Heliostat array intensity and polarization tracking

A tracking system for a solar collector is disclosed. The tracking system includes at least two polarization cameras and a tracking controller configured to: determine orientations of maximal intensity of polarized light received from the at least one heliostat mirror; generate radial lines based on the orientation of maximal intensity of polarized light from the at least one heliostat mirror; determine a position of the sun based on an intersection of the radial lines; and re-orient the at least one heliostat mirror based on the determined position of the sun. In the preferred embodiment, the sun position may be determined based on radial lines corresponding to three or more cameras mounted around the receiver aperture.

TECHNICAL FIELD

The invention relates to a solar tracking system. In particular, the invention relates to a solar collector that tracks the sun over the course of the day using a combination of light intensity, light polarization, light color, and/or light color ratios to determine the position of the sun in the sky.

BACKGROUND

There are a number of solar tracking systems that utilize small sensor arrays to determine the location of the sun in the sky relative to the solar tracker. The sensor arrays generally acquire and integrate the entire spectrum of light without differentiating between colors of light, which vary depending on the which direction the sensor is facing as well as the angular difference between the sensor and the sun. In addition, sensor arrays generally ignore the polarization of the incidence sunlight, which also varies depending on the direction the sensor is facing and its angular distance away from the sun. There is therefore a need for a solar tracking system that can determine its orientation based on a combination of light intensity, light polarization, light color, and/or light color ratios.

SUMMARY

The invention in some embodiments features a system and method for tracking the sun over the course of the day with a solar tracking system. The solar tracking system preferably comprises: at least two or four cameras including a first camera and a second camera; and a tracking controller configured to: determine orientations of maximal intensity of polarized light received from the at least one heliostat mirror; generate radial lines based on the orientation of maximal intensity of polarized light from the at least one heliostat mirror; determine a position of the sun based on an intersection of the radial lines; and re-orient the at least one heliostat mirror based on the determined position of the sun. In the preferred embodiment, the first camera and second camera are mounted on either side of the receiver where they are configured to capture images of polarized light received from the at least one heliostat mirror. Radial lines may be determined for each of a plurality of cameras mounted around the receiver aperture.

In addition to, or in the alternative, the tracking controller may be configured track the sun based on the intensity of light, preferably the ratio of white light intensity to blue light intensity. When two cameras are placed on either side of the receiver aperture, the ratio of white light to blue is substantially the same when the heliostat mirror is properly aligned. If not properly aligned, the tracking controller is configured to adjust the orientation of the at least one heliostat mirror to make the ratio the same. In some embodiments, the heliostats are oriented based on both light intensity ratios as well as polarization as described above.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention pertains to a solar tracking system implemented with an array of heliostats and a solar thermal receiver that captures energy reflected from the heliostat array. Illustrated inFIG. 1is an elevation view of heliostat array120and receiver130in accordance with a preferred embodiment. The heliostat array includes numerous identical heliostats122that are distributed in two dimensions in proximity to the receiver. Each heliostat122includes a mirror110pivotably coupled to a frame or stanchion112affixed to the ground and/or other heliostats. Each heliostat further includes a tracking controller114configured to determine the proper orientation of one or more mirrors through the day. A mirror is properly oriented when the incoming light152from the sun150is reflected to the solar thermal receiver130, specifically the receiver aperture132. If the actual orientation of the mirror differs from the proper orientation at that instant, the tracking controller energizes actuators116that drive the mirror to the proper orientation.

Illustrated inFIG. 2is an elevation view of the front side of the solar thermal receiver130and aperture132for receiving sunlight into the receiver. The receiver may further include absorbers that convert the sunlight to heat and transfer the heat to air, water, molten salt, or other working fluid (not shown). In the preferred embodiment, the receiver has mounted to it a portion of the solar tracking system, namely a plurality of cameras configured to capture images of the heliostat mirrors and the reflections therein. The left camera212and the right camera213are the same distance from the aperture. Similarly, the upper camera210and lower camera211are the same distance from the aperture. The cameras210-213may be any of a number of different types of two dimensional imagers including still cameras and video cameras. Image data observed in the multiple mirrors is combined to determine the elevation and azimuth angles of each heliostat mirror. In particular, image data from the left camera212and right camera213are combined to determine the azimuth angle of each heliostat mirror, while image data from the upper camera210and lower camera211are combined to determine the elevation angle of each mirror.

Illustrated inFIG. 3is a perspective view of an exemplary array of heliostats and receiver. In this example, the array of heliostats is located in the northern hemisphere, and the heliostats are north of the receiver130. The cameras210-214, although not visible in this figure, are configured with a field of view sufficient to capture a complete image of all the heliostat mirrors122in the heliostat array120. As explained below, the image data includes light intensity levels and/or polarization angles measured for each heliostat mirror.

Illustrated inFIG. 4is a representative image of the heliostat field210as seen from one of the plurality of cameras210-213. The image includes a plurality of heliostat mirrors410-421, each mirror reflecting light from the sky. If the mirrors are not properly oriented to reflect light to the receiver, the intensity of the light seen in each mirror will generally differ depending on the angle from the sun. For mirrors410-411in which the sun appears in the reflection, the light intensity for the mirror is very high. For other mirrors419-421that are angled far from the sun, the light intensity is relatively low and the sky appearing relatively dark. If all the mirrors are “off sun” and not reflecting light to the receiver aperture, the mirrors410-421will generally reflect varying degrees of light, as indicated by the different levels of greyscale used in the illustration.

The actual intensity of light observed in the reflection from each mirror depends on the section of sky from which the light originates. As seen inFIG. 5, the light intensity is maximal for the angular range that coincides with the sun, where the center of the sun is defined to be the origin of the angular range. The intensity drops rapidly around the nominal edge of the sun. Beyond the nominal edge of the sun, the intensity is non-zero due to scattering effects. In particular, light from the sun is scattered by the atmosphere mostly by Rayleigh scattering and Mie scattering. Rayleigh scattering, which is wavelength dependent, produces omnidirectional scattering that produces a uniform or nearly uniform bluish illumination of the sky. Mie scattering, which is weakly wavelength dependent, produces scattering at smaller angles. Mie scattering results in a white halo around the sun with a brightness gradient that decreases with angle away from the sun. Radiation from the sun is also absorbed by the atmosphere at wavelengths corresponding to the various molecules it contains. These effects combine to produce a white light intensity gradient that gradually reduces as the angular distance from the sun increases. This “off-sun” light gradient is used in the present invention for solar tracking, as explained in detail below.

Illustrated inFIG. 6is a cross section of the solar thermal receiver showing the source of light incident on the receiver and cameras210-211. When the heliostat mirror122is properly oriented, the light from the sun600is reflected onto or into the receiver aperture132. At the same time, light is captured by cameras210,211. Due to the offset between the aperture and cameras, however, the light captured at the cameras originates from sections of the sky adjacent to the sun. In the preferred embodiment, the distance between the cameras210,211and the aperture is large enough to ensure that the cameras never image the sun in the reflection from the heliostat mirror122while tracking. If the sun was visible in the images, the intensity of the light would likely saturate the camera and throw off the tracking computation.

As a result of the camera offset, upper camera210receives a reflection of light that originates from a section610of the sky below the sun600, i.e., a solid angle at a lower elevation angle than the sun. From the same heliostat mirror, the lower camera211receives a reflection of light that originates from a section611of the sky above the sun600. Although not shown, one skilled in the art will appreciate that the left camera212receives a reflection of light that originates from a section of the sky to the right of the sun600, while the right camera213receives a reflection of light that originates from a section of the sky to the left of the sun600for the same heliostat mirror.

When the images of the sky from the four cameras—as seen in one heliostat mirror—are compiled into a map of the sky, the four images reside at four different angles around the sun, which is illustrated inFIG. 7. When the mirror is properly oriented to reflect light on the aperture and the sun substantially bisects the angle between the two cameras, the intensity of light measured in the mirror by the upper and lower cameras is substantially the same. Similarly, the intensity of light measured in the mirror by the left and right cameras is the same. However, when the mirror is not properly oriented to reflect light on the aperture, the sun does not bisect the angle between the two cameras and the intensity of light measured in the mirror by opposing cameras (e.g., upper and lower cameras, or left and right cameras) is not the same.

As illustrated inFIG. 8, the light intensity of the sky adjacent to the sun is used to track the sun, in the present invention. While tracking the sun, the tracking controller monitors light levels for a plurality of mirrors, and determines if there is an imbalance in the light intensity for two opposing cameras. If a reflection of the sun from one mirror is diverted away from the center of the receiver aperture and biased toward a particular camera, the light intensity for that mirror will appear brighter in that camera than the camera on the opposite side of the aperture. This scenario is shown inFIG. 8where the camera that acquired the image810of a heliostat is closer to the sun's reflection than the camera that acquired the image812of the same heliostat. As such, the light intensity for the particular heliostat appears brighter in image810than image812. In response, the tracking controller, re-orients the heliostat mirror to direct sunlight at a lower elevation angle so that the light intensity is the same in heliostat image810as image812, which causes the sun's reflection to be vertically centered on the receiver aperture. The analogous tracking procedure is used to adjust heliostat azimuth angle using the light intensities measured in the left and right camera images813,811. When the proper azimuth angle is achieved, the light intensity observed in left and right cameras is balanced and the reflected sunlight is horizontally centered on the receiver aperture.

In some embodiments of the present invention, the tracking controller tracks the sun based on solar polarization as well as light intensity. Polarization is determined using receiver-mounted cameras210-213in the form of camera assembly900, as shown inFIG. 9. This camera assembly900includes a camera base910with charge-couple device (CCD) imager, an optical lens housing920, and a rotating polarizer930. The polarizer is configured to automatically rotate while images of the heliostat array are acquired. The maximum polarization is then used to determine a direction of the sun. As shown inFIG. 10, the polarization of sunlight is predominately tangential about the sun, and the strength of that tangential polarization increases with the angular distance from the sun.

In the preferred embodiment, the polarization for each mirror is determined for each image captured. The maximum polarization for a mirror is determined by identifying the image with the brightest light level while the polarizer930is rotated at least 180 degrees. The process of finding the maximum brightness is repeated for each for each mirror seen in each camera210-213. The polarizer angle associated with the maximum brightness of a mirror is recorded for each of the four cameras210-213. A radial line perpendicular to polarization angle is then generated to account for the fact that the polarization is tangential to the sun. The sun should then lie somewhere on the radial line. The radial line for each of the mirrors are superimposed and the position of the sun estimated based on the convergence of those radial lines. In the preferred embodiment, two or more polarization measurements are required to locate the position of the sun, but four measurements are used here for increased accuracy.

Referring toFIG. 11, the polarization of sunlight is measured for the same heliostat in the four cameras210-213. The polarization angles1110are then used to generate the four radial lines1120, which converge to indicate the position of the sun centered between the mirror images. In another example inFIG. 12, the polarization of sunlight1210is measured, the four radial lines1220generated, and the position of the sun determined to be off-center. At this point, the tracking controller issues commands to reorient the associated mirror in the proper position to reflect sunlight to the receiver aperture,

In some embodiments, the off-sun light intensity tracking procedure ofFIGS. 7-8is combined with the polarization tracking procedure ofFIGS. 11-12to enhance the accuracy of the estimate of the sun's position as well as accuracy of the tracking procedure.

Some embodiments of the present invention, the tracking controller selectively uses the red end of the optical spectrum in order to increase the signal-to-noise ratio of images used for light gradient tracking. The red wavelengths can be isolated using cameras with optical filters or by adjusting the gain values for RGB (red, green, blue) components of the images using image processing software. Red wavelengths are preferred because they are less susceptible to Rayleigh scattering than blue wavelengths. Also, the blue wavelengths, which have been scattered out of the direct beam to a greater extent than red wavelengths, make up a lower portion of the Mie scattered light that forms the gradient observed in the tracking images.

In some embodiments of the present invention, the tracking controller determines the position of the sun based, at least in part, on the ratio of colors of light or the ratio of spectra of light. In one embodiment for example, the tracking controller determines the intensity of white light and the intensity of blue light corresponding to each mirror as viewed by each of the four cameras210-213. White light intensity drops off rapidly as a function of the angular distance from the edge of the sun, while blue light is substantially uniform. The tracking controller then determines the ratio of white light intensity to blue light intensity for each mirror as viewed by each of the four cameras. For a given mirror, if this white/blue ratio is the same for two opposing cameras (top and bottom, or left and right) on either side of the aperture, then the reflected sunlight is centered on the receiver aperture. If however, the ratio is different for the opposing cameras, then the sunlight is being directed to a spot closer to the camera observing the higher ratio. In response, the tracking controller adjusts the orientation of the mirror to direct the reflected sunlight to a point substantially between the two cameras to achieve a uniform ratio of white/blue light between the two opposing cameras. This ratio may be used to track the sun in both elevation and azimuth angles.

In the embodiment described above, the tracking controller orients the mirrors based on the ratio of white light intensity to blue light intensity. In other embodiments, the ratio is a function of the red and/or green light intensity and blue light. The intensity of red light will be highest at an angle near the sun and drop off rapidly as the angular distance from the sun increase. When referring to white, red, green, and blue light herein, one skilled in the art will appreciate that these terms refer to a spectrum of light where white is broad spectrum and red, green, and blue are narrow spectrum.

One or more embodiments of the present invention may be implemented with one or more computer readable media, wherein each medium may be configured to include thereon data or computer executable instructions for manipulating data. The computer executable instructions include data structures, objects, programs, routines, or other program modules that may be accessed by a processing system, such as one associated with a general-purpose computer or processor capable of performing various different functions or one associated with a special-purpose computer capable of performing a limited number of functions. Computer executable instructions cause the processing system to perform a particular function or group of functions and are examples of program code means for implementing steps for methods disclosed herein. Furthermore, a particular sequence of the executable instructions provides an example of corresponding acts that may be used to implement such steps. Examples of computer readable media include random-access memory (“RAM”), read-only memory (“ROM”), programmable read-only memory (“PROM”), erasable programmable read-only memory (“EPROM”), electrically erasable programmable read-only memory (“EEPROM”), compact disk read-only memory (“CD-ROM”), or any other device or component that is capable of providing data or executable instructions that may be accessed by a processing system. Examples of mass storage devices incorporating computer readable media include hard disk drives, magnetic disk drives, tape drives, optical disk drives, and solid state memory chips, for example. The term processor as used herein refers to a number of processing devices including personal computing devices, servers, general purpose computers, special purpose computers, application-specific integrated circuit (ASIC), and digital/analog circuits with discrete components, for example.

Although the description above contains many specifications, these should not be construed as limiting the scope of the invention but as merely providing illustrations of some of the presently preferred embodiments of this invention.

Therefore, the invention has been disclosed by way of example and not limitation, and reference should be made to the following claims to determine the scope of the present invention.