Variable Density Heliostat Field Layout
A heliostat array having a variable field density for use in a concentrating solar power (CSP) plant. Heliostats are arranged into subgroups and configured to track the sun and reflect light to a receiver tower. Heliostats are deployed onto structures, wherein the structures can be arranged in rows separated by a service gap. The structures can comprise cross members that can be varied in size. By altering the size of cross members in a structure, heliostats in one row can be deployed farther apart or closer together than heliostats in a different row. Heliostat field density can vary with distance from the receiver tower, wherein heliostats close to the receiver are more tightly packed and heliostats further from the receiver are spaced farther apart. Heliostat subgroups can exhibit variable heliostat density using one or more of the following features: variable spacing of heliostats within the same row, variable spacing of heliostats mounted to the same structure, or by varying the width of the service gap between rows. The result is a field configuration that reduces the blocking and shading of heliostats by their neighbors.
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The present application claims priority to and the benefit of U.S. Provisional Patent Application No. 62/027,735, filed on Jul. 22, 2015, the entire disclosure of which is incorporated herein by reference.BACKGROUND OF THE INVENTION
This disclosure relates generally to a field layout of heliostats in a concentrating solar power system. In particular, the invention relates to an improved configuration of heliostat support structures, wherein the heliostats are distributed in a varying density depending on their proximity to a receiver.
In Concentrating Solar Power (CSP) plants, arrangements of heliostats reflect sunlight toward a receiver mounted atop a tower containing a working fluid. One type of receiver transfers incident radiant energy to the working fluid to produce high-pressure, high-temperature steam through the means of a heat exchanger or a phase change of the working fluid itself. The working fluid can be water, air, or a salt material heated to a molten state. The output steam can facilitate a variety of applications, such as electrical power generation, enhanced oil recovery, and desalination. Heliostats are generally mounted on the ground in an area facing or surrounding the receiver tower. Each heliostat has a reflector: a rigid reflective surface, such as a mirror, that tracks the sun through the actuation of a heliostat drive mechanism about at least one axis. Sun-tracking involves orienting the reflector throughout the day so as to optimally redirect sunlight from the sun toward the receiver and maintain the desired temperature of the working fluid.
Arrays of heliostats can be arranged into a plurality of subgroups comprising a field. The subgroups are configured to provide a preferred orientation that facilitates efficient land usage, optimizes the amount of solar flux delivered to the receiver, minimizes the blocking of outer heliostats by inner heliostats, and balances total system costs. One approach to constructing a heliostat field is to utilize a small amount of comparatively large heliostats (e.g., having an area between about 50 m2 and 150 m2). In such a power plant, having a fewer number of heliostats can necessitate the manufacture of very precise, and thus very expensive, components for the positioning of the reflective surfaces. Another approach, however, is to use a large amount of comparatively small heliostats (e.g., having an area between about 1 m2 and 10 m7), such as with reflective surfaces that measure between about 1 m and 3 m on each side. Such an approach can be more efficient at redirecting sunlight because there are more individually adjustable reflective surfaces. In addition, smaller heliostats can be cheaper to produce and easier to assemble, decreasing installation time and operations costs. The use of smaller heliostats does present its own set of challenges, however. In order to be cost effective, heliostats may require framed bracing to stabilize them when exposed to wind. To minimize the amount of bracing used, it is preferred that heliostats be deployed close together in groups or rows having shared supporting members. This increased packing density can result in some heliostats blocking and shading other reflectors depending on the sun's position or their distance from the receiver. For a large arrangement having evenly spaced heliostat rows this issue is exacerbated for the rearmost reflectors that must lower their angle of attack to reflect light onto the receiver. When a heliostat mirror is shaded or blocked, this lowers the amount of solar flux that can be delivered to the receiver. The plant must compensate for this reduction in incident energy by focusing more heliostats onto the receiver tower. Reducing the shading on heliostats therefore minimizes the amount of reflector area that must face the receiver in order to deliver the same amount of requisite flux.
One solution to this problem is to adequately space the heliostat rows from each other and to widen the gap between adjacent heliostats in a row to prevent blocking and shading of proximate reflectors. However, because the angle of reflection of light from a reflector to the receiver is lower the further the heliostat lies from the base of the tower, the optimal heliostat layout that minimizes blocking and shading while conserving land use is not the same at the front and rear sections of a large array. One option is to vary the density of heliostats in the array such that heliostats furthest from the receiver are spaced the furthest apart. This would have the side-effects of increasing the cable length required to supply power and communication distribution pathways to the rows as well as increase the size of brace members needed for structural support. The height of the receiver tower could be increased to present a more accessible target for rearmost heliostats, but this substantially increases receiver tower costs for very large heliostat fields. Therefore there exists a need to achieve variable heliostat field density while limiting overall cable length, bracing structure usage, arid receiver tower height.SUMMARY OF THE INVENTION
An improved heliostat field layout for a concentrating solar power field is described herein, wherein the heliostat field layout comprises subgroups of heliostats having a variable field density. A heliostat subgroup comprises a plurality of heliostats arranged in predetermined configuration. In the preferred embodiment of the present invention the heliostats are deployed in rows, though they can also be deployed in other suitable arrangements, such as in a radial pattern. The density of a heliostat field is considered to be the number of heliostats deployed within a defined land area. Field density can be altered by varying the distance between adjacent heliostat rows or by varying the spacing between heliostats within the same row.
A heliostat comprises a drive assembly that can actuate about two axes (for example, azimuth and elevation) and a reflector mounted to the drive. Drives are operated via control boards installed within the drive assembly. Commands issued to individual heliostats originate from a control system within a plant network and are delivered via a communication distribution topology comprising inter-drive connector cables. Each heliostat is installed onto the post of a structure fastened to the ground. Heliostat structures can comprise a single post or can comprise multiple posts for mounting multiple heliostats. Multiple structure posts can be linked via cross members and can be arranged such that the posts and cross members form a geometric shape. Cross members can be connected to each other along the span between two post members at one or more points. Additional cross members can also be included to provide additional support for the structure.
In an example of a preferred embodiment of the present invention, the heliostat structure comprises three post members, wherein each post member is connected to the other two posts via at least two cross members to form a triangle. A single triangle structure is called a “pod”. The triangular arrangement allows the structure to sit on irregular ground surfaces without a loss of contact. The triangle structures are arranged in a hexagonally-packed layout wherein adjacent triangles alternate their orientation such that two adjacent triangular structures form two parallel rows comprising three heliostats each. Multiple adjacent triangle structures in a line thereby always establish two rows of heliostats, collectively called a “row-pair”. Adjacent heliostat row-pairs can be separated from each other in the heliostat array by a gap. This gap can be sized so as to provide access for maintenance vehicles or service crews for repair and cleaning of heliostats.
Adjacent heliostats in a row are connected to each other via inter-drive cables, wherein the inter-drive cables facilitate both communication and power-delivery via constituent wiring. Each heliostat control board is integrally connected to an inter-drive cable having a male cable end and an inter-drive cable having a female end. The male or female end of one of the heliostat's inter-drive cables can be connected to the compatible cable end of an adjacent heliostat to connect the drives in series. Cable trays can be situated at one or both ends of the row-pairs or between heliostats in a row-pair to rout power to heliostats from corresponding buses and connect heliostats to the plant network. Communication interface modules can be mounted to the cable trays. The communication interface modules are capable of interfacing with inter-drive cables and can serve as intermediaries between the plant network and the heliostats. The cable trays can comprise field cables capable of interfacing with the communication interface modules to provide power and communication pathways to and from the plant network. Inter-drive cables can connect heliostats in the row-pairs to the communication interface modules mounted on the cable trays.
Heliostats can have their inter-drive cables daisy chained together, such that the same transmission line supplies power and facilitates data throughput to multiple units in a single subgroup. Heliostats in alternating and adjacent rows can also be connected to each other via inter-drive cables to create redundant power and communication transmission pathways. For example, communication interface modules can be connected to the nearest heliostats in separate, adjacent row-pairs while the heliostats at the ends of adjacent rows in the same row-pair can be connected to each other, thereby creating a power and communication transmission loop. In the event that a single component in the loop malfunctions, power and communication to an entire subgroup or a substantial portion thereof can still be maintained.
As described previously, a heliostat array exhibiting variable field density is advantageous for limiting receiver height and reducing the blocking of heliostats by their neighbors. Designing the heliostat structures to accommodate variable heliostat density allows for customization of the array in response to various plant site requirements, such as those impacted by the terrain or topography. One method of varying the field density is to lengthen or shorten the service gap between heliostat row-pairs. This increased distance can help minimize blocking and shading effects on adjacent heliostat rows.
An additional method of varying the heliostat field density is to lengthen or shorten the distance between heliostats in the same row by altering the dimensions of the structure cross members. Heliostat structures in the same or different subgroups may have the same or different cross member sizes, with the size of the cross members corresponding to the distance of the heliostat row-pair from the receiver tower. The cross members can have a fixed length or can comprise retractable or elongating members that are adjustable during field deployment. If the triangle heliostat structures are widened in a first direction to increase the distance between heliostats in a row, this can reduce the width of the triangle structures in a second direction. Thus even if the number of heliostats in a row decreases due to reduced field density, the drives in the row can still be linked with the same total length of inter-drive cable, saving on material costs and easing field deployment.
Inter-drive cables connect heliostats together by interfacing with the respective inter-drive cable connectors that are attached to the drive control boards. The cables thereby facilitate the distribution of power to the heliostat drives and create data communication pathways for monitoring, feedback, and control. Cables between heliostats in the same pod can be attached to the triangle structures with the use of one or more fastening devices such as twist ties, clamps, clips, wire, adhesive, or other suitable methods. The fastening devices serve to minimize the movement of the cables due to wind and to provide strain relief. Inter-drive cables are routed between heliostats in adjacent pods or in adjacent row-pairs by hanging the cables between structural members or by positioning them on a support member. Examples of support structures can include a wire, a rigid member, a flexible member, a slot, or an enclosed tube. A support may be used to keep the cable raised off the ground to provide additional strain relief. Cables can be routed along the outside of the surface of the heliostat triangle structures or routed within the internal surface of the post and cross members. Cables can be connected to each other in a variety of configurations. For example, chained inter-drive cables can be run along a single row, looped around the end of the row, and then extended to connect to the nearest structure of an adjacent row. Alternatively, chained inter-drive cables can be run along the cross members of a single triangle heliostat structure and then extended to connect to the nearest adjacent structure of the same or adjacent row.
is configured in a hexagon shape having a variable heliostat field density. The plurality of heliostat subgroups comprises a fixed subgroup  closest to the receiver tower , wherein the first subgroup has a hexagonal shape and comprises heliostats deployed in a constant field density in the manner shown in
Alternative embodiments of a variable density heliostat field may have configurations in other shapes, such as circles, polygons, heptagons, or other suitable n-gons. For example, heliostat fields can be deployed in a radial configuration in concentric circles around a central receiver, wherein the heliostat rows and row-pairs are no longer parallel to each other. Heliostats can also be deployed to face one or more receiver towers from one or more directions. All, or some, fraction of the heliostat subgroups can exhibit variable heliostat density using one or more of the following features described herein: variable spacing of heliostats within the same row, variable spacing of heliostats within the same pod, or variable spacing of the service gap width between heliostat row-pairs.
In yet another alternative embodiment, heliostat rows and row-pairs may be arranged lengthwise pointing directly away from a receiver tower. The triangular structures of heliostat pods in these rows may be equilateral and all heliostat pods may have the same dimensions. Variable density of the heliostat field may be achieved by varying the longitudinal spacing between adjacent pods within a heliostat row or row-pair, wherein the space between adjacent pods is determined as a function of their distance from the receiver.
In all of the preceding embodiments, varied spacing between heliostats may ensure that adjacent heliostats do not block reflected solar flux from, or shade incident solar flux onto, nearby adjacent reflectors in the same or adjacent rows or row-pairs. As distance from a receiver tower increases, the angle relative to the ground at which heliostats must be oriented to reflect sunlight towards the receiver must necessarily decrease, increasing the chances for blocking or shading. A heliostat field having a variable density as described in any of the aforementioned preferred embodiments will improve flux delivery in a concentrating solar power plant and obviate the need for a taller receiver or shorter, less densely-packed heliostats.
Various combinations and/or sub-combinations of the specific features and aspects of the above embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments may be combined with or substituted for one another in order to form varying modes of the disclosed invention. Further it is intended that the scope of the present invention herein disclosed by way of examples should not be limited by the particular disclosed embodiments described above.
1. A heliostat array comprising:
- a plurality of heliostats mounted onto structures;
- a plurality of heliostat subgroups comprising at least one heliostat structure, wherein the density of heliostats varies throughout at least one of the subgroups; and
- a receiver tower facing the plurality of subgroups from at least one side.
2. The heliostat array of claim 1, wherein each heliostat comprises a two-axis drive assembly and a reflector mounted to the drive assembly.
3. The heliostat array of claim 2, wherein the heliostats in the array are configured to track the sun and reflect light from the reflector onto the receiver tower.
4. The heliostat array of claim 2, wherein each heliostat structure comprises at least one structure post having an interface for attaching to the drive assembly of at least one heliostat, wherein the structure posts are fastened to the ground.
5. The heliostat array of claim 4, wherein adjacent structure posts are interconnected by cross members.
6. The heliostat array of claim 1, wherein the heliostats in each subgroup are arranged in rows, and wherein rows are separated by a service gap.
7. The heliostat array of claim 7, wherein the service gap between heliostat rows in the same subgroup or in different subgroups varies depending on the distance of the rows from the receiver tower.
8. The heliostat array of claim 7, wherein the distance between adjacent heliostats mounted to the same structure varies depending on the distance of the structure from the receiver tower.
9. The heliostat array of claim 7, wherein the distance between adjacent heliostats in the same row varies depending on the distance of the row from the receiver tower.
10. The heliostat array of claim 7, wherein adjacent heliostats in row or mounted to a structure are connected to each other via inter-drive cables that distribute both power and data communications.