Patent Publication Number: US-2013249297-A1

Title: Energy recovery from a photovoltaic array

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/586,036, filed Jan. 12, 2012 and incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     Embodiments of the invention relate generally to rapid reconfiguration of electrical connections between photovoltaic modules in a photovoltaic array, and more specifically to maximizing output power from a photovoltaic array by adaptive reconfiguration of serial and parallel electrical connections between photovoltaic modules. 
     BACKGROUND 
     A photovoltaic (PV) module comprises many relatively small solar cells connected together in an electrical circuit. The PV module may include a transparent cover over the solar cells to protect the solar cells from mechanical damage and may be sealed to prevent circuit faults, for example open circuits or short circuits, from water or contaminants such as dust and dirt. A PV panel comprises one or more PV modules mechanically attached to a common support substrate or frame and having combined electrical outputs through one or more electrical connectors. The PV modules on one PV panel may have a fixed arrangement of electrical connections between modules. The electrical power output from one PV panel includes the power contributed from each PV module on the panel, and the output of each PV module includes the power output from each solar cell in the module. 
     A PV array for converting solar energy to electrical power may include several hundred PV panels mounted on the roof of a building or a mechanical support structure located close to local electrical loads. A utility-scale PV array may include thousands of PV panels electrically interconnected in large groups. A reduction in output power from a small number of PV panels in a PV array may substantially reduce output power from the entire array. For example, a reduction in output power from just one PV module on a PV panel can cause a substantial reduction in the output power from an entire PV array. 
     Output power from a PV panel may be reduced by, for example, a shadow falling across part of the PV panel&#39;s photosensitive surface, high temperature in part of the PV panel (sometimes referred to as a “hot spot”), aging effects, or dust, water, or debris accumulating on the PV panel. Power output may also be reduced by mechanical damage to the relatively brittle silicon material commonly used in the manufacture of commercially available PV panels. Corrosion and electrical insulation breakdown in electrical conductors, electrical connectors, and other components may also reduce PV panel output power. 
     Power output from a PV array may be monitored to determine if PV panels within the array have malfunctioned or are otherwise operating with reduced power output. A supervisory monitoring and control system may communicate with each of the PV panels in a PV array to log values related to PV array performance, detect fault conditions, and change operating parameters in response to load changes, weather events, daily and seasonal illumination changes, and so on. Because even a modest reduction in the output current, voltage, or power from one PV panel can reduce power output from the entire array, detection of an underperforming panel, for example a partially shadowed panel or a panel with a hot spot, may cause the supervisory control and monitoring system to switch the underperforming panel out of the array. As the shadow falls across more PV panels, for example when a cloud shadow passes over the PV array, more and more PV panels may be switched out of the PV array, and array output power decreases. 
     A partially-shadowed PV panel may still produce electrical output power. Even a fully shadowed PV panel may produce a usable amount of power. However, once an underperforming PV panel is switched out of a PV array, any power the PV panel could have contributed to the array output is lost. Power that might have been produced from PV panels underperforming for reasons other than partial shadowing would also be lost when the underperforming panels are switched out of an array. 
     A PV panel may be underperforming in the sense that its output voltage and current are less than other panels in a PV array even with all the PV panels are operating in accord with their design specifications. In this sense, underperformance is relative to other panels and may result from different operating specifications for different PV panels, for example PV panels from different manufacturers. An automatic supervisory monitoring and control system may attempt to switch such mismatched panels out of an array, even though the panels are capable of contributing power to the PV array. Some PV panels may produce more electrical power under a particular set of illumination and environmental conditions than other PV panels. It may be advantageous to be able to include different types of PV panels in one PV array to take advantage of a broader range of illumination and environmental conditions or lower-cost PV panels, without degrading the output of the array to a condition related to the lowest-performing panels. 
     SUMMARY 
     An example of an embodiment of the invention includes a monitoring module for a photovoltaic (PV) panel. The example of a monitoring module includes a module controller, a serial-parallel selector control output electrically connected to the module controller, and a bypass selector control output electrically connected to the module controller. The example of a monitoring module further includes a first and a second of two redundant means of communication electrically connected to the module controller, and a sensor and indicator input and output module in data communication with the module controller. The example of a monitoring module also includes a power management and battery backup circuit adapted to receive input power from at least one photovoltaic panel and having an output for providing electrical power to the module controller. Some embodiments of an intelligent node do not include battery backup but may have connections for an optional external battery. Each of the at least two redundant means of communication are configured for exchanging data and commands between at least two of the module controller. The module controller selects one of the two redundant means of communication when the other of the two redundant means of communication is not available for communication. The module controller is adapted to control a series-parallel switching state of a serial-parallel selector connected to the serial-parallel selector control output. The module controller is further adapted to control a bypass switching state of a bypass switch connected to the bypass selector control output. 
     Another example of an embodiment of the invention comprises a method for selecting a combination of serial and parallel electrical connections between PV panels in a PV array, including connecting a plurality of PV panels in a PV array in an initial series-parallel (S-P) configuration corresponding to an initial arrangement of serial and parallel electrical connections between the PV panels, calculating an initial value of PV array output power for the initial S-P configuration, measuring an amount of output power from the PV array, and detecting a change in an amount of PV array output power in comparison to the initial value of PV array output power. The example of a method embodiment of the invention further includes reconfiguring the PV array into a plurality of new S-P combinations, and for each new S-P configuration, storing a value of PV array output power and a value representing a switching state for an S-P selector on each PV panel in the PV array, selecting the maximum value of PV array output power from the stored values of PV array output power, retrieving the value representing the switching state for an S-P selector on each PV panel in the PV array corresponding to the maximum value of PV array output power, and setting the PV array to the S-P configuration corresponding to the selected maximum value of PV array output power by setting the S-P selector on each PV panel according to the retrieved value representing the switching state. 
     This section summarizes some features of the present invention. These and other features, aspects, and advantages of the invention will become better understood with regard to the following description and upon reference to the following drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a simplified block diagram of an example of an intelligent node in accord with an embodiment of the invention in which an example of a monitoring module includes a series-parallel switch and a bypass selector. 
         FIG. 2  shows an example of a PV module which may be used with embodiments of the invention. 
         FIG. 3  shows an example of a PV panel which may be used with embodiments of the invention. 
         FIG. 4  continues the example of  FIG. 1 , showing electrical connections between parts of an intelligent node.  FIG. 4  further represents an alternative embodiment of a monitoring module in which the monitoring module includes control ports for an external series-parallel switch and an external bypass selector. 
         FIGS. 5-7  illustrate block diagrams of alternative implementations of a module controller. 
         FIG. 8  illustrates an example of the intelligent node of  FIGS. 1 and 4 . 
         FIG. 9  continues the example of  FIG. 4 , showing components in the monitoring module for performing bypass and series-parallel (S-P) switching functions. 
         FIG. 10  represents an example of a reconfigurable PV array comprising two groups of interconnected intelligent nodes whose combined power outputs are connected to a DC to AC inverter. 
         FIGS. 11-13  show another example of a reconfigurable PV array with different configurations of serial and parallel electrical connections between intelligent nodes. 
         FIG. 11  shows an integer number “n” groups of intelligent nodes electrically connected in parallel, with each of the intelligent nodes within a group electrically connected in series. 
         FIG. 12  continues the example of  FIG. 11 , showing one of the groups of intelligent nodes having two intelligent nodes electrically connected in parallel and the remaining intelligent nodes in the group electrically connected in series. 
         FIG. 13  continues the example of  FIGS. 11-12 , showing more examples of different ways in which intelligent nodes within a group may be operated in selectable combinations of series and parallel electrical connections. 
     
    
    
     DESCRIPTION 
     Some embodiments of the invention comprise an intelligent node for recovering energy from underperforming solar panels by adaptively switching electrical connections between intelligent nodes. Embodiments of the invention may switch electrical connections between intelligent nodes in a PV array in response to measured or predicted changes in incident solar radiation, magnitude of an electrical load receiving power from a PV array, an electrical fault in one or more PV panels in the PV array, to isolate one or more PV panels for maintenance, cleaning, or replacement, or for other reasons. A PV array in accord with an embodiment of the invention is capable of rapidly reconfiguring itself to deliver a maximum amount of output electrical power in response to measured, predicted, or reported changes in parameters that affect the operation of a PV array. Examples of parameters which may be measured or monitored by an embodiment of the invention include, but are not limited to, output current and voltage from a PV array, output voltage and current from each PV panel in a PV array, output current and voltage from PV modules on a PV panel, inverter output voltage and current, current and voltage supplied to an electrical load by the PV array, measurements of incident solar radiation, temperature measurements on a PV module, PV panel, battery, or other parts of a PV array, ground fault detectors, arc fault detectors, tilt angles for PV panels, motor voltages and currents for heliostats or systems for changing tilt angles of PV panels, and so on. 
     Embodiments of the invention are capable of outputting more electrical power from a partially-shadowed PV array or a PV array generating reduced output as a result of damaged or otherwise underperforming PV panels than previously known PV arrays having a fixed arrangement of serial and parallel electrical connections between PV panels. A quantitative difference in an amount of power generated by an embodiment of the invention compared to a prior-art PV array corresponds to an amount of recovered power that would have been lost in a prior-art system. 
     As used herein, an intelligent node refers to an apparatus for rapidly reconfiguring electrical connections between a PV panel connected to the intelligent node and PV panels connected to other intelligent nodes, without disconnecting and reconnecting electrical cables between intelligent nodes or between PV panels. A plurality of intelligent nodes electrically connected with one another is referred to as a reconfigurable PV array. Each intelligent node optionally includes at least one PV panel. Each PV panel includes at least one PV module, and each PV module includes a plurality of interconnected PV cells. More than one PV panel may optionally be connected as a group to one intelligent node, and the intelligent node may control electrical connections between its connected group of PV panels and groups of PV panels connected to other intelligent nodes. 
     An intelligent node in accord with an embodiment of the invention may accept commands from an external supervisory monitoring and control system to change serial and parallel electrical (S-P) connections to neighboring intelligent nodes or to bypass one or more PV panels, or the intelligent node may make such switching changes autonomously. Intelligent nodes may communicate measured values related to solar panel performance to the supervisory control and monitoring system and to other intelligent nodes. Some embodiments of the invention comprise a PV array including a plurality of interconnected intelligent nodes. Some embodiments of the invention include steps in a method for finding a combination of S-P connections between intelligent nodes in a reconfigurable PV array that result in a maximum amount of PV array output electrical power for a given set of operating conditions. 
     Embodiments of the invention are able to rapidly adapt to changing operating conditions such as, but not limited to, partially shadowed PV panels, weather changes, hot spots on one or more PV panels, or dirt or foreign objects obscuring light-sensitive surfaces on part of one or more PV panels. Embodiments of the invention are also able to maximize power output from PV arrays comprising PV panels having mismatched specifications for output voltage, current, and power. Such mismatches may be related to differences in design specifications between panels from different manufacturers or may be the result of differences in aging effects between one group of PV panels and another. Intelligent nodes are particularly well suited to recovering power from underperforming PV panels by connecting underperforming PV panels in an optimized combination of serial and parallel electrical connections to PV panels in other intelligent nodes, rather than simply switching underperforming PV panels out of the PV array as is commonly done in prior art arrays. 
     An example of an intelligent node in accord with an embodiment of the invention is shown in  FIG. 1 . An apparatus embodiment of the invention  100  comprises an intelligent node  366  having a monitoring module  300  for controlling electrical connections to other intelligent nodes in a PV array. Connections between two or more intelligent nodes are made through connectors P1  102  and P2  156 . Unless otherwise stated, “connected” will refer hereinafter to electrical connection between two components. The monitoring module  300  optionally includes measuring and status reporting capabilities. The monitoring module  300  includes a node controller  364  connected to a series-parallel (S-P) selector  138  by an S-P control line  116 . The node controller  364  is also connected to a bypass selector  120  by a bypass control line  118 . Operation of the S-P and bypass selectors will be explained in more detail in relation to  FIG. 9 . 
     The S-P selector  138  and bypass selector  120  in the example of  FIG. 1  receive current and voltage output from a PV panel  200 , or from a group of PV panels, on a V+ line  110  and a V− line  112 . The bypass selector  120  operates to selectively include or exclude current and voltage from the PV panel  200  from current and voltage present at connectors P1  102  and P2  156 . An example of a PV panel suitable for use in an intelligent node is shown in  FIG. 3 , and an example of a PV module which may be used in the PV panel of  FIG. 3  is shown in  FIG. 2 . More than one PV panel may optionally be connected to a monitoring module in an intelligent node by connecting at least two PV panels in a series electrical circuit comparable to the series electrical circuit shown in the example of  FIG. 3  for connections between PV modules in one PV panel. 
     The PV module  108  of  FIG. 2  includes PV cells  404  for converting light to electrical energy. The PV module  108  may include a plurality of PV cells  404  connected to one another in a series electrical circuit. Groups of serially-connected PV cells  404  may further be connected to one another in a parallel electrical circuit. A bypass diode  404  may be included, with the diode&#39;s cathode connected to the PV module V+ output  408  and the diode&#39;s anode connected to the V− output  410 . The PV cells  404  may be electrically modeled as a diode connected to the V+ and V− outputs of the PV module. Bypass diodes may optionally be connected across V+ and V− outside the PV module  108 . When several PV modules are connected in a series electrical circuit, for example as shown in  FIG. 3 , the bypass diode  404  may cause the polarity of the output connections ( 408 ,  410 ) to reverse when one or more of the PV modules is partially shadowed, when one or more of the modules develops a hot spot, or when a PV panel underperforms for other reasons. A shadowed or otherwise underperforming PV module decreases the output voltage, current, and power from the module&#39;s PV panel, which may in turn decrease output power from the entire PV array as previously stated. 
     The monitoring module  300  of  FIG. 1  may optionally be provided in an enclosure that is mechanically attached to a PV panel  200  as suggested in the example of an intelligent node  366  in  FIG. 8 . Direct attachment of the monitoring module  300  to the PV panel  300  may expose the monitoring module to high temperatures, high voltages, and electrical noise, so the monitoring module  300  may alternatively be provided in a case or enclosure that may be electrically connected to but mechanically separate from a PV panel  200 . A monitoring module  300  provided as a separate enclosure also permits the monitoring module to be positioned in a location that is more easily accessible for service, repair, or replacement than the PV panels may be, for example by positioning the monitoring module close to the ground when PV panels are on an elevated structure. 
     An example of an embodiment of an intelligent node is shown in  FIG. 4 .  FIG. 4  represents a simplified block diagram of an intelligent node  366  including a monitoring module  300  electrically connected to an optional PV panel  200 . The monitoring module  300  measures parameter values related to the status and performance of the PV panel  200  and optionally outputs electrical signals, visual signals, and sound signals to assist operating and maintenance personnel in identifying and locating a particular intelligent node from which reported values originated. A combination of a module controller  306 , an optional power management and battery backup module  302 , an optional sensor input module  308 , a PV panel identification (ID) memory  312 , a data and program memory  314 , and a data and communications bus  334  may optionally be provided as a node controller  364  in the monitoring module  300 . In some embodiments of an intelligent node, a sensor input module may be combined with an indicator output module to form a combined sensor and indicator I/O module. 
     The example of a monitoring module  300  in  FIG. 4  includes a module controller  306  for monitoring parameters from the PV panel  200  and comparing measured parameter values against saved values to determine if the PV panel is malfunctioning or operating inefficiently. The module controller  306  sends and receives digital and optionally analog signals over a plurality of electrical connections comprising a data and communications bus  334 . In some embodiments of a monitoring module, analog signals are converted to digital signals and digital signals are converted to analog signals by sensor input module  308 . Alternatively, some signal conversion is accomplished within the module controller  306 . The module controller  306  is adapted for communicating parameter values with an external system such as a monitoring and control system or a portable data collection system and for outputting signals for identification of the PV panel being monitored by the monitoring module  300 . Electrical signals are selectively exchanged between the module controller  306  in the monitoring module  300  and an external system, for example another intelligent node or a supervisory control system, through a communications I/O port  316 . 
     I/O port  316  represents an example of a first redundant means of communication for exchanging signals representative of data and commands between module controllers in intelligent nodes in a PV array and between a module controller in an intelligent node and an external supervisory control system. Redundant means of communication improve the reliability and availability of communications between intelligent nodes by providing for alternative communications pathways between intelligent nodes and between an intelligent node and an external system. Redundant means of communication may improve the overall reliability of a photovoltaic. Embodiments of the invention may alternatively send, receive, or send and receive the same data and commands one more than one redundant means of communication simultaneously, associate sending data and commands with one means of communication and receiving with the other, or send and receive data over one means of communication and commands over the other. 
     The I/O port  316  is adapted for sending and receiving signals representative of data and commands over a physical transmission medium such as a wired network using coaxial cables, twisted-pair interconnections, or other forms of interconnecting electrical cables, or optical communications over an optical fiber connection. Signals representative of data and commands may combine representations of data values and representations of commands into one signal or may segregate data and commands from one another. Data and commands may be represented as, for example but not limited to, electrical signals carried on an electrical conductor, radio signals, optical signals, analog signals, or digital signals. A wireless transceiver  368  represents an example of a second redundant means of communication. The wireless transceiver  368  is adapted for sending and receiving data and commands by exchange of radio frequency or optical signals between a transmitter and a receiver without an interconnecting physical transmission medium such as a cable, fiber optic, or wire between the transmitting and receiving systems. The monitoring module  300  may optionally operate autonomously or may measure, save, and report parameter values after receiving commands from an external system. 
     A module controller  306  may alternatively be implemented using discrete logic, a microprocessor, or a microcontroller, or as a customizable logic device such as an application specific integrated circuit (ASIC), a programmable logic device (PLD), a gate array, or a combination of these devices, and optionally includes a combination of digital and analog circuits. An example of a module controller  306  having a microprocessor is shown in  FIG. 5 . In  FIG. 5 , a module controller  306  comprises a microprocessor  370  having a central processing unit (CPU)  384  and a clock/calendar circuit  310 . The CPU  384  sends and receives data and commands through a plurality of lines connected to the communications I/O port  316  on the monitoring module. The CPU  384  obtains time and date information from the clock/calendar  310 , which may alternatively be implemented as a circuit in the microprocessor  370 , as a peripheral electrical circuit, for example a peripheral integrated circuit, or as software executing on the CPU  384 . The microprocessor  370  communicates with the sensor/indicator I/O circuit module  308  and one or more memory devices  372  over a plurality of lines comprising the data and communications bus  334 . The memory device may optionally include a PV panel ID memory  312  and a data and program memory  314 . Alternately, the PV panel ID memory  312  and the data and program memory  314  may be located in separate memory devices  372 . 
     An example of a module controller  306  having a microcontroller is shown in  FIG. 6 . In  FIG. 6 , a module controller  306  comprises a microcontroller  374  having a CPU  384 , a clock/calendar  310 , a PV panel ID memory  312 , a data and program memory  314 , digital I/O  376  for exchanging digital signals with the sensor/indicator I/O circuit module  308  over the data and communications bus  334 , and analog I/O  378  for exchanging analog signals with the sensor/indicator I/O circuit module  308 . Optionally, an external memory device may be connected to the microcontroller  374  to increase memory capacity, for example by connecting a memory device  372  as shown in the example of  FIG. 5 . 
     An example of a module controller  306  implemented as a customizable logic device is shown in  FIG. 7 . In the example of  FIG. 7 , the customizable logic device  382  includes a CPU  384  electrically connected to a data and communications bus  334 , a clock/calendar  310 , a PV panel ID memory  312 , a data and program memory  314 , and digital I/O circuitry  376 . Analog I/O functions, for example an analog to digital converter, a digital to analog converter, a high-current output driver, and a high-voltage output driver, may optionally be part of the sensor circuit module  308 . In other embodiments, some or all of these analog functions are included in the customizable logic device. 
     As shown in  FIG. 4 , the module controller  306  is electrically connected to a communications input/output (I/O) port  316 . Signals representative of PV panel parameter values may optionally be output by the module controller  306  on the communications I/O port  316 . Signals representative of commands to be performed by the module controller  306  may optionally be received from an external monitoring and control system on the communications I/O port  316 . The module controller may receive commands or data from other intelligent nodes on the communications I/O port  316 . Examples of commands and data include, but are not limited to, output of an identification code for the PV panel, output of time- and date-stamped parameter values for the PV panel, and error codes related to PV panel status. Data and commands exchanged between the monitoring module  300  and an external monitoring and control system via the communications I/O port  316  may pass over an external communications system, for example a communications system using electrical conductors, fiber optics, or power line communications (PLC). 
     The data and program memory  314  is adapted for storage and retrieval by the module controller  306  of commands received through the communications I/O port  316  and digital data values output from the sensor/indicator I/O circuit module  308 , the PV panel ID memory  312 , and the clock/calendar  310 . 
     The module controller may optionally perform data logging to create records of PV array performance under different conditions of air temperature, solar illumination, partial shading of the PV array, array output for different S-P configurations, and so on. Time and data values may optionally be obtained from the clock/calendar circuit  310  by the module controller  306  of  FIGS. 1-5 . The module controller  306  may associate time and date values with one or more measured parameter values and save the time, date, and parameter values in the data and program memory  314  to form a historical log of PV panel performance. A historical log may optionally include a time and date at which the module controller  310  detects a parameter value from the PV panel  200  that is outside a range of values retrieved from the data and program memory  314 . Limiting values related to a PV parameter range may optionally be received by the monitoring module  300  through the communications I/O port  316  and saved in the data and program memory  314 . Limiting values for parameter ranges may optionally be modified by the module controller  306  in response to, for example, measured values of temperature or incident illumination. 
     When a measured parameter crosses a threshold defined by a limiting value, the monitoring module may report the condition to an external monitoring system. The external monitoring system may direct the PV array to switch to an S-P configuration retrieved from the monitoring system&#39;s storage subsystem, the monitoring system may seek improved PV array output by switching the array into many different S-P configurations, or the external monitoring system may direct the intelligent nodes in the PV array to autonomously search for a new S-P configuration that provides improved output power from the array. 
     The PV panel ID memory  312  in  FIGS. 1-5  optionally retains an identification code assigned to each PV panel  200  in a PV array. The identification code may be saved in the monitoring module  300  at the time the monitoring module  300  is installed on a PV panel. Alternatively, an identification code may be received from an external system through the communications I/O port  316  and stored in the PV panel ID memory  312  by the module controller  306 . In some embodiments, the PV panel ID memory is nonvolatile memory which may optionally be reprogrammable or may alternately be programmable once. In other embodiments, an identification code is retained in the PV panel ID memory  312  as long as the memory  312  receives power from a PV panel  200  or from a battery in the monitoring module  300 . 
     The module controller  306  may exchange signals with alarm indicators and sensors through a sensor/indicator I/O circuit module  308 . In some embodiments, the sensor/indicator I/O circuit modifies output signals from the module controller  306  so the signals have sufficient voltage and current to drive a visual indicator  320 . In some embodiments of an intelligent node, inputs from sensors and outputs to indicators are partitioned into different modules. Other signals from the module controller  306  are modified so the signals are able to drive an audible indicator  322 . Sensor output signals related to PV panel parameters are also conditioned by the sensor/indicator I/O circuit before being input to the module controller  306 . For example, an optional illumination sensor  324  measures an amount of light incident upon the solar panel  200 . The signal from the illumination sensor  324  is converted to a digital value for input to the module controller  306  and is saved by the module controller  306  in the data and programming memory  314 . Alternately, an output signal from the illumination sensor  324  is converted to a corresponding digital value within the module controller  306 . Electrical signals from the illumination sensor  324  are coupled into the sensor/indicator I/O circuit module  308  through an optional cable connector P7  356  and through a corresponding optional connector J7  358  on the monitoring module  300 . 
     Output voltages V+ and V− from the PV panel  200  are output on an electrical connector J2  206 , as shown in  FIG. 4 . Cable connector P2  338  connects to J2  206  and carries voltages V+ and V− to cable connector P3  340 , which attaches to power input connector J3  342  on the monitoring module  300 . Alternatively, electrical connections to and from the monitoring module may be made with point-to-point wiring instead of with electrical connectors, for example point-to-point wiring electrically connected to terminal strips. A value of PV panel  200  output current is measured by an optional current sensor  330  in series with a power connection between J3  342  and a Power Management and Battery Backup circuit  302 . An output signal from the current sensor  330  is input to the sensor/indicator I/O circuit module  308 , converted to a form suitable for input to the module controller  306 , and a corresponding numerical value of PV panel output current is selectively stored in the data and program memory  314 . Similarly, a value of PV panel  200  output voltage is measured by a voltage sensor  328  electrically connected to the power input connector J3  342  and sensor/indicator I/O circuit module  308 , and a PV panel output voltage value is selectively saved in the data and program memory  314 . The module controller  306  may then compare measured values of current and voltage from the PV panel  200  against, for example previously saved values, or against a range of values related to an amount of illumination measured by the illumination sensor  324  to determine if the PV panel is operating efficiently or if it is producing a smaller amount of output power than expected. 
     A PV panel  200  may optionally include one or more temperature sensors  202 . Signals related to temperatures on the PV panel  200  are output from a connector  204  on the PV panel  200 , coupled to cable connector  336  and then to connector J4  346  on the monitoring module  300 . Output signals from the temperature sensor  202  pass through lines from connector J4  346  to inputs to the sensor/indicator I/O circuit module  308 . Values for measured temperatures on the PV panel  200  are selectively saved in the data and program memory  314  for subsequent comparison by the module controller  306  against a range of operating temperatures for normal operation of the PV panel. A measured temperature may also be used by the module controller  306  to modify expected values of other parameters, for example a value of output current expected at a particular temperature. A measured temperature outside a range of operating temperatures is detected by the module controller  306 , which may send a signal representing an alarm condition to the communications I/O port  316  and the sensor/indicator I/O circuit module  308 . 
     A signal representing an alarm condition may cause activation of one or more alarm indicators such as a visual indicator  320  or an audible indicator  322 . In some embodiments, for example the embodiment shown in  FIG. 4 , the visual indicator  320  comprises one or more incandescent bulbs or light-emitting diodes (LEDs) capable of being collectively turned on and off in response to a signal output from the sensor/indicator input/output circuit  308  under the control of the module controller  306 . In other embodiments, the visual indicator  320  comprises an alphanumeric display adapted to show an error code, a panel identification number, or other selected alphanumeric values. An example of a visual indicator  320  comprising an alphanumeric display  402  is shown in  FIG. 2 . In the example of  FIG. 4 , the alphanumeric display  402  displays an error code “E 2 ”, although one will appreciate that other letters and numbers could also be displayed. In the example of  FIG. 4 , the alphanumeric display  402  receives input signals representative of data to be displayed from the module controller  306 . In other embodiments, the alphanumeric display receives input signals from the sensor/indicator I/O circuit module  308 . The alphanumeric display  402  in  FIG. 4  may alternatively be implemented as an LED display, a vacuum fluorescent display, a liquid crystal display, an electromechanical display, or other types of display capable of showing characters which may be read in daylight or at night by service personnel standing several yards (meters) away from the PV panel  200 . Although the example of  FIG. 4  shows an alphanumeric display for two characters, a display for showing more than two characters may be used. 
     Signals from the sensor/indicator I/O circuit module  308  to the visual indicator  320  are optionally coupled through connector J5  350  on the monitoring module  300  and cable connector P5  348  electrically connected to the visual indicator  320 , as shown in  FIG. 4 . Signals from the sensor/indicator I/O circuit module  308  to the audible indicator  322  are optionally coupled through connector J6  354  on the monitoring module  300  and cable connector J6  352  electrically connected to the audible indicator  322 . 
     The visual indicator  320  and the audible indicator  322  are provided to assist service personnel in locating a PV panel having an out of range temperature condition as determined by the module controller  320 . Furthermore, the module controller  306  may optionally output an alarm signal for a current sensor  330  output signal or a voltage sensor  328  output signal outside a range expected for a measured amount of incident illumination. For example, a PV panel exposed to sunlight but having no output current may cause an alarm signal to be output by the module controller  306 . The module controller may optionally suppress the output of some alarm signals when the illumination sensor senses that the panel is receiving too little illumination to output usable electric power. Sounds produced by the audible indicator  322  and lights emitted from the visual indicator  320  may optionally be output in selected on-off patterns for conveying information to a person seeing or hearing the alarm indicator. Data related to selected patterns and associated error conditions are stored in the data and program memory  314  and retrieved by the module controller  306 . 
     A monitoring module  300  optionally includes a wireless transceiver  368  electrically connected to the module controller  306  over the data and communications bus  334  as shown in  FIG. 4 . The wireless transmitter  304  selectively transmits and receives radio frequency signals related to data from the module controller  306  and data and program memory  314  over a beacon antenna  318 . Electrical signals between the beacon antenna  318  and the wireless transmitter  304  pass through an optional cable connector P8  360  and a corresponding connector J8  362  on the monitoring module  300 . The wireless transceiver  368  is representative of a second redundant means of communication for input and output of data and commands related to the operation of the module controller  306 , monitoring module  300 , intelligent node  366 , and PV panel  200 . 
     Data sent from the module controller  306  to the wireless transmitter  304 , or alternately to the transceiver  368 , optionally includes, but is not limited to, a PV panel identification code, a time value, a data value, values for PV panel temperature, output current, and output voltage, a value for incident illumination, positions of bypass selector  120  and S-P selector  138 , and data related to operational status of the monitoring module  300 , for example, but not limited to, charge status of a battery in the power management and battery backup circuit  302 . One will appreciate that many other data items related to PV panel condition may optionally be sent by the module controller  306  to the wireless transmitter  304  for radio transmission to an external system. In some embodiments, the wireless transmitter  304  or the transceiver  368  conforms to a communication protocol for relatively long range communications. In other embodiments, the wireless transmitter  304  or the transceiver  368  conforms to a communications protocol for relatively short range communications, such as Bluetooth (IEEE 802.11) or similar standards for sending information to portable devices separated by a few meters from the monitoring module. Such a portable device may be carried by service personnel or carried in a vehicle for rapidly scanning output transmissions from a large number PV panels in a PV array. Any one or more of the previously described data items may be exchanged bidirectionally between the module controller  306  and an external system, for example another intelligent node or an external supervisory and control system, through either one or both of the redundant means of communication. 
     If one of the redundant means of communication is not available for communication, for example because the means of communication is not operable or is busy, the module controller  306  may autonomously select the other redundant means of communication to data and commands with other systems. Alternatively, an external system may command the module controller to select a specific one of the redundant means of communication for conducting communications with the external system or with other intelligent nodes. Some intelligent nodes in a PV array may use one of the redundant means of communication while other intelligent nodes are using a different one of the redundant means of communication. 
     Referring again to  FIG. 4 , power to operate the monitoring module  300 , optional sensors, and optional alarm indicators is supplied by the PV panel  200 . Output current and output voltage from the PV panel  200  are input to the power management and battery backup circuit  302 . The power management and battery backup circuit  302  distributes the current and voltage received from the PV panel  200  on a power bus Vcc  332  to other parts of the PV panel monitoring apparatus  100 . Optionally, the power management and battery backup circuit  302  outputs a voltage Vcc having a different value than the value of voltage output from the PV panel  200 . The power management and battery backup circuit  302  includes a backup battery and circuitry for charging the battery so that the monitoring module  300  may continue to operate when the PV panel is not producing sufficient output power, for example at night or when a shadow falls across the PV panel. 
     As shown in the example of  FIG. 1 , the intelligent node  366  includes a monitoring module  300 , at least one PV Panel  200 , an S-P selector  138 , and a bypass selector  120 . In some embodiments of a monitoring module  300 , the S-P selector  138  and bypass selector  120  are included in a common enclosure or case with other parts of the monitoring module  300 . In alternative embodiments of a monitoring module  300 , the monitoring module  300  includes a series-parallel control port  116  and a bypass control port  118 , each connected to the data and communications bus  334  and in data communication with the module controller  306 , with either one or both of the series-parallel switch and bypass selector mounted externally to the monitoring module  300  and connected to the monitoring module by cable assemblies. 
     Some embodiments of a monitoring module  300  include circuits for detecting a ground fault in a photovoltaic panel or in cables connecting a PV panel or monitoring module to other parts of a PV array. A Ground Fault Circuit Detector (GFCD)  398  in  FIG. 4  is electrically connected in parallel with V+ and V− lines from the output of the PV panel  200  to the inputs of the power management and battery backup circuit  302 . In order to reduce the risk of fire from an electrical short circuit or an electrical arc resulting from breakdown in electrically insulating materials in the PV panel  200 , monitoring module  300 , or electrical connections between these components, some embodiments include an arc fault circuit detector (AFCD)  400 , also electrically connected in parallel with V+ and V− lines from the output of the PV panel  200  to the inputs of the power management and battery backup circuit  302 . An output from the GFCD  398  and an output from the AFCD  400  are electrically connected to the data and communications bus  334 . Alternately, outputs from the GFCD  398  and AFCD  400  are electrically connected directly to inputs on the module controller  306 , for example interrupt inputs. Upon receiving a signal from the GFCD  398  or the AFCD  400 , the module controller  306  may selectively shut down parts of the monitoring module  300 , cause the PV panel  200  to be electrically bypassed or electrically disconnected from the PV array in which the PV panel resides. The monitoring module  300  optionally outputs audible or visual alarm signals to warn service personnel about ground fault or arc fault hazards. 
     A front view of an example of an intelligent node  366  comprising a monitoring module mechanically attached to a PV panel is shown in  FIG. 8 . One or more optional temperature sensors  202  are attached to the PV panel  200  to measure PV panel operating temperatures. In the example of  FIG. 8 , a temperature sensor  202  is attached to a back surface of the photosensitive area of the PV panel  200 . In some embodiments, a monitoring module  300 , illumination sensor  324 , audible indicator  322 , visual indicator  320 , and beacon antenna  318  are mechanically attached to a bracket  326 . An illumination sensor  324  may optionally be attached to a front surface of the PV panel  200 , preferably in a location which does not reduce sunlight exposure of a solar cell in the PV panel. The bracket  326  provides structural support for the monitoring module, sensors, and indicators, and further provides a standardized mechanical interface for attachment to PV panels in a PV array. Although the example of  FIG. 2  shows the bracket  326  attached to a right side of the PV panel  200 , alternative embodiments of the invention may have a bracket attached to one or more of the other sides of the PV panel. The bracket  326  may optionally provide mechanical support for the PV panel  200  and other components when the bracket  326  is attached to an external support structure. Other alternative embodiments have the beacon antenna  318 , visual indicator  320 , and other components arranged in a different order on the bracket  326 . The monitoring module  300  may optionally be positioned some distance away from the PV panel  200  and not attached to the bracket  326  while electrically connected to at least one PV panel  200 . 
     Embodiments of an intelligent node include a bypass selector and an S-P selector as described in relation to  FIG. 1 . An example of a node controller, bypass selector, and S-P selector is shown in  FIG. 9 . Switching states for the electrically controlled bypass selector  120  and the electrically controlled S-P selector Xn  138  determine how current and voltage output from the PV module  108  is combined with electrical power flowing through the first and second power connectors P1  102  and P2  156 . As shown in  FIG. 9 , the bypass selector  120  and the S-P selector Xn  138  are preferably double-pole, double-throw (DPDT) electromechanical relays. Either one or both of the selectors ( 120 ,  138 ) may alternatively be replaced by a solid state relay or solid state switching devices made from discrete electronic components. Either selector ( 120 ,  138 ) may optionally be changed from a single DPDT electrically controlled switching device to a pair of single-pole, single-throw switching devices sharing a common control line electrically connected to the node controller  114 . 
     Referring to  FIG. 9 , electric power from other intelligent nodes in a configurable PV array may optionally be connected to the intelligent node  366  on the second power connector P2  156  comprising a first terminal  158  and a second terminal  160 . Voltage and current on the P2 first terminal  158  and the P2 second terminal  160  are selectively combined with voltage and current output from the PV panel  200  according to selected switching states for the bypass selector  120  and the S-P selector Xn  138 . The P2 first terminal  158  is electrically connected to a parallel terminal  144  of a first S-P switch  140  in the S-P selector Xn  138 . The P2 first terminal  158  is further electrically connected to a series terminal  154  of a second S-P switch  148  in the S-P selector Xn  138 . The P2 second terminal  160  is electrically connected to a parallel terminal  152  of the second S-P switch  148 . 
     A series terminal  146  of the first S-P switch  140  is electrically connected to a common terminal  128  for a first bypass switch  122  in the bypass selector  120 . A common terminal  142  of the first S-P switch  140  is electrically connected to a common terminal  132  for a second bypass switch  130  in the bypass selector  120 . The common terminal  142  of the first S-P switch  140  is further connected electrically to a connector P1 first terminal  104 . A common terminal  150  of the second S-P switch  148  is electrically connected to a negative terminal  112  on the PV module  108 , to a connector P1 second terminal  106 , and to a bypass terminal  126  of the first bypass switch  122  in the bypass selector  120 . 
     A bypass selector control line  118  carries control signals from the node controller  114  to a control input of the bypass selector  120 . A first control signal from the node controller  114  on the bypass selector control line  118  sets the bypass selector  120  to a “Bypass” switching state in which output from the PV module  108  is excluded from the voltage and current on the terminals of the first power connector P1  102 . A “Bypass” switching state is also referred to herein as a “B” switching state. In a Bypass switching state, output power from the PV panel  200  is excluded from current and voltage on connectors P1  102  and P2  156 . A second control signal from the node controller  114  on the bypass selector control line  118  sets the bypass selector  120  to a “Normal” switching state in which output from the PV panel  200  is selectively combined with the voltage and current on the terminals of the connector P1  102  according to one of two alternate switching states for the S-P selector Xn  138 . A “Normal” switching state is also referred to herein as an “N” switching state. In the example of  FIG. 9 , the first bypass switch  122  and the second bypass switch  130  in the bypass selector  120  are shown in the “Normal” switching state.  FIG. 9  further shows the first bypass switch  122  normal terminal  124  and the second bypass switch  130  bypass terminal  136  as unterminated. Passive components may optionally be electrically connected to the unterminated terminals to reduce electrical noise coupled into the circuit. 
     A series-parallel selector control line  116  carries control signals from the node controller  114  to a control input of the S-P selector Xn  138 . A third control signal from the node controller  114  on the series-parallel selector control line  116  sets the S-P selector Xn  138  to a “Series” switching state, also referred to herein as an “S” switching state. A fourth control signal from the node controller  114  on the series-parallel selector control line  116  sets the S-P selector Xn  138  to a “Parallel” switching state, also referred to herein as a “P” switching state. In the example of  FIG. 2 , the first S-P switch  140  and the second S-P switch  148  in the S-P selector Xn  138  are shown in the “Series” switching state. 
       FIG. 10  shows an example of an embodiment of the invention comprising twelve intelligent nodes  366  interconnected to form a reconfigurable PV array  20 . Each of the intelligent nodes  366  in  FIG. 10  includes a PV panel  200  and the S-P and Bypass selectors described above.  FIG. 10  shows a simplified representation of a reconfigurable array  20  having two groups  10  of interconnected intelligent nodes. The array  20  has outputs ( 168 ,  170 ) connected to inputs of an inverter  172  for converting DC electrical power to AC electrical power.  FIG. 10  is representative of connections between intelligent nodes in PV arrays having a different number of intelligent nodes  366  in each group  10  and a different number of groups  10  in the PV array  20 . 
       FIGS. 11-13  represent a few of the many different S-P configurations that may be made for a PV array of a given size. An S-P configuration refers to an arrangement of serial and parallel connections between intelligent nodes in an photovoltaic array. Changing at least one serial connection or at least one parallel connection between any two or more intelligent nodes in a photovoltaic array places the array in a new S-P configuration. Embodiments of the invention are capable of rapidly switching from one S-P configuration to another without connecting or disconnecting cables or wires used to make electrical connections between PV modules on a PV panel or between PV panels in a PV array.  FIG. 11  represents “n” groups ( 10   i ,  10   j , . . .  10   n ) of intelligent nodes  366  interconnected to form a PV array  20  having power outputs  168  and  170 . In the example of  FIG. 11 , all of the intelligent nodes  366  within each group ( 10   i ,  10   j , . . .  10   n ) are connected in a series circuit. In  FIG. 12 , one of the intelligent nodes ( 14 ) in group  10   i  has been reconfigured by the S-P selectors  138  for parallel circuit connection to its neighboring intelligent node  366 . Placing intelligent node  366  ( 14 ) in a parallel circuit with one or more neighboring nodes permits electrical power from node ( 14 ) to be included in the output of the PV array  20 , thereby salvaging power from the node that would otherwise have been lost. Note that connections between intelligent nodes  366  in other groups ( 10   j , . . .  10   n ) have not been changed in  FIG. 12  compared to  FIG. 11 . 
     In  FIG. 13 , intelligent nodes  366  in group  10   i  have been reconfigured to a new arrangement of serial and parallel electrical connections. Group  10   n  has also been changed to a different configuration of connections between intelligent nodes  366 . Although  FIG. 13  shows group  10   i  with four serially-connected subsets of three parallel-connected nodes, each subset could in practice have a different number of intelligent nodes  366 . Furthermore, changing series-parallel connections in one group, for example group  10   i , may be done independently of any configuration imposed on other groups, for example groups  10   j  and  10   n  in  FIG. 13 . It will be appreciated that even for the relatively small PV array illustrated in the examples of  FIGS. 11-13 , it is not practical to include in the figures every possible permutation of serial and parallel connections between intelligent nodes  366 , which range from the configuration of  FIG. 12  to a configuration (not illustrated) in which all intelligent nodes are in a parallel electrical circuit, with array output voltage, current, and power adjustable between corresponding limits by suitable selection of serial and parallel connections between intelligent nodes. 
       FIGS. 10-13  suggest the flexibility that may be achieved in reconfiguring serial and parallel electrical connections between intelligent nodes in a PV array in accord with an embodiment of the invention. An intelligent node may be used to extract a maximum amount of output power from a PV array by selectively reconfiguring the array, measuring the output power for each array configuration, identifying the maximum power achieved and its corresponding array configuration, then returning the PV array to the configuration corresponding to maximum power output. Optimization can be accomplished without resorting to mathematical models, for example models of PV panel characteristics or PV array performance. An optimized PV array will salvage energy from shaded or otherwise underperforming PV panels that would have been wasted in prior art systems. Optimization of output power, that is, finding a maximum amount of output power corresponding to actual PV array operating conditions, can be performed even when a cause for underperformance is unknown and mathematical models may therefore be difficult to apply, or when a PV array is simultaneously subjected to more than one cause for underperformance. 
     Because of the speed with which embodiments of the invention can switch from one array configuration to another, it can be reasonable to test every possible array configuration in a relatively short time period, even for large PV arrays. For example, an embodiment of the invention is capable of switching several hundred PV panels to a new S-P configuration and measuring a new PV array output power value in about one second. Very large arrays may take no more than a few seconds per S-P configuration tested. In some cases, for example when all the underperforming PV panels are detected to be in a common group (see a common group  10   i  in  FIGS. 11-13 ), it may be necessary to reconfigure only the affected group to find a new maximum output power for the PV array. As operational experience is gained with a particular installation of a reconfigurable PV array, a supervisory control and monitoring system can store which optimized S-P configurations are best suited to previously encountered situations and quickly restore the PV array to the previously determined optimum configuration when the corresponding situation is detected again. 
     Operator experience and conventional mathematical modeling methods may be used to eliminate some combinations of series-parallel connections from the array configurations to be tested. A mathematical model may be used to predict a starting S-P configuration to be evaluated. However, even when such models are available, they may contain inaccurate or dated information about PV panels, weather conditions, panel cleanliness, panel aging effects, array impedance, load impedance, and other operational parameters that affect power output. Embodiments of the invention permit PV array output to be maximized according to actual field conditions at the time an optimization is conducted. 
     A method embodiment of the invention adaptively selects a combination of serial and parallel electrical connections between intelligent nodes in a reconfigurable PV array to produce the maximum PV array output power under measured or predicted electrical load conditions, measured, predicted, or reported environmental conditions, measured, predicted, or reported power output or status of individual PV modules and PV panels, and other operational parameters in effect at the time the method is performed. An example of a method embodiment of the invention comprises: 
     connecting a plurality of PV panels in a PV array in an initial series-parallel (S-P) configuration corresponding to an initial arrangement of serial and parallel electrical connections between the PV panels, and calculating an initial value of PV array output power for the initial S-P configuration; 
     detecting a decrease in output power from the PV array in comparison to the initial value of power output from the PV array; 
     instructing each intelligent node to place the PV array in a new S-P configuration and measuring the output voltage and current for the new S-P configuration; 
     calculating the output power corresponding to the new S-P configuration; 
     saving S-P configuration data including a value corresponding to the switching state of each bypass switch and S-P switch in the array and the output power corresponding to the S-P configuration; 
     reconfiguring the PV array into a plurality of new S-P combinations, and for each new S-P configuration, storing PV array output power and S-P configuration data, until all members of a selected set of S-P configurations have been implemented and measured and their corresponding output power values saved; 
     selecting the maximum saved value of PV array output power and its associated S-P configuration data; and 
     setting the PV array to the S-P configuration corresponding to the selected maximum value of PV array output power by setting the S-P selector on each PV panel according to the retrieved value representing the switching state. 
     The following steps are optional: 
     detecting a fault condition in a PV panel or in the PV array that would lead to a decrease in PV array output power and changing the array configuration in anticipation of a power decrease that may not yet have occurred; 
     placing the PV array in a new S-P configuration corresponding to a new maximum value of PV array output power upon detection of a ground fault in the PV array; 
     placing the PV array in a new S-P configuration corresponding to a new maximum value of PV array output power upon detection of an arc fault in the PV array; 
     placing the PV array in a new S-P configuration corresponding to a new maximum value of PV array output power when a shadow falls on at least one PV panel in the PV array; 
     detecting a polarity reversal in the output from at least one PV module and initiating a search for a new maximum power configuration of the PV array; 
     preventing a search for a new S-P configuration for decreases in PV array output power that persist for less than a selected duration of time; 
     preventing a search for a new S-P configuration for decreases in PV array output power that are less than a selected threshold value; 
     preventing the PV array from being placed into an S-P configuration having a predicted value for PV array output power that is less than a previously saved value of PV array output power; 
     changing serial and parallel electrical connections between PV panels in a subset of the PV array that includes fewer than all panels in the PV array; 
     autonomously selecting one of two redundant means of communication by a module controller connected to a PV panel when the other of the two redundant means of communication is not available for communication; 
     placing the PV array in an S-P configuration associated with a recurring event, for example a shadow that passes across part of the PV array at a predictable time each day or at certain times of year, or a preventive maintenance schedule that disconnects selected panels from the array for cleaning or repair; 
     initializing the array configuration to a combination of serial and parallel electrical connections predicted by a mathematical model, then reconfiguring and measuring PV array performance beginning from that initial configuration; and 
     eliminating from a set of S-P configurations to be tested any configurations which a mathematical model predicts will be unproductive. 
     Unless expressly stated otherwise herein, ordinary terms have their corresponding ordinary meanings within the respective contexts of their presentations, and ordinary terms of art have their corresponding regular meanings.