Patent Application
20110220094
The present disclosure relates generally to solar collection systems, and more particularly, to secondary reflectors, tertiary reflectors used with said secondary reflectors, solar collection systems having secondary reflectors, and solar collection systems having secondary reflectors and tertiary reflectors.
Current Linear Fresnel Reflector (LFR) systems generally include an array of parallel reflector lines focusing sunlight to a linear receiver above. The receiver may contain heat transfer fluids or may be photovoltaic or thermoelectric absorbers. The receiver may be mounted downward or to the side, but most current systems are downward-facing.
One drawback of such downward-facing LFR systems is that the performance of the outer mirrors or heliostats is relatively poor compared to that of the inner minors. This is because conventional downward-facing systems offer the largest apparent receiver aperture to the heliostats which have the smallest images (minors directly below the receiver) and the smallest apparent receiver aperture to those with the largest images (mirrors farthest from the receiver). Thus, while the heliostats directly below the receiver perform with relatively high efficiency, the heliostats farthest away from the receiver perform with relatively low efficiency since a large portion of the reflected solar radiation spills past the small apparent aperture of the receiver. In addition, this also has the effect of limiting the number of mirrors that can be practically or economically used, and accordingly, the flux concentration that can be achieved.
One solution that has been used in an attempt to solve the problem described above is to add a downward-facing secondary mirror placed above the receiver. This secondary mirror reflects solar radiation that spills past the receiver back onto the top of the receiver. While the additional minor increases the amount of solar radiation absorbed by the receiver, the shape and position of the mirror results in hot air becoming trapped against the surface of the mirror. The air causes the minor to increase in temperature, thereby possibly causing a degradation in the reflectance of the mirror and reducing the overall efficiency of the system. Furthermore, such arrangements tend to increase the average number of reflections required to strike the receiver, thereby diminishing system collection efficiency.
In one example, a collector system is described comprising at least two separately pivotable primary reflectors operable to reflect solar radiation and at least one solar receiver operable to receive solar radiation, wherein the at least one solar receiver is positioned on a level above the at least two separately pivotable primary reflectors, and the collection aperture of the at least one solar receiver is downward facing. The system further includes at least one secondary reflector operable to reflect at least a portion of a solar radiation reflected by the at least two separately pivotable primary reflectors onto at least a portion of the at least one solar receiver, wherein the at least one secondary reflector is positioned on a level below the at least one solar receiver, and wherein the secondary reflector comprises a first reflective surface.
In another example, the first reflective surface of the secondary reflector may form at least a portion of an ellipse. The ellipse may be defined by foci located at an end of one of the at least one solar receiver and a top edge of one of the at least two separately pivotable primary reflectors at its highest position of use. Alternatively, the ellipse may be defined by foci located with a portion of the solar receiver and a portion of one of the at least two separately pivotable primary reflectors. In another example, the first reflective surface of the secondary reflector may form at least a portion of a macrofocal ellipse.
In another example, the collector system may further include at least one tertiary reflector operable to enlarge the collection aperture of the solar receiver. In yet another example, the first reflective surface of the secondary reflector may form at least a portion of an ellipse having foci located at a portion of the solar receiver, a portion of the tertiary reflector, or a portion of one of the at least two separately pivotable primary reflectors.
In another example, the collector system may include two or more secondary reflectors. The secondary reflectors may be separated by a space operable to allow heated air to pass through.
In another example, the collector system may include at least one light barrier operable to at least partially block the solar radiation reflected by the at least one primary reflector. The light barrier may be a horizontal light barrier or a vertical light barrier.
In one example, a secondary reflector is described, the secondary reflector comprising a first reflective surface having a curvature defined by an ellipse, the reflective surface operable to reflect at least a portion of solar radiation reflected by a primary reflector directly onto a solar receiver while the secondary reflector is positioned on a level above the primary reflector and below the solar receiver.
In another example, the first reflective surface of the secondary reflector may form at least a portion of an ellipse defined by foci located with a portion of the solar receiver and a portion of one of the at least one primary reflector.
In yet another example, the secondary reflector may further include a light barrier configured to at least partially block a portion of the solar radiation directed towards a non-reflective surface opposite the first reflective surface. The light barrier may be a horizontal light barrier or a vertical light barrier.
The following description is presented to enable a person of ordinary skill in the art to make and use the various embodiments. Descriptions of specific devices, techniques, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those of ordinary skill in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the various embodiments. Thus, the various embodiments are not intended to be limited to the examples described herein and shown, but are to be accorded the scope consistent with the claims.
Various embodiments are described below relating to solar collection systems. In particular, a secondary reflector and solar collection system with one or more secondary reflectors are described below. The solar collection system may include primary and secondary reflectors for directing solar radiation to a solar receiver, e.g., a receiver having a plurality of absorber tubes. The secondary reflector may be positioned below the absorber tubes and configured to reflect solar radiation from the primary reflectors onto the absorber tubes, e.g., solar radiation reflected from the primary reflectors that might otherwise miss the absorber tubes. The absorber tubes may carry a heat transfer fluid that is heated during operation by solar radiation reflected by the primary and secondary reflectors to the absorber tubes. Additionally, the solar collection system may include one or more tertiary reflectors to increase the apparent aperture of the solar collector.
In one example, a group of primary reflectors 101 may be arranged in one or more rows and associated with a particular solar receiver 103. In another example, each row of primary reflectors 101 and, hence, each solar receiver 103 may have an overall length (into the drawing) of 400 meters. Solar receiver 103 may be supported at a suitable height above primary reflectors 101, e.g., a height of approximately 10 to 20 meters, by stanchions which may be stayed by ground-anchored guy wires. The width of primary reflectors 101 may be approximately 0.5 to 5 meters and adjacent primary reflectors 101 may be separated by 1 to 15 meters depending on both the width of the reflector chosen and its position in the field. One of ordinary skill in the art will appreciate that other similar or different sized reflectors and configurations may be employed.
As will be discussed in greater detail below, primary reflectors 101 may be driven collectively or regionally, as rows or individually, to track movement of the sun (relative to the earth) and to reflect incident radiation to respective ones of solar receiver 103.
Solar receiver 103 may include one or more absorber tubes 105 for absorbing solar radiation. Absorber tubes 105 may carry a heat exchange fluid (e.g., water or, following heat absorption, water-steam or steam, molten salt, air, or heat transfer oil). Absorber tubes 105 may be made from any thermally conductive material, such as aluminum, stainless steel, and the like.
Absorber tubes 105 may vary in diameter from about 25 mm to 500 mm depending on the number of tubes used and the overall size and optical concentration of the system. Further, the length of absorber tubes 105 may range from approximately 50 meters to 1,000 meters. Absorber tubes 105 may also be doubled around at one end to allow free thermal expansion of the tubes. Further, each of the absorber tubes 105 may be coated, along its length and around a portion of its circumference that is exposed to incident solar radiation, with a solar absorptive coating. The coating may comprise a solar spectrally selective surface coating that remains stable under high temperature conditions in ambient air or it may comprise a black paint that is stable in air under high-temperature conditions.
Solar receiver 103 may include any number of absorber tubes 105 to suit specific system requirements. In one example, solar receiver 103 may include between two and thirty absorber tubes 105. While solar receiver 103 has been described as comprising one or more absorber tubes 105, it will be appreciated by one of ordinary skill in the art that other types of solar receivers, such as photovoltaic or thermoelectric absorbers may be used as solar receivers 103.
In one example, as illustrated by
Solar collection system 100 may further include one or more secondary reflectors 107 for reflecting at least a portion of the solar radiation reflected by primary reflectors 101. Secondary reflectors 107 may be configured to reflect solar radiation from primary reflectors 101 onto one or more absorber tubes 105 of solar receiver 103. Secondary reflectors 107 may be made of any reflective material or any material coated with a reflective substance. For example, secondary reflectors 107 may be polished reflective metals or coated glass minors identical, similar, or different than primary reflectors 101.
In one example, as will be discussed in greater detail below with respect to
Secondary reflectors 107 may be disposed within solar collection system 100 using various techniques. For example, secondary reflectors 107 may be attached to a common structure supporting solar receiver 103 or one or more primary reflectors 101. Alternatively, secondary reflectors 107 may be supported by a separate structure. Further, mechanisms to rotate or translate secondary reflectors 107 may be included to adjust the position of secondary reflectors 107, e.g., for different positions of the sun, changes in relative positions of primary reflectors 101 and solar receiver 103, and so on.
In one example, solar collection system 100 may include two secondary reflectors 107 positioned below solar receiver 103 with the reflective surfaces of secondary reflectors 107 facing outwards in opposite directions. Secondary reflectors 107 may be further spaced a vertical distance below solar receiver 103, which may allow at least a portion of solar radiation reflected by primary reflectors 101 to directly strike a surface of absorber tubes 105. The reflection of solar radiation will be described in greater detail below with respect to
In one example, multiple solar collection systems 100 may be placed in a row. In such a configuration, the farthest primary reflector 101 of one solar collection system 100 is placed adjacent to the farthest primary reflector 101 of the neighboring solar collection system 100. Further, primary reflectors 101 may be driven collectively or regionally, as rows or individually, to track movement of the sun (relative to the earth) and to reflect incident radiation to respective ones of solar receiver 103.
In another example, primary reflectors 101 may be operable to flip and reorientate to reflect incident radiation onto either solar receiver 103 of two adjacent solar collection systems 100. Primary reflectors 101 may be redirected depending on the sun's position throughout the day. Mirror flipping provides improved system efficiency by placing more reflectors in an approximately sun-facing orientation that intercepts a greater amount of solar energy and reflects said radiation to the solar receiver 103 that will absorb the most solar radiation for that given position of the sun. Mirror flipping is described in greater detail in U.S. Pat. No. 5,899,199 and U.S. Pat. No. 6,131,565, which are incorporated herein by reference.
Mirror flipping improves the efficiency of solar collection system 100 by improving the efficiency of the primary reflectors 101 farthest from solar receiver 103. As will be discussed in greater detail below, sideways-facing secondary reflectors provide improved efficiency over downward-facing secondary reflectors for the primary reflectors farthest away from the solar receiver by providing the largest apparent aperture to the heliostats with the largest image (minors farthest from the receiver). Thus, using sideways-facing secondary reflectors increases the efficiency gained by mirror flipping. In one example, the performance benefit provided by minor flipping with sideways-facing secondary reflectors may range from 0% (when the sun is directly overhead) to approximately 4.5% in the late afternoon.
In another example, where primary reflectors 101 are operable to flip and reorientate to reflect incident radiation onto different solar receivers, the reflective surface of secondary reflector 107 may form an arc approximating a portion of an ellipse having a first focus located at or near the outer edge of solar receiver 103 (or outermost absorber tube 105) and a second focus located at or near the top edge of the outermost primary reflector 101 operable to reorientate towards that solar receiver 103.
In another example, instead of reflecting to a point, secondary reflector 107 may reflect an extreme ray from the top of the outermost primary reflector 101 to be tangent to the outer circumference of the outer absorber tube 105. In this example, the reflective surface of secondary reflector 107 may form an arc approximating a portion of a macrofocal ellipse.
In another example, the reflective surface of secondary reflector 107 may form an arc approximating a portion of a parabolic curvature with an optic axis parallel to the ray passing between the top of the outermost primary reflector 101 and the bottom of the secondary reflector 107.
In other examples, the reflective surface of secondary reflector 107 may form an arc approximating a portion of an ellipse having other foci than shown in
The exemplary shapes of secondary reflector 107 described above allow secondary reflector 107 to be placed in a substantially vertical orientation and at least partially below solar receiver 103. Positioning secondary reflector 107 in such a manner allows a sizeable fraction of rays coming in from the sides to directly strike the receiver. Further, the vertical orientation may also allow rays from reflectors close to the base supporting secondary reflectors 107 to bypass secondary reflectors 107. This reduces the average absorption loss in the reflectors. Additionally, the vertical orientation allows secondary reflector 107 to cool more rapidly than conventional horizontal secondary reflectors because hot air is allowed to rise up and away from the vertical reflector instead of being trapped against the underside of the horizontal reflector.
In one example, another secondary reflector 107 on the opposite side of solar collection system 100, as seen in
Thus, in one example, one or more secondary reflectors 107 may be positioned below solar receiver 103 to redirect at least a portion of the reflected solar radiation 102r from primary reflectors 101 that would otherwise pass by solar receiver 103. In this example, these portions of solar radiation may be reflected a second time by secondary reflectors 107 onto one or more absorber tubes 105 of solar receiver 103. Accordingly, more of the solar radiation reflected by primary reflectors 101 may be directed to solar receiver 103 either directly from primary reflectors 101 or from primary reflectors 101 via secondary reflectors 107 than without the secondary reflectors 107.
Secondary reflectors 107 may be positioned a vertical distance below solar receiver 103 to allow the reflected solar radiation aimed at solar receiver 103 to directly strike solar receiver 103 while still being operable to reflect at least some of the solar radiation that would otherwise pass by solar receiver 103. Since each reflection reduces the efficiency of energy transfer, disposing secondary reflectors 107 vertically below solar receiver 103 may improve the overall efficiency of solar collection system 100 by preserving much of the single reflection solar radiation (solar energy reflected by primary reflectors 101) delivered to solar receiver 103, and reflecting otherwise lost solar radiation to solar receiver 103 via secondary reflectors 107. For example, secondary reflectors 107 are disposed so as to provide secondary reflection for solar radiation that may pass by solar receiver 103, but to provide little or no secondary reflection for solar radiation that would directly strike solar receiver 103. In one example, secondary reflectors 107 may be placed such that the uppermost end of secondary reflectors 107 just intersect the most horizontal rays reflected by the outermost primary reflectors 101 that would directly strike the outermost point on solar receiver 103 if secondary reflectors 107 were not present.
In one example, solar collection system 100 may be configured such that approximately 50% of the solar radiation reflected by the outer primary reflectors 101 is aimed directly towards solar receiver 103. The remaining 50% of the solar radiation reflected by the outer primary reflectors 101 may be directed below solar receiver 103 and reflected by secondary reflectors 107. It should be appreciated by one of ordinary skill in the art that in other examples, different distributions of solar radiation reflected by primary reflectors 101 may be applied between solar receiver 103 and secondary reflectors 107.
While
In one example, if 50% of the rays collected from the outer reflectors use secondary reflectors 107 and none of the rays collected from the innermost reflectors use secondary reflectors 107, on average, approximately 25% of the total rays collected will use secondary reflectors 107. As a result, secondary reflectors 107 may add approximately 25% to the overall reflection loss of the system. For a system with a minor having a reflectance of 0.940, for example, the effective reflection loss from the primary would be 0.940 and from the secondary would be approximately 0.985 for a net of 0.926. This is better than conventional single pipe downward-facing secondary reflection systems which typically have between 50% and 100% of the total rays collected using the secondary reflectors. For example, for a non-imaging secondary such as a compound macrofocal elliptical concentrator (CMEC) (e.g., as described in Rabl, An and Winston, Roland, Ideal Concentrators for Finite Sources and Restricted Exit Angles, Applied Optics, Vol. 15, Issue 11, pp. 2880-2883 and Chaves, Julio, Introduction to Nonimaging Optics, Light Presciptions Innovators, page 3) having a secondary geometrical concentration of about 1×, the number of reflections in the secondary will be approximately 1 and the combined reflectance loss will be 0.940×0.940=0.884.
In another example, tertiary reflectors may be placed near the ends of the solar receiver to enlarge the apparent aperture of the receiver. This arrangement may allow a larger number of primary reflectors to be placed in the reflector field. Further, as will be discussed in greater detail below, this arrangement may also lessen ray spillage and increase the overall solar concentration on the solar receiver.
In another example, instead of reflecting to a point, secondary reflector 507 may reflect an extreme ray from the top of the outermost primary reflector 501 to be tangent to the outer circumference of the outer absorber tube 505. In this example, the reflective surface of secondary reflector 507 may form an arc approximating a portion of a macrofocal ellipse.
In the illustrated example, tertiary reflectors 515 are shown extending outwards from the top of each outer absorber tube 505. The reflective surfaces of the top portions of tertiary reflectors 515 may form an arc approximating an involute centered around the outer absorber tubes 505 of solar receiver 503. In one example, the top portion of tertiary reflector 515 may be the portion of the reflector located above the intersection point 519 between ray 517 and tertiary reflector 515. Ray 517 represents the ray reflected from the outer primary reflector 501 on the opposite end of solar collection system 500 that passes just below solar receiver 503. The reflective surfaces of the bottom portions of tertiary reflectors 515 (the portions below intersection point 519) may form an arc approximating a portion of ellipse 511 having focus 513 located at or near the outer edge of solar receiver 503 (or outermost absorber tube 505) and focus 514 located at or near the top edge of the outermost primary reflector 501 when said reflector top edge is at its highest position of usage.
In another example, instead of reflecting to a point, the bottom portion of tertiary reflector 515 may reflect an extreme ray from the outermost primary reflector 501 to be tangent to the outer circumference of the outer absorber tube 505. In this example, the reflective surface of the bottom portion of tertiary reflector 515 may form an arc approximating a portion of a macrofocal ellipse.
By positioning and shaping tertiary reflector 515 in the manner described above, solar radiation reflected by primary reflectors 501 and secondary reflectors 507 that would otherwise miss solar receiver 503 may be reflected onto solar receiver 503. It will be appreciated that other shapes and curves for tertiary reflector 515 may be used.
In one example, secondary reflectors 507 may be positioned a vertical distance below solar receiver 503 to allow the solar radiation aimed at solar receiver 503 and tertiary reflectors 515 to directly strike the receiver and tertiary reflector. For example, ray 521 (as well as rays aimed above ray 521) may be allowed to pass above secondary reflectors 507. This configuration increases the efficiency of solar collection system 500 by maximizing direct ray hits on the receiver and thus reducing the number of unnecessary reflections.
While
Although solar collection system 100 may be configured so that most of the solar radiation reflected by the primary reflectors is not incident to the underside of secondary reflectors 703 and 705, imperfections in the primary reflectors and imperfections in the arrangement of components of solar collection system 100 may cause stray reflections to strike the underside of secondary reflectors 703 and 705. Thus, in one example, the inner surfaces of secondary reflectors 703 and 705 may be coated with highly reflective and non-absorbing paint or material to reduce the amount of heat absorbed by the inner surfaces of secondary reflectors 703 and 705 due to reflections from the primary reflectors. For example, the inner surfaces of secondary reflectors 703 and 705 may be coated with a highly reflective white or silver coating such as paints containing Titanium Oxide. In other examples, secondary reflectors 703 and 705 may be constructed such that the silver coating is protected by glass on both sides to create a double sided reflector having a high reflectance for the inside surface of secondary reflectors 703 and 705.
In another example, secondary reflectors 703 and 705 may include light barrier 707 for blocking at least a portion of the stray reflections from the primary reflectors that may strike the underside of the secondary reflectors. Light barrier 707 may be configured to connect to the bottom portions of each reflector. Light barrier 707 may be positioned horizontally, or close to horizontally, relative to the ground. The surface of light barrier 707 may be highly reflective and non-absorbing. For example, the surface of light barrier 707 may be coated with a highly reflective white or silver coating. Additionally, when light barrier 707 is used, the inner surfaces of secondary reflectors 703 and 705 may be coated with a coating having a high emissivity to reduce heating. For example, the inner surfaces of reflectors 703 and 705 may be coated with a black paint or glass surface having a high infrared emissivity. In one example, the horizontal light barrier 707 may include holes or apertures designed to admit air for upward circulation and cooling of the underside of the secondary reflectors. In another example, light barrier 707 may also be designed to minimize light intrusion.
In another example, illustrated by
Although a feature may appear to be described in connection with a particular embodiment, one skilled in the art would recognize that various features of the described embodiments may be combined. Moreover, aspects described in connection with an embodiment may stand alone.
