Abstract:
A PV system may be used in case of emergencies. Each individual photovoltaic module receives a signal to determine if it is allowed to be operational or must shut down. Modules by default are shut off and safe to handle, absent the signal and in the presence of light.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is related to U.S. Provisional Application Ser. No. 61/252,985, filed on Oct. 19, 2009, hereby incorporated by reference herein. 
    
    
     BACKGROUND 
       FIG. 1  describes a typical photovoltaic (PV) grid-tied  100  or off-grid  110  system. A PV system consists of a number of modules  101 ; each module by itself generates power when exposed to light. A series of modules is wired together to create a higher voltage string  102 . Multiple PV strings may be wired in parallel to form a PV array  103 . The PV array connects to a DC-disconnect switch  104 , and the DC disconnect switch feeds power to a grid-tied inverter  105  which converts the DC power from the array to AC power for the grid. 
     Off-grid systems  110  connect the PV array  103  to the DC disconnect, and on to a battery charger  111 , which stores the electrical energy in batteries  112 . Off-grid residential systems typically use an off-grid inverter  113  that produces AC electricity for AC loads connected to an AC main panelboard  106 . 
     Inside a silicon cell based module  200 , shown in  FIG. 2 , there is a series of photovoltaic cells  201 , the basic building block in solar electric systems. Each cell is producing approximately 0.5 volts and a few amps (e.g. 5 A). The PV cells are also wired in series and in parallel within the module to achieve a desired voltage and current, and each module has a positive and negative module terminal  202  to connect to the PV system. A typical module used in a residential or commercial power generating system will produce in the order of 18-50V DC at 90-200 W at its electrical connectors. There are two terminals one positive and the other negative. Arrays used in residential installations will typically produce power in the range of 2 KW-10 KW with voltages up to 600V DC (grid-tied). The module voltage and power output is true for other module architectures such as thin-film (CdTe, CIGS, etc.). 
     When a PV array is installed and operational, the PV system generates power whenever there is light present. Furthermore, it is impractical to disable the system beyond shutting off the AC mains or the DC disconnect. Once wired, the array itself is never able to fully shut down in the presence of light even with the DC disconnect in the open position. The string wiring connecting all the modules in series, the wiring to the DC disconnect, and the array will all continue to generate lethal levels of voltage when exposed to light. 
     In the case of a damaged array from fire or natural disaster, an open (non-insulated) wire of the array&#39;s circuits may present itself. The exposed circuits provide a higher likelihood of an unintended electrical circuit path to ground (ground fault), and a human can become a part of this path to ground either by touching or through exposure to water. With a human body in a ground fault circuit it is very likely to be lethal. The National Fire Protection Association (NFPA) 70E defines “low voltage” somewhere near ˜50V. This low voltage is the threshold where one is able to generally survive a shock and “let go” (˜9 mA). PV systems are well above this level. This poses a serious and very real problem for firefighters when they encounter a building on fire with a PV array. 
     Even an operational and properly insulated system poses a potential problem for service technicians in the case of a PV array in need of service. In the case of the need to replace a defective module the person may be exposed to high voltages even with the DC disconnect in the “off” or “open” position. 
     In the case of earthquakes, floods, or other natural disasters, partially destroyed PV systems pose a threat to the occupants of a structure and any rescue personnel, especially untrained civilians. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates two embodiments of a solar photovoltaic (PV) system with all the major system components; 
         FIG. 2  illustrates the elements of a current PV module; 
         FIG. 3  illustrates an embodiment of a PV system with the additional system-level components; 
         FIG. 4  illustrates the elements within the system-monitor function; 
         FIG. 5  illustrates the two possible methods to control an individual module; 
         FIG. 6  illustrates a module embodiment utilizing an electromechanical relay as the means to control the module; 
         FIG. 7  illustrates a module embodiment utilizing a transformer and transistor as the means to control the module; 
         FIG. 8  illustrates a module embodiment utilizing an opto-isolator and transistor as the means to control the module; 
         FIGS. 9 and 10  illustrate a module embodiment utilizing an FET driver and transistor as the means to control the module; and 
         FIG. 11  is a depiction of the back side of one embodiment of a solar module. 
     
    
    
     DETAILED DESCRIPTION 
     Typically a firefighter disables power to a dwelling at the main AC panelboard of a home prior to dousing it with water. Shutting off power to the AC main panelboard automatically disables every individual module in a photovoltaic (PV) array in one embodiment. In natural disasters the AC mains of a building will likely be off, again making the PV system safe in one embodiment. Additional systems allow for automatic shutdown in the case of natural disasters. 
     A System-Monitor device  301  creates a “System-On” signal  302 , which each individual solar module receives to activate itself and operate normally, as shown in  FIG. 3 . This signal controls (i.e. enables or disables) each module individually. Disabling a module can be achieved, for example, by shorting out the module or by opening up the string circuit that connects all the modules together. The System-Monitor device is connected to the AC power of the system, and when the AC power is off, the “System-On” signal is no longer “true.” 
     Additionally, a manually operated switch  303  that is key operated may also be used to disable the PV system and each individual module therein, in some embodiments. The manual switch may be used by firefighters as well as service technicians to disable all the modules individually. It includes visual feedback  305  to indicate its state. 
     The System-Monitor  400 , shown in  FIG. 4 , generates the System-On signal for the modules to begin operation. In one embodiment this particular component is a simple step down transformer  401 . Typically, 240 V AC from the AC main panelboard is connected to the primary coil. The transformer generates a low voltage (e.g. ˜12V AC 60 Hz) signal pair  402  on the secondary coil. An active (e.g. 12V AC) signal indicates to the modules that the AC grid is on, or “System-On” is true. The active signal may be fed via a set of wires that is then routed to each module, for example, by “daisy chaining” the System-On signal to every module. The System-Monitor device may have a manual switch  403  with a key-lockout that disconnects the step down transformer from the AC power operated by anyone who needs to disable the PV system. 
     The System Monitor may employ internal protection fuses  404  for fault conditions. In the case of an off-grid system, the off-grid inverter  405  supplies AC power in order for the System-Monitor to operate. Depending on code or safety requirements, one leg of the System-On signal may be bonded to ground with a conductor  410 . 
     For natural or man-made disasters, a motion, water or heat sensor and switch  411  may automatically disable the “System-On,” for example, in case of earthquakes, floods, or fires. Those skilled in the art of electronic or electrical design recognize the many options to implement such a sensor switch. 
       FIG. 5  illustrates one embodiment of a mechanism to control the module&#39;s power production. Solar photovoltaic module junction box  506  may include a logic element  501  and module switch  502  may be part of the module assembly  500  (e.g. inside the module junction box) or be a separate element wired to the module (not shown). The System-On signal may be connected to the logic element with a twin lead connector  503 . The module terminals  504  deliver direct current (DC) potential to other modules to form a string. The module switch  502  can be in series with the PV cells  505  and one of the module terminals, in this case the switch  502 , disconnects the PV cells from the array. In a second embodiment, the module switch  510  can be in parallel to the PV cells, connecting to both module terminals. The junction box  506  may be a physical box that is secured to or integrated with a photovoltaic module. It may be attached by the module manufacturer at the time of manufacture or thereafter by third parties, in some embodiments. 
     Each of the module switches is electrically isolated from other module switches since each operates at a different voltage potential. This is due to the series wiring of the modules into a string and the fact that all the module switches share a common signal “System-On”. A given module switch in a given array may be operating at a high potential (e.g. 400V) to ground, and the next module in the string at 350V, and so on, assuming each module generated 50 Volts (DC). Electrical isolation between the common System-On signal and the module switch can be achieved a number of ways including but not limited to AC transformer coupling, or optical coupling inside the logic element  501 . 
     The logic element and switch circuitry can be designed in a number of ways. Those skilled in the art of electronic circuit design will understand the proper selection of the individual components, the detail of which is left out for clarity. 
     The electro-magnetic relay-based system uses electromechanical systems for isolation and switching. The signal System-On has enough power to energize a standard AC relay coil. The signal is operating at a voltage considered safe to humans (low voltage, e.g. 12V AC). To control a module using a relay, the switch may be in series or in parallel with the PV cells. 
       FIG. 6  shows the circuit of a module assembly  600  with the module switch in parallel. In a system where the signal System-On is true (e.g. 12V AC) the energized coil  602  moves the normally closed (NC) contact  601  of the relay  603  to open up and allows the cells to produce power at the module terminals. The power from the System-On signal connects to the relay&#39;s coil through the two-contact System-On connector  605 , and the isolation between the System-On signal and the module switch is provided inherently between the relay coil and the relay&#39;s contact. 
       FIG. 7  shows the circuit of a module assembly  700  utilizing a small transformer  701  along with a few other components and a transistor  702  to perform the logic element and module switch functions. A basic transformer AC couples the System-On signal, present at the connector  703 , through a primary coil to a secondary coil. The primary and secondary coils provide the needed isolation. The coupled and isolated System-On signal is now converted to a DC control signal, for example, through a 4-diode rectifier  704 , and the rectified AC ripples are reduced with a capacitor  705 . This circuit provides a positive voltage of sufficient level to turn on a power MOSFET transistor  702 . The MOSFET source terminal is connected to the negative terminal of the first in a series of cells  706 , and the MOSFET drain terminal is connected to the module negative terminal wire  707 . 
     When the System-On signal is false (0V AC), the transistor is off due to the gate voltage (Vgs) being zero, and the module is disconnected from the other modules in the array. With the signal present the transistor will be on and it will close the circuit with the other modules in the string. To ensure the transistor turns off without a system-On signal, a resistor  708  discharges the capacitor. 
     The transformer in the previous example can be replaced with an opto-isolator component, as shown in  FIG. 8 . The System-On signal at the System-On connector  801  is converted to a DC voltage with a diode rectifier  802  and capacitor  803 . The DC voltage is current limited through a series resistor  804  to operate the opto-isolator&#39;s  805  transmitter (LED). The light energy will activate a photosensitive photo detector (e.g. transistor) in the opto-isolator, the light is providing the electrical isolation. When light is present the opto-isolator&#39;s transistor is conducting current otherwise not. The opto-isolator transistor controls a MOSFET transistor  806 , able to handle the module power loads. The N-channel MOSFET is by default off (or open) since the gate is pulled down to the same level as the source with a resistor  807 . When the opto-isolator&#39;s transistor is on it will raise the voltage of the MOSFET&#39;s gate close to the level present at the positive terminal of the last cell in the module  808  if light is present. The MOSFET will be in fully saturated mode and “on”, connecting the negative terminal of the first cell  809  to the module&#39;s negative terminal  810  allowing the module current to flow through the array. The voltage feeding the gate may need to be limited to protect the MOSFET depending on the choice of components; this can be achieved with an additional resistor  811 . 
     In  FIG. 9 , solar module  900  junction box  911  utilizes a photovoltaic MOSFET (PV PET) driver  901  as the isolation function. The System-On signal is converted to DC through the rectifier  902 , capacitor  903 , and is current limited through a resistor  904  as it drives the light transmitter (LED) of the PV FET driver. The light energy will be converted to by the PV FET driver&#39;s photodiodes to a DC voltage of sufficient voltage to directly drive the MOSFET transistor  905  to a fully saturated mode. This in turn will connect the negative terminal of the first cell  906  to the negative module terminal  907 . A resistor  909  will guarantee that the transistor will be off by default, by discharging any energy stored from leakage or stray capacitance. The positive module terminal  912  is coupled to cells  908 . The System-On connector  910  may use twin leads. 
       FIG. 10  illustrates another design, which has very few components for reliability and low cost. The System-On signal current (via connector  1007 ) is limited through resistor  1002  and drives the photo-diode of the FET photovoltaic driver  1001  in the solar module  1000  junction box  1008  to generate light energy for half of the AC cycle. The light energy is converted by the FET-driver to a DC voltage, which is applied to the MOSFET transistor  1004 . The inherent gate capacitance of the MOSFET is sufficient to store the needed voltage to turn on the FET for the entire AC cycle, thus eliminating any gate charge storage device. A resistor  1003  turns off the FET to bring it to desired default state of “off” by draining the FET gate charge when the System-On signal is not present. When the System-On signal is present, the transistor is on or fully saturated. This in turn connects the negative terminal of the first cell  1005  to the negative module terminal  1006 . 
     Finally, referring to  FIG. 11 , the back side of a solar module, such as the solar module  1100 , is depicted. The back side is the side which is not exposed to receive solar energy. The back side of the module  1100  may include a back sheet  1101 . In one embodiment, the junction box  1105  may be integrally formed within the back sheet  1101 . In other embodiments, it may be a physical box, such as a plastic electrical box, accessible through the back sheet  1101 . A pair of leads  1106  may be provided with a positive terminal  1103  and a negative terminal  1104  to connect a DC potential to the neighboring modules into a string. A second pair of leads  1107  may be provided to “daisy chain” the System-On signal from one module to the next. The connectors  1108  for the System-On signal contain two contacts each. 
     Another approach is to deliver the signal representing “System-On” as a light signal to the modules. This example requires a modification to the System-Monitor device, which will be sending light instead of an AC signal. Each module receives a fiber optic cable and the light received is converted to a voltage as in the case of the MOSFET driver through a series of photo diodes to a voltage level sufficient to turn on the FET. 
     Those skilled in the art of electronics can appreciate the possible variations of connecting a common signal (System-On) with some form of energy such as an AC or DC voltage, radio waves, or light to an isolated logic element. The logic element in turn drives a module switch that enables power output from the module. The Switch itself may also be integrated into one of the cells in the series (gated cell). Furthermore the circuit that controls the PV module may be part of the module or a separate system component that the module will plug into. The previous examples illustrate a few of the possible ways to implement the principle idea. 
     In systems with a number of distributed inverters, one for each module, there is no equivalent of the DC disconnect switch, and by turning off the AC mains the PV system will shut down if the micro-inverters are operating as expected. However the ability to shut the PV system down by disabling the power generated from the module itself via the System Monitor device (using the manual lockout switch) provides an additional safety measure and more importantly a consistent and clear visual means to firefighters to ensure that the PV array is indeed off. It also provides a safe and lockable means to people servicing the modules. 
     Currently a module is “live” the moment it leaves the module factory; there is no “off” switch. Like a charged car battery, PV modules are dangerous to the untrained, and able to generate power. Once an array is wired into a PV system it is a permanent installation and is not ever typically disconnected. The array wires pose a particularly lethal level of power to people since the voltages are typically 200-600V, which by NFPA NEC (National Electric Code) definition is well above “low voltage systems”. The only practical means of switching off an array is at the singular DC disconnect point where the lethal voltage levels are present even when switched off. Firefighters are trained to shut off the DC disconnect and the AC mains to a building, however even after both of these actions occur the power generated by the array continues to be present in the array, within the modules, and the wiring on the roof or inside the home leading up to the DC disconnect. If a firefighter were to use an axe to ventilate a roof, cut a wire, cut into a module, or douse a broken array with water—the firefighter would be exposed to high voltages. A path of lethal current to (earth) ground will exist. Additionally if a PV service technician were diagnosing a faulty array for ground faults, or replacing a broken module, this person will be exposed to very high voltages, requiring very careful conduct without any mistakes to remain safe. Electricians prefer in all cases to “lock out and tag out” any circuit they are working on, however a PV array cannot be shut down by any practical means. 
     By installing a switch in each module it is possible to deactivate each individual module to a level where the voltages will be in the order of 18-50 volts or less. At these levels it is safe to handle the modules or any components of the array. 
     Each module receives a “System-On” signal, a corresponding logic element, and a switch in or near the module will perform the control of each individual module. These may take the form of simple coils and relays or in other embodiments optical and electronic components. The reliability or cost of these simple components does not pose a cost burden nor a reliability challenge for module manufacturing. Each module has a junction box with a few electronic components in it today (diodes), and this shutdown system can add a few more to the module. In addition to the module switch the PV system may use a System-Monitor device. This function may be built into the grid-tie inverter or the off-grid battery charger to lower overall parts and costs. 
     References throughout this specification to “one embodiment” or “an embodiment” mean that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one implementation encompassed within the present invention. Thus, appearances of the phrase “one embodiment” or “in an embodiment” are not necessarily referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be instituted in other suitable forms other than the particular embodiment illustrated and all such forms may be encompassed within the claims of the present application. 
     While the present invention has been described with respect to a limited number of embodiments, those skilled in the art will appreciate numerous modifications and variations therefrom. It is intended that the appended claims cover all such modifications and variations as fall within the true spirit and scope of this present invention.