 20 forms the outlet 22 for discharge of the pressurized hot gas. A flange 23 formed on 25 foot 20 serves for attachment of absorber module 10 on a holding structure. The front opening 24 for entrance of the solar radiation is open and is not cov ered by a window. At the bottom 25, the inner wall 12 of absorber wall 11 has a larger thickness than in the peripheral region because it is at the bottom 25 0~ that the channel structure 15 is guided to terminate along a linear course while entering the porous structure 21 of foot 20.

7 In the embodiment according to Fig. 2, the distributor channel 18 on front edge 17 is connected, via a surrounding channel 30 extending on the outer side of supporting wall 13, with the inlet 19 arranged on the rear half of the axial length of absorber wall 11, i.e. near foot 20. Since, in this embodiment, inlet 19 is provided at a position farther to the rear, it is located within the shaded region of the closed high-temperature solar absorber; thus, the con nection of the heat carrier medium to inlet 19 can be designed to be durable under the aspect of material technology. Also in this embodiment, channel 30 is filled with a foamed structure. Here, the outer face A of the absorber wall is 10 for the most part formed of outer wall 12a. In the embodiment according to Fig. 3, the annular channel 30 extends all the way down to flange 23 having inlet 19 arranged therein. A tube portion 31 in tegrally formed to foot 20 and supporting wall 13 extends from the flange 23 and then is continued in channel 30. According to Fig. 3, the webs 14 separating the channels of channel structure 15 are arranged on supporting wall 13. For improving the stability of inner wall 12 and for enlarging the radiation-receiving surface of the absorber module, 20 inner wall 12 is provided with ribs 32 arranged in a star-shaped configuration. In the embodiment according to Fig. 4, channel structure 15 comprises helical ribs 33 formed to inner wall 12 and defining a multiple thread. 23r In the embodiment according to Fig. 5, channel structure 15 is provided be tween inner wall 12 and supporting wall 13 as a free, sintered structure which is of an open design or is filled by a porous or foamed material. Figs. 6 and 7 illustrate the manner in which a plurality of absorber modules 10 are combined into an absorber field. Fig. 6 is a lateral view of the absorber modules as arranged on a tower-type receiver (not shown). The radiation is supplied, as usual, from the heliostat field (mirror field) below. The incident 8 radiation travels obliquely to the horizontal axes of the absorber modules. Thereby, it is safeguarded that radiation will impinge also onto the outer walls 12a and the supporting walls 13 of the absorber modules. The absorber mod ules do not form a closed wall but are arranged in spaced relationship to each 5 other. Further, each absorber module 10 is mounted by its foot 20 or its flange 23 to a holding structure 35 to freely extend therefrom in cantilevered man ner. The holding structure 35 further includes the supply and discharge lines for the heat-carrying fluid. 0O According to Fig. 7, the absorber modules 10 are arranged in horizontal rows, with the modules of mutually adjacent rows placed in a staggered relationship, resulting in an overall six-fold structure with respectively six modules arranged to form a ring around a central module. Fig. 8 shows a corresponding arrangement of hexagonal modules 10a. Also in this embodiment, intervals exist between the individual modules. By way of alternative, the modules can also have a rectangular shape. In case of a rec tangular shape of the modules, arranging them on a technical structure basi cally designed as a generally cylindrical tower-type receiver is particularly ad ;LO vantageous.

Claims

1. A high-temperature solar absorber comprising a cup-shaped absorber wall (11) exposed to the incident radiation, formed as a hollow wall and including a channel structure (15) for the convective heating of a gas, characterized in that at least a part of the outer face (A) of the absorber wall (11) facing towards the exterior of the cup is unshielded and exposed to radiation in cident obliquely to the cup axis. l0

2. The high-temperature solar absorber according to claim 1, characterized in that the absorber wall (11) comprises a supporting wall (13), an inner wall (12) facing towards the interior of the cup, and an outer wall (12a) covering at least a part of the supporting wall (13).

3. The high-temperature solar absorber according to claim 2, characterized in that the inner wall (12) and the outer wall (12a) each have a smaller thickness than the supporting wall (13). 2o

4. The high-temperature solar absorber according to any one of claims 1 to 3, characterized in that the absorber wall (11) is comprised exclusively of high-temperature ceramics.

5. The high-temperature solar absorber according to any one of claims 1 to )5 4, characterized in that a distributor channel (18) is arranged to enter a plurality of channels of the channel structure (15) which internally of the absorber wall (11) lead into a common outlet (22) provided at the bottom (25) of the cup. 10

6. The high-temperature solar absorber according to any one of claims 1 to 5, characterized in that the channels of the channel structure (15) in the absorber wall (11) extend in the circumferential direction. 5

7. The high-temperature solar absorber according to claim 5, characterized in that the distributor channel (18) comprises an inlet (19) arranged in the rear half of the axial length of the absorber wall (11).

8. The high-temperature solar absorber according to any one of claims 1 to 1%0 7, characterized in that the cup-shaped absorber wall (11) is provided with a foot (20) axially extending from the bottom of the absorber wall (11) and having the outlet (22) of the channel structure (15) extending therethrough. 19

9. The high-temperature solar absorber according to any one of claims 1 to 8, characterized in that the channel structure (15) and/or the outlet (22) comprises a porous structure (21).

10. The high-temperature solar absorber according to claim 9, characterized 'O in that the absorber wall (11) inclusive of the porous structure (21) is provided as a one-pieced ceramic portion.

11. The high-temperature solar absorber according to any one of claims 1 to 10, characterized in that a plurality of cup-shaped absorber modules (10) are mounted in a cantilevered manner to a holding structure (35), the absorber modules (10) being arranged in a non-contacting, mutually spaced configuration.