WO2015088809A1

WO2015088809A1 - Hyperbolic paraboloid contoured mirrors for trough-type solar collector systems

Info

Publication number: WO2015088809A1
Application number: PCT/US2014/067932
Authority: WO
Inventors: Kenneth Biggio
Original assignee: Abengoa Solar Llc
Priority date: 2013-12-13
Filing date: 2014-12-01
Publication date: 2015-06-18

Abstract

One embodiment comprises a parabolic trough reflector including a plurality of mirrors which are configured to reflect sunlight to a lengthwise zone of concentrated solar flux. Each mirror comprises a reflective surface. The mirror surface is defined by a primary curvature within a plane perpendicular to the lengthwise zone of concentrated solar flux. In addition, the reflective surface of one or more of the mirrors comprises a secondary curvature within a plane parallel to the lengthwise zone of concentrated solar flux. The secondary curvature provides for reduced mirror bending deflection under a load, for example, gravity or wind load. Alternative embodiments include systems for generating electricity utilizing a thermal power cycle with thermal energy is provided to a working fluid from concentrated solar flux using parabolic trough reflectors having enhanced mirrors.

Description

HYPERBOLIC PARABOLOID CONTOURED MIRRORS FOR TROUGH-TYPE

SOLAR COLLECTOR SYSTEMS

TECHNICAL FIELD

[0001] The embodiments disclosed herein relate to the reflective mirrors utilized in the parabolic trough solar collectors used in concentrated solar power (CSP) electricity generation systems. In particular, the disclosed embodiments feature parabolic trough collectors including mirror elements having secondary curvature within a secondary plane, which secondary curvature provides enhanced structural rigidity to the mirror elements.

BACKGROUND

[0002] Concentrating Solar Power (CSP) systems utilize solar energy to heat a working fluid which drives a thermal power cycle for the generation of electricity.

Considerable interest in CSP has been driven by renewable energy portfolio standards applicable to energy providers in the southwestern United States and renewable energy feed- in tariffs in Spain. CSP systems are typically deployed as large, centralized power plants to take advantage of economies of scale.

[0003] One type of CSP system uses multiple parabolic trough reflectors to concentrate sunlight and directly or indirectly heat a working fluid. The thermal energy transferred to the working fluid is then used to drive a power generation cycle. Conventional parabolic trough-based CSP systems typically include a large array of individual trough reflector elements. Individual parabolic trough reflectors are usually aligned on a north-south axis, and rotated to track the sun as it moves across the sky each day. Alternatively, the trough reflectors can be aligned on an east- west axis; but this tends to reduce the overall efficiency of the collector due to losses arising from an excessive longitudinal component within the approaching light's incidence angle. East- west trough orientation only requires the trough to be aligned with the seasonal change in solar elevation however, thereby minimizing the need for tracking motors.

[0004] The general structure of parabolic trough reflectors suitable for use as a heat concentrating element in a CSP system is shown in Fig. 1 and Fig. 2. The parabolic trough reflector 100 includes a structural support subsystem 102 supporting an array of mirrors 104. The surfaces of the mirror elements are flat and without curvature in any plane parallel to the longitudinal direction shown as "L" on Figs. 1 and 2. The surfaces of the mirror elements 104 are formed in a parabolic curve within a plane perpendicular to the longitudinal direction, shown as "W" on Figs. 1 and 2. The mirrors 104 are implemented with highly reflective metal or metallized glass facets. Therefore, the energy of incident sunlight (represented by a dashed arrow in Fig. 2) is focused along a lengthwise zone of concentrated solar flux which corresponds with the position of a solar receiver 106 in Figs. 1 and 2.

[0005] The solar receiver 106 can be implemented as a tube having surface characteristics making it suitable for absorbing solar energy. A heat transfer material, for example thermal oil, is flowed within the receiver thereby causing the heat transfer material to be heated to an operational temperature. Thermal energy stored and transported in the heat transfer material is subsequently utilized to generate electrical energy.

[0006] Parabolic trough collector mirrors are specifically shaped to deliver light to the lengthwise receiver zone or location so that solar energy may be effectively collected and used to generate electricity. In conventional parabolic trough implementations, reflection to the receiver zone is the only function of a mirror. It is recognized and accepted that the stiffness and strength of glass is low relative to other materials, particularly metals, so glass mirrors are not considered structural elements. Considerable effort is generally expended to assure that the load that a mirror must independently carry is minimized since any deflection from the designed curvature under load will reduce the energy focused in the zone of concentrated solar flux.

[0007] Undesirable mirror deflection can be caused by gravitational loads, which typically vary in magnitude and direction throughout the day as the trough reflector tracks the motion of the sun. In addition, wind loading can cause significant mirror deflection.

[0008] Minimization of the undesirable deflection of conventional mirrors is typically accomplished by utilizing a separate structural support subsystem that has been sized to react the significant environmental loads introduced by gravity, wind, and other sources. The objective of the structural support subsystem is to position each mirror in an array so that it can perform its optical purpose, and to augment the stiffness of the mirror in order to reduce deflection. Several structural configurations are employed in industry to attempt to control the position and deflection of a mirror.

[0009] As illustrated in Figs. 1 and 2, and as utilized in many types of parabolic trough reflector design, the structural support subsystems 102 include some combination of space frame and truss arrangement 108, a series of supporting arms 110 cantilevered from a central torque tube and other elements. Each of these known mirror supporting structures suffers from mechanical limitations and several common shortcomings. For example, a support structure 102 is frequently statically indeterminate, complicated, and requires many components. As a result, the pointing error from accumulated tolerance errors, misaligned parts, and thermally mismatched parts can not only be considerable, but also somewhat variable, and the degree of unpredictability increases with the number of components in the load path. Furthermore, unless the interface between the supporting frame elements and mirror is continuous, the required intermittent mirror attachments provide stiffness to the mirror at the support points only, offering no support between or beyond these support points. Therefore, even the most optimized frame must rely on glass stiffness away from the actual support points to minimize deflection and maximize the optical integrity of the reflector system.

[0010] Certain attempts have been made to improve the structural performance of the mirror itself. Backface stringers have been attached to mirrors to improve bending stiffness, but this technique increases part count, requires additional manufacturing operations, and potentially introduces new failure mechanisms. The physical properties of the glass itself can also be improved through a variety of mechanisms, the most notable being the manufacture of tempered glass. Tempered glass is more expensive than the more common float glass, and while it resists breakage much better than untempered glass, tempering does not improve stiffness, so deflection, particularly bending deflection from a transverse load, is not reduced.

[0011] As noted above, most parabolic trough reflector based CSP systems feature a large array of trough reflectors arranged with a longitudinal axis positioned along a north- south line. Typically these implementations track the sun from east to west around a single axis of rotation. Therefore, as the position of the sun changes throughout the day, the endpoints of the zone of concentrated solar flux shift to the north or south with respect to the receiver 106. Concentrated solar flux reflected beyond the end of a receiver is problematic because this energy is not collected. In addition, solar energy concentrated on support structures, hoses/piping, or other elements can damage the system or require that the affected elements be insulated or thermally hardened at additional cost. At the opposite end of the concentrated solar flux zone, a portion of the receiver may not be illuminated. Similar problems arising from the difficulty of optically matching the ends of the zone of

concentrated solar flux to the receiver element as the position of the sun changes are also inherent in east-west parabolic trough designs.

[0012] Lance, et al., US 8,039,777 B2, describes the use of mirrors curved along two axes. In the Lance reference, the doubly-curved mirrors are specifically intended for use with a photovoltaic (PV) receiver, and are described as a solution to the "string current mismatch loss" that can occur with PV cells connected in series. More significantly, the cited benefits of the Lance doubly-curved mirrors are limited to the optical characteristics of the mirror.

[0013] The embodiments disclosed herein are directed toward overcoming one or more of the problems discussed above.

SUMMARY OF THE EMBODIMENTS

[0014] One embodiment disclosed herein is a parabolic trough reflector including a plurality of mirrors which are configured to reflect sunlight to a lengthwise zone of concentrated solar flux. The disclosed parabolic trough reflector also includes a structural support system connected to and supporting the plurality of mirrors. The trough reflector further includes a solar receiver operatively positioned at or in the lengthwise zone of concentrated solar flux. The receiver contains a flow of heat transfer material within the zone of concentrated solar flux such that the heat transfer material may be heated by concentrated sunlight reflected from the mirrors.

[0015] Each mirror comprises a reflective surface. The mirror surface is defined by a primary curvature within a plane perpendicular to the lengthwise zone of concentrated solar flux. Typically, the primary curvature extends across multiple mirrors to define a parabolic curve perpendicular to the lengthwise axis of the parabolic trough reflector. In addition, the reflective surface of one or more of the mirrors disclosed herein comprises a secondary curvature within a plane parallel to the lengthwise zone of concentrated solar flux. In some embodiments all mirrors of a parabolic trough reflector include a secondary curvature. The secondary curvature provides for reduced mirror bending deflection under a load, for example, gravity or wind load.

[0016] The mirrors noted above typically are fabricated from a curved glass sheet. In certain embodiments the reflective surface of the mirrors comprises metallization applied to the surface of the curved glass sheet which is opposite the zone of concentrated solar flux in use. In other words, the reflective metallization may be applied to the back surface of the mirrors.

[0017] Typically, when viewed from above, the mirrors of a parabolic trough reflector are arranged in two side -by-side rectangular arrays. The array, or arrays, consist of rows of mirrors perpendicular to the length of the oriented zone of concentrated solar flux and columns of mirrors parallel to the length of the zone of concentrated solar flux. In some embodiments the secondary curvature of some mirrors is different from the secondary curvature of other mirrors. For example, a first subset of mirrors in the exterior rows of the trough reflector may have different secondary curvature from the secondary curvature of a second subset of mirrors in interior rows. Although all mirrors having secondary curvature realize enhanced stiffness and reduced deflection, different secondary curvatures at various locations along the columns and rows of mirrors may achieve specific purposes including but not limited to maintaining the zone of concentrated solar flux on the receiver while minimizing the heating of adjacent structural members.

[0018] The enhanced stiffness of the disclosed mirrors with a secondary curvature can result in a simplified or more effective support structure. For example, the structural support system can include multiple pairs of cantilevered support arms supporting the rows of mirrors on opposite sides of the trough centerline. The enhanced rigidity of the disclosed mirrors provides for support at the mirror edges without substantially compromising the optical surface of a given mirror through deflection. Therefore, in systems utilizing pairs of cantilevered support arms, the number of support arms can be reduced to equal the number of rows of mirrors plus one. Other modifications to the support structure facilitated by the enhanced stiffness of the disclosed mirrors are disclosed herein.

[0019] The mirrors of parabolic trough reflectors which do not include a secondary curvature are often fabricated from a curved glass sheet having a thickness of 4 mm. The enhanced stiffness of the disclosed mirrors provides for the manufacture of a parabolic trough reflector including mirrors having a thickness of less than 4 mm, for example 3 mm.

[0020] Alternative embodiments include systems for generating electricity utilizing a thermal power cycle. In system embodiments, thermal energy is provided directly or indirectly to a working fluid from solar flux concentrated using parabolic trough reflectors having enhanced mirrors as described above.

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Fig. 1 is an isometric view of a prior art parabolic trough reflector.

[0022] Fig. 2 is isometric in view of the parabolic trough reflector of Fig. 1

[0023] Fig. 3 is a schematic illustration of a prior art mirror element from a parabolic trough reflector having primary curvature in a plane which is perpendicular to the longitudinal axis of the parabolic trough reflector.

[0024] Fig. 4 is a schematic illustration of a mirror element from a parabolic trough reflector having primary curvature in a plane which is perpendicular to the longitudinal axis of the parabolic trough reflector and secondary curvature in a plane which is parallel to the longitudinal axis of the parabolic trough reflector.

[0025] Fig. 5 is a graph showing the moment of inertia of a mirror element such as the mirror element of Fig. 4 is a function of secondary curvature measured by edge displacement.

[0026] Fig. 6 is a partial isometric view of a parabolic trough reflector as disclosed herein.

[0027] Fig. 7 is a partial isometric view of a prior art parabolic trough reflector showing certain structural elements.

[0028] Fig. 8 is a schematic diagram illustration of a concentrated solar power system featuring multiple parabolic trough reflectors including mirrors with primary and secondary curvature as disclosed herein.

DETAILED DESCRIPTION

[0029] Unless otherwise indicated, all numbers expressing quantities of ingredients, dimensions, reaction conditions and so forth used in the specification and claims are to be understood as being modified in all instances by the term "about".

[0030] In this application and the claims, the use of the singular includes the plural unless specifically stated otherwise. In addition, use of "or" means "and/or" unless stated otherwise. Moreover, the use of the term "including", as well as other forms, such as "includes" and "included", is not limiting. Also, terms such as "element" or "component" encompass both elements and components comprising one unit and elements and components that comprise more than one unit unless specifically stated otherwise.

[0031] All the features described in this specification (including the claims, description and drawings) and/or all the steps of the described method can be combined in any combination, with the exception of combinations of mutually exclusive features and/or steps.

[0032] The parabolic trough reflectors of certain CSP systems are implemented with an array of glass mirrors as reflecting elements. As noted above, the surfaces of the mirror elements in a conventional parabolic trough system are flat in a longitudinal direction shown as (L) on Figs. 1 and 2. The surfaces of the mirror elements 104 are formed in a parabolic curve in a plane perpendicular to the longitudinal direction, shown as (W) on Figs. 1 and 2. The foregoing curvatures are schematically illustrated in Fig. 3 which represents a conventional, prior art mirror 300, also known as a facet, from the parabolic trough reflector 100 illustrated in Figs. 1 and 2. The conventional mirror 300 includes substantially straight edges 302 and 304 which in use are operatively positioned parallel to the lengthwise axis of the parabolic trough. The edges 306 and 308 lie within planes which are perpendicular to the longitudinal axis of the parabolic trough. Edges 306 and 308 define a parabolic curve. Thus, the three-dimensional surface of the mirror 300 defines a line in any intersecting plane which is parallel to the longitudinal axis of the parabolic trough and defines a parabolic section in any plane which is perpendicular to the longitudinal axis of the parabolic trough.

[0033] The glass mirrors used to reflect light in parabolic trough energy collectors are typically not considered structural elements. Instead, a considerable amount of additional metal framework and other supporting materials are employed to provide stiffness and strength to a load bearing back structure that supports and orients the non-structural mirrors. Even so, under no more than an inherent gravity load, the stiffness of the back structure can be insufficient to sufficiently eliminate bending in the mirrors to preclude pointing errors that are detrimental to the optics of the assembly. Pointing errors increase significantly when wind approaches the trough collector from the lateral (transverse) direction, as the longitudinal direction of the module defines the weaker axis of the mirror' s section properties.

[0034] The various embodiments disclosed herein introduce a secondary curvature to individual mirror or facet elements around the lateral axis of a collector. As detailed herein, the introduction of a secondary curvature changes the section properties of the mirror in a fashion that dramatically increases its moment of inertia about this weaker axis. This increased moment of inertia results in a relatively stiff glass element that sharply reduces deflection due to bending and is capable of carrying considerably greater bending loads than a mirror without secondary curvature without structural failure. Systems utilizing mirrors that simultaneously serve as reflective and a structural elements provide for a corresponding reduction in the amount of material required for the back structure and in certain

embodiments the thickness of the glass used to fabricate mirror elements can be reduced.

[0035] Accordingly, the enhanced mirrors or facets disclosed herein feature an additional secondary curvature. As used herein, the primary curvature of the parabolic glass lies within a plane perpendicular to the longitudinal axis of the parabolic trough and thus within a plane parallel to edges 306 or 308. The secondary curvature of the parabolic glass lies within a plane parallel to the longitudinal axis of the parabolic trough and therefore within a plane parallel to edges 302 or 304. Fig. 4 illustrates a representative facet 400 of parabolic glass to which a secondary curve has been added. Although the secondary curvature extends throughout the three-dimensional surface of the mirror 400, the secondary curvature is readily apparent at the edges 402 and 404 which are within planes parallel to the longitudinal axis of the parabolic trough.

[0036] At the longitudinal midpoint of the glass, the two facets 300 and 400 trace out an identical parabolic curve in the primary direction, as the slope of the surface that incorporates the secondary curvature does not change from that of the single-curvature facet 300 along this line. Away from this centerline, in either longitudinal direction (±L), the magnitude of the surface slope in the secondary direction increases. In Fig. 4, the radius of curvature (R) has been given a constant value, resulting in curvature that traces out an arc of a circle, but this curvature could be any function determined to be desirable. The combination of the circular secondary curvature with the parabolic primary curvature over a three- dimensional surface results in a hyperbolic paraboloid mirror surface. R could also be either convex or concave, again according to specific structural or operational requirements some of which are discussed below.

[0037] The secondary curvature is swept along the parabolic path defined by the mirror's primary curvature at its longitudinal midpoint. As a result of the secondary curvature, light approaching the mirror surface in any plane perpendicular to the longitudinal axis of the trough will no longer be reflected to strike a point on the receiver in the same plane, but rather will strike the receiver at a point slightly offset in the +L direction, depending upon which side of the centreline is being considered. Because the reflected light must travel a slightly greater distance before striking the receiver than is the case when using a single-curvature mirror, a small correction in the surface slope of the mirror may be necessary to optimize performance. With these considerations addressed, the secondary curvature can be designed to have negligible detrimental impact on the ability of the parabolic trough reflector to properly concentrate solar flux within the designated lengthwise concentrated solar flux zone.

[0038] The new secondary curvature will, however, have a dramatic impact on the bending deflections exhibited by the mirror around the longitudinal axis of the trough. As noted above, bending deflections caused by gravitational, wind or other loads can significantly compromise the flux concentration at the solar receiver. The secondary curvature increases the moment of inertia, I_yy, in cubic fashion, which in turn decreases the deflections and stress levels in the mirror.

[0039] To illustrate the foregoing, the moment of inertia for a typical parabolic mirror facet such as is currently in use may be calculated with and without added secondary curvature for a typical glass facet. In this example, the thickness of the glass is presumed to be 0.15748 inches (4 mm), and its width (measured in the L direction is 68.1101 inches (1.73 m). As Iyy = (bh³)/12 for this "infinite" radius of curvature cross section, the baseline moment of inertia is Iyy = 0.022167 in⁴.

[0040] A section property calculator (Femap vl0.3) was used to calculate the new moment of inertia as the section's radius of secondary curvature is decreased, resulting in a secondary curvature in the facet characterized by increasing slope. To assure an equitable comparison, the thickness and width of the section are held constant, with the latter being swept along a circle's arc, resulting in a slight decrease in global (longitudinal) length as it follows that arc.

[0041] As illustrated in Fig. 4, the edges 406 and 408 display a downward edge displacement when a convex secondary curve is introduced. Using this displacement as a measure of the degree of secondary curvature being introduced, the graph 500 of Fig. 5 summarizes the relationship between the moment of inertia that results from introducing a secondary curve and the displacement at the edge. Referring to the left-hand y-axis of the Fig. 5 graph, it is notable that when the edge deflection introduced by secondary curvature is only 5 times the thickness of the glass (i.e t_gi_ass = 0.15748", deflection = 0.7874"), the moment of inertia increases by more than 25 times. When displaced a little more than 17 thicknesses, Iyy increases by well over 300 times. Expressed in terms of mirror width, the secondary (right hand) y-axis indicates that the greater than 300x moment of inertia improvement results when the edge is displaced only 4% of the overall 68.1101" facet width (i.e. d ~ 2.7inches). [0042] Mirror deflections resulting from a transverse bending load decrease linearly with increasing moment of inertia for both simply- supported and cantilevered support conditions. Therefore the introduction of a secondary mirror curvature of only a few percentage points of the width of a facet can reduce bending deflections by two orders of magnitude, with a corresponding reduction of the deflection introduced stress levels in the glass. The foregoing advantages are realized without any change to mirror material properties.

[0043] The increase in moment of inertia and the associated increase in mirror stiffness resulting from the introduction of a secondary curvature as described above provide the foundation for several performance improvements and cost reduction opportunities in the design and implementation of parabolic trough CSP systems. The first of these is the performance improvement that is immediately realized as the degree of the mirror shape change experienced under gravity and lateral wind loading is decreased. In use, each mirror encounters an extensive continuum of forces that influence deflection, but a stiffer mirror is the most direct means to mitigate the effects of all of these load cases simultaneously. In one parametric study of mirror performance, it was determined that mirror deflection accounts for approximately 70% of the optical degradation of a conventional parabolic trough system when subjected to only a nominal gravity load. Improving mirror performance is therefore one of the most effective means to improve aggregate system performance. A system that is more effective when collecting light requires less collection area, suggesting that the physical size of a power plant could be decreased for a given power output. This, in turn, results in lower land acquisition costs, decreased surface preparation costs, shorter piping lengths, fewer collector loops, and other benefits. Furthermore, a stiffer mirror also results in lower internal stresses, which decreases the potential for breakage and associated mirror replacement costs.

[0044] Another path to improved optical performance involves improvement of the reflectivity of each mirror element, in particular improved reflectivity over time. For facets that incorporate a reflective surface on the back side of translucent glass, reflectivity increases when the thickness of the glass is decreased, but the use of mirrors with a thickness of less than 4 mm has often been rejected because such a thin cross section is easily broken when bending under the influence of gravity or wind about the weaker longitudinal axis. Glass of only 3 mm thickness becomes structurally viable as a substantial increase in the moment of inertia resulting from secondary curvature introduces a much improved stiffness. In this case, the cost of the mirror's raw material is decreased. Furthermore, thinner glass provides the opportunity for back-surface metallization which offers the potentially more important benefit of better reflectivity over time, which, in turn, enhances optical performance and leads to the additional benefits noted above.

[0045] Other benefits emerge when adopting a design approach that provides for the mirror glass to become a structural element. The structural requirements placed on the supporting collector back structure may be relaxed because the stiffened glass improves optical performance or because it is capable of independently supporting more load, or both. Therefore, the support structure used to support a mirror system featuring secondary curvature may prove sufficient despite being comprised of smaller, "weaker", but less expensive members. A secondary benefit of such smaller support structure members is the greater simplicity of handling, storage, transport and assembly.

[0046] Fig. 6 illustrates a portion of a parabolic trough reflector 600 including a supporting structure 602 and mirror elements 604. The mirrors 604 include secondary curvature across the mirror surface, which secondary curvature lies within planes which are parallel to the lengthwise axis of the trough "L". In Fig. 6, the degree of secondary curvature has been slightly exaggerated, to facilitate illustration. Although not readily apparent from the Fig. 6 illustration, the primary parabolic curvature of each mirror element 604, in the direction "W," within planes perpendicular to the lengthwise axis of the trough, may be tuned to best focus concentrated sunlight on the lengthwise concentrated solar flux zone at the location of the receiver 606.

[0047] It may be noted from Fig. 6 that the interior rows of mirrors, as represented by mirror 604a have a convex secondary curvature and the exterior rows of mirrors, as represented by mirror 604b have a concave secondary curvature. Each of the illustrated secondary curvatures provides the structural advantages noted above. Therefore, the shape and radius of the secondary curvature used for individual mirror elements may be selected to achieve additional optical or structural benefits. For example, the concave curvature of the end row of mirrors 604b can serve to minimize the extension of the zone of concentrated solar flux beyond the end of the receiver 606 under certain illumination conditions caused by the position and altitude of the sun. Furthermore, the secondary curvature may be selected for mirrors located at each end of the trough to minimize the undesirable focusing of solar energy on support structures such as the support 608 located at or near each end of the receiver 606.

[0048] Thus, variations on the secondary curvature selected for some or all of the mirrors in an array can include a mirror shape that concentrates reflected light over a shorter receiver length (rather than over a longer length) and/or the application of the secondary curvature to only a portion of the reflector' s width, while leaving the remainder of the width unchanged from the original single-curvature parabolic profile. Each of the foregoing curvature possibilities may be further adapted to compensate for incident solar illumination angles that are not normal to the aperture plane of the collector, for example when the solar elevation different from the ideal elevation setting of the collector. The foregoing principles may also be utilized to improve the optics for an east-west collector.

[0049] As noted above, increasing the rigidity of mirror elements reduces the rigidity required of the support structure. As a result, the complexity of individual structural support members or the number of support members may in certain embodiments be reduced. For example, as best illustrated in Fig. 7, conventional mirrors 702 are often supported by bolts 704 extending from cantilevered arms 706 at support points away from the mirror edges, near the airy points. A stiffer mirror as disclosed herein, that is capable of serving as a structural member, could be supported at its edge, for example a mirror 604 could be supported in or on a continuous track along one or more mirror edges, for example edge 610 in Fig. 6.

[0050] In addition, a comparison of Fig. 6 with Fig. 7 shows that the number of cantilevered arms can be reduced from two per mirror to one between each mirror, plus one closeout arm when stiffer mirrors fabricated with a secondary curvature are used. Thus, the use of mirrors with a secondary curvature can reduce support structure costs through a substantial reduction in part count, as well as reduced transportation and assembly costs.

[0051] CSP systems such as described herein utilize concentrated sunlight to directly or indirectly heat a working fluid which is used to drive one or more power generation cycles. The power cycles occur within machinery including but not limited to turbines and compressors or heat engines which in turn drive electric generators. Some CSP systems utilize an initial heat transfer fluid (HTF) circuit where HTF is directly heated to operational temperatures by solar energy interfacing with a separate power cycle working fluid circuit holding working fluid thermally charged by heat exchange with the HTF. For example, Fig. 8 illustrates a highly simplified CSP system which includes mirrors and parabolic trough reflectors disclosed herein The CSP system 800 featuring a receiver 802 situated at or within the longitudinal zone of concentrated solar flux of a parabolic trough 804. As noted above, any commercial implementation would include a large number of individual parabolic trough elements. The parabolic trough 804 is implemented with multiple mirrors or facets having primary curvature and secondary curvature as described in detail above. [0052] A primary HTF circuit 808 carries HTF through the receiver 802 where the

HTF is heated to an operational temperature. Thermal energy from the HTF may be stored at any point in the HTF circuit in thermal energy storage devices 810 or 812 to extend the operational timeframe of the system.

[0053] In the simplified diagram of Fig. 8, the heated HTF is conveyed to a heat exchanger 814 from the receiver 802 or from TES 810 where thermal interchange with the HTF causes the heating of pressurized working fluid flowing in a working fluid circuit 816.

[0054] The thermal energy of the working fluid is utilized to drive a thermal power cycle 818. In the particular embodiment of Fig. 8, the thermal power cycle 818 is represented as a highly simplified Brayton cycle featuring a turbine 820 and compressor 822 connected by an axle 824. Expansion of the heated working fluid within the turbine 820 converts thermal energy of the working fluid to mechanical energy, thereby outputting work

(represented by rotational arrow 826) which can be utilized to drive an electric generator. Heat 828 is rejected from the system through an air-cooled condenser 830. Other types of thermal power cycle or heat engines of any level of complexity may also be driven with thermal energy obtained initially from solar flux concentrated with a parabolic trough reflector having mirror elements with secondary curvature as described herein.

[0055] Various embodiments of the disclosure could also include permutations of the various elements recited in the claims as if each dependent claim was a multiple dependent claim incorporating the limitations of each of the preceding dependent claims as well as the independent claims. Such permutations are expressly within the scope of this disclosure.

[0056] While the embodiments disclosed herein have been particularly shown and described with reference to a number of alternatives, it would be understood by those skilled in the art that changes in the form and details may be made to the various configurations disclosed herein without departing from the spirit and scope of the disclosure. The various embodiments disclosed herein are not intended to act as limitations on the scope of the claims. All references cited herein are incorporated in their entirety by reference.

Claims

CLAIMS What is claimed is:

1. A parabolic trough reflector comprising:

a plurality of mirrors configured to reflect sunlight to a lengthwise zone of

concentrated solar flux;

a structural support system connected to the plurality of mirrors; and

a solar receiver operatively positioned above the plurality of mirrors at the lengthwise zone of concentrated solar flux, wherein the receiver contains and positions a heat transfer material in the lengthwise zone of concentrated solar flux providing for the heat transfer material to be heated by concentrated sunlight reflected from the mirrors and wherein each of the mirrors comprises a reflective surface defined by a primary curvature within a plane perpendicular to the lengthwise zone of concentrated solar flux and wherein the reflective surface of one or more of the mirrors comprises a secondary curvature within a plane parallel to the lengthwise zone of concentrated solar flux and wherein the secondary curvature provides for reduced mirror bending deflection under a load.

2. The parabolic trough reflector of claim 1 wherein the plurality of mirrors comprise a curved glass sheet and wherein the reflective surface of the mirrors comprises metallization applied to a surface of the curved glass sheet which is opposite the lengthwise zone of concentrated solar flux in use.

3. The parabolic trough reflector of claim 1 wherein the structural support system is connected to one or more of the plurality of mirrors only at selected exterior edges of the mirrors.

4. The parabolic trough reflector of claim 3 wherein the structural support system is connected to each of the plurality of mirrors only at selected exterior edges of the mirrors.

5. The parabolic trough reflector of claim 1 wherein the plurality of mirrors are arranged in a rectangular array of rows perpendicular to the lengthwise zone of concentrated solar flux and columns parallel to the lengthwise zone of concentrated solar flux and wherein a first subset of the plurality of mirrors comprise a secondary curvature which is different from the secondary curvature of a second subset of the plurality of mirrors.

6. The parabolic trough reflector of claim 5 wherein the first subset of the plurality of mirrors comprises the exterior rows of the trough reflector and wherein the second subset of the plurality of mirrors comprises the interior rows of the trough reflector.

7. The parabolic trough reflector of claim 6 wherein the secondary curvature of the first subset of mirrors causes the lengthwise zone of concentrated solar flux to not extend beyond an end of the solar receiver.

8. The parabolic trough reflector of claim 5 wherein the structural support system

comprises multiple pairs of cantilevered support arms supporting the rows of mirrors on opposite sides of a centerline of the parabolic trough reflector and wherein the number of cantilevered support arms per side of the parabolic trough reflector is less than two support arms per row of mirrors.

9. The parabolic trough reflector of claim 1 wherein the plurality of mirrors comprise a curved glass sheet having a thickness of less than 4 mm.

10. The parabolic trough reflector of claim 9 wherein the plurality of mirrors comprise a curved glass sheet having a thickness of 3 mm.

A concentrated solar power system comprising:

a parabolic trough reflector comprising:

a plurality of mirrors configured to reflect sunlight to a lengthwise zone of concentrated solar flux;

a structural support system connected to the plurality of mirrors and a solar receiver operatively positioned above the plurality of mirrors at the lengthwise zone of concentrated solar flux, wherein the receiver contains and positions a heat transfer material in the lengthwise zone of concentrated solar flux providing for the heat transfer material to be heated by concentrated sunlight reflected from the mirrors and wherein each of the mirrors comprises a reflective surface defined by a primary curvature within a plane perpendicular to the lengthwise zone of concentrated solar flux and wherein the reflective surface of one or more of the mirrors comprises a secondary curvature within a plane parallel to the lengthwise zone of concentrated solar flux and wherein the secondary curvature provides for reduced mirror bending deflection under a load;

a heat transfer material circuit in thermal communication with the solar receiver; a working fluid circuit in thermal communication with the heat transfer material circuit; and

a power generation cycle in thermal communication with the working fluid circuit.

The concentrated solar power system of claim 11 wherein the plurality of mirrors comprise a curved glass sheet and wherein the reflective surface of the mirrors comprises metallization applied to a surface of the curved glass sheet which is opposite the lengthwise zone of concentrated solar flux in use.

The concentrated solar power system of claim 11 wherein the structural support system is connected to one or more of the plurality of mirrors only at selected exterior edges of the mirrors.

The concentrated solar power system of claim 13 wherein the structural support system is connected to each of the plurality of mirrors only at selected exterior edgi of the mirrors.

15. The concentrated solar power system of claim 11 wherein the plurality of mirrors are arranged in a rectangular array of rows perpendicular to the lengthwise zone of concentrated solar flux and columns parallel to the lengthwise zone of concentrated solar flux and wherein a first subset of the plurality of mirrors comprise a secondary curvature which is different from the secondary curvature of a second subset of the plurality of mirrors.

16. The concentrated solar power system of claim 15 wherein the first subset of the

plurality of mirrors comprises the exterior rows of the trough reflector and wherein the second subset of the plurality of mirrors comprises the interior rows of the trough reflector.

17. The concentrated solar power system of claim 16 wherein the secondary curvature of the first subset of mirrors causes the lengthwise zone of concentrated solar flux to not extended beyond an end of the solar receiver.

18. The concentrated solar power system of claim 15 wherein the structural support system comprises multiple pairs of cantilevered support arms supporting the rows of mirrors on opposite sides of a centerline of the parabolic trough reflector and wherein the number of cantilevered support arms per side of the parabolic trough reflector is no more than the number of rows of mirrors plus one.

19. The concentrated solar power system of claim 11 wherein the plurality of mirrors comprise a curved glass sheet having a thickness of less than 4 mm.

20. The concentrated solar power system of claim 11 wherein the plurality of mirrors comprise a curved glass sheet having a thickness of 3 mm.