WO2010042533A2

WO2010042533A2 - Photovoltaic module performance monitoring system and devices

Info

Publication number: WO2010042533A2
Application number: PCT/US2009/059716
Authority: WO
Inventors: Michael Gostein; William Stueve; Lawrence R. Dunn; Russell Apfel
Original assignee: Atonometrics, Inc.
Priority date: 2008-10-06
Filing date: 2009-10-06
Publication date: 2010-04-15
Also published as: WO2010042533A3

Abstract

A photovoltaic module performance monitoring system and associated devices, for monitoring the electrical output of individual modules in a photovoltaic array. The system includes one or more module monitors, each of which monitors electrical parameters of an associated module and transmits signals encoded with the measured data; at least one transponder that receives signals from the module monitors and communicates the data to at least one computer which receives and analyzes the data.

Description

PHOTOVOLTAIC MODULE PERFORMANCE MONITORING SYSTEM AND DEVICES

RELATED APPLICATION DATA

[0001] The present application claims priority to U.S. Provisional Patent Application Ser. No. 61/102,933, filed Oct. 6, 2008, and entitled "PHOTOVOLTAIC MODULE PERFORMANCE

MONITORING SYSTEM, METHOD, AND STORAGE MEDIUM" which is incorporated herein by reference.

FIELD OF THE INVENTION [0002] The invention relates to performance monitoring of photovoltaic (PV) modules, also known as solar panels, used for solar energy generation. A system and devices are provided that enable monitoring electrical output of individual modules in an array of modules.

BACKGROUND OF THE INVENTION [0003] Photovoltaic (PV) modules, also known as solar panels, are used in solar energy generation systems to convert sunlight to electricity. The initial installation cost of PV systems is amortized over long system lifetimes, typically expected to be 20 to 30 years. However, system performance typically degrades over time and is negatively affected by exposure to outdoor conditions. Failure or degradation of individual elements in a PV system reduce the output power and system lifetime, thereby increasing the average cost per unit of energy generated.

[0004] Accordingly, performance monitoring of PV systems is typically desired. Monitoring allows the system owner to compare performance with design expectations and to identify system elements requiring maintenance or replacement. In many PV systems monitoring is performed continuously by automated units that transmit data to remote computers for data logging and analysis. [0005] FIGURE 1 illustrates the typical layout of a PV system. The figure depicts multiple modules 100, electrical junction boxes 102, power lines 103, and strings 104, but only one of each is labeled, for simplicity. Individual modules 100 are connected in series via power lines 103 to form one or more strings 104; multiple strings 104 are combined in parallel at one or more string-combiners 106 to aggregate power; and the output of the string combiner(s) 106 is fed to one or more inverters 110 which convert the direct current (DC) output of the PV modules 100 to alternating current (AC). Output power is optionally provided either to a load 116, such as a residential or commercial building, or to a utility electric grid 118, which can optionally also supply power to the load 116 when needed. Metering circuits 112 and 114 measure the total output power of the system and/or the net power delivered to/from the grid 118. For monitoring purposes, a local site computer 120 may collect performance data, which are optionally transmitted to a remote computer 122.

[0006] Typical PV modules in use today have operating voltages of 10 to 100 volts, or even up to 300 volts; currents of 1 to 10 amps; and power outputs of 50 to 250 watts. Series strings 104 typically contain 10 to 15 modules 100 resulting in string voltages of 600 to 1000 volts. A small 4kW residential PV system may include on the order of 10 to 100 modules 100 while a 30MW utility-scale array would include on the order of 200,000 modules 100.

[0007] When system performance does not match expectations, it is desirable to identify any components - including PV modules 100 - that may be at fault. Early detection of even small faults or degradations allows operators to take corrective actions.

[0008] However, examination of individual PV modules 100 is tedious and time-consuming, especially in large arrays. [0009] Therefore, it is common to include string-level monitoring circuits in a PV array. These circuits measure the current flowing in each module string. With reference to Figure 1, string-level monitoring may be performed with current sensors 107 (of which only one is labeled in the figure) integrated into the string combiner box 106. The sensors are read by control circuitry 108 which transmits data e.g. to a site computer 120. This allows a greater level of resolution to detect potential problems, as compared with system-level monitoring alone, because detected problems can be localized to a particular string. However, many problems may still go undetected. In addition, it is still necessary to further localize faults within the string.

[0010] To facilitate identification of individual module 100 faults, various approaches have been used to provide module-level indicators that will identify specific fault conditions. For example, these approaches include sensing elements that activate visual indicators on the module 100. While such indicators can be useful, they cannot be used for remote diagnosis or for providing quantitative information.

[0011] In view of the above, it would be advantageous to monitor the electrical output of each module in a PV array individually. Hereafter we refer to this as module-level monitoring. [0012] Various systems have been developed for module-level monitoring. However, existing systems suffer from a number of shortcomings. Principally, their cost and complexity make them impractical for use on large PV arrays.

[0013] Module-level monitoring may also be provided by power-optimizer or micro-inverter devices. Figure 1 depicts the typical PV system layout with module 100 outputs directly connected in series strings with DC-to-AC conversion at a remote inverter 110. In this layout shading of individual modules or poor module matching can result in power degradation. Various power optimizer devices

(which convert the variable DC output of individual modules to a fixed DC voltage or current) and micro- inverter devices (which convert the individual module outputs to a fixed AC voltage) have been developed to address this issue. In some cases these devices include module-level monitoring features. However, the cost and complexity of existing devices, and their potential negative effects on system reliability and efficiency, make them impractical for use on large PV arrays. BRIEF SUMMARY OF THE INVENTION

[0014] The invention provides a system and associated devices that can be used to monitor the output of individual modules 100 in a PV array. It is an object of the invention to achieve module-level monitoring with lower cost and complexity than prior approaches. [0015] The system consists of individual "module monitor" circuits each of which monitors parameters of a single module 100; one or more "transponders," which receive and then re-transmit signals from one or more module monitors; and a site computer 120 that receives signals from the one or more transponders.

[0016] Each module monitor device is preferably integrated into the assembly or the junction box of an associated PV module 100. It periodically measures parameters of its associated module 100, including at least one of a module voltage, current, power, or temperature, and transmits these data to a transponder together with an identification code that identifies the associated module.

[0017] In one embodiment, the module monitors are powered directly by their associated PV modules. [0018] In one embodiment, the module monitors operate without requiring any additional wiring beyond the DC power wires that are conventionally used to interconnect PV modules. Each module monitor transmits data to a transponder by imposing signals on the DC power line or, alternatively, by using a wireless transmitter, or both.

[0019] In one embodiment, the module monitors require only unidirectional data transmission to the transponders and each module monitor transmits independently of others, reducing hardware cost and eliminating the need for synchronization.

[0020] Each transponder receives signals from an associated group of module monitors. The transponder interprets the signals in order to generate data packets in a form suitable for transmission, and then transmits these data packets to the site computer 120. The transponder may also perform analysis or calibration functions and may aggregate data before re-transmission. Signals that are improperly received at the transponder, affected by noise, or are otherwise compromised are rejected. This includes signals from one or more module monitors that are received during overlapping time windows at the transponder such that data integrity is compromised. [0021] In one embodiment, each transponder communicates with a site computer 120 using a wireless transceiver, while in an alternative embodiment communication is performed over a wired connection.

[0022] The transponder may be powered by any of the following: a direct connection to the PV array being monitored; PV cells attached to the enclosure of the transponder; or a separate power wire(s). [0023] In one embodiment, one or more transponders may be integrated with another element, such as a string combiner 106, an inverter 110, or a site computer 120, so that they function substantially as one unit.

[0024] When the number of modules in the PV array is sufficiently small, only a single transponder may be necessary. [0025] The site computer 120 receives data packets from the transponders and performs data logging and/or analysis. Data analysis may be used to determine if the PV array is operating correctly or if a fault condition exists and/or to diagnose the nature of fault conditions. Such data analysis may optionally use data transmitted by the module monitors in combination with data from other measurement elements in the PV array. The site computer includes an interface or communication medium that enable the system to notify personnel of system status and detected faults.

[0026] The site computer 120 may apply calibration factors to determine calibrated values of the electrical parameter data transmitted by the module monitors. The site computer 120 may also apply calibration adjustments as needed to correct for sources of variation in the measurements at the module monitors.

[0027] The site computer 120 optionally transmits data to a remote computer 122 for analysis and/or remote access functions. When data are transmitted to a remote computer 122, any of the functions attributed to the site computer 120 may be performed by the remote computer 122 instead. [0028] These and other aspects of the disclosed subject matter, as well as additional novel features, will be apparent from the description provided herein. The intent of this summary is not to be a comprehensive description of the claimed subject matter, but rather to provide a short overview of some of the subject matter's functionality. Other systems, methods, features and advantages here provided will become apparent to one with skill in the art upon examination of the following FIGURES and detailed description. It is intended that all such additional systems, methods, features and advantages that are included within this description, be within the scope of the accompanying claims.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0029] The novel features believed characteristic of the invention will be set forth in the claims.

The invention itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings, wherein: [0030] FIGURE 1 depicts a typical photovoltaic array system, including elements for string- level monitoring, according to the prior art. [0031] FIGURE 2 depicts an exemplary computer system with which the disclosed subject matter could be implemented.

[0032] FIGURE 3 depicts a photovoltaic array system that includes elements for module-level monitoring according to the present invention.

[0033] FIGURE 4 depicts the flow of data between module monitors, transponders, a site computer, a remote computer, and other elements of a photovoltaic array system. [0034] FIGURE 5 depicts the functional elements of the module monitor. Dashed lines indicate alternative embodiments.

[0035] FIGURE 6 depicts an illustrative embodiment of the module monitor. Dashed lines indicate alternative embodiments. [0036] FIGURE 7 depicts an embodiment in which the module monitor device is integrated within the construction of a junction box.

[0037] FIGURE 8 depicts the functional elements of the transponder. Dashed lines indicate alternative embodiments. [0038] FIGURE 9 depicts exemplary photographs of a prototype of the module monitor.

[0039] FIGURE 10 depicts an exemplary diagram of the communication protocol between module monitor and transponder in accordance with the embodiment of a prototype.

[0040] FIGURE 11 depicts an exemplary flowchart of the module monitor and transponder control in accordance with the embodiment of a prototype.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0041] Although described with particular reference to monitoring photovoltaic modules, those with skill in the arts will recognize that the disclosed embodiments have relevance to a wide variety of areas in addition to those specific examples described below. [0042] All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein.

Computing System [0043] With reference to FIGURE 2, an exemplary system within a computing environment for implementing the invention includes a general purpose computing device in the form of a computing system 200, commercially available from Intel, IBM, AMD, Motorola, Cyrix and others. Components of the computing system 202 may include, but are not limited to, a processing unit 204, a system memory 206, and a system bus 236 that couples various system components including the system memory to the processing unit 204. The system bus 236 may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, and a local bus using any of a variety of bus architectures.

[0044] Computing system 200 typically includes a variety of computer readable media.

Computer readable media can be any available media that can be accessed by the computing system 200 and includes both volatile and nonvolatile media, and removable and non-removable media. By way of example, and not limitation, computer readable media may comprise computer storage media and communication media. Computer storage media includes volatile and nonvolatile, removable and nonremovable media implemented in any method or technology for storage of information such as computer readable instructions, data structures, program modules or other data. [0045] Computer memory includes, but is not limited to, RAM, ROM, EEPROM, flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical disk storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other medium which can be used to store the desired information and which can be accessed by the computing system 200.

[0046] The system memory 206 includes computer storage media in the form of volatile and/or nonvolatile memory such as read only memory (ROM) 210 and random access memory (RAM) 212. A basic input/output system 214 (BIOS), containing the basic routines that help to transfer information between elements within computing system 200, such as during start-up, is typically stored in ROM 210. RAM 212 typically contains data and/or program modules that are immediately accessible to and/or presently being operated on by processing unit 204. By way of example, and not limitation, an operating system 216, application programs 220, other program modules 220 and program data 222 are shown. [0047] Computing system 200 may also include other removable/non-removable, volatile/nonvolatile computer storage media. By way of example only, a hard disk drive 224 that reads from or writes to non-removable, nonvolatile magnetic media, a magnetic disk drive 226 that reads from or writes to a removable, nonvolatile magnetic disk 228, and an optical disk drive 230 that reads from or writes to a removable, nonvolatile optical disk 232 such as a CD ROM or other optical media could be employed to store the invention of the present embodiment. Other removable / non-removable, volatile / nonvolatile computer storage media that can be used in the exemplary operating environment include, but are not limited to, magnetic tape cassettes, flash memory cards, digital versatile disks, digital video tape, solid state RAM, solid state ROM, and the like. The hard disk drive 224 is typically connected to the system bus 236 through a non-removable memory interface such as interface 234, and magnetic disk drive 226 and optical disk drive 230 are typically connected to the system bus 236 by a removable memory interface, such as interface 238.

[0048] The drives and their associated computer storage media, discussed above, provide storage of computer readable instructions, data structures, program modules and other data for the computing system 200. For example, hard disk drive 224 is illustrated as storing operating system 268, application programs 270, other program modules 272 and program data 274. Note that these components can either be the same as or different from operating system 216, application programs 220, other program modules 220, and program data 222. Operating system 268, application programs 270, other program modules 272, and program data 274 are given different numbers hereto illustrates that, at a minimum, they are different copies. [0049] A user may enter commands and information into the computing system 200 through input devices such as a tablet, or electronic digitizer, 240, a microphone 242, a keyboard 244, and pointing device 246, commonly referred to as a mouse, trackball, or touch pad. These and other input devices are often connected to the processing unit 204 through a user input interface 248 that is coupled to the system bus 208, but may be connected by other interface and bus structures, such as a parallel port, game port or a universal serial bus (USB).

[0050] A monitor 250 or other type of display device is also connected to the system bus 208 via an interface, such as a video interface 252. The monitor 250 may also be integrated with a touch-screen panel or the like. Note that the monitor and/or touch screen panel can be physically coupled to a housing in which the computing system 200 is incorporated, such as in a tablet-type personal computer. In addition, computers such as the computing system 200 may also include other peripheral output devices such as speakers 254 and printer 256, which may be connected through an output peripheral interface 258 or the like. [0051] Computing system 200 may operate in a networked environment using logical connections to one or more remote computers, such as a remote computing system 260. The remote computing system 260 may be a personal computer, a server, a router, a network PC, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computing system 200, although only a memory storage device 262 has been illustrated. The logical connections depicted include a local area network (LAN) 264 connecting through network interface 276 and a wide area network (WAN) 266 connecting via modem 278, but may also include other networks. Such networking environments are commonplace in offices, enterprise-wide computer networks, intranets and the Internet. [0052] The central processor operating pursuant to operating system software such as IBM OS/2^®, Linux^®, UNIX^®, Microsoft Windows^®, Apple Mac OSX^® and other commercially available operating systems provides functionality for the services provided by the present invention. The operating system or systems may reside at a central location or distributed locations (i.e., mirrored or standalone). [0053] Software programs or modules instruct the operating systems to perform tasks such as, but not limited to, facilitating client requests, system maintenance, security, data storage, data backup, data mining, document/report generation and algorithms. The provided functionality may be embodied directly in hardware, in a software module executed by a processor or in any combination of the two. [0054] Furthermore, software operations may be executed, in part or wholly, by one or more servers or a client's system, via hardware, software module or any combination of the two. A software module (program or executable) may reside in RAM memory, flash memory, ROM memory, EPROM memory, EEPROM memory, registers, hard disk, a removable disk, a CD-ROM, DVD, optical disk or any other form of storage medium known in the art. An exemplary storage medium is coupled to the processor such that the processor can read information from, and write information to, the storage medium. In the alternative, the storage medium may be integral to the processor. The processor and the storage medium may also reside in an application specific integrated circuit (ASIC). The bus may be an optical or conventional bus operating pursuant to various protocols that are well known in the art.

Monitoring System Layout

[0055] FIGURE 3 illustrates a PV array system including module-level monitoring elements according to the present invention. Each PV module includes a module monitor device 300, which is integrated into the assembly or the junction box 102 of the module 100. For simplicity only one module 100, one junction box 102, and one module monitor 300 are labeled in the figure. Each module's 100 positive and negative terminals lead through a module monitor device 300 and are interconnected to adjacent modules in a string 104 via power lines 103 in a conventional manner. For simplicity only one string 104 is labeled in the figure.

[0056] In one embodiment no additional wiring is required. Each module monitor 300 periodically measures parameters of its associated module 100 and transmits data to a transponder 302. Preferably, the module monitor devices 300 are integrated within the assembly of the modules 100, such that installation of the modules 100 is the same as if the module monitors 300 were not present. [0057] Each transponder 302 receives signals from module monitors 300 and transmits data to a site computer 120. In one embodiment, module monitors 300 transmit signals by imposing them on the DC power lines 103 interconnecting the modules. In this case each transponder 302 is electrically wired into the PV array. For example, as illustrated in Figure 3, the positive and negative terminals at the output of a string combiner 106 are wired through a transponder 302 before input to an inverter 110, allowing the signals to be detected. In this case a transponder 302 can detect signals from all modules 100 upstream of the string combiner 106. In an alternative embodiment, module monitors 300 communicate with transponders 302' via wireless radio-frequency transmission. In this case transponders 302' need not be wired into the PV array but simply need to be located within the proximity of their associated modules 100, as indicated by the dashed-outline transponder 302'. Hereafter both wired and wireless transponder embodiments (302, 302') are designated using reference 302.

[0058] Transponders 302 communicate with site computers 120 using either a wired or a wireless connection. [0059] Transponders 302 may be incorporated within other system elements, such as a string combiner 106, an inverter 110, a site computer 120, or a peripheral of a site computer 120. [0060] A site computer 120 receives data from transponders 302 and may also receive information from other elements of the PV array including an inverter 110, meters 112 and 114, and conventional string-level monitoring circuits 107 and 108. The site computer 120 diagnoses system performance, communicates status to personnel, and optionally transmits data to a remote computer 122 for analysis and remote access. Any of the functions of a site computer 120 may be performed by a remote computer 122 instead. Note that a site computer 120 may be a relatively simple device such as a control station or a wall-mounted console. [0061] Although Figure 3 illustrates only one string combiner 106, one transponder 302, one inverter 110, one site computer 120, and one remote computer 122, it is evident that any number of these elements could be combined according to the number of modules 100 and the PV system design. [0062] FIGURE 4 illustrates the extension of the monitoring system to a larger PV array including multiple transponders 302. Each transponder 302 (of which only one is labeled) is in communication with a group of associated module monitors 300 (of which only one is labeled). A typical utility-scale PV array system will include strings of 10-15 modules 100 each with up to 12 strings 104 joined at a string combiner 106. Therefore, transponders 302 preferably are designed to receive signals from at least 180 module monitors 300. Conveniently, transponders 302 may be designed to communicate with up to 256 or 512 module monitors 300. [0063] Figure 4 also summarizes the flow of data through the system. Module monitors 300 transmit data to their associated transponders 302. In one embodiment, module monitors 300 can communicate bidirectionally, both transmitting, and, as indicated by the dashed arrows, receiving signals. In this case transmission of data from individual module monitors 300 may be coordinated to avoid collision of data packets using any number of protocols familiar in the art; such protocols include, but are not limited to, for example, causing each module monitor 300 to transmit only upon request from a transponder 302 or causing module monitors 300 to detect interfering transmissions and transmit again when the signal medium is clear. In another embodiment, only unidirectional transmission is required from module monitors 300 to transponders 302. This may result in lower cost and higher reliability for the module monitor devices 300. In this case module monitors 302 transmit periodically or at semi- random intervals and transponders 302 filter out interfering signals. Multiple communication frequencies may be used to reduce the occurrence of interferences.

[0064] As shown in Figure 4, transponders 302 communicate data to one or more site computers

120. Bidirectional communication is preferentially employed, in order to synchronize transmission. Site computers 120 optionally receive data from other system elements, such as inverters 110, meters 112 and 114, and conventional string monitors 107 and 108. Site computers are optionally in communication with one or more remote computers 122.

[0065] In Figure 3 and Figure 4 lines from the transponder to the site computer illustrate data flow, and not necessarily physical wiring.

Location Information

[0066] The location of each module monitor device 300 must be known, so that if a module fault condition is identified by a site computer 120 the fault location can be determined. In this context, location is understood to be equivalent to either a sequential position within the PV array or to physical location.

[0067] Therefore, in one embodiment the location within the array and the serial numbers of a module monitor and its associated module (which may be the same) are recorded by installation personnel when a module is installed, moved, or replaced in the array. Alternatively, the location information or sequential position may be automatically discovered and recorded by other elements of the system, such as transponders 302, site computers 120, or remote computers 122, as modules 100 are added to the array. Recorded location data may be stored within module monitor devices 300, within transponders 302, within a site computer 120, or within a remote computer 122.

Module Monitor

Functional Elements

[0068] FIGURE 5 depicts the functional elements of the module monitor device 300. Electrical connections from the PV cell array within the module are fed to the module monitor 300 at its "PV + In" 320 and "PV - In" 322 terminals. The "PV + Out" 324 and "PV - Out" 326 terminals lead to the DC power cables 103 that are conventionally used to interconnect PV modules 100. Note that the designations of "In" and "Out" are meant to indicate pass-through of the connections through the circuit, not the direction of positive current flow, which is from "-" to "+". [0069] A local power source 330 provides a regulated supply voltage to power other elements of the module monitor device 300. The supply voltage is typically in the range of 1- 12V. The power source 330 is preferably tied to the "PV +" input 320 to derive power directly from the associated PV module 100, using either a shunt regulator, a linear regulator, or a step-down DC-to-DC converter to reduce the module voltage to a suitable supply voltage. Alternatively, the power source 300 could be an energy- harvesting device that derives power from its environment, such as a thermo-voltage source powered by heat. The power source 300 could also include a rechargeable energy storage device such as a battery or capacitor.

[0070] A voltage sensor 332 measures the module voltage between the "PV+ In" 320 and "PV-

In" 322 terminals. The voltage sensor 332 may be implemented, for example, as a simple resistor divider with an op-amp buffer stage, or through other methods known in the art. Precision resistors with low temperature coefficients can be used; alternatively, the resistor values can be calibrated during the manufacture of the module monitor device.

[0071] A current sensor 334 measures the output current of the module. The current sensor 334 should be implemented without adding significant series resistance, in order to minimize power dissipation. One method is to measure the voltage drop across a low-value resistance, while another method is to use a Hall-effect current measurement device. In both cases, the current sensor 334 may be calibrated during manufacture of the module monitor device 300. Note that while Figure 5 shows the current sensor 334 measuring current exiting the module from "PV + In" 320 ("high-side sensing") in an alternative embodiment the current sensor could be arranged to measure current going into the module at "PV - In" ("low-side sensing").

[0072] An optional temperature sensor 333 may be included to measure the module monitor's

300 temperature or another temperature of the module 100. The temperature sensor 333 may be implemented through any of a number of methods known in the art. The measured temperature within the module monitor device 300 may be used to determine temperature-dependent corrections to measurements from the voltage sensor 332 and current sensor 334.

[0073] Calibration data for the voltage sensor 332, current sensor 334, or temperature sensor 333 may be stored either within the module monitor 300, within a transponder 302, within a site computer 120, or within a remote computer 122.

[0074] Analog outputs from the voltage sensor 332, current sensor 334, and temperature sensor 333 are converted into forms suitable for data transmission by an analog encoder 336. In one embodiment, the analog encoder 336 function is provided by a multi-channel analog-to-digital converter. In alternative embodiments, the analog encoder 336 function could be provided by, for example, a voltage-to-frequency converter or a voltage-to-pulse-width-modulation converter. Any of these types of encoders may require a voltage reference 338 for calibration. The voltage reference 338 should have sufficiently high precision and small temperature variation. Different analog encoder 336 types could be used with different sensors. Note also that encoder functions could be combined with sensors.

[0075] Control logic 340 initiates periodic measurement cycles during which the output of the analog encoders 336 is read. It also performs averaging, calibration, and low-level analysis functions. An identifying code, such as a serial number, is stored in non-volatile memory 342 accessible by the control logic 340. The control logic 340 initiates transmission of data to the transponder 302 via communication functions 344.

[0076] In one embodiment, the module monitor 300 communicates with a transponder 302 by imposing (and optionally also detecting) signals on the DC power line 103. In this embodiment communication functions 344 are connected to at least one of the "PV Out" terminals (324, 326) as indicated in Figure 5. In an alternative embodiment communication is performed using a wireless transceiver 352, as indicated by dashed lines in Figure 5.

[0077] An optional bypass element 354 allows current to flow directly from the "PV - Out" 326 to "PV + Out" 324 in the event that the module is reverse-biased or cannot provide sufficient current. The bypass element 354 could be a simple bypass diode such as that conventionally used in PV arrays, or a device with greater functionality.

[0078] It will be evident that the functions of the module monitor described in the preceding could be combined into a smaller number of components, or even combined onto a single integrated circuit.

Power Consumption

[0079] The power consumed by the module monitor device 300 should be minimized since any power consumed by the monitoring system reduces the total power produced by the PV array and increases the cost per energy unit generated. Preferably the module monitors will consume <1%, or even <0.1%, of the total system power. For an average module 100 size of approximately 100 W, the module monitor 300 should therefore consume <1 W and preferably <0.1 W.

[0080] Accordingly, attention is required to the following areas in the design of the module monitor 300: the quiescent current of the voltage regulator in the power source 333; the quiescent current of the sensors (332, 334), control logic 340, and communication functions (344, 352); and the series resistance of the current sensor 334. These points are further discussed below with reference to an illustrative embodiment.

[0081] The average power consumption can be significantly reduced by operating the module monitor 300 intermittently, recognizing that measurement and data transmission are required only once every few minutes. The circuit can accordingly be designed to enter a low-power state in between transmissions. In the low-power state the control logic 340 causes various circuit elements to shut down to conserve power. The device automatically wakes up from the low-power state e.g. following a designated time interval. Illustrative Embodiment

[0082] FIGURE 6 depicts an illustrative embodiment of the module monitor device 300. The figure shows a configuration for high-side current sensing, and accordingly the positive supply voltage rail "VS+" of the circuit 363 is tied to the "PV+" line 320; it will be recognized by those skilled in the art that the circuit could be re-arranged for low-side current sensing, in which case the negative supply rail "VS-" 365 would be tied to the "PV-" line 322.

[0083] The power source function is fulfilled by linear or shunt regulator 364 in combination with optional resistor 370 and/or transistor 369. Together these components produce a regulated voltage drop between the positive supply voltage "VS+" at 363 and a floating negative supply voltage "VS-" at 365. The other circuit elements are powered from VS+ to VS-. The regulator circuit can be easily designed to work with a wide range of module 100 voltages by changing resistor 370. Transistor 369 can be chosen to stand off a large range of module voltages. With proper component selection the entire module monitor device 300 can be designed to operate using «~lmA of current. For a typical module 100 operating voltage ~70V, this will result in <70mW of power dissipation at the regulator circuit. This level of quiescent power dissipation could be reduced by replacing the linear/shunt regulator 364 with a switched-mode DC-to-DC step-down converter, in which case an inductor component would be added. In either case, capacitor 368 can be used to provide extra power during brief periods of higher power consumption, in order to minimize the quiescent current design requirements. [0084] The voltage sensor function is implemented using resistors 356 and 357 as a divider with gain/buffer element 367. The voltage sensor may be calibrated as previously described.

[0085] The current sensor function is implemented by measuring the voltage drop across a current sense resistor 362 with gain/buffer element 366. For module currents of up to 8A the series resistance is preferably <0.01 ohm or even -0.001 ohm in order to reduce power dissipation to <1 W or <0.1 W. A resistance value in this range is conveniently provided by using a calibrated PCB trace length as the resistance element 362. The current sensor may be calibrated as previously described.

[0086] A microcontroller 360 integrated circuit is used to provide, in one component, the analog encoder function 336, via an analog-to-digital converter 359, the control logic 340, the non-volatile memory 342, the communication functions 344, and a temperature sensor 333. In this case the control logic 340 is implemented in software stored in the non-volatile memory 342. For communication over the output power line 103 the communication function 344 is coupled to "PV+ Out" 324 via coupler 375, which could be for example a transistor or capacitor. Optionally communication is via wireless means and the microcontroller component 360 contains a wireless transceiver 352 within the same package.

Integration With Module Assembly

[0087] Preferably, the module monitor device 300 is included within the module 100 assembly or junction box 102 during manufacture of the module 100. To simplify assembly, reduce cost, and improve reliability, it may be advantageous to reduce the number of components and electrical connections required. [0088] In one embodiment, this may be done by integrating the module monitor 300 into the construction of the junction box 102, and forming the input terminals 320 and 322 and output terminals 324 and 326 of the module monitor 300 so as to permit direct connection to, respectively, the PV cell leads and the power output cable. [0089] In typical modules 100 the PV cells are interconnected by thin metal bus strips which are then electrically tied to terminals in the junction box 102 once it is installed. The bus strips may be fastened to the terminals using, for example, soldering, a spring-loaded device, or a clamping device. Power line cables leading out of the junction box 102 are then attached to separate portions of the terminals. [0090] FIGURE 7 depicts a method of integrating the module monitor 300 with the construction of the junction box 102. In one embodiment, the module monitor input terminals 320 and 322 (cross- reference Fig. 5) are formed to allow direct connection to the bus strips exiting the module. For example, the input terminals may be formed as spring-loaded clips 373 and 374 soldered into the printed-circuit board of module monitor 300; the positive and negative bus strip leads 371 and 372 from the PV cells are then bent up and inserted within the clips. The output terminals 324 and 326 (cross-reference Fig. 5) may likewise be formed e.g. as clamping terminals 376 and 377 allowing direct connection of power line cables 103. An optional bypass diode can also be included within this structure. The figure is exemplary and is meant only to illustrate the general principle of utilizing the module monitor 300 device to provide electrical terminals in the junction box; a final product could take a different form.

Transponder

[0091] FIGURE 8 illustrates the functional elements of the transponder 302.

[0092] In one embodiment the transponder 302 is wired into the PV array, in order to receive signals transmitted by module monitors 300 and/or to use the array as a power source. The DC power lines 103 of the array enter the transponder 302 using the "PV + In" 380, "PV - In" 382, "PV + Out" 384, and "PV - Out" 386 terminals.

[0093] In one embodiment, the transponder includes a local power source 392. If the transponder is wired into the PV array, the power source 392 may be a voltage regulator or step-down DC-to-DC converter which derives power from the PV array. Alternatively, for example if the transponder 302 is not wired into the PV array, the power source 392 may comprise additional PV cells attached to the enclosure of the transponder 302. In another alternative, the transponder 302 may receive power from an external wired power source 390, designated by dashed lines in Figure 8.

[0094] In the embodiment in which a module monitor 300 transmits data to the transponder 302 by imposing signals on the DC power line 103, a signal detector 394 detects the signals and passes them to control logic 398. An optional filter 396 limits the communication signals from passing through the transponder 302, to prevent the signals from being detected by other transponders 302 in the array. [0095] In the alternative embodiment in which module monitors communicate with the transponder 302 via wireless transmission, the transponder includes a wireless receiver and/or transceiver 400, and the signal detector 394 and filter 396 are not needed.

[0096] Control logic 398, which may be, for example, a microcontroller, functions to interpret signals, reject erroneous signals, prepare data packets, temporarily store data in internal memory, and initiate data transmission to the site computer 120.

[0097] The transponder 302 preferably communicates with a site computer 120 wirelessly, using wireless transceiver 402. Alternatively, it communicates over optional wired connection 404. In both cases bidirectional communication can be employed to initiate or synchronize data transmission.

Module Monitor / Transponder Communication

[0098] In one embodiment module monitors 300 transmit data to their associated transponders 302 by imposing signals on the DC power line 103. A number of methods can be employed to achieve this. In general, the methods involve modulating the voltage or current of the power line 103 to encode the data. In the simplest approach, a digital signal is capacitively or inductively coupled directly to the power line 103. In an alternative approach, a high-frequency carrier signal is imposed on the power line 103 and the signal is then modulated to encode the data to be transmitted. The high-frequency carrier signal can be imposed by capacitive, inductive, or resistive coupling of a signal source. The amplitude, frequency, phase, or duration of the carrier signal is then modulated to encode data. Multiple frequencies can be used in order to permit multiple communication channels.

[0099] Any of a number of communication protocols may be used to transmit data. The potential for over-lapping transmissions must be considered. Embodiments employing bi-directional communication allow data transmission to be coordinated to avoid over-lapping transmissions, e.g. by initiating transmission from a particular module monitor 300 only upon request from a transponder 302. However, it may be advantageous to achieve data transmission from module monitors 300 to transponders

302 without requiring bi-directional communication, in order to eliminate the need for receivers in the module monitor devices 300 and thereby reduce cost and potentially improve reliability. In one embodiment, module monitors 300 do not receive signals but do detect the presence of potentially interfering signals on the line, and wait to transmit until the line is quiet. In another embodiment, module monitors 300 neither receive nor detect signals, but transmit data unidirectionally at semi-random intervals. In this case, transmissions from separate devices will occasionally interfere. Transmissions corrupted by interference are filtered out by transponders 302. The occurrence of interfering transmissions can be minimized to an acceptable level by keeping the data transmission duty cycle low. It can be further minimized by utilizing multiple communication channels for simultaneous transmissions.

Prototype Example

[00100] The invented system and devices are illustrated with the aid of the following Example based on a prototype of the invention. FIGURE 9 depicts the module monitor prototype circuit 412 installed in the junction box 410 of module 408. Connections 400, 402, 404, and 406 serve as the "PV+ In" 320, "PV- In" 322, "PV+ Out" 324, and "PV- Out" 326 terminals (cross-referencing Figure 5.) A scale in the inset photograph shows the prototype size; it will be apparent that the prototype is only exemplary and that the size could be significantly reduced in a final product. [00101] With reference to Figure 5, the functional elements of the module monitor prototype 412 are implemented as follows: the power source 330 is implemented as a linear voltage regulator producing a +5V output with respect to the "PV-" voltage rail; the voltage sensor 332 is implemented as a resistor divider with an op-amp buffer; the current sensor 334 is implemented using a Hall-effect current sensor integrated circuit measuring in series with the "PV+" rail; a microcontroller provides analog-to-digital conversion, control logic 340, and non-volatile memory 342; a PWM (pulse-width modulated) output of the microcontroller is coupled to the "PV+ Out" terminal 324 via a transistor drive circuit, to provide communication functions 344; and a diode provides the bypass function 354. Data transmission is accomplished by modulating the amplitude of a 100kHz carrier signal generated by the microcontroller PWM and imposing this signal onto the output via the transistor drive circuit. The prototype device 412 does not include any receiver or signal detection, but communicates in a unidirectional mode only.

[00102] The module monitor prototype 412 requires up to 0.3 W in continuous operation. This level of power dissipation is acceptable but could also be reduced significantly, as discussed above. [00103] The Example includes a prototype transponder device (not shown in the figures) that detects and decodes data transmission from the module monitor prototype 412. The transponder prototype device includes a tuned LC circuit that filters and receives the 100kHz carrier wave, a demodulation circuit that converts the filtered signal into a data stream, a microcontroller that interprets the signals, and a communication function that re-transmits the signal to a computer for logging and analysis. [00104] FIGURE 10 depicts data transmission from the module monitor prototype 412 to the transponder prototype. The 10OkHz PWM signal 460 is modulated by a digital data output 462 to produce an amplitude-modulated square wave. This is applied to the transistor drive circuit which pulls current from the module thereby producing module output voltage pattern 464. The signal is detected and filtered in the transponder to recover filtered signal 468 and then demodulated to produce data signal 470. [00105] FIGURE 11 depicts control flowcharts for the prototype module monitor 412 and transponder devices. The module monitor follows a loop with the following steps: readings are acquired from the module voltage and current sensors at steps 502 and 504; the PWM output is enabled at 506; the data packet is transmitted at 508; the PWM is disabled 510; and a semi-random sleep interval is chosen before entering a temporary low-power state at 512. The interval is chosen semi-randomly in order to prevent continual interference between transmissions from multiple devices. The transponder follows a control loop with the following steps: the device waits at 516 until a signal is detected on the line; a data packet is read 518; the packet is analyzed at 520 to determine if it is erroneous or compromised by interfering transmissions, and the packet is rejected if there is an error; correctly received data packets are transmitted to the computer at step 522. [00106] Note that, with this design and assuming a communication duty cycle of about 10% at the transponder, 180 module monitors can transmit to a single transponder about once each every 3 minutes. Each module monitor will have a 10% chance that its data transmission is rejected during this period due to overlapping transmissions. However, a data reporting interval of 10 minutes or longer is used, allowing each module monitor three or more transmission opportunities; therefore, the chance that all transmissions from a given module monitor are rejected during the data reporting interval is <0.1%, and the chance that this is repeated during the subsequent interval is negligible. The occurrence of overlapping transmissions could be further reduced by using multiple communication frequencies, as discussed above.

Calibration

[00107] As discussed above, it may be advantageous to individually calibrate the voltage, current, and/or temperature sensors of the module monitor devices 300 during or after manufacture, in order to allow the devices to be manufactured using less-costly low-precision components. Calibration data may be stored either within the module monitor 300, within a transponder 302, within a site computer 120, or within a remote computer 122. Calibration equations are then applied within any of these devices to produce calibrated readings. Furthermore, temperature readings can be used to compensate for temperature-related variations in the sensor outputs. Temperature -related measurement errors can also be compensated by using estimated module temperatures, remotely measured temperatures, or weather data, if temperature readings from the module monitor devices 300 are not available.

Fault Conditions

[00108] The site computer 120 analyzes data packets received from module monitors 300 to identify and diagnose fault conditions. Electrical parameters analyzed may include the voltage, current, and/or power of each module. Fault conditions could include, but are not limited to, any of the following: electrical parameters lower or higher than expected; electrical parameters deviating significantly from average values for other modules in the array; electrical parameters varying from one end of a string to another, indicating possible ground faults; trends in electrical parameters indicating degradation of components; fluctuations in electrical parameters indicating an intermittent fault; or absence of data transmissions from any module monitor in the array, indicating a non- functioning or bypassed module.

[00109] In an embodiment in which module monitors 300 communicate with transponders 302 over the power lines 103, the absence of data transmission from a portion of a string 104 in the array will indicate a fault in the power line 103 interconnections.

[00110] Fault detection analysis functions ascribed to the site computer 120 could also be performed by a transponder 302 or a remote computer 122.

[00111] Although particularly described with reference to a small number of module monitors

300, transponders 302, site computers 120, remote computers 122, inverters 110, and the like, this disclosure is intended to include any number of these components. [00112] Further, although example circuits and schematics to implement the elements of the disclosed subject matter have been provided, one skilled in the art, using this disclosure, could develop additional hardware and/or software to practice the disclosed subject matter and each is intended to be included herein.

[00113] In addition to the above described embodiments, those skilled in the art will appreciate that this disclosure has application in a variety of arts and situations and this disclosure is intended to include the same.

Claims

WE CLAIM:

1. A system for monitoring the parameters of individual photovoltaic (PV) modules in a PV array, comprising: one or more module monitors, said module monitor(s) transmitting data to at least one transponder, said data including measurements from at least one parameter of an associated module; said transponder(s) receiving said data from said module monitor(s) and communicating said data to at least one computer; wherein said computer(s) receives and analyzes said data.

2. The system of claim 1, wherein said parameters include at least one of said module's: voltage, current, power, or temperature.

3. The system of claim 1, wherein each said module monitor is incorporated within said module monitor's associated module.

4. The system of claim 1 , wherein each said module monitor is powered by at least one of: said module monitor's associated module; an energy storing device; an energy harvesting device; or an external power source.

5. The system of claim 1, wherein each said transponder is powered by at least one of: said PV array; one or more additional PV cells associated with said transponder; an energy storing device; an energy harvesting device; or an external power source.

6. The system of claim 1 , wherein each said module monitor contains a unique serial number.

7. The system of claim 1 , wherein said module monitor(s) and/or said transponder(s) transmit said data by imposing signals on one or more DC power lines interconnecting said module(s) and/or said transponder(s).

8. The system of claim 7, wherein said data is transmitted by imposing a carrier signal on said power line(s) and modulating the amplitude, frequency, phase, or duration of said carrier signal to encode said data.

9. The system of claim 7, wherein the absence of transmissions from portions of said array is used to identify interruptions in said module's interconnections.

10. The system of claim 1, wherein said module monitor(s) and/or said transponder(s) transmit said data via a wireless communication medium.

11. The system of claim 1 , wherein said module monitor(s) and/or said transponder(s) transmit said data via multiple frequencies.

12. The system of claim 1, wherein said module monitor(s) are communicably coupled to said transponder(s) via a bidirectional communication medium.

13. The system of claim 1, wherein said module monitor(s) transmit said data to said transponder(s) via a unidirectional communication medium.

14. The system of claim 1, wherein said transponder(s) log data received from said module monitor(s) and transmit said data to said computer upon said transponder's receipt of a transmission request from said computer.

15. The system of claim 1, wherein said transponder(s) identify and reject erroneous data received from said module monitor(s).

16. The system of claim 1, wherein said transmitted data is analyzed to identify fault conditions in said PV array.

17. The system of claim 1, wherein locations of said module monitor(s) in said PV array are used to identify one or more locations of one or more fault conditions in said PV array.

18. The system of claim 1, wherein said module monitor(s), said transponder(s), and/or said computer(s) apply calibration equations to determine calibrated quantities from said transmitted data.

19. A device for monitoring parameters of a photovoltaic (PV) module, including: input terminals, said input terminals electrically coupled between said PV module and a module monitor; a power source disposed within said module monitor; one or more sensors, said sensor(s) measuring one or more parameters of said PV module and disposed within said module monitor; one or more analog data encoding units coupled to said sensor(s) and disposed within said module monitor; control logic coupled to said analog data encoding unit(s) and disposed within said module monitor; a unique serial number; a communication medium coupled to said control logic; and output terminals electrically coupled between said module monitor and said PV module.

20. The device of claim 19, wherein said parameter(s) is at least one of: a voltage; a current; a power; or a temperature.

21. The device of claim 19, wherein said communication medium operates by imposing signals on said output terminals of said module monitor.

22. The device of claim 19, wherein said communication medium is a wireless communication medium.

23. The device of claim 19, wherein said communication medium is a bi-directional communication medium.

24. The device of claim 19, wherein said communication medium is unidirectional out of said module monitor.

25. The device of claim 19, wherein said parameter(s) include said module monitor's current, said current determined by measuring a voltage drop across a low-value resistance.

26. The device of claim 25, wherein said low-value resistance is a calibrated trace on a printed circuit board.

27. The device of claim 19, wherein calibration data for one or more of said sensors is stored in said module monitor, in a transponder communicably coupled to said module via said communications medium and external to said module monitor, or in one or more computers, said computer(s) communicably coupled to said module monitor via said communications medium and external to said module monitor.

28. The device of claim 19, wherein a measured or estimated temperature is used to calibrate said parameters.

29. The device of claim 19, wherein said power source is a switched-mode step-down DC to DC converter.

30. The device of claim 19, wherein said control logic enters a low-power state when not using said analog data encoding units or said communications medium.

31. The device of claim 19, said power source, said sensor(s), said analog data encoding unit(s), said control logic, said unique serial number, and said communication medium contained on an integrated circuit, said integrated circuit either a single integrated circuit or an integrated circuit package.

32. The device of claim 31, further comprising, external to said integrated circuit, one or more of the following: a current sense resistor; a capacitor; a voltage regulator shunt resistor; or an inductor.

33. The device of claim 19, said module monitor disposed within said PV module or a junction box, wherein: said input terminals include connectors for direct connection to leads from said PV module; and/or said output terminals include connectors for direct attachment of at least one power line cable.