SOLID PARTICLE SOLAR RECEIVER

GOVERNMENT RIGHTS

[0001] The United States Government may have certain rights to this invention pursuant to contract number DE-AC36-08GO28308 between the United States Department of Energy and Alliance For Sustainable Energy, LLC. This invention was developed under subcontract ZGJ-3-23315-01 between Alliance For Sustainable Energy, LLC. and Babcock & Wilcox Power Generation Group, Inc. under contract number DE-AC36-08GO28308.

RELATED APPLICATION DATA

[0002] This application claims the benefit of U.S. Patent Application No. 14/691002 filed April 20, 2015 and titled "Method to Construct and Support Tube Module Assemblies for Solid Particle Solar Receiver", which itself is a non-provisional of and claims priority to U.S. Provisional Application No. 61/981 ,974 filed April 21 , 2014 and titled "Method to Construct and Support Tube Module Assemblies for Solid Particle Solar Receiver". The complete texts of these applications are hereby incorporated by reference as though fully set forth herein in its entirety.

BACKGROUND

[0003] The following pertains to the solar power generation arts and related arts. In a known solar concentration design, a field of heliostats concentrates solar power onto a (typically tower-mounted) solar receiver. A flowing heat transfer medium flows through the solar receiver. This flowing heat transfer medium absorbs energy from the concentrated light and is thus heated. The hot flowing heat transfer medium may be variously used, for example being fed into a fluidized-bed boiler to generate steam for driving an electrical generator turbine.

[0004] Some such solar concentrators are described, by way of non-limiting illustrative example, in Ma, U.S. Pub. No. 2013/0257056 A1 published October 3, 2013 l which is incorporated herein by reference in its entirety, and in Ma et al., U.S. Pub. No. 2013/0255667 A1 published October 3, 2013 which is incorporated herein by reference in its entirety, and in Maryamchik et al., "Concentrated Solar Power Solids-Based System", U.S. Serial No. 14/250,160 filed April 10, 2014 and published as U.S. Pub. No. 2014/031 1479 A1 which is incorporated herein by reference in its entirety.

BRIEF SUMMARY

[0005] In some aspects disclosed herein, a solar receiver module comprises a front tube sheet including light apertures, a back plate cooperating with the front tube sheet to define a sealed gap, and light channeling tubes having first ends optically coupled with the light apertures and extending through the gap and having second ends connected with the back plate. A flowing or fluidized heat transfer medium, for example a flowing particulate medium such as silica sand or calcined flint clay, but not limited thereto, is suitably disposed in the gap over exterior surfaces of the light channeling tubes. In some embodiments the solar receiver module further comprises slip joint engagements between light apertures of the front tube sheet and first ends of most or all of the light channeling tubes. Each slip joint may be defined by an inner perimeter of the light aperture receiving the first end of the light channeling tube. In some such embodiments, each slip joint is defined by the inner perimeter of a necked down portion of the light aperture receiving the first end of the light channeling tube. In some embodiments slip joint engagements are provided between light apertures of the front tube sheet and first ends of all but a sub-set of the light channeling tubes, and welds are provided between light apertures and the first ends of the sub-set of the light channeling tubes. In some such embodiments, the sub-set of the light channeling tubes are immediately neighboring light channeling tubes engaging light apertures that are centrally located on the front tube sheet. In some embodiments the second ends of the light channeling tubes are connected with the back plate by threaded studs extending from the second ends of the light channeling tubes. In some embodiments a module support post extends away from the back plate on the opposite side of the back plate from the front tube sheet, and is secured at a center of the back plate. [0006] In some aspects disclosed herein, a solar receiver comprises a plurality of solar receiver modules as set forth in the immediately preceding paragraph arranged with adjoining front tube sheets and adjoining back plates to define the solar receiver with an outward facing surface defined by the adjoining front tube sheets and an inward facing surface defined by the adjoining back plates and further having an annular gap between the outward facing surface and the inward facing surface. In some solar receiver embodiments the solar receiver does not include sealing material interposed between the adjoining front tube sheets. In some solar receiver embodiments, the front tube sheets have jagged edges defined by peripheral light apertures and the jagged edges of adjoining front tube sheets mesh together.

[0007] In further aspects disclosed herein, a solar power generation system includes a solar receiver as set forth in the immediately preceding paragraph, a flowing or fluidized heat transfer medium disposed in the annular gap over exterior surfaces of the light channeling tubes of the solar receiver modules, and a fluidized-bed heat exchanger arranged to receive heated heat transfer medium from the solar receiver. In still further aspects disclosed herein, a method of operating a solar receiver as set forth in the immediately preceding paragraph is disclosed. The method comprises disposing a flowing or fluidized heat transfer medium in the annular gap of the solar receiver over exterior surfaces of the light channeling tubes of the solar receiver modules, and operating heliostats to concentrate solar energy onto the solar receiver wherein the concentrated solar energy is effective to induce thermal expansion of the solar receiver modules. In such a method, slip joint engagements between light apertures of the front tube sheet and first ends of most or all of the light channeling tubes suitably accommodates thermal expansion of the solar receiver modules. Additionally, the central rear support of the tube modules allows the modules to thermally expand into one another creating sealing at the front face between adjoining tube modules.

[0008] These and other non-limiting aspects and/or objects of the disclosure are more particularly described below. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The invention may take form in various components and arrangements of components, and in various process operations and arrangements of process operations. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. This disclosure includes the following drawings.

[0010] FIGURE 1 diagrammatically shows an illustrative solar concentrator system, with enlarged detail drawings of the solar receiver shown in insets identified by block arrows.

[0011] FIGURE 2 diagrammatically shows a side view of one solar receiver module of the solar receiver of FIGURE 1 , with the flow path of the flowing heat transfer medium indicated diagrammatically by a shaded block arrow.

[0012] FIGURES 3-6 diagrammatically illustrate aspects of a suitable assembly of a solar receiver module of the solar receiver of FIGURE 1.

[0013] FIGURE 7 shows an enlarged view of one light channeling tube of the solar receiver of FIGURE 1 .

[0014] FIGURE 8 shows a perspective view of the front tube sheet of the solar receiver of FIGURE 1 , with the inner perimeter of the necked-down portion of an aperture shown in an inset.

[0015] FIGURE 9 diagrammatically shows thermal expansion experienced by a solar receiver module as it is brought into operation.

[0016] FIGURES 10-13 diagrammatically illustrate aspects of a suitable assembly of adjacent solar receiver modules to construct the solar receiver of FIGURE 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] A more complete understanding of the processes and apparatuses disclosed herein can be obtained by reference to the accompanying drawings. These figures are merely schematic representations based on convenience and the ease of demonstrating the existing art and/or the present development, and are, therefore, not intended to indicate relative size and dimensions of the assemblies or components thereof.

[0018] Although specific terms are used in the following description for the sake of clarity, these terms are intended to refer only to the particular structure of the embodiments selected for illustration in the drawings, and are not intended to define or limit the scope of the disclosure. In the drawings and the following description below, it is to be understood that like numeric designations refer to components of like function.

[0019] The singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise.

[0020] A value modified by a term or terms, such as "about" and "substantially," may not be limited to the precise value specified.

[0021] It should be noted that many of the terms used herein are relative terms. For example, the terms "interior", "exterior", "inward", and "outward" are relative to a center, and should not be construed as requiring a particular orientation or location of the structure.

[0022] The terms "horizontal" and "vertical" are used to indicate direction relative to an absolute reference, i.e. ground level. However, these terms should not be construed to require structures to be absolutely parallel or absolutely perpendicular to each other. For example, a first vertical structure and a second vertical structure are not necessarily parallel to each other.

[0023] The term "plane" is used herein to refer generally to a common level, and should be construed as referring to a volume, not as a flat surface.

[0024] To the extent that explanations of certain terminology or principles of the solar receiver, boiler and/or steam generator arts may be necessary to understand the present disclosure, the reader is referred to Steam/its generation and use, 40th Edition, Stultz and Kitto, Eds., Copyright 1992, The Babcock & Wilcox Company, and to Steam/its generation and use, 41 st Edition, Kitto and Stultz, Eds., Copyright 2005, The Babcock & Wilcox Company, the texts of which are hereby incorporated by reference as though fully set forth herein.

[0025] With reference to FIGURE 1 , a solar concentrator power generation plant includes a field of heliostats 10 disposed over an area 12 occupied by the plant. The heliostats 10 are diagrammatically represented in FIGURE 1 , and typically include suitable components (not shown) such as beam-forming optics typically comprising mirrors or other reflectors and beam-steering apparatus such as multi-axis motorized system that cooperate to capture solar radiation impinging upon the heliostat and form the light into energy beams 14 that are directed to a solar receiver 16, with the multi-axis motorized systems of the heliostats 10 operating to track the (apparent) movement of the sun across the sky to keep the light beams 14 directed to the solar receiver 16 over the course of the day. (As used herein, the terms such as "light", "solar radiation", and "solar energy" are used interchangeably, and encompass all energy from the sun that is captured and concentrated by the heliostats 10 and/or other referenced system components whether such solar energy is in the form of visible light, infrared light, or ultraviolet light. In the case of components that are heated by solar radiation, the term "energy" or "solar energy" encompasses energy in the form of heat so generated.) In the illustrated configuration, the solar receiver 16 is mounted in an elevated position on a tower 18 so as to provide an unimpeded direct line-of-sight between each heliostat 10 in the field and the solar receiver 16. However, other physical arrangements are contemplated - for example, a tower could include a top-mounted mirror system that directs light downward to a solar receiver located at (or even below) ground level (variants not illustrated).

[0026] With continuing reference to FIGURE 1 including its insets, and with further reference to FIGURE 2, the solar receiver 16 comprises an assembly of solar receiver tube modules 20, one of which is shown in side view in FIGURE 2, where it is seen that the solar receiver tube module 20 includes a front tube sheet 22 defined by adjoining light apertures 24, light channeling tubes 26 connected with respective apertures 24 and extending into the receiver tube module 20, and a back plate 28 to which "rear" ends of the light channeling tubes 26 are connected. By way of non-limiting illustration, the apertures 24 and light channeling tubes 26 may comprise bent sheet metal (e.g. sheet steel) components, drawn tubes, or so forth, and the back plate 28 may comprise a metal plate (e.g. steel plate). For example, the light channeling tubes can be manufactured by a drawing process or by stamping two halves and welding with two longitudinal seam welds. As used herein, the following orientation terms are used: the "front" side of the solar receiver module 20 is faced by the front tube sheet 22 where light enters, while the "back" side of the solar receiver module 20 is the side faced by the back plate 28. The light channeling tubes 26 are in an approximately horizontal orientation, although some tilting of the light channeling tubes 26 is contemplated, for example a downward tilt of a few degrees to a few tens of degrees to up to about 45° is contemplated to better align with the upward angle of light beams 14 coming from the heliostats 10. A module support post 30 (for example, a metal pipe or rod) connects with the back plate 28 and extends rearward. Optionally, insulation and/or lagging 32 is provided on the back side of the module 20, for example contacting the back plate 28, to reduce heat loss from the back side of the module 20.

[0027] With reference back to the insets of FIGURE 1 , solar receiver modules 20 are assembled to form the solar receiver 16 as a hollow cylindrical structure having an outward-facing (faceted) cylindrical surface 42 defined by adjoining planar front tube sheets 22 of the adjoining solar receiver modules 20, and an inward-facing (faceted) cylindrical surface 48 defined by adjoining planar back plates 28 of the adjoining solar receiver modules 20. The two cylindrical surfaces 42, 48 define an annular gap 50, which gap 50 is also indicated for the single solar receiver module 20 depicted in FIGURE 2. It will be appreciated from FIGURE 2 that the light channeling tubes 26 of the solar receiver modules extend through the annular gap 50 between the two cylindrical surfaces 42, 48, and that the light beams 14 suitably input light into the light channeling tubes 26 through the apertures 24 in the outward-facing cylindrical surface 42. The solar receiver modules 20 are suitably supported by their respective module support posts 30 on a truss or other support structure 52 secured to the tower 18.

[0028] With continuing reference to FIGURES 1 and 2, a flowing heat transfer medium 56 (diagrammatically indicated in FIGURE 2 by a downward shaded block arrow) flows generally downward through the gap 50 under force of gravity. Additionally or alternatively, the flowing heat transfer medium can be fluidized with compressed air, which may improve contact between the heat transfer medium and tube surfaces to enhance heat transfer. The flowing heat transfer medium 56 thus flows over the exterior surfaces of the light channeling tubes 26. These tubes 26 are preferably constructed so as to absorb most of the channeled light, for example by including suitable absorbing coatings on their inside surfaces, optional faceting to cause light to scatter within the light channeling tubes 26 so as to increase the number of opportunities for absorption, or so forth. Thus, the solar energy channeled through the light channeling tubes 26 is transferred into heat absorbed by the tubes 26 and then transferred to the flowing heat transfer medium 56. In this manner, solar energy contained in the light beams 14 is converted to heat energy contained in the flowing heat transfer medium 56, and thus the solar energy is contained as heat in the flowing heat transfer medium 56 that exits the bottom of the solar receiver 16.

[0029] With particular reference back to FIGURE 1 , the heated flowing heat transfer medium 56 exiting the bottom of the solar receiver 16 may be variously used. In the illustrative example, the heated flowing heat transfer medium 56 exiting the bottom of the solar receiver 16 feeds into a storage silo 57 and fluidized-bed boiler or heat exchanger 58 shown diagrammatically in FIGURE 1. The storage silo 57 provides thermal storage capability, as hot particles are stored in the silo 57 and may for example be used to provide thermal energy during cloudy days or at night. Thermal storage via the silo 57 decouples energy collection and power production thereby allowing dispatchable, continuous power production. The fluidized-bed boiler or heat exchanger 58 may, for example, comprise a bubbling fluidized bed (BFB) or circulating fluidized bed (CFB) boiler or heat exchanger or so forth, in which the heated flowing heat transfer medium 56 is dispersed on the fluidized bed so as to heat water (to form steam) or other working fluids such as air or supercritical CO ₂ to drive a power cycle and turbine- generator (not shown) to produce electrical power or to perform other useful work.

[0030] The flowing heat transfer medium 56 is typically a flowing particulate medium such as silica sand or calcined flint clay (e.g. with average particle size on the order of a few hundred microns), but is not limited thereto (for example, it is contemplated to employ air as the flowing heat transfer medium). In typical embodiments in which the flowing heat transfer medium 56 is a flowing particulate medium, it is to be understood that this flowing particulate medium serves as the hot "fluid" which is dispersed onto the fluidized bed of the fluidized-bed boiler or heat exchanger 58. Said another way, the term "fluid" as used herein in reference to the flowing heat transfer medium 56 encompasses flowing particulate media. [0031] With reference to FIGURE 1 , the flowing particulate medium is suitably returned to the top of the solar receiver 16 by any suitable elevator structure, for example driven by motors, diesel engines, or so forth. The top inset of FIGURE 1 diagrammatically shows a suitable return structure 60 for this purpose comprising receiver bucket elevators and a solids distribution hopper.

[0032] With reference to FIGURE 2, the front tube sheet 22 and the back plate 28 define the gap 50 of the solar receiver module 20 through which the flowing heat transfer medium 56 flows over the exterior surfaces of the light channeling tubes 26. Thus, the front tube sheet 22 and the back plate 28 should seal against leakage of the flowing heat transfer medium 56 outside of the module 20. Moreover, the connections of the light channeling tubes 26 to the front tube sheet 22 should also seal against leakage of the flowing heat transfer medium 56 into either the apertures 24 or the light channeling tubes 26, since any such leakage will introduce blockage into the light channeling tubes 26 and/or result in egress of the flowing heat transfer medium 56 outside of the solar receiver module 20. Moreover, it will be appreciated that interfaces between adjoining modules 20 should be similarly sealed. That is, the interfaces between adjoining front tube sheets 22 of outward-facing cylindrical surface 42 and the interfaces between adjoining back plates 28 of the inward-facing cylindrical surface 48 should seal against leakage of the flowing heat transfer medium 56, so as to seal against leakage from the annular gap 50 of the solar receiver 16.

[0033] Another consideration is that the solar receiver 16 undergoes substantial thermal cycling during startup, shutdown, cloud transients and emergency trips. In some contemplated embodiments intended to operate a fluidized bed boiler or heat exchanger, the flowing heat transfer medium 56 is to be heated to a temperature of order 800°C (1470°F). Accordingly, the solar receiver 16 should be robust against thermal cycling over a range of 0°C-800°C in some embodiments, and over even larger temperature ranges in other contemplated embodiments.

[0034] With reference to FIGURES 3-8, a suitable approach for assembling the solar receiver module 20 to address these design factors (provide a low stress thermal expansion) is described. FIGURES 3, 5, and 6 illustrate diagrammatic perspective views of assembly operations. FIGURE 4 diagrammatically shows suitable welds used in the operation depicted in FIGURE 3. FIGURE 7 shows an enlarged a diagrammatic side sectional view of one light channeling tube 26 of the assembled solar receiver module. FIGURE 8 shows a diagrammatic perspective view of the front tube sheet 22, with the inner perimeter of the necked-down portion of an aperture shown in an inset.

[0035] FIGURE 3 shows a diagrammatic perspective view of a first assembly operation in which a small number (illustrative four) light channeling tubes 26 are welded to their respective corresponding apertures 24 of the front tube sheet 22. The welded light channeling tubes 26 are optionally chosen to be centrally and adjacently located so as to minimize differential thermal expansion amongst the welded light channeling tubes 26, although other selections of the light channeling tubes 26 to be welded are also suitable. FIGURE 4 illustrates a suitable welding approach for the operation depicted in FIGURE 3. In this approach, each tube to be welded is force-fitted onto or into the mating opening of the front tube sheet 22, and fillet welds 70 are formed by a suitable welding process. Some interior surfaces 72 may not be accessible for welding in the case of immediately neighboring welded tubes as shown in FIGURES 3 and 4 (although some of these surfaces could be accessible for welding if welding is performed before inserting adjacent tubes). In an alternative approach, the welded tubes are chosen to be spaced apart in the array, so that symmetrical and/or additional welds can be made on each tube in order to provide additional support/rigidity of the front tube sheet to resist wind loads for example.

[0036] FIGURE 5 depicts a diagrammatic perspective view of a next operation in which the remaining light channeling tubes 26 are inserted into their corresponding apertures 24, but are not welded. To this end, as seen in FIGURES 7 and 8 the back side of each aperture 24 of the front tube sheet 22 includes a necked-down portion 74 that defines an inside perimeter 76 facing the end of the light channeling tube 26. As seen in FIGURE 7, the outer perimeter of the light channeling tube 26 is sized slightly smaller than the matching inside perimeter 76 of a necked-down portion 74 of the aperture 24, and the insertion of the tube end into the necked-down portion 74 of the aperture 24 forms a slip joint 80 (labeled in FIGURE 7) between the end of the tube 26 and the aperture 24. Other slip joint configurations are contemplated. For example, the tube can have a larger diameter than the necked-down portion of the aperture, in which case the necked down portion of the aperture fits inside the tube end. It is noted here that the light channeling tubes 26 may have various perimeter shapes, such as triangular, circular, square or diamond, regular or flared hexagonal, or so forth. In the illustrative example of FIGURES 3-8, the light channeling tubes 26 have regular hexagonal perimeters and accordingly the inner perimeter 76 of the necked-down portion 74 of the aperture 24 is correspondingly hexagonal, as seen in the inset of FIGURE 8.

[0037] In the illustrative example of FIGURE 8, the apertures 24 also have hexagonal perimeters at the front side of the front tube sheet 22, with the necking of each aperture 24 leading to the light input (i.e. front) side of the aperture 24 being flared outward or approximately conically expanding. This enhances light collection efficiency by the aperture 24. Additionally, by making the apertures 24 with hexagonal perimeters at the front side of the front tube sheet 22, they can form a honeycomb structure as seen in FIGURE 8, which maximizes the collection area of the front tube sheet 22 that collects light into the light channeling tubes 26. Other geometries can be used, such as circular apertures, but such geometries will result in added "dead area" between the circular apertures that does not lead into the light channeling tubes thus reducing the efficiency of the receiver.

[0038] FIGURE 6 depicts a next operation in which the back plate 28 is attached to the back ends of the light channeling tubes 26 by fasteners 82, which may by way of illustrative example be washer/nut fasteners that connect with threaded studs 84 extending from the back ends of the light channeling tubes 26 (see FIGURE 5). With particular reference to FIGURE 7, in one configuration an end cap 86 including the threaded stub 84 is welded to the back end of the light channeling tubes 26.

[0039] In the solar receiver module 20 fabricated in accord with the process described with reference to FIGURES 3-8, each light channeling tube 26 is supported in cantilevered fashion from the back plate 28 by way of the fastener 82, 84. The front end of the light channeling tube 26 is laterally restrained by the slip joint 80, but is free to expand or contract in the axial direction (that is, in the direction of the tube axis). This allows independent thermal expansion due to tubes operating at different temperatures and thus expanding to different lengths. In the case of the welded light channeling tubes 26 (those of FIGURES 3-4), the thermal expansion of the tubes is not accommodated by a slip joint but rather has the effect of moving the front tube sheet 22 outward (during thermal expansion) or inward (during thermal contraction) in response to heating or cooling, respectively, of the solar receiver module 20. The slip joints of the remaining (large majority) of light channeling tubes accommodates any differences in thermal expansion amongst the array of light channeling tubes 26. The front tube sheet 22 is supported by all the light channeling tubes 26, but is secured in the tube-axial direction only by the welded tubes of FIGURES 3-4.

[0040] In an alternative embodiment, it is contemplated to employ separate designated tie rods (not shown) welded between the back plate 28 and the front tube sheet 22 to provide the tube-axial direction support, rather than obtaining this axial support by welding designated light channeling tubes 26 as in the operation of FIGURES 3-4. This alternative approach employing tie rods has the advantage of employing the slip joints 74 for all of the light channeling tubes 26 (as none are welded to the front tube sheet 22 in this alternative embodiment), but perhaps at the cost that some light receiving area is lost (i.e. select tubes removed to make room for tie rods) to accommodate the designated tie rods. Additionally, the impact of the tie rods on the flow paths of the flowing heat transfer medium 56 should be taken into account in performing numerical thermal analyses.

[0041] With particular reference to FIGURES 7 and 8, the front tube sheet 22 provides the front seal between tubes within a module for the gap 50 (see FIGURE 2). To this end, the hexagonal aperture perimeters at the front side of the front tube sheet 22 which form the honeycomb structure seen in FIGURE 8 are joined to create the seal. In one contemplated approach, the front tube sheet 22 is formed by punching hexagonal openings corresponding to the apertures 24 into a single metal sheet (e.g. single steel sheet) and then working the openings using sheet metal forming tooling to define the necked-down portion 74.

[0042] With reference to FIGURE 7, in another contemplated approach, each hexagonal aperture 24 is separately formed from sheet metal, with the six sides of the hexagonal perimeter being flat sides 87. The flat sides 87 of each pair of adjoining apertures are then welded together to assemble the front tube sheet 22. Other approaches are contemplated. For example, in a variant approach the sides are flat on one to four of the segments of the hexagonal perimeter around the perimeter of the module and straight on the remaining segments where two apertures adjoin. In this case, only the edges of each adjoining apertures are welded together to assemble the front tube sheet 22. As yet another variant, it is contemplated to employ separate or integral tube cap pieces that are welded to the back end of the tube or formed as part of the tube to facilitate making good seals.

[0043] The illustrative solar receiver module assembly approach described with reference to FIGURES 3-8 addresses both flow sealing and thermal expansion design considerations within the module.

[0044] With reference to FIGURES 9-13, approaches are described for assembling the solar receiver modules 20 to form the overall solar receiver 16 while addressing both flow sealing and thermal expansion design considerations between adjoining modules. FIGURE 9 illustrates a diagrammatic perspective view of a solar receiver module 20 in isolation, with thermal expansion directions indicated diagrammatically by arrows. (Note that in FIGURE 9 the apertures are omitted in depicting the front tube sheet 22). FIGURE 10 illustrates a diagrammatic side sectional view of a portion of the solar receiver 16 including four solar receiver modules 20 mounted by their respective module support posts 30 to a support column 88, for example using bolted connections or other fastening methods. The support column 88 may, for example be a vertical column of the truss or other support structure 52 (see FIGURE 1 ). Because each module is mounted via the module support post 30 which is centrally located (see FIGURES 2 and 10), the thermal expansion of the module in the height and width direction is reduced compared to a top, bottom, or side supported module during heating of the solar receiver module 20 (FIGURE 9), which makes it easier to seal between adjoining modules because they thermally expand from their central support point. Individual module support posts also eliminate the need for a buckstay system of the type employed with top supported modules to resist wind and earthquake loads.

[0045] In addition, this approach enables the use of smaller tube modules, which results in smaller solar heat flux gradients across the face of the module, more uniform operating metal temperatures, lower thermal stresses and reduced potential for thermal distortion compared to larger modules with larger face areas.

[0046] With reference to FIGURE 1 1 which shows a diagrammatic perspective view of the front tube sheet 22 and light channeling tubes 26 of five modules 20, the hexagonal apertures 24 of neighboring modules are arranged so that jagged edges 90 of adjoining solar receiver modules 20 mesh together, that is, with "slots" of one jagged edge receiving "bumps" of the other jagged edge. The adjoining modules 20 are designed to fit snugly together at non-operational temperature (typically room temperature) to form a seal without employing a sealant material. When brought up to operating temperature, the isotropic thermal expansion of the modules 20 about their respective centrally located module support posts 30 causes the snug fit to become tighter at operational temperature. The aperture perimeters at the jagged edges 90 include the flat sides to increase contact area to create a seal.

[0047] While using the foregoing approaches is expected to provide suitable sealing between adjoining modules 20 at their front sides, it is additionally or alternatively contemplated to employ a high temperature sealant material. However, interposing sealing material between the modules reduces the active area and thus efficiency of the solar receiver 16 for collecting and channeling light.

[0048] With reference to FIGURES 12 and 13 which show diagrammatic perspective and diagrammatic back views, respectively, of a portion of the solar receiver 16 including a 4x2 array of modules 20, the sealing between adjoining modules 20 at the back side is also required. In illustrative FIGURES 12 and 13 the back plates 28 include contoured edges 92 (e.g. sawtooth at top and bottom and pumpkin tooth edges at sides to match the contour of the tubes) that mesh together but with clearance in the non-operating condition to allow thermal expansion and prevent interference of back plates 28 between adjoining modules in the operating condition. As shown in FIGURE 13, the gap between back plates 28 along the edges and corners are then sealed with seal strips 94 and corner seal plates 96 respectively that are bolted to one but not both adjoining modules as shown to allow free thermal expansion of the back plates 28. Unlike the case for the front side, however, there is no concern about additional sealing features blocking light at the back side. [0049] An advantage of the disclosed solar receiver modules 20 is that individual modules are readily removed for repair or replacement. At non-operating temperature the modules 20 have thermally contracted to their smallest configuration, and an individual module can be removed by disconnecting its module support post 30 from the column 88 (see FIGURE 10, or more generally by disconnecting the support post from the module back plate) and pulling the module out. This is facilitated if no sealing material is employed at interfaces between adjoining modules. To facilitate such removal, in the embodiment of FIGURE 13 each sealing strip 94 is removable and is attached by fasteners to only one of the two adjoining modules 20, and at meeting corners a removable corner cover sealing plate 96 is attached to one of the sealing strips 94. This reduces the number of sealing strips that must be removed to pull out an individual module. Additionally, by securing each sealing element 94, 96 to only one module, a slip joint is effectively formed so that thermal expansion is accommodated. Repair and replacement of individual light channeling tubes 26 is similarly simplified by the disclosed assembly. To replace a light channeling tube 26, the center support tubes are unbolted from the back plate. These are the light channeling tubes that are welded to the front tube sheet 22 as described with reference to FIGURES 3 and 4. (In alternative embodiments in which separate tie rods are employed, these are unbolted from the back plate). The front tube sheet 22 is thereby released so that the slip fitted tubes 26 can unbolted and removed and replaced. To replace one of the support tubes which is welded to the front tube sheet 22, some of the surrounding support tubes may need to be removed to gain access to break the welds to the tube sheet.

[0050] In similar fashion, it is contemplated to modularize the insulation and/or lagging 32 (FIGURE 10) on the back side of the module 20, for example by employing a piece of insulation/lagging for each module, or for each set of four neighboring modules, or so forth. The insulation/lagging 32 does not perform any sealing function, and may for example be blanket-type insulation with ship lapped edges to reduce heat loss.

[0051] Illustrative embodiments including the preferred embodiments have been described. While specific embodiments have been shown and described in detail to illustrate the application and principles of the invention and methods, it will be understood that it is not intended that the present invention be limited thereto and that the invention may be embodied otherwise without departing from such principles. In some embodiments of the invention, certain features of the invention may sometimes be used to advantage without a corresponding use of the other features. Accordingly, all such changes and embodiments properly fall within the scope of the following claims. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the present disclosure be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.