WO2012061144A2 - Solar thermal receiver with concentric tube modules - Google Patents

Abstract

A solar thermal receiver with concentric tube modules is disclosed. The outer two tubes (101, 102) of each module form an annular gap (104) for the inlet flow (109) of a heat transfer fluid (HTF). A third inner tube (103) allows for the outlet flow (108) of the HTF. Each of the concentric tube modules is bottom-supported so as to allow for thermal expansion. Embodiments may include a structural element (750) to mitigate oscillation of the concentric tube modules and/or a top-mounted bell-shaped cap (534).

Description

TITLE: Solar Thermal Receiver with Concentric Tube Modules

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. Provisional Patent Application Serial No. 61/406,384, filed October 25, 2010, which is hereby incorporated herein by reference in its entirety for all purposes.

TECHNICAL FIELD

Embodiments pertain to solar thermal receivers with concentric tube modules for converting concentrated sunlight into heat.

BACKGROUND ART

Solar-Thermal Receivers convert concentrated sunlight, e.g., as coming from a heliostat field, into heat and are cooled by a heat transfer fluid (HTF) such as molten salt, oil, or water. In the prior art one example includes, a receiver having one or more panels of parallel absorber tubes. Fluid flow through said panels may be parallel or in an alternate/serpentine fashion. This may result in an additional part count, e.g., valves, manifolds; complex drainage procedures; added costs; and a significant pressure drop within the receiver. DISCLOSURE OF INVENTION

Embodiments include an assembly comprising: an inner wall of a cylindrical vessel and an outer wall of a toroidal shell forming a first HTF conduit configured to conduct HTF in an elevating direction; a distal portion of the inner wall of the cylindrical vessel and a distal portion of the outer wall of the toroidal shell configured to conduct the HTF toward a volume defined by the inner wall of the toroidal shell; the inner wall of the toroidal shell forming a second HTF conduit configured to conduct HTF in a descending direction; and a proximal portion of the cylindrical vessel and a proximal portion of toroidal shell forming a manifold configured to conduct HTF into the first HTF conduit and out of the second HTF conduit. Additional exemplary embodiments include the first HTF conduit further comprising a helical structure having an angle of ascent and configured to conduct HTF along the first HTF conduit.

Additional exemplary embodiments include a solar-thermal receiver comprising: a first tube; a second tube where the diameter of the first tube is greater than the diameter of the second tube; and a third tube where the diameter of the second tube is greater than the diameter of the third tube. The first tube, the second tube, and the third tube are mutually concentric. The second tube is disposed within the first tube, providing an annular gap between the outer surface of the second tube and the inner surface of the first tube for an input flow of a heat transfer fluid (HTF). The third tube is disposed within the second tube, where the third tube is separated from the second tube by an internal volume, and where the inside of the third tube is a conduit providing for an output flow of the HTF. In other exemplary embodiments, the first tube, the second tube, and the third tube are connected to a header at only one end. In additional exemplary embodiments, the internal volume between the second tube and the third tube further comprises at least one of: thermocouples, heating elements, heat absorbing materials, and insulation. In additional exemplary embodiments, the annular gap between the outer surface of the second tube and the inner surface of the first tube further comprises a helical structure having an angle of ascent and configured to conduct an input flow of the HTF. In additional embodiments, the helical structure may further comprise a pitch that varies along the length of the tube or a pitch that remains constant along the length of the tube. In some exemplary embodiments, the first tube may comprise an array of convex indentations disposed interstitial with an array of concave indentations on the second tube. Other exemplary embodiments may comprise at least one baffle in the annular gap between the outer surface of the second tube and the inner surface of the first tube. Other exemplary embodiments may further comprise at least one angular flange comprising at least one hole in the at least one angular flange in the gap between the outer surface of the second tube and the inner surface of the first tube. Additional exemplary embodiments may further comprise a throttle poppet valve, where the throttle poppet valve is further configured to adjust the flow rate of the HTF.

Exemplary method embodiments may comprise a method of monitoring the flow and thermal state of a HTF in a solar thermal receiver comprising: providing a temperature measurement device proximate to an input flow of HTF; providing a heater proximate to the input flow of HTF;

measuring a temperature of the input flow of HTF by the temperature measurement device proximate to the input flow of HTF; determining, by a processor having addressable memory, whether the measured temperature of the input flow of HTF is below a set point; if the temperature of the input flow of HTF is below the set point, then generating a command for the heater proximate to the input flow of HTF to turn on; and, otherwise, if the temperature of the input flow of HTF is not below the set point, then generating a command for the heater proximate to the input flow of HTF to turn off. In additional exemplary embodiments, the provided temperature measurement device proximate to an input flow of HTF may be a thermocouple.

Exemplary solar-thermal receiver embodiments comprise: at least one receiver module; and bell-shaped cap, where the bell-shaped cap is affixed to the top of the at least one receiver module, and is coextensive with a top portion of the at least one receiver module. In some exemplary embodiments, the bell-shaped cavity may be supported by a structural beam extending from a header. In additional exemplary embodiments, an inner portion of the bell-shaped cavity may have a ceramic coating. This design may allow for fewer tubes than previous receivers, as a relatively greater volume of HTF may be passed through the modules while maintaining adequate receiver characteristics such as a good heat transfer coefficient and low pressure drop. The reduced supports required by this receiver design may allow for thermal deflection. The reduced supports may also allow for the addition of more tube modules, and greater HTF flow, at the same weight as other solar thermal receivers. Alternatively, the reduced supports may allow for the creation of a lighter solar thermal receiver, which may also reduce tower costs. In embodiments where the tube modules and/or headers are manufactured separately, the number of tube module welds required may decrease. Additionally, the solar thermal receiver may be broken up into parts for greater shipping efficiency. One exemplary embodiment may include drain valves located in the bottom of the receiver modules. In the exemplary bottom-supported module, the number of drain valves may be reduced compared to prior art solar thermal receivers as the design does not include a flow path that includes several up and down serpentine passes of HTF where freeze points may be created at the low point of any U-bend created. In an exemplary embodiment, the receiver modules may contain no bottom- connected U-bends.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, and in which:

FIG. 1 A is a cross-sectional view of an exemplary tube in tube embodiment of a solar thermal receiver module with a spiral path;

FIG. IB is a cross-sectional view of an exemplary tube in tube embodiment of a solar thermal receiver module;

FIG. 1C is a cross-sectional view of an exemplary tube in tube embodiment of a solar thermal receiver module having indentations to promote flow disruptions;

FIG. 2A is a cut-away view of an exemplary first tube;

FIG. 2B depicts a view of an exemplary second tube;

FIG. 3A depicts an exemplary single tube in tube receiver embodiment of a solar thermal receiver with an exemplary bell-shaped cavity; FIG. 3B depicts an exemplary solar thermal receiver with a plurality of modules;

FIG. 4 depicts an exemplary solar thermal receiver consisting of two intersecting panels, each panel comprising a plurality of parallel vertical modules;

FIG. 5 depicts an exemplary solar thermal receiver with an exemplary bell-shaped cap;

FIG. 6 is a cross-sectional view of an exemplary tube in tube embodiment of a solar thermal receiver module having a throttle poppet;

FIG. 7A depicts an exemplary receiver module with a structural element; and

FIG. 7B depicts a cross-sectional view of an exemplary receiver module with a structural element.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 A depicts an exemplary module of an exemplary solar thermal receiver embodiment comprising three concentric tubes of different diameters: a first outer tube 101, a second inner tube 102, and a third central tube 103. The first tube 101 and second tube 102 have a small diameter difference compared to their respective diameters, allowing for an annular gap 104 between the first tube 101 and the second tube 102. The dimensions of the annular gap 104 may be determined based on the pressure drop and heat transfer coefficient between a heat transfer fluid (HTF) and the absorbed flux. In addition, the receiver module outside diameter may be determined based on these latter two characteristics as well as the structural integrity of the receiver module. The larger the receiver diameter, the more it may be rigid and self-supported. There may be a greater diameter difference between the second tube 102 and the third tube 103 than the first tube 101 and the second tube 102, providing for an internal volume 105, isolated from the HTF, between the second tube 102 and the third tube 103. The dimensions of the internal volume 105, and, accordingly, the diameter of the third tube 103, may be directly correlated to the dimensions of the annular gap 104 and/or the receiver module outside diameter.

The HTF flows upwards, i.e., opposite the local gravity vector, through the annular gap 104 as it is heated by incident sunlight, i.e., from a heliostat array directed towards the surface of the first outer tube 101. The HTF may flow directly upwards through the annular gap 104 or may be guided upwards along a spiral path 106, i.e., a spiral helix having a first pitch 107, positioned within the annular gap 104. This spiral path 106 increases the overall heat transfer flow length while allowing for a short overall axial length of the receiver module. The spiral flow may also result in a gradually increasing temperature gradient from the bottom of the receiver module to the top of the receiver module. The use of a spiral path may be used to achieve the desired temperature rise of the HTF in a single pass from the bottom to the top of the receiver module, where the overall flow length is greater than the minimum required to achieve the desired temperature rise.

Upon reaching the top of the receiver module, the heated HTF is directed down and out of the solar thermal receiver through the third concentric tube 103 located within the second concentric tube 102. The outlet flow 108 may be isolated from the inlet flow 109 by the internal volume 105, which may or may not contain insulation. The concentric tubes may be made of steel, high temperature alloys, silicon carbide, graphite, ceramics, or other suitable materials known in the art. The receiver module may be exposed to incident flux on more than one side of the tube. In embodiments with a spiral flow path 106, the flow of heated HTF may reduce any temperature differential between different surface areas in the first tube circumference. The flow of HTF up a spiral path 106 may also reduce any temperature differential around the entire tube perimeter. In a case where one section of the receiver module is exposed to a zone of higher flux, the HTF may flow from the zone of higher flux to the other sections of the module, which may add heat to those other sections more effectively than by conduction through the tube surface. The exemplary receiver module design may therefore mitigate undesirable stresses and bowing of the tubes resulting from a thermal expansion differential between various sections of the module. The spiral flow path of the HTF in the annular gap 104 gives rise to a velocity differential between the HTF which flows adjacent to the inner surface of the first tube 101, i.e., the outer wall of the annular gap 104, versus the HTF which flows adjacent to the outer surface of the second tube 102, i.e., the inner wall of the annular gap 104. This type of differential promotes high flow turbulence, which increases the overall heat transfer coefficient between the HTF and the heat absorbing surface. In addition, flow disturbances may be used to increase flow turbulence. In an exemplary embodiment with a spiral flow in the annular gap 104, the created turbulence may self- clean the receiver module and remove any sediments accumulating in the annular gap 104 that would otherwise reduce the cross-sectional area. The turbulence created by the spiral flow embodiment may be great enough to remove accumulated sedimentations in a single pass.

The internal volume 105 between the second tube 102 and the third tube 103 may contain a plurality of thermocouples or other instrumentation to measure the temperature profile and the flow rate of the HTF. This internal volume 105 may also contain heating elements capable of pre-heating the receiver or melting HTF freezes, e.g., in a molten salt application. This internal volume 105 may also be filled with heat-absorbing materials such as alumina, graphite, some metals, or other materials known in the art to provide additional thermal mass to mitigate cloud transients, i.e., short- term storage or buffering. Additionally, a layer of insulation may be present in the internal volume 105 to insulate the outlet flow 108 of hot HTF from the inlet flow 109 of cold HTF to be heated by concentrated sunlight. The addition of measurement devices and/or heaters may allow for the direct monitoring and control of temperatures and flow rates in real-time or near real-time. This monitoring and control may be used to identify and prevent the freezing of molten salt or other HTFs. It may also be used to adjust the heat transfer fluid flow rate for the optimization of receiver efficiency.

The receiver module may be connected at the bottom to a header, which may provide structural support for the module. This header may comprise an inlet flow header 111 and an outlet flow header 112. In some exemplary embodiments, the outlet flow header 1 12 may comprise additional insulation 113 to prevent heat loss of the outlet flow 108 of the HTF.

In one exemplary embodiment shown in FIG. IB, the spiral fins may not be present at all, i.e., HTF flows in sheet manner along the annular gap 104. In another embodiment, baffles and fins may be joined, brazed, extruded, or welded to the inner diameter of the first tube or the outer diameter of the second tube, or both. In yet another embodiment, one or more annular flanges may be positioned along the gap, with each such flange containing holes to disrupt and spread the HTF flow with the purposes of enhancing heat transfer. Other embodiments may include a number of nested flow paths and/or flow disruptors to influence heat transfer.

In one exemplary embodiment shown in FIG. 1C, the solar thermal receiver module may have indentations 114 to promote flow disruptions. Each indentation may be configured with a depth similar to the annular gap 104 width. The first tube 101 and the second tube 102 may contain a large number of indentations 114 to promote flow disturbances in the annular gap 104. These indentations 114 may be pre-fabricated on the circumference of each of the tubes 101,102. The indentation pattern design may increase flow turbulence and, therefore, heat transfer.

As shown in FIGS. 2A and 2B, a protruding spiral fin 210 may be present about the second tube 202. In some embodiments, this fin 210 may be integrated by non-welding means, e.g., brazing or by extruding the fin to the inner wall of the first tube 201. In other embodiments, the fin 210 may be welded onto the second tube 202. Cold HTF, such as molten salt, oil, or a coolant fluid, enters the annular gap created between the inner surface of the first tube 201 and the outer surface of the second tube 202 from the bottom and flows upward as it is progressively heated due to absorbed solar radiation. On reaching the top, the HTF is then re-directed into the third tube, or a conduit therein, and flows to the bottom of the receiver where it is conveyed out, e.g., to a hot tank storage. The pitch of the spiral path may remain constant throughout the length of the receiver module. In other exemplary embodiments, the pitch of the spiral path may be varied along the tube length to achieve variable HTF velocity during operation and to achieve a desired convective heat transfer coefficient. This feature, along with the proper choice of gap width, allows for the optimization of heat transfer and pressure drop along the receiver module.

As shown in FIG. 3A, a top-mounted bell-shaped cap 320 may be included which creates a cavity and may significantly reduce radiation losses from the portions of the receiver containing higher temperature fluid, i.e., towards the top of the receiver. In the exemplary embodiment depicted in FIG. 3 A, the entire solar thermal receiver may be a single tube module 321. This single module 321 may have a larger diameter than a receiver comprising a plurality of modules 322, as shown in FIG. 3B, and the tubes may require a thicker diameter to compensate for added pressure. The single module 321 may be useful in certain applications to reduce manufacturing costs and/or in operation with varying heliostat array 323 sizes.

Embodiments of a solar thermal receiver are presented comprising a plurality of tube in tube modules. FIG. 4 depicts one such embodiment comprising two intersecting panels 431,432, each panel comprising a plurality of parallel vertical modules 433, where each receiver module comprises three concentric tubes (as in FIG. 1). Each panel may be supported from the bottom by a special header, which allows flow into each module's annular gap, and collects the flow out of each module's third tube, or a conduit therein. The top tips may be tied together with compliant structural members, e.g., a spring or buckstay, to increase the overall structural stiffness of the receiver assembly, but may not require any substantial support structures and/or additional insulation.

Because the receiver modules are bottom-supported they are free to thermally expand upward independent of each other to accommodate non-uniform fluxes and module-to-module temperature variations.

The receiver modules may be replaced and/or repaired independent of the remaining receiver modules, as they may be independently attached to the header at only one end. In one exemplary embodiment, the receiver modules may be detachably attached to the bottom manifold assembly. In embodiments where the receiver modules are detachably attached by means other than welding, the receiver modules may be readily assembled and/or repaired on-site. The use of detachably attached receiver modules may also allow for mass production on an assembly line rather than requiring that the entire solar thermal receiver be built and assembled in one place. By accomplishing a significant portion of the assembly off-site, the assembly time on-site could be significantly reduced and the overall process could be accomplished more efficiently. In other exemplary embodiments, the receiver modules and inlet flow header may be pre-manufactured. The header may be manufactured to have slots in the inlet flow header which set the receiver modules in the correct orientation relative to the header. Accordingly, any final assembly and welding to this configuration may be done on- site.

FIG. 5 depicts another exemplary solar-thermal receiver configured as two intersecting panels 531,532, each panel comprising several parallel vertical modules 533, with a bell-shaped cavity 534 affixed to the top of the receiver. This X-receiver may work with the bell-shaped cavity 534 to minimize heat losses due to the cavity effect. In one exemplary mode of operation, cold HTF flows within the annular gap created between the inner surface of the first tube and the outer surface of the second tube of each receiver module from the bottom to the top as it is heated, creating a temperature profile substantially monotonic with receiver height. FIG. 3B illustrates an X-receiver of this type mounted atop a tower surrounded by ground mounted heliostats.

The use of a bottom-supported module in a solar thermal receiver may simplify reflector pointing strategies allowing for the addition of a bell-shaped cavity cap. The bell-shaped cavity cap may reduce radiation and convection losses from the hottest portion of the solar thermal receiver. The bell-shaped cap may be supported by a structural beam positioned in the middle of the receiver modules, i.e., in the middle of the X in the X-receiver embodiment shown in FIGS. 3B, 4, and 5. The bell-shaped cap may also be supported in such a manner so as to accommodate the expansion of individual receiver modules. In some embodiments, it may be desirable to direct less flux to the top portion of the receiver modules, because they may not be able to accept as much flux as the cooler lower portions. Embodiments utilizing a bell-shaped cavity cap 534 that extends down over the top portion of the receiver modules may prevent heliostats positioned distant from the solar thermal receiver from directing flux onto the top portion of the receiver modules. Accordingly, such a cap 534 may allow only closely situated heliostats to direct flux onto the top portion of the receiver modules. In some embodiments, the cap 534 may have a ceramic coating on the inner portion. In additional embodiments, the cap 534 may be used in conjunction with heliostat array pointing strategies to achieve optimal heating of the HTF. The cap 534 may reduce radiation and/or convection losses.

FIG. 6 depicts an exemplary receiver module containing a throttle poppet valve 640 wherein the throttle poppet 640 may have a throttle poppet shaft within the third tube 603 in contact with outlet HTF flow 614. The poppet valve 640 may allow for automatic flow rate adjustment based on outlet HTF temperature on a per-module basis, which may be especially useful when the flux is nonuniform. This control may be achieved through an individual flow-regulation valve per module or set of modules. The poppet valve may also work in conjunction with embedded thermocouples 641, instrumentation, and/or heaters to achieve real-time changes in the HTF flow rate. The throttle poppet 640 may include a small gap which allows for expansion of the poppet regardless of the thermal properties of the material used. In one embodiment, a pressurized conduit and valve may be connected to the top of the receiver, for the purposes of assisting and speeding up drainage of HTF from the gap, normally based only on gravity.

FIG. 7A depicts an exemplary receiver module with a structural element 750, e.g., a structural beam. In this exemplary embodiment, the bottom of the module may be connected to a header while the top may be loosely connected to the structural element 750. This connection may be done using a spring type material 751, e.g., spring steel, in combination with an insulation 752 to allow sufficient compliancy for thermal expansion. The spring steel cantilever beam 751 may be welded, or otherwise connected, into the structural beam element 750. The spring steel beam width may be between 1/4 and 1/3 of the receiver module diameter. An insulative material 752 may be attached to the spring steel beam 751 using a high temperature adhesive. At ambient temperatures, the module may have a small interference with the insulative material. At high operating

temperatures, the module may expand both horizontally outward and vertically upward, and thus react with the spring steel beam 751. The spring steel beam 751 is designed to have enough compliancy to accommodate a large deflection of the receiver module which may result in a small reaction force between the receiver and the spring steel beam 751. In addition, the receiver module is free to expand upward by sliding against the insulation 752. This structural element 750 may mitigate oscillation phenomena of the receiver caused by high wind conditions, stormy weather, and other natural phenomena.

FIG. 7B depicts a cross-sectional view of an exemplary receiver module with a structural element 750. The spring steel beam 751 extends down a portion of the receiver module 753. In some exemplary embodiments, the length of the spring steel beam 751 may be approximately 1/10 of the overall receiver module length.

It is contemplated that various combinations and/or sub-combinations of the specific features and aspects of the above embodiments may be made and still fall within the scope of the invention.

Accordingly, it should be understood that various features and aspects of the disclosed embodiments may be combined with or substituted for one another in order to form varying modes of the disclosed invention. Further it is intended that the scope of the present invention herein disclosed by way of examples should not be limited by the particular disclosed embodiments described above.

Claims

What is claimed is:

1. An assembly comprising:

an inner wall of a cylindrical vessel and an outer wall of a toroidal shell that form a first heat transfer fluid (HTF) conduit configured to conduct HTF in an elevating direction;

a distal portion of the inner wall of the cylindrical vessel and a distal portion of the outer wall of the toroidal shell are configured to conduct the HTF toward a volume defined by the inner wall of the toroidal shell;

the inner wall of the toroidal shell forms a second HTF conduit configured to conduct HTF in a descending direction; and

a proximal portion of the cylindrical vessel and a proximal portion of toroidal shell form a manifold configured to conduct HTF into the first HTF conduit and out of the second

HTF conduit.

2. The assembly of claim 1 wherein the first HTF conduit further comprises a helical structure having an angle of ascent and configured to conduct HTF along the first HTF conduit.

3. A solar-thermal receiver comprising:

a first tube;

a second tube wherein the diameter of the first tube is greater than the diameter of the second tube; and

a third tube wherein the diameter of the second tube is greater than the diameter of the third tube;

wherein the first tube, the second tube, and the third tube are mutually concentric, the second tube is disposed within the first tube providing an annular gap between the outer surface of the second tube and the inner surface of the first tube for an input flow of a heat transfer fluid (HTF), the third tube is disposed within the second tube, wherein the third tube is separated from the second tube by an internal volume, and wherein the inside of the third tube is a conduit providing for an output flow of the HTF.

4. The solar thermal receiver of claim 3 wherein the first tube, the second tube, and the third tube are connected to a header at only one end.

5. The solar thermal receiver of claim 3 wherein the internal volume between the second tube and the third tube further comprises at least one of: thermocouples, heating elements, heat absorbing materials, and insulation.

6. The solar thermal receiver of claim 3 wherein the annular gap between the outer surface of the second tube and the inner surface of the first tube further comprises a helical structure having an angle of ascent and configured to conduct an input flow of the HTF.

7. The solar thermal receiver of claim 6 wherein the helical structure further comprises a pitch that varies along the length of the tube.

8. The solar thermal receiver of claim 6 wherein the helical structure further comprises a pitch that remains constant along the length of the tube.

9. The solar thermal receiver of claim 3 wherein the first tube comprises an array of convex indentations disposed interstitial with an array of concave indentations on the second tube.

10. The solar thermal receiver of claim 3 further comprising an at least one baffle in the annular gap between the outer surface of the second tube and the inner surface of the first tube.

11. The solar thermal receiver of claim 3 further comprising an at least one angular flange

comprising an at least one hole in the at least one angular flange in the gap between the outer surface of the second tube and the inner surface of the first tube.

12. The solar thermal receiver of claim 3 further comprising a throttle poppet valve, wherein the throttle poppet valve is further configured to adjust the flow rate of the HTF.

13. A method of monitoring the flow and thermal state of a heat transfer fluid (HTF) in a solar thermal receiver comprising:

providing a temperature measurement device proximate to an input flow of HTF;

providing a heater proximate to the input flow of HTF;

measuring a temperature of the input flow of HTF by the temperature measurement device proximate to the input flow of HTF;

determining, by a processor having addressable memory, whether the measured temperature of the input flow of HTF is below a set point;

if the temperature of the input flow of HTF is below the set point, then

generating a command for the heater proximate to the input flow of HTF to turn on; and otherwise, if the temperature of the input flow of HTF is not below the set point, then generating a command for the heater proximate to the input flow of HTF to turn off.

14. The method of claim 13, wherein the provided temperature measurement device proximate to an input flow of HTF is a thermocouple.

15. A solar-thermal receiver comprising:

at least one receiver module; and

a bell-shaped cap, wherein the bell-shaped cap is affixed to the top of the at least one receiver module and is coextensive with a top portion of the at least one receiver module.

16. The solar thermal receiver of claim 15 wherein the bell-shaped cavity is supported by a

structural beam extending from a header.

17. The solar thermal receiver of claim 15 wherein an inner portion of the bell-shaped cavity has a ceramic coating.