HELIOSTAT CHARACTERIZATION USING STARLIGHT

Abstract

The present invention offers an improvement to existing canting, slope error, and/or pointing measurement approaches, by using one or more cameras to observe the reflections of points of light in the firmament, such as the reflections of stars and/or planets as visible within the night sky in the heliostat facets. An illustrative heliostat measurement system comprises a plurality of heliostats, and at least one camera that observes at least one heliostat. The heliostats reflect an image of the firmament that can be observed by the at least one camera. The system further comprises (i) at least one captured image of the firmament reflected from at least one of the heliostats; and (ii) a computer comprising programming that determines a heliostat imperfection from the captured image, wherein the heliostat imperfection is selected from at least one of a slope error, a canting error, and a pointing error.

Claims

1. A method of measuring one or more heliostat imperfections selected from at least one of a slope error, a canting error, and a pointing error, comprising the steps of: a) providing a plurality of heliostats and at least one camera that observes at least one heliostat, wherein the heliostats reflect an image of the firmament that can be observed by the at least one camera; b) reflecting an image of the firmament from at least one of the heliostats: c) using at least one camera to capture the reflected image of the firmament; d) using the image comprising the reflected image of the firmament to measure the heliostat imperfection.

2. The method of claim 1, wherein the image of the firmament comprises an image of starlight, and wherein step (c) comprises using at least one camera to capture an imaging comprising reflected starlight, and wherein step (d) comprises using the reflected starlight to measure the heliostat imperfection.

3. The method of claim 1, wherein step (b) comprises reflecting an image of the firmament from a plurality of the heliostats, step (c) comprises capturing reflected images of the firmament from the plurality of the heliostats, and step (d) comprises measuring heliostat imperfections of the plurality of the heliostats.

4-8. (canceled)

9. The method of claim 1, wherein step (d) comprises: i. comparing the position of a reflected point of the firmament in a captured image with data regarding known positional information of the point; and ii. determining an error with respect to at least one of a facet slope error, a facet canting error, and a heliostat pointing alignment.

10. (canceled)

11. The method of claim 1, wherein step (c) comprises capturing an image map of the firmament and step (d) comprises using the image map to determine an imperfection of a heliostat facet.

12. The method of claim 1, wherein step (c) comprises recording a firmament illumination pattern as the heliostat is controllably articulated to a predetermined orientation.

13. The method of claim 1, wherein step (c) comprises recording a firmament illumination pattern as the heliostat is controllably articulated along a predetermined path.

14. The method of claim 1, wherein step (c) comprises using a plurality of cameras.

15. The method of claim 1, wherein step (c) comprises using a plurality of cameras at multiple locations.

16. The method of claim 1, wherein step (c) comprises observing the reflections of a star at a plurality of points on a heliostat facet.

17. The method of claim 1, wherein step (c) comprises observing a plurality of points on a heliostat while keeping the heliostat stationary.

18. The method of claim 1, wherein step (c) comprises observing a plurality of star transits on a heliostat;

19. The method of claim 18, further comprising the step of, after observing a star transit, articulating the heliostat to a different position and observing an additional star transit.

20. The method of claim 1, wherein step (b) comprises controlling a heliostat to reflect a point of the firmament from a specific point on the heliostat.

21. The method of claim 1, wherein an imperfection corresponds to an image shift of a reflected image of the firmament.

22. The method of claim 1, wherein an imperfection corresponds to an image distortion of a reflected image of the firmament.

23. (canceled)

24. The method of claim 1, wherein the step (b) comprises reflecting an image of a planet; step (c) comprises capturing the reflected planet image; and step (d) comprises using the image comprising the reflected image of the planet to measure the heliostat imperfection.

25. A heliostat measurement system, comprising: a) a plurality of heliostats, and b) at least one camera that observes at least one heliostat, and wherein the heliostats reflect an image of the firmament that can be observed by the at least one camera; and wherein the system further comprises (i) at least one captured image of the firmament reflected from at least one of the heliostats; and (2) a computer comprising programming that determines a heliostat imperfection from the captured image, wherein the heliostat imperfection is selected from at least one of a slope error, a canting error, and a pointing error.

26. The system of claim 25, wherein the system comprises a plurality of captured images of starlight reflected from a heliostat.

27. The system of claim 25, wherein the system comprises a plurality of captured images of starlight reflected from a plurality of heliostats.

28-31. (canceled)

32. The system of claim 25, wherein the system comprises a plurality of captured images of a planet reflected from a heliostat.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0088] FIG. 1 illustrates a starlight observation capability coupled to a concentrating solar power system.

[0089] FIG. 2 illustrates starlight rays being reflected from a heliostat facet.

[0090] FIG. 3 illustrates how a camera views a reflection of the firmament from a heliostat facet.

[0091] FIG. 4 is an image of the firmament formed by a camera viewing the heliostat facet of FIG. 3.

[0092] FIG. 5 shows a star transiting across a stationary heliostat due to earth rotation.

[0093] FIG. 6 shows how an improperly canted facet results in displacement of a segment of a star transit.

[0094] FIG. 7 shows how slope error results in the distortion of the star transit.

[0095] FIG. 8 shows how a plurality of transits may be used to build a more complete map of the heliostat mirror surface.

[0096] FIG. 9 illustrates transits that may result from appropriately commanded heliostat movements.

[0097] FIG. 10 shows how slope and canting errors are measured via a method where stars are controlled to appear at specific points on the heliostat.

[0098] FIG. 11 illustrates how pointing error measurements can be made by pointing at multiple stars.

DESCRIPTION OF THE INVENTION

[0099] Embodiments described herein are exemplary and do not represent all possible embodiments of the principles taught by the present invention. In particular, embodiments of the present invention have direct application in the field of concentrating solar power, particularly concentrating solar power including the use of heliostats to redirect sunlight onto a fixed focus in which concentrated sunlight may be converted into other forms of energy such as heat or electrical energy. Nevertheless, the apparatus and methods described herein can be applied and adapted by those skilled in the art for use in alternative applications in which the optical characteristics of a mirror must be measured from a distance.

[0100] As shown, in FIG. 1, an exemplary CSP system 1 can include an array of heliostats 3 that redirect and concentrate sunlight onto a focus area 7 of a tower 17. Each heliostat 3 may include one or more mirror facets 5.

[0101] An embodiment of the invention includes a digital imaging device, preferably a camera 9, which can observe the reflections of stars or planets in one or more of the individual facet mirrors 5 of one or more heliostats 3. The camera 9 may be mounted on the power plant's central tower 17, but it may also be mounted in any convenient place which provides a desirable vantage point for viewing one or more heliostats 3. Multiple cameras at multiple locations may be used.

[0102] Canting and slope error, as discussed above, may be measured by observing the reflections of stars at a plurality of points in each heliostat facet 5. Heliostat pointing, also discussed above, can be measured by observing the reflection of one or more stars or planets a one or more points in the heliostat mirrors.

[0103] In one embodiment, a plurality of points may be obtained while keeping the heliostat stationary. As the earth rotates, stars or planets are observed to transit the heliostat. This is shown in FIG. 5. FIG. 5 shows heliostat 81, which is oriented so as to reflect a portion of the night sky into a camera such as camera 9 of FIG. 1. FIG. 5 shows the scene as would be observed by the camera. Note that the heliostat is intentionally shown at an angle, to underscore the fact that the heliostat need not be facing the camera, and in fact, may be in any orientation that reflects the night sky into the camera.

[0104] As the earth rotates, the reflection 83 of a star may appear in one of the facets 85, as shown. As the earth continues to rotate, the image of the star will tend to transit across the face of the heliostat mirror, tracing out an arcing path 87. The figure shows the transit path for a heliostat in which all the facets are flat, and all parallel to each other. For heliostats with other facet shapes, such as being slightly concave, the transit path can also be determined using known mathematical formulas of geometry. In any case, it is one aspect of the present invention to determine an expected path based upon the occurrence of a star reflection at a given location. In certain methods of the present invention, it may be desirable to compare a determined path to the actual transit for making measurements in accordance with the present invention.

[0105] If a flat-faceted, parallel-canted heliostat has a facet with a canting error, the transit will be offset in the corresponding facet, as shown in FIG. 6. In FIG. 6, heliostat 91 has facet 93 that is canted upward from its correct canting, and a segment 97 of transit 95 is displaced vertically as a result. Similar offsets would be seen with other faceted shapes as well, although the path may be different.

[0106] Similarly, FIG. 7 shows a heliostat 101 in which a facet 103 has a slope error. In this case, a segment 107 of transit 105 is distorted.

[0107] By capturing and recording this transit, preferably digitally from an imaging device, we can infer the mirror normal at each point where we see the reflection of the star, since we know the star position (by virtue of knowing the time and the location on the earth) and the relative geometry of the camera and heliostat.

[0108] In order to build up a more complete map of a heliostat, in one embodiment, continued observations can be conducted, waiting for one or more additional star transits to occur. As shown in FIG. 8, a rich map of the heliostat mirror surface from a plurality of transits 111 can be eventually built up. This map can then be translated directly into canting and slope measurements.

[0109] In another embodiment, rather than passively waiting for stars or planets to transit, the heliostat can instead be repointed after a transit is complete, essentially allowing a repeat of the transit at a slightly different position. This is shown in FIG. 9. Here a nominal transit 121 is illustrated. At the end of this transit, the star image leaves the heliostat at point 123. After recording this transit, the heliostat can be moved so that the same star reappears at point 125. Then, by waiting for a period, a star transit 127 can be observed.

[0110] By commanding appropriate motions of the heliostat, as many transits as are desired may be obtained. The figure shows additional transits 129, 131, 133, 135, and 137.

[0111] In another embodiment, the use of greater heliostat control can eliminate the need to wait for transits. For example, the heliostat can be controlled to move so as to reflect the star from a specific point on the heliostat. This is shown in FIG. 10. Control includes a step of commanding the heliostat to position the star at point 141. That can be followed up by additional steps to command the star to points 143, 145, and 147.

[0112] Moving on to the next facet, further commands to move the reflection of the star to points 149, 151, 153, and 155 can be performed. As is illustrated in FIG. 10, the second facet has a slope error at point 153. As a result, the star image appears shifted. This shift can be converted directly to slope error, and the shift of groups of stars can be converted to canting error.

[0113] Finally, an embodiment of measuring overall heliostat pointing error is illustrated in FIG. 11. Here, the heliostat is controlled so as to point at stars 161, 163, and 165 in the firmament, with the goal of placing the reflection of each star image exactly in the center of the one of the heliostat's facets, such as at point 167. While the center of a facet is convenient, any predetermined point or set of points may be used.

[0114] In this example, the heliostat is able to correctly point at stars 161 and 163, but the image of star 165 appears at an incorrect point 169. This information (both the correct pointing of 161 and 163, and the pointing error for star 165 (the distance between actual image point 169 and desired image point 167) form pointing measurements that can be used to solve the heliostat geometry.

[0115] The following methodologies for making measurements in accordance with aspects of the present invention are noted.

[0116] The ability to make a slope measurement at a point on a mirror is equivalent to measuring the mirror's normal vector at that point. Making a slope measurement for a point on a mirror therefore comprises the steps of: [0117] 1) Observing a star at some point in a heliostat mirror. [0118] 2) Computing the mirror normal vector for that point on the mirror by applying the law of reflection. Based on the position of the camera, position of the star, and position of the mirror, the normal vector is the bisector of the mirror-to-camera vector and the mirror-to-star vector.

[0119] Making a canting measurement for a mirror facet comprises the steps of: [0120] 1) Making one or more slope measurements at one or more points on the mirror facet. [0121] 2) Averaging or otherwise combining the slope measurements to produce a net slope. This net slope is the canting of the facet.

[0122] Making a pointing measurement for a heliostat comprises the steps of: [0123] 1) Positioning the heliostat so that it reflects a star into the camera. [0124] 2) Observing the position of the star in the heliostat mirror. [0125] 3) Comparing the observed position to the predicted position based on the geometric model of the heliostat, the position of the star, and the position of the camera. The difference is the error in the prediction. [0126] 4) Repeat steps 1 through 3, looking at different stars and/or at different times, to collect additional data points. The exact number required depends on the geometric model being used, but can be as few as one or as many as four or even more. [0127] 5) Solve for an improved geometric model based on the measured prediction errors. Any technique familiar to one skilled in the art may be used, including least squares, Bayesian estimation, or the like. Some techniques may operate in batch mode on all collected points at once, while other techniques may process each point as it arrives, improving the geometric model iteratively.

[0128] [92] Also in accordance with the present invention, a means is also preferably provided for digitally assisting with the measurement aspects of the present invention and more preferably with the analyzing and comparing captured digital images with other digital information of the stars. Star information can be obtained electronically in many ways, such as by utilizing the data from the Yale Bright Star Catalog, which can be downloaded from http://tdc-www.harvard.edu/catalogs/bsc5.html. Such a means can comprise one or more general purpose computers. A computer can include software for capturing the image as taken from an imaging device. The computer can include or have access to a data base with star information. The computer can also include digital data comparative programming (as commercially available) for comparing captured images to known data, namely star data for the present invention. From such a comparison, measurements can be determined based upon the above noted methodologies for the type of measurement to be determined. Analytical software can also be utilized for calculating the measurements utilizing star data, heliostat positioning and geometries. It is contemplated as well that a digital image can be compared to a known or image produced based upon known data by any visual examination including that of a human observer.

[0129] All patents, patent applications, and publications cited herein are incorporated by reference as if individually incorporated. Unless otherwise indicated, all parts and percentages are by weight and all molecular weights are number average molecular weights. The foregoing detailed description has been given for clarity of understanding only. No unnecessary limitations are to be understood therefrom. The invention is not limited to the exact details shown and described, for variations obvious to one skilled in the art will be included within the invention defined by the claims.