Abstract:
This invention relates to a system for tracking a solar energy collector and diagnosing the solar collector&#39;s operational status. More specifically, it relates to a system employing a 3-axis accelerometer to determine the orientation of a solar collector, the vibration experienced by that solar collector, whether that solar collector has experienced an impact, and initiating system position control and diagnostics based on that information.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority per 35 U.S.C. §119(e)(1) to the Provisional Application No. 61/304,397, filed Feb. 12, 2010, which is incorporated by reference in its entirety. This application is a continuation-in-part of U.S. patent application Ser. No. 12/962,650 entitled Concentrated Photovoltaic and Thermal Solar Energy Collector, by David Correia, et al., which was filed on Dec. 7, 2010, and is incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    In a tracking solar collector system, such as a concentrating parabolic reflector system, there is a need to keep the panel aligned with the sun as it moves through the sky. Alignment can be accomplished using predictions of sun position based on location of the collector, day of the year and time of day. To use such requires sensing of the collector&#39;s tilt and accurate positioning based on the time of day and day of the year. Tilt accuracy required is determined by the focusing properties of the solar collector. A 20× parabolic concentrating system requires tilt accuracy of approximately 0.5 degrees, while higher concentration systems require a greater degree of accuracy. 
         [0003]    A tracking solar collector is subject to damage by high winds and flying objects that may be “flung at” the collector during a wind storm. It is desirable for such damage to be detected, the collector involved identified, and a service warning given, all by automatic diagnostic equipment. 
         [0004]    Solar collector system lifetimes must be in excess of 20 years to effect efficient payback and return on investment. Long lifetime typically favors non-moving parts and sealed components. Cost is an issue and sensors and mechanisms used in solar collectors must be very cost effective. 
         [0005]    An accelerometer is a sensor typically utilized for measuring acceleration forces. These forces may be static, like the constant force of gravity, or they can be dynamic, caused by moving or vibrating the accelerometer. An accelerometer may sense acceleration or other phenomena along one, two, or three axes or directions. From this information, the movement or orientation of the device in which the accelerometer is installed can be ascertained. 
         [0006]    One growing use for micro electromechanical system (MEMS) accelerometers is in protection systems for a variety of devices. These protection systems ideally function to safeguard a device from shocks and vibration. 
         [0007]    By way of background, Application Note AN3107 (hereinafter referred to as “AN3107”), published May 2005, by Freescale Semiconductor, teaches the use of a low-g accelerometer to measure and quantify tilt. This application note, promoting the use of the company&#39;s products, states that static tilt resolution of 0.057 degrees to 1.63 degrees can be resolved using a 12 bit analog to digital conversion of the output of their device models MMA6260Q or MMA7260. 
         [0008]    U.S. Published Application No. US2009/0014057 A1 to Croft (“Croft”) describes a photovoltaic module with integrated devices, which could be an accelerometer. Croft is incorporated herein by reference in its entirety. The device is used to detect a change of one or more parameters affecting at least one of the photovoltaic cells. The accelerometer is used to detect whether the module is properly oriented and to detect trauma but Croft does not disclose a solar concentrator or the use to detect buffeting. Croft also does not disclose the use with a mid-range concentrator and due to the accuracy of accelerometers, it is not contemplated for continuous tracking of rotational and tilt position. Croft does not disclose use of a three axis accelerometer. 
         [0009]    U.S. Published Application No. US2008/0011288 A1 to Olsson (“Olsson”) teaches the use of an accelerometer on a reflecting mirror to track the sun and is incorporated herein by reference in its entirety. The system described is one with a remote mirror such as that used by high concentration PV systems in which multiple remote mirrors focus and concentrate the sun&#39;s radiation on a single remote photovoltaic cell. As with Croft, Olsson does not contemplate the use of a three axis accelerometer for continuous tracking of rotational and tilt position of the PV system. 
         [0010]    U.S. Published Application No. US2009/0229597 A1 to Choi (“Choi”) teaches a combination of servo control and predetermined azimuth adjustment and is incorporated herein by reference in its entirety. As is standard in the art, a pinion and worm gear is used to effect rotation and an angle sensor is attached to the shaft of the worm gear. 
         [0011]    U.S. Published Application No. US29314279A1 to Karim (“Karim”) teaches a method of tracking the sun using photo sensors and is incorporated herein by reference in its entirety. This approach avoids position encoders and predetermined azimuth calculation by actively tracking to peak sunlight regardless of the absolute position. This technique is acceptable as long as the sun is direct and not diffuse. However, when it is significantly overcast and/or the sunlight significantly diffused, the sensor will not be able to find a solar maximum. 
         [0012]    U.S. Published Application No. US20090101135A1 to Tsai (“Tsai”) teaches the use of an accelerometer to measure the wind force on a solar panel mounted on a pole and is incorporated herein by reference in its entirety. If excessive wind force is detected the pole is automatically lowered, presumably to bring the panel lower to the ground and out of the wind. Tsai, however, does not disclose the measurement of tilt, rotation, or a strike force. 
         [0013]    U.S. Pat. No. 7,569,764 to Shan, et al. (“Shan”) teaches linking multiple collector assemblies and controlling the tilt angle using feedback from device that senses the angle of the incoming sunlight and is incorporated herein by reference in its entirety. 
         [0014]    U.S. Pat. No. 7,667,833 to Diver (“Diver”) teaches the alignment of mirror facets of a parabolic trough solar concentrator with respect to the target using optical and clinometer methods, and the linking of a plurality of trough concentrator modules and is incorporated herein by reference in its entirety. 
         [0015]    U.S. Pat. No. 6,597,709 to Diver (“Diver”) also teaches the alignment of facets of a solar concentrator with respect to a target using lasers and is incorporated herein by reference in its entirety. 
         [0016]    Thus, there presently exists the need for a sensor for a tracking solar collector that provides tilt and rotation sensing to assist in orienting the collector; wind buffeting sensing to signal potentially damaging wind conditions; and impact sensing to determine if potential damage to the collector has occurred, all with a device that is sealed, has no moving parts, is low cost, and durable such that it exhibits a long lifetime. 
       BRIEF SUMMARY OF PREFERRED EMBODIMENTS 
       [0017]    In one preferred embodiment, the tilt sensor embodiments described herein can be used with a solar concentrator, for example, such as that described in U.S. patent application Ser. No. 12/962,650, which is incorporated by reference herein in its entirety. The tilt and rotational sensor may also be used with other solar concentrators or other devices that are preferably oriented to receive solar energy. 
         [0018]    One embodiment employs an accelerometer and a microprocessor to determine the orientation, both rotationally to track the sun across the daytime sky and in vertical tilt to track and orient the incident light based on the time of year. The accelerometer is mounted on the solar collector itself and provides data relating to the three axis (x, y and z) orientation of the solar collector, sending such data to the microprocessor, which employs the algorithm to determine the orientation of the solar collector, for example, a mid-range solar concentrator of 20× to 50× concentration. In a preferred embodiment, the data is compared with stored information regarding the proper orientation according to the time of day and the year. The microprocessor, in response to the data, adjusts the concentrator to equilibrate the data from the accelerometer with the corresponding orientation based on the location of the unit, the time of day and the time of year. 
         [0019]    In a preferred embodiment, the data from the accelerometer is used synergistically with output data to obtain continuous maximum output. For example, the accelerometer can be used for example as the coarse tuning aspect of the solar collector and the energy output from the photovoltaic cells can for example then be used to fine tune the orientation of the concentrator based on either a two axis rotational adjustment or a three axis rotational and tilt adjustment. 
         [0020]    The microprocessor also employs the data from the accelerometer to determine the vibration of the solar collector caused by high winds, for example. This is also referred to as wind buffeting. If the vibration is determined to be above a set limit, indicating conditions that may damage the solar device, the microprocessor causes the solar collector to rotate into a protected or parked position. 
         [0021]    The accelerometer may also be used to determine whether the collector has experienced trauma from wind, hail, wildlife or projectiles. If so detected, the microprocessor may, for example, perform a diagnostic such as a collector output compared with other linked solar collectors or historic data to determine whether it has been damaged. Such a review may also be initiated should the vibration data be above a set limit. In one embodiment, the diagnostic may for example include alerting a database, operator, or service center to the data reading. 
         [0022]    In a preferred embodiment an accelerometer is mounted on one or more mechanically-linked solar collectors. The data from this accelerometer is then provided to a microprocessor, which employs its algorithm to determine the orientation of the single collector and, with the single collector being linked to the group of collectors, this determines the orientations of the entire group. The accelerometer vibration data from the single collector may also be used to determine whether the linked collectors should be moved into a protected orientation, or have their output reviewed. The microprocessor may also analyze the accelerometer data to determine whether the single collector, or the mechanically-linked group, has experienced an impact and whether to review the collector output. In one preferred embodiment a number of the solar collectors are equipped with accelerometers, allowing the microprocessor to determine which collector, from those equipped with accelerometers, has experienced an impact. After sensing vibration data or a strike force indicative of a potential foreign object strike or other damage causing incidents or conditions, the unit output may perform a diagnostic to determine if it is operating properly. 
         [0023]    In one embodiment the solar collector may be linked in operation with one or more other solar collectors. In such an embodiment, the solar collector experiencing data indicating potential trauma may be compared to other units linked thereto to determine if the output of the potentially damaged unit is diminished, indicating possible damage. Upon such a determination, a signal indicating needed repair or inspection may be sent to, for example, an operator or source of maintenance. Where a single unit is in operation, output data after the potential damage may be compared with historical data to determine if damage is likely. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0024]    The foregoing and other aspects and advantages of the embodiments described herein will be better understood from the following detailed descriptions of particular embodiments with reference to the drawings. 
           [0025]      FIG. 1  is an excerpt from Application Note AN3107, by Freescale Semiconductor, of the sensing axis for the MMA6260Q accelerometer with x, y, and z-axis for sensing acceleration; 
           [0026]      FIG. 2  is an excerpt from Application Note AN3107, by Freescale Semiconductor, of the gravity component of a tilted x-axis accelerometer; 
           [0027]      FIG. 3  is an excerpt from Application Note AN3107, by Freescale Semiconductor, of the gravity component of a tilted z-axis accelerometer; 
           [0028]      FIG. 4  is a perspective view of a preferred embodiment; 
           [0029]      FIG. 5A  is a top view of a connected assembly of four solar collectors from the preferred embodiment illustrated in  FIG. 4 ; 
           [0030]      FIG. 5B  is a side view of a connected assembly of four solar collectors from the preferred embodiment illustrated in  FIG. 4 ; 
           [0031]      FIG. 5C  is an additional side view of a connected assembly of four solar collectors from the preferred embodiment illustrated in  FIG. 4 . 
       
    
    
       [0032]    Like reference numerals refer to corresponding elements throughout the several drawings. 
       DETAILED DESCRIPTION 
       [0033]    A tracking solar collector is subject to damage by high winds and flying objects that may be “flung at” the collector during a wind storm and damaged. Solar collectors are exposed to the elements in a storm and even if “parked” large flying debris can strike and damage them. Ideally such damage can be avoided. However, such potential damage conditions can be detected, the collector involved identified and a service warning given, all by automatic diagnostic equipment. 
         [0034]    One means to avoid damage is to rotate the collector to a safe position, with the collection surface facing downward and the rotation mechanism locked into secure position. This is known as “parking”. Ideally an automatic sensor is used to command the collector to rotate to the parked position during high winds. 
         [0035]    One embodiment employs a three axis low g-force accelerometer applied as a tilt and vibration sensor to a tracking solar collector. As opposed to a mechanical position sensor or encoder a preferred embodiment provides a tilt-sensing device that is sealed, has no moving parts, is low cost, and has very long lifetime by using a solid state low g-force accelerometer in the static sensing mode to sense the gravitation force indicating orientation and in the dynamic sensing mode to sense vibration indicating high winds or an impact. 
         [0036]    Referring to  FIG. 4 , a preferred embodiment provides an accelerometer  1 , mounted on one of a plurality of collectors  3  and used in the static sensing mode. Collectors  3  are constrained by links  4  to all be oriented in the same position. Data from accelerometer  1  is interpreted by microprocessor  5  to determine the orientation of collectors  3 . Microprocessor  5  directs rotation mechanism  6  to influence links  4  and cause collectors  3  to move into a desired position as determined by an algorithm within microprocessor  5 . In a preferred embodiment accelerometer  1  is a solid-state low g-force accelerometer. 
         [0037]    Still referring to  FIG. 4 , a preferred embodiment provides a wind sensor capable of signaling potentially damaging wind conditions by using accelerometer  1  in the dynamic mode to sense buffeting, leading to vibration, caused by wind forces. Excessive vibration can be used to indicate potential storm conditions and allow the collectors  3  to be “parked” in a face down secure position by feeding the accelerometer data to a microcontroller  5 , which in turn directs rotation mechanism  6  to rotate collects  3  by influencing links  4 . 
         [0038]    Remaining with  FIG. 4 , a preferred embodiment provides an impact sensor capable of signalling a potentially damaging impact by using accelerometer  1  to identify high impulse g-forces indicative of a hit or strike. The determination of a hit or strike triggers diagnostics to determine if the collector is still operating normally or needs maintenance or repair. As was the case for high vibrations, the determination of a strike or impact can also cause microcontroller  5  to initiate the “parking” of collectors  3 . 
         [0039]    In a preferred embodiment, an accelerometer  1  is mounted on each of collectors  3 , and microprocessor  5  is provided with data on the power outputs of each of collectors  3 , and the data from each of accelerometers  1 . Should the algorithm contained within microcontroller  5  record an excessive g-force indicating a hit, it then compares the output of the collector sustaining the “hit” to others in assembly  2  and recommends inspection for damage if the output of the impacted collector is low. 
         [0040]    A preferred embodiment uses an Analog Devices AD325Z 3-axis accelerometer. The static output from the device yields tilt information and the dynamic output yields vibration, indicated by multiple axis accelerations, information. The peak g-forces encountered in the static output are used to indicate striking forces, indicative of damage or potential damage to the collector. 
         [0041]    While referring to  FIGS. 1 ,  2 , and  3  for reference, calculation of the tilt angle based on the static output is described by the following excerpt from AN3107:
       In order to determine the angle of tilt, Θ, the A/D values from the accelerometer are sampled by the ADC channel on the microcontroller. The acceleration is compared to the zero g offset to determine if it is a positive or negative acceleration, e.g., if value is greater than the offset then the acceleration is seeing a positive acceleration, so the offset is subtracted from the value and the resulting value is then used with a lookup table to determine the corresponding degree of tilt (See Table 1 for a typical 8-bit lookup table), or the value is passed to a tilt algorithm. If the acceleration is negative, then the value is subtracted from the offset to determine the amount of negative acceleration and then passed to the lookup table or algorithm. One solution can measure 0° to 90° of tilt with a single axis accelerometer, or another solution can measure 360° of tilt with two axis configuration (XY, X and Z), or a single axis configuration (e.g. X or Z), where values in two directions are converted to degrees and compared to determine the quadrant that they are in. A tilt solution can be solved by either implementing an arccosine function, and arcsine function, or a look-up table depending on the power of the microcontroller and the accuracy required by the application. For simplicity, we will use the equation: Θ=arcsin(x). The arcsin(y) can determine the range from 0° to 180°, but it cannot discriminate the angles in range from 0° to 360°, e.g.) arcsin(45°)=arcsin(135°. However, the sign of x and y can be used to determine which quadrant the angle is in. By this means, we can calculate the angle β in one quadrant (0-90°) using arcsin(y) and then determine Θ in the determined quadrant.       
 
         [0000]        V out= V offset+{ΔV/Δg×1.0 g×sin Θ}  [1]
           Where:
               Vout=Accelerometer output in volts   Voffset=Accelerometer 0 g offset   ΔV/Δg=Sensitivity   1 g=Earth&#39;s gravity   Θ=Angle of tilt   
                       
 
         [0049]    Solving for the angle: 
         [0000]      Θ=arcsin {[ V out− V offset]/[ΔV/Δg]}  [2]
 
         [0050]    This equation can be used with the MMA6260Q as an example: 
         [0000]        V out=1650 mV+800 mV×sin Θ
 
         [0051]    Where the angle can be solved by 
         [0000]      Θ=arcsin ([ V out−1650 mV]/[[800 mV/g]}
       From this equation, you can see that at 0° the accelerometer output voltage would be 1650 mV and at 90° the accelerometer output would be 2450 mV.       
 
         [0053]    Calculation of dynamic forces indicative of wind “buffeting” involves monitoring the amplitude and frequency of the dynamic vibrations. Those skilled in the art understand how to interpret these signals and when such vibration is approaching the tolerable limits of the collector can construct an automatic park algorithm. 
         [0054]    Analysis of the accelerometer signals for any instantaneous signal above a predetermined g-force, or above previous average g-force, can be used to indicate a possible “hit” and trigger follow up testing or user warnings. 
         [0055]    When mounted inside the volume of the collector  3  accelerometer  1  is protected from the environment, extending its useful life (e.g., 20 years). The device can be mounted anywhere on a solar collector that rotates with the unit, unlike position encoders it need not be mounted on the rotating shaft of the collector. For example, in one embodiment, the accelerometer is mounted within the concentrator tube housing photovoltaic cells, as disclosed in co-pending U.S. application Ser. No. 12/962,650. In a preferred embodiment, only one accelerometer  1  is required to sense tilt of collectors  3  in all 3 dimensions, x, y, and z. It is also very low cost, compared to the two encoders and other sensors it replaces and reduces mechanical complexity because it does not interface with rotation mechanism  6  (which in a preferred embodiment are motion drive gears), or links  4 . 
         [0056]    Referring again to  FIG. 4 , which shows a preferred embodiment, a single tilt sensor  1  is mounted on a connected assembly  2  of four collectors  3 . With the collectors  3  linked mechanically by links  4  only the tilt of one collector  3  is equipped with an accelerometer. Accelerometer  1  supplies acceleration data to microprocessor  5 , which interprets that data according to an internal algorithm and instructs rotation mechanism  6 , to position collectors  3  accordingly. In a preferred embodiment, microprocessor  5  is contained within a computer. 
         [0057]    Optionally, a tilt sensor  1  can be mounted on each of the collectors  4 . This arrangement provides the capacity for a diagnostics. By comparing the output of the multiple accelerometers, microprocessor  5  can determine whether linkage  4  was broken or if one of the panels  3  sustained a blow from a flying object. With a cost of approximately $5 per tilt sensor multiple sensors are feasible and introduce only a modest cost per value tradeoff. 
         [0058]    While the foregoing description and drawings represent embodiments of the sensor system disclosed herein, it will be understood that various additions, modifications and substitutions may be made therein without departing form the spirit and scope of the sensor system disclosed and claimed herein. In particular, it will be clear to those skilled in the art that the system may be embodied in other specific forms, structures, arrangements, proportions, and with other elements, materials, and components, without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive.