TECHNICAL FIELD
The present invention relates to the fields of solar energy conversion and in particular to devices for collecting solar energy as heat and storing heat whereby its use is not directly linked to the availability of sunlight.
BACKGROUND OF THE INVENTION
Worldwide there is an increasing awareness of the need to reduce reliance on fossil fuels and increase the use of renewable energy sources. One major renewable energy source that is effectively unlimited in the foreseeable future is solar energy, however solar energy has the disadvantage that it is not available at night and during cloudy periods and so conversion systems need to include some form of energy storage if they are to become a viable replacement for fossil fuel as a source of energy.
Existing solar energy conversion systems fall into several categories:—
1) Photovoltaic (PV) systems, in which solar energy is absorbed into materials that convert the solar energy directly into electricity;
2) Concentrating Solar Power (CSP), in which solar energy is used to heat a fluid and that heated fluid is used to directly or indirectly drive a mechanical device (such as a turbine) to convert the heat energy into electrical energy.
To enable solar radiation to be used as heat for a thermodynamic cycle to produce process steam or electricity, it must be first concentrated to achieve higher temperatures, as solar radiation reaches the earth at a density too low to directly produce such temperatures. A variety of technologies are being developed for use in CSP Systems including:—
Trough and “Fresnel” type linear collector systems, which comprise an elongate reflector, and one collector tube or assembly of tubes running along the focal point of the reflector. The tube(s) contains a fluid which is heated and then pumped to a heat engine (e.g. a turbine);
Tower systems which collect solar energy concentrated to a target from a large number of mirrors which track the sun and focus the large number of images at one collection point (heliostats), where the high temperatures achieved are used to heat a fluid which is transmitted to a heat engine (e.g. a turbine). Tower systems may include single towers and multiple tower arrangements;
Dish/Engine systems, where a small heat engine is placed at the focal point of a parabolic dish and driven directly by the concentrated solar energy.
Within the category of tower systems one arrangement that has shown promise is the use of a graphite body as a solar receiver in which heat exchanger tubes are embedded where the heat exchange fluid used to drive an engine such as a turbine is directly heated by heat energy stored in the graphite body. Such systems are in their infancy and existing configurations have a variety of disadvantages typical of early stage technologies including high cost of manufacture and structural integrity problems. In particular, present designs (or at least those which have been shown to be practical) comprise a gas tight containment housing which encompasses most of the remainder of the assembly and this design places constraints on the manufacture of the receiver such as:—
- If receivers are to be of a size that achieves reasonable efficiency and cost effectiveness, they must be fully assembled and tested at or near site, typically from outsourced sub-assemblies. Due to the gas tight nature of the housing this assembly operation is not trivial and adds considerably to the expense of manufacture;
- If assembly were to be contemplated at a site remote from the installation site, overall dimensions of receivers are constrained by road transport limitations and even with onsite assembly there is a limitation on the size of subassemblies that can be readily transported;
- Graphite and heat exchanger piping costs amount to only 45% of total cost of the receiver. Other major costs relate to the containment housing and other structural items 16%, insulation 10% (of which 7% is for the shield protecting the base structure);
- The design requires many sub-assemblies and the resultant supply chain has many vendors;
- The need to ship unassembled parts and to assemble the parts on site makes it expensive. Alternatively, shipping assembled units would be difficult, if not impossible and prohibitive in cost;
- The present receiver designs have limited scalability options without total redesign and so the external dimensions, graphite mass and potential power handling capacity are effectively fixed;
- Preparation of the receiver at site before installation on the tower is time consuming;
- Dimensional constraints imposed on the receiver design by transport limitations restrict utilization of graphite in previous designs to at best 70% to 75% because the constraints on the design do not allow maximized usage of the standard manufactured sizes of graphite block.
Throughout this specification, unless otherwise specified, panels of solar receivers will be described in a vertical orientation with vertical side walls at least one of which is a solar energy receiving wall. The panels and their components will be described as having a top and a bottom and two ends relative to the vertical side walls, and will include a top and bottom walls, and end walls, however the panels may be used in other orientations in which, for example a horizontal orientation in which the side wall may be at the top and a top wall may be at the side.
SUMMARY
According to one aspect, the present invention consists in a solar energy receiver comprising a panel, the panel comprising a graphite core, a substantially gas tight housing encasing the graphite core, a heat exchanger comprising heat exchanger tubing including a heat exchanger inlet and a heat exchanger outlet, the heat exchanger tubing at least partially embedded in the graphite core, the heat exchanger inlet and the heat exchanger outlet extending through the housing and the housing sealed around the heat exchanger inlet and the heat exchanger outlet.
According to a second aspect, the present invention consists in method of manufacturing a solar energy receiver comprising a panel, the panel comprising a graphite core, a substantially gas tight housing encasing the graphite core, a heat exchanger comprising heat exchanger tubing including a heat exchanger inlet and a heat exchanger outlet the heat exchanger tubing at least partially embedded in the graphite core, the heat exchanger inlet and the heat exchanger outlet extending through the housing and the housing sealed around the heat exchanger inlet and the heat exchanger outlet, the method comprising:
- a) fabricating the heat exchanger in a serpentine coil shape;
- b) inserting grooved planks of graphite between individual coils of the heat exchanger to form the graphite core such that the coils are encompassed in the grooves;
- c) inserting the graphite and heat exchanger into the housing; and
- d) sealing the housing.
The heat exchanger may further comprise a heat exchanger drain which also extends through the housing and the housing is sealed around the heat exchanger drain.
The heat exchanger drain may also act as an inlet/outlet such that only one other inlet/outlet is required to pass through the housing wall. The working fluid may pass between the inlet/outlets in either direction depending on the location of the panel in the solar receiver installation:
The drain may be configured to also be used as the inlet to the heat exchanger and the outlet will be located at the top of the receiver panel such that flow of working fluid through the panel is from bottom to top through the panel.
The housing may have two spaced apart side walls joined together about their periphery by one or more additional walls to form a closed container. One of, or a portion of the one or more additional walls is a bottom wall forming a base of the housing and in one embodiment the graphite core will be located in thermal communication with the base and at least one of the two side walls of the housing. At least 2 walls of the housing may be in thermal communication with the graphite core. The bottom wall of the housing may also be formed by bending a single piece of wall material into a “U” shape having curved bends in which the bottom wall transitions into each of the side walls to which it is connected via one of the curved bends. The curved bends at the edges of the bottom wall will reduce stresses in the housing wall allowing the use of lighter wall construction and eliminating the need for further structural support in the base wall. The walls of the housing may be fabricated from 253MA austenitic stainless steel or any other high temperature thermally conductive material (e.g. 800H or Inconel alloys) finished to mill finish class 2B. Depending upon the location of the surfaces within the final receiver configuration and the geographic location of the installation, some surfaces may be provided with a specific thermal emittance while others may be provided with a specific thermal absorptance to enhance performance. Surface treatments or surface coatings may be applied to achieve specific emissivity in the range of 0.2-1.0. For example, if some surfaces are required to be emittive they may be left natural (specific emissivity 0.7) may be polished (specific emissivity 0.2-0.3), or may be coated with or surface treated to achieve a specific emissivity in the range of 0.3-0.8 while other surfaces which are required to be highly thermally absorptive may be coated with or surface treated to achieve a highly heat absorbing surface (such as a black surface with a specific absorptivity of 0.8-1.0, preferably 0.9-1.0).
The other walls of the housing may also comprise a top wall opposite the bottom wall, and two end walls. The housing may also include a plurality of mounting flanges extending from the housing and capable of suspending and supporting the weight of the receiver element. The mounting flanges may extend from joins between adjoining walls of housing and may include holes for attachment to a mounting frame. For example the flanges may extend from joins between the side walls and the end walls of the housing. Each mounting flange may comprise an extension of one of the end walls beyond the respective side wall to which it is joined. Alternatively each mounting flange may comprise an extension of one of the end walls beyond the top wall. The mounting flanges may extend from an end wall that in use is typically oriented vertically. By suspending the receiver element rather than supporting it from below, the resulting tension in the side walls due to gravity of the graphite core acting on the housing allows them to resist buckling to maintain good thermal communication with the graphite core. The shape of the housing also tends to keep the metal walls pressed against the graphite core. In addition, by suspending the receiver from above, the base of the housing is exposed to, and can absorb, solar irradiation which would otherwise be reflected by shielding tiles used in prior art designs to protect the base structure from overheating.
The graphite core may be shaped to conform to the internal shape of the housing and in particular has a portion shaped to conform to the shape of the curved bends of the bottom wall of the housing. The graphite core may comprise a plurality of stacked graphite planks, at least a lower one of which is profiled to match the shape of the curved transitions between the base and the lower portions of the side walls.
Graphite cores in prior art receivers were surrounded by insulation (except for the energy receiving surfaces) and the core and insulation were housed in an inert gas environment to prevent chemical reaction of the core. The inert gas was generally maintained at a positive pressure to prevent leakage of air into the complex housing structure. In contrast, embodiments of the present receiver have a simplified housing which has greater structural integrity. There is also no internal insulation within the housing and the graphite core conforms to the inner shape of the housing. Therefore the amount of space left in the housing is quite small after the graphite and heat exchanger are inserted and the housing is sealed, and it is possible to leave this remaining space filled with air. On the first heating of the receiver element a small amount of graphite will oxidize until the oxygen in the air is consumed, leaving the spaces substantially filled with nitrogen and carbon dioxide protecting the graphite from further oxidation when exposed to high temperatures in subsequent thermal cycles. However if the operating temperature of the panel is to exceed 700° C. a reduction reaction may occur causing the carbon dioxide to reduce to carbon monoxide with further carbon being consumed by the liberated oxygen. Subsequent cooling can lead to some of the carbon monoxide decomposing to carbon and oxygen which again forms carbon dioxide. This can lead to deterioration of the physical structure of the graphite over time. Therefore if the operating temperature of the panels is expected to exceed 700° C. in a particular installation, it will be desirable at manufacture to replace the air filling the spaces in the panel between the graphite and the walls with an inert gas such as argon or helium. Alternatively the spaces in the panel may be filled with thermally conductive material that exists in a solid, liquid or gaseous state at least in the working temperature range of the panel, such as tin, zinc, mercury, or a molten salt such as potassium nitrate, potassium nitrite, sodium nitrate, sodium nitrite, other nitrate, nitrite, chloride or fluoride salts or a mixture of such salts or graphite powder. A reservoir containing graphite powder may be located in communication with the interior of the housing, whereby graphite powder is supplied from the reservoir to the interior of the housing to fill additional void space created by expansion of the housing.
The points where the heat exchanger inlet and heat exchange outlet pass through the housing may be in close proximity and may be at one end of the top wall of the housing, to assist with mounting and manifolding the pipes with other receivers. Alternatively, the drain, which is located at the lowest point of the heat exchanger, may double as one of the inlet/outlets, in which case only one inlet/outlet is required to be provided at the top of the heat exchanger.
At least some of the heat exchanger tubes may be fabricated in a coiled or serpentine form suitable for compression (like a spring) during assembly, such that when the container expands due to thermal expansion, the resulting stresses from the movement of the pipe configuration do not exceed the mechanical properties of the pipe material.
The heat exchanger coils may comprise a plurality of straight tube portions arranged to be parallel to each other and connected at their ends to form one or more serpentine or coil shapes. In some embodiments the straight tube portions are arranged in parallel planes forming rows of straight tube portions. The straight tube portions may be arranged in coils where rows of straight tube portions are interconnected at their ends and each row is connected to the row above and below to form a single coil structure. Alternatively the straight tube portions may each be connected at respective ends to a straight tube portion above and below to form parallel serpentine constructions. Where the straight tube portions form parts of coils there may be an even number of straight tube portions (such as two straight tube portions) in each row. The straight tube portions in each row may be aligned with the straight tube portions in adjacent rows such that they also form planes perpendicular to the first mentioned parallel planes. At a first end of the heat exchanger (which will become the insertion end for subsequent insertion of graphite planks) the straight tube portions in each row may be connected in pairs by first U-shaped connecting tube portions. At a second end of the heat exchanger (the non-insertion end), the straight tube portions in each row may be connected to straight tube portions in each of the two adjacent rows by second U-shaped connecting tube portions. In the case where there are two straight tube portions per row, the two straight tube portions in each row may therefore be connected together at the first (insertion) end and each of the two straight tube portions in each row are respectively connected to straight tube portions of each of two adjacent rows at their second (non-insertion) end. A heat exchanger inlet tube and a heat exchanger outlet tube may be connected to one of the straight tubes in each of top row of straight tube portions and bottom row of straight tube portions via interconnecting tube portions.
Where the straight tube portions form serpentine constructions, there may be an even number or an odd number of straight tube portions (such as two or three straight tube portions) in each row. The straight tube portions in each row may be aligned with the straight tube portions in adjacent rows such that they also form planes perpendicular to the first mentioned parallel planes. At a first end of the heat exchanger each of the straight tube portions in each row may be connected by first U-shaped connecting tube portions to straight tube portions in the row below. At a second end of the heat exchanger, each of the straight tube portions in each row may be connected to straight tube portions in row above by second U-shaped connecting tube portions. In this arrangement the graphite planks are inserted from alternate ends of the serpentine structure such that the planks are always inserted between two rows of straight tube portions at an end opposite the end at which those two rows are interconnected.
The configuration of the heat exchanger tubing and drain may be arranged to allow drainage of liquid from top to bottom of the heat exchanger both when the heat exchanger is in a vertical orientation (i.e. where the are coils stacked vertically above one another) and when the heat exchanger is angled from the vertical orientation (with the mounting points on the upper side) as when the panels are configured in an inverted “V” configuration. In one embodiment where the straight tube portions form a coil structure, the heat exchange may be angled at an angle of up to 21°, however this angle is dictated by the angle of the second “U” shaped connecting tube portions which interconnect straight tube portions of different rows of straight tube portions and may be varied depending on the angle of interconnection of adjacent rows of straight tube portions. The angle by which the heat exchanger deviates from the vertical orientation should not exceed the angle of the second “U” shaped connecting tube portions (with respect to the plane of a row of straight tube portions), such that condensed liquid in the heat exchanger is not required to flow up hill to reach the drain. In the serpentine structure the heat exchanger may be readily angled at angles of up to 45° and possibly even approaching 90° to the vertical.
After the heat exchanger, is fabricated, pre-shaped planks of graphite are positioned to be located between each row of tubes and capping planks are placed over the inlet end rows of straight tube portions and the outlet end row of straight tube portions. The abutting surfaces of the graphite planks may have a surface finish which is N8 or better (ISO 1302). The graphite planks (excluding the capping planks) each include two grooved surfaces, on opposite surfaces thereof, where the grooves may be semi-circular in cross-section conforming to the shape and radius of the straight tube portions and interconnecting tube portions at the first (insertion) end of the heat exchanger when the tube portions and the surrounding graphite are at their working temperature such that when assembled between rows of straight tube portions adjacent pairs of the planks encompass and closely conform to the respective straight tube portions and first connecting tube portions. To achieve close conformity of the heat exchanger tubes with the grooves in the graphite at the internal working temperature of the panel, which is up to 800° C., the grooves are made approximately 1.6% bigger than the nominal outside diameter of the tubes to allow for the radial expansion of the tube at working temperature with a tolerance of approximately +0.00/−1.00%. For example, when the heat exchanger tubes (made for example from 253MA austenitic stainless steel, or any other suitable high temperature thermally conductive material like 800H austenitic steel or alloys such as Inconel) have a nominal outside diameter of 26.67 mm the grooves will be 27.1 mm (+0.00/−0.25 mm) in diameter. Alternatively when the heat exchanger tubes made from the same or a similar material have a nominal outside diameter of 42.16 mm the grooves will be 42.9 mm (+0.00/−0.25 mm) in diameter. The smoothness of the surface of the grooves will have a bearing on heat transfer with smooth surfaced grooves having a higher contact surface than rough surface grooves, however the smoother the surface the more expensive the cost of finishing the grooves.
The surface within the grooves may have a surface finish which is N7 or better (ISO 1302). Rather than being semicircular, the grooves may also be a half obround shape with a radius which is slightly greater (by about 1.6% with a straight section about 1.6% of the radius in the direction perpendicular to (i.e. across) the parallel groove, to accommodate lateral movement of the tube when the coils expand. However this has the disadvantage that the tubes will not be as closely encompassed in the grooves and in some cases it may be preferable to accommodate expansion of the coils by other means such as by allowing them to expand into the cavity which accommodates the second connecting portions.
At the second (non-insertion) end of the heat exchanger, ends of the graphite planks are recessed to accommodate the second connecting tube portions joining straight tube portions from adjacent rows of straight tube portions.
Capping planks are provided at either end of the stack of graphite planks. A lower capping plank is grooved on one surface facing an adjacent graphite plank the grooves conforming to the shape and radius of the straight tube portions and interconnecting tube portions at the first (insertion) end of the heat exchanger. Edges of the lower capping plank between the face opposite the adjacent graphite plank and the sides of the lower capping plank are radiused. An upper capping plank is preferably recessed on a surface facing an adjacent graphite plank to accommodate inlet and outlet tubing without constraining thermal expansion thereof. As an inlet and outlet of the heat exchanger are fixed to the top wall of the housing where they pass through the housing, the recess in the upper capping plank accommodates longitudinal expansion of the tubes as well as allowing the coils, to separate or compress with differential movement between the graphite and the housing as the housing expands and contracts with heating and cooling between ambient and its upper working temperature, which can be as high as 1000° C. In the present embodiment the volume of void spaces within the housing not occupied by graphite or tubing is generally in the range of 4-10% and typically 5-7% of the internal volume of the housing (at the working temperature). Correspondingly the side panel of the housing, which is the irradiated surface of the panel when in use, is generally backed by the graphite core over all but 1-5% of its area and typically 2-3% (at the working temperature) in the preferred embodiment.
The first (insertion) ends of the straight tube portions may be sprung apart slightly to allow the planks to be easily inserted into the fabricated heat exchanger tubing past the first connecting tube portions and between adjacent rows of coils. Alternatively during fabrication of the heat exchanger coils, they may be spaced by a spacing greater than or equal to a plank thickness of the graphite planks between which they are located in the final assembly (or at least the first (non-insertion) ends may be so spaced), such that the planks may be easily inserted into the fabricated heat exchanger tubing between adjacent rows of coils and the coils of the tubing may then be compressed into contact with the graphite after insertion of the graphite planks between the coils.
A solar energy receiver may comprise two or more receiver panels configured and mounted to form a downward opening cavity. The cavity may be formed with a combination of receiver panels and insulation panels. Outside surfaces of the receiver panels forming the solar energy receiver may also be covered by insulation.
The solar energy receiver may comprise a plurality of receiver panels arranged to form an opening, which is in a shape of a rectangular prism. The top of the rectangular prism opening may be closed by one or more additional receiver panels or may be closed by insulation on transparent panels such as fused quartz. An underside of the insulation closing the top of the rectangular prism shaped opening may have a high emissivity surface facing into the opening.
Alternatively the solar energy receiver may comprise a plurality of receiver panels arranged in an inverted “V” configuration to form an opening which is in a shape of a triangular prism (with the lower parallelogram side horizontal). The ends of the triangular prism shaped opening may be closed by further receiver panels or may be closed by insulation panels or panels of optically transparent material such as fused quartz which can pass solar energy directed from the heliostats to the skies of the receiver while confining convection within the opening and can withstand the high temperatures created by absorption Of the solar energy. Again, an inside surface of the insulation closing the ends of the triangular prism shaped opening may be a high emissivity surface facing into the opening.
Outer surfaces of the receiver panels of the solar energy receiver are preferably covered with insulation, which may comprise prefabricated insulation panels, to prevent heat loss from the outer surfaces by radiation or conduction.
The rectangular or “V” shaped openings in the bottom of panel assemblies may be partially closed by insulating tiles or fused quartz panels (not illustrated) to restrict heat loss by convection while leaving smaller apertures through which solar energy may be directed from the heliostats.
The solar energy receiver may be mounted suspended from a tower and energy is directed at the openings in the receiver by heliostats located around a base of the tower. The solar energy receiver may be mounted on a side of the tower facing away from the equator and the heliostats may be located predominantly on the opposite side of the tower to the equator although an East-West orientation is also possible.
BRIEF DESCRIPTION OF THE DRAWINGS
Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:
FIG. 1 shows a Concentrating Solar Power (CSP) installation, including a thermal storage receiver mounted on a tower in a heliostat field;
FIG. 2 shows a housing of a solar receiver panel used in the receiver of FIG. 1 shown in plan, elevation, end elevation and perspective views;
FIG. 3 shows a perspective view of a heat exchanger coil used in the panel of FIG. 2;
FIG. 4
a shows a partial perspective view of the heat exchanger coil of FIG. 3 sitting on a base capping graphite plank;
FIG. 4
b shows a partial perspective view of the heat exchanger coil of FIG. 3 showing insertion of a graphite plank adjacent to the base capping plank seen in FIG. 4a (viewed from a non-insertion end);
FIG. 4
c shows a partial perspective view of the heat exchanger coil of FIG. 3 showing insertion of a graphite plank adjacent to the base capping plank as seen in FIG. 4b when viewed from the opposite end (i.e. the insertion end);
FIG. 4
d shows a cross-section of one of the planks seen in FIGS. 4a, b & c illustrating a semicircular groove;
FIG. 4
e shows a cross-section of one of the planks seen in FIGS. 4a, b & c illustrating a half obround groove;
FIG. 5
a shows a partial perspective view of the heat exchanger coil of FIGS. 3, 4a & 4b, & 4c with a number of a graphite planks inserted viewed from a second (non-insertion) end;
FIG. 5
b shows a perspective view of the heat exchanger coil of FIGS. 3, 4a & 4b, & 4c fully embedded in graphite planks, with a top capping plank removed, viewed from a second (non-insertion) end;
FIG. 5
c shows a perspective view of an alternative coil and graphite arrangement comprising two of the heat exchanger coils of FIGS. 3, 4a & 4b, & 4c fully embedded in graphite planks in parallel, with a top capping plank removed, viewed from a second (non-insertion) end;
FIGS. 6, 7, 8 & 9 show in perspective, plan, end elevation and elevation, an assembly of receiver panels forming a solar receiver having a rectangular prism opening;
FIGS. 10 to 15 show in perspective, plan and elevation, two further assemblies of receiver panels forming solar receivers scaled up from the FIG. 6 assembly;
FIGS. 16 to 23 show in perspective and end elevation views, six further assemblies of receiver panels forming solar receivers having a triangular prism opening (with open ends typically closed by insulation or optically transparent panels—not shown);
FIGS. 24 to 31 show in perspective and end elevation views, six further assemblies of receiver panels forming solar receivers having triangular prism openings, similar to those of FIGS. 16 to 23 with the ends of the openings closed by further receiver panels;
FIGS. 24 to 47 show in perspective and end elevations views, eighteen further assemblies of receiver panels forming solar receivers having triangular prism openings, similar to those of FIGS. 16 to 23 with the openings rotated towards the energy source (with open ends typically closed by insulation or optically transparent panels—not shown);
FIG. 48 is a sectional side view of an assembly of receiver panels forming solar receiver having two rectangular prism openings in a stepped or vertically offset arrangement, showing insulation around the receiver panels and panels closing the openings;
FIG. 49 is a sectional plan view of the receiver assembly of FIG. 48;
FIGS. 50 and 51 are perspective views from above and below of the assembly of FIGS. 48 and 49;
FIGS. 52 & 53 are perspective views from above and below of the assembly of FIGS. 48 and 49 before the insulation is added;
FIGS. 54 & 55 are perspective views from above and below of an assembly similar to that of FIGS. 48 and 49 with narrower openings, before the insulation is added;
FIGS. 56 & 57 are perspective views from above and below of an assembly similar to that of FIGS. 48 and 49 with openings of double the width, before the insulation is added;
FIG. 58 is a sectional side view of yet another assembly of receiver panels forming solar receiver having two triangular prism openings in a stepped or vertically offset arrangement similar configuration to the arrangement of FIG. 46, showing insulation around the receiver panels;
FIG. 59 is a sectional plan view of the receiver assembly of FIG. 58;
FIGS. 60 and 61 are perspective views from above and below of the assembly of FIGS. 58 and 59;
FIG. 62 is a sectional side view of yet another assembly of receiver panels forming solar receiver having two triangular prism openings in a stepped or vertically offset arrangement similar configuration to the arrangement of FIG. 38, optically clear panels closing the ends of the openings;
FIG. 63 is a perspective view of the assembly of FIG. 62;
FIGS. 64 & 65 show perspective views from above and below of still another assembly of receiver panels forming solar receiver having two triangular prism openings in side by side arrangement;
FIG. 66 is a sectional front elevation of the receiver assembly of FIGS. 64 & 65;
FIG. 67 shows a perspective view of an alternative heat exchanger coil used in the panel of FIG. 2;
FIG. 68 shows a partial perspective view of the heat exchanger coil of FIG. 67 sitting on a base capping graphite plank and showing insertion of a graphite plank adjacent to the base capping plank (viewed from a non-insertion end);
FIG. 69 shows a partial perspective view of the heat exchanger coil of FIGS. 67 & 68 with a number of a graphite planks inserted viewed from a second (non-insertion) end;
FIG. 70 shows a perspective view of the heat exchanger coil of FIGS. 67, 68 & 69 fully embedded in graphite planks, with a top capping plank removed, viewed from a second (non-insertion) end;
FIG. 71 shows a housing of a solar receiver panel similar to the housing of FIG. 2, but incorporating the heat exchanger arrangement of FIGS. 67 to 70, shown in plan, elevation, end elevation and perspective views;
FIG. 72 shows a cross-section of two of the planks seen in FIGS. 68, 69, 70 and 71 illustrating a half obround groove;
FIG. 73 is a perspective view from above of another assembly of receiver panels forming solar receiver having two rectangular prism openings in side by side arrangement; and
FIG. 74 is a sectional view of an absorber panel showing a powder, such as graphite powder which fills voids between the graphite core and the housing and a reservoir containing additional powder to accommodate changes in the volume of the void spaces in the absorber panel.
DETAILED DESCRIPTION
Referring to FIG. 1 of the drawings an example of a graphite solar energy receiver 102 is shown mounted on a tower 101 within a field of heliostats 106 whereby solar energy is reflected from the heliostats onto receiving surfaces 107 of the solar energy receiver 102. The field of heliostats 106 and the solar energy receiver 102 are each preferably located generally on the side of the tower facing away from the equator although an East-West orientation is also possible. The solar energy receiver 102 is suspended from the tower by suspension elements 105 such as cables or rods. A door 103 is hinged from one side of the solar energy receiver 102 such that the solar energy receiver 102 can be covered to conserve energy during periods of low insolation (i.e. during the night or during periods excessive of cloud cover. The inner surface 104 of the door 103 may be highly reflective (or highly emissive) to reflect energy falling on this surface onto the receiving surface 107 when the cover is open. The solar energy receiver 102 is configured using a plurality of standard panels 111 of a type illustrated in FIGS. 1 to 5 and described below. Note however that this receiver is illustrated by way of example and many different receiver configurations are possible, some of which are described below.
In FIG. 2 an example of the outer housing of a receiver panel 111 is illustrated in plan, elevation, end elevation and perspective views. The panel housing comprises two large substantially flat parallel side walls 12, 13 bounded by a bottom wall 14, end walls 15, 16 and a top wall 17 to form a closed container. In use the panel 111 will typically be oriented vertically with the bottom wall 14 typically be located at a lower end of the panel. In some cases however a panel 111 might be used horizontally with the side face 12 as the lower face. With reference to FIG. 2, in one form the housing has dimensions of 2214 (A)×1829 (B)×428 (C) mm, however these dimensions may vary to optimize usage of graphite cut from standard dimension blanks and to optimize packing of complete receiver panels into a standard shipping container.
The bottom wall 14 of the housing may be integrally formed with the two side walls 12, 13 by bending a single piece of wall material into a “U” shape in which the base transitions into each of the side walls via a curved bend of radius R which in the present example is in the range of 50 to 180 mm and nominally 80 mm. The wall material is preferably a sheet steel material capable of retaining structural integrity to support the enclosed graphite core, the heat exchanger and any heat exchange fluid contained therein at elevated temperatures of at least 1000° C.
Mounting flanges 21 are provided extending from the end walls 15, 16 and include respective upper and lower mounting holes 23, 24. The flanges 21 are used to suspend the panel from a mounting frame (not shown) by bolting them to the frame via the mounting holes 23, 24. Each flange may comprise an extension of one of the end walls 15, 16 beyond the respective side wall 13 to which it is joined (i.e. the flange may be cut from the same piece of sheet material as the end walls 15, 16 from which they extend). By suspending the receiver panel from the flanges 21 rather than supporting it from below, the resulting tension in the side walls due to gravity of the graphite core acting on the housing allows them to resist buckling to maintain good thermal communication with the graphite core. The curved shape of the housing where the side walls 15, 16 join the bottom wall 14 through a bend also tends to keep the metal walls pressed against the graphite core. In addition, suspending the receiver from above leaves the base of the receiver free of supporting structure and associated protective insulation such that it may be exposed to and can absorb solar radiation from which it would otherwise be protected, allowing more of the surface of the receiver panel 111 to be used for energy absorption.
Vents 51 are provided in the top wall 17 of the housing to allow venting during welding together of the housing walls. These holes may be plugged (e.g. by welding after the panel walls are joined, or they may be used to accommodate sealed cable ports through the wall to pass instrumentation cables such as thermocouple wires into the housing, as fill ports to provide an Argon blanket to the graphite core, to accommodate a filling nozzle to fill the void space and/or an internal reservoir with graphite powder or other thermally conductive media, or to accommodate a connection to an external reservoir to maintain the level of such materials, when the graphite core and housing expand and contract during thermal cycling.
Referring to FIG. 3, an example of a heat exchanger 20 is shown in perspective. The heat exchanger 20 is embedded in a graphite core as seen in FIGS. 4a, 4b, 4c, 4d, 4e, 5a, & 5b (and in a different configuration in FIG. 5c). The heat exchanger 20 comprises heat exchanger tubing 25, 26, 27, 28, 39, 40 and first and second heat exchanger inlet/outlet 18, 19. The first and second heat exchanger inlet/outlets 18, 19 are interchangeable as inlet or outlet depending on the direction in which it is desired to flow the heat exchange fluid through the heat exchanger in a particular application.
The heat exchanger tubes may be made, for example, from 253MA austenitic stainless steel (or any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Inconel), and may have a nominal outside diameter of for example 42.16 mm in the present embodiment but the outside diameter may vary to be greater or smaller than this depending on the particular circumstances of the application. For example, in other embodiments the heat exchanger tubes may be made from the same or a similar material and may have a nominal outside diameter of 26.67 mm. The heat exchanger tubing 25, 26, 27, 28, 39, 40, the drain 29 and associated inlet/outlet tubes 18, 19 are preferably formed with at least some of the tube assembly taking a coiled or serpentine form suitable for compression (like a spring) during assembly, such that when the housing 111 expands due to thermal expansion, the resulting stresses from the movement of the pipe configuration does not exceed the mechanical properties of the pipe material.
In an alternative arrangement, the drain 29 may also act as an inlet or outlet in which case the inlet/outlet 18 will be redundant and might be removed along with its connecting tubes 25 and 40 and passage of fluid through the heat exchanger will be (in either direction) between the drain 29 and the inlet/outlet 19.
The heat exchanger tubing (as seen in FIG. 3) is preferably almost fully embedded in the graphite core (see FIGS. 5a, 5b & 5c), the first and second heat exchanger inlet/outlets 18, 19 extend through openings 55, 56 in a top graphite capping plank 52, shown removed in FIG. 5b and the walls of the housing 111 as seen in FIG. 2. An end 38 of the heat exchanger drain 29 also extends through the housing (see FIGS. 2 and 4c) through a channel 54 in the graphite base plank 31. A riser 25 connects the lower portion of the heat exchanger coil 26, 27, 28 to the first inlet/outlet tube 18 via a further tubing section 40, the riser passing inside the connecting portions 27. The riser 25 and further tubing section 40 are unconstrained by the graphite between the lower end of the riser 25 where it joins to one of the straight tube portions 26 of the lowest row, and the end of tubing section 40 where it joins to the inlet/outlet tube 18. Thus these tubing sections are able to move to accommodate expansion of the heat exchanger tubing in use, without exceeding the material limits of the tubing. A pair of longer coils 39 connect the second inlet/outlet tube 19 to one of the straight tube portions 26 of the highest row. These longer tubing coils are also unconstrained by the capping plank 52 of graphite (See FIG. 5b) which is placed over them and are able to move to accommodate expansion of the heat exchanger tubing in use, without exceeding the material limits of the tubing. The top pair of longer coils 39 may be compressed (i.e. sprung together) prior to it being fixed to the top plate 17 (FIG. 2) during assembly of the heat exchanger and graphite into the housing to allow for thermal effects when the panel is in use. When the panel is heated from ambient to its working temperature, the outer walls of the panel (particularly the side walls 12, 13) may reach up to 1000° C., while the interior temperature of the graphite and the heat exchanger tubing may reach 800° C. In these conditions the walls will expands by about 19 mm per meter of length or about 40 mm in the preferred arrangement, however the graphite will only expand by about 3 mm per meter, resulting in a vertical growth of about 6 mm in the preferred arrangement. Thus the top of the graphite core will drop by about 34 mm. Therefore by compressing the longer coils 39 when the panel is assembled these coils can spring apart as the top of graphite drops in the housing to accommodate the increases difference in distance between the top coil of the coils held captive in the graphite and the second inlet/outlet tube 19 which is welded into the top of the housing. The problem is less pronounced for the first inlet/outlet tube 18 as the riser 25 will expand at a similar rate to the housing, however the horizontal tube 40 will spring to accommodate any differences that do occur.
Referring to FIG. 5c, an alternative arrangement is illustrated in which two sets of essentially identical heat exchanger tubing (as seen in FIG. 3 but with the riser 25 positioned slightly differently) is preferably almost fully embedded in the graphite core in parallel, the first and second heat exchanger inlet/outlets 18, 19 extend through walls of the housing 111 similarly to the arrangement seen in FIG. 2 but with a wider housing and two pairs of inlet/outlet tubes 18, 19 corresponding to the two heat exchangers. Risers 25 connect the lower portions of each of the heat exchanger coils 26, 27, 28 to the respective first inlet/outlet tube 18 via the further tubing sections 40. The risers 25 and further tubing sections 40 are unconstrained by the graphite between the lower end of the risers 25 where they join to one of the straight tube portions 26 of the lowest row of each heat exchanger, and the end of tubing sections 40 where they join to the inlet/outlet tubes 18. Thus these tubing sections of each heat exchanger are able to move to accommodate expansion of the heat exchanger tubing in use, without exceeding the material limits of the tubing. In each heat exchanger, a pair of longer coils 39 connect the second inlet/outlet tube 19 to one of the straight tube portions 26 of the highest row. These longer tubing coils are also unconstrained by the capping plank 152 of graphite (see FIG. 5c) which is placed over them and are able to move to accommodate expansion of the heat exchanger tubing in use, without exceeding the material limits of the tubing.
The housing is sealed around the first and second heat exchanger inlet/outlets 18, 19, and the end 38 of the drain 29 where they exit the housing such that air cannot enter the housing after it is sealed. The plurality of openings 51 in the top wall 17 of the housing (as seen in FIG. 2) act as vents during welding together of the wall panels. These vents may be sealed by welding after the rest of the panel has been welded together or they may be used as sealed cable ports for sensors such as thermocouples used to monitor conditions inside the panel in operation, as fill ports to provide an Argon blanket for the graphite core or to accommodate a filling nozzle to fill the void space with a graphite powder or other thermally conductive media. By excluding air from entering the housing, once it is heated to operating temperature a first time, any oxygen in the housing will oxidize a small quantity of graphite forming carbon dioxide and after that no further reaction will occur within the housing between the graphite and the residual air contained in the housing. However as discussed above if the operating temperature of the panel is likely to exceed 700° C. in a particular installation, it will be desirable at manufacture to replace the air filling the spaces in the panel between the graphite and the walls with an inert gas such as argon or helium.
The points where the first and second heat exchanger inlet/outlet 18, 19 pass through the housing 111 are preferably in close proximity and preferably exit through the top wall 17 of the housing, to assist with mounting and manifolding the pipes with other receivers.
The heat exchanger coils comprise a plurality of straight tube portions 26 arranged in parallel and connected at their ends by connecting portions 27, 28 to form a serpentine coil. Preferably the straight tube portions 26 are arranged in parallel planes forming rows of pairs of straight tube portions. The straight tube portions 26 in each row are aligned with the straight tube portions in adjacent rows such that they also exist in vertical planes perpendicular to the first mentioned parallel planes.
In the example illustrated in FIG. 3, the two straight tube portions 26 in each row are connected together at their first end (the graphite insertion end) by first connecting tube portions 28 and at their second end (non-insertion end) each of the two straight tube portions of each row are connected to straight tube portions 26 of each of two adjacent rows by second connecting tube portions 27. A first heat exchanger inlet/outlet tube 18 is connected to one of the straight tube portions 26 in the bottom row via the riser tube portion 25 and the further tube portion 40. A second heat exchanger inlet/outlet tube is connected to one of the straight tube portions 26 in a top row (the upper row in FIG. 3) via the longer tube coils 39. As seen in FIGS. 4a, 4b & 5a, the connecting portions 27 are joined in the middle of their curve by a weld 37 allowing the coils to be formed as individual rows comprising two straight tube portions 26 joined at the first (graphite insertion) end by the first connecting portion 28 and having two halves of second connecting portions 27 angled respectively upwardly and downwardly for connection (by welds 37) to the corresponding mating halves of the second connecting portions 27 of the rows above and below. Thus a supplier can manufacture the individual coil sections in bulk, as single pieces bent from one piece of tube, each comprising two straight tube portions 26 joined by a first connecting tube portion 28 and terminated by two half connecting tube portions 27, and these can be easily transported to the assembly factory for assembly into the finished coil and subsequent assembly into the receiver panel 111.
After the heat exchanger is fabricated, pre-shaped planks of graphite 31, 32 & 52 are positioned to encompass most of the heat exchanger tubes. Referring to FIG. 4a, first a lower capping plank 31 is positioned beneath the lowest row of straight tube portions 26. A second capping plank 52 (shown removed in FIG. 5b to expose its lower surface) is positioned over the upper row of straight tube portions 26 after the remaining graphite planks 32 are in position. The lower capping plank 31 is grooved on one (upper) surface groove having a semicircular cross-section conforming to the shape and radius of the straight tube portions 26 and first connecting tube portion 28 at the first end of the heat exchanger. The edges 33 of the lower capping plank 31, between the face opposite the grooved surface (i.e. the downward facing surface in FIG. 4a) and the sides of the capping plank 31, are radiused to correspond with the curved transition between the side walls 12, 13 and the base wall 14 of the housing. The upper capping plank 52 has squared edges between the opposite face (i.e. the outward facing surface) and the sides of the capping plank and the surface that abuts the topmost of the intermediate graphite planks 32 is recessed 53 to accommodate the tube section 40, the longer tube coils 39 and the inlet/outlet tubes 18, 19 allowing them room to move with expansion and contraction as the heat exchanger, the graphite and the housing heat and cool. The inlet/outlet tubes 18, 19 pass through the capping plank 52 (not visible) to access the openings in the housing top wall 17. To accommodate two longer coils 39, the top capping plank 52 is thicker than the remaining graphite planks.
Referring to FIGS. 4b, 4c, 5a & 5b, the bulk of the graphite planks 32 are positioned between the rows of straight tube portions 26 and the respective first connecting tube portions 28. The graphite planks 32 each include two opposite surfaces in which the semicircular grooves 35, 36 are formed (only upper grooves are visible in FIGS. 4a, 4b & 4c), conforming to the shape and radius of the straight tube portions 26 and the first connecting tube portions 28 at the first (insertion) end of the heat exchanger. Referring to FIG. 4d, a partial cross section of two abutting planks 32 shows two pairs of aligned semicircular grooves 35 encompassing a pair of pipes 26. When assembled between rows of straight tube portions 26 adjacent pairs of the planks 32 encompass and closely conform to the respective straight tube portions 26 and first connecting tube portions 28. Referring to FIGS. 4a, 4b & 4c the second ends of the graphite planks 32 are provided with a recess 34 to accommodate the second connecting tube portions 27 joining straight tube portions 26 from adjacent rows of straight tube portions. The weld joins 37 in the centre of each connecting portion 27 are also located in the recesses 34 to allow inspection of the weld joins after assembly. The recesses 34 also accommodate the riser 25 which connects the lower coil to the first inlet/outlet tube 18 and allows the graphite to almost completely fill the housing while allowing space for the riser 25 and the second connecting portions 27 which each must pass through the end of the graphite planks 32. Because the graphite planks extend to the ends of the housing and almost fully occupy the space within the housing; the load of the graphite is spread evenly across the bottom wall 14 of the housing, allowing thinner material to be used. Also by maximizing the area of graphite in contact with the walls and consequentially minimizing void space, the heat transfer into the graphite by insolation is maximized. Minimizing void space also minimizes the amount of trapped air that is available to react with the graphite when the panel is heated to it operating temperature. In the present embodiment the volume of void spaces within the housing not occupied by graphite or tubing is generally in the range of 4-10% and typically 5-7% of the internal volume of the housing (at the working temperature). Correspondingly the side panel of the housing, which is the irradiated surface of the panel when in use, is generally backed by the graphite core over all but 1-5% of its area and typically 2-3% (at the working temperature) in the preferred embodiment.
Referring to FIG. 67, another example of a heat exchanger 670 is shown in perspective. This embodiment is primarily intended for superheater-only operation and comprises three parallel serpentine shaped tube assemblies each having an independent input and output. The heat exchanger 670 is again embedded in a graphite core as seen in FIGS. 68, 69 & 70. The heat exchanger 670 comprises heat exchanger tubing 678, 682, 683, 684, 685, 686, 687 & 688, heat exchanger inlets 674, 675, 676 and heat exchanger outlets 671, 672, 673. The lower tube section 686, 687, 688 provide the three inlets 674, 675, 676 and connect to the lower end of the main tube assembly comprising tube sections 678. The heat exchanger inlets 674, 675, 676 also act as drains. The upper tube section 683, 684, 685 provide the three outlets 671, 672, 673 and connect via tube sections 682 to the upper end of the main tube assembly comprising tube sections 678. The tube sections 678, 682, 683, 684, 685, 686, 687 & 688 are joined together by welds 681. The flow may be reversed in various applications such that the inlets may be 671, 672 and 673 and the outlets may be 674, 675 and 676.
The heat exchanger tubes may again be made, for example, from 253MA austenitic stainless steel (or any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Inconel), and may have a nominal outside diameter of for example 42.16 mm in this embodiment but the outside diameter may vary to be greater or smaller than this depending on the particular circumstances of the application.
The heat exchanger tubing 678, 682, 683, 684, 685, 686, 687 & 688 are formed with at least some of the tube assembly taking a coiled or serpentine form suitable for compression (like a spring) during assembly, such that when the housing 111 expands due to thermal expansion, the resulting stresses from the movement of the pipe configuration does not exceed the mechanical properties of the pipe material.
The heat exchanger tubing (as seen in FIG. 67) is preferably almost fully embedded in the graphite core (see FIGS. 68, 69 & 70), the heat exchanger inlet tubes 686, 687, 688 extend through openings 697 in a bottom graphite capping plank 698 and heat exchanger outlet tubes 683, 684, 685 extend through openings 703, 704, 705 in a top graphite capping plank 701, shown removed in FIG. 70. Referring to FIG. 71, the heat exchanger outlet tubes 683, 684, 685 extend through openings 711, 712, 713 in the top of the housing 111 and the heat exchanger inlet tubes 686, 687, 688 extend through openings 714, 715, 716 in the bottom of the housing 111. As with the previous embodiments, these tubing sections are able to move to accommodate expansion of the heat exchanger tubing in use, without exceeding the material limits of the tubing.
The housing is sealed around the heat exchanger inlet tubes 686, 687, 688 where they exit the housing such that air cannot enter the housing after it is sealed. The plurality of openings 51 in the top wall 17 of the housing (as seen in FIG. 71) act as vents during welding together of the wall panels. These vents may be sealed by welding after the rest of the panel has been welded together or they may be uses as sealed cable ports for sensors such as thermocouples used to monitor conditions inside the panel in operation, as fill ports to provide Argon blanket to graphite core or as filling nozzle to fill void space with graphite powder or other thermally conductive media.
After the heat exchanger is fabricated, pre-shaped planks of graphite 689, 692, 701 are positioned to encompass most of the heat exchanger tubes. Referring to FIG. 68, first a lower capping plank 689 is positioned beneath the lowest row of inlet tube portions 686, 687, 688. A second capping plank 701 (shown removed in FIG. 70 to expose its lower surface) is positioned over the upper row of outlet tube portions 683, 684, 685 after the remaining graphite planks 692 are in position. The lower capping plank 689 is grooved 691 on one (upper) surface groove having a semicircular (or preferably obround) cross-section conforming to the shape and radius of the outlet tube portions 683, 684, 685 of the heat exchanger. The edges 706 of the lower capping plank 689, between the face opposite the grooved surface (i.e. the downward facing surface in FIGS. 68, 69 & 70) and the top edges of the capping plank 689, are angled to correspond with the transition between the side walls 12, 13 and the base wall 14 of the housing (see FIG. 71). The upper capping plank 701 has squared edges between the opposite face (i.e. the outward facing surface) and the sides of the capping plank and the surface that abuts the topmost of the intermediate graphite planks 692 is recessed 696 to accommodate the tube section 683, 684, 685.
Referring to FIGS. 68, 69 & 70, the bulk of the graphite planks 692 are positioned between the rows of tube portions 678. The graphite planks 692 each include two opposite surfaces in which the semicircular (or preferably semi-obround) grooves 691, 696 are formed, conforming to the shape and radius of the tube portions 678. When semi-obround grooves are used they are elongated in the vertical direction (i.e. two grooves abut to form an obround cross section with a vertical major axis) to accommodate expansion of the tube assembly in the vertical direction (as viewed in FIG. 70). Referring to FIG. 72, a partial cross section of two abutting planks 692 shows three pairs of aligned semi-obround grooves (691, 696) encompassing a pair of pipes 678.
Preferably the abutting surfaces of the graphite planks of FIGS. 4a, 4b, & 4c and FIGS. 68, 69 & 70 will have a surface finish which is N8 or better (ISO 1302). Such that when assembled between rows of straight tube portions adjacent pairs of the planks encompass and closely conform to the respective straight tube portions and first connecting tube portions at the internal working temperature of the panel, which is up to 800° C., the grooves are made approximately 1.6% bigger than the nominal outside diameter of the tubes with a tolerance of approximately +0.00/−1.00%. For example, when the heat exchanger tubes are made from 253MA austenitic stainless steel (any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Inconel) and have a nominal outside diameter of 26.67 mm, the grooves will preferably be 27.1 mm (+0.00/−0.25 mm) in diameter. Alternatively, when the heat exchanger tubes are made from the same or similar material and have a nominal outside diameter of 42.16 mm, the grooves will preferably be 42.9 (+0.00/−0.25 mm) in diameter. To achieve a high contact surface without excessive expense, the surface of the graphite within the grooves will preferably have a surface finish which is N7 or better (ISO 1302). By maximising the contact of the graphite with the surface of the grooves by designing the grooves to be sized appropriately for the tube diameter at the working temperature and by providing appropriate surface finish, the operation of the heat exchanger within the graphite is enhanced.
Embodiments may also be manufactured in which the grooves 35, 36 (FIG. 4d) are elongated in cross section, rather than being semicircular. In this case the grooves may be a half obround shape with a radius which is slightly greater than the tube it encompasses (by about 1.6%) with a straight section about 1.6% of the radius in the direction perpendicular to (i.e. across) the parallel groove, to accommodate lateral movement of the tube when the coils expand. Referring to FIG. 4e, a partial cross section of two abutting planks 32 shows two pairs of aligned half obround grooves 45 encompassing a pair of pipes 26. In the embodiment of FIGS. 67 to 72, the grooves 691, 696 (FIGS. 68, 69, 70 & 72) are also preferably elongated in cross section, rather than being semicircular, however in this case the half obround shape is oriented with its major axis in the vertical direction in FIGS. 68, 69, 70 & 72. Referring to FIG. 72, the grooves 691, 696 may have a radius which is slightly greater than the tube it encompasses (by about 1.6%) with a straight section about 0.8% of the thickness of graphite plank, to accommodate vertical movement of the tube when the coils expand (refer to FIG. 69). However all of these arrangements have the disadvantage that the tubes will not be as closely encompassed in the grooves and therefore this arrangement might not always be appropriate and expansion of the coils may be accommodated by other means.
Because, in the embodiment of FIGS. 2, 34a, 4b, 4c, 4d, 4e, 5a, 5b and 5c, the tube spacing between adjacent rows of tubes is less than a plank thickness in the finished receiver panel, the straight tube portions need to be separated at the first (insertion) end while the planks are inserted between the heat exchanger coils. This may be achieved by springing the first (insertion) ends of the straight, tube portions 26 slightly apart to allow the planks to be easily inserted into the fabricated heat exchanger tubing past the first connecting tube portions 28 and by sliding them between adjacent rows of straight tube portions 26 as illustrated in FIG. 4b. Alternatively during fabrication of the heat exchanger coils, if they are spaced by a spacing greater than or equal to a plank thickness of the graphite planks 32 between which they are located in the final assembly (or if at least the first (insertion) ends are so spaced), the planks may be easily inserted into the fabricated heat exchanger tubing between adjacent rows of coils and the coils of the tubing may then be compressed into contact with the graphite after insertion of the graphite planks 32 between the coils. Such springing is not required in the embodiment shown in FIGS. 67 to 72.
The panel described with reference to FIGS. 67 to 72 and the panes described with reference to FIGS. 2 to 5 may each be used in any of the assemblies described herein.
Once the graphite planks 31, 32, 52 are assembled to encompass the heat exchanger 20, (or planks 689, 692, 701 are assembled to encompass the heat exchanger 670) the assembly is inserted into the housing, locating tubes are inserted into the holes 41 (or 702) extending through all of the planks to maintain alignment. At least one of the locating tubes will engage a locating pin projecting from the base of the housing (not shown) to locate the graphite core 31, 32, 52 within the housing. The housing is then welded closed, including sealing the openings through which the inlet/outlet tubes 18, 19 (or 686, 687, 688, 683, 684, 685) and the drain 29 (or 686, 687, 688) pass through the housing to form the finished panel 111 (see FIGS. 2 & 71). The vent holes 51 are also sealed either by welding or by inserting sealing plugs or a port fitting that sealingly allows passage of transducer cables such as thermocouple wires into the interior of the panel. The vent holes 51 might also be fitted with port fittings to be used as fill ports to provide Argon blanket to graphite core or as filling nozzles to fill void space with graphite powder or other thermally conductive media.
In the alternative arrangement shown in FIG. 5c, two heat exchangers are used in parallel, each essentially the same as the heat exchanger of FIG. 3. After the heat exchangers are fabricated, pre-shaped planks of graphite 131, 132 & 152 are positioned to encompass most of the heat exchanger tubes as in the single coil case. A lower capping plank 131 is positioned beneath the lowest row of straight tube portions 26 and a second capping plank 152 (shown removed in FIG. 5c to expose its lower surface) is positioned over the upper row of straight tube portions 26 after the remaining graphite planks 132 are in position. The lower capping plank 131 is grooved on one (upper) surface conforming to the shape and radius of the straight tube portions 26 and first connecting tube portion 28 at the first end of the heat exchanger. The edges 133 of the lower capping plank 131, between the face opposite the grooved surface (i.e. the downward facing surface in FIG. 5c) and the sides of the capping plank 131, are radiused to correspond with the curved transition between the side walls 12, 13 and the base wall 14 of the housing. The upper capping plank 152 has squared edges between the opposite face (i.e. the outward facing surface) and the sides of the capping plank and the surface that abuts the topmost of the intermediate graphite planks 132 is recessed 153 to accommodate the tube section 40, the longer tube coils 39 and the inlet/outlet tubes 18, 19 allowing them room to move with expansion and contraction as the heat exchanger, the graphite and the housing heat and cool. The inlet/outlet tubes 18, 19 pass through the capping plank 52 (not visible) to access the openings in the housing top wall 17. To accommodate two longer coils 39 of each heat exchanger, the top capping plank 152 is thicker than the remaining graphite planks.
Referring to FIG. 5c, the bulk of the graphite planks 132 are positioned between the rows of straight tube portions 26 and the respective first connecting tube portions 28. The graphite planks 132 each include two opposite surfaces in which grooves are formed (not shown but similar to grooves 35, 36 in FIGS. 4a, 4b & 4c), conforming to the shape and radius of the straight tube portions 26 and the first connecting tube portions 28 at the first (insertion) end of the heat exchanger. When assembled between rows of straight tube portions 26 adjacent pairs of the planks 132 encompass and closely conform to the respective straight tube portions 26 and first connecting tube portions 28. Referring to FIG. 5c the second ends of the graphite planks 132 are provided with a recess 134 to accommodate the second connecting tube portions 27 joining straight tube portions 26 from adjacent rows of straight tube portions. The weld joins 37 in the centre of each connecting portion 27 are also located in the recesses 134 to allow inspection of the weld joins after assembly. The recesses 134 also accommodate the risers 25 which connect the lower coils to each of the first inlet/outlet tubes 18 and allows the graphite to almost completely fill the housing while allowing space for the risers 25 and the second connecting portions 27 which each must pass through the end of the graphite planks 132. Again, because the graphite planks extend to the ends of the housing and almost fully occupying the space within the housing, the load of the graphite is spread evenly across the bottom wall 14 of the housing, allowing thinner material to be used. Assembly in the double heat exchanger example illustrated in FIG. 5c is similar to that of the single heat exchanger example described with reference to FIGS. 4a, 4b, 4c, 5a & 5b.
Preferably a solar energy receiver will comprise two or more receiver panels configured to form a downward opening cavity. The cavity may be formed with a combination of receiver panels and insulation panels. Outside surfaces of the receiver panels forming the solar energy receiver will preferably be covered by insulation to minimize unwanted heat loss.
Referring to FIGS. 6, 7, 8 and 9 one possible configuration of a solar energy receiver 102 (FIG. 1) is illustrated in perspective, plan, front and side elevation respectively and comprises six heat absorbing receiver panels 111 arranged in a rectangle around an opening 62. The opening is capped by a panel 61 which may be either another heat absorbing receiver panel similar to the other heat absorbing receiver panels 111 or alternatively an insulating panel. The decision as to whether the panel 61 should be a heat absorbing or insulator panel will be determined by whether energy from the heliostats 106 (FIG. 1) is incident on the panel. This will depend on the aspect ratio (height to width) of the side panels 111 and the layout of the heliostats 106 in the heliostat field. Usually energy will be reflected into the opening 62 and onto bottom walls 14 of the vertical panels, but the outer surfaces would not usually be energy receiving surfaces.
FIGS. 10, 11 & 12 show perspective, plan and side elevations of a solar energy receiver 102 similar to that of FIGS. 6, 7, 8 and 9 except that it has two rectangular openings 62 formed by adding a further four panels 111 to the arrangement of FIGS. 6, 7, 8 and 9. Similarly FIGS. 13, 14 and 15 show perspective, plan and side elevations of a solar energy receiver 102 similar to that of FIGS. 10, 11 & 12 except that it has three rectangular openings 62 formed by adding a further four panels 111 to the arrangement of FIGS. 10, 11 & 12. In use the arrangements of FIGS. 6 to 15 would include layers or panels of insulation (not shown in these views) around the outside (and over the top when panel 61 is a heat absorbing panel) of each assembly to minimize heat loss though the non absorbing outer surfaces (side wall 13 and top walls 17 and some end walls 15, 16 in FIG. 2).
FIGS. 16, 17, 18 and 19 show perspective views and a side elevation respectively of another three possible configurations of a solar energy receiver 102. The side elevation of FIG. 19 is common to each of the embodiments shown in FIGS. 16, 17 & 18. The included angle of 36° is indicative only and can be up to 90° depending on the application, location and solar field design. The FIG. 16 embodiment comprises six heat absorbing receiver panels 111 arranged in an inverted “V” shape to create an opening 162. Usually energy will be reflected into the opening 162 and onto the bottom walls 14 of the vertical panels, but the outer surfaces would not usually be energy receiving surfaces. The FIG. 17 embodiment has four panels and the FIG. 18 embodiment has two panels forming receivers that are respectively ⅔ and ⅓ of the width of the FIG. 16 embodiment.
FIGS. 20, 21, 22 & 23 show perspective views and a side elevation respectively of another three possible configurations of a solar energy receiver 102 similar to those of FIGS. 16, 17, 18 and 19 except that they are configured in two inverted “V” shapes with two openings 162 by doubling the number of panels used in each case. As before in use the arrangements of FIGS. 16 to 23 would include layers or panels of insulation (not shown) over the outside surfaces of each panel 111 of each assembly to minimize heat loss though the non absorbing outer surfaces (side wall 13 and top walls 17 and some end walls 15, 16 in FIG. 2). The ends of the openings 162 could also be blocked by insulating panels or optically transparent panels such as fused quartz panels (not shown in these views).
The configurations of solar energy receivers 102 shown in FIGS. 24 to 31 are similar to those of FIGS. 16 to 23 except that the openings 162 are closed at their ends by additional heat absorber panels 241 similar to the panels 111. FIGS. 32 to 47 show configurations similar to FIGS. 16 to 31 except that in each case the solar energy receiver 102 is tilted toward the heliostat field 106 (in FIG. 1) such that panels 111 forming the back surface of the opening 162 (i.e. furthest from the heliostat field are vertical in use such that the inverted “V” shape and the openings 162 are tilted to more directly face the heliostat field 106, but in FIGS. 40 to 47 the end panels 401 (similar to panels 111) are not tilted, such that an edge of the panel 401 aligns with an edge of the rearmost panel 111 unlike end panels 241 in FIGS. 24 to 31.
A particularly advantageous configuration of a solar energy receiver 102 is illustrated in sectional end elevation in FIG. 48 and in sectional plan view in FIG. 49. FIGS. 50 and 51 are upper and lower perspective views of the receiver of FIGS. 48 & 49. Perspective views without insulation are seen in FIGS. 52 & 53, a version with a narrower opening is illustrated in FIGS. 54 & 55 (insulation no shown) and a two panel wide version is illustrated in FIGS. 56 & 57 (insulation no shown). In the FIGS. 48 & 49 configuration, a plurality of receiver panels 111 are positioned in an offset fin configuration (as seen in FIG. 48 viewed from the side) in which the panels 111 form a plurality of spaced vertical fins 481 which are progressively offset upwardly from rearmost to foremost (with respect to the heliostat field) to form openings 486. Referring to FIG. 49, additional receiver panels 111 form end closures 491 which close the ends of the openings 486. In FIGS. 48 and 49 the solar energy receiver 102 is shown as having three fins but it will be appreciated that it could have two fins or it could have four or more fins. Similarly solar the energy receiver 102 is shown with only one panel forming each fin 481, whereas the fins might be 2, 3 4 or more panels long. The horizontal spacing “d” of the panels will be in the order of 1 to 3 times the panel thickness (C in FIG. 2). The vertical offset “a” of each panel relative to the one behind it will be in the order of 0-2 times the panel thickness (dimension C in FIG. 2). The offsets “a” in FIGS. 39 & 47 will have similar values.
The walls of each housing are preferably fabricated from 253MA austenitic stainless steel (or any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Inconel) finished to mill finish class 2B. The surfaces 191 of panels 111 which face inwardly of the opening 486 and are forward facing with respect to the heliostat field 106, have a natural class 2B mill finish to the stainless steel material to provide a degree of emissivity which causes a portion of the incident solar energy to be re-radiated onto the surface 192 of the opposing panel 111, which is a rearward facing surface with respect to the heliostat field 106. The rearward facing surface 192 on the other hand will preferably be coated with a robust high temperature heat absorbing (black—specific absorptivity 0.80-1.0) paint, surface treatment or other suitable coating. Inwardly facing surfaces of the side receiver panels 491 also have a natural class 2B mill finish (specific emissivity 0.7) or polished surface (emissivity 0.2) or may be provided with a further surface treatment or coating to achieve a medium emissivity surface (specific emissivity in the range of 0.3-0.8) such that some of the solar energy falling on these panels is re-radiated to other internal surfaces within the opening 162.
The sides and top of the solar energy receiver 102 are surrounded with insulating panels as with the earlier described arrangements. In particular the top of the openings 486 are closed with insulating panels 485 which include high emissivity surfaces 487 facing into the opening 486 to reflect any solar energy reaching the top of the openings 486 back towards the heat absorbing surfaces of the fins 481. Insulating panels 483 are also located over the front (heliostat facing) surface of the front fin 481 and further insulating panels 483 are located over the rear (non heliostat facing) surface of the rear fin 481. Insulating panels 492 also cover the outside surfaces of the side closure heat absorbing panels 491.
Referring to FIGS. 58 to 61, another advantageous configuration of a solar energy receiver 102 is illustrated in sectional end elevation in FIG. 58 and in sectional plan view in FIG. 59. This panel configuration is similar to the FIGS. 46 & 47 embodiment, in which two triangular prism openings are formed in a stepped or vertically offset arrangement. In this example however the panels are shown with surrounding insulation. FIGS. 60 and 61 are upper and lower perspective views of the receiver of FIGS. 58 & 59.
In the FIGS. 58 & 59 configuration, a plurality of receiver panels 111 are positioned in an offset inverted “V” configuration (as seen in FIG. 58 viewed from the side) in which the panels 111 form a pair of spaced inverted “V” shaped openings 162 which are progressively offset upwardly from rearmost to foremost (with respect to the heliostat field) to form the openings 162. Referring to FIG. 59, additional receiver panels 111 form end closures 401 which close the ends of the openings 162 to prevent convection losses from the opening 162 and to capture solar energy directed from heliostats located towards the sides of the heliostat field 106, either by direct absorption or by reflection onto another surface within the opening 162. In FIGS. 58 and 59 the solar energy receiver 102 is shown as having two inverted “V” shaped openings but it will be appreciated that it could have one inverted “V” shaped opening or it could have three or more inverted “V” shaped openings. Similarly solar the energy receiver 102 is shown with each inverted “V” shaped opening being only one panel long, whereas the openings might be 2, 3 4 or more panels long. The vertical offset “a” of each pair of panels relative to the one pair behind it will be in the order of 0-2 times the panel thickness (dimension C in FIG. 2) as was the case in earlier examples.
The walls of the housing are preferably fabricated from 253MA austenitic stainless steel (or any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Inconel) finished to mill finish class 2B. The surfaces 191 of panels 111 which face inwardly of the opening 162 and are forward facing with respect to the heliostat field 106, have a natural class 2B finish to the stainless steel material to provide a degree of emissivity which causes a portion of the incident solar energy to be re-radiated onto the surface 192, which is a rearward facing surface with respect to the heliostat field 106. The rearward facing surface on the other hand will preferably be coated with a robust high temperature heat absorbing (black—specific absorptivity 0.80-1.0) paint, surface treatment or other suitable coating. Inward facing surfaces of the additional receiver panels 111 which form end closures 401 have a natural class 2B mill finish (specific emissivity 0.7) to the stainless steel material, or may be polished (specific emissivity 0.2) or polished surface (emissivity 0.2) or may be provided with a further surface treatment or coating to achieve a medium emissivity surface (in the range of 0.3-0.8) which causes a portion of the incident solar energy to be re-radiated onto other internal surfaces of the opening 162.
As illustrated in FIGS. 60 & 61, the outward facing sides, fronts, backs and top of the inverted “V” assemblies of the solar energy receiver 102 are surrounded with insulating panels as with the earlier described arrangements. In, particular front (heliostat facing) outside surfaces of the panels 111 of each inverted “V” shaped assembly are covered by insulating panels 583 & 585. Further insulating panels 582 & 584 are located over the rear (non heliostat facing) surfaces of each inverted “V” shaped assembly. Insulating panels 492 also cover the outside surfaces of the side closure heat absorbing panels 401. The tops of the receiver panels 111 are each covered by additional insulation panels 586 & 587 while edges of the panels 111 are covered by insulation panels 591, 592 and 593. Additional insulation panels 589 cover those portions of the inwardly facing surfaces of the side panels 401 extending above the sloping panels forming the front of the inverted “V” shape.
Referring to FIGS. 62 & 63, a further advantageous configuration of a solar energy receiver 102 is illustrated in end elevation in FIG. 62 and in perspective view in FIG. 63. In this panel configuration two triangular prism openings 162 are formed in a stepped or vertically offset arrangement similar configuration to the arrangement of FIG. 38. In this example however the panels are shown with optically clear panels 621 closing the ends of the openings (insulation panels not shown).
In the FIGS. 62 & 63 configuration, a plurality of receiver panels 111 are positioned in an offset inverted “V” configuration (as seen in FIG. 62 viewed from the side) in which the panels 111 form a pair of spaced inverted “V” shaped openings 162 which are progressively offset upwardly from rearmost to foremost (with respect to the heliostat field) to form the openings 162. Transparent panels 621, which may be fused quartz for example, form end closures which close the ends of the openings 162 to limit heat loss by convection while allowing solar energy to enter the opening 162 from the sides of the opening. In FIGS. 62 and 63 the solar energy receiver 102 is shown as having two inverted “V” shaped openings but it will be appreciated that it could have one inverted “V” shaped opening or it could have three or more inverted “V” shaped openings. Similarly solar the energy receiver 102 is shown with each inverted “V” shaped opening being only one panel long, whereas the openings might be 2, 3 4 or more panels long. The vertical offset “a” of each panel relative to the one behind it will be in the order of 0-2 times the panel thickness (dimension C in FIG. 2).
The walls of the housings in FIGS. 62 & 63 are preferably fabricated from 253MA austenitic stainless steel (or any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Inconel) finished to mill finish class 2B. The surfaces 191 of panels 111 which face inwardly of the opening 162 and are forward facing with respect to the heliostat field 106, may have a natural class 2B finish to the stainless steel material (specific emissivity 0.7) or a polished surface (specific emissivity 0.2-0.3), or may be provided with another suitable surface coating or treatment (specific emissivity in the range of 0.3-0.8) which causes a portion of the incident solar energy to be re-radiated onto the surface 192 on the inside of the opening, which is a rearward facing surface with respect to the heliostat field 106. The rearward facing surfaces 192 on the other hand will preferably be coated with a robust high temperature heat absorbing (e.g. black—specific absorptivity in the range of 0.8-1.0, preferably 0.90-1.0) paint, surface treatment or other suitable coating.
A further embodiment is illustrated in FIGS. 64, 65 & 66 which shows a plurality of receiver panels 111 which are configured as a pair of inverted “V” assemblies 641, 642, which are positioned side by side (i.e. in parallel) and suspended from a tower 101 (not shown in this Figure). The configuration in this embodiment is similar to the configuration of FIG. 29, except that in this case the assembly of receiver panels is intended to be rotated so that the long axes of the openings 162 are directed towards the heliostat field. The panels 111 form a pair of inverted “V” shaped openings 162 and each inverted “V” assembly 641, 642 comprises four panels arranged as two side by side pairs angled together to form a two panel long inverted “V” shape assembly. In an alternative arrangement (not shown) each assembly 641, 642 may have its plurality of inverted “V” shaped pair of panels progressively offset upwardly (by a distance “a”) from rearmost to foremost (with respect to the heliostat field—i.e. away from the tower) to provide better exposure of the solar energy to the entire assembly. As in the FIG. 29 embodiment one end (the rearward or tower end) of each of the openings 162 is closed by another receiver panel 241. However unlike the FIG. 29 embodiment, the other end of each opening 162 (the end facing the heliostat field) is partially closed by a transparent panel 621, which may be fused quartz for example, form end closures which close the forward ends of the openings 162 to limit heat loss by convection while allowing more solar energy to enter the opening 162 from the centre of the heliostat field. In FIGS. 64, 65 and 66 the solar energy receiver 102 is shown as having two side by side inverted “V” shaped openings 621 but it will be appreciated that it could have one inverted “V” shaped opening or it could have three or more inverted “V” shaped openings. Similarly the solar energy receiver 102 is shown with each inverted “V” shaped opening being two panels long, whereas the openings might be 1, 3 4 or more panels long. The possible vertical offset “a” of each pair of panels relative to the pair behind it (mentioned above but not illustrated) will be in the order of 0-2 times the panel thickness (dimension C in FIG. 2).
The openings 62, 162, 486 in the bottom of panel assemblies 102, 641,642 may be fully or partially closed by insulating tiles or fused quartz panels (not illustrated) to restrict heat loss by convection while leaving smaller apertures through which solar energy may be directed from the heliostats.
The walls of the housings in FIGS. 64, 65 & 66 are again preferably fabricated from 253MA austenitic stainless steel (or any suitable high temperature thermally conductive material such as 800H austenitic steel or alloys such as Inconel) finished to mill finish class 2B. The surfaces 643 of panels 111 which face inwardly of the opening 162 and are forward facing with respect to the heliostat field 106, may have a natural finish to the stainless steel material (specific emissivity 0.7) or a polished surface (specific emissivity 0.2-0.3), or may be provided with another suitable surface coating or treatment (specific emissivity in the range of 0.3-0.8) which causes a portion of the incident solar energy to be re-radiated onto the surfaces 644, which are sideways facing surfaces with respect to the heliostat field 106. The sideways facing surfaces 644 on the other hand will preferably be coated with a robust high temperature heat absorbing (e.g. black—specific absorptivity in the range of 0.8-1.0, preferably 0.90-1.0) paint, surface treatment or other suitable coating.
The outside surfaces of the sides fronts, backs and top of the inverted “V” shaped panel assemblies of the solar energy receiver 102 will be surrounded with insulating panels similar to those previously described, for example in the description of the FIG. 58-61 embodiment, with the exception that the transparent panels 621 will not be covered.
Referring to FIG. 73, another embodiment is illustrated which is a variation of the embodiments of FIGS. 6 to 15. In this case the absorber panels 111 are similar to those of FIG. 71, with a plurality of parallel serpentine tube assemblies. The outlets 683, 684, 685 are located similarly to those of the FIG. 71 embodiment, however in this embodiment the inlets 686, 687, 688 are located to enter through the lower extremity of the rear end wall 16 (i.e. the wall furthest from the heliostat field), where they are protected from the solar energy reflected onto the assembly by the heliostat field. In this case, the panels 111 are configured to create a plurality of openings 62 having shapes which are rectangular prisms similar to those in FIGS. 6 to 15, however the mounting arrangement is different as the panels are hung by flanges 731, located at the top of the panels, via mounting holes 732. The flanges 731 may be extensions of the end walls 15 & 16. The tops of the openings 62 are closed by refractory panels 61 having lower (i.e. internal) surfaces which may be emissive (similar to emissive surfaces described earlier). To minimise heat loss, surfaces of the receiver panels 111 which are external surfaces of the receiver assembly may be covered by insulating panels (not shown for clarity but refer to earlier examples). The rear of the assembly is closed by panels 734 which may be refractory panels (shown) with emissive surfaces (again similar to emissive surfaces described earlier) or may be absorber panels (such as panels 111). The front of the assembly is closed by panels 733 which may be transparent quartz panels or refractory panels with emissive surfaces facing the interior of the cavity 62 (again similar to emissive surfaces described earlier). The variations described in this embodiment may be incorporated in any of the previously described embodiments. In the top wall of the panels, openings 51 allow expansion of the internal air during manufacture as with the previously described embodiments and may be welded closed or used as ports. One of the openings 51 is shown with a filling nozzle 735 attached to permit filling of void spaces with graphite powder (refer to description of FIG. 74 below).
FIG. 74 shows a receiver panel 111 with one side wall removed showing the graphite planks 689, 692, 701 forming the graphite core. Voids will exist between the graphite planks and the walls of the housing (e.g. between the planks 689, 692, 701 visible in FIG. 74 and the wall which has been removed). A larger void 741 forms a reservoir between the top of graphite core and the top of the housing. The reservoir 741 and the voids in this case are filled with graphite powder. The graphite powder enhances heat transfer between walls of the housing and the graphite core. A filling nozzle 735 is in communication with the reservoir 741 to enable filling of the voids in the housing and topping up of the reservoir 731. The reservoir 731 stores additional graphite powder which prevents spaces opening up when expansion and contraction of the housing and core occur during thermal cycling. This arrangement may be employed in any of the previously described embodiments.
By using modular receiver panels that can be assembled into a variety of solar energy receiver configurations, simple and fast site installation can be achieved with minimal on site preparation requirements. The receiver panels can be shipped to site assembled and fully tested for simple mounting on a tower along with associated insulation panels in a preferred one of a variety of optional configurations.
Reduced cost is achieved by simplification of design, having only 1 basic panel design and a single basic configuration which is scalable by adding panels in a repeating pattern, thereby maximizing utilization of key materials and maximise graphite and heat exchanger piping costs as a percentage of the overall cost of installation.
In the preferred embodiment the receiver panel can be manufactured at a single (off site) location from prefabricated parts supplied by perhaps 2 to 4 suppliers. After assembly the panels may be sealed and QC tested at the manufactured site. Hence no further assembly or testing of the panels is required on site. The panel design also optimizes usage of graphite and high pressure tubing which is manufactured in a very small number of standard dimensions. Assembly of the panels into a solar energy receiver on site is achieved by hanging the panels and the arrangement can be configured to suit different applications with varying heat storage capacities by using multiples of the panel combined to increase the size of the total assembly. Because the arrangement is suspended the lower surfaces 14 of the panels may be exposed to solar radiation and is no supporting base structure under the solar energy receiver or heat shields to protect the base. Thus additional heat is captured by the hung panel through its base.
By using the planks of graphite stacked one on top of the other and hung within a clad ‘skin’ of the housing, the skin will be tensioned due to gravity acting on the graphite core such that when the ‘skin’ expands at temperature, skin buckling which would otherwise reduce the transfer of heat to the graphite is eliminated or at least minimized.
Patent Application
20150122244
