Abstract:
A photovoltaic system includes a solar module having at least one solar cell. In order to reduce the power loss of the solar module when a solar module is in shade or is defective, a semiconductor switching element is connected in parallel with the solar module and drive electronics block the semiconductor switching element when the solar module is generating power and otherwise switch the semiconductor switching element to low impedance in order to bridge the solar module.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority, under 35 U.S.C. §119, of German Patent Applications DE 10 2006 006 967.6, filed Feb. 14, 2006, and DE 10 2006 026 661.7, filed Jun. 8, 2006; the prior applications are herewith incorporated by reference in their entirety. 
       BACKGROUND OF THE INVENTION 
     Field of the Invention 
       [0002]    The present invention relates to a photovoltaic system having a solar module with at least one solar cell. 
         [0003]    A photovoltaic system usually includes a multiplicity of solar modules which are connected in series to form so-called strings. The solar modules usually contain a plurality of solar cells. The string or strings feed(s) an inverter which supplies the energy of the solar modules to an AC power supply system or a load in the correct phase and with the correct voltage. 
         [0004]    The layout of a solar module according to German Utility Model DE 88 15 963 U1, is illustrated in  FIG. 1  and described in more detail below. Such solar modules have series-connected solar cells which are each bridged by a respective shunt diode. 
         [0005]    In the case of the solar module described in German Utility Model DE 88 15 963 U1, an indicating element in the form of a light-emitting diode or a moving-coil instrument is also connected in parallel with the solar modules. That indicating element indicates the flow of current through the shunt diode so that a solar module which is in shade or is defective can be detected. 
         [0006]    German Utility Model DE 20 2005 001 044 U1 discloses a photovoltaic system having a plurality of strings of series-connected solar modules and an indicating unit being connected downstream of each string. The indicating unit uses a light-emitting diode to indicate whether or not the string in question is providing power. 
         [0007]    One disadvantage of the above-described solar modules having a shunt diode connected in parallel with the solar module is that those shunt diodes generate a relatively high power loss when a current flows. If, for example, a string current of 5 A flows through such a shunt diode having a forward voltage of 1 V, 5 W are converted into heat in the shunt diode. On one hand, that energy is lost to the system and can no longer be fed into the inverter. On the other hand, the shunt diode becomes hot and heats its surroundings, as a result of which an adjacent solar cell may possibly be damaged. 
       SUMMARY OF THE INVENTION 
       [0008]    It is accordingly an object of the invention to provide a photovoltaic system having a solar module, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type in such a manner that a power loss of a module is reduced in comparison with a conventional solar module in the case of a solar module which is in the shade or is defective. 
         [0009]    With the foregoing and other objects in view there is provided, in accordance with the invention, a photovoltaic system. The photovoltaic system comprises a solar module having at least one solar cell. A semiconductor switching element is connected in parallel with the solar module. Drive electronics block the semiconductor switching element during power generation by the solar module and otherwise switch the semiconductor switching element to low impedance to bridge the solar module. 
         [0010]    Replacing the shunt diode of a conventional solar module, which is connected in parallel with the solar module, with a semiconductor switching element which can be driven and has a low impedance in the on state, in particular has a considerably lower impedance than a shunt diode, achieves a considerably lower power loss of the solar module, in the case of a solar module which is in shade or is defective, and reduces heating. In other words, more energy can be supplied to the inverter by the solar module or by the string containing a plurality of such solar modules. 
         [0011]    In accordance with another feature of the invention, the semiconductor switching element is formed by a MOSFET transistor, in particular a power MOSFET transistor. The use of a MOSFET transistor is particularly advantageous because a parallel diode which can be effective as a shunt diode is integrated in its structure. In the case of a faulty transistor function of this semiconductor switching element, the function of the shunt diode may thus still be present, so that the solar module remains operational albeit with the disadvantages of a conventional solar module. 
         [0012]    In accordance with a further feature of the invention, a series circuit including a diode and a capacitor is connected in parallel with the solar module. The capacitor is charged by the solar module through the diode in the case of solar radiation and otherwise feeds the drive electronics. 
         [0013]    In accordance with an added feature of the invention, the drive electronics contain a multivibrator having an output signal which is short-circuited when the solar module is generating power. Alternatively, the drive electronics may be formed by an integrated circuit. 
         [0014]    In accordance with an additional feature of the invention, the drive electronics generate a pulsed control signal having short pulse pauses. In this way, during the time window generated by the short pulse pauses, the solar module can respectively return to its normal method of operation with a semiconductor switching element which is turned off. 
         [0015]    In accordance with a concomitant feature of the invention, the solar module is one of at least one string of a plurality of series-connected solar modules. 
         [0016]    Other features which are considered as characteristic for the invention are set forth in the appended claims. 
         [0017]    Although the invention is illustrated and described herein as embodied in a photovoltaic system having a solar module, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
         [0018]    The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a simplified schematic circuit diagram illustrating two series-connected solar modules according to the prior art; 
           [0020]      FIG. 2  is a simplified schematic and block diagram of a solar module according to one preferred exemplary embodiment of the present invention; 
           [0021]      FIG. 3  is a timing diagram of an output signal generated by drive electronics of the solar module of  FIG. 2 ; and 
           [0022]      FIG. 4  is a schematic diagram of the solar module of  FIG. 2  with a detailed layout of exemplary drive electronics. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Referring now to the figures of the drawings in detail and first, particularly, to  FIG. 1  thereof, there is seen a layout of a solar module which is disclosed, for example, in German Utility Model DE 88 15 963 U1. Solar modules  10  having series-connected solar cells  12  are each bridged by a respective shunt diode D 1 , D 2 . In the case of solar radiation, that is to say when the solar module  10  is generating power, the shunt diode D 1 , D 2  is reverse-biased and thus does not influence the function of the solar module. If a solar module in a string is temporarily shaded, that solar module  10  no longer generates a voltage and the shunt diode D 1 , D 2  accepts string current from the remaining solar modules in the string. The solar module which is in the shade is protected and the functionality of the string is retained. In the case of a defective solar panel  12 , the shunt diode D 1 , D 2  functions in a similar manner and the functionality of the remaining string is retained. 
         [0024]    A fundamental layout and fundamental method of operation of a solar module for a photovoltaic system according to the present invention will now first of all be described with reference to  FIGS. 2 and 3 . An exemplary embodiment of drive electronics of such a solar module will then be explained in more detail with reference to  FIG. 4 . 
         [0025]    A solar module, as is described in more detail below, is used, for example, in a photovoltaic system having one or more strings containing a plurality of series-connected solar modules of this type. The strings feed an inverter which supplies electrical energy generated by the solar modules to an AC power supply system or directly to a load in a correct phase and with a correct voltage. 
         [0026]    Referring now to  FIG. 2 , there is seen a solar module  10  which contains a plurality of solar cells  12  and a semiconductor switching element T 1  being connected in parallel with the module  10 . This semiconductor switching element T 1  is preferably a MOSFET transistor, in particular a power MOSFET transistor. A diode (indicated in an equivalent circuit diagram in  FIG. 2 ) which can be used as a shunt diode, is integrated in the structure of the transistor T 1 . Even in the case of a faulty transistor function of the MOSFET transistor T 1 , this diode can still ensure the functionality of the solar module  10  in a similar manner to a conventional solar module (see  FIG. 1 , for example). 
         [0027]    Drive electronics  14  are associated with the semiconductor switching element T 1 . These drive electronics  14  receive their supply voltage from a capacitor C 1 , for example an electrolytic capacitor, which is connected in parallel with the solar module  10 . In the case of solar radiation, the capacitor C 1  is charged by the solar module  10  through a diode D 1  which, for this purpose, is connected in parallel with the solar module  10  and is connected in series with the capacitor C 1 . The diode D 1  is forward-biased in the case of solar radiation. If the solar module  10  is switched off due to shade, for example, the diode D 1  changes to the reverse direction and decouples the capacitor C 1  from the voltage of the solar module  10 . 
         [0028]    The drive electronics  14  are constructed to have a high impedance for a very small power consumption. They generate an output signal U D  which is applied, as a control signal, to a control electrode of the semiconductor switching element T 1 . A temporal profile of the output signal U D  is illustrated in  FIG. 3 . However, while the solar module  10  is generating power, a voltage pattern of the drive electronics  14  is suppressed, that is to say U D =0 V, so that the semiconductor switching element T 1  remains turned off. 
         [0029]    In the case of a solar module  10  which is in the shade or is defective, the voltage pattern illustrated by way of example in  FIG. 3  is applied to the semiconductor switching element T 1 . The voltage pattern U D  contains, in particular, periodic control pulses with short pulse pauses  15  in between. During pulse durations  16 , the semiconductor switching element T 1  is turned on, so that the solar module  10  is bridged for at least one pulse duration. In this case, the on state of the semiconductor switching element. T 1  has a considerably lower impedance than that of a shunt diode D 1 , D 2  (see  FIG. 1 ) which is conventionally used. During the short pulse pauses, the (shunt) diode integrated in the semiconductor switching element T 1  accepts the string current. If the solar module  10  is irradiated again and generates power, the control pulses generated by the drive electronics  14  are suppressed again beginning in a pulse pause, so that the solar module  10  can return to its normal function again. If the solar module  10  is defective, the semiconductor switching element T 1  is still continuously driven by the drive electronics  14  using the output signal U D  illustrated in  FIG. 2 . 
         [0030]    The drive electronics  14  may, for example, contain a multivibrator, as is illustrated by way of example in  FIG. 4 , or may be in the form of an integrated circuit. 
         [0031]    Referring to  FIG. 4 , the drive electronics  14  contain, in particular, an operational amplifier OPV which is connected in the form of an astable multivibrator and has an operating voltage which is provided by the above-mentioned capacitor C 1 . The output signal U D  from the operational amplifier OPV is supplied through a resistor R 3  to a gate connection of the MOSFET transistor T 1 . A non-inverting input of the operational amplifier OPV is connected to a center tap of a voltage divider, which is formed of two resistors R 7  and R 8  and is connected in parallel with the capacitor C 1 . The output of the operational amplifier OPV is fed back through a resistor R 6  to its non-inverting input. 
         [0032]    The inverting input of the operational amplifier OPV is connected through a capacitor C 2  to a base potential of its operating voltage. The output of the operational amplifier OPV is also fed back through a parallel circuit to its inverting input. The parallel circuit includes, on one hand, a first series circuit of a forward-biased diode D 2  and a resistor R 5  and, on the other hand, a second series circuit of a reverse-biased diode D 3  and a resistor R 4 . 
         [0033]    The drive electronics  14 , which are constructed as described above, generate, at the output of the operational amplifier OPV, the output signal U D  having the voltage pattern which is illustrated in  FIG. 3  and is applied to the semiconductor switching element T 1  as explained above. The duration of the control pulses and the duration of the pulse pauses of the voltage pattern of the output signal U D  can be set independently of one another by using the diodes D 2  and D 3  and the resistors R 4  and R 5 . 
         [0034]    If 0.1 Hz, for example, is selected as the oscillation frequency of the multivibrator, the duration of the control pulses could be selected to be 9.9 seconds and the duration of the pulse pauses could be selected to be 0.1 second, for example. After the solar module  10  has been shaded, a time window of 0.1 second would thus result every ten seconds, and the solar module  10  would be able to return to its normal function in the string again in that time window. 
         [0035]    While the solar module  10  is generating power, the output signal U D  from the operational amplifier OPV is suppressed so that the MOSFET transistor T 1  remains turned off. This is achieved through the use of the circuit described below. 
         [0036]    The drive electronics  14  also contain a transistor T 2  having a collector which is connected to the output of the operational amplifier OPV and to the gate connection of the MOSFET transistor T 1  and having an emitter which is connected to the base potential of the operating voltage of the operational amplifier OPV. A base of the transistor T 2  is connected to a center tap of a voltage divider formed of two resistors R 1  and R 2  which are connected in parallel with the semiconductor switching element T 1 . 
         [0037]    While the solar module  10  is generating power, that is to say is neither in shade nor defective, the transistor T 2  is turned on through the voltage divider having the resistors R 1  and R 2 . The gate connection of the MOSFET transistor T 1  is thus at its source potential, with the result that the MOSFET transistor T 1  turns off. In this operating state, the diode of the MOSFET transistor T 1  also turns off and the capacitor C 1  is charged through the diode D 1 . 
         [0038]    If the solar module  10  is in shade, for example, and does not generate any power, the transistor T 2  turns off, with the result that the output signal U D  from the operational amplifier OPV is applied to the gate connection of the MOSFET transistor T 1 . As described above, the MOSFET transistor T 1  is on with a low impedance during the control pulses and is on through the integrated diode during the pulse pauses in this operating state.