FIELD OF THE INVENTION

The invention relates to a solar receiver for absorbing high-flux concentrated solar radiation and transferring the absorbed energy to a working fluid as high-temperature heat. The invention is particularly applicable to central solar receivers.

BACKGROUND OF THE INVENTION

In solar receivers of the kind specified, the working fluid has to be held separated from the ambient and is heated up either directly or indirectly. In the direct heating type, the working fluid flows through a fully or partly transparent chamber and is heated up by direct irradiation. In the indirect heating type the working fluid flows through a non- transparent housing having a heat-conducting wall which is heated up by the incident solar radiation and transfers its heat to the working fluid by indirect heat exchange.

The present invention is concerned with solar receivers of the second type. Such indirect-absorption type solar receivers, also known as non-volumetric solar receivers, comprise a housing for the throughflow of the working fluid which has a non-transparent front wall with an outer layer facing the incident solar radiation and an inner layer contacting the working fluid. The front wall is capable of absorbing the incident solar radiation, of conducting the resulting heat to the interior and of transferring the conducted heat to the working fluid.

Generally, in order to achieve high efficiency of operation, a solar receiver which operates at medium or high temperatures requires high energy flux. When an indirect-absorption by the solar receiver is subjected to such a high energy flux, the need for energy transfer across the non- transparent wall leads to significant energy losses, which renders the operation of the indirect-absorption solar receiver inefficient. These losses are generally caused by reflection of some of the incident solar radiation from the outer surface of the front wall, by thermal emission therefrom and by heat conduction, and the higher the energy flux of the radiation impinging the front wall, the higher the radiation and heat losses therefrom. In consequence, known indirect-absorption central solar receivers operate under relatively low energy fluxes which, in turn, dictates the use of large area front walls which, however, does not enable to raise the temperatures of the working fluid to a level desirable for many thermal and thermo- chemical applications.

It is accordingly the object of the present invention to provide a more efficient solar receiver with indirect solar radiation absorption adapted to absorb and transfer high energy fluxes.

SUMMARY OF THE INVENTION

In accordance with the invention there is provided a solar radiation receiver comprising a housing having an inner space for the throughflow of a working fluid, which housing is formed with a front wall having an outer layer capable of absorbing incident solar radiation and an inner layer capable of transferring heat to a working fluid in contact therewith, characterized by a plurality of recesses in said front wall, which recesses have side walls reaching into said inner space and being so oriented as to essentially face the incident solar radiation.

The term "front wall" used herein signifies the wall that confronts the incident solar radiation and does not necessarily relate to the structure of the receiver. Thus, in case of a frusto-conical tubular solar receiver, the radiation-wise front wall is the structure -wise inner wall.

The recesses in the front wall of a solar receiver housing according to the present invention act as solar radiation traps in that any radiation reflected from one side wall portion of a recess, is redirected to another side wall portion so that reflection losses are negligible and the effective absoφtion of the radiation by the front wall is thus significantly increased. Due to the above recessed design of the front wall, its area is increased whereby the local energy flux which the front wall has to transfer and the temperature gradient across the wall are reduced. Furthermore, with the increase of the front wall area, the contact area between the inner layer and the working fluid in the inner space is also significantly increased, whereby the heat transfer from the inner layer of the front wall to the working fluid is significantly improved. All the above renders the solar receiver according to the present invention capable of operation under high energy fluxes, which is particularly useful for central solar receivers. The recesses may, for example, be in the form of relatively narrow cylindrical, conical or frusto-conical cavities; in the form of elongated grooves having, for example, a rectangular or V-shaped cross- sectional shape; and may quite generally have any geometry suitable for ensuring that no significant amount of radiation entering a recess is rejected. If desired, the front wall of a receiver according to the invention may have a variety of differently shaped recesses, with the geometry of the recesses, their orientation and mutual arrangement being selected in accordance with the characteristics and directional distribution of the incident radiation. By proper design of the recesses, it is possible to achieve that each recess behaves nearly like a black absorber.

Preferably, the recesses are arranged in staggered rows. Furthermore, in a solar radiation receiver according to the invention, the front wall of the housing or only the recesses thereof may be processed, e.g. coated, to improve the radiation absoφtion capacity. The front wall of the solar receiver housing may be substantially flat, e.g. when the housing is of a planar shape, or it may be curved, e.g. concave.

A preferred embodiment of the present invention is a central solar receiver designed to receive concentrated solar radiation.

In a solar energy receiver according to the present invention the working fluid may be circulated by pumping or may take place by spontaneous convection.

DESCRIPTION OF THE DRAWINGS

For better understanding, the invention will now be described, by way of example only, with reference to the annexed drawings, in which: Fig. 1 is a schematic, perspective cutaway view of a solar receiver according to one embodiment of the present invention;

Fig. 2 is a schematic, perspective cutaway view of a solar receiver according to another embodiment of the present invention; and

Figs. 3a, 3b and 3c illustrate alternative shapes of recesses formed in a front face of a solar receiver of the present invention.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Fig. 1 illustrates a solar receiver for concentrated solar radiation according to one embodiment of the present invention. As seen, the solar receiver comprises a substantially flat solar receiver housing 1 having a front wall 2 and rear and side walls 3 and 4 preferably lined from within with an insulation layer (not shown) to prevent heat losses therefrom. Walls 2, 3 and 4 define an inner space for the throughflow of working fluid which ingresses into the housing 1 through inlet conduit 5 and egresses therefrom through outlet conduit 6.

The front wall 2 is preferably made of a material with high thermal conductivity. It has an outer layer 7 adapted for the absoφtion of incident concentrated solar radiation R and an inner layer 8 contacting with the working fluid. As seen, the front wall 2 is formed with a plurality of recesses 9 in the form of conical cavities whose side walls 9' reach into the inner space of housing 1 forming therein an array of pin-like heat exchanger

elements. The recesses 9 open into the outer layer of front wall 2 and are so oriented as to essentially face the incident solar radiation R.

Fig. 2 illustrates a solar receiver for concentrated solar radiation according to another embodiment of the present invention. As seen, this solar receiver comprises a frusto-conical, tubular receiver housing 11 having a heat-conducting front wall 12, a back wall 13 and annular end walls which together define the housing's inner space. The back and end walls of the housing are preferably lined from within with an insulation layer (not shown). The front wall 12 encloses a centrally disposed conical cavity 11' whose large diameter end Ila constitutes an aperture for the entry of incident concentrated solar radiation R and whose small diameter end l ib is fitted with a reflector 10. Housing 11 is fitted with inlet and outlet conduits 15 and 16 for the ingress and egress of a working fluid into and from the housing's inner space. Similarly to the front wall 2 of the solar receiver housing 1 of

Fig. 1, the front wall 12 of the housing 11 has an outer layer 17 adapted for the absoφtion of the incident concentrated solar radiation R penetrating through the inlet aperture I la, and an inner layer 18 contacting the working fluid. As seen, the front wall 12 is formed with a plurality of recesses 19 having in the form of cylindrical cavities whose side walls 19' reach into the housing's inner space. The recesses 19 are preferably arranged in staggered rows or circles and oriented to essentially face the incident radiation R.

During operation of a solar receiver of the kind shown in Fig. 1 or Fig. 2, the incident solar radiation R impinges the outer layer 7, 17 of the front wall 2, 12 of the solar receiver housing 1, 11, penetrating into the recesses 9, 19 thereof. The absorbed radiation converts into heat which is conducted from the outer layer 7, 17 across front wall 2, 12 to the inner layer 8, 18. Working fluid ingresses the solar receiver housing 1, 11 gaining heat from the inner layer 8, 18 of the front wall 2, 12, and the heated working fluid egresses through the outlet conduit 6, 16.

In the embodiment of the present invention, shown in Fig. 2, any radiation which reaches the small diameter end lib of the central cavity 11' is reflected by the reflector 10 towards the front wall 12.

In the course of operation, the working fluid may be pumped through the inner space of the solar receiver housing 1, 11 by a suitable pump or compressor, or may circulate by spontaneous convection.

The solar receiver according to the present invention may have features different from those described in the embodiments specifically described herein. Thus, for example, alternative shapes of recesses formed in the front surface of the solar receiver are shown in Figs. 3a, 3b and 3c. By a proper arrangement of adjacent recesses, it is possible to create arrays of heat-transfer elements that resemble rib-type fin array, tube-bundle or pin-fin type heat exchangers. In addition, the front wall of the solar receiver housing or only the recesses thereof may be suitably finished, e.g. coated, so as to improve the radiation absoφtion capacity.