Solar device with an air receiver and air return

A solar device has an air receiver with a housing and an absorber connected within the housing. The absorber is heated by solar radiation. The absorber has an upstream side and a downstream side, wherein a stream of air is guided through the absorber from the upstream side to the downstream side and heated to generate heated air and wherein said heated air is cooled to produce useable heat energy and cooled air. A device for returning at least a portion of the cooled air as return air in a plurality of partial streams to the absorber is provided for guiding the partial streams through the absorber counter to the stream of air without mixing therewith from the downstream side to the upstream side. A device for deflecting the partial stream at the upstream side is provided for deflecting the partial streams toward the upstream side.

BACKGROUND OF THE INVENTION 
The invention relates to a solar device in which an air stream is guided 
through and heated by an absorber of an air receiver that is heated by 
solar radiation, the heated air stream is cooled by removing useful heat 
energy, especially by generating steam, and at least a portion of the 
cooled air stream is returned in a plurality of partial streams to the 
upstream side of the absorber. 
From CH-Z "Technische Rundschau Sulzer" 3/1989, P. 9-14, especially FIG. 
10, such a solar device is known in which the not completely cooled air is 
guided from the steam generator into a warm air chamber upstream of the 
absorber. From there the air flows through suitable nozzles toward the 
absorber. It is stated that, for example, 60% of the not completely cooled 
heated air is returned into the circuit. A similar device is known from EP 
399 381 A1 in which the outer edges of the absorber are also surrounded by 
a warm air chamber. In this arrangement a protective air stream is 
provided in front of the return air for preventing heat loss, which 
protective air stream flows to the exterior. 
With respect to the technology of an air receiver reference is further made 
to EP 124 769 B1. 
In the known method it is thus desired to return the air cooled by removal 
of useful heat energy, especially by generating steam, which cooled air 
still contains a considerable amount of detectable heat, be returned to 
the receiver substantially completely so that the air sucked through the 
receiver is preheated. In the known arrangements a warm air channel is 
arranged about the absorber from which the warm return air is blown toward 
the surface of the receiver that is radiated by the sun. In the known 
arrangement it is difficult to distribute the returned warm air 
corresponding to the radiation intensity profile that is present at the 
absorber over the entire receiver surface, especially when during 
operation of the device a strong wind is present and/or the upstream side 
of the absorber is relatively large. It should be noted that for a solar 
device having a steam output of 100 MW.sub.th the receiver must have a 
diameter of approximately 15 m. 
It is therefore an object of the present invention to provide a solar 
device of the aforementioned kind in which a desired distribution of the 
returned air across the upstream side of the receiver is achieved without 
substantially affecting the efficiency with respect to the conversion of 
solar radiation to heat. 
SUMMARY OF THE INVENTION 
The solar device according to the present invention is primarily 
characterized by: 
an air receiver comprising a housing and an absorber connected within the 
housing, the absorber heated by solar radiation; 
the absorber having an upstream side and a downstream side, wherein a 
stream of air is guided through the absorber from the upstream side to the 
downstream side and heated to generate heated air and wherein the heated 
air is cooled to produce useful heat energy and cooled air; 
means for returning at least a portion of the cooled air as return air in a 
plurality of partial streams to the absorber such that the partial streams 
are guided through the absorber counter to the stream of air without 
mixing therewith from the downstream side to the upstream side; and 
means for deflecting the partial stream at the upstream side such that the 
partial streams are deflected toward the upstream side. 
Expediently, for a uniform distribution of the partial streams over the 
upstream side the return air is divided into partial streams of different 
strengths. Preferably, each of the partial streams exiting from the 
absorber at the upstream side is forcibly deflected. 
Advantageously, the means for returning includes a return air distributor 
connected within the housing downstream of the absorber in a flow 
direction of the heated air. The return air distributor is preferably 
spaced at a predetermined distance from the absorber and has a size that 
corresponds substantially to a size of an end face of the absorber facing 
the return air distributor. The return air distributor comprises outlet 
openings distributed in a predetermined manner about the return air 
distributor. The return air distributor further has for each outlet 
opening one return pipe, the return pipes connected to the outlet openings 
and extending to the upstream side of the absorber. 
In a preferred embodiment of the present invention, the outlet openings are 
substantially uniformly distributed. 
Preferably, the means of deflecting comprises a deflecting member for each 
return pipe. Each return pipe has a mouth, and the deflecting members are 
positioned at the mouth of the return pipes. Preferably, the deflecting 
member serves also as a radiation protection device for the mouth of the 
return pipe. 
The deflecting member is preferably mushroom-shaped and has a leg extending 
toward a center of the mouth of the return pipe. The mushroom-shaped 
deflecting member is preferably of a massive construction. 
In a preferred embodiment of the present invention, a chamber is positioned 
within the housing adjacent to the downstream side of the absorber. The 
mushroom-shaped deflecting member has a head and a leg, wherein the head 
comprises an air-permeable absorbent material and the leg is hollow and 
connected to the chamber. 
In another embodiment of the present invention, the deflecting member is a 
plate that is spaced at a distance from the mouth. 
The deflecting member preferably consists of a radiation-impermeable 
material or of a material that is partially radiation-impermeable. 
When the deflecting member is radiation-permeable, the return pipe further 
comprises a plug inserted into the mouth, the plug consisting of an 
absorbent material. 
The absorber is advantageously comprised of individual absorber modules, 
and the return pipes serve as holding elements for the absorber modules. 
For adapting the partial stream to a solar radiation profile, the return 
pipes expediently have different diameters or, in the alternative, the 
return pipes have identical diameters and comprise nozzle inserts of 
different bores. 
The return air distributor is expediently comprised of an air inlet pipe 
(central pipe) penetrating the housing and extending radially within the 
housing and further comprised of transverse pipes (branch pipes) extending 
perpendicularly from the air inlet pipe in a common radial plane to both 
sides of the air inlet pipe, wherein the outlet openings are positioned at 
the inlet pipe and at the transverse pipes in a predetermined 
distribution. 
This object is solved by guiding the partial streams of the return air 
counter to the cold air stream without mixing therewith from the 
downstream side of the absorber in a predetermined distribution through 
the absorber and by deflecting the partial stream at the upstream side of 
the absorber in the direction toward the upstream side. 
In this manner a predetermined distribution of the return air over the 
entire surface of the upstream side of the receiver is achieved without 
shading parts of the upstream side with respect to the solar radiation 
with a distributing. Simultaneously, the return air exits in close 
vicinity to the upstream side and is sucked from there into the receiver. 
Cold air from the surroundings is substantially prevented from flowing 
into the receiver. 
Naturally, a radiation intensity profile with varying radiation is observed 
at the surface of the receiver. It is thus advantageous, when a uniform 
distribution of the partial streams over the upstream side is present, 
that the return air be divided into partial streams of different strengths 
in order to guide the return air in required volumes to the location where 
it is needed for heat removal. An adaptation can also be achieved by a 
non-uniform distribution of partial streams of identical strength. 
When in addition to the return air the receiver also sucks in cold air to a 
substantial amount, the deflection of the partial streams can be achieved 
solely by the vacuum generated by the absorber in sucking in the cold air 
stream. For the desired object of sucking in only a minimal amount of cold 
air and to circulate only return air within the circuit, it is however 
expedient when each partial stream exiting at the upstream side of the 
absorber is subjected to a forcible deflection. 
A very simple design of the inventive solar device is provided when the 
housing of the air receiver containing the absorber has arranged therein a 
return air distributer which is positioned behind the absorber in the flow 
direction of the heated air stream and placed at a predetermined distance 
from it and which covers a surface area of the absorber, whereby its 
outlet openings are distributed in a predetermined, preferably uniform, 
manner and each have coordinated therewith a return pipe extending to the 
upstream side of the absorber. 
Then it is furthermore expedient that the outlet opening of each return 
pipe is provided with a deflection device. This is preferably also the 
radiation protection device for the mouth of the return pipe in case that 
the return pipe is not stable with respect to radiation. 
In a preferred manner the deflecting member is a deflecting mushroom having 
a leg that is aligned with the center of the mouth of the return pipe. The 
mushroom may be of a massive (non-hollow, non-airpermeable) construction. 
However, it is also possible that the mushroom has a head with an 
air-permeable absorbent material and the leg is hollow and connected with 
the chamber located at the downstream side of the absorber. In this 
manner, on the one hand, the mouth of the return pipe is protected against 
radiation, and, on the other hand, an optimal use of the incident 
radiation is achieved since the heated air above the mushroom head is 
introducible into the heated air stream downstream of the downstream side 
of the absorber. 
A simple design of the deflecting member is provided when it is in the form 
of plate that is positioned at a distance from the mouth of the return 
pipe. 
The deflecting member may be made of a radiation-impermeable material, for 
example, a heat-resistant ceramic material, or a material that is at least 
partially radiation permeable, such as, for example, quartz glass. 
When a radiation permeable deflecting member is used, it is expedient to 
provide a plug made of absorbent material in the mouth of the return pipe 
since in this manner the incident radiation at the cross-section of the 
mouth is also utilized. 
According to CH-Z "Technische Rundschau Sulzer", page 12, right column, 
first paragraph, the absorber may be manufactured of square absorber 
modules of wire mesh strips etc. whereby each absorber module is provided 
with an aperture plate at the downstream side having openings for 
determining the amount of air flowing through the absorber module. With 
such a measure, a further adaptation possibility for the radiation 
intensity profile is provided. 
In such an arrangement it is advantageous that the return pipes are 
simultaneously used as holding elements for the absorber modules. For 
adaptation of the partial streams to the required amount of air pursuant 
to the radiation profile, the return pipes of the return air distributer 
may have different diameters or return pipes of identical diameter may be 
provided with nozzles of different bores inserted into their mouth. When 
arranged in groups, the return pipes, of course, may have nozzles of 
identical diameters. 
The return air distributer can preferably be comprised of an air inlet 
pipe, extending radially within the housing and penetrating the housing of 
the air receiver, and transverse tubes extending transverse to the air 
inlet pipe in the form of a Christmas tree, whereby in the air inlet pipe 
and in the transverse pipe outlet openings are provided in a 
predetermined, preferably uniform, distribution. Of course, other 
embodiments are possible, for example, a plurality of air inlet pipes may 
be provided that uniformly supply only one sector of the housing 
cross-section.

DESCRIPTION OF PREFERRED EMBODIMENTS 
According to FIG. 1, the air receiver 1 has a housing 2 with an absorber 3 
supported with supports 4a, 4b connected at one of its end faces. The 
absorber is heated by the incident solar radiation S and which is 
penetrated by the sucked-in air stream L. Downstream of the absorber 3 a 
chamber 5 is provided from which the heated air H heated by the absorber 
may be sucked. The sucked heated air H is subjected to a heat exchange 
process, for example, in a non-represented steam generator, and at least a 
partial stream of the air that has not been completely cooled within the 
heat exchanger is returned as return air R. The return air R is returned 
with a return air distributer 6 that is positioned at a distance from the 
absorber 3 within the housing 2. The return air distributer 6 is comprised 
of a central pipe 6a radially penetrating the heated air collecting 
chamber 5 and a plurality of branch pipes 6b that extend branch-like from 
the central pipe 6a whereby the central pipe 6a and the branch pipes 6b 
are provided with bores 7 such that a substantially uniform distribution 
of bores 7 over the cross-section of the heated air collecting chamber 5 
is achieved. To each bore a return pipe 8 is connected that first 
penetrates an aperture plate 9, coordinated with the downstream side 3b of 
the absorber, and then extends through the absorber to the upstream side 
3a. The return air partial streams RT that exit from the return pipes 8 
are deflected at the upstream side 3a. 
The return pipes 8 in FIG. 1 are represented with identical diameters. When 
it is desired for a uniform distribution to provide different streams of 
volume to different locations of the receiver, the pipes may have 
different diameters, whereby with respect to the radiation intensity 
profile in the central area of the upstream side 3a pipes of a greater 
diameter and at the outer areas pipes of a smaller diameter may be used. 
It is also possible to insert nozzle inserts 10 that have different bore 
widths into pipes of identical diameters, is represented in a dashed line 
in FIGS. 3 and 4. 
When the return pipes 8 are rigidly connected with the return air 
distributer that in itself is a rigid structure, the return pipes 8 may be 
used as holding elements for the absorber, especially when the absorber is 
designed in a known manner from individual absorber modules 3. 
In order to improve the deflection of the partial streams of the return air 
R through the pipes 8 at the upstream side 3a, it is possible to provide 
at the mouth of the pipes a deflector mushroom 11, as shown in FIG. 3, 
that is supported with suitable stays 12 at the pipe. The leg 11a is 
coordinated with the mouth of the pipe 8. According to FIG. 3, the head of 
the mushroom 11b may be designed such that a 90.degree. deflection takes 
place or the head 11b' can be designed such that a deflection of more than 
90.degree. occurs. (Both head designs are possible.) Due to the high 
surface load of the incident solar energy, the mushroom is preferably 
manufactured from a highly heat resistant ceramic material. 
With a suitable material selection the stays 12 or the holders 14 may serve 
as ribs for heat return since they are convectively cooled by the return 
air. 
In the embodiment according to FIG. 4 the mouths of the pipes 8 have 
coordinated therewith a transparent quartz plate 13 as the deflecting 
member that is connected to the pipe 8 with holders 14. Since the plate 13 
is transparent for the radiation, it does not shade any area of the 
absorber. In this case, the pipe must be made of a radiation-resistant 
material such as ceramic, or at least the mouth of the return pipe must be 
protected by other means with forced cooling. 
It must be noted that the return pipes penetrating the absorber do displace 
a part of the absorber volume relative to the volume of the prior art 
absorbers; however, due to the uniformness of the distribution of a return 
air an overall efficiency improvement of the solar device is achieved. 
As is shown in FIG. 5 it is possible, when using a radiation permeable 
deflecting device 13, to reduce the aforementioned displacement effect by 
providing a plug 15 made of radiation absorbent material. 
In FIG. 6 a mushroom-shaped deflecting member 16 is represented having a 
hollow leg 16a that engages the mouth of the return pipe 8 and that is in 
flow connection via two pipe sections 17, penetrating the wall of the 
return pipe from the inside to the outside, with the heated air collecting 
chamber 5. The hollow leg 16a is connected with a heated air collecting 
chamber 18 in the head 16b of the mushroom. The cover of the collecting 
chamber 18 is formed by a cup-shaped aperture plate 19 that has connected 
to its outer side an absorbent material 20 that is comparable to the 
absorber 3. With this arrangement the mouth of the return pipe 8 is 
protected against radiation, and, on the other, hand the radiation is 
collected by the absorbent material. The air sucked through the head is 
heated and is guided via the branch pipes 17 into the heated air 
collecting chamber. 
The present invention is, of course, in no way restricted to the specific 
disclosure of the specification and drawings, but also encompasses any 
modifications within the scope of the appended claims.