EP3111146A1

EP3111146A1 - Advanced cavity receivers for parabolic solar troughs

Info

Publication number: EP3111146A1
Application number: EP14870350.7A
Authority: EP
Inventors: Oliver J. Brambles; Emil CASHIN; Troy O. Mcbride; Joel STETTENHEIM; Scott Aaron SNYDER; Albert K. KIM; Paul H. HOGAN; Ellen C. MEYER; Ariana M. SOPHER; Samuel F. BAUER
Original assignee: Norwich Technologies Inc
Current assignee: Norwich Technologies Inc
Priority date: 2013-12-11
Filing date: 2014-12-11
Publication date: 2017-01-04
Also published as: WO2015089273A1; EP3111146A4

Abstract

A heat-absorbing element partly enclosed in an insulating layer or jacket, has absorbing surface that is accessible to solar radiation. The thermal insulation is designed to provide entry to solar radiation by way of a cavity. In cross-section, the insulation jacket has approximately the form of a letter "C" or "U" oriented so that the opening of the "C" or "U" faces toward a parabolic collector that focuses light thereon. The absorbing surface can be substantially planar (i.e., flat) rather than another shape (for example, in one embodiment, an arc) which reduces the absorbing surface's area for a given aperture width. This reduces the area that can re-radiate heat energy away from the heat absorbing element. The heat absorbing element is supported so that its distance relative to the parabolic collector does not change as the heat absorbing element expands upon heating and contracts upon cooling.

Description

ADVANCED CAVITY RECEIVERS FOR PARABOLIC SOLAR TROUGHS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to and the benefit of co-pending U.S.

provisional patent application Serial No. 61/914,646, filed December 11, 2013, co-pending U.S. provisional patent application Serial No. 61/914,726, filed December 1 1, 2013, copending U.S. provisional patent application Serial No. 61/914,795, filed December 11, 2013, and co-pending U.S. provisional patent application Serial No. 62/078,742, filed November 12, 2014, each of which applications is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under DE-EE0006687 awarded by the DOE. The government has certain rights in the invention.

FIELD OF THE INVENTION

[0003] The invention relates to concentrating solar power in general and particularly to a system that employs reflective troughs to focus solar radiation upon a linear receiver.

BACKGROUND OF THE INVENTION

[0004] Solar parabolic troughs, which focus sunlight on tubes carrying a fluid that conveys heat for steam generation (e.g. to a steam-driven electric generator, for industrial process heat, or for the generation of process steam) or to a body of material for energy storage, are a proven, reliable, and relatively low-cost technology for collecting energy. They are the most widely deployed form of concentrating solar power (CSP) at this time. The tubes upon which the light is focused in such systems are typically termed "receiver tubes," "receivers," or "heat collection elements." At a typical electric generation plant employing solar parabolic troughs, many receivers (for example, in one embodiment, thousands) are arrayed with reflective troughs in parallel rows to form a "field" that can collect sufficient energy for a generating system of economical size. At present, receivers represent approximately 12% of the capital cost of a concentrating solar power installation employing solar parabolic troughs. [0005] In a typical receiver constructed according to the prior art, a central liquid- carrying tube with an outer optical absorption coating is surrounded by a vacuum held within a transparent concentric envelope. Light focused on the receiver by a mirror (also known as a "collector") passes through the transparent concentric envelope and through the vacuum and impinges on the central liquid-carrying tube. The coating on the central tube absorbs most (preferably all, although this cannot be realized in practice) of the energy incident upon it and is thus heated. This heat is transmitted by conduction through the wall of the central tube and thence to the tube's liquid contents. The heated liquid is pumped through the receiver and, in general, through additional receivers, being thus raised to a high temperature (for example, in one embodiment, 400 °C) before being pumped to a boiler, or energy storage device (for example, in one embodiment, reservoir of hot fluid). The function of the vacuum between the inner, fluid-carrying tube and the outer, transparent envelope is to prevent loss of heat from the receiver by convection and conduction to the outer envelope and thence, by radiation and conduction, to the environment.

[0006] A number of problems in the use of standard vacuum-containing receiver tubes have been observed. These include, but are not limited to, the following: (a) The absorption coatings on the inner, fluid-carrying tube are expensive to manufacture, (b) Degradation of a receiver's vacuum entails increased thermal losses from the receiver and, if severe enough, requires replacement of the receiver. In practice, vacuum degradation causes failure of 1-5% of receiver tubes per year, (c) The tubular outer glass envelope of a conventional receiver should preferably be thick enough to withstand the stresses imposed by containing a vacuum as well as by wind and its own weight. This strength requirement increases the cost of the envelope, (d) The receiver not only absorbs radiant energy but emits it, particularly in the infrared part of the spectrum. Emission losses increase with temperature T approximately as the fourth power of T (i.e., as Γ⁴). Energy thus emitted is for the most part lost to the environment, diminishing the receiver's efficiency. Moreover, the absorptive coating may be destroyed by sufficiently high T. Prohibitively large T⁴ radiation losses, coupled with high-temperature instability of the absorber coating, today prevent practical operation of solar parabolic -trough generating plants at elevated temperatures (for example, greater than 500 °C). Yet, for fundamental thermodynamic reasons it is more efficient to operate any thermal generating plant at higher peak T. [0007] A receiver design that offers advantages over the standard, vacuum-containing receivers of the state of the art has been disclosed and discussed in U.S. patent application

Serial Number 13/831,632, issued on , 201- as U.S. Patent No. -,— ,— , hereinafter the

Stettenheim patent, the disclosure of which is incorporated herein by reference in its entirety.

[0008] There is a need for improved receivers for parabolic-trough solar power applications.

SUMMARY OF THE INVENTION

[0009] According to one aspect, the invention features an improved receiver useful in a solar power application. The improved receiver comprises a radiation-absorbing element configured to contain a fluid heat-transfer medium, the radiation-absorbing element configured to operate in a gaseous environment rather than in a vacuum, the radiation- absorbing element having a solar absorber coating on at least a portion of an exterior surface of the radiation-absorbing element to form an absorbing surface, the radiation-absorbing element having first and second apertures at respective ends thereof, the apertures configured to allow the fluid heat-transfer medium to pass through the radiation-absorbing element; a thermally insulating jacket disposed around at least a portion of the radiation-absorbing element other than the absorbing surface; a shell configured to contain said radiation- absorbing element and said thermally insulating jacket; an optical cavity defined at least in part by said radiation absorbing element, a plurality of cavity sidewalls, and a lower transparent aperture configured to permit solar radiation to pass therethrough; a mirror configured to reflect solar radiation into the aperture; and a plurality of fixed rigid supports configured to support the radiation-absorbing element in fixed optical alignment with the mirror without vertical motion irrespective of an expansion or a contraction of the radiation- absorbing element.

[0010] In one embodiment, the radiation-absorbing element comprises a tube.

[0011] In another embodiment, the tube has a cross section selected from the group of cross sections consisting of a circular cross section, a rectangular cross section, a rectangular cross section having rounded corners, an elliptical cross section, and a lenticular cross section.

[0012] In another embodiment, the radiation-absorbing element has a planar absorbing surface. [0013] In yet another embodiment, the plurality of fixed rigid supports are configured to allow free axial movement of the radiation-absorbing element with respect to the shell.

[0014] In still another embodiment, the plurality of fixed rigid supports comprise slider supports.

[0015] In a further embodiment, the plurality of fixed rigid supports comprise rollers.

[0016] In yet a further embodiment, the shell comprises a plurality of sections, at least one of the plurality of sections having a telescope sleeve at one end thereof.

[0017] In an additional embodiment, the thermally insulating jacket comprises two or more graded layers of insulation.

[0018] In one more embodiment, the two or more graded layers of insulation are distinct layers.

[0019] In still a further embodiment, the two or more graded layers of insulation are disposed with a higher-grade insulation proximate to the radiation-absorbing element, while a lower-grade insulation is less proximate to the radiation-absorbing element.

[0020] In one embodiment, the thermally insulating jacket comprises blended insulation.

[0021] In another embodiment, the improved receiver lacks a vacuum envelope and lacks a baffle which would be required for supporting the vacuum envelope.

[0022] According to another aspect, the invention features an improved receiver useful in a solar power application. The improved receiver comprises a radiation-absorbing element configured to contain a fluid heat-transfer medium, the radiation-absorbing element configured to operate in a gaseous environment rather than in a vacuum, the radiation- absorbing element having a solar absorber coating on at least a portion of an exterior surface of the radiation-absorbing element to form an absorbing surface, the radiation-absorbing element having first and second apertures at respective ends thereof, the apertures configured to allow the fluid heat-transfer medium to pass through the radiation-absorbing element; a thermally insulating jacket disposed around at least a portion of the radiation-absorbing element other than the absorbing surface; a shell configured to contain the radiation- absorbing element and the thermally insulating jacket; an optical cavity defined at least in part by the radiation absorbing element, a plurality of cavity sidewalls, and a lower transparent aperture configured to permit solar radiation to pass therethrough; a mirror configured to reflect solar radiation into the aperture; and a plurality of fixed rigid supports configured to support the radiation-absorbing element in fixed optical alignment with the mirror without vertical motion irrespective of an expansion or a contraction of said radiation- absorbing element.

[0023] According to a further aspect, the invention features an improved receiver useful in a solar power application. The improved receiver comprises a radiation-absorbing element configured to contain a fluid heat-transfer medium, the radiation-absorbing element configured to operate in a gaseous environment rather than in a vacuum, the radiation- absorbing element having a solar absorber coating on at least a portion of an exterior surface of the radiation-absorbing element to form an absorbing surface, the radiation-absorbing element having first and second apertures at respective ends thereof, the apertures configured to allow the fluid heat-transfer medium to pass through the radiation-absorbing element; a thermally insulating jacket disposed around at least a portion of the radiation-absorbing element other than the absorbing surface; a shell configured to contain the radiation- absorbing element and the thermally insulating jacket; an optical cavity defined at least in part by the radiation absorbing element, a plurality of cavity sidewalls, and a lower transparent aperature configured to permit solar radiation to pass therethrough; a mirror configured to reflect solar radiation into the aperture; wherein the radiation-absorbing element has a nearly continuous absorbing surface such that a cross section of a lower portion of said radiation-absorbing element is nearly identical from said inlet end to said outlet end.

[0024] In another embodiment the radiation-absorbing element is displaced from a focal point of the mirror configured to reflect solar radiation into the aperture.

[0025] In a further embodiment, the tube has a cross section that has a first region that is substantially circular and a second region that is substantially planar, the first and the second regions in thermal contact.

[0026] The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.

[0027] Furthermore, it is to be understood that the features of the various

embodiments described herein are not necessarily mutually exclusive and may exist in various combinations and permutations. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] The objects and features of the invention can be better understood with reference to the drawings described below, and the claims. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views.

[0029] FIG. 1A is a schematic diagram of a cavity receiver with a cylindrical tube.

[0030] FIG. IB is a schematic diagram of a cavity receiver with a rectangular tube having rounded corners.

[0031] FIG. 2A is a schematic diagram illustrating that the optical focal point of the trough may be offset from the geometric center of the tube.

[0032] FIG. 2B is a line plot of the relationship between optical efficiency and absorber surface opening width, given cylindrical tube of fixed diameter.

[0033] FIG. 3 is a schematic diagram of a receiver comprising protruding cavity walls.

[0034] FIG. 4 is a schematic diagram of a receiver comprising graded insulation layers.

[0035] FIG. 5 is a schematic diagram of a receiver comprising a conformally shaped cylindrical shell with a conformal aperture cover.

[0036] FIG. 6 is a schematic diagram of a receiver comprising a conformally shaped cylindrical shell and protruding cavity walls with a planar aperture cover.

[0037] FIG. 7 is a schematic diagram of a receiver comprising a conformally shaped cylindrical shell, a conformal aperture cover, protruding cavity walls, and graded insulation.

[0038] FIG. 8A is a schematic diagram of aspects of tube geometry pertaining to thermal performance.

[0039] FIG. 8B is a schematic diagram of aspects of tube geometry pertaining to thermal performance.

[0040] FIG. 9 is a diagram that shows a range of nonstandard cross-sectional tube shapes.

[0041] FIG. 10 is an illustration of a lay-up method for shaping foam in the foam cavity of a receiver. [0042] FIG. 11 is an illustration of a press-forming method for shaping foam in the foam cavity of a receiver.

[0043] FIG. 12 is an illustration of a foam-injection method for shaping foam in the foam cavity of a receiver.

[0044] FIG. 13 A is a schematic diagram of an illustrative trough CSP system comprising a receiver supported by swing-jointed struts, according to prior art, in a cool state.

[0045] FIG. 13B is a schematic diagram of an illustrative trough CSP system comprising a receiver supported by swing-jointed struts, according to prior art, in a hot state.

[0046] FIG. 14A is a schematic side view of a continuous running tube suspension system for a run of cavity receivers in a cool state.

[0047] FIG. 14B is a schematic side view of a continuous running tube suspension system for a run of cavity receivers in a hot state.

[0048] FIG. 15A is a schematic side view of a bound running tube suspension system for a run of cavity receivers in a cool state.

[0049] FIG. 15B is a schematic side view of a bound running tube suspension system for a run of cavity receivers in a hot state.

[0050] FIG. 16A is a schematic side view of a thermal expansion joint between cavity receiver sections in a cold state.

[0051] FIG. 16B is a schematic side view of a thermal expansion joint between cavity receiver sections in a hot state.

[0052] FIG. 17A is a schematic diagram (axial view) of a roller-type system for supporting a cavity receiver tube without constraining horizontal movement of the tube.

[0053] FIG. 17B is a schematic diagram (lateral view) of a roller-type system for supporting a cavity receiver tube without constraining horizontal movement of the tube. \

[0054] FIG. 18A is a schematic diagram (axial view) of a system for supporting a cavity receiver tube without constraining horizontal movement of the tube.

[0055] FIG. 18B is a schematic diagram (lateral view) of a system for supporting a cavity receiver tube without constraining horizontal movement of the tube.

[0056] FIG. 19A is a schematic diagram (axial view) of a second roller-type system for supporting a cavity receiver tube without constraining longitudinal movement of the tube.

[0057] FIG. 19B is a schematic diagram (lateral view) of a second roller-type system for supporting a cavity receiver tube without constraining longitudinal movement of the tube. [0058] FIG. 20A is a schematic diagram (axial view) of a third roller-type system for supporting a cavity receiver tube without constraining longitudinal movement of the tube.

[0059] FIG. 20B is a schematic diagram (lateral view) of a third roller-type system for supporting a cavity receiver tube without constraining longitudinal movement of the tube.

[0060] FIG. 21 is a schematic side view of a wire suspension system for supporting a cavity receiver tube without constraining longitudinal movement of the tube.

[0061] FIG. 22 is a schematic side view of an indirect roller-type system for supporting a cavity receiver tube without constraining longitudinal movement of the tube.

[0062] FIG. 23 is a schematic side view of a tube suspension system according to another embodiment of the invention.

DETAILED DESCRIPTION

[0063] We describe improved receivers for parabolic -trough solar power applications that optimize nonstandard receiver tube and shell geometries under a range of real-world physical and cost constraints, feature improved manufacturability, feature methods of mechanical support and management of thermal expansion which reduces complexity while improving system performance, and employ solar-selective coatings with lower cost, tuned absorption spectra, and long service life in air at high temperature. We describe

improvements in areas that include receiver geometry design for co-optimized thermal and optical performance, material usage minimization, manufacturability, mechanical support and management of thermal expansion, and solar-selective coatings.

[0064] In particular, the invention addresses the following issues and provides advantages over the prior art. Receivers are provided that, relative to state-of-the-art receivers, (1) do not suffer from vacuum degradation, (2) have absorptive coatings and other components that are less costly and simpler to manufacture, (3) operate durably with undiminished or improved overall energetic efficiency at today's typical operating temperatures (for example, in one embodiment, 350 C) or at higher temperatures (for example, in one embodiment, at 600 C or above), (4) can in various embodiments be used either in the construction of new power plants or as drop-in replacements for conventional receivers in existing power plants, (5) employ nonstandard cross-sectional tube and shell geometries co-optimized for thermal and optical performance under a number of

simultaneous constraints, (6) feature improved manufacturability vis-a-vis the forming of an insulation enclosure of the high-temperature, fluid-carrying receiving tube, (7) are mechanically supported by novel means that cope with thermal expansion while suppressing undesirable positional changes of the receiver, (8) employ improved solar-selective coatings to maximize absorption of incident solar energy, and offer advantages additional to those just described. Such improved receivers are expected to lower the cost of energy for

concentrating solar power produced using solar parabolic troughs. The receivers disclosed herein preserve and extend the advantages of the receivers disclosed in the Stettenheim patent, including operation up to higher temperatures than have hitherto been standard in solar trough concentrating solar power systems, while adding novel advantages over the prior art.

[0065] As used hereinafter, a "tube" or object having a "tubular" form is any elongated, two-ended, hollow body whose cross-sectional form is a simple closed figure (for example, in one embodiment, a circle, a rectangle, or a rectangle with rounded corners). A tube may be either closed or open at its ends. The invention pertains to a tubular receiver or heat-absorbing element for use in concentrating solar power systems. Light is focused along the length of the receiver by a trough-shaped collector having a reflective surface that is typically parabolic in cross-section.

[0066] According to one aspect, the invention features a central tube or pipe (herein also termed "the radiation-absorbing element") through which a fluid heat-transfer medium flows. A portion of the energy focused upon the receiver by the collector is ultimately absorbed by the fluid medium (for example, in one embodiment, the fluid medium is heated and/or undergoes a phase change). The fluid is then circulated through the radiation- absorbing element and circulated through piping in order to transport the heat energy to a boiler, to a storage unit, or to another destination.

[0067] The invention also features a solar absorber coating on part or all of the exterior surface of the radiation-absorbing element. The portion of the radiation-absorbing element's surface that is coated with the absorbent coating is herein termed the "absorbing surface." The solar absorber coating is designed to absorb a large portion of the light that impinges upon it, converting this energy to the form of heat; is designed to be stable at temperatures up to and in excess of 400 °C; and is designed to have both high optical absorptance and low thermal emissivity. That is, the solar absorber coating effectively absorbs light, especially in the visible part of the spectrum, but tends not to re-radiate the energy thus absorbed as infrared light. Thus, energy collected by the radiation-absorbing element tends to be retained rather than dissipated to the environment in the form of infrared radiation.

[0068] The invention also features a thermally insulating jacket around at least a portion of the radiation-absorbing element other than the portion of the radiation-absorbing element that is coated with the solar absorber coating. In some embodiments, the thermally insulating jacket is opaque.

[0069] The invention also features an enclosure (herein termed the "shell"), which can be a second tube that surrounds the radiation-absorbing element and thermally insulating jacket. The shell admits light through a non-opaque strip or segment (herein also termed the "aperture"), planar or non-planar, that runs lengthwise along the shell.

[0070] In various embodiments, the aperture may be covered partly or wholly by one or more strips of a solid, transparent material (for example, in one embodiment, glass) or may consist partly or wholly of an unobstructed opening. The heat-absorbing element is located within the shell, aligned with the shell, and separated from the shell by an intervening space at most points. The space between the pipe and the shell is herein also termed the "cavity." The absorbing surface of the heat-absorbing element is exposed to light that enters through the aperture. Light that has entered through the aperture may impinge directly on the absorbing surface, or may undergo one or more reflections or be absorbed and re-emitted one or more times before impinging on the absorbing surface. The portion of the cavity through which light can pass after entering the aperture or being reflected or re-emitted within the cavity is herein termed the "optical cavity"; the absorbing surface is exposed to (i.e., forms one surface or wall of) the optical cavity. The portion of the cavity between the heat- absorbing element and the shell that is not the optical cavity is filled with the thermally insulating jacket and is herein termed the "insulation cavity." Portions of the cavity not occupied by some solid material are occupied by a gas (for example, in one embodiment, ordinary air) at or near ambient atmospheric pressure.

[0071] In one embodiment, the optical cavity and the insulation cavity are separated by two barriers that are herein termed the "sidewalls." The sidewalls bound the sides of the optical cavity and may in some embodiments comprise (and in other embodiments consist essentially of) strips of a relatively thin material. The sidewalls prevent or impede the mixing of gas in the insulation cavity with gas in the optical cavity. The aperture, the absorbing surface, and the sidewalls are positioned and sized so that when the receiver is approximately aligned with the focus of a trough-shaped mirror of specific dimensions, light reflected from any portion of the mirror enters the aperture along a path that leads directly to the absorbing surface of the collector. Light focused from the mirror does not impinge substantially upon the sidewalls. The sidewalls may be opaquely absorbent, opaquely reflective, or transparent. Further features of the cavity -type receiver are comprised by various embodiments of the present invention, including but not limited to reflective parabolic cavity sidewalls and lensing aperture cover, planar absorbing surface of the heat-absorbing element, displaceable aperture cover, and aerodynamic shaping of the receiver's outer surface.

ASPECTS PERTAINING TO OPTICAL-THERMAL CO-OPTIMIZATION

[0072] Several aspects of the present invention relate to co-optimization of optical and thermal performance, primarily through design geometry, with simultaneous

accommodation of additional constraints such as minimization of materials usage. The co- optimization problem can be stated as follows: Energy is delivered to the receiver optically but is converted to thermal energy by the absorber surface and transferred thence to the fluid contents of the absorber tube. Energy losses may therefore occur in both optical and thermal forms (for example, in one embodiment, optical loss occurs with failure to direct light accurately to the absorber surface, and as a result of reflection of light away from the receivers, while thermal loss occurs with radiation of energy as infrared (IR) light from various receiver surfaces and conduction of heat from receiver components to support structures or ambient air). Ideally, both optical and thermal efficiency would be at the maximum dictated by thermodynamic limits, but this is technically unachievable. There are inherent trade-offs between optical collection and thermal retention of energy. By way of example, any area exposed to collect incident photons may also radiate IR photons. The hotter the tube the more heat all portions of the receiver will tend to radiate as given by the Stefan -Boltzmann law (or T⁴ radiation law). Although ideal optical delivery of energy and thermal conversion and retention of energy are not achievable, system design can co-optimize optical performance and thermal performance, subject to materials usage and other constraints, in order to reduce to a practical minimum total per-unit cost of thermal energy delivered by the receiver. Thus, in a typical design process for various embodiments, thermal performance data (measured and/or simulated) can advantageously be developed iteratively with optical performance data to identify designs having the best overall performance. In an illustrative actual study, the thermal and optical performance of a fully specified cavity receiver design can be studied with and without concentrating secondary optics (for example, in one embodiment, compound parabolic concentrator ("CPC") sidewall reflectors in the optical cavity). In one such study, it is found that a CPC-incorporating design similar to that of FIG. IB has better thermal performance than that of a non-CPC design but worse optical performance due to the imperfectly reflectivity of the CPC surface (modeled as 95% as a realistic best case). In a non-CPC design, light from the concentrating trough does not reflect directly from the sidewalls. At lower temperatures, optical performance tends to be more important than thermal performance because heat loss is lower. Up to 650°, the illustrative study found that the receiver without CPC secondary optics performs better than a design with near-ideal CPC. Given the reduced cost and complexity of the non-CPC design relative to the CPC design, a design choice might on these grounds be made in favor of the non-CPC design. Essentially all other features, dimensions, and materials considered for a specific advanced cavity receiver design, including designs comprising embodiments of the invention, may be comparatively and jointly evaluated by means of such numerical simulations.

[0073] In various embodiments, the invention features cavity walls (for example, in one embodiment, planar or parabolic) that extend beyond the body of the shell in order to minimize shell bulk while assuring adequate gathering of light into the aperture and adequate cavity depth to minimize heat loss.

[0074] In yet another embodiment, the invention features two or more graded layers of insulation within the cavity, distinct or blended, arranged around the tube (or radiation- absorbing element) so that higher-grade insulation is proximate to the high-temperature tube, while lower-grade insulation is less proximate. The result is increased economy of manufacture while maintaining satisfactory control of thermal through the shell.

[0075] In still another embodiment, the exterior of the shell may be conformal to the shape of the tube. Herein, a "conformal" shape or component is one that is geometrically similar to another shape or component and co-axial or approximately so with that other shape or component: for example, in one embodiment, a cylindrical shell surrounding a cylindrical tube is a conformal shell. Uniform insulation depth around the tube may thus be maintained while minimizing insulation and shell material usage and overall receiver weight. Receiver weight savings propagate to material savings in support structures.

[0076] In further embodiments, novel tube cross-sectional shapes of arbitrary form are employed that optimize thermal performance (overall thermal efficiency of the receiver) under a set of simultaneous constraints, including collection of energy by the absorbing surface of the tube, radiation of energy by the absorbing surface of the tube, radiation of energy by the insulated surface of the tube, ratio of tube volume to total tube area, efficient flow of fluid within the tube, cost of materials, cost of manufacture, mechanical strength of the tube (for example, in one embodiment, until self-weight load in a horizontal position), and more. The resulting novel tube shapes depart from state-of-the-art cylindrical (circular cross-section) receiver tubes, which have advantages in manufacturing simplicity, strength for a given weight, and volume-to-total-surface-area ratio, but which do not optimize thermal performance with respect to the asymmetric thermal environment of a partly-insulated tube residing in a cavity receiver. The novel, multiply-constrained tube designs of various embodiments also depart from receiver tubes having a rounded-rectangle cross section, which are shown and described in the Stettenheim patent and which feature a flat absorber surface but which do not optimize with respect to material usage and insulated tube surface area.

ASPECTS PERTAINING TO MANUFACTURE

[0077] A cavity receiver comprises a number of components that do not exist in state- of-the-art vacuum-based receivers. A component having a bearing on the performance of the cavity receiver is the insulation that essentially fills the insulation cavity. This insulation should preferably conform to the shape of the cavity volume defined jointly by the tube, shell, and cavity walls. Prior art for the filling of the insulation cavity has primarily featured subtractive machining and multi-part layup (the crafting of multiple, precisely shaped parts that are then encapsulated, for example by placing a film of suitable encapsulant on their surfaces, and emplaced in the insulation cavity during receiver assembly). In various embodiments, the cavity insulation of the receiver is shaped and emplaced using techniques not hitherto applied to the manufacture of solar cavity receivers, including net-shape molding processes using the shell as a mold, or a mold shaped similarly to the shell. It is expected that these manufacturing innovations will reduce or eliminate insulation waste during

manufacture and enable greater mechanization of receiver manufacture. Moreover, encapsulation of insulation to exclude moisture and contain particles is achieved in the prior art primarily by application of foil to the shaped surface. In various embodiments, depending on the mechanical properties of the insulation or insulations to be used, foil may be pre- emplaced in the insulation cavity; or, spray-type encapsulation is used to permit

mechanization of the encapsulation stage of shaped insulation manufacture. In other embodiments encapsulation of the insulation is applied to only to some of the insulation surfaces as other surfaces are protected by other structures such as the shell.

ASPECTS PERTAINING TO MECHANICAL SUPPORT AND ACCOMMODATION OF THERMAL EXPANSION

[0078] Receivers should preferably be mechanically supported at a precise location above the reflecting trough. Differential heating of receiver components (hottest possible tube, coolest possible shell) causes differential expansion of components. Joining receivers end-to-end, supporting them in place, and managing differential thermal expansion and contraction are managed in the state of the art using complex and suboptimal arrangements, which include jointed swing-arm supports that allow lateral receiver movement as well as end-bellows that allow tubes to expand and contract without losing vacuum or fluid integrity. Swing arm supports can entail vertical motion of the receiver, which may vary complexly along the length of a trough run, compromising optical efficiency. Bellows are complex, welded, metal structures and accounts for ~7% of heat loss from the receiver run under realistic assumptions. See, for example, Wu et al, "Three-dimensional numerical study of heat transfer characteristics of parabolic trough receiver," Applied Energy, Vol. 1 13, Jan. 2014, pp. 902-211. Accommodation of thermal expansion becomes proportionally more difficult as thermal expansion increases with higher operating temperature, as in systems now envisaged for concentrating solar power with cavity receivers. In various embodiments, disadvantages of the prior art are overcome by the suspension of the tube and shell in a manner that permits free axial movement of the expanding tube with respect to the shell; bellows-free coupling of receiver sections; and accommodation of thermal expansion without vertical displacement of the receiver and without the associated optical issues.

ASPECTS PERTAINING TO COATINGS [0079] The absorber coating of a non-vacuum cavity receiver is also a consequential component. The coating should preferably have favorable absorptive and emissive properties, endure a long service lifetime at high temperature while exposed to air, and be affordably manufacturable. In various embodiments of the present invention, the coating may comprise any coating having acceptable optical properties, durability in air at high temperature, and manufacturability. Examples of such coatings include, for example, the Liu nanochain coating; the multilayer Ni-Si-Al "S Solar SolGel" coating developed by SunStrip,

Inc.; Si-Ge nano material coatings or nanoceramic material coatings developed at the

University of San Diego; and the "black chrome" coating developed by Olymco.

[0080] FIG. 1A and FIG. IB pertain to illustrative forms of the advanced cavity receiver to which the present invention adds novel features, as will be made clear by subsequent Figures and accompanying discussion. Before describing the cavity receivers of

FIG. 1A and FIG. IB in detail, we note the following general considerations, which pertain to various embodiments of the invention: Various embodiments, including those depicted in

FIG. 1 A and FIG. IB, comprise a tubular heat-absorbing element (also herein termed "the tube") partly enclosed in an insulating layer or jacket, with a recess or cavity (also herein termed the "optical cavity") filled with air or a neutral gas, transparent to light on one side along an aperture, adjacent an exposed surface (also termed "the absorbing surface") of the heat-absorbing element. In cross-section, the insulation jacket has approximately the form of a letter "C" or "U" that can be turned, or is permanently turned, so that the opening of the

"C" or "U" faces toward a parabolic collector that focuses light thereon. In this analogy, the surface of the letter seen through the gap in the "C" or "U" corresponds to the optical cavity and the width of the gap in the "C" or "U" corresponds to the width of the receiver aperture.

A portion of the heat-absorbing element, herein termed the "absorbing surface," is exposed to light entering the optical cavity through the aperture. Making the absorbing surface planar

(i.e., flat) rather than another shape (for example, in one embodiment, an arc) reduces the absorbing surface's area for a given aperture width. This is advantageous because reducing the absorbing surface's area reduces thermal loss via emission of infrared radiation from the surface. Other non-planar surfaces while having somewhat larger radiating surface areas may have preferable properties with respect to structural stability and fluid dynamics including end to end pressure drop or pumping head loss and efficiency of thermal transfer between the absorber tube and the heat carrying internal fluid. [0081] An additional design consideration in various embodiments is the inclusion or omission of a transparent aperture cover (for example, in one embodiment, of glass) enclosing the recessed optical cavity. Omitting the aperture cover tends to improve optical performance but reduces thermal performance by allowing relatively static stratified air- temperature zones within the cavity that reduce thermal loss to be disturbed by external gas flows (for example, in one embodiment, by wind). Omitting the aperture cover also tends to expose the absorber surface to potential contamination (for example, in one embodiment, by dust). In certain embodiments, removal of the glass may be preferred, for example, when the entire mirror structure is enclosed in a transparent greenhouse structure.

[0082] Receiver designs incorporating (a) a tubular heat-absorbing element partially surrounded by an insulating jacket, (b) a recessed optical cavity with sidewalls (either passive sidewalls or compound parabolic concentrators), and (c) a portion of the heat-absorbing element that is exposed to light in at least some states of operation of the receiver and which, when exposed to light, acts as absorbing surface (either planar or curved), are herein also referred to as illustrative of an "advanced cavity receiver geometry."

[0083] Various embodiments of the invention seek to co-optimize optical and thermal performance of the advanced cavity receiver, to reduce materials usage, to manufacture the insulation component of the receiver more efficiently, to mechanically support the receiver in a manner that reduces complexity and confers other advantages, and to coat the receiver tube using high-performance, readily-manufactured, durable coatings.

[0084] FIG. 1 A is a cross-sectional diagram that depicts features of an illustrative receiver 100 that shows one possible response to some of the design considerations discussed in the foregoing paragraphs. This design is a dramatic departure from the construction of standard, vacuum-based receiver tubes. This advanced cavity receiver is simpler to manufacture and maintain than a conventional vacuum-tube receiver because it does not contain a vacuum and reduces the exposed glass component. Elimination of the vacuum confers a number of advantages, including obviation of airtight seals and hydrogen-absorbing "getter" inserts, elimination of vacuum failure (responsible for failure of -1-5% of conventional vacuum-containing receivers per year in real-world usage), and easing of design constraints on end-bellows or equivalent contrivances for accommodating thermal expansion (since there is no airtight seal that needs to be protected against mechanical failure). [0085] An illustrative advanced cavity receiver 100 comprises a shell 102; a heat- absorbing element or tube 104 having a cylindrical cross section, through which a heat- transfer fluid 106 can flow; planar sidewalls 108, 1 10, an aperture 1 12; a transparent aperture cover 1 14 spanning the aperture 1 12; an insulating jacket 1 16 partially surrounding the heat- absorbing element 104; an absorbing surface 118 coated with an absorbent coating and exposed to light entering the aperture 1 12; and an optical cavity 120 bounded by the absorbing surface 118, sidewalls 108, 1 10, and aperture cover 114. The shell 102 and insulation 1 16 may be opaque or transparent in the visible portion of the electromagnetic spectrum but will preferably be opaque in the infrared portion of the spectrum. The receiver 100 is approximately uniform in cross-section along its entire length, apart from mounting and other hardware (not shown). The dimensions Dl (shell width), D2 (minimum shell-to- tube insulation thickness), D3 (tube external diameter), D4 (minimum tube-to-aperture-cover distance), and D5 (linear or chord width of absorbing portion of tube surface) are indicated in FIG. 1A. In an illustrative embodiment of receiver 100, these dimensions have the values Dl = 6.6 in, D2 = 1.27, D3 = 3.0 in, D4 = 1.18 in, and D5 = 2.21.

[0086] The optical cavity 120 or other portions of the collector 100 may be evacuated or filled with a gas (for example, in one embodiment, air) at approximately ambient atmospheric pressure. If the gas is air, the advantage is gained over the prior art that no provision need be made to exclude ambient air from any portion of the receiver 100 except the fluid-containing tube 104.

[0087] The specific dimensions of the components of a receiver having a schematic cross-section like receiver 100 are chosen to optimize performance (i.e., solar energy collected) when the receiver 100 is mounted at the focus of a trough-shaped collector of specific dimensions whose reflective surface is parabolic in cross-section.

[0088] In brief, receiver 100 operates as follows: light rays are focused across a range of angles upon the aperture 112 by a parabolic collector (not shown). All light rays passing through the transparent aperture cover 1 14 strike the absorber surface 118 after either (a) traversing the optical cavity 120 directly or (b) reflecting off a sidewall 108 or 110. The absorber surface 1 18 absorbs a large portion of the solar radiation incident upon it and is heated thereby.

Reference is now made to FIG. IB. The illustrative advanced cavity receiver 122 of FIG. IB is similar in features and dimensions to that depicted in FIG. 1A except that the tube 126 is square in cross section rather than circular, with the result that absorber surface 124 of the tube 126 is approximately planar rather than being a circular arc in cross-section (as is the case for absorber surface 1 18 of receiver 100). A planar absorbing surface 124 minimizes absorber area for a given size aperture 112 in comparison to any non-planar absorbing surface, which in turn minimizes IR radiation from the absorber surface 124. Thus, for a given coating composition at a given temperature, the planar absorber surface of FIG. IB will radiate less energy than the non-planar absorber surface of FIG. 1A simply because the area of the former is smaller. The dimensions Dl (shell width), D6 (minimum shell-to-tube insulation thickness), D7 (minimum tube-to-aperture-cover distance), and D8 (linear width of absorbing portion of tube surface, same as width of square tube 126) are indicated in FIG. IB. In an illustrative embodiment of receiver 122, these dimensions have the values Dl = 6.6 in, D6 = 1.76, D7 = 1.18 in, and D8 = 2.5 in.

[0089] FIGS. 2A through 9 all illustrate features of the invention that pertain to various embodiments that are optimized for optical, thermal and/or cost performance.

[0090] Reference is now made to FIG. 2A. FIG. 2A is a schematic cross-section of an illustrative cavity receiver 200 comprising a cylindrical tube 104 and exposing an absorber surface 118 whose areal size is determined by the opening width 202. The focal point of the trough mirror (not shown) is, in the conventional prior art of vacuum-based, non-insulated, non-cavity receivers, co-located with the axis of the tube. Numerical analysis shows, however, that in advanced cavity receivers (for example, in one embodiment, of the type shown in FIGS. 1A and IB), maximum optical efficiency is achieved when the trough focal point is offset from the tube axis by some Offset distance (indicated in Fig. 2A). FIG. 2B is a plot 200 of illustrative numerical simulation data showing the relationship between Offset

(marked in FIG. 2A) and cylindrical absorber diameter (marked in FIG. 2A; also, dimension

D3 in FIG. 1A) for optimum optical efficiency in a cavity receiver comparable to receiver

100 of FIG. 1A. The horizontal axis is Offset of Absorber Center from the focal axis of the trough, and the vertical axis is the diameter of the cylindrical absorber. The angular width of the optical cavity for this simulation is 55 degrees; the rim-to-rim reflecting trough aperture is

5 m; and the optical cavity aperture width (aperture width 1 12 in FIG. 1A) is 8 cm. The x- axis intercept (absorber offset 0) is at approximately Offset = 0.7. The relationship between offset and absorber diameter is approximately linear except for the diameter region ~8-9 cm.

In embodiments where tube cross-section is non-cylindrical, the relationship of offset to absorber dimensions may generally resemble that shown in FIG. 2B but will differ in detail. Other design variations, such as the employment of secondary focusing by parabolic sidewall reflects in the cavity 120 of FIG. 1A, or the depth of the optical cavity (dimensions D4 in FIG. 1A and D7 in FIG. IB) may also alter the details of the relationship between offset and efficiency.

[0091] FIG. 3 is a cross-sectional schematic of an illustrative cavity receiver 300 according to an embodiment of the invention. In receiver 300, the optical cavity 120 is flanked by sidewalls 108, 1 10 that are extended beyond the width of the shell 102. By geometric analysis it is evident that the longer the sidewalls 108, 1 10 are made (at a given angle), the deeper will be the optical cavity depth 302, that is, the distance between the nethermost point of the tube 104 and the aperture cover 1 14. Control of optical cavity depth 302 is advantageous because too-shallow depth 302 does not permit the formation of air layers stratified by temperature within the cavity 120. The formation of such stratifications has been found by numerical modeling and experiment to improve the thermal efficiency of an advanced cavity receiver (for example, in one embodiment, receiver 300) by reducing convective transport of heat between the tube 104 and the aperture cover 114. In particular, numerical simulation studies of the a range of advanced cavity receiver designs similar to those shown in FIG. 1 show that it performs well with an air-filled cavity due in part to thermal stratification of air within the cavity. Daily sun tracking of the collector changes the receiver operating angle, generating a convective loop in the cavity across a range of angles. Losses through conduction are minimal at 0° tilt, since stable stratified layering of air is established in the cavity, with the hottest temperatures strictly at the top of the cavity. As tilt increases, the stratified layer of air breaks down and a one-cell convection pattern is generated in the cavity. Heat is convected from the absorber to the aperture cover, where it is lost via conduction through the glass. At 80° tilt, the losses by conduction through the aperture cover are comparable to losses by conduction through the insulation. Depending on receiver geometry, the increase in heat loss at 650 °C is between 15-25% from vertical to full (80°) tilt.

[0092] Sidewall extension enables the depth 302 of the optical cavity 120 to be designed independently of the thickness of the insulation layer 306. This is advantageous because the thickness of insulation layer 306 may be reduced (for example, in one embodiment, by using more efficient insulation) while retaining an optical cavity depth 302 sufficient to allow thermal stratification of the air therein. Thus, extension of the sidewalls 108, 1 10 allows two aspects of the thermal efficiency of the receiver 300, the optical cavity depth 302 and the thickness of insulation layer 306, to be addressed independently.

Moreover, in various embodiments the extended sidewalls 108, 1 10 may be advantageously insulated, partly or wholly, using insulating materials the same as or different from those employed in other parts of the receiver: insulation of sidewalls will reduce energy losses from heated air contained in the optical cavity.

[0093] FIG. 4 is a cross-sectional schematic of an illustrative cavity receiver 400 according to some embodiments of the invention. In receiver 400, the tube 104 is concentrically surrounded by graded layers of insulation, for example, in one embodiment, a first layer 402 and a second layer 404. Preferably the first layer 402 will in some embodiments comprise (and in other embodiments consist essentially of) insulation having a high R value (or low thermal conductivity), which in some embodiments can be a high-grade form of insulation such as one of the microporous insulations made by MicroTherm, Thermodyne, or Zircar. For example, the microporous insulation manufactured by Zircar is a combination of ultra- fine silica powders, specially processed refractory oxides and glass reinforcing fibers. Such insulation can withstand the high temperatures reached by the tube 104. The second layer 404 preferably in some embodiments comprises (and in other embodiments consists essentially of) an insulation material having higher thermal conductivity, and/or less heat-resistant, but also less costly and with thermal performance comparable to that of the material used in the first layer 402. The total material cost of the two layers 402, 404 may thus be reduced without compromising the thermal performance of the receiver 300. Table 1 lists the properties of illustrative candidate for use in the lower- temperature second layer 404, with engineering information current as of the time of filing of this application.

TABLE 1

[0094] In various other embodiments, three or more graded insulation layers might be comprised by the insulation component of the receiver. In still other embodiments, a continuously graded rather than discretely layered body of insulation might be constructed around the tube 104. Even where per-unit- volume cost of insulating material in more outward portions of the insulation cavity is not lower, if a lower-density material can be employed then total receiver weight is decreased. Weight savings in the receiver tend to propagate, with advantage, to the structural components of the trough solar power system: i.e., lighter receivers can be supported by lower-strength, lower-cost mechanical members.

[0095] FIG. 5 is a cross-sectional schematic of an illustrative cavity receiver 500 according to some embodiments of the invention. In receiver 500, the tube 104 is cylindrical; in various similar embodiments, the tube 104 may have a non-standard shape optimized for a set of constraints, as will be made clearer with reference to FIG. 8 and FIG. 9. In receiver

500, the shell 502 and aperture cover 504 are conformal, that is, approximately parallel the shape of the tube 104; in this case, the tube 104 and the shell 502 and aperture cover 504 have the form of concentric cylinders. An insulation layer 506 is present. Advantages of the receiver shown in 500, with cylindrical cross-section, are that a cylinder contains maximal volume per unit of surface area and thus of tube, shell, and cover material; the cylindrical shell 502 has minimal (i.e., radiating) surface area for the volume it contains; a conformal shell 502, cylindrical or otherwise, enables the thickness of the insulation layer 506 to be uniform or approximately so; a cylinder is mechanically strong for a given type and thickness of material; and light rays 508, 510 focused by a parabolic mirror upon the receiver 500 will tend to impinge upon the aperture cover 504, as shown by dashed arrows in FIG. 5, from normal or approximately normal angles, minimizing light reflection from the aperture cover 504 and thus increasing optical efficiency. For a flat aperture cover 1 14 such as that shown in FIG. 1A, FIG. IB, and FIG. 3, some of the light rays focused on the receiver by a parabolic reflector will be incident at non-normal angles and some fraction of incident light will tend to be reflected from, rather than transmitted through, the aperture cover, as described by Snell's Law. In various embodiments the shell and aperture components need not be distinct parts: for example, the shell and aperture may in some embodiments comprise (and in other embodiments consist essentially of) a single tube (for example, in one embodiment, a cylindrical glass tube), thus avoiding any need for joints, seals, and retention features.

[0096] FIG. 6 is a cross-sectional schematic of an illustrative cavity receiver 600 according to some embodiments of the invention. The receiver 600 comprises a cylindrical tube 104, a conformal shell 602, an insulation layer 506, and extended optical cavity sidewalls 108, 1 10. In various similar embodiments, the tube 104 may have a non-standard shape optimized for a set of constraints. As for receiver 300 of FIG. 3, sidewall extension enables the depth 302 of the optical cavity 120 to be designed independently of the thickness of the insulation layer 506. The design of receiver 600 combines advantages described for receiver 500 of FIG. 5 with advantages described for receiver 300 of FIG. 3.

[0097] FIG. 7 is a cross-sectional schematic of an illustrative cavity receiver 700 according to some embodiments of the invention. The receiver 700 comprises a cylindrical tube 104, a conformal shell 602, a conformal aperture cover 504, a first insulation layer 704, a second insulation layer 706, and extended optical cavity sidewalls 708, 710. In various similar embodiments, the tube 104 may have a non-standard shape optimized for a set of constraints. The design of receiver 700 combines advantages described for receiver 300 of FIG. 3, receiver 400 of FIG. 4, receiver 500 of FIG. 5, and receiver 600 of FIG. 6. Receiver 700 is illustrative of the way that various aspects of the invention may be combined to co- optimize receiver optical performance and thermal performance under set of constraints. In other embodiments of the receiver 700, the conformal aperture cover 504 can be extended and placed along or at the bottom of the optical cavity sidewalls 708, 710. In further embodiments the conformal aperture cover 504 can be planar. Various other embodiments comprise features and advantages not depicted herein, such as compound parabolic reflective sidewalls for the optical cavity 712, as described in the Stettenheim patent.

[0098] FIG. 8A is a cross-sectional schematic of geometric aspects of an illustrative cavity receiver 800. The purpose of FIG. 8A is to illustrate some aspects of tube geometry optimization pertinent to the thermal performance of the receiver 800; however, additional aspects of tube and receiver design (for example, in one embodiment, materials usage, cost of manufacture, mechanical strength), not mentioned with reference to FIG. 8A, are in general also pertinent to design optimization.

[0099] In FIG. 8A, a circle 802 represents a cross-sectional tube geometry. A absorbing surface of linear or chord width W exposes an absorbing portion of surface of the tube to light focused symmetrically along the length of the tube 800 by a parabolic mirror (not shown). A circular cross section 802 minimizes tube surface-to-volume ratio, which is advantageous because this minimizes (a) material per unit length of tube for given wall thickness and (b) total area over which heat is lost from the tube. A circular cross section also assures relatively simple manufacture and offers high mechanical strength for a given weight. However, departures from circularity may be advantageous in the highly asymmetric thermal environment of the receiver. The main asymmetry of the tube's environment is the division between an absorber area 1 18 and an insulated area 806. The absorber area 1 18 can be minimized by flattening to the line 808. The interior tube volume per unit length above the line 808 (i.e., above the plane defined by line 808) is labeled VI in FIG. 8A, and the volume per unit length below the line 808, if any exists, is labeled V2. The total interior tube volume per unit length is V = VI + V2.

[00100] For absorber coatings that are relatively indifferent to angle of incidence, area minimization will likely optimize thermal performance, because area minimization minimizes

IR radiation from the absorber. Flattening reduces V by setting V2 = 0 (no volume below the line 808). For absorber coatings, if any, that are sensitive to angle of incidence (for example, in one embodiment, for which a closer-to-normal angle of incidence improves absorption, a curved absorber surface 1 18 may be preferable; for example, in one embodiment, a parabolic cross section 810, conformal to the parabolic mirror focused upon the receiver, might have an average angle of incidence closer to normal over its whole surface. However, under some conditions (for example, in one embodiment, due to focusing error), a planar absorber surface might have an average angle of incidence closer to normal than any curved absorber surface. A parabolic absorber cross-section 810 would increase V by increasing V2 relative to an absorber surface 118 with circular arc cross-section.

[00101] Heat is lost not only from the absorber surface (for example, in one embodiment, surface 118), but also from the insulated portion 806 of the tube surface.

Reducing this surface area will therefore also tend to reduce thermal losses through the insulation (not shown). For a given VI, insulated surface area 806 is minimal for an insulated-surface cross section having the form of a circular arc. Therefore, by areal considerations alone (a simplified case), a circular arc cross-section would appear to be optimal. Moreover, reducing the arc length of insulated area 806 will reduce both insulated surface area 806 and VI. Circular arc 812 shown as a dotted line is illustrative of a cross- section for insulated area with lower VI than for circular arc cross-section 802. As the radius of the circular arc approaches infinity, the arc approaches the straight line 808 and insulating area approaches a minimum defined by W.

[00102] However, various constraints apply. Minimizing both absorber area 118 and insulated area 806 would reduce V to zero and the tube would carry no fluid. An arc 812 that reduces insulated area while retaining nonzero V, paired with a planar, minimal-area absorbing surface at line 808, would produce an interior tube cross section having sharp interior corners (for example, in one embodiment, 814). Fluid flow along the tube would likely be impeded, increasing drag and reducing heat transport. Moreover, a too-flat tube cross section will likely suffer significantly decreased mechanical strength in flexure as compared to the original cylindrical cross section 802. Also, any departure from the cylindrical cross-section 802 will consume more tube wall material, insulation, and shell material per unit of fluid volume accommodated.

[00103] Further, issues of heat transference between the absorber surface and the fluid within the tube arise that are particular to the design of advanced cavity receivers. In general, an advanced cavity receiver geometry is effective at reducing heat loss largely because it covers the non-absorbing surfaces of the absorber tube with state-of-the-art insulation. In addition, the surface area available to absorb the incoming solar radiation is reduced due to the increased collector focal length appropriate for such a receiver (i.e., a longer focal length concentrates light upon the absorber tube over a narrower angular range, permitting a smaller optical cavity and relatively larger, insulation-covered non-absorbing surface with consequently lower heat loss). Thus, the incoming solar flux is more concentrated on the exposed absorber surface than in prior-art receiver designs. If the coefficient of heat transfer between the absorber tube and fluid remains constant, the increased solar flux concentration with smaller area will require a larger ΔΤ (temperature difference) between the fluid with the absorber tube and the tube itself to maintain rate of heat transfer to the fluid. However, it is desirable to minimize this ΔΤ both to reduce both heat loss and to ease thermal constraints on the absorber tube and its coating. For example, computational fluid dynamics models of heat transfer from pipe to heat-transfer fluid (HTF) suggest that for a given mass flow rate, achieving a heat transfer of 4000 W/m from pipe to HTF requires a ΔΤ of 15-20 K for a Schott PTR-70 receiver (state-of-the-art) and 30 K for one embodiment of a cavity receiver. Such models show that due to the heat-loss reductions of the advanced cavity receiver design, the efficiency advantage of an illustrative square-tube cavity receiver (for example, in one embodiment, that of FIG. IB) is actually increased when maximum absorber temperatures are considered, despite the greater ΔΤ required for un-enhanced heat transfer in a square tube. Further modeling compares the merits of an illustrative cylindrical-tube design to an illustrative design similar except in having a square tube. Such modeling shows poorer relative performance for the square-tube design in terms of heat transfer efficiency to the fluid, due to locally low heat-transfer coefficients in the slow-flowing corner regions.

However, the square-tube design shows superior heat-loss performance, which follows from the fact that the square tube has a smaller exposed surface area than the optimized circular pipe geometry and a comparable ΔΤ between the exposed surface of the absorber tube and fluid.

[00104] All these constraints, as well as others not mentioned here, should preferably be evaluated simultaneously, generally by numerical computer simulation methods, in order to specify a tube cross-section optimized relative to an ultimate, overall figure of merit, typically cost per unit of thermal energy delivered by the receiver. Some such tube cross- sections are nonstandard in comparison to the prior art, which comprises cylindrical and rounded-rectangular tube geometries.

[00105] FIG. 8B is an illustrative cross-section 816 used for such optimization. The circle 802 is shown for visual reference. The dashed line 822 corresponds to the cross- section 816. A planar absorber area 818 is mated by corner- free matching curves 820, 820' to a circular arc 822. Together, the matching curves 820 and circular arc 822 form the cross- section of the insulated tube area 824. The resulting nonstandard tube shape 816 is representative of a class of tubes herein referred to as "lenticular." A range of lenticular and other nonstandard (i.e., non-cylindrical, non-rounded-rectangular) tube designs, their precise characters to be determined by analysis subject to optical, thermodynamic, material, manufacturing, and other constraints, potentially including the co-design of other receiver components and other system components (for example, in one embodiment, trough mirror), is contemplated and within the scope of the invention. In various embodiments, nonstandard tube cross-sections include geometric shapes such as ellipses, semicircles, circle-on-base shapes, and others.

[00106] FIG. 9 depicts five illustrative tube cross-sections 900 (standard cylindrical),

902 (cylindrical tube mated conductively to planar absorber), 904 (elliptical), 906 (lenticular),

908 (rounded rectangular), all with extended optical cavity sidewalls. Other illustrative cross sections includes those that could be formed by extruded processes in which interior members such as vertical planes or otherwise could be included to improve heat transfer, mechanical strength or other operational parameters. The lenticular cross-section 906 most closely resembles the illustrative lenticular cross-section 816 of FIG. 8B.

[00107] FIGS. 10 through 12 illustrate features of the invention that pertain to manufacture of an advanced cavity receiver according to various embodiments.

[00108] FIG. 10 is a cross-sectional schematic depiction of a first illustrative method for shaping and emplacing the insulation component of an advanced cavity receiver. In the method of FIG. 10, two receiver shell halves 1002, 1004 are manufactured separately. For each shell half 1002, 1004, an insulation block 1006, 1008 is shaped by subtractive machining, molding, or other shaping of an originally larger insulation blank (not shown) so that the insulation blocks 1006, 1008 fit the receiver shell halves and other components (for example, in one embodiment, receiver tube) that bound the insulation cavity of the receiver.

After suitable encapsulation of the insulation to exclude moisture and preserve insulation block integrity, the blocks 1006, 1008 are emplaced in the shell haves 1002, 1004, which are then joined, as indicated by horizontal dashed lines. The method of FIG. 10 is a multi-part layup method. Other multi-part layup methods may entail the formation of more or fewer than 2 insulation blocks. Repeated application of the method of FIG. 10 may produce graded layers of insulation such as those depicted in FIG. 4 and FIG. 7.

[00109] FIG. 11 is a cross-sectional schematic depiction of a second illustrative method for shaping and emplacing the insulation component of an advanced cavity receiver. In the method of FIG. 11, in Step 1, a receiver shell 1 100 is manufactured and placed within a supportive mold support 1102 having a complementary shape. The shell 1 100 acts as a mold, reinforced by the mold support 1 102. A quantity of insulation material 1104 is placed within the receiver shell 1100. The insulation material 1104 may be dry, foamed, viscous, or otherwise capable of changing its shape in response to mechanical pressure. A die 1 106 is aligned with, and above, the shell 1 100. In Step 2 of the method, the die 1106 is pressed into the insulating material 1 104, which is deformed to fill the shell 1100. The mold support 1 102 prevents the shell 1 100 from being deformed during the shaping process. In or after Step 2, the insulating material may be heated, cooled, or permitted to harden to enable it to retain the shape given by the die; or, the insulating material 1 104 may simply retain its new shape. In Step 3 of the method, the die and mold support are removed, leaving the formed insulation 1 104 within the shell 1100. The method of FIG. 11 is a die-and-mold method. Clearly, repeated application of the method of FIG. 1 1 using differently-shaped dies may produce graded layers of insulation such as those depicted in FIGS. 4 and 7. The method of FIG. 1 1 is advantageous relative to that of FIG. 10 in that little or no insulating material is consumed other than that which ends up inside the receiver, in contrast to subtractive machining of an insulation blank, and no complex precision machining of the insulating material is required. Moreover, contact seams between insulation sections are eliminated, minimizing loss of heat via air movement through seams. Concurrent with or after the molding process, encapsulation of the open portion of the insulation may be achieved. In one embodiment, a metal foil is placed on the surface of the die or above the shell and left in place after the forming process. The foil may be attached to the shell by welding, or may be attached by other methods such as a press fit or capture grooves in the extruded shell. In this manner, the foil is shaped and applied concurrently with the molding and shaping process of the insulation in as few as one step.

[00110] FIG. 12 is a cross-sectional schematic depiction of a third illustrative method for shaping and emplacing the insulation component of an advanced cavity receiver. In the method of FIG. 12, in Step 1, a receiver shell 1200 is manufactured. In Step 2 of the method, the receiver tube 1202 is suspended in its proper location with respect to the shell 1200 and a quantity of insulation material 1204 is placed within the receiver shell 1 100, for example, in one embodiment, by foaming, injection, or pouring. The shell 1200 and tube 1202 together act as a mold, constraining the shape of the incoming insulation material. Following Step 2, the insulating material may be heated, cooled, or permitted to harden to enable it to retain the shape given by the shell 1200 and tube 1202; or, the insulating material 1204 may simply retain its new shape. In a Step 3 of the method, the upper surfaces of the insulating material

1204 are shaped to accommodate the optical cavity sidewalls 1206, 1208. The method of

FIG. 12 is an injection-molding method. The method of FIG. 12 is advantageous relative to that of FIG. 10 in that little or no insulating material is consumed other than that which ends up inside the receiver, in contrast to subtractive machining of an insulation blank, and no complex precision machining of the insulating material, other than for shaping of the insulation faces accommodating the sidewalls 1206, 1208, is required. In other embodiments, not shown, the insulation may be formed into shape by a continuous manufacture process such as extrusion. In the extrusion process, the shell may or may not act as a form. In one embodiment, the insulation is continuously forced through a template to generate an extended length of shaped insulation. Encapsulation of the insulation may be achieved through metal foil, polymer, or other protectant layer. Encapsulation may be applied mechanically or in embodiments where a polymer encapsulation is used, may be sprayed upon the insulation.

[00111] FIG. 13A is a side-view schematic depiction of an illustrative "swing-arm" suspension system in view 1300 for a single receiver tube according to prior art. A standard cylindrical receiver tube 1302 is supported in a level orientation by a rigid strut or member

1304 that is supported at its nether end to a hinged joint 1306 and at the tube end by a second hinged joint 1308. The hinged joints 1306, 1308 are free to rotate in the plane of the drawing

(i.e., to the right or the left along the axis of the tube 1302). The nether hinge 1306 is affixed to a support structure (for example, in one embodiment, a portion of a reflective trough, not shown) and does not translate, and the tube 1308 is constrained to remain horizontal by attachments not shown in FIG. 13A. A dashed line indicates the location of the axis 1310 of the tube 1302. In FIG. 13A, the strut 1304 is vertical and the tube 1302 is in a "cool" condition, that is, at a relatively low temperature such as might prevail when the sun is down and the reflecting trough is not focusing light upon the tube 1302. The horizontal position of the tube 1302 in FIG. 13A is presumed to be constrained at the left end of the tube 1302, e.g. by the presence of a second receiver tube (not shown): that is, the tube 1302 is one of a plurality of similar tubes in an end-to-end row, all of which are undergoing approximately the same temperature changes and expansions: tube 1302 may therefore, in general, be made to shift axially as other tubes in the row also expand. [00112] FIG. 13B depicts the system in view 1300 of FIG. 13A in a second, "hot" state in view 1350 of operation, such as might prevail when the sun is high and the reflecting trough is focusing light upon the tube 1302. In its hot state, the tube 1302 will have expanded linearly by some amount 1312 (greatly exaggerated in FIG. 13B for clarity). The tube 1302 is at a much hotter temperature than all components of the system 1300 other than insulation in close proximity to the tube 1302, so the tube 1302 expands much more. Since horizontal movement of the tube 1302 is required and the tube 1308 is constrained to remain horizontal, the expansion length change 1312 can only be accommodated by net axial movement of the tube 1302, with accompanying rightward rotation of the strut 1304 (indicated by arc arrows in FIG. 13B) and downward movement of the axis 1310 of the tube 1302 from its cool position by some amount 1314. In discussions of component movements herein,

"downward" refers to movement toward a trough and "upward" refers to movement away from a trough; these directions will not necessarily correspond exactly to gravitational "down" and "up" except when the trough is in a horizontal position (untilted).

[00113] In a receiver run in a solar field supported by the swing-arm system of FIG.

13A and FIG. 13B, tubes will expand differentially along the run (due to the presence of cooler fluid within the receivers at the start of the run than at the end of the run, when focused sunlight will have heated the fluid to a maximum temperature). Different light brightnesses due to haze and clouds will also lead to different maximum operating temperatures at different times. There is therefore a tendency for the axes of individual receiver tubes to be vertically displaced by different amounts under different operating conditions, although the focus of the parabolic trough mirror is fixed. Optimal precision placement of receiver tubes at the trough focus cannot be maintained over the full range of real -world operating conditions.

[00114] The radiation-absorbing element of the present invention in various embodiments is supported so that its distance relative to the parabolic collector does not change as the radiation-absorbing element expands upon heating and contracts upon cooling, as will now be described in greater detail. In particular, we now describe various embodiments of a plurality of fixed rigid supports configured to support the radiation- absorbing element in fixed optical alignment with the mirror without vertical motion irrespective of an expansion or a contraction of the radiation-absorbing element. [00115] FIG. 14A through FIG. 22 illustrate features of the invention that pertain to the mechanical support and suspension of the advanced cavity receiver and the accommodation of changes in length caused by thermal expansion, especially as regards receivers installed in a run comprising a plurality of individual receivers in a solar field.

[00116] FIG. 14A is a side-view schematic depiction of an illustrative "continuous running tube" suspension system 1400 for a run of advanced cavity receivers according to various embodiments of the invention. A cavity receiver 1402 contains a receiver tube 1404; the upper portion of the receiver 1402 comprises a shell 1406 essentially filled with insulation 1408. Sunlight focused upward by a trough (not shown) enters the optical cavity on the nether side of the cavity receiver 1402 and heats fluid in the tube 1404. In FIG. 14A, a plurality of individual lengths of receiver have been fused or rigidly attached end-to-end to produce what is effectually a single long receiver whose length is that of the run. Only a portion of the receiver run is depicted. The receiver run is supported above the trough by a plurality of fixed rigid supports, e.g. supports 1410, 1410', which are affixed to a support structure (for example, in one embodiment, trough) by mounts, for example, in one embodiment, mounts 1412, 1412', that neither rotate nor translate. In some embodiments, the supports 1410, 1410' are vertical. The tube 1404 is supported at the upper end of the supports 1410 by "slider supports," for example, in one embodiment, supports 1414, 1414', where each slider support may be a roller cuff, low- friction cuff, or other support mechanism that constrains the tube 1404 laterally but permits the tube 1404 to move without constraint along the tube's axis. The system 1400 is herein termed a "running tube" herein because the tube 1404 is not attached or positioned rigidly with respect to the trough or ground; the system 1400 is herein termed a "continuous" running tube system because a plurality of individual receiver tubes have been fused or rigidly attached to form what is effectually a single long tube the length of the receiver run. Small clearances (not shown) between the insulation 1408 and the tube 1404, as well as the optical cavity sidewalls (not shown) and the tube 1404, reduce friction. The non-tube components of the cavity receiver are supported in a fixed position by extensions 1416, 1416' of each member 1410. Other support schemes for both the tube and non-tube components of the receiver run the schemes that depicted in FIG. 14A are employed in various other embodiments to achieve essentially the same results, namely, a freely-expanding cavity receiver tube supported at a fixed height above a reflecting trough, i.e., in a fixed relationship to the focus of the trough. In principle, receiver 1402 can be fixed at at most one point along its length by a support.

[00117] FIG. 14A depicts the system in view 1400 in a cool condition. At this temperature, the tube 1404 is approximately the same length as the insulation 1408 and shell 1406 components of the receiver run.

[00118] FIG. 14B depicts the system in view 1450 in a hot condition. At this elevated temperature, the tube 1404 has expanded and lengthened, as indicated by arrows in FIG. 14B. The tube 1404 is at a much hotter temperature than all components of the system 1400 other than insulation in close proximity to the tube 1404, so the tube 1404 expands much more. However, the tube 1404 is free to move and expand axially through the slider supports 1414, 1414'. The system of 1400 is advantageous compared to the prior-art system 1300 of FIG. 13A and FIG. 13B because no vertical displacement of the tube occurs between the cool and hot conditions, or between hot conditions of different temperatures. Since the weight of the tube 1404 is borne by the supports 1414, 1414', even the relatively small transverse (i.e., vertical, in FIG. 14B) expansion of the tube 1404 from cool to hot will not change the vertical distance of the nether (i.e., absorbing) surface of the tube 1404 from the trough, and thus will not impact optical efficiency. Other components, such as the supports 1410, 1410', remain at relatively constant temperature and do not in general change size significantly between cool and hot system states or between hot systems states of different temperatures. Thus, in the system 1400, the tube 1404 retains a more nearly fixed relationship to the focus of the trough, regardless of temperature, than a receiver tube supported according to the prior art of FIG. 13A and FIG. 13B.

[00119] FIG. 15A is a side-view schematic depiction of an illustrative "sectional running tube" suspension system in view 1500 for a run of advanced cavity receivers according to various embodiments of the invention. In the system 1500, a run is comprised of a plurality of cavity receivers. Each cavity receiver 1502 contains a receiver tube 1504 whose upper portion comprises a shell 1506 essentially filled with insulation 1508. As in system 1400 of FIG. 14A and FIG. 14B, the tube 1504 is free to move axially with respect to other components of the system 1500. Sunlight (or other electromagnetic radiation) focused upward by a trough (not shown) enters the optical cavity on the nether side of the cavity receiver 1502 and heats fluid in the tube 1504. In FIG. 15A, a plurality of individual lengths of receiver tube have been fused or rigidly attached, end-to-end, to produce what is effectually a single long receiver tube whose length is that of the run. Internal struts or supports (for example, in one embodiment, support 1509) support the tube 1504 with respect to the reset of the receiver 1502 but are flexible, rolling, or otherwise accommodate internal expansion of the tube 1504. However, the non-tube portions of each receiver— for example, in one embodiment, receiver 1502, receiver 1512 (shown in part at left), and receiver 1514 (shown in part at right)— are not rigidly attached to each other. Rather, one end of each receiver (for example, in one embodiment, receiver 1502) comprises a telescope sleeve 1516 that closely surrounds the non-sleeve-bearing end of the next receiver in the run. In Fig. 15 A, the sleeve of receiver 1510 is telescoped around the non-sleeve end of receiver 1502, and the sleeve of receiver 1502 is telescoped around the non-sleeve end of receiver 1512. The space between the inner surface of each sleeve 1518 and the shell of the receiver telescoped by the sleeve 1518 is spanned by a brush or gasket 1518 that prevents the entry of particulates and moisture and the loss of heat by convection. Moreover, each receiver (for example, in one embodiment, receiver 1502) is supported by one or more rigid vertical struts or supports 1520 that are affixed to the receiver at their upper ends and to rollers 1522 at their nether ends. Thus, each receiver individually is free to move axially on its roller-mounted supports, while the tubes, individually and jointly, are free to move axially with respect to other receiver components.

[00120] The system 1500 is termed a "running tube" because the tube 1504 (or plurality of fused tubes along the length of the trough run) is not attached or positioned rigidly with respect to the trough or ground except possibly at one support point. The system 1500 is termed a "sectional" running tube system because the tube portion of each receiver is attached to and supported within each receiver (for example, in one embodiment, by strut 1509), so the expansive motion of each cavity receiver's tube is constrained with respect to each receiver's non-tube components.

[00121] FIG. 15A depicts the system in view 1500 in a cool condition. At this temperature, the tube 1504 is approximately the same length as the insulation 1508 and shell

1506 components of the receiver 1502. The sleeves (for example, in one embodiment, sleeves 1516, 1516') of the receivers are in a maximally telescoped position.

[00122] FIG. 15B depicts the system in view 1550 in a hot condition. The tube portion

1504 of the receiver 1502 has increased in length, as have the tube portions of the other receivers in the run. FIG. 15B depicts how the ends of the lengthened tube portions have caused adjacent receivers to move farther apart: for example, in one embodiment, receiver 1510 has rolled to the left and receiver 1512 has rolled to the right. (The receiver 1502 of FIG. 15B, which happens to occupy a central position with respect to other receivers, has not been depicted as moving, though it is free to move and in general all receivers in a run will tend to move somewhat.) The telescoping sleeves have reached a maximally untelescoped position. No component has undergone significant vertical displacement from the cool to the hot condition. Thus, in the system 1500 as in system 1400, the tube 1504 retains a more nearly fixed relationship to the focus of the trough, regardless of temperature, than a receiver tube supported according to the prior art of FIG. 13A and FIG. 13B. In various other embodiments, the rollers 1522 are located at the tops of the supports 1520, and other variations on the arrangements of illustrative system 1500 are comprised.

[00123] FIG. 16A is a schematic cross-sectional depiction of another illustrative system in view 1600 according to embodiments of the invention and similar to system 1500 of FIGS. 15A and 15B, except that the telescoping sleeve 1602 comprises an insulation layer 1604 along a portion of the length of the sleeve 1602 sufficient to preserve an unbroken run of insulation between adjacent tubes during a state of maximal tube expansion. Also, the system 1600 comprises a telescoping aperture cover section 1606. The space between the inner surface of each aperture sleeve 1606 and the aperture covers 1608, 1608' is spanned by a brushes or gaskets 1612, 1612' that prevent the entry of particulates and moisture and the loss of heat by convection. Parts similarly represented in FIGS. 15 A, 15B, 16A, and 16B are to be similarly interpreted even if not explicitly labeled and described.

[00124] FIG. 16A depicts the system in view 1600 in a cool condition. At this temperature, the sleeves (for example, in one embodiment, sleeves 1602, 1606) of the receivers are in a maximally telescoped position.

[00125] FIG. 16B depicts the system in view 1650 in a hot condition. The telescoping sleeves 1602, 1606 have reached a maximally untelescoped position. No component has undergone significant vertical displacement from the cool to the hot condition.

[00126] The system in view 1400 of FIG. 14A and in view 1450 of FIG. 14B, the system in view 1500 of FIG. 15A and in view 1550 of FIG. 15B, and the system in view 1600 of FIG. 16A and the system in view 1650 of FIG. 16B are illustrative of a class of embodiments, all of which are contemplated and within the scope of the invention, that permit free axial movement of the absorber tube with respect to other components of the system as the absorber tube undergoes differential heating and thermal expansion. Such free axial movement obviates vertical movement that compromises optical efficiency. Moreover, the class of embodiments illustrated by systems 1400 and 1500 obviate the use of metallic end-bellows to accommodate absorber tube expansion in hot states, which has the advantages of simplifying receiver design and construction and eliminating a significant source of thermal loss from systems constructed according to the prior art.

[00127] A class of embodiments of which 1400 is illustrative in some embodiments comprise a "slider support," that is, a device that partly or wholly supports the weight of an absorber tube while permitting the tube to move axially without constraint. In various embodiments, a slider support does not entail direct physical contact between any component of the slider support and the absorber tube the slider support upholds: for example, sufficiently powerful permanent magnets comprised by a ring set around the absorber tube may be oriented with respect to sufficiently powerful permanent magnets comprised by the slider support in such a manner that magnetic forces support the tube with air-filled clearance between the tube and all support components, preventing the tube from coming into contact with the support under loads encountered during stable system operation. In various other embodiments, a slider support is in direct physical contact with the absorber tube it supports, as, for example, if the weight of the tube is supported by resting upon a surface, roller, ball bearing, or other component that does not constrain the axial motion of the tube. Where there is direct contact between a support and a tube, conduction of heat from the hot tube into the support will provide an avenue for energy loss. It is therefore generally desirable to (a) make the components of the support that contact the tube out of low-conductivity material, (b) reduce the contact area between the support and the tube to a minimum, or to do both of (a) and (b).

[00128] FIG. 17A is a schematic cross-sectional diagram of an illustrative first system

1700 for supporting a cavity receiver tube 1702 without constraining axial movement of the tube 1702 according to some embodiments of the invention. In FIG. 17A, a tube 1702, the outer surface of which is indicated by a dashed circle, is suspended within a sleeve or cuff comprising an upper half 1704 and a lower half 1706 by contact with at most six cylindrical posts or spacers (e.g., spacer 1708). The dimensions of the cuff and posts are set relative to the size and properties of the tube by the requirement that at highest tube temperature

(greatest expansion), the tube must not be stressed by expansive pressure against the posts, which may fracture the tube, damage the posts, or both. The posts are set into conformal grooves in the cuff and are preferably made essentially of a material with low thermal conductivity and a low coefficient of friction. In various other embodiments, the posts may vary in number from 3 to any larger number; may have non-circular cross sections (e.g., may be elliptical, triangular, or otherwise shaped in cross-section); may be permitted to rotate axially and/or slide laterally with respect to the cuff; may be mounted upon springs or other flexible components within their grooves that permit the surface of the tube to press the posts further into their grooves, if thermal expansion changes the tube's diameter so as to require such motion; and may be coated with, rather than made essentially of, a material with low thermal conductivity and a low coefficient of friction.

[00129] FIG. 17B is a schematic cross-sectional diagram of an illustrative second system 1710 for supporting a cavity receiver tube 1702 without constraining axial movement of the tube 1702 according to some embodiments of the invention. In FIG. 17B, a tube 1702, the outer surface of which is indicated by a dashed circle, is suspended within a sleeve or cuff comprising an upper half 1712 and a lower half 1714 by contact with six ball bearings (e.g., bearing 1716). The dimensions of the cuff and bearings are set relative to the size and properties of the tube by the requirement that at highest tube temperature (greatest expansion), the tube must not be stressed by expansive pressure against the bearings. The bearings are embedded in sockets set into the material of the cuff; in this cross-sectional view, only the portion of each bearing protruding from the bearing's socket is depicted. The bearings are preferably made essentially of a material with low thermal conductivity and/or a low coefficient of friction. In various other embodiments, the bearings may vary in number from 3 to any larger number; may be socketed against springs or other flexible components that permit the surface of the tube to press the bearings further into their sockets, if thermal expansion changes the tube's diameter so as to require such motion; and may be coated with, rather than made essentially of, a material with low thermal conductivity and/or a low coefficient of friction.

[00130] FIG. 17C is a schematic cross-sectional diagram of an illustrative third method

1718 of supporting a cavity receiver tube without constraining axial movement of the tube according to some embodiments of the invention. In FIG. 17C, a collar 1720 surrounds the tube. A liner 1722, preferably made essentially of a material with low thermal conductivity and a low coefficient of friction, lines the inside surface of the cuff. The tube, at maximum expansion, is essentially in contact with the liner 1722 (hence the absence of a dotted line indicating tube surface in FIG. 17C, as such a line would be superimposed on the line denoting the interior surface of the liner).

[00131] All three systems 1700, 1710, and 1718 permit free lateral motion of the tube 1702. Moreover, in various embodiments, the cuffs of all three systems 1700, 1710, and 1718 may be conformally shaped to support and laterally constrain a tube of square or other nonstandard cross-section without the introduction of further inventive novelty. However, in the system 1700 of FIG. 17A the tube 1702 is in contact with the whole length of each post, encouraging thermal loss by conduction. The system 1710 of FIG. 17B offers less contact area with the tube 1702 and therefore less opportunity for conductive loss, but requires, in practical designs, a relatively large number of ball bearings (e.g., 18), with attendant cost for this high degree of complexity. In the design 1718 of FIG. 15C, maximal tube expansion must be completely accommodated by cuff and liner size: overexpansion will likely fracture the tube. Moreover, once slip-mounted upon a receiver tube, the system 1718 cannot be removed from the tube without detaching the tube from one of its neighbors in the trough run, which may be impractical without tube breakage if tubes are welded or otherwise tenaciously sealed end-to-end.

[00132] FIG. 18A depicts an illustrative system in view 1800 and FIG. 18B depicts an illustrative system in view 1850 that possesses some of the advantages of systems 1700, 1710, and 1718 while offering other advantages.

[00133] In particular, FIG. 18A is a schematic diagram (axial cross-sectional view) of an illustrative transverse-roller system in view 1800 for supporting a cylindrical cavity- receiver tube 1702 without constraining axial movement of the tube 1702 according to some embodiments of the invention. The system 1800 comprises a triangular frame 1802 consisting essentially of three frame sections 1804, 1806, 1808 that are hinged at their three joints by bolts or pins (e.g., pin 1810, which hinges frame section 1804 to frame section 1806). A suitable locking mechanism (e.g., latch; or, nut tightened on each pin or bolt end), not shown, secures the three frame sections 1804, 1808, 1810) in position around the tube 1702. The frame 1802 may be opened or disassembled, allowing removal of the tube 1702 from the frame 1802 or of the frame 1802 from the tube 1702, with undoing of the locking mechanism. Further, each frame section 1804, 1808, 1810 comprises a roller (e.g., roller 1812 of frame section 1804) whose axis is at right angles to the axis of the tube 1702. The weight of tube 1702 rests upon at least one of the three rollers, depending on orientation of the frame 1802. The rollers are preferably made essentially of, or coated with, a material with low thermal conductivity and/or a low coefficient of friction. In various other embodiments, each frame section 1804, 1808, or 1810 comprises a fixed post rather than a roller. The number of parts required for system 1800 is likely less than that required for an embodiment similar to system 1710 in FIG. 17B; the contact area between tube 1702 and rollers (and, consequently, conductive loss) is smaller than for system 1700 of FIG. 17 A; and the system 1800 can be removed or opened without disturbing the tube 1702 or a run of such tubes. In various other embodiments, the rollers may vary in number from 2 to any larger number; may be mounted flexibly in a manner that permits the surface of the tube to press the rollers further into the frame sections, if thermal expansion changes the tube's diameter so as to require such motion; and may be coated with, rather than made essentially of, a material with low thermal conductivity and/or a low coefficient of friction. FIG. 18B is a central cross-sectional side view of the system 1800 of FIG. 18A. Two frame sections 1804, 1806 are visible in cross- section; the tube 1702 is indicated by dashed lines; a roller 1812' is visible in tilted side view; a roller 1812 is visible in cross-axial cross-section; and a pin 1810' is visible in axial cross- section.

[00134] FIG. 19A depicts a second illustrative system in view 1900 and FIG. 19B depicts a second illustrative system in view 1950 for supporting a cylindrical cavity-receiver tube 1702 without constraining axial movement of the tube 1702 according to some embodiments of the invention. In particular, FIG. 19A is a schematic axial view of system

1900 that utilizes linear bearings 1901, 1902 or other rollers, as in system 1600 but without the triangular frame of that system. Instead, system 1900 comprises brackets 1903, 1904 that may be formed by bending bar stock, or by stamping sheet stock, or by other fabrication method that produces a shape of sufficient rigidity to secure the rollers 1901, 1902 in position around the tube 1702, indicated by the dashed circle. Dashed lines indicate the planar sidewalls 108, 110 of the cavity opening that is not obstructed by any of the system 1900 components. The system may be attached to the shell (not shown) directly by bracket 1903 or indirectly by additional fastening elements. Linear bearing 1901 may be fitted onto a shaft

1905 that can be threaded at one or both ends, where it may serve to join multiple brackets

1903, 1904 through holes in those brackets which may be tapped. Shaft 1905 may also be secured with nuts 1906, pins, or other fastener, and may serve to stiffen the system 1900 against transverse deformation. Cam follower-type linear bearings 1902 may be used as rollers that are mounted on threaded studs 1907 which may fasten directly to tapped holes in brackets 1904, and which may be unscrewed to permit installation or removal of the absorber tube. In various other embodiments, the rollers and brackets may occur at different angles or in separate planes, possibly alternating sides, at regular or variable intervals along the length of the tube.

[00135] FIG. 19B is a central cross-sectional side view of the system 1900 of FIG.

19A. Two bracket sections 1903, 1904 are visible in cross-section; the tube 1702 is indicated by dashed lines; a roller 1901 is visible in cross-axial cross-section; and a cam follower-type roller 1902 is visible in oblique view.

[00136] FIG. 20A depicts a third illustrative system in view 2000 and FIG. 20B depicts a third illustrative system in view 2050 for supporting a cylindrical cavity-receiver tube 1702 without constraining axial movement of the tube 1702 according to some embodiments of the invention. In particular, FIG. 20A is a schematic axial view of system 2000 that utilizes linear bearings 2001, 2002 or other rollers, as in system 1900, but unlike that system may use a single bracket or frame 2003 to secure the rollers in their respective positions around the tube 1702 (shown as a dashed circle). Additionally, in some embodiments of the invention, a strap, wire, or other tension element 2004 may serve to secure together the free ends of rollers 2002 or of their shafts, counteracting the outward thrust imposed by the weight of the tube on the lower rollers 2003 while using a minimum of material within the space of the cavity. Dashed lines indicate the planar sidewalls 108, 1 10 of the cavity opening.

[00137] FIG. 20B is a central cross-sectional side view of the system 1900 of FIG.

19A. The bracket section 2003 is visible in cross-section; the tube 1702 is indicated by dashed lines; a roller 2001 is visible in cross-axial cross-section; and roller 2002 is visible in oblique view.

[00138] FIG. 21 is a schematic side view of an illustrative wire suspension system for supporting a cavity receiver tube 1702 without constraining longitudinal movement of the tube 1702. The system 2100 consists of thin wires 2101, preferably of material having low thermal conductivity, wrapped around the tube 1702 in a single or double helix, in a net formation, or as multiple loops. The thinness of the wires 2101 provides minimal area for heat loss by conduction. The wires 2101 are held at discrete points by hooks 2102 or other means of attachment to the interior surfaces of the shell (not shown). [00139] FIG. 22 is a schematic side view of an indirect roller-type system for supporting a cavity receiver tube 1702 without constraining longitudinal movement of the tube 1702. The system 2200 consists of thin wire or other hanger 2201, preferably of material having low thermal conductivity, supporting the tube 1702 and in turn suspended by means of a roller bearing 2202 or other frictionless mounting to a bracket 2203 that is fixed to the shell (not shown) by fasteners 2204. The surface of the bracket 2203 along which the roller 2202 glides is sufficiently wide and rigid to provide lateral stability, and may be either flat or curved. In the curved variation, the profile of the bracket 2203 may be shaped such that the axis of the roller 2202 moves away from the axis of the tube 1702 when, due to thermal expansion, the tube 1702 elongates in the axial direction as both the tube 1702 and hanger 2201 also expand transversely, in order to maintain the absorber surface in a near constant position relative to the height of the focal plane. Additionally, a curved profile could provide a self-locating mechanism whereby the roller 2202 and hanger 2201, under the influence of gravity or pre-tension forces, tend towards the closest point to the tube 1702 on the curved bracket 2203. At various intervals along the length of the receiver, brackets 2203 may be fixed to the top or side inner surfaces of the shell to provide restraint in all lateral directions.

[00140] FIG. 23 is a schematic side view of a tube suspension system according to another embodiment of the invention. In one illustrative example of this suspension system, a cut-out plate 2302 is attached to the top of the tube 1702. Mating with the cut-out plate 2302 is a suspension tab 2304 that has pins or rollers 2306 or other elements that come in contact with the top inner edge of the cut-out plate 2302 and allow lateral motion as the tube 1702 thermally expands and contracts. The suspension tab 2304 may have sections of material removed 2308 to reduce the thermal conductivity of the suspension tab 2304. In various embodiments, the suspension tab 2304 is attached to the receiver shell or some other rigid structure such that the receiver tub 1702 is supported.

[00141] Another advantage of the present invention over the prior art that relies on vacuum to reduce convective losses is the following. The prior art requires baffles to support the envelope that maintains the vacuum around the central thermal absorber structure.

Because loss of vacuum is deleterious, baffles that terminate vacuum envelopes are used at distances much shorter than the entire length of a central thermal absorber array. Therefore, there are significant numbers of such baffles present in the prior art devices. Given that the solar absorber array is generally fixed in a given orientation (such as North-South orientation so that illumination is captured as the Sun moves from East to West in the sky), there is at most one angular position of the Sun along the North-South axis that avoids having the baffles shade the central thermal absorber. As the seasons change, and the Sun travels over a more northerly or a more southerly arc, the baffles reduce the sunlight reaching the central thermal absorber by shading. The present invention does not use such baffles to maintain a vacuum envelope (and actually lacks a vacuum envelope and lacks baffles which would be required for supporting a vacuum envelope), and so such shading can be eliminated or avoided in part or in its entirety.

[00142] In current vacuum tube receivers a typical length for each unit is four meters.

In such systems there are, therefore, at least two vacuum baffles every four meters. In a typical installation a continuous string of receivers is on the order of fifty meters or more long. Given this, there are on the order of one hundred vacuum baffles or more for a string of receivers. This series of vacuum baffles introduces a significant series of interruptions in the absorber surface where solar radiation cannot be absorbed. By contrast in various embodiments of the invention, the removal of the vacuum allows for a nearly continuous absorber surface from the inlet aperture to the outlet aperture for a receiver run. In this region from the inlet to the outlet aperture, the lower portion of a cross section of the radiation absorbing element is not altered. In one embodiment the radiation absorbing element forms a continuous run of unimpeded tubing from inlet to outlet aperture. In contrast to this as mentioned above, the lower portion of a cross section of a radiation absorbing surface for a traditional vacuum receiver is altered periodically by the presence of a vacuum baffle that is used to connect to and support the outer glass shell. Again in contrast in various

embodiments of the present invention, the glass aperture at the bottom of the optical cavity is supported by the shell and not the radiation absorbing element. In various embodiments the inlet aperture is the point at which a heat transfer fluid enters a receiver run and begins increasing in temperature as energy from absorbed solar radiation is transferred to it. The outlet aperture is the point at which the heat transfer fluid leaves the receiver run with the corresponding piping being encased in opaque insulation so that no additional absorbed solar energy is transferred to the heat transfer fluid in these encased sections.

THEORETICAL DISCUSSION [00143] Although the theoretical description given herein is thought to be correct, the operation of the devices described and claimed herein does not depend upon the accuracy or validity of the theoretical description. That is, later theoretical developments that may explain the observed results on a basis different from the theory presented herein will not detract from the inventions described herein.

INCORPORATION BY REFERENCE

[00144] Any patent, patent application, patent application publication, journal article, book, published paper, or other publicly available material identified in the specification is hereby incorporated by reference herein in its entirety. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material explicitly set forth herein is only incorporated to the extent that no conflict arises between that incorporated material and the present disclosure material. In the event of a conflict, the conflict is to be resolved in favor of the present disclosure as the preferred disclosure.

[00145] While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be affected therein without departing from the spirit and scope of the invention as defined by the claims.

Claims

What is claimed is:

1. An improved receiver useful in a solar power application, comprising:

a radiation-absorbing element configured to contain a fluid heat- transfer medium, said radiation-absorbing element configured to operate in a gaseous environment rather than in a vacuum, said radiation-absorbing element having a solar absorber coating on at least a portion of an exterior surface of said radiation-absorbing element to form an absorbing surface, said radiation-absorbing element having first and second apertures at respective ends thereof, said apertures configured to allow said fluid heat-transfer medium to pass through said radiation-absorbing element;

a thermally insulating jacket disposed around at least a portion of said radiation-absorbing element other than said absorbing surface;

a shell configured to contain said radiation-absorbing element and said thermally insulating jacket;

an optical cavity defined at least in part by said radiation absorbing element, a plurality of cavity sidewalls, and a lower transparent aperture configured to permit solar radiation to pass therethrough; a mirror configured to reflect solar radiation into said aperture; and a plurality of fixed rigid supports configured to support said radiation- absorbing element in fixed optical alignment with said mirror without vertical motion irrespective of an expansion or a contraction of said radiation-absorbing element.

2. The improved receiver useful in a solar power application of claim 1, wherein said radiation-absorbing element comprises a tube.

3. The improved receiver useful in a solar power application of claim 2, wherein said tube has a cross section selected from the group of cross sections consisting of a circular cross section, a rectangular cross section, a rectangular cross section having rounded corners, an elliptical

section.

4. The improved receiver useful in a solar power application of claim 1, wherein said radiation-absorbing element has a planar absorbing surface.

5. The improved receiver useful in a solar power application of claim 1, wherein said plurality of fixed rigid supports are configured to allow free axial movement of said radiation-absorbing element with respect to said shell.

6. The improved receiver useful in a solar power application of claim 1, wherein said plurality of fixed rigid supports comprise slider supports.

7. The improved receiver useful in a solar power application of claim 1, wherein said plurality of fixed rigid supports comprise rollers.

8. The improved receiver useful in a solar power application of claim 1, wherein said shell comprises a plurality of sections, at least one of said plurality of sections having a telescope sleeve at one end thereof.

9. The improved receiver useful in a solar power application of claim 1, wherein said thermally insulating jacket comprises two or more graded layers of insulation.

10. The improved receiver useful in a solar power application of claim 9, wherein said two or more graded layers of insulation are distinct layers.

1 1. The improved receiver useful in a solar power application of claim 9, wherein said two or more graded layers of insulation are disposed with a higher- grade insulation proximate to said radiation-absorbing element, while a lower-grade insulation is less proximate to said radiation-absorbing element.

12. The improved receiver useful in a so

wherein said thermally insulating jacket comprises blended insulation.

13. The improved receiver useful in a solar power application of claim 1, wherein said improved receiver lacks a vacuum envelope and lacks a baffle which would be required for supporting said vacuum envelope.

14. An improved receiver useful in a solar power application, comprising:

a thermally insulating jacket disposed around at least a portion of said radiation-absorbing element other than said absorbing surface; a shell configured to contain said radiation-absorbing element and said thermally insulating jacket;

an optical cavity defined at least in part by said radiation absorbing element, a plurality of cavity sidewalls, and a lower transparent aperature configured to permit solar radiation to pass therethrough; and

a mirror configured to reflect solar radiation into said aperture, wherein said radiation-absorbing element has a nearly continuous absorbing surface such that a cross section of a lower portion of said radiation-absorbing element is nearly identical from said inlet end to said outlet end.

15. The improved receiver useful in a so

wherein said radiation-absorbing element is displaced from a focal point of said mirror configured to reflect solar radiation into said aperture.

16. The improved receiver useful in a solar power application of claim 14, wherein said tube has a cross section that has a first region that is substantially circular and a second region that is substantially planar, said first and said second regions in thermal contact.