US20090084374A1

US20090084374A1 - Solar energy receiver having optically inclined aperture

Info

Publication number: US20090084374A1
Application number: US12/157,879
Authority: US
Inventors: David R. Mills; Philipp Schramek
Original assignee: Ausra Inc
Current assignee: Areva Solar Inc
Priority date: 2007-06-13
Filing date: 2008-06-13
Publication date: 2009-04-02
Also published as: WO2009023063A2; WO2009023063A3

Abstract

The present application provides a solar energy receiver comprising an effective absorption aperture that is biased, so that solar radiation from a certain direction can be preferentially absorbed by a solar radiation absorber in the receiver. The effective absorption aperture is inclined relative to a physical aperture. Thus, in an elevated receiver comprising a downward facing physical aperture defining a plane that is relatively parallel to ground, the effective absorption aperture of the receivers described herein may be inclined relative to ground, but the physical aperture may remain generally parallel to ground. The biased receivers may be used in Linear Fresnel Reflector solar arrays.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 60/934,549, entitled “Solar Energy Receiver Having Optically Inclined Aperture,” filed Jun. 13, 2007, which is incorporated by reference herein in its entirety. This application also claims priority to U.S. Provisional Patent Application Ser. No. 61/007,926, entitled “Linear Fresnel Solar Arrays,” filed Aug. 27, 2007, which is incorporated by reference herein in its entirety.

FIELD

The present application relates to a solar energy receiver arranged to be illuminated by solar radiation from a reflector field and to transfer absorbed energy. The receiver is adapted so as to present an effective aperture for illuminating radiation that is optically inclined relative to ground.

BACKGROUND

A variety of solar energy collector systems are known, including those that rely on thermal solar energy collector systems, photovoltaic energy collector systems, and thermoelectric systems. For example, solar energy collector systems of the type referred to as Linear Fresnel Reflector (LFR) systems are relatively well known. LFR arrays include a field of linear reflectors that are arrayed in parallel side-by-side rows. The reflectors may be driven to track the sun's motion. In these systems, the reflectors are oriented to reflect incident solar radiation to an elevated distant receiver that is capable of absorbing the reflected solar radiation. The receiver typically extends parallel to the rows of reflectors to receive the reflected radiation for energy exchange. The receiver typically can be, but need not be, positioned between two adjacent fields of reflectors.
To track the sun's movements, the individual reflectors may be mounted to supports that are capable of tilting or pivoting. Examples of suitable supports are described in International Patent Publication Number WO05/003647, filed Jul. 1, 2004, and International Patent Publication Number WO05/0078360, filed Feb. 17, 2005, each of which is incorporated herein by reference in its entirety.
A need exists for improved receivers for use in solar energy collector systems. For example, receivers are needed that result in improvement in overall solar energy collection of a system, and/or in improvement in solar energy collection of a system over a desired period of time.

SUMMARY

Broadly, the present application provides a solar energy receiver comprising an effective absorption aperture that is biased, so that solar radiation from a certain direction is preferentially absorbed by a solar radiation absorber in the receiver. In general, the effective absorption aperture may be optically inclined relative to a physical aperture in the receiver. Thus, in an elevated receiver comprising a downward facing aperture defining a plane that is relatively parallel to ground, the effective absorption aperture of the receivers described herein may be inclined relative to ground.
As used herein “vertical” and “horizontal” are used in reference to ground. Further, descriptions such as “substantially horizontal,” “substantially vertical,” and the like are meant to encompass the relevant properties and minor deviations therefrom, e.g., deviations of about 10%, or about 5% or less. Thus, a “substantially horizontal” aperture may be generally parallel to ground, e.g., within about +/−10 degrees or less, within about +/−8 degrees, within about +/−5 degrees, within about +/−3 degrees, or within about +/−1 degree of a horizontal direction, relative to ground. As used herein, the terms “a” “an” and “the” are meant to encompass singular as well as plural referents unless the context clearly indicates otherwise. Numerical ranges as used herein are meant to be inclusive of any endpoints indicated for the ranges, as well as any numerical value included in the ranges.
In some variations, a solar energy receiver comprises a cavity having opposing side walls (e.g., two opposing side walls) and a physical aperture defined between the side walls. A solar radiation absorber is disposed within the cavity and is arranged to be illuminated by solar radiation directed through the physical aperture. A first reflector element is located at least partly within the cavity and is configured to reflect incident solar radiation toward the solar radiation absorber and so establish an effective absorption aperture that is inclined relative to a plane defined by the physical aperture. The physical aperture in some variations may be oriented generally parallel to ground. The solar radiation absorber may comprise a plurality of solar radiation absorber tubes, each configured to contain a heat transfer fluid, that are arranged side-by-side in the cavity and extend longitudinally along a length of the cavity. In some cases, the cavity in a receiver may be formed by an inverted trough, and the physical aperture may be defined between two opposing side walls (e.g., flared side walls) of the trough.
Some variations of receivers may comprise a second reflector element located at least partly within the cavity, wherein the second reflector element is arranged to be asymmetric in the receiver with respect to the first reflector element, and the first and second reflector elements are configured to reflect incident solar radiation toward the absorber and so establish an effective absorption aperture that is inclined relative to a plane defined by the physical aperture.
The first reflector element and/or second reflector elements in a receiver may have a relatively planar reflective surface, or may have a concave curved reflective surface facing toward the solar radiation absorber. If a reflective surface of a reflector element in a receiver is curved, it may have an elliptical concave curvature facing toward the solar radiation absorber, and one focus of the elliptical reflective surface may be at or near an edge of the absorber and the other focus of the elliptical reflective surface may be selected to be at or near an outer edge of a reflector field directing incident solar radiation to the receiver. Thus, in a receiver comprising two reflector elements to form the optically inclined aperture, either one or both of the reflector elements may have a planar reflective surface, and one or both of the reflector elements may have a curved reflective surface concave toward the absorber, which may or may not be an elliptical curved reflective surface.
In a receiver comprising two reflector elements, the reflector elements may be asymmetric with respect to a receiver in a variety of ways. For example, the first and second reflector elements may have different lengths extending from a base of the cavity outwardly toward the physical aperture that admits incoming solar radiation. Alternatively or in addition, the first and second reflector elements may extend outwardly from a base of the cavity at different angles relative to the base so as to create an effective absorption aperture that is inclined relative to a plane defined by the physical aperture.
The one or more reflector elements may be installed into a cavity of a receiver using any suitable means. In some variations, a reflector element may optionally be secured within a receiver cavity in such a way as to permit relative movement between the reflector element and the cavity, e.g., to accommodate differential thermal expansion between a reflector element and a trough. For example, a reflector element may be slidably mounted to a cavity by way of mounting brackets or clasps that permit relative movement between the reflector element and the cavity. In some variations, a reflector element may be more fixedly secured to a cavity in a receiver, e.g. by fasteners or an adhesive cement or the like to a wall of the cavity. In certain variations, a position of at least one of the reflector elements may be adjustable, e.g., a length of a reflector element extending from a base of the cavity may be adjusted so as to tune the receiver for a particular application.
A reflector element may have any suitable construction and/or composition. For example, the reflector element may optionally comprise a polished metal element (e.g., a metal strip) or may comprise a reflective coating on a metal substrate. In certain circumstances, the reflector element desirably comprises a thermally stable (e.g., Pyrex™) silvered glass mirror. In some cases, the silver coating may be laminated between two plates of thermally stable glass, e.g., to protect the silver coating against heat damage and/or environmental damage.
The optically inclined absorption aperture of the receivers described herein may be biased to accommodate any illumination scheme from a field of reflectors. For example, a reflector field may be configured to be asymmetric with respect to a receiver to accommodate seasonal variations in illumination from the sun and/or daily variations in illumination. In certain cases, a biased receiver may be used in instances when it desired to preferentially collect light during certain periods of a diurnal cycle, e.g., during afternoon or morning hours. In some cases, a receiver may comprise an effective aperture biased to accommodate an east-west oriented reflector field. For example, east-west oriented arrays may be used to increase an annualized collection from a solar energy collector system, and/or used in certain locations to accommodate seasonal variations in sun position. In other instances, a receiver may comprise an effective aperture biased to accommodate a symmetric or asymmetric north-south oriented reflector field, e.g., one that is designed to preferentially reflect sunlight during certain periods of a diurnal cycle, such as an afternoon period.
Although in general the receivers may be configured so that a plane defined by a physical aperture of a receiver is generally horizontal (parallel to ground), in some cases a receiver body can be tilted so that receiver comprises an inclined physical aperture as well as an optically inclined absorption aperture.
Thus, solar energy receivers are disclosed here that comprise a linearly extending trough having side walls and a physical aperture defined by longitudinally extending marginal edges of the side walls, a plurality of linearly extending absorber tubes located side-by-side within the trough and arranged (when the receiver is located in situ) to be illuminated by solar radiation. Also, a linearly extending reflector element may be located at least in part within the trough adjacent one of the side walls and may be arranged to reflect incident solar radiation toward the absorber tubes and so establish an optically inclined absorption aperture.
Other embodiments of solar energy receivers are disclosed herein. These variations of receivers comprise a cavity and a physical aperture defined between side walls of the cavity and a solar radiation absorber disposed within the cavity. The solar radiation absorber is arranged to be illuminated by solar radiation directed through the physical aperture. A first optical element is located at least partly within the cavity or proximate to the cavity so as to establish an effective absorption aperture that is inclined relative to a plane defined by the physical aperture. In these receivers, the first optical element may diffract incident solar radiation toward the absorber, may refract incident solar radiation toward the absorber, or may reflect incident solar radiation toward the absorber. Thus, the first optical element may comprise a grating, or a lens, or a reflector. In certain variations, these receivers may comprise one or more additional optical elements that are located at least partly within the cavity or proximate to the cavity. For example, a receiver may comprise first and second optical elements, each located at least partly within the cavity or proximate to the cavity so as to establish an effective absorption aperture that is inclined relative to a plane defined by the physical aperture. In some cases, a physical aperture of the receiver may be substantially parallel to ground.
Solar energy collector systems are disclosed here. These systems comprise one or more reflector fields, and an elevated receiver comprising a solar radiation absorber that is configured to receive and absorb solar radiation directed from the one or more reflector fields through a physical aperture of the receiver. The elevated receiver comprises an effective absorption aperture that is inclined relative to a plane defined by the physical aperture. In some instances, the plane of the physical aperture may be substantially parallel to ground.
In some solar energy collector systems, the one or more reflector fields may be arranged asymmetric with respect to the elevated receiver, and the effective absorption aperture may be inclined toward a designated side of the one or more reflector fields. A reflector field in a solar energy collector system may be oriented in a north-south direction or in an east-west direction.
Further, the solar energy collector systems may be configured (e.g., through a combination of reflector field arrangement and a configuration of a biased receiver optically inclined toward a designated side of a reflector field) to preferentially collect solar radiation over a certain period during a diurnal cycle (e.g., during the afternoon or morning) and/or over a certain time of year (e.g., a season). For example, a system may employ an east-west reflector field (e.g., an asymmetric east-west oriented reflector field). An east-west field may for example be employed to accommodate a latitude at which the solar energy collector system is located or to increase an annualized collection of the system. Some systems may employ a north-south oriented reflector field, which may in some cases be arranged to preferentially reflect light to a biased receiver at a certain time of day, e.g., in the afternoon hours. A north-south solar energy collector system may also be configured to preferentially collect solar radiation to increase an annualized collection of that system.
Methods for collecting solar radiation are also disclosed here. In general, the methods comprise reflecting solar radiation from one or more reflector fields through a physical aperture of an elevated receiver to be incident on a solar radiation absorber, wherein the receiver comprises an effective absorption aperture that is inclined relative to ground so as to preferentially receive and absorb solar radiation from a designated side of the receiver.
In some methods, an amount of solar radiation reflected from a first side of the one or more reflector fields to the receiver is greater than an amount of solar radiation reflected from a second side of the one or more reflector fields. In those methods, the effective absorption aperture may be inclined toward the first side of the one or more reflector fields.
In the methods, the effective absorption aperture in the receiver may be established by mounting a first reflector element at least partly within a cavity of the receiver, the cavity housing the absorber. The first reflector element is configured to reflect incident solar radiation toward the absorber and so establish an effective absorption aperture that is inclined relative to ground. Certain methods may comprise establishing the effective absorption aperture by mounting a second reflector element at least partly within the cavity. In these methods, the second reflector element is arranged to be asymmetric in the receiver with respect to the first reflector element, and the first and second reflector elements are configured to reflect incident solar radiation toward the absorber and so establish an effective absorption aperture that is inclined relative to ground.
The methods may for example be adapted for preferentially collecting solar radiation during a certain time of year and/or during a certain portion of a diurnal cycle, e.g., by configuring the reflector fields to preferentially reflect solar radiation during the selected time period and arranging the optically inclined aperture of the biased receiver to receive and absorb the preferentially reflected solar radiation. Thus, the methods may be used in connection with asymmetric reflector fields to preferentially collect solar energy at a certain time of day (e.g., afternoon or morning) and/or during a certain period in a year, e.g., during a certain season. The methods may be adapted for increasing an annualized collection of a solar energy system. In certain variations, the methods may be used in connection with an asymmetric east-west oriented reflector field, e.g., depending on the latitude of the reflector field and/or in connection with an asymmetric north-south oriented reflector field, e.g., one that has been biased toward collecting sunlight at a certain time of day, e.g., during the afternoon hours.
Thus, the present application provides methods of establishing an optically inclined absorption aperture within a solar energy receiver having a plurality of linearly extending side-by-side absorber tubes, wherein a linearly extending reflector element is located at least in part within a cavity of the receiver so one side of the absorber tubes and is disposed to reflect incident solar radiation toward the absorber tubes.
Methods for biasing solar radiation collection in a solar energy collector system are disclosed here. These methods comprise reflecting solar radiation from reflectors in one or more reflector fields to an elevated receiver, and biasing the receiver to preferentially collect solar radiation from a subset of the reflectors. The methods may utilize any of the biased receivers as described herein. The methods may for example comprise preferentially collecting reflected solar radiation from reflectors located on an eastern side of the receiver, e.g., to preferentially collect solar radiation during afternoon hours. Some of these methods may comprise biasing the receiver to preferentially collect reflected solar radiation from a subset of the receivers to increase an annualized collection for the solar energy collector system.
The invention will be more fully understood from the following description of exemplary embodiments of the solar energy receivers and related methods and systems. The description is provided, by way of example, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a variation of a solar energy receiver having an optically inclined aperture.

FIG. 2 illustrates another variation of a receiver with an optically inclined aperture.

FIG. 3 illustrates another example of a receiver with an optically inclined aperture.

FIG. 4 illustrates yet another variation of a receiver with an optically inclined aperture.

FIG. 5 shows a largely diagrammatic representation of a LFR system that comprises a field of ground mounted reflectors that are arrayed in rows, and associated receivers.

FIG. 6 illustrates an example of a receiver with an effective absorption aperture that is optically inclined.

FIGS. 7A-7E illustrate another example of a receiver with an effective absorption aperture that is optically inclined.

FIG. 8 illustrates typical reflections of solar radiation toward an elevated receiver.

FIG. 9 illustrates an example of a solar array including a receiver having an optically inclined aperture.

DETAILED DESCRIPTION

Solar energy receivers are described here that are biased toward absorbing solar energy directed thereto from a certain direction. The receivers comprise a physical aperture that admits solar radiation into a receiver cavity that houses a solar radiation absorber. The physical aperture of the receiver may be substantially horizontal, and may open downward. The receivers also comprise an effective aperture that is optically inclined relative to a plane defined by the physical aperture, so that solar radiation from a certain direction is preferentially absorbed by the solar radiation absorber in the receiver. Thus, in an elevated receiver comprising a downward facing aperture defining a plane that is generally parallel to ground, the effective absorption aperture of the receivers described herein may be inclined relative to ground. Solar energy collector systems incorporating such receivers, and methods of collecting solar energy utilizing such receivers are also described herein.
In many solar energy collector systems, reflectors that are configured to direct solar radiation to an elevated receiver are arranged symmetrically with respect to that receiver. For example, in most LFR systems, the receiver or receivers and the respective rows of reflectors are positioned to extend linearly in a north-south direction, with the reflector fields symmetrically positioned relative to the receivers. The reflectors can be pivotally mounted and driven to track (apparent) east-west motion of the sun during successive diurnal periods. The reflectors may be driven through an angle approaching about 90° to track motion throughout a single diurnal period. East west extending LFR arrays have also been proposed. Examples of east-west extending LFR arrays are disclosed in DiCanio et al, Final Report 1977-79 DOE/ET/20426-1 and International Patent Publication No. WO 2008/022409, each of which is hereby incorporated by reference in its entirety.
In some cases, it may be desired to arrange reflectors in a solar energy collector system asymmetrically with respect to an elevated receiver that is configured to receive and absorb solar radiation directed thereto from the reflectors. For example, when the receiver is located at one end of a reflector field having north-south extending linear reflectors or when the receiver is positioned within an inherently asymmetrical reflector field having east-west extending linear reflectors. In both of these situations so-called cosine losses can occur at the receiver, with a resultant loss of collection efficiency. Reflectors may be asymmetrically arranged relative to a receiver to account for seasonal variations, which vary as a function of latitude at which the array is placed, or to account for daily variations in illumination. By making one or more fields in a solar array asymmetric with respect to a receiver, an array output may be increased during certain seasons and/or at certain times of day. Thus, a solar array may be configured to be asymmetric to increase output during high demand periods, high power price periods, relatively low insolation periods, to increase output near the end of a day, e.g., so as to shorten a thermal energy storage time requirement overnight, and/or to increase an annualized collection from the array. The biased receivers as described herein may be used in connection with any solar array configuration and/or reflector field configuration in which preferential absorption of solar radiation from a certain direction is desired, e.g., a symmetric or asymmetric east-west extending array, or a symmetric or asymmetric north south extending array.
Asymmetric arrays may be addressed in part by tilting the receiver so that the physical aperture of the receiver presents equally to radiation reflected from near and far field positions relative to the aperture axis. However, this approach may result in the establishment of thermal convection currents within the receiver and a consequential significant fall in collection efficiency.
Although the receivers comprising an optically inclined aperture are discussed herein primarily in connection with LFR arrays, it should be understood that the concepts described herein may be adapted to other types of receivers in other types of solar energy collector systems. For example, the receivers having optically inclined apertures may be adapted for any type of solar thermal energy collector system, photovoltaic system, or thermoelectric system.
Certain of the receivers described herein comprise a cavity having opposing side walls and a physical aperture defined between the side walls. A solar radiation absorber (which can be any suitable type of solar radiation absorber) is disposed within the cavity and arranged to be illuminated by solar radiation directed through the physical aperture. One or more optical elements are located at least partly within the cavity or proximate to the cavity so as to establish an effective absorption aperture that is inclined relative to a plane defined by the physical aperture. It should be noted that the effective absorption aperture may be larger or smaller than the physical aperture. Further, although adjusting the one or more optical elements affects the effective absorption aperture, in general the physical aperture to the receiver may be substantially unchanged by the adjustment of the one or more optical elements.
The one or more optical elements used to form the inclined absorption aperture can be any suitable elements that are capable of directing solar radiation to an absorber. In some circumstances, but not all circumstances, an optical element may function to capture solar radiation that passes through the physical aperture, but without the presence of the optical element, would not be incident on the absorber. Thus, an optical element may comprise a diffractive element, a refractive element, or a reflective element. For example, an optical element may comprise a diffraction grating, a graded index region, a lens, or a reflector. The configuration of the optical element within a cavity, partly within a cavity, or proximate to a cavity (e.g., adjacent to a cavity) may be selected so as to diffract, refract, or reflect a desired amount of incident solar radiation toward the absorber.
Referring now to FIG. 1, a schematic illustration is provided that shows one variation of an elevated receiver having an effective absorption aperture that is not parallel to (is inclined relative to) a plane defined by the physical aperture of the receiver. In FIG. 1, receiver 100 comprises a solar radiation absorber 101. The receiver 100 comprises a housing 111 having an interior cavity 102 that houses the absorber 101. Although illustrated as an inverted trough in FIG. 1, a housing may have any suitable shape and configuration, e.g., a housing may comprise a plate, a cylindrical housing, or a box-like housing. A downward opening physical aperture 104 admits solar radiation into the cavity 102 where it can be absorbed by absorber 101. Thus, as indicated by arrows 103, radiation that is reflected by one or more reflectors (not shown) can be directed through aperture 104. The physical aperture 104 can define a plane 105. The receiver 100 may be oriented such that the plane 105 is generally horizontal (i.e., generally parallel to ground). The receiver comprises a reflector element such that solar radiation directed through the physical aperture from a designated direction is preferentially absorbed over solar radiation directed through the physical aperture from another direction. Thus, in this particular example, the reflector element 106 is placed along a first side wall 107 of the cavity 102 of receiver 100. The reflector element 106 may be disposed within the cavity 102 as shown, or may at least partially extend outside of cavity 102, e.g., out through aperture 104. Solar radiation that is directed through the aperture 104 along a direction 112B is incident upon the reflective surface 108 of reflector element 106, and the reflective surface 108 is configured to reflect that solar radiation toward the absorber 101. Conversely, solar radiation that is directed through the aperture along direction 112A is incident upon a second side wall 109 of the receiver 101 and does not reach the absorber 101. The presence of the reflector element 106 along the first side wall 107 of the cavity 101 with no corresponding reflector element along the second side 109 of the cavity 101 results in an effective absorption aperture 110 that is inclined relative to the plane 105 defined by the physical aperture 104. Thus, receiver 100 is effectively tilted toward incoming solar radiation from direction 112B without physically tilting aperture 104.
Some variations of the receivers with optically inclined apertures may comprise two or more reflector elements. In those variations, the two or more reflector elements may have a variety of asymmetric configurations relative to an absorber in a receiver so as to make an effective absorption aperture that is asymmetric relative to the absorber. The two or more reflectors may be arranged to be asymmetric in the receiver with respect to the absorber in a variety of ways. For example, the reflector elements may have different lengths, be disposed at different angles, and/or be situated at different displacements relative to an absorber in a receiver. Reflectors may be curved, and/or may comprise multiple sections. Reflectors may be positioned on opposing sides of a receiver, or may be placed adjacent to each other.
An example of a receiver having two reflector elements of different lengths relative to an absorber in the receiver is illustrated in FIG. 2. There, receiver 200 comprises first and second reflector elements 206 and 213, respectively, that are directed outwardly from a base or rear portion 214 of the receiver toward physical aperture 204 in the receiver housing 211. The first and second reflector elements may be contained within a receiver cavity 202 that houses absorber 201, or one or both of the reflector elements may extend out of the cavity, e.g., through aperture 204. Incident solar radiation 203 enters the receiver cavity 202 through the physical aperture 204. A first reflector element 206 is disposed along a first side wall 207 of housing 211, and a second reflector element 207 is disposed along a second opposing side wall 209 of housing 211. However, because a length 215 of reflector element 206 is longer than a length 217 of reflector element 213, radiation from direction 212B is preferentially directed to absorber 201 over radiation from direction 212A, so that the effective absorption aperture 210 is tilted relative to a plane 205 defined by the physical aperture 204.
An example of a receiver comprising two reflector elements disposed at different angles relative to an absorber in the receiver is shown in FIG. 3. There, first and second reflector elements 306 and 313, respectively, are directed outwardly from a base or rear portion 314 of a cavity 302 of receiver 300, and toward physical aperture 304 in the receiver housing 311. The first and second reflector elements 306 and 313 are located at least partially within the cavity 302 housing the absorber 301, although in some cases one or both of the reflector elements may extend out of the cavity, e.g., through aperture 304. Incident solar radiation 303 enters a receiver cavity 302 that houses absorber 301 through the physical aperture 304. The first reflector element 306 is disposed along a first side wall 307 of housing 311 and is generally orthogonal to base 314. The second reflector element 313 is disposed along a second side wall 309 of housing 311 but is not generally orthogonal to the base 314, and is instead angled away from absorber 301. This asymmetry between the two reflectors 306 and 313 leads to an effectively asymmetric absorption aperture 310 that is able to preferentially admit radiation 303 from direction 312B over radiation from direction 312A. Thus, the effective aperture 310 is tilted relative to a plane 305 defined by the physical aperture 304.
Another variation of a receiver having an effectively tilted absorption aperture is shown in FIG. 4. There, the receiver 400 comprises first and second reflector elements 406 and 413, respectively. The first reflector element 406 is disposed along a first side wall 407 of the cavity 402, and the second reflector element 413 is disposed along a second sidewall 409 of the cavity 402. The reflector elements 406 and 413 are each located at least partially within the cavity 402 housing absorber 401, although in some cases one or both of the reflectors may extend out of the cavity 402, e.g., through aperture 404. In this particular variation, reflector element 406 is closer to absorber 401 than is reflector element 413. Therefore, the effective aperture 410 created by the two reflector elements 406 and 413 is effectively tilted relative to a plane 405 defined by the physical aperture 404. Solar radiation 403 that is directed through aperture 405 from direction 412B will be preferentially absorbed over radiation directed through aperture 405 from direction 412A.
Although the reflector elements illustrated in FIGS. 1 to 4 are depicted as having planar reflective surfaces for ease of illustration, a reflective surface of any of the reflector elements may be curved (concave or convex) or planar. In some cases a reflector may comprise multiple sections, which may for example be arranged in a generally circumferential manner with respective to a receiver. If a receiver comprises two reflectors, and the two reflector elements have different reflective surfaces with different curvatures, those differences in curvature may create or contribute to an asymmetric absorption aperture in the receiver. Further, in some cases, reflectivities of two reflector elements in a receiver may be different, which can also create or contribute to an optically inclined absorption aperture.
By optically tilting the aperture of a solar energy receiver, e.g., as described above, the benefits of tilting the aperture toward solar radiation from a certain direction can be realized without encountering difficulties that may be associated with physically tilting an aperture and/or a receiver, e.g., increased convective losses, instabilities in mounting, increased wind resistance, and the like. In some variations, and as will be described in more detail below, a physical aperture may be offset relative to a center of an absorber in addition to creating an inclined absorption aperture to further bias the receiver toward receiving and absorbing radiation from a particular direction.
The extent of inclination of an effective absorption aperture and, hence, the disposition of the reflector element may vary from one collector system installation to another and can be determined by such factors as the geometrical relationship of the receiver to the associated reflector field (in terms of height, reflection path angle, reflection path length, aperture width, etc.). However, the absorption aperture parameters for a given collector installation may readily be calculated trigonometrically by those skilled in solar collection field design. As described above, a reflector element may optionally extend beyond a physical aperture of a receiver cavity (e.g., a trough), depending upon the reflector field, and the geometric relationship of the receiver to the reflector field, with which the receiver is in use associated, but the reflector element desirably is located wholly within the trough.
In some variations, reflector element may be concave elliptical in curvature facing toward a solar radiation absorber, rather than flat. The elliptical reflector's two foci may be located, for example, with one focus at or near the edge of the absorber (e.g., an array of solar radiation absorber tubes) farthest from the elliptical reflector element and the other focus at or near the edge of the longer reflector field farthest from the receiver unit. Such an arrangement may allow a smaller absorber (e.g., a smaller absorber tube array, or a higher spatial concentration of absorber tubes) for a given aperture opening. Such an arrangement may create greater local peaks of radiation on the absorber tubes which may or may not be desired under some heat transfer regimes. Also, this particular approach may use larger reflectors, which may in some circumstances lead to relatively increased absorption in the reflector compared to variations using a flat reflector. It should be noted that reflector elements may have any suitable curvature and are not limited to flat, substantially flat, or elliptical as described above. In particular, reflector elements having any suitable curvature between flat and elliptical may be used. In two-reflector element variations, each reflector may have any suitable curvature and thus any suitable combination of curvatures may be used.
A reflector element used in a receiver may have any suitable construction and/or composition. For an elongated receiver cavity, a reflector element may be elongated and extend along side walls of a receiver cavity, e.g., as illustrated and described in connection with FIGS. 1 to 4 above. A reflector element may optionally comprise a polished metal element (e.g., a metal strip) or comprise a reflective coating on a metal substrate. In some cases, a reflector element desirably comprises a thermally stable (e.g., Pyrex™) silvered glass mirror. In some cases, a silvered coating may be laminated between two plates of a thermally stable glass, e.g., to protect the silver coating against heat damage and/or any other environmental exposure and/or contaminant.
A reflector element may be secured within a receiver, e.g., within a receiver cavity such as a trough-shaped cavity, using any suitable technique and fixture. In some cases, a reflector element may be secured within a receiver cavity in such a way that the reflector element and a receiver housing creating the receiver cavity can move relative to each other, e.g., to accommodate differential thermal expansion. For example, a reflector element may be mounted to a receiver housing (e.g., a trough) by way of mounting brackets or clasps that permit relative movement (e.g., sliding) between the reflector element and the trough. In other cases, a reflector element may be fixedly secured to a receiver housing (e.g., a wall of a trough), e.g., by fasteners, and/or an adhesive.
In certain variations, a position of a reflector element may be adjustable in a receiver so as to tune an optically inclined absorption aperture. For example, a reflector element may be positioned so as to increase or decrease a length that a reflector extends (from a rear portion or base of a receiver cavity that is opposite its physical aperture). A reflector element may be positioned so as to adjust an angle of the reflector element in the cavity so as to direct more or less radiation to the absorber. Such adjustments may be made manually or automatically.
In any of the variations of receivers comprising inclined absorption apertures, e.g., in connection with FIGS. 2-4, the solar radiation absorber can be any suitable absorber. In the case of receivers for LFR solar arrays, the absorber may comprise a plurality of longitudinally extending solar radiation absorber tubes that contain a heat transfer fluid, e.g., water and/or steam.
FIG. 5 illustrates a typical LFR system that may employ a receiver as described herein. As shown there, the LFR system 500 comprises a field of ground mounted reflectors 510 that are arrayed in rows 511 and further comprises parallel elevated receivers 512, each of which may be constituted by aligned receiver structures 513 and comprise a downward facing aperture 518. Several reflector rows 511 form reflector fields 540 that are disposed on opposite sides of elevated receivers 512. The reflectors may be supported on space frames 520 and supported and pivotally driven on hoop-like supports 516. The system as illustrated in FIG. 5 may be considered as a representative portion only of a, typically, larger LFR system, and the reflectors 510, frames 520 and supports 516 may for example be of the type described in International Patent Application No. PCT/AU2004/000883, filed Jul. 1, 2004, International Patent Application No. PCT/AU2004/000884, filed Jul. 1, 2004, U.S. patent application Ser. No. 12/012,821, filed Feb. 5, 2008, U.S. patent application Ser. No. 12/012,829, filed Feb. 5, 2008, and U.S. patent application Ser. No. 12/012,920, filed Feb. 5, 2008, each of which is incorporated herein by reference in its entirety.
The reflectors 510 are driven collectively or regionally, as rows or individually, to track movement of the sun (relative to the earth) and they are orientated to reflect incident radiation to respective ones of the elevated receiver 512. Also, some or all of the reflectors 510 may be driven so as to reorientate, when required, to change the direction of reflected radiation from receiver 512 to another.
As illustrated in FIG. 5, and as might typically be the case in an LFR system that has its reflectors 510 extending linearly in rows 511 in a north-south direction, each receiver 512 receives reflected radiation symmetrically from several (e.g., 4 to 20, 10 to 16, or 12) rows 511 of reflectors 510. Thus, in a typical north-south oriented LFR array, each receiver 512 receives reflected radiation symmetrically from the same number of reflector rows (e.g., 2 to 10 rows or 6 or 8 rows) at one side of the receiver as from the other side of the receiver (e.g., 2 to 10 rows or 6 or 8 rows).
Each row 511 of reflectors 510 and, hence, each receiver 512 might typically have an overall length of about 300 metres, and the parallel receivers 512 might typically be spaced apart by about 30 to about 35 metres. The receivers 512 may be supported at a height of approximately 11 to approximately 15 metres by stanchions 514 which may be stayed by ground-anchored guy wires 515, although other support arrangements might be employed, e.g., those described in U.S. patent application Ser. No. 12/012,920, filed Feb. 5, 2008, which has already been incorporated by reference herein in its entirety.
As indicated previously, each of the receivers 512 comprises a plurality of receiver structures 513 that are connected together co-linearly to form an elongated elevated receiver comprising a row of the structures. Each receiver structure might typically have a length of the order of about 12 meters and an overall width of the order of about 1.4 meters. In other variations, a receiver structure may have a length of about 10 meters to about 20 meters, and a width of about 1 meter to about 3 meters.
Apart from the inclusion of one or more reflector elements at least partially placed in a receiver cavity housing a solar radiation absorber, the receivers described herein may have a construction substantially similar to or the same as that described in the previously referenced International Patent Application No. WO2005/078360 or U.S. patent application Ser. No. 12/012,829, each of which is incorporated by reference herein in its entirety.
An example of a receiver construction is illustrated in FIG. 6. There, an end sectional view of a receiver unit 613 is shown. Receiver unit 613 may for example be used in a solar array such as that illustrated in FIG. 5. In this particular variation, the receiver unit 613 comprises an inverted trough 614 which might typically be formed from stainless steel sheeting and which, as best seen in FIG. 6, has a longitudinally extending channel portion 615 and flared side walls 616 that, at their margins, define a downwardly facing aperture of the trough. The trough 614 may be supported by and provided with structural integrity by side rails 617 and transverse bridging members 618, and the trough optionally may be surmounted by a roof 619, e.g., a corrugated steel roof. The void between the trough 614 and the roof 619 may be filled with a thermal insulating material 620.
A window 621 interconnects the side walls 617 of the trough. The window 621 may be formed from glass but it may in some instances be formed from a transparent heat resistant plastics material. The window 621 may be relatively planar or curved, as shown.
In the receiver 613 as illustrated in FIG. 6, a plurality (e.g., 4 to 20, or 10 to 16) of longitudinally extending absorber tubes 622 (e.g., stainless steel or carbon steel) are provided for carrying a heat exchange fluid, typically water or, following heat absorption, water vapour. However, the actual number of absorber tubes may be varied to suit specific system requirements, provided that each absorber tube has a diameter that is small relative to the dimension of the trough aperture between the side walls 616 of the trough. The plurality of absorber tubes 622 does, in the limit, effectively simulate a flat plate absorber, as compared with a single-tube collector in a concentrating trough. The absorber tubes 622 may be freely supported by a series of parallel support rails 623 which extend between side supports 624.
Another variation of a receiver configuration that may be used is provided in FIG. 7A-7E. In FIG. 7A, an end section view of receiver structure 713 is shown. Receiver structure 713 may for example be used in lieu of receiver structure 513 in a solar energy collector system such as that illustrated in FIG. 5. In this particular variation, the receiver structure 713 comprises an inverted trough 724, which may for example be formed from stainless steel sheeting. The trough 724 has a longitudinal channel portion 726 and side walls 727, which may be flared. The trough 724 may for example be similar to the trough illustrated in receiver unit 613 in FIG. 6. In the receiver structure 713, the trough 713 may be supported by and provided with structural integrity by longitudinal members 760 a-760 c and arches 762. Longitudinal members may be formed for example from tube steel and welded together, for example to form an approximately semi-cylindrical framework 764. Trough 724 may be further supported and provided with structural integrity by transverse bridging member 766 in framework 764. An outer shell, e.g., a smooth outer shell 768 of, for example, galvanized steel may be attached to framework 764 with for example adhesive. The smooth outer shell 768 may provide a low wind profile and shed water and thus may reduce structural (e.g., strength and/or rigidity) requirements of receiver structure 713, and/or reduce moisture ingress into the receiver. Optionally, a void between trough 724 and outer shell 768 may be at least partially filled with a thermal insulating material 732, which may comprise the same or similar materials as described above with respect to receiver unit 613 and which provides the functions there described.
A physical aperture 775 is defined between side walls 727 of trough 724, e.g., between slot 770 and ledge 772 that extend from side walls 727. The physical aperture 775 defines a plane 779. A longitudinally extending window 725 (which may comprise glass or heat resistant plastic) may for example be supported by the slot 770 and the ledge 772 (if present) to interconnect side walls 727 of trough 724 to form a closed, heat retaining cavity 733 within the trough. Optionally, gas (e.g., filtered air) may be fed into the cavity 733 via port 774, e.g., to provide a laminar flow along window 725 to remove dust or other contaminants. A window 725 may be unitary in nature, or may comprise several segments, e.g., lapped segments. Solar radiation 783 incident from reflectors (not shown) of an LFR array enters cavity 733 through the aperture 775. Additional variations of receivers and receiver structures, including window configurations and slot and ledge configurations are described in U.S. patent application Ser. No. 12/012,829, filed Feb. 5, 2008, which has already been incorporated by reference herein in its entirety.
Similar to receiver structure 613 illustrated in FIG. 6, receiver structure 713 comprises a plurality of longitudinally extending (e.g., stainless steel or carbon steel) absorber tubes 734 for carrying a heat transfer fluid (e.g., water and/or steam) to be heated by solar radiation absorbed by the tubes. The absorber tubes 734 may for example be supported by a rolling support tube 735 that may be configured to accommodate differential thermal expansion of the tubes during use. Further, an outside diameter of the tubes may be small relative to the aperture 775 that admits solar radiation into the cavity 733, e.g., so that the plurality of absorber tubes approximates a flat plate absorber. Other examples of tube configurations and supports that may be used are described in U.S. patent application Ser. No. 12/012,829, filed Feb. 5, 2008, which has already been incorporated by reference herein in its entirety.
Each of the absorber tubes in any of the receivers described herein may be coated along its length with a solar absorptive coating that comprise a solar selective surface coating that remains stable under high temperature conditions in ambient air, or for example a black paint that is stable in air under high temperature conditions. Examples of suitable solar spectrally selective coatings are disclosed in U.S. Pat. Nos. 6,632,542 and 6,783,653, each of which is incorporated by reference herein in its entirety.
In some cases, a physical aperture may be offset relative to a center of a solar radiation absorber, e.g., a plurality of absorber tubes. Such an asymmetric aperture arrangement may be used to accommodate an asymmetrical arrangement of reflectors around a receiver, as described in more detail below. Referring now to FIG. 7E, receiver structure 713 comprises a trough 724, with an aperture 775 defined between two sidewalls 727 (which may or may not be flared as illustrated). In this particular variation, slot 770 and ledge 772 extend from sidewalls 727, and therefore aperture 775 is defined between the slot 7770 and ledge 772. The aperture 775 is offset relative to a longitudinal center line 791 of the group of absorbers 734. Such an offset aperture may be arranged for example so that a ray 778 reflected by the outer edge of a first reflector row 712-1 farthest from the receiver on one side is incident at the largest angle α₁by which it may be incident on the absorber tube 734 nearest to reflector row 712-1, and so that a ray 780 reflected by the outer edge of a second reflector row 712-2 farthest from the receiver on the opposite side is incident at the largest angle α₂which it may be incident on the absorber tube nearest to reflector row 712-2. The angle α₂may not be equal to the angle α₁for an asymmetric array.
The receiver units as described thus far, e.g., in connection with FIGS. 6 and 7A-7E may be suitable for use in a symmetrical reflector field, e.g., as illustrated in FIG. 5. However, referring now to FIG. 8, in some cases illuminating solar radiation may be reflected from asymmetrical reflector fields 8A and 8B (with field 8B extending a longer distance than field 8A from elevated receiver 812 supported by stanchion 814). To accommodate such a situation and to reduce (cosine) collection losses due to fringing at the receiver, the receiver itself may be inclined slightly so that the receiver physical aperture 875 presents more favourably to the reflector field 8B. However, as previously indicated, such an approach may give rise to undesired convection losses, and this may be avoided in the receivers illustrated herein by the novel approach of mounting one or more reflector elements (e.g., flat or substantially flat reflector elements) at least partially within a cavity containing a solar radiation absorber.
Examples of possible configurations of such reflector elements are described and illustrated in connection with FIGS. 1 to 4 above. For example, as illustrated in FIGS. 2-4, two reflector elements may be mounted in receiver 812 with one at either side of the solar radiation absorber 822 (e.g., an array of absorber tubes). As illustrated in FIG. 2, the reflector elements may be of different lengths (i.e., extend downward from the receiver unit different distances), with the larger reflector element typically positioned facing toward, and on the on the far side of receiver 812 from, the longer reflector field (e.g., field 8B). In variations including two reflector elements, the reflector elements may both be flat or substantially flat, the reflector elements may both be elliptical, or either one of the reflector elements may be elliptical with the other flat. The foci of an elliptical reflector element in these variations may be located, for example, with one focus at or near the edge of the absorber (e.g., an absorber tube array) farthest from the elliptical reflector element and the other focus at or near the farthest edge of the reflector field faced by the elliptical reflector element. Similarly to the single elliptical reflector variations, use of one or more elliptical reflector elements in a two-reflector variation may allow a smaller absorber tube array (higher concentration) for a given aperture opening but may also cause relatively increased absorption in the reflector compared to variations using flat reflectors.
The reflector element configurations illustrated FIGS. 1 to 4 may be readily adapted to receiver units such as those illustrated in FIGS. 6 and 7A-7E. Referring to FIG. 6, receiver unit 613 comprises a reflector element 625, e.g., in the form of a silvered glass mirror positioned at least partially within the trough 614 and to one side of the absorber tubes 622 in a manner to establish an inclined absorber aperture 626 within the margins of the trough aperture. Thus, the receiver 613 may be oriented such that a physical aperture 675 defined between side walls 616 is directed substantially downward, the effective absorption aperture 626 is tilted away from a horizontal plane 676 defined by the physical aperture 675. In this particular example, only a single reflector element 625 is provided to form the inclined absorption aperture 626. However, any configuration of one or more reflector elements as described herein may be used to form an inclined absorption aperture. Further, in this particular variation, the reflector element 625 extends beyond a plane 676 defined by the physical aperture 675, but is still contained with the cavity 615 by virtue of curved window 621. In other variations, e.g., those discussed below in connection with FIG. 7A, a flat window may be used to close a physical aperture in a receiver, and a reflector element may not extend beyond a plane defined by the physical aperture.
Referring now to FIGS. 7A-7E, receiver structure 713 comprises a reflector element 750 which may be in the form of a silvered glass mirror as described above. The reflector element 750 is positioned at least partially within the cavity 733 defined by the trough 724. The reflector element 750, which may have a substantially planar reflective surface or a curved reflective surface as described above, reflects incident solar radiation 783 toward the absorber tubes 734. The presence of the reflector element along one side of the receiver unit 713 but not along the opposing side creates an inclined aperture similar to that illustrated in FIG. 1 and FIG. 6 above. The reflector element 750 may extend longitudinally along the receiver unit 713, e.g., generally parallel to side rails 760 a-760 c of frame 764. It should be noted that the physical aperture 775 may be offset relative to a longitudinal center line of the absorber tubes 734 in addition to being optically inclined by one or more reflector elements 750.
As indicated above, in some cases, the one or more optical elements used in the receivers to create an optically inclined absorption aperture may not be reflector elements but may instead comprise a refractive element or a diffractive element. Such elements may be placed along a side of a receiver as indicated for the reflector elements above, or they may incorporated into a window disposed on a physical aperture to a receiver cavity. Thus, referring back to FIGS. 6 and 7A, window 621 or window 725 may each comprise a diffractive portion or a refractive portion that results in preferential absorption of light by one side of the absorber. For example, a receiver window may comprise a diffraction grating, a graded index region, a lensed region, or may be wedged. If a grating is used, such grating may be formed, e.g., etched or molded, into a surface of the window.
In certain variations of the receivers, an absorber may be configured to be asymmetric to accommodate asymmetric illumination due to the optically inclined absorption aperture. An absorber may be made asymmetric in a variety of ways, e.g., by adjusting a shape or configuration of the absorber to favour illumination in one direction. For example, in the case of a solar radiation absorber that comprises a plurality of solar radiation absorber tubes, the tubes may be spaced asymmetrically, e.g., spaced closer together in regions of higher illumination.
Solar energy collector systems incorporating any type of reflector array, e.g., a reflector array that is oriented north-south, or east-west, and that has a reflector array that is symmetric or asymmetric with respect to a receiver, may be used in connection with the biased receivers as described herein. A solar array may be configured to use a biased receiver as described herein to increase output during high demand periods, periods of high energy prices, relatively low insolation periods, and/or to increase output near the end of a day so as to shorten a thermal energy storage time requirement overnight. For example, a solar array incorporating a biased receiver may allow for an overnight thermal energy storage period to be reduced from about 8 hours to about 4 hours by biasing collection towards a later part of a day.
For example, the biased receivers may be used in connection with a north-south oriented array. The biased receivers may be used to accommodate an asymmetric north-south array, or may be used even though the reflectors are arranged symmetrical with respect to the receiver. The biased receivers may for example be used with a north-south array so as to preferentially collect solar radiation at a certain time of day, e.g., during the morning or afternoon hours. In those variations, the reflectors in the north-south array may be configured to be asymmetric with respect to the elevated, biased receiver, e.g., as schematically illustrated in FIG. 8. For example, in an asymmetric north-south oriented array, there may be more reflectors on one side of a receiver than on another side of the receiver. Further, reflectors may be packed differently (e.g., spacings between reflector rows may be different) on one side of the receiver than on another side of the receiver, and/or reflectors on one side of a receiver may have a different distance to the receiver than reflectors on another side of the receiver. In any one of these asymmetric arrays, one or more receivers having a tilted effective absorption aperture may be used to accommodate asymmetry in the reflector field.
As stated above, an asymmetric array may be set up to allow preferential collection of light at certain times of day. For example, in an asymmetric north-south oriented array, the array may be configured to have more reflectors on the eastern side of the array to preferentially collect solar radiation during the afternoon hours. The distances between reflectors (e.g., inter-row spacings), and/or distances between reflectors and a receiver may different on the eastern side of an array relative to a western side of an array. Correspondingly, a receiver that is optically inclined toward the eastern side of the array may be used to preferentially receive and absorb solar radiation from the eastern side. In other variations, an asymmetric north-south oriented array may be configured to have more reflectors on the western side of the array to preferentially collect solar radiation during morning hours. Here again, the distances between reflectors (e.g., inter-row spacings), and/or distances between reflectors and a receiver may different on the eastern side of an array relative to a western side of an array. In those variations, a biased receiver that is optically inclined toward the western side of the array may be used.
The biased receivers may also be used in connection with east-west oriented solar arrays. For example, a solar energy collector system may be configured to be oriented east-west to increase an annualized collection from that system. Examples of east-west oriented LFR solar arrays are provided in International Patent Publication No. WO 2008/022409 and in U.S. Provisional Patent Application Ser. No. 61/007,926, each of which is incorporated by reference herein in its entirety.
The reflector fields in an east-west extending LFR array may be configured to be asymmetric with respect to a receiver, and may utilize the biased receivers as described herein to accommodate the asymmetry. For example, an east-west extending array may comprise a polar reflector field located on the polar side of a receiver, and an equatorial reflector field located on the equatorial side of the receiver. The polar reflector field may be configured to be different than the equatorial field, e.g., to increase an annualized collection of the array. Thus, an east west extending array may include asymmetric numbers of reflector rows and/or asymmetric row spacings between polar and equatorial reflector fields. In some cases, reflectors in the equatorial field may be located closer to the receiver than corresponding reflectors in the polar field. In any one of these asymmetric arrays, one or more receivers having a tilted effective absorption aperture may be used to accommodate asymmetry in the reflector field. For example, inter-row spacings in the equatorial field may be smaller than corresponding inter-row spacings in the polar reflector field. A polar reflector field may be configured to include more reflectors than an equatorial reflector field. Such configurations depend generally on the latitude at which the array is located.
An example of an asymmetric east-west extending LFR array (903) is depicted in FIG. 9. A polar reflector field (located to the northern side N of a center-line 901 of the receiver) in the case of an array located in the northern hemisphere) 910P comprises reflectors positioned in M parallel side-by-side rows 912P_{1, . . . , M}extending generally in an east-west direction, and an equatorial reflector field (located to the southern side S relative to center-line 901 of the receiver in the case of an array located in the northern hemisphere) 910E comprises reflectors positioned in N parallel side by side rows 912E_{1, . . . , N}also extending generally in an east-west direction. In the polar field, reflector row 912P₁is spaced apart from reflector row 912P₂by a spacing 915P_1,2, and so on. In the equatorial field, reflector row 912E₁is spaced apart from reflector row 912E₂by a spacing 915E_1,2, and so on. An elevated receiver 905 is configured to receive reflected solar radiation from the reflector fields 910P and 910E. The elevated receiver 905 is biased toward the polar side, and hence includes optical element (e.g., a reflector element) 906 configured to create an optically inclined aperture in receiver 905. Optical element 906 may for example be analogous to reflector element 750 illustrated in FIG. 7A or reflector element 625 illustrated in FIG. 6.
The reflectors in each field 910P and 910E are configured to reflect incident solar radiation (e.g., ray 913) to the receiver 905 during diurnal east west motion of the sun, and to be pivotally driven to maintain reflection of the incident solar radiation to the receiver during cyclic diurnal north-south motion of the sun. Additionally, the reflectors are pivotally driven to maintain reflection of the incident solar radiation to the receiver 905 during cyclic diurnal north-south motion of the sun in the (inclining and declining) directions indicated by arrow 21.
In some cases, an east-west solar array may be configured so that the number of reflector rows M in a polar field is greater than the number of reflector rows N in an equatorial field. For example, referring to FIG. 9, an angle of incidence θ₁(relative to a perpendicular axis Z of the reflector) for a reflector in an equatorial field may be greater than an angle of incidence of a reflector in a polar field. Thus optical aberrations such as astigmatism and the like may be decreased. Hence it may be possible that a reflector in a polar field may have a greater effective incident surface area as well as an ability to produce a better focus at the receiver than a corresponding reflector in the equatorial field positioned the same distance from the receiver. Hence, it may in some circumstances be advantageous to increase a number of reflectors in a polar field relative to that in an equatorial field. Correspondingly, a biased receiver as described herein may be used that has an effective absorption aperture inclined toward the polar field, but that is not necessarily physically tilted toward the polar field.
An east-west array may be made asymmetric between polar and equatorial arrays in other ways, e.g., by making a polar field or an equatorial field closer to an elevated receiver, and/or by making inter-row spacings in a polar field (e.g., 915P_1,2) different than inter-row spacings in an equatorial field (e.g., 915E_1,2). Examples of such arrays are provided in International Patent Publication No. WO 2008/022409 and in U.S. Provisional Patent Application Ser. No. 61/007,926, each of which is incorporated by reference herein in its entirety.
It should be understood that each of the features illustrated FIG. 9 for an east-west array apply also to north-south oriented arrays that may be used with the biased receivers as described herein. Thus, for a north-south oriented array, a solar array is envisioned wherein a receiver analogous to receiver 905 extends longitudinally in a north-south direction rather than in an east-west direction, and reflectors analogous to reflectors 912P_{1, . . . , M}are located on an eastern or western side of the receiver instead of on a polar side, and reflectors analogous to reflector 912E_{1, . . . , N}are located on an eastern or western side of the receiver instead of on an equatorial side.
Methods for collecting solar radiation are also disclosed here. In general, the methods comprise biasing the collection of solar radiation by reflecting solar radiation from reflectors in one or more reflector fields to an elevated receiver, and biasing the receiver to preferentially collect solar radiation reflected from a subset of the reflectors in the one or more reflector fields. Thus, the methods may utilize any elevated receiver having an optically inclined aperture as described herein to preferentially collect reflected solar radiation from a subset of the reflectors. Further, although the methods may be used in connection with reflectors arranged symmetric with respect to a receiver, in many cases, the methods may be used in connection with reflectors arranged asymmetric with respect to a receiver, as described above.
The methods may be adapted for preferentially collecting solar radiation during a certain time of year and/or during a certain portion of a diurnal cycle, e.g., by configuring the reflector fields to preferentially reflect solar radiation during the selected time period and arranging the optically inclined aperture of the biased receiver to receive and absorb the preferentially reflected solar radiation. Thus, the methods may be used in connection with asymmetric reflector fields to preferentially collect solar energy at a certain time of day and/or during a certain period in a year, e.g., during a certain season. In certain variations, the methods may be used in connection with an asymmetric east-west oriented reflector field, e.g., depending on the latitude of the reflector field and/or in connection with an asymmetric north-south oriented reflector field, e.g., one that has been biased toward collecting sunlight at a certain time of day, e.g., during the afternoon hours. The methods may be adapted for preferentially collecting solar radiation from a subset of the reflectors to increase an annualized collection from the solar energy collector system.
More specifically, the methods may be adapted for collecting solar radiation from one or more asymmetric reflector fields, where an amount of solar radiation reflected from a first side of the one or more reflector fields to the receiver is greater than an amount of solar radiation reflected from a second side of the one or more reflector fields. In those methods, the effective absorption aperture may be inclined toward the first side of the one or more reflector fields. Thus, the methods may comprise preferentially collecting reflected solar radiation from reflectors located on an eastern side of a north-south array, e.g., to preferentially collect solar radiation during afternoon hours, or preferentially collecting solar radiation from reflectors located on a western side of a north-south array, e.g., to preferentially collect solar radiation during morning hours. Certain methods may comprise preferentially collecting solar radiation from reflectors located on an equatorial side of an east-west array, or from a polar side of an east-west array. In some situations, e.g., when an array is east-west oriented, the methods may comprise preferentially collecting solar radiation from a subset of the reflectors to increase an annualized collection from the solar energy collector system.
As stated above, the methods may employ any of the biased receivers as described herein. Thus, methods may comprise reflecting solar radiation from one or more reflector fields through a physical aperture of an elevated receiver to be incident on a solar radiation absorber. The receiver used in these methods comprises an effective absorption aperture that is inclined relative to ground so as to preferentially receive and absorb solar radiation from a designated side of the receiver.
In the methods, the effective absorption aperture in the receiver may be established as described above, e.g., by mounting a first reflector element at least partly within a cavity of the receiver, the cavity housing the absorber, where the first reflector element is configured to reflect incident solar radiation toward the absorber and so establish an effective absorption aperture that is inclined relative to ground. Certain methods may comprise establishing the effective absorption aperture by mounting a first and a second reflector element at least partly within the cavity. In these methods, the second reflector element is arranged to be asymmetric in the receiver with respect to the first reflector element, and the first and second reflector elements are configured to reflect incident solar radiation toward the absorber and so establish an effective absorption aperture that is inclined relative to ground.
Thus, the present application provides methods of establishing an optically inclined absorption aperture within a solar energy receiver having a plurality of linearly extending side-by-side absorber tubes, wherein a linearly extending reflector element is located at least in part within a cavity of the receiver so one side of the absorber tubes and is disposed to reflect incident solar radiation toward the absorber tubes.
The embodiment of the invention as described with reference to the drawings is presented solely as an example of one possible form of the invention. Variations and modifications may be made in the invention as described without departing from the scope of the invention. This disclosure is illustrative and not limiting. Further modifications will be apparent to one skilled in the art in light of this disclosure and such modifications are intended to fall within the scope of the appended claims. Each publication and patent application cited in the specification is incorporated herein by reference in its entirety as if each individual publication or patent application were specifically and individually put forth herein.

Claims

1. A solar energy receiver comprising:

a cavity having opposing side walls and a physical aperture defined between the side walls;

a solar radiation absorber disposed within the cavity and arranged to be illuminated by solar radiation directed through the physical aperture; and

a first reflector element located at least partly within the cavity and configured to reflect incident solar radiation toward the solar radiation absorber and so establish an effective absorption aperture that is inclined relative to a plane defined by the physical aperture.

2. The solar energy receiver of claim 1, wherein the physical aperture is substantially parallel to ground.

3. The solar energy receiver of claim 1, wherein the side walls are two opposing side walls that are part of an inverted trough that defines the cavity.

4. The solar energy receiver of claim 1, wherein the solar radiation absorber comprises a plurality of solar radiation absorbing tubes arranged side-by-side in the cavity and extending longitudinally along the cavity, wherein the absorber tubes are configured to contain a heat transfer fluid.

5. The solar energy receiver of claim 1, wherein the first reflector element comprises a reflective surface having a concave curvature facing toward the solar radiation absorber.

6. The solar energy receiver of claim 1, further comprising a second reflector element located at least partly within the cavity and configured to reflect solar radiation toward the solar radiation absorber, wherein the second reflector element is arranged to be asymmetric in the receiver with respect to the first reflector element.

7. The solar energy receiver of claim 6, wherein the second reflector element comprises a reflective surface having a concave curvature facing toward the solar radiation absorber.

8. The solar energy receiver of claim 5, wherein the first reflector element comprises a reflective surface having an elliptical concave curvature facing toward the solar radiation absorber, and one focus of the reflective surface is at or near an edge of the absorber and the other focus is selected to be at or near an outer edge of a reflector field directing solar radiation to the receiver.

9. The solar energy receiver of claim 6, wherein at least one of the first and second reflector elements comprises a reflective surface having an elliptical concave curvature facing toward the solar radiation absorber, and one focus of the reflective surface is at or near an edge of the absorber and the other focus is selected to be at or near an outer edge of a reflector field directing solar radiation to the receiver.

10. The solar energy receiver of claim 9, wherein each of the first and second reflector elements comprise a reflective surface having an elliptical concave curvature facing toward the absorber and one focus of the reflective surface is at or near an edge of the absorber, and the other focus is selected to be at or near an outer edge of a reflector field directing solar radiation to the receiver.

11. The solar energy receiver of claim 6, wherein the first and second reflector elements have different lengths extending from a base of the cavity outwardly toward the physical aperture.

12. The solar energy receiver of claim 6, wherein the first and second reflector elements extend outwardly from a base of the trough at different angles so as to created an effective absorption aperture that is inclined relative to a plane defined by the physical aperture.

13. The solar energy receiver of claim 3, wherein the first reflector element is slidably secured to the trough.

14. The solar energy receiver of claim 1, wherein the first reflector element comprises a polished metal element.

15. The solar energy receiver of claim 1, wherein the first reflector element comprises a silvered glass mirror.

16. The solar energy receiver of claim 1, configured for use in a Linear Fresnel Reflector array.

17. A solar energy collector system comprising:

one or more reflector fields; and

an elevated receiver comprising a solar radiation absorber that is configured to receive and absorb solar radiation directed from the one or more reflector fields through a physical aperture of the receiver,

wherein the elevated receiver comprises an effective absorption aperture that is inclined relative to a plane defined by the physical aperture.

18. The solar energy collector system of claim 17, wherein the physical aperture is substantially parallel to ground.

19. The solar energy collector system of claim 17, wherein the one or more reflector fields are arranged asymmetric with respect to the elevated receiver, and the effective absorption aperture is inclined toward one side of the one or more reflector fields.

20. The solar energy collector system of claim 17, wherein one or more reflector fields is oriented in a north-south direction.

21. The solar energy collector system of claim 17, wherein one or more reflector fields is oriented in an east-west direction.

22. The solar energy collector system of claim 17, wherein the one or more reflector fields are configured and the effective absorption aperture of the receiver is inclined so as to preferentially collect solar radiation directed thereto at a certain period during a diurnal cycle.

23. The solar energy collector system of claim 22, configured so as to preferentially collect solar radiation directed thereto after noon.

24. The solar energy collector system of claim 17, wherein the one or more reflector fields are configured and the effective absorption aperture is inclined so as to preferentially collect solar radiation directed thereto during a certain time of year.

25. The solar energy collector system of claim 17, wherein the one or more reflector fields are configured and the effective absorption aperture of the receiver is inclined so as to increase annualized solar energy collection.

26. A method for collecting solar radiation, the method comprising reflecting solar radiation from a reflector field through a physical aperture of an elevated receiver to be incident on a solar radiation absorber, wherein the receiver comprises an effective absorption aperture that is inclined relative to the physical aperture so as to preferentially receive and absorb radiation directed thereto from a designated side of the receiver.

27. The method of claim 26, wherein the physical aperture is oriented substantially parallel to ground.

28. The method of claim 26, wherein an amount of solar radiation reflected from a first side of the reflector field to the receiver is greater than an amount of solar radiation reflected from a second side of the receiver, and the effective absorption aperture is inclined toward the first side of the reflector field.

29. The method of claim 26, comprising establishing the effective absorption aperture in the receiver by mounting a first reflector element at least partly within a cavity of the receiver, the cavity housing the absorber, wherein the first reflector element is configured to reflect incident solar radiation toward the absorber and so establish an effective absorption aperture that is inclined relative to ground.

30. The method of claim 26, comprising establishing the effective absorption aperture in the receiver by mounting a second reflector element at least partly within the cavity, wherein the second reflector element is arranged to be asymmetric in the receiver with respect to the first reflector element, and the first and second reflector elements are configured to reflect incident solar radiation toward the absorber and so establish an effective absorption aperture that is inclined relative to ground.

31. The method of claim 26, comprising configuring the reflector field and establishing the inclined effective absorption aperture for preferentially collecting solar radiation during a certain time of year.

32. The method of claim 26, comprising configuring the reflector field and establishing the inclined effective absorption aperture for preferentially collecting solar radiation during a certain part of a diurnal cycle.

33. The method of claim 32, comprising configuring the reflector field and establishing the inclined effective absorption aperture for preferentially collecting solar radiation during afternoon.

34. A method for biasing solar radiation collection in a solar energy collector system, the method comprising:

reflecting solar radiation from reflectors in one or more reflector fields to an elevated receiver; and

biasing the receiver to preferentially collect reflected solar radiation from a subset of the reflectors.

35. The method of claim 34, comprising utilizing an elevated receiver having an optically inclined aperture to preferentially collect reflected solar radiation from a subset of the reflectors.

36. The method of claim 34, comprising preferentially collecting reflected solar energy from reflectors located on an eastern side of the receiver to preferentially collect solar radiation during afternoon hours.

37. The method of claim 34, comprising preferentially collecting reflected solar radiation from a subset of the reflectors to increase an annualized collection from the solar energy collector system.

38. A solar energy receiver comprising:

a first optical element located at least partly within the cavity or proximate to the cavity so as to establish an effective absorption aperture that is inclined relative to a plane defined by the physical aperture.

39. The solar energy receiver of claim 38, wherein the first optical element diffracts incident solar radiation toward the absorber.

40. The solar energy receiver of claim 38, wherein the first optical element refracts incident solar radiation toward the absorber.

41. The solar energy receiver of claim 38, wherein the first optical element reflects incident solar radiation toward the absorber.