Abstract:
A low cost method and apparatus for detecting faults in a solar module comprising the steps of monitoring the voltage across each string of cells within the module&#39;s junction box and signaling an alarm at the module and or at a remote location if any of the voltages across the strings of cells drop below an acceptable voltage threshold, thereby indicating a fault condition.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to an apparatus and method for measuring the performance of a solar cell module and more particularly to a low cost apparatus and method for detecting faults in a solar cell module and signaling such fault condition. 
         [0003]    2. Description of the Background Art 
         [0004]    Photovoltaic (PV) devices for producing electrical energy directly from sunlight have become increasingly popular in recent years. Worldwide production of PV cells in 2010 exceeded 17,500 MW, with power output determined under standard test conditions (1 kW/m 2  light intensity, Air Mass 1.5 Global spectrum, and cell at 25 degree C.). With these solar cells typically encased in a module having a selling price of approximately $1.5/W, the 17,500 MW production represents a $26.25 billion a year industry. Furthermore, the worldwide industry output, measured in MW/year, has had a compounded annual growth rate in excess of 30%. 
         [0005]    Presently there exist methods for detecting the failure in a solar module by measuring the voltage output of the solar module, preferably under load conditions. If the voltage output of the module is significantly lower than the standard operating voltage, a failure of an individual string is likely. Unfortunately, measuring the voltage output is very labor intensive and requires disconnecting the module from the others to make the measurement. 
         [0006]    As set forth in JP Patent Application No 8-97456, the disclosure of which is hereby incorporated by reference herein, another method for detecting the failure in a solar module comprises inspecting the light emitting state of a light-emitting bypass diode within the module. However, disadvantages of this prior art include (1) LEDs result in a significant loss of power because of their high voltage drop over conventional diodes, (2) economical LEDs cannot carry the current required for conventional modules and (3) LEDs are very difficult to see in the sunlight thereby requiring an inspector to visually inspect each and every module from a fairly close distance. Moreover, in order to see the LEDs on the front of the module, a significant design change would have to be made to the module to allow the LEDs to be visible between the cells on the front side or on the side between the interconnect ribbons. While an easier way to mount the LEDs would be within the junction box on the back, they would be hidden on the backside and not visible from the front, thereby requiring the inspector to look under each module. 
         [0007]    Still other methods for detecting the failure in a solar module comprise (1) measuring the temperature of the bypass diode and signal a fault if the temperature rises above a predetermined threshold and (2) providing a means for changing color by a current flowing in a bypass diode of the solar module and indicating the fault condition. The problem with these two methods is setting the threshold—temperature near the bypass diode in a module can reach temperatures greater than 80 degrees C. in standard conditions and, moreover, may vary greatly between summer and winter conditions. 
         [0008]    Therefore, it is an object of this invention to provide an improvement which overcomes the aforementioned inadequacies of the prior art methods for detecting the fault of a solar module and provides an improvement which is a significant contribution to the advancement of detecting faults within a solar module. 
         [0009]    Another object of this invention is to provide a fault detector for a solar module having a plurality of strings of photovoltaic cells including a microcontroller whose inputs comprise analog-to-digital converters, a plurality of bypass diodes whose cathodes and anodes are connected in parallel across the respective strings of photovoltaic cells, the cathodes of respective bypass diodes being connected to the inputs of said microcontroller; and an alarm connected to an output of said microcontroller. 
         [0010]    Another object of this invention is to provide a fault detector for a solar module, wherein the fault detector is located in a junction box of the solar module. 
         [0011]    Another object of this invention is to provide a fault detector for a solar module, wherein the alarm comprises an audible alarm such as a buzzer or loud speaker or a visual alarm. 
         [0012]    Another object of this invention is to provide a fault detector for a solar module, wherein the alarm comprises a radio frequency (RF) transmitter or DC power line communication interface that sends fault detection information to a remote computer base station for data acquisition and alarm. 
         [0013]    Another object of this invention is to provide a fault detector for a solar module, wherein the alarm indicates the specific string of photovoltaic cells whose voltage falls below a predefined minimum voltage. 
         [0014]    Another object of this invention is to provide a fault detector for a solar module, wherein the voltages of the strings of photovoltaic cells are indicated. 
         [0015]    Another object of this invention is to provide a fault detector for a solar module which measures the electrical current flowing between each string of photovoltaic cells. 
         [0016]    The foregoing has outlined some of the pertinent objects of the invention. These objects should be construed to be merely illustrative of some of the more prominent features and applications of the intended invention. Many other beneficial results can be attained by applying the disclosed invention in a different manner or modifying the invention within the scope of the disclosure. Accordingly, other objects and a fuller understanding of the invention may be had by referring to the summary of the invention and the detailed description of the preferred embodiment in addition to the scope of the invention defined by the claims taken in conjunction with the accompanying drawings. 
       SUMMARY OF THE INVENTION 
       [0017]    For the purpose of summarizing this invention, this invention comprises an apparatus and method for monitoring the voltage across each string of cells of a solar module and signaling an alarm at the module&#39;s junction box and or at a remote location if any of the voltages across the string of cells drop below an acceptable voltage threshold, thereby indicating a fault condition. 
         [0018]    More particularly, in each embodiment, the invention comprises a bypass diode connected across a string of photovoltaic cells (e.g., 20 cells to a string). It is noted that bypass diodes are commonly employed to isolate each string of photovoltaic cells to which it is connected in the event one or more cells in such string becomes defective or shaded (i.e., no longer producing any significant power and presenting a high resistance in the string, thereby blocking the flow of power). 
         [0019]    In the various embodiments of the invention, the cathodes of the bypass diodes are each connected to an analog-to-digital converter (ADC) to convert the analog outputs of the bypass diode to a digital signal. The outputs of the ADCs are then connected to a microprocessor for detecting the voltage level at the cathodes of the bypass diodes. The microprocessor detects when the voltage has dropped appreciably, indicative of one or more of the strings having one or more failed or shaded photovoltaic cells. It is noted that the bypass diodes are optional inasmuch as the microprocessor would be still be able to detect the output voltages of the strings. 
         [0020]    In a first embodiment, the microprocessor outputs a signal to an audible piezoelectric buzzer in a pattern that is indicative of the number of the defective string (e.g., three buzzes if the third string of the solar module has become defective). In a second embodiment, the microprocessor outputs a signal to a loudspeaker for vocalizing the number of the defective string (e.g., vocalize through the loudspeaker “string 3 has become defective”). In a third embodiment, the microprocessor outputs a signal to a radio frequency transmitter, which is then in turn received by a computer monitoring system of a base station for alarm reporting (and to be shared via a website connected to the world wide web). The ADCs and microprocessor may comprise a single chip microcontroller (e.g., PIC® Microcontroller). 
         [0021]    A fourth embodiment of the invention comprises measuring the current flowing between each solar module by means of a shunt resistor of the microprocessor whose voltage is measured and supplied to a computer base station. 
         [0022]    A fifth embodiment of the invention utilizes an optocoupler whose photodiode is connected in parallel across the bypass diode to drive a phototransistor, the output of which is connected through a blocking diode to a transistor switch driving an audible piezoelectric buzzer. Unlike the first embodiment that indicates the defective string, this highly cost-effective fifth embodiment sounds the buzzer when there are one or more defective strings. 
         [0023]    A sixth embodiment of the invention comprises a DC power line communication interface circuit that communicates the fault information along the DC wiring to a computer monitoring system optionally including an internet connection for remote monitoring. In a seventh embodiment, the DC power line communication interface includes means for monitoring the electrical current flowing between adjacent solar modules. 
         [0024]    The foregoing has outlined rather broadly the more pertinent and important features of the present invention in order that the detailed description of the invention that follows may be better understood so that the present contribution to the art can be more fully appreciated. Additional features of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0025]    For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings in which: 
           [0026]      FIG. 1  is a schematic diagram of the first embodiment of the invention; 
           [0027]      FIG. 2  is a flowchart of the microprocessor firmware thereof; 
           [0028]      FIG. 3  is a schematic diagram of the second embodiment of the invention; 
           [0029]      FIG. 4  is a flowchart of the microprocessor firmware thereof; 
           [0030]      FIG. 5  is a schematic diagram of the third embodiment of the invention; 
           [0031]      FIG. 6  is a flowchart of the microprocessor firmware thereof; 
           [0032]      FIG. 7  is a schematic diagram of the fourth embodiment of the invention; 
           [0033]      FIG. 8  is a flowchart of the microprocessor firmware thereof; 
           [0034]      FIG. 9  is a schematic diagram of the fifth embodiment of the invention; 
           [0035]      FIG. 10  is an elevational view of a standard junction box of a solar panel showing the manner in which the invention implemented via a printed circuit board may be easily connected in parallel to the existing wiring thereof; 
           [0036]      FIG. 11  is a schematic diagram of the sixth embodiment of the invention; 
           [0037]      FIG. 12  is a schematic diagram of the seventh embodiment of the invention; 
       
    
    
       [0038]    Similar reference characters refer to similar parts throughout the several views of the drawings. 
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0039]    Referring to  FIG. 1 , the first embodiment of the invention comprises a fault detector  10  including a microcontroller  12  whose inputs comprise analog-to-digital converters ADC 1−n  (internally contained within the microcontroller  12  or separately provided) connected to the cathodes of respective bypass diodes D 1−n  connected in parallel across respective strings  5   1−n  of photovoltaic cells V 1−n . The output of the microcontroller  12  is connected to drive an alarm such as an alarm buzzer  20 . Power to the microcontroller  12  may be provided from one of the strings  5   1−n  by means of a voltage regulator  22  and filter capacitors  24 . 
         [0040]    By way of background, when a photovoltaic cell is defective or shaded, it will not produce as much electrical current. The output current of the cell declines proportionally to the amount of shading. Since the cells are connected in series, shading a single cell can cause the output of the entire string of cells to fall to the same level as the shaded cell, thereby greatly reducing the power of the entire system. Because of this, bypass diodes may be used in to allow the current from the other strings to flow around the defective/shaded string and through the bypass diode. 
         [0041]    Bypass diodes, typically located in the junction box of the solar module, are connected in parallel, but with the opposite polarity to a string of solar cell. Under normal operation, each string of solar cells will be forward biased with the diode being reverse biased. However, if a cell becomes defective or shaded, the current through the string of cells will drop and may cause the defective/shaded solar cell to reverse bias due to the mismatch in short circuit current of the other series connected cells. This will cause the bypass diode to conduct and allow the current from the good cells to flow through the diode thus allowing current to pass around the string of defective/shaded cells. The bypass diode holds the string of defective/shaded cells to a voltage of approximately −0.6 volts (i.e., the forward bias voltage of the diode), thus limiting the reduction in the output. 
         [0042]    The firmware employed in the microcontroller  12  of  FIG. 1  is illustrated in the flow chart of  FIG. 2 . At the Start of the program, TestFlag and AlarmFlag are set to False. The voltages at strings  5   1−n  are each measured. The voltage of S 3  is checked and if less than a Low Voltage Limit (e.g., 18 vdc for a 60 cell module), the program simply returns. This Low Voltage Limit prevents the module from emitting nuisance alarms at start-up in the morning and shut-down at night. However, if the voltage of S 3  is not less than the Low Voltage Limit, the program proceeds to determine which of the strings  5   1−n  of photovoltaic cells V 1−n  is defective or shaded. 
         [0043]    More particularly, if the voltage of ((S 2 −S 1 )−S 1 ) is greater than a Fault Voltage Limit (e.g., 5 vdc for a 20 cell string) or if the voltage of ((S 3 −S 2 )−S 1 ) is greater than the Fault Voltage Limit, the program outputs to the alarm buzzer  20  to beep once, indicating the first string S 1  of cells V 1 -V 20  contains a defective cell or is otherwise shaded. The Test Flag and the Alarm Flag are then set true and after a delay of one second, the program continues. 
         [0044]    Upon continuing, if the voltage of (S 1 -(S 2 −S 1 )) is greater than a Fault Voltage Limit or if the voltage of (S 3 −S 2 )−(S 2 −S 1 ) is greater than a Fault Voltage Limit, the microcontroller  12  outputs to the alarm buzzer  20  to beep twice, indicative of a defective cell V 21 -V 40  in the second string S 2 . The TestFlag and the AlarmFlag are then set to true and after delay the program continues. If neither of the voltages (S 1 −(S 2 −S  1 )) or ((S 3 −S 2 )−(S 2 −S 1 )) is not greater than the Fault Voltage Limit as described above, the program simply continues. 
         [0045]    Upon continuing, if the voltage of (S 1   31  ( 5   3 −S 1 )) is greater than a Fault Voltage Limit or if the voltage of (S 2 −S 1 )−(S 3 −S 2 ) is greater than a Fault Voltage Limit, the microcontroller  12  sounds the alarm buzzer  12  three times indicative of the third string S 3  containing a defective cell V 41 -V 60 . The TestFlag and the AlarmFlag are then set to true and after a delay, the program continues. If neither of the voltages (S 1 −(S 3 −S 1 )) or ((S 2 −S 1 )−(S 3 −S 2 )) is not greater than the Fault Voltage Limit as described above, the program simply continues. 
         [0046]    It is noted that forgoing is repeated n times to coincide with the n number of strings S n  of photovoltaic cells V 1−n . 
         [0047]    The program continues by checking the status of the TestFlag and the AlarmFlag and if either the TestFlag is False or the AlarmFlag is True, the program simply returns. However, if neither the TestFlag is False nor the AlarmFlag is True, the program proceeds by setting the TestFlag False and then audibly indicating the numerical value of the voltage at S 3  by calculating the Tenths Digit (N 1 ) and beeping N 1  times, then calculating the Ones Digit (N 2 ) from S 1  and beeping N 2  times and then calculating the Tenths Digit (N 3 ) from S 3  and beeping N 3  times. This is useful for determining the voltage of a good module. A fault condition may be simulated by intentionally shading a cell to determine the system is working properly. Upon removal of the simulated fault condition, the TestFlag will be true thereby indicating the numerical value of the voltage. 
         [0048]    As shown in  FIG. 3 , the second embodiment of the invention is similar to the first embodiment of  FIG. 1 , with the exception that the microcontroller  12  outputs to a loud speaker  30 . Likewise, as shown in  FIG. 4 , the firmware of the second embodiment functions similarly to the firmware of the first embodiment, with the exception of audibly playing the N 1  Digit Sound File, the N 2  Digit Sound File and the N 3  Digit Sound File to audibly convey the voltage S 3  via the loud speaker  30 . 
         [0049]    As shown in  FIG. 5A , the third embodiment of the invention is similar to the second and third embodiments as previously described with the exception of the microcontroller  12  outputting to a radio frequency (RF) transmitter  40 . A shown in  FIG. 5B , the transmission of transmitter  40  is received at a Computer Base Station via its antenna  42 . The received transmission of transmitter  40  is then recovered via a RF Receiver Base Station  44  and processed by a Computer Monitoring System  46 . An Internet connection  48  may optionally be provided for remote monitoring via the web or the like. Likewise, as shown in  FIG. 6 , the firmware of the Microcontroller  12  in the third embodiment is similar to that of the first and second embodiments, with the exceptions that (1) the Alarm Signals S 1 , S 2  and S 3  are transmitted to the computer Base Station and (2) the S 3  voltage reading is transmitted to the computer Base Station periodically (e.g. every 10 minutes.) 
         [0050]    Referring now to  FIG. 7A , the fourth embodiment of the invention comprises means for monitoring the current by incorporating a Shunt Resistor Rsh inline between adjacent solar modules and then measuring the voltage across the Shunt Resistor by a Shunt Resistor Analog to Digital Converter (ADC) connected to a microcontroller  12 . The output of the Microcontroller  12  is supplied to the RF Transmitter  40  which then as described above, communicates with a computer Base Station (see  FIG. 7B ). 
         [0051]    As shown in  FIG. 8 , the firmware in the Microcontroller  12  functions to transmit the current reading to the Base Station Receiver periodically (e.g., every 10 minutes.) 
         [0052]    As shown in  FIG. 9 , the fifth embodiment of the invention comprises an ultra low cost version having an optocoupler  50  connected across the positive and negative terminals of each string S 1n  within the module. This connection is preferably made in the module&#39;s junction box along with the associated electronics. The output of the phototransistor Q 1-3  of each optocoupler  50  is connected in parallel through diodes D 7-9  to a common transistor Q 4  to drive it in pull-down mode. The collector of transistor Q 4  is connected to a piezoelectric buzzer  20 . When the strings S 1-3  are working normally, the output of the phototransistors Q 1−3  are pulled low not allowing transistor Q 4  to conduct, thereby keeping the buzzer off. When a string S 1-3  malfunctions, the voltage begins to drop and when the voltage drops below a certain point, the bypass diode D 1-3  begins to conduct. As the voltage drops and goes negative −0.07 volts, the output of the resistivity of phototransistor Q  1-3  goes high and allows the voltage on the base of transistor Q 4  go high and conduct through the emitter. This allows the collector of Q 4  to flow current thereby causing the piezobuzzer  20  to alarm. 
         [0053]    As shown in  FIG. 10 , the components of the Fault Detector  10  of the invention are preferably mounted onto a printed circuit board  60  having an elongated rectangular shape so as to be able to fit within a cavity inside a typical junction box  62  of a solar panel  64 . In this manner, the electrical wires providing analog voltage to the ADCs 1 -s 3  may be simply clipped-into the conventional wire connectors  66  typically used in such junction box  62 . 
         [0054]      FIG. 11  shows a sixth embodiment of the invention in which the output of the microcontroller  12  is connected to a DC power line communication interface similar in concept to X 10 ® and Insteon® that communicates the fault information outputted by the microcontroller  12  along existing wiring. However, this embodiment modulates the signal on the same DC wiring connecting the solar modules. A low pass filter  34  may be provided to isolate the DC power line communication from adversely affecting the operation of the ADCs 1 -s 3 . In a seventh embodiment shown in  FIG. 12 , the microcontroller  12  outputs to the DC power line communication interface  32  the amount of electrical current flowing between adjacent solar modules. As shown both  FIGS. 11 and 12 , the computer monitoring system  46  with its optional internet connection  48  allows for remote monitoring via a complementary DC communication interface circuit  32 . Finally, it is noted that alternatively the computer monitoring system  46  may output to the alarms such as the alarm buzzer  20  (or loudspeaker  30 ) or to the RF transmitter  40  for further ease in monitoring. 
         [0055]    The present disclosure includes that contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a certain degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangement of parts may be resorted to without departing from the spirit and scope of the invention. 
         [0056]    Now that the invention has been described,