Abstract:
Methods and systems for measuring heliostat reflectivity with a control processing unit configured to receive an image of a heliostat, receive an image of the Sun, process the received images, and determine a reflectivity estimate based on a comparison of the processed images.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and the benefit of U.S. Provisional Patent Application Ser. No. 61/543,923, filed Oct. 6, 2011, which is hereby incorporated by reference in its entirety for all purposes. 
    
    
     TECHNICAL FIELD 
     The invention relates to the field of concentrated solar thermal power and heliostat fields and more particularly to devices, systems, and methods for measuring the reflectivity of portions of solar collector arrays. 
     BACKGROUND 
     The performance of a solar thermal power system is dependent on the magnitude of the solar flux delivered by a heliostat field. Many factors affect the delivered flux, including, for example, the soiling of heliostat mirrors by dust or other contaminants. 
     Measuring the reflectivity of a heliostat field is a difficult task using prior art. Standard practice is to position a handheld reflectometer on the surface of each heliostat to be tested. This process is time-consuming and expensive, but more importantly it is subject to systematic biases. The users of the device may systematically choose parts of the mirror to measure that are clearer or dirtier than average, depending on their incentives. Furthermore, such devices typically only sample a subset of the solar spectrum and do not necessarily measure the amount of light reflected in a solid angle that is most relevant to the solar thermal application. 
     SUMMARY 
     Exemplary device embodiments may include a thermopile, where the thermopile has radiation shielding, and a camera configured to produce digital imagery. The thermopile may be connected proximate to the camera and the field of view of the thermopile may be within the field of view of the camera. The radiation shielding of the thermopile may have an opening disposed in a plane perpendicular to the aperture of the camera. The camera may have a neutral density filter, have a resolution of at least five megapixels, and/or be a pinhole camera. The device may have at least one gimbal for re-orienting the device. 
     An exemplary system embodiment may have a control processing unit having a processor and addressable memory where the control processing unit may be configured to receive an image from an imaging device that has a field of view, receive a reading of total energy absorbed by a thermopile that has a field of view within the field of view of the imaging device, determine a total number of pixels in the received image that contain part of a image of the solar disc, and determine a reflectivity estimate based on the determined total number of pixels in the received image that contain part of the image of the solar disc and the received reading of total energy absorbed by the thermopile. In other embodiments, the determined reflectivity estimate may be further based on an angular size of a pixel in the received image and a surface brightness of the solar disc. In additional embodiments, the control processing unit may be further configured to calibrate the angular size of a pixel based on the number of pixels in a received image of an object with a known size and where the object is a known distance from the imaging device. In additional embodiments, the control processing unit may be further configured to receive a measurement of the direct normal insolation provided by the solar disc, receive an angular size of the solar disc, and determine the surface brightness of the solar disc based on the received measurements of direct normal insolation and angular size. In further embodiments, the control processing unit may be further configured to determine a scaling factor based on a received reading of total energy absorbed by a thermopile for a received image of a surface with known reflectivity. The determined reflectivity estimate may be further based on the scaling factor. In additional embodiments, the scaling factor may be the product of the total number of pixels in the received image containing part of an image of the solar disc and the known reflectivity of the surface divided by the received reading of total energy absorbed by the thermopile. In additional embodiments, the determined reflectivity estimate may be the received reading of total energy absorbed by the thermopile divided by the product of the total number of pixels in the received image containing part of an image of the solar disc and the scaling factor. In additional embodiments, the determined reflectivity estimate may be the received reading of total energy absorbed by the thermopile divided by the product of the total number of pixels in the received image containing part of an image of the solar disc, the angular size of a pixel in the received image, and the surface brightness of the solar disc. 
     Another exemplary system embodiment may have a control processing unit having a processor and addressable memory where the control processing unit may be configured to receive an image of a heliostat from a imaging device having a field of view taken at a first point in time; receive an image of the Sun from the imaging device having a field of view taken at a second time; determine a brightness of the received image of the heliostat; determine a brightness of the received image of the Sun; and determine a reflectivity estimate based on a comparison of the determined brightness of the received image of the heliostat and the determined brightness of the received image of the Sun. In additional embodiments, the control processing unit may be further configured to: receive a measurement of the direct normal insolation provided by the Sun at the first point in time; receive a measurement of the direct normal insolation provided by the Sun at the second time; scale the brightness of the received image of the heliostat based on the received measurement of the direct normal insolation provided by the Sun at the first point in time; and scale the brightness of the received image of the Sun based on the received measurement of the direct normal insolation provided by the Sun at the second time. In additional embodiments, the determined brightness of the received image of the heliostat and the determined brightness of the received image of the Sun may be a Pixel Intensity Metric (PIM), where the control processing unit is further configured to determine PIM by detecting a blob of the Sun in a received image, eroding away pixels on the edge of the blob, and calculating an average pixel value from the remainder. In additional embodiments, the control processing unit may be further configured to: process the received image of a heliostat from the imaging device having a field of view taken at the first point in time; and process the received image of the Sun from the imaging device having a field of view taken at the second time; where the processing is based on at least one of: a camera calibration constant, a camera non-flatness correction profile, a camera non-linear response correction profile, and a camera bias subtraction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which: 
         FIG. 1  depicts an exemplary reflectivity measurement system connected to a solar thermal receiver positioned between two heliostat arrays; 
         FIGS. 2A-2F  depict the positioning of an exemplary reflectivity measurement system; 
         FIG. 3  depicts an exemplary reflectivity measurement system; 
         FIG. 4A  is a view of an exemplary solar thermal power system with a reflectivity measurement system measuring heliostat field reflectivity; 
         FIG. 4B  is a view of an exemplary solar thermal power system with a reflectivity measurement system measuring direct normal insolation provided by the Sun; 
         FIG. 5  is an exemplary digital image of a plurality of heliostats; 
         FIG. 6  is an exemplary digital image of a surface with known dimensions and surface reflectivity; 
         FIG. 7  depicts another exemplary embodiment of a reflectivity measurement system; and 
         FIG. 8  is a flowchart of another exemplary embodiment of a reflectivity measurement system. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  depicts an exemplary reflectivity measurement system  108  connected to a solar thermal receiver  106  positioned between a first heliostat array  102  and a second heliostat array  104 . In an exemplary embodiment utilizing a tower, the reflectivity measurement system  108  may be mounted on a solar thermal receiver  106  at an elevation providing a field of view of one or more heliostat arrays  102 , 104 . Mounting the reflectivity measurement system  108  in such an elevated and/or central position may reduce the movement required by the heliostats of the one or more heliostat arrays  102 , 104  to be measured. The reflectivity measurement system  108  may be fixed in position or may be connected by a mount, e.g., a gimbal, to an outer surface of the solar thermal receiver  106  allowing for the rotation of the reflectivity measurement system  108  about two or more axis. In an alternative embodiment, the reflectivity measurement system  108  may be mounted on preexisting structures such as a first separate tower  110  and/or a second separate tower  112 , e.g., camera towers, that are proximate to the one or more heliostat arrays  102 , 104 . 
       FIGS. 2A-2F  depict the positioning of an exemplary reflectivity measurement system  108 . The reflectivity measurement system  108  may be fixed directly (not shown) to a mounting surface  204  or may be attached to the mounting surface  204  via a gimbal  202 . Calibration of the reflectivity measurement system  108  may be readily effected when the reflectivity measurement system  108  is gimbaled and repeatedly reoriented. In an exemplary embodiment, the reflectivity measurement system  108  may be aimed at the Sun in order to determine the direct normal insolation (DNI) provided by the Sun (see  FIG. 4B ).  FIG. 2A  depicts a side view of an exemplary reflectivity measurement system  108  having movement about at least two axis. The reflectivity system  108  may be re-oriented along a vertical axis, as in  FIG. 2B , to a new position, e.g., aimed at a selected heliostat, as in  FIG. 2C .  FIG. 2D  depicts a front view of an exemplary reflectivity measurement system  108 . The reflectivity measurement system may be re-oriented along a horizontal axis to align with one or more heliostats of a heliostat array, as in  FIGS. 2E-2F . 
       FIG. 3  depicts an exemplary reflectivity measurement system  108  embodiment. The system may be constructed by rigidly mounting a thermopile  302  immediately adjacent to a camera  304 , e.g., within 10 cm, where the thermopile  302  and the camera  304  face in the same direction. The thermopile  302  may be shielded from all radiation except for a subset of the light that enters from a set of directions that are also visible in the field of view of the camera  304 . The reflectivity measurement system  108  may be connected to one or more control processing units  314  having a processor  310  and memory  312  addressable with a data bus  308 . The one or more processing units  314  may be used to determine values, e.g., a solar disc pixel count and a reflectivity estimate, based on inputs from the camera  304  and the thermopile  302 . This reflectivity measurement system  108  may then be mounted in a location, in the vicinity of one or more heliostat arrays, and aimed at a set of one or more heliostats in the one or more heliostat arrays, such that the set of targeted heliostats reflects sunlight into the camera  304  and simultaneously onto the thermopile  302 . The camera  304  may feature a small aperture  306  and/or a neutral density filter to prevent damage to an imaging sensor of the camera  304  from flux, flaring, or other distortion of the image. 
     A challenge in estimating the number of pixels with Sun presence is that some pixels may contain only a part of the image of the Sun. For this reason, a high-resolution camera  304 , e.g., 5 or more megapixels, may be used to capture images of the Sun reflected in the one or more heliostats. The ratio of the number of edge pixels to non-edge pixels in the image of the Sun is directly proportional to the angular length of each pixel. The number of “edge” pixels, and hence, the opportunity for error, diminishes in higher resolution images, because the total number of pixels with Sun presence scales as the square of the angular length of each pixel. The angular size of a pixel may be provided by the camera manufacturer. Otherwise, it may be calibrated by imaging objects of known sizes at known distances from the camera. 
     The amount of absorbed radiation is measured using a thermopile  302 . In order to accurately measure a quantity relevant to solar thermal power plants, it is appropriate for this device to include power over all wavelengths, or to use a device painted with a similar material, as is used on the surface of the solar receiver. If this system is used to determine effective reflectivity for a concentrated photovoltaic application, the thermopile  302  may be replaced with a photometer which integrates over the solar spectrum weighted by an approximation of the sensitivity of the photovoltaic receiver. 
       FIG. 4A  is a view of an exemplary solar thermal power system with a reflectivity measurement system  108  measuring heliostat field reflectivity. To obtain an estimate of heliostat reflectivity, the heliostat to be measured  408  may be reoriented to be aimed at the reflectivity measurement system  108 . The reflectivity measurement system  108  may be repositioned to point towards the heliostat to be measured  408  (see  FIGS. 2A-2F ) such that the surface of the heliostat to be measured  408  is within the field of view  404  of the reflectivity measurement system  108 . In some embodiments, additional heliostats  410  may continue to reflect flux from the Sun  402  onto a solar thermal receiver  106 . 
     The reflectivity measurement system  108  uses one or more processing units having a processor and memory addressable with a data bus to estimate heliostat field reflectivity by taking in measurements from a thermopile in combination with a digital camera. The reflectivity measurement system  108  may be positioned relatively very far, e.g., 100 m, from the heliostat to be tested  408 . By comparing the solid angle of sunlight as seen by the camera to the heat received by the thermopile, an estimate of reflectivity may be made. 
       FIG. 4B  is a view of an exemplary solar thermal power system with a reflectivity measurement system  108  measuring DNI provided by the Sun  402 . The surface brightness of the Sun  402  is the total amount of DNI provided by the Sun divided by the angular size of the Sun. The angular size of the Sun  402  is well known, and may be determined for a given time point from astronomical data. The DNI may be measured by either aiming the field of view  404  of this system, e.g., the sensor suite that may comprise at least one of the thermopile and the digital camera, directly at the Sun  402  frequently, or by using a separate system dedicated to measuring this quantity, e.g., a pyrheliometer or a well-calibrated camera mounted on a tracking device (not shown). The heliostats  408 , 410  of the at least one heliostat array may continue to reflect flux from the Sun  402  onto a solar thermal receiver  106 , while the DNI is measured by the reflectivity measurement sensor  108 . If the atmosphere between the camera and the reflector surface being measured is not transparent, the attenuation of sunlight along this path may also be taken into account when estimating the expected radiation. Devices to measure the transmission of the atmosphere are readily available for purchase. Expected radiation should be scaled as:
 
Expected radiation including attenuation=Expected radiation otherwise*exp(−Distance between camera and heliostat/Length over which sunlight loses 1 /e  of its intensity due to atmospheric interference)
 
       FIG. 5  is an exemplary digital image of a plurality of heliostats  502 , 504 , 506 , 508 , 510 . The thermopile measurement area  514  must be less than the field of view of the camera  512  to obtain an accurate reading. In some embodiments, more than one heliostat  502 , 504 , 506 , 508 , 510  at a time may be measured. A reflectivity estimate may be calculated by dividing the total amount of energy absorbed by the thermopile by the total number of pixels in the camera&#39;s image which contain part of an image of the Sun, and scaling the result appropriately:
 
Reflectivity=(Absorbed Radiation)/(Expected Radiation for a perfectly reflective surface)=(Absorbed Radiation)/((Number of pixels with Sun presence)*(Solid angular size of a pixel)*(Surface brightness of the Sun as seen from the location of the experiment)
 
     In the above equation, a key observation is that the amount of radiation expected is directly proportional to the number of pixels with Sun presence in the camera&#39;s image. The reason why this holds is because the Sun has nearly uniform surface brightness—neglecting, of course, the limb darkening effect. As viewed from a camera, the image of the Sun on a flat reflector will appear as a nearly circular disk, independent of the orientation of this surface with respect to either the Sun or the camera, and independent of the distance between the reflector and the camera. The pixels containing an image of the Sun  516  form a nearly circular disk around the pixels containing no image of the Sun  518 , with edge pixels  520  between both. If the image of the Sun is only partially present in the reflector as seen from the camera, the expected radiation incident on the thermopile is concordantly less. If the reflector surface is not flat, the image of the Sun will be distorted, and the expected radiation from the Sun may be higher or lower due to this effect. In any case, however, the expected radiation maintains its direct proportionality with the number of pixels of the camera&#39;s image in which the image of the Sun is present. 
       FIG. 6  is an exemplary digital image  512  and thermopile measurement area  514  of a surface  602 , e.g., a cleaned heliostat, with known dimensions (D 1 , D 2 ) and surface reflectivity. In some embodiments, the reflectivity measurement system may be calibrated based on manufacturer specifications. In other embodiments, a calibration sample may be required to obtain the angular length of a pixel and/or reflectivity. For example, one may obtain a measurement of reflectivity from a surface with a known reflectivity, e.g., a clean heliostat, and use this to determine a scaling factor between the number of illuminated pixels and the absorbed radiation:
 
Scaling Factor=(Number of pixels)*Known Reflectivity/(Absorbed radiation)
 
     Using the scaling factor, the reflectivity of a surface which was otherwise unknown can be determined:
 
Reflectivity=(Absorbed radiation)/(Number of pixels)*(Known Scaling Factor)
 
       FIG. 7  depicts another exemplary reflectivity measurement system  108 . In this exemplary embodiment, a single optical camera  704  is placed on the reflectivity measurement system and mounted to a tower near one or more heliostat arrays, e.g., a solar thermal receiver tower or optional camera tower, and aimed at the one or more heliostat arrays. The camera  704  may have a relatively narrow field of view, e.g., 3-4 degrees across. The reflectivity measurement system  108  may aim the camera  704  at a particular heliostat, which will reflect sunlight into the camera. The camera  704  may have a gain, filtering, and exposure setting so that the image of the Sun may not be saturated in the camera  704 . The reflectivity of the particular heliostat may be determined by comparing the brightness, measured via the pixel value of the captured image, of the Sun-image in the heliostat to a similar image taken by redirecting the camera  704  to capture an image of either the Sun directly (see  FIG. 4B ), or the Sun-image in a reflector of known reflectivity, e.g., a clean mirror (see  FIG. 6 , without a thermopile measurement area). The reflectivity measurement system  108  may also comprise a separate device dedicated to measuring DNI  702 , e.g., a pyrheliometer, which must be placed relatively close, e.g., 100 m or less, to the camera  704 . This device  702  may be located near the heliostat field or on a separate tower and/or preexisting structure. The delay between the DNI measurement and the image of the heliostat to be measured should be minimized to yield a more accurate result. Any error introduced by such a delay depends on the variability of the DNI in both space and time. For example, on a clear day, a reasonable error due to this effect may be obtained by measuring DNI within approximately five minutes of obtaining an image and/or by a distance of the DNI measuring device  702  to the camera  704  of approximately 100 m. On a day with variable weather conditions, the time between the measurements obtained and/or the distance between the measuring devices may need to be reduced to obtain a result within an acceptable range of error. The reflectivity measurement system  108  may be connected to one or more control processing units  714  having a processor  710  and memory  712  addressable with a data bus  708 . The one or more processing units  714  may be used to determine values, e.g., a solar disc pixel count and a reflectivity estimate, based on inputs from the camera  704  and the DNI measuring device  702 . 
       FIG. 8  is a flowchart of another exemplary reflectivity measurement system. Several considerations must be accounted for by the reflectivity measurement system  108  for this photometric approach of  FIG. 7  to yield accurate results. First, the system starts with a raw image of the Sun reflection (step  802 ). Then, the system applies bias subtraction to the image (step  804 ). The bias may depend on camera gain, but should be independent of camera filtering and camera exposure. In dark images, the real “zero” value may usually be reported as a non-zero value, e.g., ten. Once this non-zero value is identified, it is subtracted off. In some embodiments, the bias subtraction step (step  804 ), may be included in the mapping from flux to pixel intensity (step  806 ), where that relationship may have a non-zero intercept, and resulting in the same calibration. 
     Next, the system corrects for camera response (step  806 ). To produce an image of the Sun that is proportional to the actual incident flux, for this example, the camera&#39;s response to incident flux must be appropriately accounted for. Cameras may exhibit a non-linear response to incident flux, and this response may be characterized on an individual camera basis by collecting multiple images of the Sun at different DNI values. Since the true intensity of the incident light is known and the response is measured in each image, the overall response to flux may be determined There may be variations from camera-to-camera due to either manufacturing tolerances of the camera itself, the filters, the optics assembly, and/or any analogue settings such as the aperture or zoom. Therefore, each camera should be individually calibrated. 
     Next, the system divides by a flat image (step  808 ). To produce an image which has the same mapping from incident flux to pixel intensity everywhere in the image, any non-flatness must be corrected for in a pre-processing phase. These non-flatness profiles may be caused by vignetting, e.g., due to the shape of the optics of the lens, which may cause the center of the image to be brighter than the edge of the image, by intrinsic differences in pixel sensitivities, and/or by the read-out properties of the imaging sensor itself. These effects may be characterized in a single experiment. One must construct an image which, in an ideal camera, would be perfectly flat. The resulting—typically non-flat—image is a map of the combination of these effects for the given configuration of the camera. One may produce this “flat” image by, for example, placing the camera very close to a frosted glass and/or plastic translucent surface and illuminating that surface from the side opposite the one facing the camera with a bright light. If the camera is focused at infinity, or at least at a very far distance as compared to the camera&#39;s distance to the frosted surface, the image will appear as completely blurred and the incident flux will be uniform across all pixels. Then, the system multiplies by a calibration constant (step  810 ). 
     Next, the system extracts the Pixel Intensity Metric (PIM) (step  812 ). Once calibrated images of the Sun have been obtained (step  810 ), a single metric of “brightness”, the PIM, must be extracted. This step may be exacerbated by two noise issues: mirror-non-flatness distorting the image of the Sun, and non-uniform contamination of the reflector surface. Due to non-flatness, the distribution of pixel intensities in the Sun-image may not be constant, and therefore a metric that uses only that distribution may not be valid. Because of non-uniform contamination, the brightest pixel may not necessarily be representative of the average cleanliness, and so a “brightest pixel” metric similarly may not be used. The preferred technique is for the system to detect the blob of the image of the Sun, erode away pixels near the boundary, and calculate an average pixel value from the remainder. 
     Next, the system divides by DNI (Step  814 ). Due to the changing brightness of the Sun over time as seen from the ground, due to motion across the sky and atmospheric changes, the image brightness of the image captured by the camera may be negatively affected. To correct for this effect, the DNI may be measured by a separate device, which may be located nearby at the plant, and within a determined distance, e.g., 100 m or less, to the camera. The image brightness may then be scaled by the change in DNI between the first point in time the control image was taken and the second point in time the heliostat image was measured. In some embodiments, the first point in time and the second point in time may be simultaneous or close together. Finally, the system determines the reflectivity of the surface in the image (step  816 ). The reflectivity of the image may be calculated as:
 
Reflectivity of test image=(Reflectivity of control image, which is 100% in the case of a direct Sun measurement)*(DNI during control image)/(DNI during test image)*(PIM during test image)/(PIM during control image)
 
     It is contemplated that 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.