Abstract:
An automated method to monitor performance of a terrestrial solar cell array tracking the sun. The solar cell system includes drive means that adjust a position of the array along different respective axes with respect to the sun using the drive means. The techniques include predicting the position of the sun during a time period, and sampling an output parameter of the array indicative of performance. The sampled data may be used to identify a fault in the solar cell array, for example a misalignment or a failure of one or more solar cells, in which case a notification of that fault may be generated for the operator or a control signal may be output for correcting the fault. Alternatively, an output signal may be sent to an external system associated with the solar cell system. Various alignment testing routines for checking the solar tracking are described. These routines may involve moving a solar cell array to a reference position at the start of, or during, an alignment routine in order to improve accuracy of position measurement during the routine.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation in part of U.S. patent application Ser. No. 12/498,135 filed Jul. 6, 2009; Ser. No. 12/468,747 filed May 19, 2009, and Ser. No. 12/258,253 filed on Oct. 24, 2008, the disclosures of each of which are hereby expressly incorporated herein by reference. 
     
    
     TECHNICAL HELD 
       [0002]    This disclosure relates to solar tracking for terrestrial solar cell arrays and more particularly to techniques for monitoring performance metrics of the array during solar tracking, and adjusting the motion of the array so that it is more accurately aligned with the sun. 
       BACKGROUND 
       [0003]    in solar tracking systems, generally one or more terrestrial solar cell arrays track the motion of the sun and convert sunlight into electrical energy. Accurate solar tracking is necessary because the amount of power generated by a given solar cell is related to the amount of sunlight that impinges on that solar cell. This is a particular concern for a concentrating solar cell array which uses lenses to focus sunlight onto respective solar cells because a tracking misalignment of only a few degrees can significantly reduce the amount of sunlight impinging on the solar cells and hence the power output of the solar cells. Solar tracking is achieved by properly orientating the array relative to the sun at an initial time instant and then using motors and actuators to move the array (e.g., in roll and pitch or in azimuth and elevation directions) along a predetermined path that properly tracks movement of the sun. However, from time to time deviations may still occur from accurate alignment of the solar array with the sun. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0004]      FIG. 1  illustrates a row of solar cell arrays which track the position of the sun during the day. 
           [0005]      FIG. 2  illustrates a portion of the row of solar cell arrays from  FIG. 1 , illustrating motor controls for adjusting a roll position and a pitch position of the solar cell arrays of the row. 
           [0006]      FIG. 3  is a perspective view of a mount connected to a longitudinal support according to one embodiment. 
           [0007]      FIG. 4  is a block diagram of an implementation of a terrestrial solar tracking system. 
           [0008]      FIG. 5  is a plot of actual azimuth and elevation values for the sun, for sun elevation angles over 15 degrees, during three exemplary days. 
           [0009]      FIG. 6  illustrates the actual path of the sun in comparison with an actual alignment direction resulting from a misaligned solar tracking. 
           [0010]      FIG. 7  is a flow diagram of a process for monitoring an output parameter of a solar cell array or group of solar cell arrays for determining an optimized roll position. 
           [0011]      FIG. 8  is a flow diagram of a process for monitoring an output parameter of a solar cell array or group of solar cell arrays for determining an optimized pitch position. 
           [0012]      FIG. 9  illustrates a system for monitoring performance of a solar cell array. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    An exemplary embodiment of a terrestrial solar power system is illustrated in  FIGS. 1 to 3 , which illustrate a solar cell array farm  100  configured as a row of solar cell arrays each capable of tracking the sun throughout the day. Each solar cell array comprises four solar cell modules  115 , with each solar cell module  115  including a generally rectangular sub-array of solar cell devices. In this embodiment, each solar cell device is a concentrating solar cell device having a rectangular concentrator lens  140  (in this embodiment a Fresnel lens) which, when properly aligned, focuses sunlight onto a solar cell receiver including a triple-junction III-V compound semiconductor solar cell. 
         [0014]    The amount of power generated by the array is directly related to the amount of sunlight impinging upon the constituent solar cells. It is advantageous, therefore, to arrange the concentrating lenses  140  of the solar cell devices such that when the plane of the concentrator lenses  140  is orthogonal to the incoming rays of the sun, the concentrating lenses  140  direct impinging sunlight onto their corresponding solar cells so that power generation is maximized. To that end, a solar tracking mechanism is employed that ensures that the plane of concentrator lenses  140  results in a sun beam being projected on the center of the respective solar cells in a continuous manner as the sun traverses the sky during the day, thereby optimizing the amount of sunlight impinging upon the solar cells. As will be discussed in more detail hereafter, to verify correct alignment with the sun, the terrestrial solar power system is also able to perform an alignment test. 
         [0015]    While only a single row of solar cell arrays is shown in  FIG. 1 , the solar tracking and alignment testing techniques discussed herein may be implemented on solar cell array farms that are formed of rows and columns of solar cell arrays, each operating independently, in full unison, or in partial unison during solar tracking. 
         [0016]    As shown, the row of solar cell arrays is supported by an elongated frame assembly  110  configured to mount clusters of solar cell modules  115  in a longitudinally-extending and spaced-apart arrangement. Frame assembly  110  includes a longitudinal support  120  positioned above a surface  300  by spaced-apart vertical supports  130 . In this embodiment, the longitudinal support  120  is a pipe with a diameter of about 4 inches and includes a thickness of about 0.167 inches. The pipe includes a length of about 192″ and weighs about 110 lbs. 
         [0017]    The longitudinal support  120  may be extended by adding one or more discrete sections  121  that are connected together in an end-to-end arrangement. The lengths and construction of each section  121  may be the same or may be different. In the embodiment illustrated in  FIG. 1 , the longitudinal support  120  has been extended by the addition of two sections  121 , each sized to mount a single solar cell array. The modular design allows a user to construct the frame assembly  110  to a length needed to support any desired number of solar cell modules  115 . That is, additional sections  121  may be added to an existing frame assembly  110  to accommodate additional solar cell arrays, as may be necessary for the row of solar cell arrays to produce the desired power output. 
         [0018]    Referring to  FIGS. 2 and 3 , the solar cell modules  115  within each solar cell array are connected to the frame assembly  110  using mounts  160 . The solar cell modules  115  are mounted to the mounts  160  by way of, for example, a jackscrew, threaded rod, and pivot point (e.g., a hinge) arrangement so that rotation of the jackscrew causes movement of the threaded rod resulting in rotation about the pivot point. In this way, the solar cell modules  115  may be aligned relative to the mounts  160 . 
         [0019]    The vertical supports  130  are spaced apart along the length of the longitudinal support  120 . The vertical supports  130  include a length adequate to position the solar cell modules  115  above the surface  300  for rotation about the axis of the longitudinal support  120 . Therefore, the vertical supports  130  are longer than a height of the mounts  160  and the solar cell modules  115 . 
         [0020]    The vertical supports  130  are positioned along the longitudinal support  120  away from the mounts  160  to prevent interference with the movement of the solar cell modules  115 . As illustrated in  FIG. 1 , the vertical supports  130  are in a non-overlapping arrangement with the solar cell modules  115 . Various numbers of vertical supports  130  may be positioned along the length of the longitudinal support  120 . In the embodiment of  FIG. 1 , a vertical support  130  is positioned between each solar cell array. In other embodiments, the vertical supports  130  are spaced a greater distance apart along the longitudinal support  120 . In one specific embodiment, the vertical supports  130  include a 4 inch by 4 inch rectangular shape, and include a thickness of about 0.188 inches. The vertical supports  130  may also be supported in a concrete pad. 
         [0021]    Each mount  160  includes a pair of mount frames, each formed by transverse members  162  and longitudinal members  163 , with each mount frame supporting a pair of solar cell modules  115 . Mounts  160  may have mount frames of different sizes to accommodate different numbers of solar cell modules  115 . 
         [0022]    The mounts  160  are positioned at various spacings along the length of the longitudinal support  120 . As shown in  FIGS. 2 and 3 , the mounts  160  are spaced along the longitudinal support  120 . For each mount  160 , the pair of mount frames are offset on opposing sides of the longitudinal support  120  directly across from one another, so that one pair of solar cell modules  115  are positioned on one side of the longitudinal support  120  and another pair of solar cell modules  115  are positioned on the other side of the longitudinal support  120 . Such offset positioning assists to balance the solar cell array, thereby facilitating rotation about the axis of the longitudinal support  120 . Other configurations are possible, including providing uneven numbers of solar cell modules  115  on opposing sides of the longitudinal support  120 . 
         [0023]    Each mount  160  has a pivot rod  165  that facilitates pivoting motion of the solar modules  115  about the axis of the pivot rod  165 . In this embodiment, the pivot rod  165  extends through a base  166 , on which the mount frames are mounted, from an upper base plate  166   b  to a lower base plate  166   a.  Other configurations of pivot rod are possible. Further, the pivot rod  165  may be a single elongated member or may be constructed of separate members that are positioned in an end-to-end orientation and connected to the base  166 . 
         [0024]    As shown in  FIG. 2 , each of the solar cell modules  115  comprises a 3×5 array of solar cell devices each having an associated concentrator lens  140 . One of ordinary skill would understand that any arrangement of solar cell devices may be possible in a module, such as, for example, a 1×1, 2×2, or 5×8 array of solar cell devices. As shown in  FIGS. 1-3 , a solar cell array if formed by grouping solar cell modules  115  into clusters (or groups). The grouping of the modules  115  into clusters allows the roll and pitch of each cluster to be controlled separately from the other clusters. While a 2×2 cluster of four solar cell modules  115  is shown, any of a variety of arrangements are possible such as, for example, a 1×1 cluster or a 1×3 cluster. The solar cell modules  115  may also be configured as stand alone modules for which no groups are formed at all. 
         [0025]    A roll drive means  170  is connected to the longitudinal support  120  to provide a force to rotate the longitudinal support  120  about an axis A generally aligned with the longitudinal axis of the longitudinal support  120 . Such movement will hereafter be referred to as rolling the solar cell modules  115  or rotating the solar cell modules  115  in a roll direction. In this embodiment, the roll drive means  170  is positioned at one end of the longitudinal support  120 . Roll drive means  170  may include a drive train with one or more gears that engage with the longitudinal support  120 . Additional roll drive means  170  may be connected along the length of the longitudinal support  120  to provide additional rotational force. 
         [0026]    A pitch drive means  175  is also connected to the longitudinal support  120  to rotate each cluster of solar cell modules  115  along a respective axis B which is substantially perpendicular to axis A. In particular, the axis B is generally aligned with the pivot rod  165  for the cluster of solar cell modules  115 . Such movement will hereafter be referred to as pitching the solar cell modules  115  or rotating the solar cell modules  115  in a pitch direction. 
         [0027]    As shown in  FIGS. 2 and 3 , in this embodiment the pitch drive means  175  is connected, via a connector  180 , to a mount  160  having first and second arms  181 ,  182 , connected together at a neck  183 , on opposing sides of the base  166 . The pitch drive means  175  provides a force to rotate that mount  160  about axis B, and connections are provided between that mount  160  and the other mounts  160  so that a corresponding rotation occurs for the other mounts  160 . The pitch drive means  175  may include a drive train with one or more gears that engage with the mount  160 . In an alternative embodiment, the pitch drive means  175  and the roll drive means  170  may share a common actuator to provide rotational force selectively around the first axis A and the second axis B. 
         [0028]    Each solar cell module  115  is moved into position to track the sun by rotating the module  115  about the first axis A and the second axis B using the roll drive means  170  and the pitch drive means  175 . A controller  190  controls the movement of the terrestrial solar tracking array  100 . The controller  190  includes a microcontroller with associated memory. In one embodiment, the controller  190  includes a microprocessor, random access memory, read only memory, and an input/output interface. The controller  190  controls operation of one or more roll motors  311  which form part of the roll drive means  170  and rotate the longitudinal support  120  and the solar cell modules  115  about the first axis A. The controller  190  also controls the one or more pitch motors  310  which form part of the pitch drive means  175  to rotate the clusters of solar cell modules  115  about their respective second axes B. The controller  190  includes an internal timing mechanism such that the operation of the motors corresponds to the time of day enabling the solar cell modules  115  to track the azimuth and elevation of the sun. 
         [0029]    Each solar cell module  115  includes a printed circuit board (PCB)  195  connected to controller  190 . The PCBs  195  send information such as measured power output from each solar cell receiver in the solar cell module  115 . In another example, each PCB  195  sends the collective measured power for the entire solar cell module  115  to the controller  190 . In another example, a PCB may be assigned to each solar cell cluster (i.e., the 2×2 cluster of four solar cell modules  115  between supports  130  in the illustrated example) and send the collective measured power output from the solar cell array to the controller  190 . The transmitted data may be encoded to identify which receiver, module, and/or solar cell array the measured power data has originated from. If additional sensors (e.g., alignment triggering sensors) are provided, then the PCBs  195  may transmit sensed data as well. The PCBs  195  may also be used to receive information from the controller  190  based on the time of day for the solar cell modules  115 . 
         [0030]      FIG. 4  is a block diagram schematically showing the basic functioning of a terrestrial solar tracking system used in this embodiment. The system receives as input data (a) the date and a future time data  301  and (b) position data  302 , including, e.g., longitude, latitude and elevation above sea level. In this implementation, the system utilizes future time data  301  rather than the current time data so that rather than lagging behind the sun, the array can be oriented to align with the sun at its expected position at the future time. 
         [0031]    At the start of the day, the future time specified in the future time data  301  can be a start up future time when the solar cell modules  115  are first oriented toward the sun for tracking during the daytime. In some implementations, the start-up future time can be sunrise, so that upon sunrise, the solar cell module  115  is aligned with the sun at sunrise. In other implementations, the start up future time is at some predetermined time after sunrise. Using a start up time after sunrise allows the solar cell modules  115  to start tracking the sun from a tilted start up position, instead of from a fully (or nearly) vertical position facing the sunrise on the horizon. In some examples, the start up position corresponding to this start up time may be the same as the “parked” (or storage) position of the solar cell modules during a night mode. For example, the start up position may be a predetermined angular position of 15 degrees, instead of 0 degrees. Following the start up time, to maintain alignment the system can repeat its alignment calculation periodically (e.g., every minute) or continuously, each calculation using an appropriate future time. 
         [0032]    Based on at least the input data  301 ,  302 , a sun position predictor in the form of a sun position algorithm  303  calculates the position of the sun (e.g., its azimuth and elevation) relative to the solar cell array at that future time  304 . In some implementations, the sun position algorithm  303  includes the Solar Position Algorithm (SPA) available from the National Renewable Energy Laboratory (see http://rredc.nrel.gov/solar/codesandalgorithms/spa/ and http://www.nrel.gov/docs/fy08osti/34302.pdf, both of which are incorporated herein by reference). 
         [0033]    The sun&#39;s azimuth and elevation at the future time  304  as output by the sun position algorithm  303  are input data to a kinematic model  305 , which correlates the sun&#39;s azimuth and elevation with respective positions of the pitch motor  310  and the roll motor  311  to align the solar cell modules  115  with the impinging sunlight. As such, the kinematic model  305  outputs appropriate pitch motor position data to a pitch motion calculator  306  which generates and sends an appropriate control signal to the motor controller  309  for the pitch motor  310 , and outputs appropriate roll position data  307  to a roll motion calculator  307  which generates and sends an appropriate control signal to a motor controller  308  for the roll motor  311 , so that the solar cell modules  115  are aligned with the sun&#39;s elevation and azimuth at the future time  304 . Each of the motors  310 ,  311  includes a position encoder that determines the current position of each respective motor (e.g., measured by the rotational position of the drive shaft, represented as an integral number of “counts” or increments of a predetermined numbers of degrees, starting from zero and continuing through 360 degrees for one turn, 720 degrees for two turns, etc.). For control purposes, the position data is fed back to the respective motor position calculator  306 ,  307 . The position encoder may determine position based on a baseline position corresponding to a start up position of the solar cell modules  115 . As described, this start up position for solar tracking may be one in which the solar cell modules  115  start tracking the sun at sunrise. However, in other implementations, this start up position corresponds to a position at a future time after sunrise, at which the solar cell modules  115  are to start tracking the sun. 
         [0034]    Motor controllers  308 ,  309  allow the low-power logic signals based on the algorithms to control the relatively high-power circuits of the motors  310 ,  311 . 
         [0035]    The data and algorithms of blocks  301 - 307  can be stored in one or more data stores (e.g., magnetic media, solid state media, or other suitable memory structure). The processing of, e.g., blocks  303  and  305 - 307  can be performed by, e.g., one or more microprocessors or special or general purpose computers. 
         [0036]      FIG. 5  is a plot of actual azimuth and elevation values for the sun, for sun elevation angles above 15 degrees. Plot  400  shows one actual path of the sun in both azimuth and elevation during an example day. Plots  410  and  420  each show a plot during two other days. As can be seen, plot  410  generally shows an azimuth and elevation of the sun much higher than plot  400 , the “current” azimuth and elevation, while plot  420  generally shows an azimuth and elevation much lower than plot  400 . Thus, it may be concluded that plots  410  and  420  occur during different seasons. For example, Plot  410  may occur closer to the summer months while plot  420  may occur closer to the winter months. 
         [0037]    While a solar tracking algorithm predicts or models the position of the sun throughout a typical year, misalignments between the position of the sun and the alignment of a solar cell array may occur for various reasons. For example, there may be errors in the sun position algorithm or the kinematic model. Even if the sun position algorithm and kinematic model are correct at one time instance, movements of the solar cell arrays either on a slow timescale (e.g. a gradual sagging of the longitudinal support  120  or subsidence of the whole row of solar cell arrays) or a fast timescale (for example earthquake movement) can lead to misalignment occurring.  FIG. 6  illustrates the actual path of the sun in comparison to the actual alignment of a solar cell array having misaligned solar tracking. 
         [0038]    To stay properly aligned with the sun during solar tracking, in this embodiment the terrestrial solar tracking system performs an alignment testing routine to correct misalignment relative to the sun. The alignment testing routine may be initiated manually by a user operation or automatically by the controller  190  based on a triggering event, which may be a sensed event or a predetermined time. As discussed hereafter, the alignment testing routine moves the solar cell modules off a current position in the roll and/or pitch directions while measuring (e.g., “scoring”) the performance of a cluster of four solar cell modules  115 , and analyses the measured performance data. 
         [0039]    In this embodiment, the alignment testing routine separately increments the roll position and the pitch position for a solar cell array and monitors the output current and/or power of the solar cell array. (For simplicity, we may also use the term “power” herein although either the parameter current or power may actually be measured).  FIG. 7  illustrates the process steps for monitoring the power of a solar cell array using an increment delta adjustment on the roll. Similarly,  FIG. 8  illustrates the process steps for monitoring the power of a solar cell array using an increment delta adjustment on the pitch. Both  FIGS. 7 and 8  illustrate techniques that individually or in combination may be used to correct for misalignment in solar tracking, as well as any of the other uses for monitored data as discussed herein. 
         [0040]    As shown in  FIG. 7 , a performance-monitoring test is started at block  500  with the solar array in a set position determined by the solar tracking system. The roll position of the solar cell array is adjusted to a position an increment amount, delta, from the set position at block  502 . The current output of the cluster of solar cell modules  115  at the new incremented position is then measured at block  504 . Then the roll position is then adjusted to a position decremented from the set position by an amount delta at block  506 . The current output of the cluster of solar cell modules  115  at the decremented position is then measured at block  508 . Based on the measured current or power from the incremented and decremented positions and the power at the original set position, a re-evaluation of the position of maximum power for the roll is performed at block  510  and the roll position is set to the position of “maximum power.” In particular, in this embodiment the re-evaluation of the maximum power position involves performing a weighted averaging of the incremented and decremented roll positions using the corresponding measured power outputs as weighting factors for the incremented and decremented roll positions. The power of the original set position may be sampled during block  510  or alternatively, data measured prior to starting the test at block  500  may be used as a power measurement for the original set position. 
         [0041]    If it is determined that the power has increased in the re-evaluated maximum power position compared with the original set position, then the alignment testing routine is repeated again using the re-evaluated maximum power position as the set position until the measured power is no longer increasing. If the measured power is no longer increasing, then at block  514  the system sets the cluster of solar cell modules  115  to the determined roll position. 
         [0042]    Similarly,  FIG. 8  illustrates a corresponding set of process steps for the pitch position. In the process implemented in  FIG. 8 , a performance-monitoring test for the pitch position is started at block  600 . The pitch position of the solar cell array is adjusted to a position an increment amount, delta, from an original set position at block  602 . The power output of the cluster of solar cell modules  115  at the new incremented position is then measured at block  604 . Then the pitch position is adjusted to a position decremented by an amount delta from the original set position at block  606 , and the power output of the cluster of solar cell modules  115  at the decremented position is measured at block  608 . Based on the measured power from the incremented and decremented positions, a re-evaluation of the position of maximum power for the pitch position is performed at block  610  using a weighted averaging as described with reference to  FIG. 7 . If the power output has increased from that in the original set position at block  612 , then the alignment testing routine is repeated again using the re-evaluated pitch position as the original set position for further increments and decrements until the measured power is no longer increasing, if the measured power is no longer increasing, then at block  614  the system sets the cluster of solar cell modules  115  to the determined pitch position. 
         [0043]    The adjustment of the roll and pitch positions performed in  FIGS. 7 and 8  above is complicated in this embodiment by the fact that the solar cell arrays are unbalanced and the positional feedback data provided from the position encoder in the roll and pitch motors exhibits some hysteresis (in other words, the previous movement history will affect the relationship between the positional feedback data and the actual position). To address this problem, in this embodiment during the alignment testing routine when the process of  FIG. 7  is performed the roll position is first moved to a reference roll position to one side of the set position and then the roll position is varied in a single direction to the incremented position and subsequently to the decremented position. The roll position is then returned to the reference roll position, and then the roll position is moved to the position of maximum power. The process of  FIG. 8  is performed in an analogous manner. The reference roll position and the reference pitch position may correspond to a parked position for the solar cell array. In this way, by always moving in the same direction from a reference position more accurate positioning is achieved. 
         [0044]    The monitored output parameter data from the cluster of solar cell modules  115  may be used purely for data logging to facilitate future manual adjustment of the solar tracking system. Further, the monitored data may be collected, based on periodic measurements, and compared against historical monitored data to identify trends in the monitored data, such as increasingly, progressive failure of the terrestrial solar tracking system. In such examples, the regression analysis or other means may be used to determine if the monitored data reaches a threshold level warranting action, and in response to reaching the threshold level notification indicating action is required is generated and output. 
         [0045]      FIG. 9  provides a schematic illustration of a system  700  in which a first subsystem  702 , such as the alignment testing routine described above, monitors performance of a solar cell array  704 . The monitoring system  702  may be triggered manually or automatically based on discrete events or periodic measurements. The triggering may be based on actual measured event, heuristic assumptions of conditions pertinent to operation, or other triggers. The triggering events may change over time or in response to the type of monitoring. For example, if the monitoring system  702  is to monitor for a first type of performance metric, then one triggering scheme may be used, while a different triggering scheme may be used for a different type of performance metric. 
         [0046]    The monitored data from the system  702  is provided to a second subsystem  706  which takes the monitored data, analyzes the data, and then provides an output which preferably performs some modification, transformation, or actuation, etc. based on that analysis. The second subsystem  706  may contain a processor for data analysis and a memory storage means for storing physical data corresponding to solar cell array  704 . In other examples, the subsystem  706  may be a motor control subsystem for the solar cell array  704 , a motor control subsystem for an entirely different solar cell array, or some other type of actuation based system. 
         [0047]    As discussed above, the second subsystem  706  may compare the measured performance with a threshold value, and output a warning to an operator if the measured performance crosses the threshold value. Alternatively, the subsystem  706  may be a processor-based system executing instructions to take monitored data and automatically adjust the kinematic model for the solar tracking system or the sun position algorithm. 
         [0048]    In other embodiments, the subsystem  706  could be a separate system that is interdependent upon the performance of the solar cell array  704 . For example, the subsystem  706  could be a system (e.g., a facility, dwelling, group of buildings, device, assembly or the like) that has a power demand dependent upon the solar cell array  704 . The subsystem  706 , therefore, may output a power usage regulation control for that system, whereby the subsystem  706  adjusts power usage in response to monitored performance data. 
         [0049]    In this embodiment, the monitored data corresponds to the power output of a solar cell array formed by a cluster of solar cell modules  115 . Alternatively, the monitored data can be collected on a per solar cell module basis to monitor performance of each solar cell module  115 . In this way, a misalignment between solar cell modules, or clusters of solar cell modules may be detected and an output signal generated indicating the misalignment. Further, the failure of a solar cell module  115  may be detected and an output signal generated indicating that failure. 
         [0050]    In other embodiments, however, the monitored data can be aggregated data from the terrestrial solar power system. Such monitoring allows a system to monitor and control a series of arrays together for performance optimization. 
         [0051]    In the illustrated embodiment, monitored data from one solar cell array is used to adjust the tracking for all solar cell arrays. This allows one or a smaller number of solar cell modules  115  to be individually mechanically adjusted, if need be, through the delta ranges on pitch, with respect to the position of other solar cell modules in the array. i This could also be useful in that at a given time certain solar cell modules  115  may be in direct view of the sun and thus more useful in monitoring performance than solar cell modules under cover of cloud and blocked from direct sunlight. 
         [0052]    The output parameter can be the normal output parameter measured for the solar cell module  115 , for example, the output current or the output power. Typically, during illumination the arrays constantly produce current, and therefore it is straightforward to provide electrical taps at the module level, the cluster level, or the array level, to capture and measure the produced current associated with any one module, cluster or array, and transmit such parameters to a system monitor for real time performance monitoring and analysis, or to data storage, so that the data can be accessed at a future time for analysis. The frequency with which the data is analyzed for alignment can vary, and can be at automatic intervals or up from automatic or manual triggering events. The techniques may be executed automatically each day, each week, each month, or seasonally. Of course, these are by way of example, as the techniques may be executed more frequently as well, for example. 
         [0053]    The system may consider numerous factors for measuring output parameter data. In fact, such factors can be used to determine when to initiate the alignment testing routine altogether, i.e., whether the output parameter data should be analyzed in the first instance. Other factors may be used to determine whether to execute alignment adjustment of the solar cell array based on the measured output parameters. For example, a decisional model may be used that analyzes power measured by the solar cell array and considers whether there has been a sudden drop in power that suggests an re-alignment is needed (such as a wind storm or ground shift) or a sudden drop that does not (such as a cloud moving between the sun and array). Other factors may include the average power measured over the day while solar tracking, historical data from previous, similar tracking periods (e.g., the day before, a similar day the year before, etc.). In any event, the decisional model may consider all such relevant factors in determining whether to actually analyze the measured parameter and determine whether a measured value suggests that alignment is needed or not. 
         [0054]    The algorithms of  FIGS. 7 and 8  are example applications of the monitored data measured by the system. These algorithms can be initiated manually by operator initiation or automatically, such as at predetermined intervals or at specifically determined events such as at start up. Other triggering events include a sudden decrease in a measured output parameter, which may indicate that the solar tracking routine is not aligned, or more slowly varying errors such as currently measured output parameters that are smaller in value than previous measurements from the same measurement period. 
         [0055]    In an alternative embodiment, instead of measuring the power at incremented and decremented positions relative to the set position, the alignment testing routine may monitor the power and measure the positions on either side of the set position at which a certain percentage drop in power has occurred for both the roll position and the pitch position. The maximum power position may then be determined by calculating the midpoint between the two measure positions. Again, the process is repeated in an iterative manner if the power at the maximum power position is greater than the power at the set position. 
         [0056]    The delta values discussed in  FIGS. 7 and 8  may or may not be fixed. The same adjustment values may be made for pitch and roll in some examples. But in others the adjustment values will be different. These values may be static, that is a fixed angle. In other examples, the amount of adjustment may be determined by the actual monitored output parameter. Typically, the delta values will be less than 5°. 
         [0057]    The performance monitoring and alignment algorithm of  FIGS. 7 and 8  may be performed as an auxiliary operation of a solar tracking mechanism that optimally predicts the location of the sun at a future time, and orients the array such that it aligns with the sun at that future time. The algorithm therefore is able to add further optimization to such a routine. 
         [0058]    In addition to triggering based on sudden decreases, the algorithms may be triggered by accumulated decreases over time, such as a continuous unexplained decrease in output power. 
         [0059]    While in the embodiments discussed above, the power performance of the solar cell array is monitored, in alternative embodiments the position of the solar cells could be directly monitored. Separate sensors could be used to measure the position of the longitudinal or vertical supports, for example, to measure against beam twisting or sagging. In response to a threshold value being reached, a warning signal could be output to an operator. Other examples are described herein and yet others will now become apparent to persons of ordinary skill in the art. 
         [0060]    The above embodiments have been described in the context of a terrestrial solar power system in which a plurality of solar cell arrays are mounted in a spaced configuration along a longitudinal support for rotation in tandem about an axis A generally aligned with the longitudinal support, and with each array also being rotatable about a respective axis B generally orthogonal to the longitudinal support. A skilled person will recognize that the monitoring techniques described above can be used with other configurations of solar power system. For example, the monitoring techniques could be used with a terrestrial solar tracking system having an array of solar cell modules mounted on a single tower, with the array of solar cell modules being adjustable in the azimuth direction and the elevation direction in order to track the sun. 
         [0061]    Embodiments of the subject matter and the functional operations described in this specification can be implemented in digital electronic circuitry, or in computer software, firmware, or hardware, including the structures disclosed in this specification and their structural equivalents, or in combinations of one or more of them. Embodiments of the subject matter described in this specification can be implemented as one or more computer program products, i.e., one or more modules of computer program instructions encoded on a computer readable medium for execution by, or to control the operation of, data processing apparatus. The computer readable medium can be a machine-readable storage device, a machine-readable storage substrate, a memory device, or a combination of one or more of them. The term “data processing apparatus” encompasses all apparatus, devices, and machines for processing data, including by way of example a programmable processor, a computer, or multiple processors or computers. The apparatus can include, in addition to hardware, code that creates an execution environment for the computer program in question, e.g., code that constitutes processor firmware, a protocol stack, a database management system, an operating system, a runtime environment or a combination of one or more of them. A propagated signal is an artificially generated signal, e.g., a machine-generated electrical, optical, or electromagnetic signal, that is generated to encode information for transmission to suitable receiver apparatus. 
         [0062]    A computer program (also known as a program, software, software application, script, or code) can be written in any form of programming language, including compiled or interpreted languages, and it can be deployed in any form, including as a stand alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program does not necessarily correspond to a file in a file system. A program can be stored in a portion of a file that holds other programs or data (e.g., one or more scripts stored in a markup language document), in a single file dedicated to the program in question, or in multiple coordinated files (e.g., files that store one or more modules, sub programs, or portions of code). A computer program can be deployed to be executed on one computer or on multiple computers that are located at one site or distributed across multiple sites and interconnected by a communication network. 
         [0063]    The processes and logic flows described in this specification can be performed by one or more programmable processors executing one or more computer programs to perform functions by operating on input data and generating output. The processes and logic flows can also be performed by, and apparatus can also be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application specific integrated circuit). 
         [0064]    Processors suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a read only memory or a random access memory or both. The essential elements of a computer are a processor for performing instructions and one or more memory devices for storing instructions and data. Generally, a computer will also include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto optical disks, or optical disks. However, a computer need not have such devices. Moreover, a computer can be embedded in another device. Computer readable media suitable for storing computer program instructions and data include all forms of non volatile memory, media and memory devices, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto optical disks; and CD ROM and DVD-ROM disks. The processor and the memory can be supplemented by, or incorporated in, special purpose logic circuitry. 
         [0065]    While operations are depicted in the drawings in a particular order, this should not be understood as requiring that such operations be performed in the particular order shown or in sequential order, or that all illustrated operations be performed, to achieve desirable results. In certain circumstances, multitasking and parallel processing may be advantageous. 
         [0066]    A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the claims.