WO2013018013A1 - Spillage reducing improvements for solar receivers - Google Patents

Abstract

Spilled solar radiation resulting from such unpredictable variations in insolation or edge effects can be reflected back towards the receiver using a reflector placed adjacent to the receiver. The addition of reflectors can create a larger aperture, or intercept area, for the capture of solar radiation for a given emission aperture through which thermal radiative losses occur. The capture aperture of the receiver can be larger than a size of the focal spot of at least some of the heliostats such that the flux through the capture aperture can be controlled by aiming the heliostats. Radiation from heliostats aimed close to the edges of the receiver can strike the one or more reflectors. A control system can employ an algorithm responsive to the reflection properties of the reflector to achieve a uniformity goal such as flux or temperature uniformity.

Description

SPILLAGE REDUCING IMPROVEMENTS FOR SOLAR RECEIVERS

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims the benefit of U.S. Provisional Application No.

61/514,452, filed August 2, 2011, which is hereby incorporated by reference herein in its entirety.

FIELD

The present disclosure relates generally to the conversion of solar radiation to usable forms of energy such as heat and/or electricity, and, more particularly, to the reduction of solar receiver spillage.

SUMMARY

A solar power generation system can have a thermal-electric power generation component, in which incident solar radiation is concentrated on a thermal receiver to heat a heat transfer or working fluid for use in electricity generation. A field of heliostat-mounted mirrors can reflect and optionally concentrate incident solar radiation onto the thermal receiver. The distribution of reflected solar radiation around an intended aiming point on a target surface may be unpredictable or may be graded. As such, in attempting to achieve a target uniformity over the surface of the receiver, and at the same time directing the focus of some heliostats near the receiver edges, some spillage of concentrated light may occur. That is, some of the reflected radiation aimed near the edges of the boiler misses the receiver. This spilled solar radiation may be reflected back towards the receiver so as to minimize the amount of flux that would otherwise be lost. In order to achieve this, a reflector placed adjacent to the thermal receiver of the thermal electric power generation component may be positioned so as to redirect the spilled radiation to the thermal receiver. The addition of reflectors adjacent to the thermal receiver creates a larger aperture, or intercept area, for the capture of solar radiation for a given emission aperture through which thermal radiative losses occur.

Embodiments of the disclosed subject matter are systems in which the capture aperture of the receiver is larger than a size of the focal spot of at least some of the heliostats such that the flux through the capture aperture can be controlled by aiming the heliostats. For example, the heliostats may be controlled to establish temperature or flux uniformity over the receiving surface of the receiver. In that case, some of the heliostats may be aimed close to the edges of the receiver thereby striking one or more of the reflectors. A control system may employ an algorithm that is responsive to the reflection properties (such as reflectivity and angle, or angle distribution, of reflection) of the reflector to achieve a uniformity goal such as flux or temperature uniformity. For example, embodiments may employ a predictive algorithm that takes into account the reflection of the reflector as well as the aiming point of the adjacent part of the receiver to achieve a net uniformity target. In such embodiments, the target is an aperture area which maps to the receiver surface with a discontinuity in the mapping at the points where the reflectors lie adjacent the receiver surface. There may also be discontinuities at various points on the reflectors depending on their shapes.

In one or more embodiments, a method of improving the efficiency of a solar

concentrating system can include controlling a plurality of heliostats to track the apparent movement of the sun to reflect incident solar radiation on a receiver, and redirecting a portion of said reflected incident solar radiation which spills over an edge of the receiver via a reflector. The reflector can be adjacent to an edge of the receiver and can direct the spilled portion of the incident solar radiation to the receiver.

In one or more embodiments, a method of reducing the amount of lost flux can include providing a reflector adjacent an edge of a solar receiver such that a surface of the reflector reflects at least part of the spilled flux towards a face of the receiver.

In one or more embodiments, a method of converting solar energy to electricity can include controlling a plurality of heliostats to track the apparent movement of the sun to reflect incident solar radiation on a thermal receiver configured to transfer all the incident solar radiation received thereby as thermal energy to a heat transfer fluid. The method can further include redirecting a portion of the reflected incident solar radiation which spills over an edge of the receiver via a reflector onto the receiver. The method can also include maintaining temperature uniformity on the receiver at least in part by controlling a portion of said plurality of heliostats to direct incident solar radiation away from said receiver.

In one or more embodiments, a solar concentrating system can include a receiver, a plurality of heliostats, a controller, and at least one reflector. The receiver can have a top edge, a bottom edge and at least one side edge extending from the top edge to the bottom edge. Each heliostat can be configured to direct incident solar radiation at the receiver so as to heat a heat transfer or working fluid flowing through the receiver. The controller can be configured to compute respective aiming points and to control the heliostats such that they are aimed at the respective aiming points on an external surface of the receiver and to track the apparent movement of the sun so as to maintain the aim of the heliostats at the respective aiming points. The at least one reflector can project from the top edge, the bottom edge or the at least one side edge of the receiver. The at least one reflector can be configured to direct a spilled portion of the incident solar radiation to the receiver. The controller can be further configured to control the heliostats in response to a prediction of the total flux on the receiver, including components incident on the receiver directly and components incident on the receiver via the at least one reflector, and to select aiming points to prevent the temperature of the receiver from exceeding a predetermined level at any location thereof.

In one or more embodiments, a solar energy conversion system can include a receiver, a plurality of heliostats, a conveyance device, and at least one reflective device. Each heliostat can be configured to direct incident solar radiation at the receiver so as to heat a working fluid flowing through the receiver. The conveyance device can be configured to transport the heated working fluid from the receiver to an electric power generating plant. The heated working fluid can be used by an electric power generating plant in the generation of electricity. The at least one reflective device can be arranged adjacent to the receiver so as to redirect a spilled portion of the incident solar radiation back toward the receiver.

Objects and advantages of embodiments of the disclosed subject matter will become apparent from the following description when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments will hereinafter be described with reference to the accompanying drawings, which have not necessarily been drawn to scale. Where applicable, some features may not be illustrated to assist in the illustration and description of underlying features.

Throughout the figures, like reference numerals denote like elements.

FIG. 1 shows a solar power tower system, according to one or more embodiments of the disclosed subject matter.

FIG. 2 shows a solar power tower system with secondary reflector, according to one or more embodiments of the disclosed subject matter.

FIG. 3 shows a solar power tower system including multiple towers, according to one or more embodiments of the disclosed subject matter.

FIG. 4 shows a solar power tower system including multiple receivers in a single tower, according to one or more embodiments of the disclosed subject matter.

FIG. 5 is a schematic diagram of a heliostat control system, according to one or more embodiments of the disclosed subject matter.

FIGS. 6A-6B are isometric and elevation views, respectively, of a receiver illustrating the orientation of reflectors projecting from top and bottom edges of the receiver, according to one or more embodiments of the disclosed subject matter. FIG. 6C is a cross-sectional view illustrating the orientation of reflectors projecting from top and bottom edges of the receiver, according to one or more embodiments of the disclosed subject matter.

FIGS. 7A-7B are plan views of an octagonal shaped receiver with reflectors projecting from edges of the receiver, according to one or more embodiments of the disclosed subject matter.

FIG. 8 is a projection view of a receiver illustrating the orientation of reflectors projected from side edges of the receiver, according to one or more embodiments of the disclosed subject matter.

FIG. 9A illustrates a reflector with a back side coating installed at a top edge of the receiver, according to one or more embodiments of the disclosed subject matter.

FIG. 9B illustrates a reflector with electricity production modules on a back side thereof installed at a top edge of the receiver, according to one or more embodiments of the disclosed subject matter.

FIG. 10A-10B are diagrammatic elevation views of heliostats in solar field and a solar thermal tower illustrating implementation of a reflector during reflecting and dumping periods, respectively, according to one more embodiments of the disclosed subject matter.

FIGS. 11A-11B illustrates moveable reflectors installed at a top edge of a receiver, according to one or more embodiments of the disclosed subject matter.

FIG. 12 compares capture aperture and emission aperture for a receiver with a reflector installed and without a reflector installed, according to one or more embodiments of the disclosed subject matter.

FIG. 13A illustrates operation of an installed reflector to direct spillage onto a desired end region of the receiver, according to one or more embodiments of the disclosed subject matter.

FIG. 13B illustrates operation of an installed reflector direct reflections from the receiver itself back onto the receiver, according to one or more embodiments of the disclosed subject matter.

FIG. 14 illustrates curved surface reflectors installed at top and bottom edges of a receiver, according to one or more embodiments of the disclosed subject matter.

FIG. 15 illustrates a reflector having multiple elements on a common substrate, according to one or more embodiments of the disclosed subject matter. DETAILED DESCRIPTION

A central receiver system, such as one with a receiver supported on a tower, can include at least one solar receiver and a plurality of hehostats. Each heliostat tracks to reflect light to a target on a tower or an aiming point on such a target. The hehostats can be arrayed in any suitable manner. For example, heliostat spacing and positioning can be selected to provide optimal financial return over a life cycle according to predictive weather data and at least one optimization goal, such as total solar energy utilization, energy storage, electricity production, or revenue generation from sales of electricity.

Insolation can be used by a solar tower system to generate superheated steam and/or supercritical steam and/or to heat molten salt. In FIG. 1, a solar tower system can include a solar tower 50 that receives reflected focused sunlight 10 from a solar field 60 of hehostats (individual hehostats 70 are illustrated in the left-hand portion of FIG. 1 only). For example, the solar tower 50 can have a height of at least 25 meters, 50 meters, 75 meters, 100 meters, 125 meters, or higher. The hehostats 70 can be aimed at solar energy receiver system 20, for example, a solar energy receiving surface of one or more receivers of system 20. Hehostats 70 can adjust their orientation to track the sun as it moves across the sky, thereby continuing to reflect sunlight onto one or more aiming points associated with the receiver system 20. A solar energy receiver system 20, which can include one or more individual receivers, can be mounted in or on solar tower 50. The solar receivers can be constructed to heat water and/or steam and/or supercritical steam and/or any other type of solar fluid using insolation received from the hehostats.

Alternatively or additionally, the target or receiver 20 can include, but is not limited to, a photovoltaic assembly, a steam-generating assembly (or another assembly for heating a solid or fluid), a biological growth assembly for growing biological matter (e.g., for producing a biofuel), or any other target configured to convert focused insolation into useful energy and/or work.

The term "receiver," by itself, is used herein to refer to the portion of the device targeted by the receiver which captures and converts incident flux to heat and which are actively cooled by a heat transfer or working fluid as opposed to portions that are primarily reflective or simply used to re-radiate or convect heat such as thermal tiles or refractory shades. The receiver may be the aggregate of concentrated light-receiving portions of a boiler, heat exchanger,

superheater, or other device used for converting sunlight to heat in a fluid.

The solar energy receiver system 20 can be arranged at or near the top of tower 50, as shown in FIG. 1. In another embodiment, a secondary reflector 40 can be arranged at or near the top of a tower 50, as shown in FIG. 2. The secondary reflector 40 can thus receive the insolation from the field of hehostats 60 and redirect the insolation (e.g., through reflection) toward a solar energy receiver system 20. The solar energy receiver system 20 can be arranged within the field of heliostats 60, outside of the field of heliostats 60, at or near ground level, at or near the top of another tower 50, above or below reflector 40, or elsewhere.

More than one solar tower 50 can be provided, each with a respective solar energy receiving system thereon, for example, a solar power steam system. The different solar energy receiving systems can have different functionalities. For example, one of the solar energy receiving systems can heat water using the reflected solar radiation to generate steam while another of the solar energy receiving systems can serve to superheat steam using the reflected solar radiation. The multiple solar towers 50 can share a common heliostat field 60 or have respective separate heliostat fields. Some of the heliostats can be constructed and arranged so as to alternatively direct insolation at solar energy receiving systems in different towers. In addition, the heliostats can be configured to direct insolation away from any of the towers, for example, during a dumping condition. As shown in FIG. 3, two solar towers can be provided, each with a respective solar energy receiving system. A first tower 50A has a first solar energy receiving system 20A while a second tower 50B has a second solar energy receiving system 20B. The solar towers 50A, 50B are arranged so as to receive reflected solar radiation from a common field of heliostats 60. At any given time, a heliostat within the field of heliostats 60 can be directed to a solar receiver of any one of the solar towers 50A, 50B. Although only two solar towers with respective solar energy receiving systems are shown in FIG. 3, any number of solar towers and solar energy receiving systems can be employed.

More than one solar receiver can be provided on a solar tower. The multiple solar receivers in combination can form a part of the solar energy receiving system 20. The different solar receivers can have different functionalities. For example, one of the solar receivers can heat water using the reflected solar radiation to generate steam while another of the solar receivers can serve to superheat steam using the reflected solar radiation. The multiple solar receivers can be arranged at different heights on the same tower or at different locations (e.g., different faces, such as a north face, a west face, etc.) on the same tower. Some of the heliostats in field 60 can be constructed and arranged so as to alternatively direct insolation at the different solar receivers. As shown in FIG. 4, two solar receivers can be provided on a single tower 50. The solar energy receiving system 20 thus includes a first solar receiver 21 and a second solar receiver 22. At any given time, a heliostat 70 can be aimed at one or both of the solar receivers, or at none of the receivers. In some use scenarios, the aim of a heliostat 70 can be adjusted so as to move a reflected beam projected at the tower 50 from one of the solar receivers (e.g., 21) to the other of the solar receivers (e.g., 22). Although only two solar receivers and a single tower are shown in FIG. 4, any number of solar towers and solar receivers can be employed. Heliostats 70 in a field 60 can be controlled through a central heliostat field control system 91, for example, as shown in FIG. 5. For example, a central heliostat field control system 91 can communicate hierarchically through a data communications network with controllers of individual heliostats. FIG. 5 illustrates a hierarchical control system 91 that includes three levels of control hierarchy, although in other implementations there can be more or fewer levels of hierarchy, and in still other implementations the entire data communications network can be without hierarchy, for example, in a distributed processing arrangement using a peer-to-peer communications protocol.

At a lowest level of control hierarchy (i.e., the level provided by heliostat controller) in the illustration there are provided programmable heliostat control systems (HCS) 65, which control the two-axis (azimuth and elevation) movements of heliostats (not shown), for example, as they track the movement of the sun. At a higher level of control hierarchy, heliostat array control systems (HACS) 92, 93 are provided, each of which controls the operation of heliostats 70 (not shown) in heliostat fields 96, 97, by communicating with programmable heliostat control systems 65 associated with those heliostats 70 through a multipoint data network 94 employing a network operating system such as CAN, Devicenet, Ethernet, or the like. At a still higher level of control hierarchy a master control system (MCS) 95 is provided which indirectly controls the operation of heliostats in heliostat fields 96, 97 by communicating with heliostat array control systems 92, 93 through network 94. Master control system 95 further controls the operation of a solar receiver (not shown) by communication through network 94 to a receiver control system (RCS) 99.

In FIG. 5, the portion of network 94 provided in heliostat field 96 can be based on copper wire or fiber optic connections, and each of the programmable heliostat control systems 65 provided in heliostat field 96 can be equipped with a wired communications adapter, as are master control system 95, heliostat array control system 92 and wired network control bus router 100, which is optionally deployed in network 94 to handle communications traffic to and among the programmable heliostat control systems 65 in heliostat field 96 more efficiently. In addition, the programmable heliostat control systems 65 provided in heliostat field 97 communicate with heliostat array control system 93 through network 94 by means of wireless communications. To this end, each of the programmable heliostat control systems 65 in heliostat field 97 is equipped with a wireless communications adapter 102, as is wireless network router 101, which is optionally deployed in network 94 to handle network traffic to and among the programmable heliostat control systems 65 in heliostat field 97 more efficiently. In addition, master control system 95 is optionally equipped with a wireless communications adapter (not shown). It should be noted that the size of the heliostat field in a solar tower system for a given rated electrical output may not be generally determined in accordance with the maximum expected level of solar radiation but rather by an optimization of expected financial return projected from the system when taking into account the expected distribution of solar radiation over the course of a year as well as other factors which can include, for example, differential tariffs. The result of this optimization is that there are some hours of peak solar radiation during the year in which the total energy available to the solar field exceeds the rated capacity. As a result of optimizing for financial return, during such peak hours, some heliostats are "defocused" (target of defocused heliostats is moved off of the receiver so that the energy is dissipated) from the tower to avoid exceeding the rated capacity of the system or one of its components such as a boiler, turbine or transformer, or alternatively to avoid exceeding an output rating mandated by contract or by regulation. This practice is typically referred to as "dumping" and the energy not captured by the system as a consequence is called "dumped" energy.

In addition or alternatively, at times, reflected and/or concentrated radiation by the heliostat-mounted mirrors may be aimed at a distribution of targets on the receiver such that some spillage of light past the edge of the receiver aperture facing some heliostats occurs. This may result because the shape of the focal spot (the area at the receiver illuminated by a given heliostat) or reflected light distribution of a given heliostat does not line up perfectly with the edge or because of some imprecision in the aiming of the heliostat. Also, the intensity distribution of the focal spot may be variable or imperfectly defined so that edge illumination with areal flux uniformity is impossible to achieve without losing some light past the edge.

A solar tower system can be designed to utilize solar radiation reflected by heliostats at a concentration corresponding to the requirements of the system utilizing the energy and various design and material specifications. In some embodiments, to efficiently utilize such concentrated solar radiation, it is desirable to ensure that the distribution of flux on the surface of the receiver falls within predetermined limits and ranges of flux. The desired distribution can generally be accomplished by assigning individual heliostats to a target on the receiver and subsequently controlled to maintain focused insolation on the target by tracking the apparent movement of the sun. These targets may be known as aiming points.

Different heliostats may be directed to different aiming points and a heliostat can be directed to different aiming points at different times. As discussed above, distribution of reflected solar radiation around an intended aiming point on a target surface is approximately Gaussian. It is desirable to designate the aiming points and to assign the heliostats while taking into account physical and operational constraints, which may include, for example, material temperature or strength limitations of components of the receiver. The distribution of reflected solar radiation around an intended aiming point on a target surface may be unpredictable or may be graded. As such, in attempting to achieve a target uniformity over the surface of the receiver, and at the same time directing the focus of some heliostats near the receiver edges, some spillage of concentrated light may occur. That is, some of the reflected radiation aimed near the edges of the boiler misses the receiver. In order to minimize the amount of flux that would otherwise be lost, this spilled solar radiation may be reflected back towards the receiver. In order to achieve this, a reflector placed adjacent to the thermal receiver may be positioned so as to redirect the spilled radiation to the thermal receiver.

Reflectors can be provided extending from the top and/or bottom edges of the receiver, for example, as shown in FIGS. 6A-6C. As shown in FIG. 6A, reflector 408 may be arranged adjacent to a top edge of receiver 402, while reflector 410 may be provided adjacent to a bottom edge of the receiver 402. Each panel 408a, 410a of the respective reflectors 408, 410 may have a side 405 facing an external surface of the receiver 402 (i.e., capable of reflecting insolation from heliostats onto a radiation receiving external surface of the receiver 402) and a side 406 not facing an external surface of the receiver 402.

The side 405 of the reflector facing the receiver 402 can have a reflective surface. The reflective surface may be formed of a reflective material. Alternatively, the surface 405 of the reflector facing an external surface of the receiver 402 may be coated or painted with a reflective material. For example, the reflective material may include, but is not limited to, a silver containing compound. The reflective material may be applied onto the surface of the reflector via electroplating or via deposition. The reflective material may be constructed so as withstand the high temperatures, thermal stresses, and rugged environmental conditions commonly associated with solar thermal systems. The surface of the reflector, i.e. the substrate, may be a metal substrate, a glass substrate, a ceramic substrate, or any other suitable substrate.

The surface 405 of the reflector facing an external surface of the receiver 402 may also be coated with adhesion or barrier layers, which can be used in conjunction with the reflective material. The adhesion and/or barrier layers can be placed directly on the reflector substrate while the reflective coating can be coated over or directly on these layers. In such a

configuration, the adhesion and/or barrier layers can promote adhesion of the reflective surface to the substrate and/or prevent (or at least reduce) corrosion of the reflector substrate.

Additionally or alternatively, the adhesion and/or barrier layers may be coated on top of the reflective surface in order to provide protection for the reflective surface against external or environmental elements. In another configuration, the adhesion and/or barrier layers can be provided as a coating both above and below the reflective material layer. The adhesion and/or barrier layers can include, but are not limited to, nickel-chromium, nickel-chromium oxide, chromium, chromium(II) oxide, chromium(III) oxide, silicon dioxide, and silicon nitride.

The surface 405 of the reflector facing an external surface of the receiver 402 can include a stacked coating of materials that provide both high absorptivity and high emissivity. For example, such a coating stack can include an optional surface treatment layer, a coating that has high solar reflectivity and high emissivity (also high thermal emissivity), and an optional protective layer. The surface treatment layer may promote adhesion of the

reflectivity/emissivity coating to the reflector substrate and/or to prevent corrosion of the substrate. The high reflectivity/emissivity coating may reduce the overall temperature of the reflector as compared to a reflector without such a coating. The color of reflectivity/emissivity coating can be selected to maximize the reflection in the visible spectrum. The coating can have a glossy finish. The protective layer can provide protection for the reflective surface against external or environmental elements and can also serve as anti-reflective layer to reduce reflection and thereby improve performance.

Optionally, as shown in FIG. 9A, the surface 406 of the reflector not facing an external surface of the receiver 402 can also be provided with one or more coatings 902 thereon. For example, the surface 406 can be provided with an infrared (IR) reflective layer, such as, but not limited to, polished gold or aluminum. By providing an IR reflective coating for the back surface 406, energy incident on the front surface 405 of the reflector and conducted to the back surface 406 will be reflected back toward the front surface 405, thereby minimizing the thermal losses that would otherwise escape from the back surface 406 of the reflector. The amount of thermal energy re-radiated from the front surface 405 of the reflector can thus be maximized or at least increased. This may also function to minimize external heating of the surface 406 of the reflector from solar radiation (i.e., surface 406 of the top reflector 408 would be exposed to the sun) and from heat radiating from the thermal receiver.

In another configuration, the reflector may include a dielectric mirror (or Bragg mirror). In such a configuration, the reflector may have a glass or ceramic substrate within thin layers of dielectric material placed thereon, the materials being arranged in alternating layers of high and low refractive indices. The thicknesses of the layers can be chosen to provide constructive interference and high reflection.

During operation, the spilled solar radiation may heat the reflector to temperatures as high as 300°C to 500°C. To protect the reflector from thermal stresses, a cooling system can be provided. The cooling system may be active or passive. For example, the reflector can be cooled by passing cooling water over the back side of the reflector, by passing a coolant through cooling pipes integrated into the reflectors, by air cooled heat sinks (which may in turn include heat pipes), or other suitable cooling devices. Alternatively or additionally, the reflector can be thermally insulated. Such thermal insulation can be placed on the back surface 406 of the reflector not facing an external surface of the receiver 402.

The receiver 402 can have a rectangular cross-section, as illustrated in FIGS. 6A-6B. The receiver 402 can have a width 416, for example, of approximately 4 meters, although other dimensions are also contemplated. In another example, the width 416 is 25 meters. The receiver 402 can have a height 414 of approximately 25 meters. Each reflector 408, 410 can extend a height 412 beyond an edge of the receiver. For example, each reflector 408, 410 can extend 5- 10 meters above the top or bottom of the receiver 402. The length of reflectors as measured from the edge of the thermal receiver 402 to the outer edge of reflectors 408 and 410 can, for example, be between 4 and 7 meters.

The reflectors can be sized based on actual or anticipated sizes of focal spots. For example, the reflectors can have a dimension of approximately half the size of the focal spot. In another example, the reflectors can have a dimension of at least the size of the focal spot. The focal spot size may be taken as one or two standard deviations of the maximum light intensity or some other approximating value. The arrangement of the reflectors can cover an entire a perimeter of the receiver, for example, to form a circular arrangement around the receiver.

Accordingly, the reflectors can be shaped based on a shape of the receiver. For example, the shape of the reflectors may follow an edge shape of the top of the receiver.

Other cross-sectional shapes for the receiver are also possible according to one or more contemplated embodiments. For example, the receiver can have an a substantially octagonal shape, as illustrated in FIGS. 7A-7B. Referring to FIG. 7 A, top and bottom reflectors can be provided extending from respective edges of the receiver 402, although only the top reflectors are illustrated in the figures for clarity. The reflector can include a plurality of segments to provide complete coverage of the perimeter of the octagonal- shaped receiver 402. As shown in FIG. 7A, the reflector can include eight rectangular segments 420 joined to each other by eight triangular segments 422. For example, each rectangular segment 420 may have an edge length of between 10 and 15 meters and a width (i.e., from a closest edge of receiver 402 to the edge of the reflector remote from the receiver 402) of, for example, between 4 and 7 meters.

Alternatively, as shown in FIG. 7B, the reflector can be formed of trapezoidal segments 424 which connect together to cover the perimeter region of the receiver 402, thereby reducing the number of parts required for the reflector.

According to any of the embodiments disclosed, angles for the face of each reflector may be established responsively to the positions of the heliostats selected to use it. The angle may be optimized in a manner that includes a search for a selection set of the "client" heliostats using an optimization procedure as discussed herein or any suitable optimization procedure. Such an optimization approach may take account of the amount of solar radiation which may be efficiently directed by candidate client heliostats, the reflectivity of the reflector, the

configuration of the receiver, etc.

As shown in FIG. 6C, the face of the reflector 408a may be arranged at an angle 418 with respect to a vertical surface of the receiver 402. Alternatively, the angle of the reflector 408a can be measured with respect to the horizontal. For example, the reflector 408a can arranged at an angle between 10° and 40° with respect to the horizon, for example,

approximately 30° above the horizon. For example, the face of reflector 410a may be arranged at an angle 420 of 45 ° or less below the horizon. The angle of inclination of reflectors 408a and/or the angle of declination of reflector 410a may be chosen based on the angles of reflected radiation by the heliostats to maximize energy production. Accordingly, the above dimensions and angles are for purposes of illustration only, and other dimensions and angles are also possible according to one or more contemplated embodiments.

Environmental conditions may adversely impact the efficiency of a thermal receiver. For example, prevailing wind patterns interacting with the surface of the receiver may increase heat loss due to convection. Accordingly, reflectors may be added to the receiver to modify the airflow patterns around the thermal receiver so as to reduce and/or minimize thermal losses due to convection. Such reflectors may extended radially from the thermal receiver and are positioned where they can effectively reduce convective heat losses from the receiver by disrupting normal airflow related to prevailing wind patterns. The reflectors can serve to alter the boundary layer adjacent to surfaces of the thermal receiver, thereby minimizing convective heat loss due to prevailing wind conditions.

As shown in FIG. 8, receiver 402 can be provided with side reflectors 450 extending from one or more side edges 452 of the receiver 402. Side reflectors 450, like top and bottom reflectors 408, 410, can reflect spillage from edge 452 onto the radiation receiving surface of receiver 402 for use thereby. Alternatively or additionally, reflectors 450 can be provided in combination with one or more reflectors extending from the top edges 454 or bottom edges 456 of receiver 402.

Referring now to FIG. 10A, radiation 28 from the sun 25 is incident on a group of heliostats 38 equipped with mirrors, and the reflected radiation 29 is directed on receiver 1 on a first tower 43. As previously mentioned, in order to achieve a desired distribution of flux on the receiver, some of the radiation directed from heliostats 38 is aimed towards the edges of receiver 1. Consequently, some of this radiation which is aimed towards the edges of the receiver does not hit the receiver. In order to reduce the amount of flux which is spilled and consequently cannot be used to generate electricity, reflectors 32, 34 can be positioned adjacent to receiver 1.

Reflectors 32, 34 can be configured to maximize the amount of spilled solar radiation that is directed back towards the receiver. The configuration is beneficially selected responsively to the size of the reflector and the angle of the reflector relative to the thermal receiver.

According to some configurations, two solar receivers are mounted on the same tower, wherein the first receiver is positioned above a second receiver the tower. In this instance a first reflector is adjacent to and projects from the top edge of the uppermost receiver and a second reflector is adjacent to and projects from the bottom edge of the bottommost receiver.

Referring to FIG. 12, the addition of reflectors (e.g., reflector 1102) adjacent to the thermal receiver 402 may also create a larger effective area 1204 for the direction and subsequent absorption of solar radiation (for example, compare area 1214 without reflector with area 1204 with reflector 1102) while at the same time reducing the effective area 1206 from which the radiation will be emitted or escape from the receiver to the atmosphere (for example, compare the area 1206 of the atmosphere visible at a point at the bottom of the receiver 402 to the area 1216 of the atmosphere visible at the same point without the reflector 1102 installed). The receiver 402 can have a direct aperture size (e.g., the intercept area facing toward a group of heliostats that are trained to direct light at said area) that is greater than the spot size of the heliostats of the group of heliostats targeting the aperture. In such an embodiment, the receiver captures a larger amount of radiation directed toward it by virtue of having an effectively larger total aperture area which includes the intercept area provided by the reflectors as well as the direct aperture size.

In any of the embodiments, the angle of orientation and the position of the reflectors can be dynamically adjustable such that the heliostats and the reflectors form a dynamic optical system. Referring to FIG. 11 A, reflector 1102 can moved by displacement module 1104. The displacement module 1104 may allow the reflector 1102 to rotate in a vertical plane.

Alternatively or additionally, the displacement module 1104 can be configured to displace the reflector 1102 into an interior region thereof, for example, to protect the reflector during inclement weather or other environmental hazards. Displacement of the reflector 1102 into the module 1104 may occur when the reflector 1102 is rotated into a substantially horizontal orientation.

In another configuration shown in FIG. 11B, reflector 1102 may be rotated in a vertical plane using a displacing piston 1108 of motion control module 1106. The reflector 1102 can be pivoted at a point on or near the edge of the receiver 402. Movement of the piston 1108 in a horizontal direction causes the reflector 1102 to rotate about its pivot, thereby changing the angle of the reflector 1102 with respect to the horizontal. Although two different displacement configurations have been discussed for controlling motion of the reflector 1102, other displacement and/or rotation mechanisms are also possible according to one or more

contemplated embodiments.

The ability to dynamically adjust the orientation and/or the position of the reflectors may allow for the solar radiation to be directed by the reflectors to specific locations on the receiver, for example, in response to changing insolation or operating conditions. If the distribution of flux on the surface of the receiver requires adjustment, at least one of the reflectors may be oriented so as to direct the spilled solar radiation to those locations which are deficient in flux relative to others where flux or heat uniformity is pursued by the targeting system.

In embodiments, the heliostats are not controlled to aim at the reflector. Rather, the reflector is configured to reflect solar radiation which undesirably spills from edges of the thermal receiver. In alternative embodiments, the reflectors may be partly utilized as part of the overall optical system. In such a system, the reflectors may be larger and used to direct energy in alternative ways to achieve performance maximization under different conditions.

As shown in FIG. 13 A, the reflector 1102 can direct spilled portions of the incident solar radiation onto the same surface of the receiver from which the incident solar radiation spills. For example, beam 1302 can be directed by a heliostat onto receiver 402. The beam width can be defined by a first ray 1302a and a second ray 1302b, while the centroid of the beam can be designated by a third ray 1302c. As shown in FIG. 13 A, the beam centroid may be aimed at or near an edge region 1306 of the receiver, such that at least a portion of the beam would be spillage if not for the reflector 1102. Ray 1302a is incident on reflector 1102 and is reflected thereby back onto the receiver 402. The reflector orientation can be selected such that the reflection 1304 is directed back at that region 1304 intended to be illuminated by the beam 1302.

At times when the total amount of light that may be directed to the receiver is excessive, to avoid exceeding the tolerances of the receiver where reflectors 32, 34 are employed, heliostats 38 may be controlled to be defocused from a tower with a thermal receiver 1, and may be moved into the dumping position, as illustrated in FIG. 10B. Alternatively, the heliostats may be aimed at a second tower 243, which is described in U.S. Publication No. 2009/0229264 and hereby incorporated by reference. The second tower may include a thermal receiver 201 or photovoltaic converters in order to capture energy that would have been otherwise dumped. This may utilize incident solar radiation more efficiently; in that (i) the reflectors reduce the amount of solar radiation which would otherwise be lost and (ii) heliostats which may not have been otherwise used due to the "extra" flux provided by the reflectors would now be used in other aspects of the solar energy conversion system. Alternatively or additionally, the dumping may be directed at the one or more of reflectors at edges of the receiver, which can dynamically adjust to direct the insolation away from the receiver or on a stationary reflector that would otherwise reflect the insolation away from the receiver. Displacement of the heliostats to return from a dumping orientation to normal operating orientation may thereby be minimized or at least reduced.

In another configuration, the reflectors can be controlled to direct solar radiation which is reflected from the receiver surface back towards the receiver surface. Although the receiver is designed to capture most of the solar radiation which directed towards it, a small percentage of this radiation is reflected from the receiver surface and thus unused for heating the heat transfer or working fluid. Therefore, in order to "save" this otherwise lost radiation, the reflectors may be arranged so as reflect this radiation back towards the receiver and be converted to electricity. Such a configuration is shown in FIG. 13B, where reflector 1102 includes one or more components 1312 on a surface thereof. Radiation 1310 undesirably reflected from the surface of the receiver 402 can be incident on the one or more components 1312 to reflect the insolation back to the surface at 1314. For example, components 1312 can be retroreflectors or other optical components capable of reflecting light back toward the receiver. Alternatively, the reflector 1102 can have a planar surface without components 1312, and appropriate orientation of the surface can result in reflection back onto the receiver surface.

Referring to FIG. 9B, the reflectors can be provided with one or more components 904 on back surface 406. For example, an electricity generation device 904 can be provided on the back surface 406 so as to convert otherwise unused insolation or heat to electricity. For example, the electricity generation device can be a photovoltaic converter. Thus, when the surface 405 is provided a photovoltaic converter, radiation reflected from at least one heliostat and directed thereon can be captured and converted to electricity. In addition, when the surface 405 faces the sun, directed insolation can be captured and converted to electricity by the photovoltaic converter. Alternatively or additionally, the electricity generation device 904 can be a thermoelectric generator constructed to convert heat generated by solar exposure of the back surface 405 or heat conducted through the thickness of the reflector to the back surface 405 into electricity.

According to some embodiments, a receiver can include a boiler section which includes tubes, pipes, or the like, in which a fluid is heated, a reflector disposed adjacent to the receiver and additionally includes a photovoltaic section with one or more photovoltaic cells made of or based upon crystalline silicon. The photovoltaic section can be configured and disposed so as to produce electricity from spilled solar radiation that is not reflected from the reflector back to the receiver. As shown in FIG. 14, one or more of the reflectors 1402 can have a substantially curved surface instead of a planar surface. For example, the shape of the reflector 1402 may be a hyperbolic sinusoidal curve. A curved surface of the reflector 1402 may provide improved control with regard to the specific points or areas on the thermal receiver 402 which the spilled radiation is directed towards. For example, reflector 1402 can be curved such that spilled radiation 1404 is directed at point 1406 on the receiver.

According to some embodiments, the reflector 1502 can include multiple elements 1508, as shown in FIG. 15. Reflector 1502 can be constructed such that each element 1508 is aimed at a specific point or area 1506 on the thermal receiver 402. Each element 1508 can be supported on a single substrate 1510. The substrate may be made of any material which is strong enough to support the reflector. For example, the substrate can be made of stainless steel. At certain times, elements 1508 can be aimed at sections of a single receiver or other receivers where temperature uniformity control boundary targets are broader than at other sections. For example, a receiver or sections thereof may provide low temperature heating such as reheat or evaporation, as opposed to higher temperature superheating or supercritical heating.

Alternatively, each element 1508 of reflector 1502 can be aimed at any other section of the thermal receiver. In one configuration, elements 1508 can be substantially static and fixed with respect to substrate 1510. Variations in direction of reflection by elements 1508 may be achieved by appropriate reorientation of the entire reflector 1502. In another configuration, each element can be independently addressable so as to redirect insolation at desired aiming dynamically. For example, the elements may form a digital mirror array on substrate 1510.

In another embodiment, a method of converting solar energy to electricity includes controlling a plurality of heliostats to track the apparent movement of the sun which then reflect incident solar radiation on a thermal receiver which is configured to transfer all of the incident solar radiation received thereby as thermal energy to a fluid. A portion of the solar radiation which spills over the edge of the thermal receiver is directed back towards the receiver via a reflector which in some instances is located adjacent to the receiver. As previously discussed, the solar radiation which is redirected back towards the receiver increases the amount of flux on the receiver surface. Therefore, in order to maintain the temperature uniformity on the receiver surface, some embodiments may include controlling a portion of the heliostats which are currently directed towards the receiver to direct incident solar radiation away from the thermal receiver. In still another embodiment, the temperature may be maintained by controlling the heliostats such that the spilled solar radiation originates from heliostats which are positioned at different points in a heliostat field. In yet another embodiment, the reflected radiation incident on the reflector is radiation aimed substantially at the fluid-heating receiver or receiver section. The temperature uniformity may also be obtained by providing reflectors which consist of multiple elements. The reflector may be designed such that each element is aimed at a specific point or area on the thermal area. According to some embodiments, an element of the reflector may be directed towards a low temperature section of the thermal receiver where the uniformity of the temperature is less critical. This may include the evaporator section of the receiver, or alternatively the superheater or supercritical section of the receiver. Generally, steam which is superheated or reheated in the thermal receiver (or receivers) requires a relatively low concentration, for example, less than about one hundred suns.

In another example of providing temperature uniformity on the receiver, the reflectors may have a curved surface. The curved surface of a reflector would allow the spilled solar radiation to be redirected to specific points or areas on the thermal receiver.

In a further embodiment, a method for improving the efficiency of a solar concentrating system includes aiming at least some heliostat-mounted mirrors substantially at a receiver or receiver section which includes tubes, pipes, or the like, in which a fluid is heated. The spilled solar radiation which is caused by the heliostats which are aimed at the edge of the receiver is redirected back towards the face of the receiver by a reflector which is located adjacent to the receiver. As this would increase the amount of flux on the solar receiver, a fewer number of heliostats are required to be aimed at the receiver. The controller may then control the heliostats to be moved into the dumping position. As the same amount of usable energy can be obtained from a smaller amount of insolation, i.e., by deploying fewer heliostat the efficiency of the solar concentration system is improved.

In yet another embodiment, a method for reducing the amount of lost flux in a solar power generation system includes providing a reflector which is adjacent to an edge of a solar receiver, wherein a surface of the reflector reflects at least part of the errant flux towards a face of the receiver. In other words, a spilled portion of incident solar radiation is directed towards the receiver via a reflector which is adjacent to at least one edge of the solar receiver. In attempting to achieve target uniformity over the surface of the receiver, the focus of some heliostats is directed near the receiver edges. Therefore some spillage of flux may occur. In order to capture this lost flux, an element adjacent to the receiver and projecting from the receiver reflects at least some of this spilled flux back onto the surface of the receiver.

Although the present specification contemplates that aiming points of the heliostats are fixed on the receiver, at least for intervals of time between new assignments of the heliostats to different assigned points, it is possible, to direct the heliostats somewhat differently. In alternative embodiments, for example, it may be possible for the heliostats to be aimed at respective aiming points that are continuously changed so as to maintain the flux or temperature uniformity goal state range of the receiver. For example, as one spot moves progressively away from a location other spots move progressively such that the uniformity condition is maintained.

It will be appreciated that the modules, processes, systems, and sections described above can be implemented in hardware, hardware programmed by software, software instruction stored on a non-transitory computer readable medium or a combination of the above. A system for controlling the heliostats, the receiver, and/or the reflector can be implemented, for example, using a processor configured to execute a sequence of programmed instructions stored on a non- transitory computer readable medium. The processor can include, but is not limited to, a personal computer or workstation or other such computing system that includes a processor, microprocessor, microcontroller device, or is comprised of control logic including integrated circuits such as, for example, an Application Specific Integrated Circuit (ASIC). The instructions can be compiled from source code instructions provided in accordance with a programming language such as Java, C++, C#.net or the like. The instructions can also comprise code and data objects provided in accordance with, for example, the Visual Basic™ language, or another structured or object-oriented programming language. The sequence of programmed instructions and data associated therewith can be stored in a non-transitory computer-readable medium such as a computer memory or storage device which can be any suitable memory apparatus, such as, but not limited to read-only memory (ROM), programmable read-only memory (PROM), electrically erasable programmable read-only memory (EEPROM), random-access memory (RAM), flash memory, disk drive, etc.

Furthermore, the modules, processes, systems, and sections can be implemented as a single processor or as a distributed processor. Further, it should be appreciated that the steps discussed herein can be performed on a single or distributed processor (single and/or multi- core). Also, the processes, modules, and sub-modules described in the various figures of and for embodiments above can be distributed across multiple computers or systems or can be co- located in a single processor or system. Exemplary structural embodiment alternatives suitable for implementing the modules, sections, systems, means, or processes described herein are provided below, but not limited thereto. The modules, processors or systems described herein can be implemented as a programmed general purpose computer, an electronic device programmed with microcode, a hard-wired analog logic circuit, software stored on a computer- readable medium or signal, an optical computing device, a networked system of electronic and/or optical devices, a special purpose computing device, an integrated circuit device, a semiconductor chip, and a software module or object stored on a computer-readable medium or signal, for example. Moreover, embodiments of the disclosed method, system, and computer program product can be implemented in software executed on a programmed general purpose computer, a special purpose computer, a microprocessor, or the like.

Embodiments of the method and system (or their sub-components or modules), can be implemented on a general-purpose computer, a special-purpose computer, a programmed microprocessor or microcontroller and peripheral integrated circuit element, an ASIC or other integrated circuit, a digital signal processor, a hardwired electronic or logic circuit such as a discrete element circuit, a programmed logic circuit such as a programmable logic device (PLD), programmable logic array (PLA), field-programmable gate array (FPGA), programmable array logic (PAL) device, etc. In general, any process capable of implementing the functions or steps described herein can be used to implement embodiments of the method, system, or a computer program product (software program stored on a non-transitory computer readable medium).

Furthermore, embodiments of the disclosed method, system, and computer program product can be readily implemented, fully or partially, in software using, for example, object or object-oriented software development environments that provide portable source code that can be used on a variety of computer platforms. Alternatively, embodiments of the disclosed method, system, and computer program product can be implemented partially or fully in hardware using, for example, standard logic circuits or a very-large- scale integration (VLSI) design. Other hardware or software can be used to implement embodiments depending on the speed and/or efficiency requirements of the systems, the particular function, and/or particular software or hardware system, microprocessor, or microcomputer being utilized. Embodiments of the method, system, and computer program product can be implemented in hardware and/or software using any known or later developed systems or structures, devices and/or software by those of ordinary skill in the applicable art from the function description provided herein and with a general basic knowledge of solar thermal energy production, heliostat control systems, and/or computer programming arts.

Features of the disclosed embodiments may be combined, rearranged, omitted, etc., within the scope of the invention to produce additional embodiments. Furthermore, certain features may sometimes be used to advantage without a corresponding use of other features.

It is thus apparent that there is provided in accordance with the present disclosure, spillage reducing improvements for solar receivers. Many alternatives, modifications, and variations are enabled by the present disclosure. While specific embodiments have been shown and described in detail to illustrate the application of the principles of the present invention, it will be understood that the invention may be embodied otherwise without departing from such principles. Accordingly, Applicants intend to embrace all such alternatives, modifications, equivalents, and variations that are within the spirit and scope of the present invention.

Claims

1. A method of improving the efficiency of a solar concentrating system, the method comprising:

controlling a plurality of heliostats to track the apparent movement of the sun to reflect incident solar radiation on a receiver; and

redirecting a portion of said reflected incident solar radiation which spills over an edge of the receiver via a reflector;

wherein the reflector is adjacent to an edge of the receiver and directs the spilled portion of the incident solar radiation to the receiver.

2. A method of reducing the amount of lost flux, the method comprising:

providing a reflector adjacent an edge of a solar receiver, such that a surface of the reflector reflects at least part of the spilled flux towards a face of the receiver.

3. A method of converting solar energy to electricity, the method comprising: controlling a plurality of heliostats to track the apparent movement of the sun to reflect incident solar radiation on a thermal receiver configured to transfer all the incident solar radiation received thereby as thermal energy to a heat transfer fluid;

redirecting a portion of said reflected incident solar radiation which spills over an edge of the receiver via a reflector onto the receiver; and

maintaining a temperature uniformity on the receiver at least in part by controlling a portion of said plurality of heliostats to direct incident solar radiation away from said receiver.

4. The method of claim 3, wherein the reflector is adjacent to the receiver.

5. The method of claim 3, wherein the maintaining is further provided by controlling the heliostats such that the spilled solar radiation originates from heliostats positioned at different points in a heliostat field.

6. The method of claim 3, wherein the redirecting includes moving the reflector to redirect said portion onto the receiver.

7. The method of claim 3, wherein the reflector has a plane surface.

8. The method of claim 3, where the reflector has a curved surface.

9. The method of claim 3, wherein the reflector is dynamically movable.

10. The method of claim 9, wherein the reflector has multiple elements, wherein at least some of the elements are aimed at a different point and/or area than others of the elements.

11. The method of claim 10, wherein said different point is on a low-temperature section of the receiver.

12. The method of claim 10, wherein said different point is on an evaporator section of the receiver.

13. The method of claim 10, wherein the different point is on a supercritical section of the receiver.

14. The method of claim 10, wherein the different point is on a reheater section of the receiver.

15. The method of claim 3, wherein the reflector is adjacent to a top edge, a bottom edge, or a side edge of the receiver.

16. A solar concentrating system, comprising:

a receiver having a top edge, a bottom edge and at least one side edge extending from the top edge to the bottom edge;

a plurality of heliostats, each heliostat being configured to direct incident solar radiation at the receiver so as to heat a heat transfer or working fluid flowing through the receiver;

a controller configured to compute respective aiming points and to control the heliostats such that they are aimed at the respective aiming points on an external surface of the receiver and to track the apparent movement of the sun so as to maintain the aim of the heliostats at the respective aiming points; and

at least one reflector projecting from the top edge, the bottom edge or the at least one side edge of the receiver, such that the at least one reflector is configured to direct a spilled portion of the incident solar radiation to the receiver,

wherein the controller is further configured to:

control the heliostats in response to a prediction of the total flux on the receiver, including components incident on the receiver directly and components incident on the receiver via the at least one reflector, and

select aiming points to prevent the temperature of the receiver from exceeding a predetermined level at any location thereof.

17. The system of claim 16, wherein the spilled portion of the incident solar radiation which is directed by the at least one reflector to the receiver increases the flux in such an amount that the total flux on the receiver exceeds the predetermined level, thereupon the controller directs a portion of the plurality of heliostats to aim away from said receiver.

18. The system of claim 16, wherein the at least one reflector has a first side facing an external surface of the receiver and a second side opposite the first side where the first side has a reflective surface.

19. The system of claim 18, wherein the second side is thermally insulated.

20. The system of claim 16, wherein the second side includes one of an infrared reflector, a photovoltaic conversion device, and a thermoelectric generator.

21. The system of claim 16, wherein the position and the angle of orientation of the at least one reflector are dynamically controlled to form a single aiming system to permit light from the heliostats to be distributed flexibly over a receiving surface of the receiver.

22. The system of claim 16, wherein the at least one reflector is a static reflector.

23. The system of claim 16, wherein a length of the reflector from the edge of the receiver to an edge of the reflector remote from the receiver is at least 2 meters, at least 4 meters, or between 4-7 meters.

24. The system of claim 16, wherein the reflector projects from the top edge of the receiver and the angle of the at least one reflector is between 10 and 40 degrees above the horizon.

25. The system of claim 24, wherein the angle of the at least one reflector is approximately 30 degrees above the horizon.

26. The system of claim 16, wherein the at least one reflector projects from the bottom edge of the receiver and the angle of the at least one reflector is at most 45 degrees below the horizon.

27. The system of claim 16, wherein the at least one reflector has a plane surface.

28. The system of claim 16, where the at least one reflector has a curved surface.

29. The system of claim 16, wherein the at least one reflector includes multiple static elements, wherein each element is aimed at a specific area on the receiver.

30. The system of claim 16, wherein the at least one reflector includes a digital reflector array having a plurality of individually addressable mirrors on a substrate.

31. The system of claim 16, wherein the thermal receiver is located on top of a tower.

32. The system of claim 31, further comprising at least one second receiver positioned below the first receiver on the same tower, a first one of the reflectors projecting from a top edge of the first receiver, a second one of the reflectors projecting from a bottom edge of a bottom-most second receiver.

33. The system of claim 16, wherein the at least one reflector is arranged such that the spilled portion of the incident solar radiation is directed onto the surface of the receiver from which the incident solar radiation spills.

34. The system of claim 16, wherein the at least one reflector does not capture all of the spilled radiation.

35. The system of claim 16, wherein the at least one reflector is arranged to direct solar radiation reflected from the surface of the receiver back toward the receiver.

36. The system of claim 16, wherein the reflective surface includes a silver compound.

37. The system of claim 36, wherein the reflective surface further comprises adhesion compounds and/or barrier compounds.

38. The system of claim 36, wherein the reflective surface includes an electroplated or deposited silver layer.

39. The system of claim 16, wherein:

a side of the receiver facing an external surface of the receiver has thereon a surface treatment layer, a coating which has high solar reflectivity and high emissivity, and a protective layer; and

a side of the receiver not facing an external surface of the receiver has thereon a surface treatment layer and an infrared (IR) reflective layer.

40. The system of claim 16, further comprising an active cooling unit constructed to cool the at least one reflector.

41. The system of claim 16, further comprising a passive cooling unit constructed to passively dissipate heat from the at least one reflector so as to cool the at least one reflector.

42. The system of claim 16, wherein a side of the receiver facing an external surface of the receiver includes a glass substrate and a stack of thin layers of dielectric material, the stack of dielectric material being on the side of the glass opposite the surface of the receiver.

43. The system of claim 16, wherein the at least one reflector includes a dielectric mirror on a glass substrate.

44. A solar energy conversion system comprising:

a receiver;

a plurality of heliostats, each heliostat being configured to direct incident solar radiation at the receiver so as to heat a working fluid flowing through the receiver;

a conveyance device configured to transport the heated working fluid from the receiver to an electric power generating plant, the heated working fluid being used by the electric power generating plant in the generation of electricity; and

at least one reflective device arranged adjacent to the receiver so as to redirect a spilled portion of the incident solar radiation back toward the receiver.