Patent Publication Number: US-2021190044-A1

Title: Multi-thermal storage unit systems and related components

Description:
RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Ser. No. 61/613,948, filed Mar. 21, 2012, and entitled “Multi-Thermal Storage Unit Systems and Related Components”; U.S. Provisional Patent Application Ser. No. 61/613,947, filed Mar. 21, 2012, and entitled “Fluid Flow Control Devices for Solar Power Systems”; U.S. Provisional patent application Ser. No. 61/613,950, filed Mar. 21, 2012, and entitled “Low Pressure Solar Receivers with Double-Walled Windows and Uses Thereof”; and U.S. Provisional patent application Ser. No. 61/613,954 filed Mar. 21, 2012, and entitled “Low Pressure Solar Receivers with Segmented Windows and Uses Thereof.” Each of these applications is incorporated herein by reference in its entirety for all purposes. 
    
    
     FIELD OF INVENTION 
     Systems, devices, and methods related to concentrated solar power generation using solar receivers are generally described. 
     BACKGROUND 
     Mounting concerns over the effect of greenhouse gases on global climate have stimulated research focused on limiting greenhouse gas emissions. Solar power generation is particularly appealing because substantially no greenhouse gases are produced at the power generation source. 
     Concentrated solar power (CSP) generation using solar receivers is known in the art. Briefly, concentrated solar power systems use lenses, mirrors, or other elements to focus sunlight incident on a relatively large area onto a small area called a solar receiver. The concentrated sunlight can be used to heat a fluid within the solar receiver. The fluid heated within the solar receiver can be used to drive a turbine to generate power. 
     SUMMARY OF THE INVENTION 
     Inventive concentrated solar power systems using solar receivers, and related devices and methods, are generally described. In some embodiments, the concentrated solar power systems include one or more low pressure solar receivers. In some embodiments, the concentrated solar power systems include one or more low pressure solar receivers that operate at or below atmospheric pressure. In addition, inventive solar power systems and methods making use of multiple thermal storage units are provided, which can be useful, for example, in providing substantially continuous operation. Systems and methods in which the solar receiver and the turbine are kept fluidically isolated from each other are also described. In some embodiments, fluid flow control devices are provided that facilitate switching of airflow between or among one or more thermal storage systems and one or more heat exchange systems used to transfer heat from a low-pressure fluid (e.g., at or below about 2 atmospheres) to a high-pressure Brayton cycle fluid (e.g., above about 2 atmospheres). 
     In one aspect, a power generation system is provided. The power generation comprises, in some embodiments, a solar receiver, a compressor, a turbine, a first thermal storage system, a second thermal storage system, and a valving subsystem. In certain embodiments, the valving subsystem is configured such that in a first valving position, a first fluidic pathway is present between the solar receiver and the first thermal storage system, and a second fluidic pathway is present between the compressor, the turbine, and the second thermal storage system; and in a second valving position, a third fluidic pathway is present between the solar receiver and the second thermal storage system, and a fourth fluidic pathway is present between the compressor, the turbine, and the first thermal storage system. 
     In some embodiments, the power generation system comprises a first fluidic pathway fluidically interconnecting a solar receiver and a first thermal storage system; and a second fluidic pathway fluidically interconnecting a compressor, a turbine, and a second thermal storage system. In certain embodiments, the first thermal storage system is fluidically isolated from the second thermal storage system. In some embodiments, the system is configured such that the solar receiver heats fluid within the first fluidic pathway during at least a portion of the time during which the turbine is used to generate power using fluid within the second fluidic pathway. 
     In certain embodiments, a fluid flow control device is provided. The fluid flow control device comprises, in certain embodiments, a housing constructed of an investment casted metal alloy and defining a fluid chamber, the fluid chamber having a first opening, a second opening, and a third opening, each opening being configured to permit flow of a fluid into or out from the fluid chamber; a disc-shaped member having a circumferential surface about which are positioned one or more removable circumferential sealing elements, the disc-shaped member being rotatably positioned within the fluid chamber such that each of the one or more circumferential sealing elements interfaces with an inner surface of the fluid chamber, and such that in a first operating position the disc-shaped member defines within the fluid chamber a fluid flow path between the first and second openings that is fluidically isolated from the third opening, and in a second operating position the disc-shaped member defines within the fluid chamber a fluid flow path between the second and third openings that is fluidically isolated from the first opening; and a stem component having a first end positioned outside the fluid chamber, an elongated shaft extending through a passage through the housing into the fluid chamber, and a second end connected to the disc-shaped member inside the fluid chamber, the stem component being configured such that rotation of the stem component causes the disc-shaped member to rotate within the fluid chamber between at least the first and second operating positions, wherein the fluid-flow device is configured to operate at a temperature in a range of 1000° F. to 2500° F. 
     In some embodiments, the fluid flow control device comprises a housing defining a fluid chamber, the fluid chamber having a first opening, a second opening, and a third opening, each opening being configured to permit flow of a fluid into or out from the fluid chamber; a disc-shaped member having a circumferential surface about which are positioned at least two circumferential sealing elements, the disc-shaped member being rotatably positioned within the fluid chamber such that the each of the at least two circumferential sealing elements interfaces with an inner surface of the fluid chamber, and such that in a first operating position the disc-shaped member defines within the fluid chamber a fluid flow path between the first and second openings that is fluidically isolated from the third opening, and in a second operating position the disc-shaped member defines within the fluid chamber a fluid flow path between the second and third openings that is fluidically isolated from the first opening; and a stem component having a first end positioned outside the fluid chamber, an elongated shaft extending through a passage through the housing into the fluid chamber, and a second end connected to the disc-shaped member inside the fluid chamber, the stem component being configured such that rotation of the stem component causes the disc-shaped member to rotate within the fluid chamber between at least the first and second operating positions. In some such embodiments, the fluid-flow device is configured to operate at a temperature in a range of 1000° F. to 2500° F., and wherein the at least two circumferential sealing elements are positioned on opposite sides of where the stem component connects to the disc-shaped member, optionally wherein the two circumferential sealing elements are constructed of a cobalt base material. 
     The fluid flow control device comprises, in some embodiments, a housing constructed of a precision investment casted metal alloy and defining a fluid chamber, the fluid chamber having a first opening and a second opening, each opening being configured to permit flow of a fluid into or out from the fluid chamber; a disc-shaped member having a circumferential surface about which are positioned one or more circumferential sealing elements, the disc-shaped member being rotatably positioned within the fluid chamber such that each of the one or more circumferential sealing elements interfaces with an inner surface of the fluid chamber, and such that in a first operating position a fluid flow path exists between the first and second openings, and in a second operating position the disc-shaped member fluidically isolates the first and second openings; and a stem component having at least one end positioned outside the fluid chamber, and an elongated shaft extending through at least one passage through the housing into the fluid chamber, the stem component being connected to the disc-shaped member inside the fluid chamber and being configured such that rotation of the stem component causes the disc-shaped member to rotate within the fluid chamber between at least the first and second operating positions. In certain such embodiments, the fluid-flow device is configured to operate at a temperature in a range of 1000° F. to 2500° F. 
     In one set of embodiments, a solar receiver is provided which comprises a low pressure fluid chamber configured for operating at pressures up to 2 atmospheres, and comprising a fluid inlet, a fluid outlet, and an opening for receiving concentrated solar radiation; a solar absorber housed within the low pressure fluid chamber; a first transparent object that defines at least a portion of a wall of the low pressure fluid chamber; and a second transparent object in juxtaposition with the first transparent object, wherein the first transparent object and the second transparent object are configured such that a first fluid flow path is defined between the first transparent object and the second transparent object. In some such embodiments, concentrated solar radiation received through the opening passes through the first transparent object and second transparent object into the low pressure fluid chamber and impinges upon the solar absorber. 
     In some embodiments, a solar receiver is provided which comprises a low pressure fluid chamber configured for operating at pressures up to 2 atmospheres, and comprising a fluid inlet, a fluid outlet, and an opening for receiving concentrated solar radiation; a solar absorber housed within the low pressure fluid chamber; and a plurality of transparent objects that define a segmented wall of the low pressure fluid chamber. In some such embodiments, concentrated solar radiation received through the opening passes through the segmented wall and between transparent objects to pass into the low pressure fluid chamber and impinges upon the solar absorber. 
     Other advantages and novel features of the present invention will become apparent from the following detailed description of various non-limiting embodiments of the invention when considered in conjunction with the accompanying figures. In cases where the present specification and a document incorporated by reference include conflicting and/or inconsistent disclosure, the present specification shall control. If two or more documents incorporated by reference include conflicting and/or inconsistent disclosure with respect to each other, then the document having the later effective date shall control. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting embodiments of the present invention will be described by way of example with reference to the accompanying figures, which are schematic and are not intended to be drawn to scale. In the figures, each identical or nearly identical component illustrated is typically represented by a single numeral. For purposes of clarity, not every component is labeled in every figure, nor is every component of each embodiment of the invention shown where illustration is not necessary to allow those of ordinary skill in the art to understand the invention. In the figures: 
         FIGS. 1A-1C  are schematic diagrams of concentrated solar power generation systems including low pressure solar receivers, according to one set of embodiments; 
         FIGS. 2A-2C  include, according to some embodiments, exemplary schematic diagrams of heat recovery configurations that can be used with a concentrated solar power generation system; 
         FIGS. 3A-3C  include exemplary schematic illustrations of thermal storage units that can be used in a thermal storage system; 
         FIGS. 4A-4B  include, according to one set of embodiments, schematic illustrations of power tower systems; 
         FIG. 5  includes an exemplary schematic diagram of a concentrated solar power generation system including a high pressure solar receiver; 
         FIG. 6  includes a schematic diagram of a concentrated solar power generation system including multiple solar receivers, according to some embodiments; 
         FIGS. 7A-7I  include exemplary schematic illustrations of low-pressure solar receivers; 
         FIGS. 8A-8C  include exemplary schematic illustrations of a secondary concentrator; 
         FIGS. 9A-9C  include exemplary schematic illustrations of high-pressure solar receivers; 
         FIGS. 10A-10F  include exemplary schematic illustrations of concentrated solar power generation systems; and 
         FIGS. 11A-11C  include exemplary schematic illustrations of fluid flow devices. 
     
    
    
     DETAILED DESCRIPTION 
     Inventive concentrated solar power systems using solar receivers, and related devices and methods, are generally described. In some embodiments, the concentrated solar power systems include a solar receiver used to heat a fluid at a relatively low pressure. Heat from the low-pressure fluid heated by the solar receiver can be transferred to a relatively high-pressure fluid, which can be used to power a gas turbine as part of, for example, a Brayton cycle. The heat exchange between the low- and high-pressure fluids can be accomplished via the use of a heat exchange system. In certain embodiments, heat provided by the solar receiver(s) can be transferred to a separate fluidic pathway in which a relatively high-pressure fluid is transported to a turbine by changing valve configurations between one, two, or more thermal storage systems within the power generation system. 
     In some embodiments, the exhaust of the gas turbine can be transported to the solar receiver and used as the low-pressure heated fluid. Optionally, a blower can be used to provide additional ambient air to the low-pressure solar receiver, which can be useful, for example, for regulating the flow of fluid through the system. In some embodiments, a controller can be used to regulate the flow rate of the gas from the blower. The controller can be constructed and arranged to adjust the flow rate of the gas transported from the blower to the solar receiver based at least in part on a condition of the gas transported from the gas turbine to the solar receiver. For example, the controller can be constructed and arranged such that the flow rate of the gas transported from the blower to the solar receiver depends on one or more of the temperature, pressure, and/or flow rate of the gas transported from the gas turbine to the solar receiver. 
     The low-pressure fluid from the solar receiver, in addition to providing heat to the high-pressure working fluid within the Brayton cycle, can be used to provide heat to a thermal storage system, which can operate, for example, by storing sensible heat from the low-pressure fluid. The thermal storage system can be useful for operating the power cycle during periods of low sunlight, for example, by providing heat to the low-pressure fluid in addition to or in place of the heat provided by the solar receiver. In some embodiments, the thermal storage system can be operated at the high pressure of the turbine, for example, by transporting a pressurized fluid through the thermal storage unit to heat the pressurized fluid prior to, for example, transporting the pressurized fluid to a turbine. 
     In certain embodiments, multiple thermal storage systems (each of which can include one or more thermal storage units) can be used such that one thermal storage system can be used to store sensible heat provided by the solar receiver while another thermal storage system can be used to provide heat to the working fluid of a Brayton cycle. In some such embodiments, the first thermal storage system can be operated at a pressure near or below atmospheric pressure while the second thermal storage system can be operated at an elevated pressure. By operating the power generation system in this way, one can eliminate the need for a heat exchange system that must exchange heat between a pressurized fluid and a non-pressurized fluid, a task which is often challenging. In addition, operating the system in this manner can ensure that a heat source is always available for providing heated working fluid to the Brayton cycle. Thus, in some embodiments, substantially continuous operation of the Brayton cycle can be achieved without directly exchanging heat between a high-pressure fluid and a low-pressure fluid. 
     In some embodiments, the airflow from the solar receiver can be switched between the thermal storage system and the heat exchange system used to transfer heat from the low-pressure fluid to the high-pressure Brayton cycle fluid. In some embodiments, a blower can be connected to transport heated air from the thermal storage system into the solar receiver. 
     In some embodiments, fluid flow control devices (e.g., valves, three-way valves, flapper valves) are provided that facilitate switching of airflow between or among one or more thermal storage systems and one or more heat exchange systems used to transfer heat from a low-pressure fluid to a high-pressure Brayton cycle fluid. In some embodiments, a fluid flow control device comprises a housing defining a fluid chamber, in which the fluid chamber has a plurality of openings (e.g., a first opening, a second opening, and a third opening). In some embodiments, each opening is configured to permit flow of a fluid into or out from the fluid chamber. In certain embodiments, the fluid flow device is configured to operate at a temperature in a range of 1000° F. to 2500° F. In certain embodiments, the fluid flow device is configured to operate at a temperature in a range of 1150° F. to 2200° F. In certain embodiments, the fluid flow device is configured to operate at a temperature in a range of up to 1150° F. 
     In certain embodiments, the housing of the fluid flow device is constructed using precision investment casting. In some embodiments, precision investment casting is a casting process in which an expendable pattern is surrounded by an investment compound (e.g., a ceramic component) and then baked so that the investment is hardened to form a mold and the pattern material (e.g., a wax) is melted and removed. In such embodiments, a metal (e.g., a metal alloy) is entered into the mold, and allowed to solidify, after which the mold is removed (e.g., by hammering the mold into pieces) to obtain the cast metal object. 
     In some embodiments, the housing and/or other components (e.g., stem components, disc-shaped members) of the fluid flow control device are constructed of a metal alloy. In certain embodiments, the metal alloy is a stainless steel alloy, or a nickel base superalloy, such as IN-718, IN-713 LC or CMSX4. In some embodiments, cobalt materials are used for rubbing surfaces in fluid flow devices configured to operate at high temperatures because the cobalt forms a lubricating oxide layer with the oxygen in air or working fluid. 
     In some embodiments, the fluid flow devices are insulated on the outside, which may facilitate internal sealing at operating temperatures &amp; pressures. 
     In some embodiments, the housing defines a connection interface for a fluid conduit at one or more of the first, second and third openings. In some embodiments, the connection interface is (i) a band flange, such as a “V” band flange, optionally which is configured for quick change assembly, (ii.) a bolted flange, optionally which is configured with a gasket surface, (iii.) a low pressure thermal fit/slip joint, (iv.) a male or female screw-type connection interface, (v.) a radiator clamp, optionally which is configured for light weight ducting, or (vi.) another appropriate connection interface. 
     In some embodiments, the fluid flow device also comprises a disc-shaped member (e.g., a flapper) having a circumferential surface about which are positioned one or more removable circumferential sealing elements, in which the disc-shaped member is rotatably positioned within the fluid chamber such that the one or more circumferential sealing elements interface with an inner surface of the fluid chamber. In some embodiments, the disc-shaped member is rotatably positioned within the fluid chamber such that in a first operating position the disc-shaped member defines within the fluid chamber a fluid flow path (e.g., airflow path) between the first and second openings that is fluidically isolated from the third opening, and in a second operating position the disc-shaped member defines within the fluid chamber a fluid flow path (e.g., airflow path) between the second and third openings that is fluidically isolated from the first opening. In some embodiments, the one or more circumferential sealing elements are constructed of a cobalt base material. In some embodiments, the one or more circumferential sealing elements are piston rings. 
     In some embodiments, the fluid flow control device also comprises a stem component having a first end positioned outside the fluid chamber, an elongated shaft extending through a passage through the housing into the fluid chamber, and a second end connected to the disc-shaped member inside the fluid chamber. In some embodiments, the stem component is configured such that its rotation causes the disc-shaped member to rotate within the fluid chamber between at least the first and second operating positions. In some embodiments, the stem component is connected to an automatically controlled motor. In some embodiments, the stem component is configured to rotate the disc-shaped member 360 degrees. In some embodiments, the angle of rotation of the disc shaped member between the first and second operating positions is up to about 90 degrees. In some embodiments, the angle of rotation of the disc shaped member between the first and second operating positions is in a range of 45 degrees to 60 degrees, 45 degrees to 135 degrees, or 60 degrees to 120 degrees. 
     In some embodiments, the stem component passes through a bushing that is positioned within the passage through the housing. In some embodiments, at least a surface of the bushing that confronts the stem component is constructed of a cobalt base material. In some embodiments, the stem component is constructed of a cobalt base material. Circumferential sealing elements (e.g., piston ring seals), in some embodiments, are provided on the disc-shaped member on both forward &amp; aft sides of the rotating stem component to provide effective sealing around (e.g., 360 degrees of) the circumference of the member. Circumferential sealing elements (e.g., piston ring seals), in some embodiments, are made from cobalt base material or similar material to enable low friction movement, long life, and minimal or no galling when operating at high temperature. In some embodiments, stem bushings are also of a cobalt or similar material to provide; low friction, low wear, long life, &amp; low galling at high operating temperatures. 
     In some embodiments, the fluid flow control device comprises an adaptor/sleeve positioned within the housing at an opening to facilitate manufacturing, assembly and low leakage. In some embodiments, the sleeve is constructed of the same material as the housing. 
     In some embodiments, a fluid flow control device is provided that has a housing defining a fluid chamber having a first opening and a second opening. In certain embodiments, each opening is configured to permit flow of a fluid into or out from the fluid chamber. In some embodiments, the fluid flow control device comprises a disc-shaped member having a circumferential surface about which are positioned one or more circumferential sealing elements. In some embodiments, the disc-shaped member is rotatably positioned within the fluid chamber such that the at least two circumferential sealing elements interface with an inner surface of the fluid chamber. In some embodiments, the disc-shaped member is rotatably positioned within the fluid chamber such that in a first operating position a fluid flow path is defined between the first and second openings, and in a second operating position the disc-shaped member fluidically isolates the first and second openings. In some embodiments, the fluid flow control device comprises a stem component having at least one end positioned outside the fluid chamber, and an elongated shaft extending through at least one passage through the housing into the fluid chamber. In some embodiments, the stem component is connected to the disc-shaped member inside the fluid chamber and is configured such that rotation of the stem component causes the disc-shaped member to rotate within the fluid chamber between at least the first and second operating positions. In some embodiments, the fluid flow control device functions as a low pressure damper. 
     Depicted in  FIG. 11A  is a cross-sectional view of an exemplary fluid flow device  1100  comprising a housing  1105  (e.g., constructed of an investment casted metal alloy) that defines a fluid chamber  1102 , in which the fluid chamber  1102  has a first opening  1101   1 , a second opening  1101   2 , and a third opening  1101   3 . Each opening in this device is configured to permit flow of a fluid into or out from the fluid chamber  1102 . The fluid flow device  1100  also comprises a disc-shaped member  1103   1-2  (shown here as a top view, in two positions) having a circumferential surface about which is positioned a removable circumferential sealing element  1104   1-2 . In this device, the disc-shaped member  1103   1-2  is rotatably positioned within the fluid chamber  1102  such that each of the circumferential sealing elements interfaces with an inner surface  1107   1-2  of the fluid chamber  1102 , and such that in a first operating position the disc-shaped member  1103   1  defines within the fluid chamber  1102  a fluid flow path between the first opening  1101   1  and second opening  1101   2  that is fluidically isolated from the third opening  1101   3 , and in a second operating position the disc-shaped member  1103   2  defines within the fluid chamber  1102  a fluid flow path between the second opening  1101   2  and third opening  1101   3  that is fluidically isolated from the first opening  1101   1 . The fluid flow device  1100  also comprises a stem component (not shown) that has a first end positioned outside the fluid chamber, an elongated shaft extending through a passage through the housing into the fluid chamber, and a second end connected to the disc-shaped member (at the x in the center of the disc shaped member  1103   1-2 ) inside the fluid chamber  1102 . The stem component is configured such that rotation of the stem component causes the disc-shaped member  1103   1-2  to rotate within the fluid chamber  1102  between at least the first and second operating positions. 
       FIG. 11B  depicts an example of a disc-shaped member  1103   3  that has two circumferential sealing elements  1104   1-2  positioned on opposites sides of a location  1106  where a stem component connects to the disc shaped member ( 1103   3 ). 
     The overall efficiency of the system can be improved, in some cases, by recovering heat from the low-pressure fluid exiting the heat exchange system used to transfer heat to the high-pressure fluid. For example, in some cases, the low-pressure fluid exiting the heat exchange system can be used to generate steam to power a steam turbine in a Rankine cycle. In some instances, the low-pressure fluid exiting the heat exchange system can be used to provide heat to an absorption chiller, which can be used, for example, to produce chilled water for an air conditioner and/or refrigeration system. The low-pressure fluid from the heat exchange system can also be used to provide heat for general space heating purposes (e.g., via an air to liquid heat exchanger). 
     Some embodiments of the invention can be used in coordination with solar power tower systems (also known as central tower solar power plants or heliostat solar power plants). Such systems include a plurality of heliostats arranged to redirect sunlight toward the top of a collector tower, sometimes called a central tower, on which one or more solar receivers are mounted. In some such embodiments, the gas turbine and/or the compressor can be mounted, along with the solar receiver, at the top of the solar tower. Other components, such as a thermal storage system can also be mounted at the top of, or within other parts of, the tower. In some embodiments, provision is made for adjusting the angle of the solar receiver relative to the ground and to ground-mounted heliostats. The angle can be adjusted, in certain embodiments, based on tower height, heliostat field size and/or shape, and/or latitude. 
     In some embodiments, low pressure solar receivers are provided that may be used in conjunction with the power generation systems disclosed herein. The solar receivers function, at least in part, to convert solar radiation energy to thermal energy of a working fluid, e.g., a working fluid of a power generation or thermal storage system. The solar receivers typically comprise a low pressure fluid chamber that is designed and constructed, at least in part, to provide an insulated casing that acts to reduce or eliminate thermal losses from the solar receiver, to contain a low pressure working fluid and/or to provide a support structure for a solar absorber. The low pressure solar receivers also typically comprise a transparent object (e.g., window) positioned adjacent to an opening in the receiver for receiving solar radiation. The transparent object functions, at least in part, to contain the low pressure working fluid, to permit solar radiation to pass into the solar receiver (where the radiation impinges the solar absorber) and to eliminate or reduce thermal losses associated with re-radiation from the solar absorber. 
     Because the low pressure receiver operates at low pressure (e.g., below 1 atmosphere, below 2 atmospheres) the chamber can be typically constructed using less material and fewer design constraints than is needed for chambers that are subjected to higher pressures. Moreover, the low pressure design enables the use of relatively large (e.g., 1 meter to 5 meters in diameter) transparent objects that enable a high solar collection capacity. Thus, according to some aspects, the low pressure solar receivers have lower cost of production and significantly larger collection capacity than currently available solar receivers. 
     In further embodiments, high pressure receivers are provided that may be used in conjunction with the power generation systems disclosed herein. The high pressure solar receivers function, at least in part, to convert solar radiation energy to thermal energy of a working fluid, e.g., a working fluid of a power generation system or thermal storage system. In some embodiments, the high pressure receivers include an insulated casing housing a high pressure solar absorber that acquires thermal energy by absorbing incident solar radiation. The high pressure fluid (e.g., fluid at a pressure of above 2 atmospheres to 50 atmospheres) entering the receiver passes through one or more fluid passages within the high pressure solar absorber and acquires thermal energy therein, in part, through contact with the passage wall(s). The high pressure solar absorber often has a black surface coating to promote absorption of incident solar radiation and is typically constructed from a single crystal super alloy, e.g., a nickel-based single crystal super alloy. 
     Current high-pressure receivers typically use metals that are often limited with respect to maximum temperatures at which they can function. For example, certain high-pressure receivers employ stainless steel or other alloys for the pressurized receiver components and these materials typically limit the receiver exit temperatures to levels that are insufficient to enable (at least at high efficiencies) certain downstream uses, such as use within a Brayton power cycle. The high-pressure solar receivers provided herein employ significantly higher temperature materials, e.g., high temperature single crystal super alloys, for the heat-exchanger elements and therefore can be operated at significantly higher temperatures. In some embodiments, high-pressure absorbers are produced from nickel-based high-temperature super alloy (e.g., using precision investment casting), and enable relatively high maximum exit temperatures (e.g., temperatures of up to ˜1150° C.) from the receiver. Thus, in some embodiments, the receivers may be used within a Brayton cycle system to achieve high power output &amp; high overall electrical efficiency. 
     In certain embodiments, additional heat transfer features are provided into the internals of the heat-exchanger elements (e.g., improved cross sectional shape) to facilitate heat transfer efficiency. In some embodiments, the cast single crystal tubes are attached to headers &amp; manifolds of similar materials via a unique vacuum brazing process known as (ADB) activated diffusion bonding or (TLP) transient liquid phase. This joining technique enables, in some embodiments, a joint to retain full strength &amp; temperature capability. In some embodiments, the high-pressure receivers also incorporate a transparent object (e.g., a Quartz glass front window). In some embodiments, the transparent object has an anti-radiation reflection coating on the inside to limit the effects of re-radiation on thermal efficiency. Moreover, in some embodiments, high resistance insulation is applied to the receivers to improve thermal efficiency. 
     In some embodiments, secondary concentrators are provided. The secondary concentrator provides, at least in part, a mechanism for collecting concentrated solar radiation from a primary concentrator, e.g., a heliostat field, or other source, and directing that solar radiation into the opening of a solar receiver. The secondary concentrator typically improves the solar collection efficiency of the solar receiver. In some embodiments, the second concentrator is constructed with a plurality of reflective panels, each reflective panel typically having a reflective surface and a predetermined shape. The plurality of reflective panels are typically arranged in a configuration that facilitates reflection of incident solar radiation toward the receiver opening. In certain embodiments the secondary concentrator includes cooling pipes that function in part to deliver cooling fluid to and from a cooling passage within each reflective panel. 
     Certain embodiments of the inventive systems and methods described herein can provide certain advantage(s) over traditional concentrated solar power techniques in certain applications. For example, low-pressure components (e.g., solar receivers, storage containers, etc.) can be relatively inexpensive to manufacture and relatively safe to operate. In addition, low-operating pressures allow for the use of relatively large windows within the solar receiver, compared to pressurized systems in which large windows can rupture at high pressures. The Brayton cycle systems described herein have a higher thermal efficiency relative to systems that employ, for example, Rankine cycles. The ability to switch the flow of low-pressure fluid between heat exchange for power generation and low-pressure storage can allow for operation at night and other low-sunlight conditions. The heat integration methods described herein can also improve overall system performance. 
       FIGS. 1A-1B  are schematic illustrations of a system  10  in which concentrated solar energy is used to generate power. The fluid streams in  FIGS. 1A-1B  can be generally divided into streams comprising relatively high-pressure fluid (illustrated as dotted lines in  FIGS. 1A-1B ), streams comprising relatively low-pressure fluid (illustrated as solid lines in  FIGS. 1A-1B ), and streams through which substantially no fluid is transported (illustrated as dash-dot lines in  FIGS. 1A-1B , such as streams  30 - 33  in  FIG. 1A  and streams  16 - 17  and  23 - 24  in  FIG. 1B ). It should be noted that these conventions are used for illustration purposes only, and are not meant to indicate that the pressures in all relatively low-pressure streams are the same and/or that the pressures in all relatively high-pressure streams are the same. 
     System  10  includes a solar receiver  102  constructed and arranged such that at least a portion of the receiver, such as face  104  in  FIGS. 1A-1B , is exposed to incident solar radiation  106 . The energy from the incident solar radiation can be used to heat a fluid within the solar receiver. In some embodiments, the solar receiver can be constructed and arranged to operate at relatively low pressures. For example, the pressure of the fluid within the solar receiver (and/or within any fluidic pathway that includes the solar receiver) can be up to and including about 2 atmospheres, less than about 1.5 atmospheres, less than about 1.25 atmospheres, less than about 1.1 atmospheres, less than about 1 atmosphere, between about 0.9 and about 2 atmospheres, between about 0.9 and about 1.5 atmospheres, between about 0.9 and about 1.25 atmospheres, between about 0.9 and about 1.1 atmospheres, or between about 0.9 and about 1 atmosphere. In some cases, the solar receiver can be constructed and arranged such that the fluid within the receiver is not substantially compressed, with the exception of incidental compression that might occur due to the heating and/or transport of the fluid, before being transported to the receiver. For example, the fluid transported to the solar receiver can be substantially equal to the pressure of the surrounding environment, in some cases. The reduced pressures at the receiver may allow a “window” of the receiver (e.g., a transparent portion of the receiver through which sunlight passes to heat the fluid in the receiver) to be made significantly larger than in other relatively high pressure receivers. For example, prior receivers may be limited to a window size of about 60 cm diameter, whereas a receiver in some embodiments of the invention may have a size up to about 150 cm or more. In some embodiments, the receivers have a window size of 4 meters or more. Additional details related to the design and operation of the solar receiver are described in more detail below. 
     System  10  also includes a turbine. In the set of embodiments illustrated in system  10 , power is primarily generated using a Brayton cycle. The Brayton cycle illustrated in  FIGS. 1A-1B  includes gas turbine  122 . While a single turbine is illustrated in  FIGS. 1A-1B , it should be understood that the invention is not so limited, and that, in some embodiments, multiple turbines can be employed. For example, in some embodiments, the power generation system includes at least 2, at least 3, at least 4, at least 5, or more turbines. A single gas turbine and/or the combination of multiple gas turbines can be capable of producing any suitable amount of power (e.g., at least about 100 kW, at least about 500 kW, at least about 1 MW, at least about 4 MW). One of ordinary skill in the art would be capable of selecting an appropriate gas turbine and/or combination of gas turbines to use, given a desired power output requirement. 
     In order to increase system efficiency, the gas supplied to gas turbine  122  should be relatively hot and relatively highly-pressurized. To accomplish this, compressor  124  can be used to compress a relatively low-pressure gas (e.g., ambient air) in stream  20  to produce relatively high-pressure stream  21 . In some embodiments, the compressor can be used to produce a fluid stream (e.g., a gas stream) with a pressure above 2, at least about 3, at least about 4, at least about 5, at least about 10, or at least about 15 atmospheres. 
     In the set of embodiments illustrated in  FIGS. 1A-1B , independent fluidic pathways are maintained. For example, in  FIG. 1A , fluidic pathways  11  and  12  are maintained. In  FIG. 1B , a third fluidic pathway and a fourth fluidic pathway are maintained, as described in more detail below. In  FIG. 1A , streams  30 - 33  are not used to transport fluid because three-way valves  13 A- 13 D have been switched to allow flow in a first configuration, which does not include flow through streams  30 - 33 . On the other hand, in  FIG. 1B , valves  13 A- 13 D have been switched such that streams  30 - 33  are used to transport fluid, and streams  16 ,  17 ,  23 , and  24  (which were used to transport fluid in the configuration illustrated in  FIG. 1A ) are not used to transport fluid. 
     Referring back to  FIG. 1A , first fluidic pathway  11  (comprising streams  15 - 18  in  FIG. 1A ) can fluidically interconnect solar receiver  102  and thermal storage system  134 A such that a relatively low-pressure fluid can be transported between solar receiver  102  and thermal storage system  134 A. In this way, heat from the solar receiver can be used to heat the media within the thermal storage system. The pressure within the low-pressure fluid stream can be, for example, up to and including about 2 atmospheres, less than about 1.5 atmospheres, less than about 1.25 atmospheres, less than about 1.1 atmospheres, less than about 1 atmosphere, between about 0.9 and about 2 atmospheres, between about 0.9 and about 1.5 atmospheres, between about 0.9 and about 1.25 atmospheres, between about 0.9 and about 1.1 atmospheres, or between about 0.9 and about 1 atmosphere. Second fluidic pathway  12  can fluidically interconnect compressor  124 , thermal storage system  134 B, and turbine  122 . Second fluidic pathway  12  can be used to transport a relatively high-pressure fluid from compressor  124  through a thermal storage system and subsequently to turbine  122  to generate power. The pressure of the fluid within second fluidic pathway  12  (comprising streams  21 - 26  and  118  in  FIG. 1A ) can be, for example, above 2, at least about 3, at least about 4, at least about 5, at least about 10, or at least about 15 atmospheres. By operating system  10  in this way, power can be generated without directly transporting heat from fluidic pathway  11  to fluidic pathway  12 . Rather, a valving subsystem (e.g., comprising three-way valves  13 A- 13 D in  FIGS. 1A-1B ) can be used to modify the fluidic pathways such that fluidic isolation is maintained between fluidic pathway  11  and fluidic pathway  12 , and between the third and fourth pathways that are present when the valves are switched, as described in more detail below. 
     An exemplary method of operating system  10 , as illustrated in  FIGS. 1A-1B  is now described. A low-pressure fluid can be transported to solar receiver  102  via an inlet, such as inlet line  14  in  FIGS. 1A-1B . Generally, fluid is transported through the solar receiver when the sun is available to provide energy to heat the fluid. The relatively low-pressure fluid can originate from any suitable source. For example, in certain embodiments, the relatively low pressure fluid can originate from the ambient atmosphere. In certain embodiments, at least a portion of the low-pressure fluid can be recycled within a low-pressure fluid pathway, illustrated as fluidic pathway  11  in  FIG. 1A . In certain embodiments, the low-pressure fluid pathway can include a fluidic loop, such that substantially no fluid is expelled from the pathway during operation. 
     Once the relatively low-pressure fluid has been heated within the solar receiver, it can be transported out of the receiver, for example, via stream  15  in  FIG. 1A . At least a portion of the fluid within stream  15  can be transported via valve  13 D and stream  16  to a first thermal storage system  134 A, where the heat can be used to heat a thermal storage medium. Thermal storage system  134 A can include a single thermal storage unit, while in other embodiments, the thermal storage system can include a plurality of thermal storage units. 
     After the heat from the relatively low-pressure stream has been used to heat the medium within thermal storage system  134 A, the relatively low-pressure fluid can be transported out of thermal storage system via stream  17 . The relatively low-pressure fluid can then be redirected via valve  13 C to stream  18  and subsequently to solar receiver  102 . In certain embodiments, optional blower  140  can be used to transport additional low-pressure fluid in optional makeup stream  19  to solar receiver  102 . Any suitable type of blower can be included in the system. The blower can comprise, for example, an electric driven induction flow fan. 
     While blower  140  and makeup stream  19  have been illustrated in  FIG. 1A , it should be understood that, in other embodiments, they may not be present, and stream  18  can be fed directly to solar receiver  102 . In still other embodiments, blower  140  and makeup stream  19  can be present, and stream  18  can be directly vented (i.e., in such embodiments, there is substantially no recycling of low-pressure fluid back to the solar receiver). 
     As noted above, a second fluidic pathway  12  can also be present within system  10 , in certain embodiments. In certain embodiments, fluid can be transported through second fluidic pathway  12  during at least a portion of the time (which can be, for example, at least 5 minutes, at least 30 minutes, at least 1 hour, at least 6 hours, or at least 12 hours) that fluid is transported through first fluidic pathway  11 . 
     In second fluidic pathway  12 , inlet fluid (e.g., fluid from the ambient atmosphere or fluid originating from an exhaust of another unit operation) can be transported to compressor  124  via stream  20 . Optionally, the relatively high-pressure fluid exiting compressor  124  via stream  21  can be pre-heated using residual heat exiting turbine  122  and heat exchanger  112 . The use of heat exchanger  112  is optional, and in other embodiments, the high-pressure fluid exiting compressor  124  is not pre-heated. 
     In  FIG. 1A , relatively high-pressure fluid (either in stream  21  directly from compressor  124  or in stream  22  from optional heat exchanger  112 ) can be transported to valve  13 A, where it can be redirected to thermal storage system  134 B via stream  23 . In some embodiments, thermal storage system  134 B can be pre-heated, and therefore, can be used to heat relatively high-pressure fluid within stream  23 . Thermal storage system  134 B can be pre-heated, for example, by transporting a hot fluid (e.g., a low-pressure heated fluid from solar receiver  102 ) through thermal storage system  134 B at a relatively hot temperature (e.g., about 2000° F.). Once the relatively high-pressure fluid has been heated within thermal storage unit  134 B, the fluid can be transported out of thermal storage unit  134 B via stream  24 . The fluid within stream  24  can be redirected by valve  13 B through stream(s)  25  (e.g., potentially a single stream when optional supplemental heater  130  is not present and potentially as few as two streams ( 25 A and  25 B) when optional supplement heater  130  is present) to turbine  122 . At turbine  122 , the high-pressure fluid can be used to generate power. After the high-pressure fluid has been at least partially decompressed within turbine  122 , it can be discharged via stream  26 . In certain embodiments, optional heat exchanger  112  can be used to transport residual heat from the fluid in stream  26  to compressor exhaust stream  21 , as described above. In certain embodiments, turbine exhaust (either from stream  26  directly or from stream  118 ) can contain residual heat, which can be recovered within heat recovery system  120  to increase system efficiency. Systems and methods for recovering the residual heat from the exhaust stream of the primary heat exchange system are described in more detail below. 
     In some embodiments, optional blower  140 B can be included. While blower  140 B is illustrated in  FIG. 1A  as being downstream of optional heat exchanger  112 , blower  140 B (when present) may be included at any position downstream of solar receiver  102 . Including blower  140 B downstream of solar receiver  102  can facilitate operating the gas within the solar receiver at a pressure close to atmospheric pressure (e.g., at a pressure less than or equal to about 1.1 atmospheres, such as from about 0.9 atmospheres to about 1.1 atmospheres). 
     After system  10  has been operated as illustrated in  FIG. 1A  for a given period of time, the heat storage media within thermal storage system  134 B can cool down to a point where the fluid within stream  24  is not heated to a sufficiently high temperature to efficiently operate turbine  122 . In addition, the heat storage media within thermal storage system  134 A can become relatively hot, inhibiting further transport of heat from the fluid within stream  16  to the thermal storage system. When this state is achieved, it can be desirable to shift the fluid flow pathways such that thermal storage system  134 A is used to heat fluid being transported to turbine  122  and thermal storage system  134 B is used to store heat absorbed by the fluid within solar receiver  102 . 
     As mentioned above, such fluidic switching can be achieved by actuating valves  13 A- 13 D, which together can form at least a part of a valving subsystem, to alter the flow of fluid, as shown in  FIG. 1B . In  FIG. 1B , valves  13 A- 13 D within the valving subsystem have been actuated to create third and fourth fluidic pathways. For example, in  FIG. 1B , a third fluidic pathway fluidically interconnects solar receiver  102  and thermal storage system  134 B. In  FIG. 1B , the third fluidic pathway is generally indicated by solid stream lines (e.g., including streams  15 ,  32 ,  33 ,  18 , and  14 ). In addition, in  FIG. 1B , a fourth fluidic pathway fluidically interconnects compressor  124 , turbine  122 , and thermal storage system  134 A. The fourth fluidic pathway includes streams indicated by dotted lines (e.g., including streams  21 ,  22 ,  30 ,  31 ,  25 A,  25 B,  26 , and  118 ). As illustrated in  FIG. 1B , the third and fourth fluidic pathways are fluidically isolated from each other. 
     In  FIG. 1B , valve  13 A has been switched such that fluid within stream  22  is transported to thermal storage unit  134 A via stream  30 , rather than to thermal storage unit  134 B. In addition, valve  13 B has been switched such that the fluid within stream  25  originates from stream  31 , which is expelled from thermal storage system  134 A. Thus, in the valving arrangement illustrated in  FIG. 1B , the working fluid that is fed to the turbine is transported through thermal storage system  134 A, rather than through thermal storage system  134 B. 
     In addition, in  FIG. 1B , valve  13 D has been switched such that fluid within stream  15  is transported to thermal storage unit  134 B via stream  32 , rather than to thermal storage unit  134 A. In addition, valve  13 C has been switched such that the fluid within stream  18  originates from stream  33 , which is expelled from thermal storage system  134 B. Thus, in the valving arrangement illustrated in  FIG. 1B , the relatively low-pressure fluid that is heated by solar receiver  102  is transported through thermal storage system  134 B, rather than through thermal storage system  134 A (as illustrated in  FIG. 1A ). 
     In certain embodiments, the valving subsystem comprising valves  13 A- 13 D can be operated such that the first and second fluidic pathways are eliminated and third and fourth fluidic pathways are established substantially simultaneously. For example, using valves  13 A- 13 D, one can switch between a first mode of operation in which heat is provided from solar receiver  102  to thermal storage system  134 A to a second mode of operation in which heat is provided from solar receiver  102  to thermal storage system  134 B essentially instantaneously (e.g., with a switching time of less than about 1 minute, less than about 30 seconds, less than about 10 seconds, or less than about 5 seconds). In some embodiments, using valves  13 A- 13 D, one can switch between a first mode of operation in which heat is provided from thermal storage system  134 B to the fluid used to produce power in turbine  122  to a second mode of operation in which heat is provided from thermal storage system  134 A to the fluid used to produce power in turbine  122  essentially instantaneously (e.g., with a switching time of less than about 1 minute, less than about 30 seconds, less than about 10 seconds, or less than about 5 seconds). In some such embodiments, thermal storage systems  134 A and  134 B can be thought of as being “swapped out.” For example, thermal storage system  134 A can be switched from a “storage” mode (i.e., in which heat from solar receiver is transferred to the media within thermal storage system  134 A) to a “providing” mode (i.e., in which heat from thermal storage system  134 A is provided to the turbine  122 ) at substantially the same time as thermal storage system  134 B is switched from a “providing” mode to a “storage” mode. 
     In some instances, shortly after valves  13 A- 13 D have been switched to re-route the flow of fluid through system  10 , the temperature of the fluid exiting the thermal storage system in the high-pressure fluidic pathway can be relatively low. In some such cases, an optional supplemental heater  130  can be employed to supply additional heat to the fluid in stream  25 A, producing stream  25 B, which can be transported to gas turbine  122 . Supplemental heater  130  can comprise, for example, an auxiliary combustor, sometimes called a boost combustor, that burns fuel to supply additional heat. One of ordinary skill in the art would be capable of selecting an appropriate device to provide the required amount of supplemental heat, given the power demands and operating conditions of a given system. For example, heater  130  might comprise an induced flow combustor. 
     In one set of embodiments, the relatively low temperature of the fluid within the high-pressure stream shortly after switching the valves can be compensated for by adjusting the power demand of the system. For example, just after switching valves  13 A- 13 D, the power demand of the system can be reduced (e.g., to zero or to a level of power than can be produced by a pressurized fluid at the temperature and flow rate available after switching). As the system approaches steady state, the power demand can be increased until, when steady state is reached, 100% of the system power can be demanded. 
       FIG. 1C  is a schematic illustration of another system  100  in which concentrated solar energy is used to generate power. Similar to the convention employed in  FIGS. 1A-1B , the fluid streams in the set of embodiments illustrated in  FIG. 1C  can be generally divided into streams comprising relatively high-pressure fluid (illustrated as dotted lines in  FIG. 1C ) and streams comprising relatively low-pressure fluid (illustrated as solid lines in  FIG. 1C ). As noted above, these conventions are used for illustration purposes only, and are not meant to indicate that the pressures in all relatively low-pressure streams are the same and/or that the pressures in all relatively high-pressure streams are the same. 
     System  100  includes a solar receiver  102  constructed and arranged such that at least a portion of the receiver, such as face  104  in  FIG. 1C , is exposed to incident solar radiation  106 . The energy from the incident solar radiation can be used to heat a fluid within the solar receiver. In some embodiments, the solar receiver can be constructed and arranged to operate at relatively low pressures. For example, the pressure of the fluid within the solar receiver can be up to and including about 2 atmospheres, less than about 1.5 atmospheres, less than about 1.25 atmospheres, less than about 1.1 atmospheres, less than about 1 atmosphere, between about 0.9 and about 2 atmospheres, between about 0.9 and about 1.5 atmospheres, between about 0.9 and about 1.25 atmospheres, between about 0.9 and about 1.1 atmospheres, or between about 0.9 and about 1 atmosphere. In some cases, the solar receiver can be constructed and arranged such that the fluid within the receiver is not substantially compressed, with the exception of incidental compression that might occur due to the heating and/or transport of the fluid, before being transported to the receiver. For example, the fluid transported to the solar receiver can be substantially equal to the pressure of the surrounding environment, in some cases. The reduced pressures at the receiver may allow a “window” of the receiver (e.g., a transparent portion of the receiver through which sunlight passes to heat the fluid in the receiver) to be made significantly larger than in other relatively high pressure receivers. For example, prior receivers may be limited to a window size of about 60 cm diameter, whereas a receiver in some embodiments of the invention may have a size up to about 150 cm or more. In some embodiments, the receivers have a window size of 4 meters or more. 
     Fluid can be transported to the solar receiver via an inlet, such as inlet line  108  in  FIG. 1C . Generally, fluid is transported through the solar receiver when the sun is available to provide energy to heat the fluid. In some cases, the relatively-low pressure fluid transported to the solar receiver can comprise the outlet stream of a turbine used to generate power within the system. However, the relatively low-pressure fluid can also originate from other sources, in addition to or in place of the exhaust stream of a turbine. For example, in some cases, relatively low-pressure fluid transported to the solar receiver can originate from the ambient environment (e.g., atmospheric air). Additional details related to the design and operation of the solar receiver are described in more detail below. 
     Once the relatively low-pressure fluid has been heated within the solar receiver, it can be transported out of the receiver, for example, via stream  110  in  FIG. 1C . At least a portion of the fluid within stream  110  can be transported to heat exchange (or recuperator) system  112  via stream  114 . Heat exchange system  112  can be used to transfer heat from the relatively low-pressure fluid stream (e.g., from a solar receiver and/or from a thermal storage system) to a relatively high-pressure fluid stream  116 , which can be used to drive a gas turbine, as described in more detail below. 
     After the heat from the relatively low-pressure stream has been transported to the relatively-high pressure stream, the relatively low-pressure fluid can be transported out of heat exchange system  112  via stream  118 . In some embodiments, stream  118  can contain residual heat, which can be recovered within heat recovery system  120  to increase system efficiency. Systems and methods for recovering the residual heat from the exhaust stream of the primary heat exchange system are described in more detail below. 
     In some embodiments, optional blower  140 B can be included. While blower  140 B is illustrated in  FIG. 1C  as being downstream of optional heat exchange system  112 , blower  140 B (when present) can be included at any position downstream of solar receiver  102 . Including blower  140 B downstream of solar receiver  102  can facilitate operating the gas within the solar receiver at a pressure close to atmospheric pressure (e.g., at a pressure less than or equal to about 1.1 atmospheres, such as from about 0.9 atmospheres to about 1.1 atmospheres). 
     In the set of embodiments illustrated in system  100 , power is primarily generated using a Brayton cycle. The Brayton cycle illustrated in  FIG. 1C  includes gas turbine  122 . While a single turbine is illustrated in  FIG. 1C , it should be understood that the invention is not so limited, and that, in some embodiments, multiple turbines can be employed. For example, in some embodiments, the power generation system includes at least 2, at least 3, at least 4, at least 5, or more turbines. A single gas turbine and/or the combination of multiple gas turbines can be capable of producing any suitable amount of power (e.g., at least about 100 kW, at least about 500 kW, at least about 1 MW, at least about 4 MW). One of ordinary skill in the art would be capable of selecting an appropriate gas turbine and/or combination of gas turbines to use, given a desired power output requirement. 
     In order to increase system efficiency, the gas supplied to gas turbine  122  should be relatively hot and relatively highly-pressurized. To accomplish this, compressor  124  can be used to compress a relatively low-pressure gas (e.g., ambient air) in stream  126  to produce relatively high-pressure stream  116 . As mentioned above, relatively high-pressure stream  116  can be heated by transferring the heat from heated, low-pressure stream  114  (e.g., from solar receiver  102  and/or from thermal storage system  134 ) to stream  116  via heat exchange system  112  to produce relatively high-pressure, relatively high-temperature stream  128 . In some embodiments, the compressor can be used to produce a fluid stream (e.g., a gas stream) with a pressure above 2, at least about 3, at least about 4, at least about 5, at least about 10, or at least about 15 atmospheres. 
     As illustrated in  FIG. 1C , primary heat exchange system  112  includes two heat exchangers (or recuperators),  112 A and  112 B. It should be understood that, while the figures illustrate the use of two heat exchangers, the invention is not limited to the use of heat exchange systems including two heat exchangers, and, in some embodiments, a single heat exchanger or more than two heat exchangers (e.g., 3, 4, 5, or more heat exchangers) can be used in the heat exchange system. In  FIG. 1C , the first heat exchanger  112 A can be used to exchange heat at relatively high temperatures, for example, removing heat from high-temperature stream  114  to produce intermediate-temperature stream  115  while transferring heat to intermediate-temperature stream  117  to produce high-temperature stream  128 . The second heat exchanger  112 B can be used to exchange heat at relatively low temperatures, for example, removing heat from intermediate-temperature stream  115  to produce low-temperature stream  118 , while transferring heat to low-temperature stream  116  to produce intermediate-temperature stream  117 . High temperature heat exchange (e.g., at temperatures between about 800° C. and about 1250° C.) can involve the use of very expensive materials, such as specially engineered ceramics and/or high temperature super alloys. The use of multiple heat exchangers (e.g., one relatively small inexpensive heat exchanger and one relatively small expensive heat exchanger) instead of a single large, relatively expensive heat exchanger can allow one to achieve efficient heat exchange while reducing cost. While heat exchange system  112  in  FIG. 1C  is illustrated as including two heat exchangers, it should be understood that, in some embodiments, a single heat exchanger can be employed. In addition, in some cases, more than two heat exchangers can be employed in heat exchange system  112 . 
     At least one of the heat exchangers in the heat exchanger system can be configured, in some embodiments, such that the heat exchanger simultaneously contains the first, low pressure fluid (e.g., gas) and the second, high pressure fluid (e.g., gas), which may, in certain embodiments involve simultaneous flow of the first and second fluids through the heat exchanger. For example, in some embodiments, at least one heat exchanger in the heat exchanger system comprises a first inlet through which gas at a relatively low pressure (e.g., a pressure of less than or equal to 2 atmospheres) is transported into the heat exchanger and a second inlet through which gas at a relatively high pressure (e.g., above 2 atmospheres) is transported into the heat exchanger while the first gas is transported into the heat exchanger. By configuring one or more heat exchangers in this manner, the amount of heat transferred from the high temperature fluid to the low temperature fluid can be enhanced, relative to situations in which the first and second fluids are transported subsequently through the heat exchanger (e.g., due to heat dissipation from the heat exchanger during the period between fluid flow). Heat exchangers configured in this manner can be configured to operate in countercurrent or concurrent mode (with flow in the same or opposite directions). 
     In some embodiments, one or more of the heat exchangers used to transfer heat from the relatively low-pressure fluid to the relatively high-pressure fluid (e.g., heat exchangers  112 A and/or  112 B in  FIG. 1C ) can be a rotary heat exchanger (e.g., a ceramic rotary heat exchanger). Suitable rotary heat exchangers (e.g., rotary regenerators) for use in the systems described herein include those described, for example, in U.S. Pat. No. RE37134, issued on Apr. 17, 2001, filed Mar. 25, 1995, entitled “Heat Exchanger Containing a Component Capable of Discontinuous Movement”; U.S. Publication No. 2007/0089283, published on Apr. 26, 2007, filed Oct. 17, 2006, entitled “Intermittent Sealing Device and Method”; U.S. Publication No. 2008/0251234, published on Oct. 16, 2008, filed Apr. 16, 2007, entitled “Regenerator Wheel Apparatus”; U.S. Publication No. 2009/0000761, published on Jan. 1, 2009, filed Jun. 29, 2007. entitled “Regenerative Heat Exchanger with Energy-Storing Drive System”; U.S. Publication No. 2009/0000762, published on Jan. 1, 2009, filed Jun. 29, 2007, entitled “Brush-Seal and Matrix for Regenerative Heat Exchanger and Method of Adjusting Same”; and U.S. Publication No. 2006/0054301, published on Mar. 16, 2006, filed Dec. 16, 2004, entitled “Variable Area Mass or Area and Mass Species Transfer Device and Method.” Ceramic rotary heat exchangers can be capable of operating at relatively high temperatures (e.g., up to about 2100° F. (1200° C.), which can allow one to supply higher temperature gas to the gas turbine, thereby increasing system efficiency. Of course, the invention is not limited to the use of rotary heat exchangers, and, in some embodiments, one or more of the heat exchangers (e.g., the heat exchangers used to transfer heat from the relatively low-pressure fluid to the relatively high-pressure fluid such as heat exchangers  112 A and/or  112 B in  FIG. 1C ) can be any of a wide variety of suitable heat exchanger configurations, including, but not limited to, a plate heat exchanger, a tube heat exchanger (e.g., a shell and tube heat exchanger), etc. 
     In some embodiments, at least one of the heat exchangers can be a metallic heat exchanger. The first and second heat exchangers can be of different types. For example, in some embodiments, one of the heat exchangers within the heat exchange system can be a ceramic heat exchanger (e.g., a ceramic rotary heat exchanger, a ceramic plate heat exchanger, a ceramic tube heat exchanger, etc.) while a second of the heat exchangers can be a metallic heat exchanger. For example, gas from the solar receiver can be transported to a ceramic heat exchanger (where a relatively high maximum temperature might be observed), and subsequently transported to a metallic heat exchanger (where the maximum temperature might be lower than that observed in the ceramic heat exchanger). 
     In some embodiments, the system can include a heat exchanger that is configured to be operated at a relatively high temperature. For example, in some embodiments, the system can include one or more heat exchangers (e.g., heat exchangers  112 A and/or  112 B in  FIG. 1C ) that can be operated above temperatures of 1500° F. and in some embodiments at temperatures of up to 1800° F. In some embodiments, the system can include one or more heat exchangers that can be operated at temperatures of up to 2100° F. or even to 2500° F. High temperature heat exchangers can comprise one or more materials configured to withstand high temperature operation including, for example, one or more ceramics (e.g., aluminum oxides, iron oxides, silicon oxides, magnesium oxides, etc.). In some embodiments, the heat exchanger can comprise one or more metals (e.g., a super alloy such as those comprising nickel, chromium, titanium, tungsten, molybdenum, tantalum, columbium, and the like, including any of the super alloys described elsewhere herein. As specific examples, all or part of a high temperature heat exchanger can be formed of Alloy 230®, Alloy 214®, and/or Alloy 556® from Haynes International. 
     In some embodiments, the fluid within high-pressure, high-temperature stream  128  can be transported directly to gas turbine  122 , where it can be used to produce power. The gas turbine can be constructed and arranged to operate using incoming gas streams with relatively high pressures. In some embodiments, the gas stream fed to the gas turbine has a pressure of above 2, at least about 3, at least about 4, at least about 5, at least about 10, or at least about 15 atmospheres. In some instances, for example during startup or during periods when the temperature of the fluid in stream  114  is relatively low (e.g., during periods of low sunlight and/or when storage container  112  (described below) is not sufficiently heated to supply high-temperature fluid), an optional supplemental heater  130  can be employed to supply additional heat to the fluid in stream  128 , producing stream  128 B which can be transported to gas turbine  122 . Supplemental heater  130  can comprise, for example, an auxiliary combustor, sometimes called a boost combustor, that burns fuel to supply additional heat. One of ordinary skill in the art would be capable of selecting an appropriate device to provide the required amount of supplemental heat, given the power demands and operating conditions of a given system. For example, heater  130  might comprise an induced flow combustor. 
     Once the gas in stream  128  (or  128 B) has been expanded, a relatively low-pressure, low-temperature turbine exhaust stream  132  can be produced. As mentioned above, in some embodiments, the turbine exhaust stream  132  can be fed to the solar receiver  102 , where it can be reheated and used to supply heat to heat exchange system  112 . Routing the turbine exhaust in this way can be beneficial, as the turbine exhaust may contain residual heat that would otherwise be lost if the exhaust were vented directly to the atmosphere. 
     In some embodiments, system  100  can include optional thermal storage system  134 . In some embodiments, the thermal storage system can include a single thermal storage unit, while in other embodiments, the thermal storage system can include a plurality of thermal storage units. The thermal storage system can be used to store heat (e.g., sensible heat) for use during periods of relatively low sunlight and/or during startup of the system. During periods of relatively high sunlight, at least a portion of the fluid exiting the solar receiver (e.g., via stream  110 ) can be transported to the thermal storage system  134  (e.g., via stream  136 ), where the heat can be retained for later use. During periods of low sunlight, a relatively low-temperature fluid can be transported into the thermal storage system via stream  138 . The heat stored within thermal storage system  134  can be used to heat the relatively low-temperature fluid to produce high-temperature fluid, which can be transported to heat exchange system  112 , e.g., via streams  136  and  114 . In some embodiments, the fluid supplied to stream  138  can comprise the exhaust stream  132  of turbine  122 . For example, in some cases during periods of low sunlight, little or no fluid might be supplied to solar receiver  102  via stream  108 , and low-pressure fluid from the exhaust stream  132  of turbine  122  (and, optionally, some fluid from the ambient atmosphere) can be re-directed to thermal storage system  134  via conduit  138 . In some embodiments, a controller and valves can be used to regulate the distribution of low-pressure fluid through solar receiver  102  and thermal storage system  134 . In some embodiments, optional blower  140  can be incorporated into the system. 
     Any suitable type of blower can be included in the system; the blower can comprise, for example, an electric driven induction flow fan. The blower can be used, for example, to transport the gas turbine exit stream (e.g., stream  132  in  FIG. 1C ) through the solar receiver during periods of relatively high sunlight. In addition, blower  140  can be used to provide the power to circulate hot air through the thermal storage system during periods of high sunlight. In some embodiments, blower  140  can be used to transport heated air from thermal storage system  134  to the solar receiver  102  (e.g., via pathway  138 ) to provide pre-heated air to the solar receiver (e.g., during periods of relatively low sunlight). In some embodiments, blower  140  can be shut down during periods of relatively low sunlight when the thermal storage system can provide heat for the system. 
     The blower can be arranged, in some cases, to accept ambient air or fluid from another source in addition to the exhaust gas from the gas turbine. In this way, the blower can be used to control the overall flow rate of the fluid within the relatively low-pressure section (i.e., from the exhaust of the gas turbine, through heat exchange system  112 , and through optional heat recovery system  120 ). For example, when higher flow rates through the low-pressure section are desirable, the blower can take in a relatively large amount of fluid from the ambient or another, non-turbine exhaust source. When lower flow rates through the low-pressure section are desirable, the blower can take in a relatively small amount of (or no) fluid from the ambient or another, non-turbine exhaust source. 
     As noted above, residual heat within stream  118  (e.g., from heat exchanger  112  in  FIGS. 1A-1B  or from heat exchange system  112  in  FIG. 1C ) can be exchanged within optional heat recovery system  120 . Heat recovery system  120  can include a variety of configurations. For example, in some cases, a Rankine bottoming cycle can be employed to recover residual heat.  FIG. 2A  includes a schematic diagram illustrating the recovery of energy from stream  118  using a Rankine cycle. In  FIG. 2A , stream  118 , originating from heat exchanger(s)  112 , is fed to a heat exchange boiler  210 . The heat within stream  118  can transferred to another fluid stream containing water (e.g., stream  211  in  FIG. 2A ), which can result in the production of steam or hot water. The steam produced during this exchange of heat can exit via stream  212 . Boiler  210  can also produce effluent stream  213 , which can include cooled fluid from stream  118 . Stream  212  can be fed to steam turbine  214 , where it can be used to produce energy and exhaust stream  216 . Exhaust stream  216  can be condensed to water in optional condenser  218 , to produce heat (which can be used in other areas of the process) a condensed stream. As shown in  FIG. 2A , the condensed stream from the condenser is illustrated as being used as heat exchange boiler inlet stream  211 , which can be re-heated to generate steam. Optionally, steam turbine exhaust stream  216  can be used as heat exchange boiler inlet stream  211 . 
     In some instances, an absorption chiller can be used to recover residual heat from stream  118 .  FIG. 2B  includes a schematic illustration of one such set of embodiments. In  FIG. 2B , stream  118  is transported to absorption chiller  220 , where the heat from stream  118  is used to provide energy to the absorption chiller necessary to cool a relatively warm fluid in stream  222  (e.g., ambient air) to produce a cooled fluid stream  224 . In addition to producing cooled fluid stream  224 , the absorption chiller can produce exhaust stream  226 , which contains fluid from stream  118  that has been cooled. Cooled stream  224  can be used, for example, as part of an air conditioning system. As another example, cooled stream  224  might be used to cool system components (e.g., the gas turbine), for example, during operation in very hot climates (e.g., temperatures of 100° F. or above). One of ordinary skill in the art would be capable of selecting a suitable absorption chiller based upon the required cooling load, temperature and flow rate of incoming fluid stream  118 , and other design parameters. 
     In still other cases, the residual heat within stream  118  can be used to provide heat (e.g., within other areas of the process and/or to areas outside the power generation process).  FIG. 2C  includes an exemplary schematic illustration of one such set of embodiments. In  FIG. 2C , stream  118  is transported to heat exchanger  230 , where it is used to heat relatively cool fluid in stream  232  (e.g., ambient air) to produce a heated fluid stream  234 . In addition, exhaust stream  236 , which contains fluid from stream  118  that has been cooled, can be produced. Heated stream  234  can be used, for example, to produce hot water or other liquids for use in a running water system, a space heating system (e.g., to provide heat one or more rooms within a building or other suitable structure), or any other suitable system in which heated fluids are required. One or ordinary skill in the art would be capable of selecting a suitable heat exchanger based upon the required heating load, temperature and flow rate of incoming fluid stream  118 , and other design parameters. 
     The inclusion of heat recovery system  120  can lead to relatively large increases in overall system efficiency. Generally, overall system efficiency is calculated as the power produced by the system (in the form of electricity and/or in the form of a heated or cooled stream that can be used in another system, such as streams  224  and  234  in  FIGS. 2B and 2C ) divided by the power of the solar energy incident on the solar receiver and multiplied by 100%. In embodiments employing the Rankine bottoming cycle illustrated in  FIG. 2A , the overall efficiency can approach about 50% (e.g., between about 40% and about 50%). For systems that include an absorption chiller such as the system illustrated in  FIG. 2B , overall system efficiency can approach about 60% (e.g., between about 40% and about 60%). The overall efficiency of power generation systems that employ a heat exchanger to provide heat to other parts of the power generation system and/or external systems can approach about 80% (e.g., between about 40% and about 80%). 
     As noted above, the thermal storage system  134  in system  100  can include one or more thermal storage units. The thermal storage unit(s) can enable a practical and cost effective method to achieve thermal storage of CSP energy for use in generating electricity during hours with no or low sunlight. In some embodiments, a thermal storage unit can comprise a tank in which solid media with passages through which the fluid flows is located to store the thermal energy at relatively high temperatures (e.g., at least about 1800° F., at least about 2000° F., or higher). 
     Exemplary illustrations of suitable thermal storage units are shown in  FIGS. 3A-3C .  FIG. 3A  includes a cross-sectional view of a storage unit  310 , including lines  312  and  314 , each of which can function as an inlet or an outlet. Unit  310  also includes plate  316 A that includes a plurality of passageways. Plate  316 A is designed in this manner so fluid can be transported through the plate while the plate supports thermal storage media within volume  318 , preventing the thermal storage media from entering line  314 . In addition, unit  310  can include plate  316 B, which can also comprise a plurality of passageways. By designing plate  316 B in this manner, fluid can be transported from volume  318  and through plate  316 B without entraining thermal storage media as the fluid exits line  312 . By avoiding the transport of thermal storage media out of the thermal storage unit  310 , one can reduce damage (e.g., erosion damage) to the turbine, blower, solar receiver, or other system components that may occur when the media contact these components. 
     Thermal storage unit  310  can be fabricated using a variety of materials including, for example, metals (e.g., stainless steel). In some embodiments, thermal storage unit  310  is configured such that it is a certified pressure vessel (e.g., ASME-certified, EN13445 certified, or a pressure vessel meeting a similar set of certification standards). In addition, plates  316 A and  316 B can be fabricated from any suitable material, including metals (e.g., stainless steel, refractory metals such as tungsten, and the like), ceramics, and/or combinations of these materials. 
     Thermal storage unit  310  can be fabricated in sections, such that two or more of the sections may be bolted together to assemble a storage unit having a desired volumetric capacity. Fabrication of the storage unit in sections facilitates factory construction, transport and onsite assembly of storage units having relatively large volumetric capacity. In some configurations sections of the storage unit may be in the range of 8 feet to 16 feet in length and 8 feet to 16 feet in diameter. In some configurations sections of the storage unit may be approximately 12 feet in length and 10 feet to 12 feet in diameter. 
     The passageways within plate  316 A and/or  316 B can be configured to impart a desired flow profile within volume  318 . For example, the sizes (e.g., diameters, lengths), cross-sectional shapes, and/or flow angles of the pathways within plates  316 A and/or  316 B can be selected to achieve a desired flow profile. The openings of the fluid passageways one or both sides of plates  316 A and/or  316 B can be arranged in any suitable pattern including, for example, a honeycomb pattern. 
     As noted above, lines  312  and  314  can each function as an inlet or an outlet. For example, in some instances in which the thermal storage media within unit  310  is being heated by an incoming fluid (e.g., at a temperature of between 1800° F. and 1900° F.), line  312  can function as an inlet and line  314  can function as an outlet. In such cases, plate  316 A can prevent thermal storage media from being entrained in the heating fluid and being transported through outlet  314 . In some instances in which the thermal storage media within unit  310  is being used to heat an incoming fluid (e.g., at an incoming temperature of between 100° F. and 1200° F.), line  314  can function as an inlet and line  312  can function as an outlet. In such cases, plate  316 B can prevent thermal storage media from being entrained in the heated fluid and being transported through outlet  312 . The direction of fluid flow within unit  310  can be freely changed, depending on the mode of operation of the power generation system. 
     In some cases, a first portion of the gas heated by the solar receiver can be transported to the thermal storage system and a second portion of the gas heated by the solar receiver can be transported to the heat exchange system (i.e., the airflow from the solar receiver can be switched between the thermal storage system and the heat exchange system used to transfer heat from the low-pressure fluid to the high-pressure Brayton cycle fluid). In some cases, substantially all of the gas from the solar receiver is transported to the thermal storage system over a first period of time, and substantially all of the gas from the solar receiver is transported to the heat exchange system over a second period of time that does not overlap with the first period of time. For example, substantially all of the exhaust stream from the solar receiver might be transported to the thermal storage system over a first period of time, and at a later time, the flow from the solar receiver can be switched such that substantially all of the solar receiver exhaust is transported to the heat exchange system. In other cases, a first portion of the gas from the solar receiver is transported to the thermal storage system over a first period of time, and a second portion of the gas from the solar receiver is transported to the heat exchange system over the first period of time. Stated another way, the exhaust stream from the solar receiver can be split such that, simultaneously, a first portion of the solar receiver exhaust is transported to the thermal storage system and a second portion of the solar receiver exhaust is transported to the heat exchange system. 
     In some embodiments, the thermal storage system (including any thermal storage unit(s) within the thermal storage system) can be constructed and arranged to operate at relatively low pressures during at least a portion of the period of time over which system  100  is operated. For example, the pressure of the fluid within the thermal storage system (including any thermal storage unit(s) within the thermal storage system), for example, during heating of the thermal storage system and/or during heating of a fluid being transported through the thermal storage system, can be up to and including 2 atmospheres, less than about 1.5 atmospheres, less than about 1.25 atmospheres, less than about 1.1 atmospheres, less than about 1 atmosphere, between about 0.9 and about 2 atmospheres, between about 0.9 and about 1.5 atmospheres, between about 0.9 and about 1.25 atmospheres, between about 0.9 and about 1.1 atmospheres, or between about 0.9 and about 1 atmosphere. In some cases, the thermal storage system can be constructed and arranged such that the fluid within the thermal storage system is not substantially compressed, with the exception of incidental compression that might occur due to the heating and/or transport of the fluid, before being transported to the thermal storage system. For example, the fluid within the thermal storage system can be substantially equal to the pressure of the surrounding environment, in some cases. In some embodiments, the thermal storage system  134  is operated at relatively low pressures when being heated by a low-pressure fluid (e.g., low pressure fluid in stream  136  from solar receiver  102 ). In some embodiments, thermal storage system  134  is operated at relatively low pressures when being used to pre-heat a fluid that is to be transported to turbine  122 . 
     It should be understood that the invention is not limited to the use of low-pressure fluid within thermal storage system  134 , and that, in some embodiments, high-pressure fluid can be transported through thermal storage system  134  during operation of system  100 . For example, in some embodiments, a relatively high-pressure fluid (e.g., at a pressure of above 2 atmospheres, at least about 2.1 atmospheres, at least about 2.25 atmospheres, at least about 2.5 atmospheres, at least about 3 atmospheres, at least about 4 atmospheres, at least about 5 atmospheres, at least about 10 atmospheres, or at least about 15 atmospheres, and, in some embodiments, up to 50 atmospheres) can be transported through and heated by thermal storage system  134  (e.g., after thermal storage system  134  has been heated by a fluid, such as a low-pressure fluid from solar receiver  102 ). After the high-pressure fluid is heated by thermal storage system  134 , it can be transported to turbine  122  to generate power, in some embodiments. 
     In many previous thermal storage systems, high pressures are employed, which can increase the expense required to construct the systems. Other previous thermal storage systems have used a variety of salts or other materials that undergo a phase change, many of which materials were also very expensive. By being unpressurized and fully factory produced along with low-cost fill (thermal storage) media this approach dramatically reduces CSP thermal storage system cost. Using the CSP thermal storage tanks in modular form as part of the CSP tower could further improve the capital costs associated with the system. 
     The thermal storage units within the thermal storage system can be modular, in some cases, which can allow one to easily scale a system in order to allow a CSP system to operate to produce a given power for a given time without low or no sunlight. For example,  FIG. 3B  includes a system in which a single unit is used to store thermal energy. In  FIG. 3C , two thermal storage units are connected (e.g., by rotating caps  320  such that lines  312  face each other) to double the thermal storage capacity. Of course, the amount of energy delivered by the thermal storage unit(s) can be altered in other ways by, for example, only partially filling one or more units with media and/or limiting the degree to which one or more sections of a single unit are heated (which might include establishing a thermal gradient along the longitudinal axis of one or more storage units). 
     In some embodiments, the tanks can be sized to achieve relatively easy transport. For example, each thermal storage unit could be between about 2 and about 12 feet in diameter, and up to 40 feet long to enable easy shipment. The thermal storage units can be filled on site or prior to delivery to the site, which can allow for cost effective production and reduce on site construction cost and/or schedule delays. 
     A variety of fill media can be used in the thermal storage unit(s) in the thermal storage system. The fill media can comprise a variety of materials with high heat capacities that are able to retain their structures at high temperatures, such as ceramics and other refractory materials. Exemplary materials include, but are not limited to, materials comprising aluminum oxides, iron oxides, silicon oxides, and/or magnesium oxides such as fire brick, mullite, magnetite, PYRO GRAN 35/38, PYRO KOR 60NR, PYRO KOR 95NK, and/or PYROFER 70. In some embodiments, the thermal storage media has a heat capacity of at least about 600 J/kg K, at least about 800 J/kg K, or at least about 900 J/kg K. It can be advantageous, in some embodiments, to use materials with relatively low densities (e.g., less than about 5 g/cm 3 , less than about 3 g/cm 3 , or less than about 2 g/cm 3 ). 
     The thermal storage media within the thermal storage unit(s) can be of any suitable form factor and size. For example, pellets (e.g., substantially spherical pellets or pellets with any of the shapes described below) with maximum cross-sectional diameters in mm, cm, or larger length scales can be used as the thermal storage media, in some instance. In some embodiments, the thermal storage media can comprise pellets, and at least about 50%, at least about 75%, at least about 90%, at least about 95%, or at least about 99% of the pellets have maximum cross-sectional diameters of less than about 100 cm, less than about 10 cm, less than about 1 cm, between about 1 mm and about 100 cm, or between about 1 cm and about 100 cm. Suitable pellet shapes include, but are not limited to, shapes that are substantially rectangular prisms (e.g., bricks, substantially cubic shapes), substantially triangular prisms, substantially spheres, bow ties, honeycombs, saddles, and the like. In one set of embodiments, the thermal storage media can comprise elongated tubes through which heated fluid is transported. 
     In certain embodiments, the thermal storage media within the thermal storage unit(s) can be selected such that a step-function change in temperature is not observed. That is to say, in such systems, the temperature profile of thermal storage media, when plotted linearly as a function of the distance along which fluid is transported through the thermal storage unit(s) would exhibit a non-step-function behavior. In certain such embodiments, one end of the thermal storage unit would be at a relatively hot temperature while the other end of the thermal storage unit would be at a relatively cool temperature, with a non-step function change in temperature (e.g., a linear change in temperature, a concave up change in temperature, or a concave down change in temperature) in between. Such profiles can be achieved, for example, by using thermal storage media with relatively low thermal conductivities, such as ceramics. 
     In some embodiments, the interior of the thermal storage unit(s) can be lined with a thermally insulating material and/or the outside of the thermal storage unit(s) can be covered with a thermally insulating material to reduce heat loss to the atmosphere. For example, when the tank is manufactured out of metal, the tank can be lined with and/or covered with a refractory material (e.g., ceramics such as alumina, silica, magnesia, and the like). In some embodiments, the refractory material can be cast in place and/or can comprise a multi-layered structure in which the density and/or heat capacity can vary from layer to layer. In some embodiments, the thickness of the thermally insulating lining within the unit(s) can be between about 5 inches and about 15 inches (e.g., for a tank with a diameter up to 12 feet and a length of up to 40 feet). In some embodiments, the thickness of the thermally insulating material on the exterior of the thermal storage unit(s) can be up to 1 foot or up to 2 feet in thickness. 
     As noted above, the components of the CSP system can be positioned in various parts of a solar power tower.  FIG. 4A  includes a schematic illustration of one set of power tower embodiments. The system  400  in  FIG. 4A  includes solar receiver  410  fluidically connected to turbine package  412 , which can include a gas turbine and a compressor. In this embodiment, the turbine package  412  is made as a single modular unit that can be completely assembled at a factory, shipped to an operation site (e.g., by truck or railroad car), and placed (e.g., by crane) onto a tower structure  414 . The turbine package  412  may include not only a compressor and gas turbine, but also a heat exchanger, or recuperator, unit, an electric generator and related power electronics, a supplemental heater and/or control valves and other components to control the operation of one or more portions of the power generation system. The turbine package  412  may be made and shipped as a single unit with the receiver  410  (which in this embodiment also includes the secondary collector), or the package  412  and receiver  410  may be made as separate modular units and assembled together on the tower structure  414  at the operation site. By making turbine and receiver sections in a modular arrangement, manufacturing of the modular units may be made more efficient. That is, the turbine package  412  and receiver  410  may be made in a factory setting, with skilled technicians building and testing the package  412  and receiver  410  before being sent to an operation site. The package  412  and receiver  410  may be tested in real conditions, e.g., on a tower or other structure that received sunlight from a heliostat field, or in more artificial conditions. For example, the turbine package  412  may be tested by supplying heated air or other fluid to the package  412  that is heated by fuel combustor or other suitable arrangement. Similarly, the receiver  410  may be tested by illuminating the receiver  410  with artificial light or other radiation that does not originate from a heliostat field. In this way, the package  412  and receiver  410  may be tested individually, or as a functioning whole, under different conditions (such as low light levels, high light levels, high and/or low ambient temperature conditions, high and low power output conditions, etc.) As a result, steps may be taken to help ensure that fully functioning turbine package  412  and/or receiver  410  units are shipped to an operation site. 
     In some embodiments, one or more thermal storage units can be incorporated as part of a tower structure  414  which can, for example, allow for relatively easy assembly and further reduce the overall cost of the CSP system. For example, thermal storage media can be stored within tower structure  414 , which can serve as the thermal storage unit. For example, in the set of embodiments illustrated in  FIG. 4A , tower structure  414  can be filled with thermal storage medium and provide thermal storage capability for system  400 . In another embodiment, the tower structure  414  may be arranged to house one or more thermal storage tanks like that shown in  FIGS. 3A-3C . If two or more tanks are provided, the tanks may be stacked within the tower structure  414 . Arranging the tanks within the tower structure  414  may provide different features such as reducing the overall footprint of the power generating unit, providing additional thermal cover for the tanks, and/or enhancing the strength of the tower structure  414 . For example, the tanks may be incorporated into the tower structure  414  so as to not only provide thermal storage, but also provide structural support for the tower structure  414 . Like the receiver  410  and turbine package  412 , the energy storage tanks and/or tower structure  414  may be made in one or more modular units that are shipped to an operation site and assembled together. 
       FIG. 4B  illustrates another set of embodiments that includes thermal storage unit  416 , independent of tower structure  414 . That is, in this embodiment, a thermal storage tank  416  is mounted to the top of the tower structure  414  with the receiver  410  and turbine package  412 . The receiver  401 , turbine package  412  and tank  416  may be made as a single modular unit that may be manufactured at a factory and shipped (e.g., by truck or railcar) to an operation site and placed on top of a tower structure  414 , or the receiver  410 , turbine package  412  and/or tank  416  may be made as separate modular units. A modular structure may significantly reduce assembly costs at the operation site, e.g., because the receiver  410 , turbine package  412  and tank  416  may be placed by crane on the tower structure  414  and be ready for operation with only relatively minimal assembly at the operation site. For example, if made as a single unit, placement of the receiver  410 , turbine package  412  and tank  416  may require only electrical power hookups and connection to the tower  414  for the system to be ready for energy generation. 
     One advantage provided by aspects of the invention relates to the decreased overall weight of the power generation system, e.g., including the receiver  410  and turbine package  412  of  FIG. 4A . In one set of embodiments, the total weight of the receiver  410  and turbine package  412  may be approximately 50 tons per MWe power output. For example, a 1MWe system may have the receiver  410  and turbine package  412  weigh approximately 100,000 pounds (or 50 tons). Of course, the weight of the components may vary depending on a variety of factors, and thus the weight per MWe power output may vary from about 25 to 100 tons/MWe or more. (It is envisioned that tower-based solar power generation system according to aspects of the invention may be constructed for power output ranging from about 100 kWe to 5MWe. Of course, smaller and larger output systems are possible, but may not be economically feasible (e.g., small output systems may not be economically justify installation costs) or technically feasible (e.g., large output systems may have receiver and turbine package weights that are too large for sensible tower deployment.) 
     Although the embodiments shown in  FIGS. 4A and 4B  show the receiver  410 , turbine package  412  and/or tank  316  arranged in a modular format, the various components of the power generation system need not be arranged in modular units. Instead, in some embodiments, the individual pieces of the system (such as a gas turbine, compressor, recuperators, receiver, collector, etc.) may be assembled in place on the tower. Thus, aspects of the invention are not necessarily limited to modular arrangements of components that are attached to a tower structure  414 . 
     As mentioned above, in some embodiments, the power generation system can include a solar receiver operating at a relatively high pressure (e.g., above 2 atmospheres, at least about 3 atmospheres, at least about 4 atmospheres, at least about 5, at least about 10, or at least about 15 atmospheres) in place of or in addition to the solar receiver operating at a relatively low pressure. 
       FIG. 5  includes an exemplary schematic diagram of a concentrated solar power generation system  500  including a high pressure solar receiver. In  FIG. 5 , fluid stream  526  (e.g., comprising ambient air) is fed to compressor  524 , where it is compressed to a relatively high pressure. High pressure stream  516  from compressor  524  is then fed to heat exchange system  512 , which can comprise one or more heat exchangers. In the set of embodiments illustrated in  FIG. 5 , two heat exchangers ( 512 A and  512 B) are shown, although in other embodiments a single heat exchanger or more than two heat exchangers may be used. For example, in some cases, a single rotary heat exchanger (e.g., a ceramic rotary heat exchanger, a metal rotary heat exchanger) can be used in heat exchange system  512 . In other cases, two or more rotary heat exchangers (e.g., ceramic rotary heat exchangers, metallic rotary heat exchangers, or combinations of the two) can be used in heat exchange system  512 . 
     In the set of embodiments illustrated in  FIG. 5 , heated high pressure stream  528  from heat exchange system  512  is transported to high pressure solar receiver  502 , where it is further heated via incident solar radiation  506  passed through surface  504 . High pressure, high temperature stream  510  is then transported to turbine  522  (e.g., a gas turbine, which can be part of a Brayton cycle), where the stream is used to produce power. Exhaust stream  514  from gas turbine  522  can be transported to heat exchange system  512 , where the residual heat in the stream can be used to pre-heat the compressor exhaust stream  516 . Exhaust stream  518  can, in some cases, be used to provide energy to a heat recovery system  520 , which can comprise any of the components described above in relation to heat recovery system  120 . The high pressure receiver system outlined in  FIG. 5  can be useful, for example, in embodiments in which a single crystal nickel receiver is employed. 
       FIG. 6  includes an exemplary schematic illustration of a power generation system  600  in which two solar receivers are employed. In the set of embodiments illustrated in  FIG. 6 , fluid stream  626  (e.g., comprising ambient air) is fed to compressor  624 , where it is compressed to a relatively high pressure. High pressure stream  616  from compressor  624  is then fed to heat exchange system  612 , which can comprise one or more heat exchangers. In the set of embodiments illustrated in  FIG. 6 , two heat exchangers ( 612 A and  612 B) are shown, although in other embodiments a single heat exchanger or more than two heat exchangers may be used. For example, in some cases, a single heat exchanger (e.g., a rotary heat exchanger such as a ceramic rotary heat exchanger or a metal heat exchanger, or any other type of high temperature heat exchanger) can be used in heat exchange system  612 . In other cases, two or more heat exchangers (e.g., two or more rotary heat exchangers such as ceramic rotary heat exchangers or metallic heat exchangers, or combinations of the two, or two or more of another type of high temperature heat exchanger) can be used in heat exchange system  612 . 
     In  FIG. 6 , a portion of heated high pressure stream  628  from heat exchange system  612  is transported to high pressure solar receiver  602 , where it is further heated via incident solar radiation  606  passed through surface  604 . In some cases, a portion of high pressure stream  628  can be transported to combustor  630 , where it can be further heated. A portion of high pressure, high temperature stream  610  exiting the high pressure solar receiver  602  can be transported to combustor  630 , where it can be further heated, if necessary. 
     In some cases, at least a portion of high pressure, high temperature stream  610  exiting the high pressure solar receiver  602  can be transported to a thermal storage system  634 . Stream  610  can be used to deliver energy to the thermal storage system, in some embodiments. In some cases, thermal storage system  634  can absorb heat from a low pressure stream exiting low pressure solar receiver  603 , described in more detail below. In some such cases, a high pressure, high temperature stream  635  exiting thermal storage system  634  can be transported to combustor  630 , where it can be optionally further heated. The fluid streams transported to combustor  630  can be subsequently transported to gas turbine  622  via stream  637 , where they can be used to generate power. 
     The turbine exhaust stream  614  can be transported to heat exchange system  612 , where the residual heat can be used to pre-heat compressor exhaust stream  616  before it is transported to high pressure solar receiver  602 . In some cases, a portion (or all) of the exhaust stream  618  from heat exchange system  612  can be transported to a second, low pressure solar receiver  603  (in some cases, via optional blower  640 ). The fluid within the low pressure solar receiver  603  can be heated via incident solar radiation  607  transmitted through surface  605 . The low pressure receiver exhaust stream  650  can be transported to thermal storage system  634 , where it can be used to supply heat (which can be used, for example, to heat all or part of high pressure solar receiver exhaust stream  610 ). The low pressure stream  652  exiting thermal storage system  634  can be used, in some embodiments, within thermal recovery region  621 , which can include any of the components described above in relation to heat recovery system  120 . 
     In some cases, the thermal storage system can include a first portion constructed and arranged to be operated at a relatively high pressure (e.g., at least about 3, at least about 4, at least about 5, at least about 10, or at least about 15 atmospheres), and a second portion constructed and arranged to be operated at a relatively low pressure (e.g., equal to or less than about 2, less than about 1.5, less than about 1.25, or less than about 1.1 atmospheres, less than about 1 atmosphere, between about 0.9 and about 2 atmospheres, between about 0.9 and about 1.5 atmospheres, between about 0.9 and about 1.25 atmospheres, between about 0.9 and about 1.1 atmospheres, or between about 0.9 and about 1 atmosphere). For example, in the set of embodiments illustrated in  FIG. 6 , thermal storage system  634  includes a first portion constructed and arranged to handle the flow of low-pressure stream  650  and a second portion constructed and arranged to handle the flow of high-pressure stream  610 . 
     In some embodiments, a portion of exhaust stream  618  from heat exchange system  612  can be transported to thermal recovery region  620 , which can include any of the components described above in relation to heat recovery system  120 . 
     Many of the components illustrated in the figures are fluidically connected. As a specific example, receiver  102  and heat exchange system  112  in  FIG. 1C  are illustrated as being directly fluidically connected. In addition, in  FIG. 1C , heat recovery system  120  and gas turbine  122  are illustrated as being fluidically connect (although not directly fluidically connected). Two components are said to be “fluidically connected” when they are constructed and arranged such that a fluid can flow between them. In some cases, two components can be “directly fluidically connected,” which is used to refer to a situation in which the two components are constructed and arranged such that a fluid can flow between without being transferred through a unit operation constructed and arranged to substantially change the temperature and/or pressure of the fluid. One of ordinary skill in the art would be able to differentiate between unit operations that are constructed and arranged to substantially change the temperature and/or pressure of a fluid (e.g., a compressor, a condenser, a heat exchanger, etc.) and components are not so constructed and arranged (e.g., a transport pipe through which incidental heat transfer and/or pressure accumulation may occur). It should be understood that, while two components might be illustrated as being directly fluidically connected in the figures, other embodiments can include arrangements in which they are fluidically connected but not directly fluidically connected. 
     In some embodiments, solar receivers, such as those shown in  FIGS. 7A-D , are designed and constructed to be used in conjunction with the power generation systems provided herein. The exemplary solar receivers in  FIGS. 7A and 7B  comprise a low pressure fluid chamber  700  that is designed and constructed to provide an insulated casing  700 , which acts to reduce or eliminate thermal losses from the solar receiver and to contain a low pressure working fluid. The low pressure solar receivers comprises a transparent object  703  positioned at the front of the low pressure fluid chamber  700  adjacent to the opening  708  for receiving solar radiation. 
     In the embodiments depicted in  FIGS. 7A and 7B , a fluid path is defined within the low pressure fluid chamber  700 , such that a relatively low temperature working fluid (e.g., a fluid having a temperature in a range of 300° C. to 800° C.) entering the fluid inlet  702  at the rear of the receiver, passes through the receiver around the periphery of a liner into a front region of the fluid chamber  700  and across the transparent object  703  (e.g., a window). By passing across the transparent object  703 , the relatively low temperature working fluid acts, in part, to cool the transparent object  703 , which is heating, in part, by incident solar radiation and thermal radiation from a solar absorber  704 . The relatively low temperature working fluid passes through the solar absorber  704  wherein it is further heated by the solar absorber  704 . Within the solar absorber the relatively low temperature working fluid is converted to a relatively high temperature working fluid (e.g., a fluid having a temperature in a range of above 800° C. to 1200° C.). The relatively high temperature working fluid exits the low pressure fluid chamber  700  through a fluid outlet  701 . After leaving the solar receiver, in some embodiments, the relatively high temperature working fluid is directed to a gas turbine system, a thermal storage system (e.g., a sensible heat storage system), or other thermal energy system, as is exemplified elsewhere herein. 
     Typically, a solar absorber, such as that depicted in  FIG. 7A , is constructed of a material that can withstand relatively high temperatures (e.g., temperatures in excess of 1000° C.) and that has sufficient thermal properties (e.g., thermal conductivity, emissivity) to absorb thermal energy from incident solar radiation and transfer thermal energy to a working fluid passing within the solar receiver. In some cases, the solar absorber is constructed of a material such as a metal, (e.g., high-temperature alloy, heat resistant cast alloy), a refractory material (e.g., a ceramic) or a carbon-based material. The solar absorber is often constructed of a ceramic material such as a glass ceramic, silicon carbide, silicon nitride, alumina, or silicon oxide. 
     The solar absorber  704  of a low pressure receiver, such as that depicted in  FIG. 7A , typically has a high surface area to facilitate the transfer of thermal energy to a working fluid passing within the solar receiver. The solar absorber  704 , in some embodiments, is designed and constructed to have a network (e.g., a honeycomb network, a shell and tube network, a foam network, etc.) of fluid passages through which the working fluid passes. The solar absorber  704  is immobilized within the low pressure chamber such that a relatively low temperature working fluid traveling within the fluid flow path of the low pressure chamber  700  is directed to enter the solar absorber  704 , passing through the network of fluid passages of the solar absorber  704 , wherein the working fluid acquires heat from the solar absorber  704 . 
     While the solar absorber  704  depicted in  FIGS. 7A and 7B  has an angular shape, the solar absorbers are not so limited and other suitable shapes may be constructed and used with the solar receivers disclosed herein. For example, a solar absorber may have a planar shape, an elliptical shape, a parabolic shape, a disc shape, a polyhedron shape or other suitable shape. 
     The transparent object  703  of the solar receiver depicted in  FIG. 7A  is positioned at the front of the low pressure fluid chamber  700  adjacent to the opening  708  for receiving solar radiation. The outer rim of the transparent object  703  is fitted within a recess  705  of the low pressure fluid chamber  700 . The transparent object  703  and the low pressure chamber  700  are typically constructed of materials having different coefficients of thermal expansion. For example, the transparent object  703  is typically constructed of a glass material (e.g., silica, quartz, etc.), whereas the low pressure chamber  700  is typically constructed of a metal (e.g., stainless steel, aluminum). When the transparent object  703  and low pressure chamber  700  are subjected to thermal fluctuations, such as those which occur between activity and lack of activity of the solar receiver, there is often differential thermal expansion and contraction of the two components. Thus, the connection between the transparent object  703  and the low pressure fluid chamber  700  must typically be designed and constructed to accommodate differential movement between the two components. 
     In some embodiments, a flexible seal is provided between an interface on the low pressure fluid chamber  700  and the transparent object  703 . The interface may be within a recess  705  within which the transparent object  703  is positioned and/or secured. The recess  705  may have an internal diameter that exceeds the outside diameter of the transparent object  703 , thereby allowing expansion of the transparent object  703  within the recess  705 . The seal is often subjected to relatively high temperatures (e.g., temperatures in excess of 500° C.), and thus, is typically produced from a material that can withstand relatively high temperatures. The seal may be produced, in some embodiments, from a metal, a carbon-based material, or a silicone-based material. In some embodiments, the seal is produced from a room-temperature vulcanizing (RTV) silicone elastomer. In some embodiments, the seal is a metallic gasket. Other appropriate seal materials will be apparent to the skilled artisan. 
     The low pressure fluid chamber  700 , in certain embodiments, is designed and constructed to have a maximum allowable working pressure of up to and including 2 atmospheres. The low pressure fluid chamber  700 , in certain embodiments, is designed and constructed to have a maximum allowable working pressure of 1 atmosphere, 1.1 atmospheres, 1.2 atmospheres, 1.3 atmospheres, 1.4 atmospheres, 1.5 atmospheres, or 2 atmospheres. As used herein, the term “maximum allowable working pressure” refers to the maximum pressure a pressure vessel can maintain, e.g., the maximum pressure that the weakest component of an assembled solar receiver can maintain. Often the maximum allowable working pressure is determined by conducting a hydrostatic pressure test. Methods for conducting a hydrostatic pressure test are well known in the art and will be apparent to the skilled artisan. In one embodiment, the maximum allowable working pressure of a solar receiver is determined by essentially completely assembling the solar receiver, capping off the fluid inlet(s) and the fluid outlet(s), and pressurizing the low pressure chamber of the solar receiver with an inert gas, e.g., air. The low pressure chamber is pressurized, in this embodiment, with the inert gas at a relatively slow rate, e.g., at a rate in a range of 1 psi/second to 5 psi/second, until the low pressure chamber can no longer maintain pressure. The highest pressure maintained during the test is the maximum allowable working pressure of the solar receiver. In certain embodiments, the weakest component of the solar receiver, such as the solar receiver depicted in  FIG. 7A , is the seal between the transparent object and the low pressure chamber. 
     As illustrated by  FIGS. 7A and 7B , the transparent object  703  may have a variety of shapes. For example, the transparent object  703  may have a planar shape (as depicted in  FIG. 7A ) such as a planar disc or a planar object having a polygonal cross-section such as a rectangular or square cross-section. The transparent object may have a relatively slight curvature inward (as depicted in  FIG. 7B ) toward the solar absorber. 
     The transparent object may have a semi-circular shape, a parabolic shape, an elliptical shape, etc. In some embodiments, a curvature inward toward the solar absorber serves to minimize tensile stress due to thermal expansion in the transparent object. Thus, in certain embodiments, the transparent object has a certain radius of curvature. The transparent object may, for example, have a radius of curvature of 1 foot to 50 feet, 1 foot to 10 feet, 1 foot to 5 feet or 1 foot to 2 feet. The transparent object may have a radius of curvature of up to 1 foot, 2 feet, 3 feet, 4 feet, 5 feet, 10 feet, 25 feet, 50 feet, or more. 
     The solar receivers depicted in  FIGS. 7A-7C  operate at low pressure (e.g., up to and including 2 atmospheres). Because the transparent object  703  is subjected to relatively small hydrostatic stresses under normal operation, it may be constructed to have a relatively large diameter and relatively small thickness. In some embodiments, the transparent object has a diameter in a range 0.5 meter to 5 meters, 2 meters to 4 meters or 0.5 meter to 2 meters. In some embodiments, the transparent object has a diameter of 0.5 meter, 1 meter, 1.2 meters, 1.4 meters, 1.6 meters, 1.8 meters, 2 meters, 3 meters, 4 meters, 5 meters or more. In some embodiments, the diameter of a transparent object (e.g., a transparent object that has a certain radius of curvature) is the diameter of the rim of the transparent object (e.g., the edge of the transparent object  703  that fits with a recess  705  of the low pressure chamber  700 ). 
     The thickness of the transparent object  703  may influence the extent to which the transparent object  703  absorbs solar radiation, with relatively thick transparent objects typically absorbing more solar radiation than relatively thin transparent objects. Consequently, the thickness of the transparent object influences the extent to which the transparent object is subjected to thermal stress during operation of the solar receiver. It is therefore often desirable for the transparent object to have a relatively small thickness, in order to minimize thermal stress. In some embodiments, the thickness of the transparent object is in a range of 0.25 inch to 4 inches, 0.5 inch to 2 inches, or 0.5 inch to 1 inch. In some embodiments, the thickness of the transparent object is 0.25 inch, 0.5 inch, 1 inch, 1.5 inch, 2 inches, 3 inches, 4 inches or more. However, the invention is not limited to transparent objects having these thicknesses. Other thicknesses may be suitable in some cases. 
     The solar absorber is typically constructed of a material that can withstand relatively high temperatures, that can absorb incident radiation and that can readily transfer thermal energy to a working fluid that is in contact with the absorber. For example, solar absorbers may be constructed of a metals, stainless steels, ceramics, heat-resistant cast alloys, high temperature metallic materials, refractory materials, thoria-dispersed alloys, graphite, or carbon-fiber-reinforced carbon-based materials. Appropriate ceramics for solar absorbers include, for example, glass ceramics, silicon carbide, silicon nitride, alumina, and silicon oxide. The solar absorber may have any of a variety of forms. Typically, the solar absorber is designed and constructed to have a relatively high surface area for contact with a working fluid. The solar absorber typically comprises a plurality of channels or passages through which a working fluid may pass. In passing through the fluid channels or passages of the solar absorber, the working fluid acquires thermal energy through contact with the absorber. The absorber may have a wire mesh, honeycomb or foam configuration, for example. Often, the solar absorber comprises a black surface coating, covering at least a portion of the absorber surface, to facilitate absorption of incident solar radiation. In some embodiments of the foregoing solar receivers, the solar absorber is a combination of both a wire mesh absorber and a ceramic absorber. In such embodiments, the wire mesh absorber can be the air inlet absorber and the ceramic absorber can be the air exit absorber. In other embodiments, the ceramic absorber can be the air inlet absorber and the wire mesh absorber can be the air exit absorber. 
     In some embodiments of the solar receivers, the multiple segments of the solar absorber are held in position by a ceramic support structure system. In some embodiments of the solar receivers, flow-regulation structures are placed on the air inlet and/or outlet/exit side of the absorber. In some embodiments, the flow-regulation structure is an orifice plate having one or more orifices through which air may pass. In some embodiments of the foregoing solar receivers, the flow-regulation structures are made of ceramic, metal or other suitable material in which small holes are present for the passage of air. In some embodiments, the flow-regulation structure is configured to produce a relatively laminar air flow. In some embodiments, the flow-regulation structure is configured to direct air flow in a relatively uniform manner across an absorber. 
     The low pressure solar receivers depicted in  FIGS. 7A-7B  are fitted with a secondary concentrator  706 . The secondary concentrator  706  serves to collect concentrated solar radiation from a primary concentrator, e.g., a heliostat field, or other source, and direct that solar radiation into the opening  708  of the solar receiver. The secondary concentrator  706 , in some embodiments, improves the solar collection efficiency of the solar receiver. The second concentrator  706  is often constructed with a plurality of reflective panels  707 , each reflective panel typically having a reflective surface and a predetermined shape. The plurality of reflective panels  707  are typically arranged in a configuration that facilitates reflection of incident solar radiation toward the receiver opening  708 . In some embodiments, the plurality of reflective panels are arranged such that the secondary concentrator has an overall parabolic shape, although other shapes may be suitable. For example, the secondary concentrator may have an elliptical shape, a semi-circular shape, a hyperbolic shape, etc. 
     A cross-section of the secondary concentrator that is parallel with the opening of the receiver may also have a variety of shapes. For example, the cross-section of the secondary concentrator that is parallel with the opening of the receiver may have a circular shape, an elliptical shape, a polygonal shape, a rectangular shape, etc. 
     The size and shape of the secondary concentrator  706  (e.g., the diameter of the outer most portion of the secondary concentrator, the depth of the secondary concentrator, etc.) may vary depending on a variety of factors, including, for example, the desired collection efficiency, the size and arrangement of the primary concentrator(s), the size of the opening of the receiver, the thermal properties of the solar absorber, etc. In some embodiments, the ratio of the depth of the secondary concentrator to the diameter of the opening of the receiver is 1, 1.25, 1.5, 2, 2.5, 3, 4, 5, or more. In some embodiments, the ratio of the depth of the secondary concentrator to the diameter of the opening of the receiver is in a range of 1 to 1.5, 1 to 2.5, 1 to 3, 1 to 4, or 1 to 5. In some embodiments, the ratio of the outer most diameter of the secondary concentrator to the diameter of the opening of the receiver is 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5.5, 6 or more. In some embodiments, the ratio of the depth of the secondary concentrator to the diameter of the opening of the receiver is in a range of 1.5 to 2, 1.5 to 3, 1.5 to 4, 1.5 to 5, or 1.5 to 6. 
       FIG. 7C  depicts an exemplary solar receiver having a fluid inlet  701  and fluid outlet  702  that enter and exit, respectively, the solar receiver on different sides of a low pressure fluid chamber. In this embodiment, the solar receiver is connected to a secondary concentrator  706  that has a rectangularly shaped opening. 
       FIG. 7D  depicts an exemplary solar receiver having a low pressure fluid chamber  716  comprising one or more fluid inlets  715   1-2  and a fluid outlet  714 , a solar absorber  712 , and a regenerator structure  717  housing a rotary regenerator matrix  713 . In this embodiment, thermal energy from concentrated solar radiation is directed and concentrated, at least in part, by a secondary concentrator  718  into the low pressure fluid chamber  716  through a transparent object  711  and impinges a solar absorber  712  thereby heating the solar absorber  712 . The solar absorber  712  transfers thermal energy to a relatively low temperature working fluid passing within the low pressure fluid chamber  716 , thereby creating a relatively high temperature working fluid. The relatively high temperature fluid leaving the low pressure fluid chamber passes through a rotary regenerator matrix  713  and transfers thermal energy to the rotary regenerator matrix  713 . 
     The rotary regenerator matrix  713 , in  FIG. 7D , rotates between two fluidically isolated conduits. The first conduit being a flow path for fluid exiting the low pressure fluid chamber  716  of the solar receiver, and the second conduit being a flow path of a second fluid system. For example, the rotary regenerator matrix  713  may transfer thermal energy from the relatively high temperature fluid leaving the low pressure fluid chamber  716  to a second fluid passing through a conduit adjacent to the receiver that is in fluid communication with the rotary regenerator matrix. The second fluid may be, for example, a fluid, e.g., ambient air, entering the compressor of a gas turbine, or a fluid used to heat a secondary thermal storage material. 
     Figure  FIG. 7E  depicts a cross-sectional view of an exemplary solar receiver  719  that transfers thermal energy from concentrated solar radiation to a low pressure working fluid. The solar receiver  719  includes an outer housing  720  defining a first fluid conduit  721  and a fluid inlet  722 . The outer housing  720  further defines an aperture  723  for receiving solar radiation at the front end of the solar receiver  719 . A transparent object  724  is connected to the outer housing  720  through a flange assembly  725  at the aperture  723 . The solar receiver  719  also includes an inner housing  726  that defines a second fluid conduit  727  and a fluid outlet  728 . As described further below, the second fluid conduit  727  is co-axial with the first fluid conduit  721 . In addition, a solar absorber  732  is connected to the inner housing  726  at a position in proximity to the aperture  723 . While inlet conduits and outlet conduits may be co-axial, it should be appreciated that, in some embodiments, the inlet conduit and outlet conduit are not co-axial. 
     In an exemplary implementation, concentrated solar radiation is directed to the aperture  723 , passes through the transparent object  724  and, after passing through the transparent object  724 , and impinges the solar absorber  732 , thereby heating the solar absorber  732 . The solar receiver  719  defines a fluid path beginning from the fluid inlet  722 , traversing forward through the first fluid conduit  721  toward the aperture  723 . The fluid traverses across the inner side of the transparent object  724 , passes through a plurality of passages in the solar absorber  732 , passes through the second fluid conduit  727 , and exits the solar receiver  719  through the fluid outlet  728 . 
     In certain embodiments, a fluid inlet  722  is at a position in relative proximity to the transparent object  724  such that fluid enters the receiver in relative proximity to the transparent object  724 . When fluid enters the receiver in this manner it can more readily pass through the solar absorber in some configurations, such as, for example, when the receiver has a relatively large diameter. 
     The fluid inlet  722  may be fluidically connected with a gas turbine exhaust outlet or other working fluid supply conduit, such that a relatively low temperature (e.g., approximately 1100° F.) fluid enters the solar receiver  719 . The solar absorber  732  transfers thermal energy to the relatively low temperature fluid as it travels through the plurality of passages in the solar absorber  732 , thereby heating the fluid to a relatively high temperature (e.g., approximately 1800° F.). The fluid outlet  728  may be fluidically connected with a gas turbine compressor inlet, a heat storage unit, or other downstream component that uses the relatively high temperature fluid. 
     The solar receiver  719  includes a flange assembly  725  for connecting the transparent object  724  at the aperture  723 . The flange assembly  725  includes an outer flange  729  that is connected to an inner flange  730 . The flange assembly  725  is generally composed of materials that are tolerant to operation at relatively high temperatures (e.g., capable of operating at temperatures in the range of 1700 to 2000° F.). Use of a high temperature tolerant materials ensures that the flange assembly  725  will not fail (e.g., melt or degrade) if concentrated solar radiation (e.g., radiation directed to the receiver from a heliostat field) is improperly directed such that excess solar radiation impinges on the flange assembly  725 . Moreover, in certain embodiments the flange assembly  725  is composed of materials having thermal properties similar to that of the transparent object  724  to minimize the risk of damage to the flange assembly  725  or transparent object  724  due to differential thermal expansion of the components. For example, if the transparent object  724  is composed of quartz, then it may be advantageous to select materials for the flange assembly  725  that have a similar coefficient of thermal expansion as quartz. Exemplary materials for the flange assembly  725  include, for example, ceramics and other high temperature tolerant materials disclosed herein or otherwise known in the art. The outer flange  729  and inner flange  730  are connected, in the illustrated embodiment, by a plurality of bolts positioned around the flange assembly  725 . Flange assembly  725  is also bolted to the outer housing  720 . In some configurations, bolts connecting the outer flange  729  and inner flange  730  serve to join the entire flange assembly  725  to the outer housing  720 . In other configurations, separate bolts join the flange assembly  725  to the outer housing  720 . The holes for bolts joining the flange assembly  725  to the outer housing  720  may be shaped as radial slots to permit differential thermal expansion of the flange assembly  725  and outer housing  720 , and to allow for bolt movement within the slots. 
     The transparent object  724  is connected to the aperture  723  by way of a flange assembly  725 . Flexible seals  731   1-3  are positioned in seal cavities within the flange assembly  725  and provide contact between the flange assembly  725  and transparent object  724  at front, rear and circumferential surfaces of the transparent object  724 . The flexible seals  731   1-3  allow for differential thermal expansion (thermal growth) between the flange assembly  725  and the transparent object  724  in the axial direction (forward and rearward expansion) and radial direction (circumferential expansion). The flexible seals  731   1-3  prevent direct contact (hard points) between the transparent object  724  and the flange assembly  725  by providing sealing surfaces with relatively low contact stress. 
     The flexible seals  731   1-3  provide support for the transparent object  724  during operation and shipping, and spread the sealing contact load to enhance component life. The flexible seals  731   1-3  may be made from ceramic fiber rope or an equivalent sealing material suitable for high temperature operation and for conforming to the dimensions of the seal cavity in the flange assembly  725 . 
     The solar receiver  719  includes a transparent object  724  (which may be referred to as a window) that is composed quartz silica glass. The transparent object  724  may have a curved shaped to contain and distribute internal pressure, and to tolerate thermal stresses from differential temperature exposure. The curved shape of the transparent object  724  also limits the formation of destructively high tensile stresses. The transparent object  724  may be designed to accommodate implementations that give rise to relatively high temperatures at its center portion and relatively cooler temperatures at portions in proximity to the flange. Thus, the transparent object  724  may function in some implementations as a thermal hinge to accommodate thermal growth without developing destructively high tensile stresses. The curved (or bowl shape) of the transparent object  724  in the illustrated embodiment also facilitates, and to an extent directs, flow of a relatively low temperature fluid toward and through the solar absorber  732 . The relatively low temperature fluid may also function to cool the transparent object  724  as it passes over the internal surfaces of the transparent object  724 . 
     In some embodiments the transparent object is constructed of one piece, e.g., a single solid quartz silica glass window. However, in other embodiments, the transparent object is constructed of several segments that are fitted together, joined together or butted together. In some embodiments, a transparent object having a diameter in a range of 2 meters to 4 meters, or more, is constructed of multiple segments (e.g., 2, 3, 4 or more segments). 
     The solar receiver  719  is configured and arranged with coaxial (co-annular) first and second fluid conduits, with the first fluid conduit  721  providing a passage for a relatively low temperature fluid and the second fluid conduit  727  providing a passage for a relatively high temperature fluid that has acquired thermal energy from the solar absorber  732 . The solar receiver  719  accommodates a relatively low temperature fluid (e.g., approximately 1100° F.) passing through the first fluid conduit  721  and relatively high temperature fluid (e.g., approximately 1800° F.) passing through the second fluid conduit  727  with minimal insulation, and minimal thermal losses, in certain embodiments. For example, thermal losses from the second fluid conduit  727  are transferred into the first fluid conduit  721  and thus not lost in the overall thermal cycle. 
     Moreover, the low pressure operation (e.g., operation at up to 1.1 atm) of the solar receiver  719  can allow for the housings that define the first and second fluid conduits  721 ,  727  to be constructed of light weight and low cost materials, and enable factory fabrication and easy on-site installation. In certain configuration, the outer housing  720  is constructed of materials suitable for operation at temperatures in a range of 1000° F. to 1200° F. (e.g., approximately 1100° F.). For example, the outer housing  720  may be constructed of stainless steel or other similar material. The outer housing  720  may have an external insulation to conserve thermal energy and provide a safe work environment. The inner housing  726  in a typical configuration is constructed of materials suitable for operation at temperatures in a range of 1700° F. to 2000° F. (e.g., approximately 1800° F.). For example, the inner housing  726  may be constructed of nickel-based super alloy or other similar material. The inner housing  726  may have insulation to minimize the extent to which thermal energy is transferred back to the low temperature fluid in the first fluid conduit  721 . Because of the low pressure operation conditions of the receiver, in some embodiments, the outer housing  720  and/or inner housing  726  has a thickness in a range of 0.001 to 0.1 inch (e.g., approximately 0.05 inch). 
     A bellows  733  is connected between the outer housing  720  and inner housing  726  and allows for differential thermal expansion between the two housings. The bellows  733  is typically constructed of a high temperature tolerant material such as for example a nickel-based super alloy or other suitable material. The bellows  733  may be connected to the outer and inner housings  720 ,  726  by brazing or welding or other suitable method. It should be appreciated, that the solar receiver  719  may be configured with any suitable component to control the axial and radial centering of the two housings and to allow for differential thermal expansion between the two housings. Vertical support and slip joints may be included between the housings, for example. 
     The solar absorber  732  may be constructed of a porous material that defines a plurality of passages traversing through the absorber. The solar absorber  732  may for example have a honey comb or foam structure. The solar absorber  732  in certain embodiments is constructed of a silicon carbide material. In other embodiments, the solar absorber  732  may be constructed of other suitable materials, including any of the materials disclosed herein for solar absorbers. The solar absorber  732  is positioned in a recess  735  defined by the inner housing  726 . The inner housing  726  is fixed to the outer housing  720  at the position of the recess  735  by bolts  734   1-2 , which comprise set pins at their ends that enter into holes in the solar absorber  732  to position the solar absorber  732 . 
     The overall shape of the solar absorber  732  may be curved. For example, the solar absorber  732  may have radius of curvature that is similar to that of the transparent object  724 . The overall shape of the solar absorber  732  may alternatively be substantially planar. The solar absorber  732  may be single solid object or may be arranged as a set of segmented components. The solar absorber  732  may be arranged, for example, as a set of pie-shaped segments in a bowl configuration that fits within the solar receiver  719 . The segmented design allows for differential thermal expansion of the different segments and thus accommodates uneven temperature distributions across the solar absorber. In some configurations temperature distributions across the solar absorber  732  may be controlled, at least to an extent, by including an orifice plate (e.g., a ceramic orifice plate) at the front end of the solar absorber. The orifice plate may include a series of orifices configured and arranged to facilitate a substantially even distribution of fluid passing into the solar absorber  732  across the entire absorber fluid inlet  722  face. The orifice plate may be retained in the receiver around its outer rim to control axial and radial movement. In some embodiments, orifice plates are designed to achieve uniform flow. In other embodiments, orifice plates are designed to have different numbers and/or diameters of holes with respect to a specific location in the plate. In some embodiments, the orifice plates are designed to vary flow through one or more regions of an absorber. In some embodiments, orifice plates that vary flow through one or more regions of an absorber are configured to manage heat transfer from the absorber to air flowing the absorber and thereby to manage temperatures on the exit side of the absorber. In some embodiments, orifice plates that vary flow are advantageous because solar flux may vary from one part of the absorber to another and the orifice plate can compensate for differences in absorber temperature by altering the flow over the absorber to create a more uniform heat transfer. 
     It should be appreciated that the solar receiver  719  may operate at pressures of up to 1.1 atm, up to 1.2 atm, up to 1.3 atm, up to 1.4 atm, up to 1.5 atm, or up to 2 atm. In particular embodiments, the receiver is configured and arranged for operating at pressures in the range of above 1 atm to 1.5 atm. In other embodiments, the receiver is configured and arranged for operating at pressures in the range of above 1 atm to 1.2 atm. Moreover, in some embodiments, the solar absorber  732  has a radius of curvature (A) in a range of 50 to 250 inches. In some embodiments, the solar absorber  732  has a radius of curvature (A) in a range of 150 to 200 inches. In some embodiments, the solar absorber  732  has a radius of curvature (A) of 170 to 190 inches. Alternatively, the solar absorber may be substantially planar. In some embodiments, the transparent object  724  has a radius of curvature (B) in a range of 50 to 250 inches. In some embodiments, the transparent object  724  has a radius of curvature (B) in a range of 150 to 200 inches. In some embodiments, the transparent object  724  has a radius of curvature (B) of 170 to 190 inches. In some embodiments, at least a portion of the inner housing  726  has an internal diameter (C) in a range of 10 to 50 inches. In some embodiments, at least a portion of the inner housing  726  has an internal diameter (C) in a range of 20 to 40 inches. In some embodiments, at least a portion of the inner housing  726  has an internal diameter (C) in a range of 30 to 35 inches. In some embodiments, at least a portion of the outer housing  720  has an internal diameter (D) in a range of 25 to 65 inches. In some embodiments, at least a portion of the outer housing  720  has an internal diameter (D) in a range of 35 to 55 inches. In some embodiments, at least a portion of the outer housing  720  has an internal diameter (D) in a range of 40 to 50 inches. In some embodiments, the thickness (E) of the transparent object is in a range of about 0.5 inch to about 3 inches. In some embodiments, the thickness (E) of the transparent object is in a range of about 1 inch to about 2.5 inches. In some embodiments, the thickness (E) of the transparent object is in a range of about 1.5 inches to about 2 inches. In some embodiments, the flange assembly defines an opening having a diameter (F) in a range of 46 inches to 86 inches. In some embodiments, the flange assembly defines an opening having a diameter (F) in a range of 56 inches to 76 inches. In some embodiments, the flange assembly defines an opening having a diameter (F) in a range of 60 inches to 70 inches. In some embodiments, the distance (G) between the inner face of the transparent object  724  and the outer face of the solar absorber  732  is in a range of 2 to 12 inches. In some embodiments, the distance (G) between the inner face of the transparent object  724  and the outer face of the solar absorber  732  is in a range of 5 to 8 inches. However, other sizes may be suitable in some configurations. 
       FIGS. 7F and 7G  each depict an example of a low pressure solar receiver having a low pressure fluid chamber comprising a fluid inlet  741   1-2 , a fluid outlet  742 , and an opening for receiving concentrated solar radiation; and a solar absorber  744  housed within the low pressure fluid chamber, similar to that depicted in  FIG. 7E . The low pressure solar receivers in  FIGS. 7F-G  each include a first transparent object  736  that defines at least a portion of a wall of the low pressure fluid chamber; and a second transparent object  737  in juxtaposition with the first transparent object  736 . The first transparent object  736  and the second transparent object  737  are configured such that a first fluid flow path  740  is defined between the first transparent object  736  and the second transparent object  737 . In each example, concentrated solar radiation received through the opening passes through the transparent objects into the low pressure fluid chamber and impinges upon the solar absorber, in a similar manner to that depicted in  FIG. 7E . 
     In the configurations depicted in  FIGS. 7F and 7G , external cooling air enters the first fluid flow path  740  at external air inlets  738   1-2 , and exits the first fluid flow path  740  through outlet  739 . External air passing through the first fluid flow path  740  between the first transparent object  736  and second transparent object  737  cools the first transparent object  736  and second transparent object  737 . In some configurations, the low pressure fluid chamber defines a second fluid flow path  743  extending from the fluid inlet(s)  741   1-2  to the fluid outlet(s)  742 , in which, between the fluid inlet(s)  741   1-2  and the fluid outlet(s)  742 , the second fluid flow path  744  extends across at least a portion of a transparent object  736  and through one or more passages within the solar absorber  744 . 
     In the configuration depicted in  FIG. 7F , the first transparent object  736  and the second transparent object  737  are configured such that the first fluid flow path  740  is fluidically isolated from the second fluid flow path  743 . Also, in the configuration depicted in  FIG. 7F , the second transparent object  737  comprises one or more openings  739  that fluidically connect the first fluid flow path  740  with the environment surrounding the solar receiver. In the configuration depicted in  FIG. 7G , the first transparent object  736  comprises one or more openings  739  that fluidically connects the first fluid flow path  740  and the second fluid flow path  743 . 
     In some embodiments, a solar receiver is provided that comprises a low pressure fluid chamber comprising a fluid inlet, a fluid outlet, and an opening for receiving concentrated solar radiation; a solar absorber housed within the low pressure fluid chamber; and a plurality of transparent objects that define a segmented wall of the low pressure fluid chamber.  FIGS. 7H and 7I  illustrate examples of pluralities of transparent objects arranged to produce segmented walls for a receiver. The segmented wall  745  in  FIG. 7H  is composed of four longitudinally arranged transparent objects S 1 -S 4 . The segmented wall  746  in  FIG. 7I  is composed of eight radially arranged transparent objects S 1 -S 8 . These illustrations are not intended to be limiting. Other appropriate arrangements and pluralities of transparent objects may be used to produce segment walls that permit the passage of solar radiation into the low pressure fluid chamber. In some embodiments, the plurality of transparent objects are half tubes (e.g., tubes that have been cut length-wise) and are arranged side by side at the opening of the receiver. In some embodiments, the tubes are up to 9 inches or more in diameter. In some embodiments, the convex side of a tube faces the absorber, and the concave side faces heliostats that direct concentrated solar radiation toward the opening of the receiver. In other embodiments, the concave side of a tube faces the absorber, and the convex side faces the heliostats. In some embodiments, the tubes are made of quartz silica. 
     In some embodiments, the solar receiver is configured such that concentrated solar radiation received through the opening passes through the plurality of transparent objects into the low pressure fluid chamber and impinges upon the solar absorber. In some embodiments, the low pressure fluid chamber defines a fluid flow path from the fluid inlet to the fluid outlet, in which, between the fluid inlet and the fluid outlet, the fluid flow path extends across at least a portion of the plurality transparent objects and through one or more passages within the solar absorber. In some embodiments, the plurality of transparent objects are configured such that when the pressure within the low pressure fluid chamber is lower than the environmental pressure surrounding the solar receiver, a fluid (e.g., ambient air) surrounding the solar receiver is drawn into the low pressure fluid chamber through gaps between transparent objects of the plurality. In some embodiments, the low pressure fluid chamber is configured such that the fluid (e.g., ambient air) that is drawn into the low pressure fluid chamber through gaps between transparent objects of the plurality enters the fluid flow path within the low pressure fluid chamber and passes through the solar absorber. In some embodiments, as the fluid (e.g., ambient air) is drawn into the low pressure fluid chamber through gaps between transparent objects of the plurality it cools the transparent objects.  FIGS. 8A-8C  illustrate a secondary concentrator  800  having an integrated fluid cooling system. The secondary concentrator  800  depicted in  FIG. 8A  includes a plurality of connected reflective panels  801 . Each of the plurality of reflective panels  801  has a planar shape having a polygonal cross-section. Each reflective panel has an inner reflective surface that is positioned to face the inner side of the secondary concentrator  800  and an outer surface. The reflective panels  801  are arranged such that the secondary concentrator  800  deflects concentrated solar radiation to the opening of the receiver to which the secondary concentrator  800  is connected. In some embodiments, the reflective panel, e.g., as depicted in  FIG. 8A , has a thickness in a range of 0.1 inch to 1 inch or 0.1 inch to 0.5 inch. 
     In the secondary concentrator  800  depicted in  FIG. 8A , the reflective panels are arranged to form three conical rings. The arrangement of conical rings is such that the conical ring having the smallest diameter is positioned to the rear of the secondary concentrator  800  and the conical ring having the largest diameter is positioned to the front of the secondary concentrator  800 . In  FIG. 8A  the secondary concentrator  800  includes two relatively large diameter cooling pipes  802 ,  803  that function in part to deliver cooling fluid to and from a cooling passage within each reflective panel and also to provide a support for arranging and immobilizing the reflective panels  801  into the predetermined shape that facilitates concentration of incoming solar radiation and reflection of the incoming concentrated solar radiation to the opening of a low pressure receiver. 
       FIG. 8B  depicts an alternative view of the secondary concentrator  800  showing the supply conduit  802 , an outlet pipe  803 , a smaller diameter pipe  804  in fluid communication with the supply conduit  802  and a cooling passage  806  of a reflective panel. The inset at  805  depicts an inlet to a cooling passage  806  of a reflective panel. 
       FIG. 8C  provides an expanded view of the inset  805  in  FIG. 8B . As shown, the supply conduit  802  is in fluid communication with cooling passage  806  of the reflective panel  801 . A series of open slots  810  define passages through which cooling fluid flows from the supply conduit  802  to the cooling passage  806  within the reflective panel  801 . The casing  808  of the reflective panel  806  and the inner reflective surface  807  are also shown. In some embodiments, the casing  808  is a metal sheet having a thickness in a range of 0.01 inch to 0.5 inch or 0.1 inch to 0.5 inch. 
     Any appropriate cooling fluid may be used to cool a reflective panel having a cooling system such as is depicted in  FIGS. 8A-8C . In some embodiments, the cooling fluid is a mixture of water and a refrigerant, e.g., ethylene glycol. In some embodiments, the cooling fluid is a 50:50 mixture of water and a refrigerant, e.g., ethylene glycol. 
     Reflective panels of a secondary concentrator may comprise any of a variety of materials. Typically metals, polymers, glass, or combinations thereof are used. Reflective panels may comprise a metal, such as aluminum, silver, or a combination thereof. Reflective panels may comprise a non-reflective material having a reflective coating, e.g., a reflective silver or reflective aluminum coating. Reflective panels may comprise a glass substrate, a reflective layer for reflecting solar energy, and optionally an interference layer (e.g., a layer between the glass and reflective layer comprised of, for example, titanium dioxide). Typically, the reflective panel has at least one surface for reflecting solar radiation. 
       FIGS. 9A-9C  depict exemplary high pressure receivers that may be used in conjunction with the power generation systems disclosed herein. In these embodiments, the high pressure receivers include an insulated casing  900  having, a working fluid inlet  901 , a working fluid outlet  902 , and an opening  904  connected to the rear portion of a secondary concentrator  906 . In some embodiments, such as is depicted in  FIGS. 9B-9C , the high pressure receiver includes a transparent object  905 , e.g., a window, adjacent to the opening  904  for receiving solar radiation. As in the low pressure receiver context, the secondary concentrator  906  serves to collect concentrated solar radiation from a primary concentrator, e.g., a heliostat field, or other source, and direct that solar radiation into the opening  904  of the solar receiver. 
     The high pressure fluid (e.g., fluid at a pressure of above 2 atmospheres to 50 atmospheres) passing through the receiver is retained within the high pressure solar absorber  903 . The high pressure absorber  903 , as exemplified in  FIGS. 9A-9C , typically comprises a network of passages (e.g., a tubular network) for containing a high pressure fluid and directing flow of the high-pressure fluid into and out of the high-pressure solar absorber  903 . 
     The high-pressure working fluid enters into the high pressure solar absorber  903 , passes through the network of passages and acquires thermal energy therein, in part, through contact with the passage walls. The high pressure solar absorber  903  often has a black surface coating to promote absorption of incident solar radiation. The surface coating may be applied using methods well known in the art including, for example, by chemical vapor deposition (e.g., a pack cementation process, a gas phase coating process, etc.). Moreover, the high pressure absorber is typically constructed from a material that can withstand extremely high temperatures, including, for example, temperatures in excess of 1000° C. 
     The high pressure solar absorber  903  can be designed and constructed in any of a variety of forms. As exemplified in  FIGS. 9A and 9B , the tubular network may be a network of tubular coils. As exemplified in  FIG. 9C , the tubular network may have a shell and tube-type form. Still other configurations, such as, for example, a plate type heat exchanger, are envisioned. In some embodiments, the high pressure solar absorber comprises a tubular network, wherein tubes of the network have a diameter in a range of 0.5 inch to 5 inches in diameter and, in some embodiments, a wall-thickness in a range of 0.1 inch to 1 inch. In some embodiments, the high pressure solar absorber comprises a tubular network, wherein tubes of the network have a diameter in a range of 1 inch to 3 inches in diameter and, in some embodiments, a wall-thickness in a range of 0.1 inch to 0.5 inch. 
     In some embodiments, the high-pressure solar absorber is constructed from a single crystal super alloy. Often the super alloy contains a nickel base, chromium at a level sufficient for oxidation resistance (e.g., at a level of about 10%), aluminum and/or titanium (e.g., at levels of about 2.5% to 5%) for the formation of the strengthening gamma prime phase and refractory metals such as tungsten, molybdenum, tantalum and columbium (e.g., at a level of about 2.5% to 5%) as solid solution strengtheners. Typically, nickel base super alloys also contain cobalt (e.g., at a level of about 10%) and carbon (e.g., at a level of about 0.1%) which acts as a grain boundary strengthener and forms carbides which strengthen the alloy. Boron and zirconium are also often added in small amounts as grain boundary strengtheners. 
     Exemplary single crystal super alloys that may be used in the construction of high-pressure solar absorber are disclosed in the following United State Patents, the contents of which, relating to single crystal super alloys, are incorporated herein by reference in their entireties: U.S. Pat. Nos. 4,371,404, 4,222,794; 4,514,360; 4,643,782; 4,765,850; 4,923,525; 5,047,091; 5,077,004; 5,100,484; 5,154,884; 5,366,695; 5,399,313; 5,540,790; and 6,074,602. 
     Components (e.g., tubes, plate walls, etc.) of the high-pressure solar absorber may be manufactured by any appropriate techniques, e.g., extruded or cast. Furthermore, components of the high-pressure solar absorber may be bonded together using any one of a variety of methods known in the art, including, for example, laser welding, electron beam welding, activated diffusion bonding, etc. 
     The transparent object  905  provides a barrier for reducing re-radiation losses, whereby the transparent object  905  allows transmission of concentrated solar radiation in the non-infrared range (e.g., in the visible range) into the solar receiver but does not allow transmission of radiation in the infrared range. Thus, thermal re-radiation, which emits in primarily the infrared range from the heated high pressure solar absorber, is reflected back into in the receiver by the transparent object  905 . 
     The casing  900  of the receiver is designed and constructed to insulate and provide structural support for the high pressure absorber  903  and to mount the secondary concentrator  906 . In the exemplary high-pressure receivers depicted in  FIGS. 9A-9C , the casing  900  operates essentially at ambient pressures. As a result, the transparent object, in these embodiments, is not subjected to substantial hydrostatic pressure induced stress. Thus, the transparent object can be designed and constructed to relatively large sizes (e.g., sizes in excess of 5 meters, e.g., 5 meters to 10 meters) without concern for hydrostatic pressure induced stress. In some embodiments, the transparent object is constructed as a combination of multiple transparent objects (e.g., a combination of overlapping transparent objects) to obtain a transparent window that functions to prevent re-radiation losses. In certain embodiments, the multiple transparent objects are flat. In certain embodiments, the multiple transparent objects are curved. In certain embodiments, the multiple transparent objects stand substantially vertically. In certain embodiments, the multiple transparent objects are arranged substantially horizontally. In certain embodiments, the multiple transparent objects are in the shape of cylinders that have been sliced in the longitudinal direction into segments. In certain embodiments, the multiple transparent segments represent one-half of the cylinder. In certain embodiments, the multiple transparent segments represent one-third of the cylinder, or tube. In certain embodiments, the multiple transparent segments represent one-quarter of the cylinder. In certain embodiments, the multiple transparent segments represent one-sixth of the cylinder. In certain embodiments, the multiple transparent segments are arranged with the concave face of the curved shape directed toward the opening. In certain embodiments, the multiple transparent segments are arranged with the convex face of the curved shape directed toward the opening. In certain embodiments, the multiple transparent segments are arranged with substantially no gaps, or no gaps, between them. In certain embodiments, the multiple transparent segments are arranged with gaps between them. 
     Unless otherwise indicated, all pressures described herein refer to absolute pressures. 
     The following patents and patent applications are incorporated herein by reference in their entirety for all purposes: U.S. Patent Publication No. 2002/0124991, published on Sep. 12, 2002, filed Feb. 1, 2002, entitled “Low Cost High Efficiency Automotive Turbines”; U.S. Pat. No. 6,681,557, issued on Jan. 27, 2004, filed Feb. 1, 2002, entitled “Low Cost High Efficiency Automotive Turbines”; U.S. Pat. No. 5,259,444, issued on Nov. 9, 1993, filed Nov. 5, 1990, entitled “Heat Exchanger Containing a Component Capable of Discontinuous Movement”; U.S. Pat. No. RE37134, issued on Apr. 17, 2001, filed Mar. 25, 1995, entitled “Heat Exchanger Containing a Component Capable of Discontinuous Movement”; U.S. Publication No. 2007/0089283, published on Apr. 26, 2007, filed Oct. 17, 2006, entitled “Intermittent Sealing Device and Method”; U.S. Publication No. 2008/0251234, published on Oct. 16, 2008, filed Apr. 16, 2007, entitled “Regenerator Wheel Apparatus”; U.S. Publication No. 2009/0000761, published on Jan. 1, 2009, filed Jun. 29, 2007. entitled “Regenerative Heat Exchanger with Energy-Storing Drive System”; U.S. Publication No. 2009/0000762, published on Jan. 1, 2009, filed Jun. 29, 2007, entitled “Brush-Seal and Matrix for Regenerative Heat Exchanger and Method of Adjusting Same”; U.S. Publication No. 2006/0054301, published on Mar. 16, 2006, filed Dec. 16, 2004, entitled “Variable Area Mass or Area and Mass Species Transfer Device and Method”; and International Patent Application Serial No. PCT/US2011/052051, filed Sep. 16, 2011, and entitled, “Concentrated Solar Power Generation Using Solar Receivers.” The following three U.S. provisional patent applications, filed on even date herewith, are also incorporated herein by reference in their entirety for all purposes: a U.S. Provisional patent application, filed on Mar. 21, 2012, under Attorney Docket Number WO644.70020US00, entitled Fluid Flow Control Devices for Solar Power Systems”; a U.S. Provisional patent application, filed on Mar. 21, 2012, under Attorney Docket Number WO644.70021US00, entitled “Low Pressure Solar Receivers with Double-Walled Windows and Uses Thereof”; and a U.S. Provisional patent application, filed on Mar. 21, 2012, under Attorney Docket Number WO644.70022US00, entitled “Low Pressure Solar Receivers with Segmented Windows and Uses Thereof.” 
     The following examples are intended to illustrate certain embodiments of the present invention, but do not exemplify the full scope of the invention. 
     Example 1 
     This example describes a concentrated solar power generation system in which a pressurized solar receiver is used.  FIG. 10A  includes a schematic diagram of such a system. In this example, compressed air from the compressor is fed to the solar receiver and heated while pressurized. The heated effluent from the solar receiver is then expanded through the gas turbine to produce power. The exhaust from the gas turbine is used to pre-heat the pressurized gas from the compressor before it is transported to the solar receiver. 
     Example 2 
     This example describes a concentrated solar power generation system in which a thermal storage system is incorporated.  FIG. 10B  includes a schematic diagram of the exemplary system. Air is used as the fluid in this system. The temperatures of process streams are indicated in the figure. In this example, ambient air at 59° F. is supplied to the compressor, where it is compressed and heated to a temperature of 1700° F. in a heat exchange system comprising one, two or more recuperator heat exchangers. The 1700° F. air is passed through a turbine to generate power, which produces an exhaust stream at 1100° F. The turbine exhaust and a portion of the heat exchange system exhaust are transported to the solar receiver, where they are heated to 1800° F. A portion of the solar receiver-heated air can be transported to the thermal storage system (similar to a cowper stove) for storage. The balance of the solar receiver-heated air is passed to the two-stage heat exchanger, where it is used to heat the compressed air upstream. It should be noted that other components, such as a startup combustor and/or a thermal recovery unit, could also be included in this example. 
     Example 3 
     This example describes a concentrated solar power generation system in which thermal storage is not included.  FIG. 10C  includes a schematic diagram of the exemplary system. Air is used as the fluid in this system, and the temperatures of process streams are indicated in the figure. Similar to the system described in Example 1, ambient air at 59° F. is supplied to the compressor, where it is compressed and heated to a temperature of 1700° F. in a heat exchange system comprising one, two or more recuperator heat exchangers. The 1700° F. air is passed through a turbine to generate power, which produces an exhaust stream at 1100° F. The turbine exhaust (and optionally, a portion of the heat exchange system exhaust) is transported to the solar receiver, where they are heated to 1800° F. The solar receiver-heated air is then passed to the two-stage heat exchanger, where it is used to heat upstream compressed air. 
     Example 4 
     This example describes a concentrated solar power generation system in which one, two or more thermal storage units and one, two or more rotary heat exchangers are used within the system.  FIG. 10D  includes a schematic diagram of the exemplary system. In this example, rather than using a two-stage heat exchanger to heat the compressed air from the compressor, a single rotary heat exchanger is used. A cross-sectional view of the rotary heat exchanger is shown in the upper-left corner of the figure. 
       FIG. 10D  also includes two thermal storage units. The units can be configured such that none, one, or both of the units is able to accept heated air from the solar receiver and/or provide heated air to the rotary regenerator for heating the compressed air stream to the turbine. 
     Example 5 
     This example describes a concentrated solar power generation system in which two or more compressors and two or more turbines are used to produce energy.  FIG. 10E  includes a schematic diagram of the exemplary system. As in the previous examples, stream temperatures are provided in the figure. 
     Example 6 
     This example describes a concentrated solar power generation system in which a high-pressure solar receiver and a low-pressure solar receiver are used in a single system.  FIG. 10F  includes a schematic diagram of the exemplary system. As in the previous examples, stream temperatures are provided in the figure. The layout of the components in this example is similar to the layout described in association with  FIG. 10E . 
     While several embodiments of the present invention have been described and illustrated herein, those of ordinary skill in the art will readily envision a variety of other means and/or structures for performing the functions and/or obtaining the results and/or one or more of the advantages described herein, and each of such variations and/or modifications is deemed to be within the scope of the present invention. More generally, those skilled in the art will readily appreciate that all parameters, dimensions, materials, and configurations described herein are meant to be exemplary and that the actual parameters, dimensions, materials, and/or configurations will depend upon the specific application or applications for which the teachings of the present invention is/are used. Those skilled in the art will recognize, or be able to ascertain using no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. It is, therefore, to be understood that the foregoing embodiments are presented by way of example only and that, within the scope of the appended claims and equivalents thereto, the invention may be practiced otherwise than as specifically described and claimed. The present invention is directed to each individual feature, system, article, material, and/or method described herein. In addition, any combination of two or more such features, systems, articles, materials, and/or methods, if such features, systems, articles, materials, and/or methods are not mutually inconsistent, is included within the scope of the present invention. 
     The indefinite articles “a” and “an,” as used herein in the specification and in the claims, unless clearly indicated to the contrary, should be understood to mean “at least one.” 
     The phrase “and/or,” as used herein in the specification and in the claims, should be understood to mean “either or both” of the elements so conjoined, i.e., elements that are conjunctively present in some cases and disjunctively present in other cases. Other elements may optionally be present other than the elements specifically identified by the “and/or” clause, whether related or unrelated to those elements specifically identified unless clearly indicated to the contrary. Thus, as a non-limiting example, a reference to “A and/or B,” when used in conjunction with open-ended language such as “comprising” can refer, in one embodiment, to A without B (optionally including elements other than B); in another embodiment, to B without A (optionally including elements other than A); in yet another embodiment, to both A and B (optionally including other elements); etc. 
     As used herein in the specification and in the claims, “or” should be understood to have the same meaning as “and/or” as defined above. For example, when separating items in a list, “or” or “and/or” shall be interpreted as being inclusive, i.e., the inclusion of at least one, but also including more than one, of a number or list of elements, and, optionally, additional unlisted items. Only terms clearly indicated to the contrary, such as “only one of” or “exactly one of,” or, when used in the claims, “consisting of,” will refer to the inclusion of exactly one element of a number or list of elements. In general, the term “or” as used herein shall only be interpreted as indicating exclusive alternatives (i.e., “one or the other but not both”) when preceded by terms of exclusivity, such as “either,” “one of,” “only one of,” or “exactly one of.” “Consisting essentially of,” when used in the claims, shall have its ordinary meaning as used in the field of patent law. 
     As used herein in the specification and in the claims, the phrase “at least one,” in reference to a list of one or more elements, should be understood to mean at least one element selected from any one or more of the elements in the list of elements, but not necessarily including at least one of each and every element specifically listed within the list of elements and not excluding any combinations of elements in the list of elements. This definition also allows that elements may optionally be present other than the elements specifically identified within the list of elements to which the phrase “at least one” refers, whether related or unrelated to those elements specifically identified. Thus, as a non-limiting example, “at least one of A and B” (or, equivalently, “at least one of A or B,” or, equivalently “at least one of A and/or B”) can refer, in one embodiment, to at least one, optionally including more than one, A, with no B present (and optionally including elements other than B); in another embodiment, to at least one, optionally including more than one, B, with no A present (and optionally including elements other than A); in yet another embodiment, to at least one, optionally including more than one, A, and at least one, optionally including more than one, B (and optionally including other elements); etc. 
     In the claims, as well as in the specification above, all transitional phrases such as “comprising,” “including,” “carrying,” “having,” “containing,” “involving,” “holding,” and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases “consisting of” and “consisting essentially of” shall be closed or semi-closed transitional phrases, respectively, as set forth in the United States Patent Office Manual of Patent Examining Procedures, Section 2111.03.