Solar cell module and method for operating a solar cell module

Abstract

A solar cell module and a method for operating a solar cell module. The solar cell module includes a plurality of strings which are each formed from a plurality of solar cells connected to one another in a series circuit, wherein each string is connected to a bypass circuit assigned thereto. The solar cell module is also characterized in that the bypass circuit has a switching element and is configured to reduce an electrical current inside the string by switching the switching element when a return current occurs within the associated string.

Claims

1. A solar cell module, comprising: a plurality of strings formed in each case from a plurality of solar cells connected to one another in a series circuit, wherein each string is connected to a bypass circuit assigned thereto, and the bypass circuit has a switching element and is configured, upon an occurrence of a back current within an associated string, to reduce an electric current within the associated string by switching of the switching element, wherein the bypass circuit has a control circuit configured to: cyclically reduce the electric current within the associated string by controlling the switching element, upon the occurrence of the back current within the associated string due to a shading situation, to reduce the electric current within the associated string by controlling the switching element for a predefined time period, and after the predefined time period, and when the shading situation persists, to switch the switching element again in order to limit the electric current in the associated string to cyclically reduce the electric current within the associated string by controlling the switching element.

2. The solar cell module as claimed in claim 1, wherein the switching element is connected to the solar cells of the associated string in a series circuit.

3. The solar cell module as claimed in claim 1, wherein the switching element is an active switching element.

4. The solar cell module as claimed in claim 3, wherein the active switching element is a transistor.

5. The solar cell module as claimed in claim 4, wherein the active switching element is a field effect transistor.

6. The solar cell module as claimed in claim 1, wherein the bypass circuit has a bypass diode connected to the associated string in a parallel circuit.

7. The solar cell module as claimed in claim 1, wherein the control circuit has a comparator circuit and/or an inverter circuit.

8. The solar cell module as claimed in claim 1, wherein at least one junction box in which the bypass circuit and/or the control circuit are/is accommodated.

9. A method for operating a solar cell module comprising a plurality of strings formed in each case from a plurality of solar cells connected to one another in a series circuit, wherein each string is connected to a bypass circuit assigned thereto and having a switching element, and a control circuit, wherein the method has a step, upon an occurrence of a back current within an associated string due to a shading situation, of reducing an electric current within the associated string by controlling the switching element and switching of the switching element for a predefined time period, and after the predefined time period, and when the shading situation persists, to switch the switching element again in order to limit the electric current in the associated string to cyclically reduce the electric current within the associated string by controlling the switching element.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is explained below on the basis of exemplary embodiments with reference to the figures. Here in each case in a circuit diagram:

(2) FIG. 1 shows a circuit of a solar cell module comprising three strings having parallel-connected bypass diodes in accordance with the prior art;

(3) FIG. 2 shows a circuit of a solar cell module comprising three strings in accordance with one preferred embodiment with a bypass circuit provided per string in addition to the bypass diode;

(4) FIG. 3 shows a string with a bypass circuit in accordance with one preferred embodiment with a switching element and a control circuit; and

(5) FIG. 4 shows a control circuit for controlling the switching element in accordance with one preferred embodiment.

DETAILED DESCRIPTION

(6) A schematic circuit diagram of a solar cell module having a multiplicity of solar cells 11 in accordance with the prior art is shown in FIG. 1. The solar cells 11 are interconnected to form three strings 1, 2, 3 consisting in each case of a plurality of solar cells 11 connected to one another in a series circuit. Each string 1, 2, 3 is connected to a bypass diode BD in a parallel circuit. If one or more solar cell(s) 11 in a string 1 is/are partially or completely shaded, then the (partially) shaded solar cell 11 becomes a consumer and the current direction in the string reverses (back current case). In this case, the bypass diode BD assigned to the string 1 is turned on and conducts the current of the solar cell module past the associated string 1. At the same time, however, the current generated in the non-shaded solar cells 11 of the affected string 1 is consumed in the (partially) shaded solar cell 11, which, after all, now acts as a consumer. Depending on the length of the string 1, on account of this the (partially) shaded solar cell 11 may heat up and thus lead to damage to the solar cell 11 or the encapsulation or lamination of the solar cell module. A similar problem arises if a solar cell 11 no longer generates current or sufficient current on account of a defect, instead of as a result of shading.

(7) The solution to this problem, according to the invention, is illustrated schematically in FIG. 2. Here, in addition to the bypass diode BD, a respective bypass circuit BS is added to each string 1. The bypass circuit BS has a switching element, which, upon the determination of a back current case within the associated string 1, leads to a disconnection of the string 1 from the rest of the solar cell module. For this purpose, the switching element in the associated string 1 is connected to the solar cells 11 in a series circuit. In FIG. 2, the bypass circuit BS is illustrated as an element separate from the bypass diode BD. However, the bypass diode BD may also be regarded as part of the bypass circuit BS. In actual fact, the bypass circuit BS is preferably additionally arranged in parallel with the bypass diode BD since it can detect the back current case for example by means of monitoring the voltage drop across the bypass diode BD. This is evident from the subsequent FIG. 3, which shows a preferred embodiment of the bypass circuit BS in which a voltage drop across the bypass diode BD is detected.

(8) A preferred embodiment of a string 1 comprising a plurality of interconnected solar cells 11 with a bypass circuit is illustrated in FIG. 3. A field effect transistor T1 is used here as switching element T1. In the normal case, i.e. when none of the solar cells 11 in the string is shaded, the rated voltage of, for example, 12 V relative to a first node N0 is present downstream of the bypass diode BD at a marked second node N1. In the bypass case or back current case, i.e. when the bypass diode BD is turned on, the voltage at the second node N1 decreases. The voltage at the second node N1 will ideally decrease to 0 V. In practice, however, it is also possible for the voltage at the node N1 to decrease to only approximately 3-4 V in the back current case, i.e. to approximately one quarter or third of the rated voltage.

(9) In order that the switching element T1 is controlled correctly in the case of different voltage value reactions in the back current case, a control circuit SC is provided. The control circuit is arranged between the two nodes N1 and N0 and generates a voltage as output signal Gate, which voltage controls the switching element T1. In the present case, the switching element T1 is a field effect transistor T1, the gate output of which receives the output signal Gate of the control circuit SC.

(10) A circuit diagram of a preferred embodiment of the control circuit SC is represented in FIG. 4. The control circuit SC comprises a voltage supply section formed from a diode D3 and a backup capacitor C3, a comparator formed from a first resistor R1, a Zener diode D1 and an operational amplifier U1, and also an inverter comprising two complementary MOSFETs, namely an N-type MOS or NMOS M1 and a P-type MOS or PMOS M2. Consequently, a CMOS inverter is involved here.

(11) The grounding node of the control circuit SC is connected to the first node N0 shown in FIG. 3. The second node N1 is connected to the anode of the diode D3. The string voltage present between the two nodes N0 and N1 is thus present across the series circuit comprising diode D3 and backup capacitor C1. During operation, for example 12 V can be present between the nodes N0 and N1. On account of this, the backup capacitor C1 is charged via the diode D3. The diode D3 prevents the backup capacitor C1 from draining into the string upon the collapse of the supply voltage.

(12) The supply voltage for the downstream comparator and for the inverter is tapped off ahead of the diode D3. This is illustrated in the diagram in FIG. 4 by the tap marked Vcc, which taps off the voltage on the cathode side at the diode D3 and feeds it to the comparator and to the inverter. The size of the backup capacitor C1 has to be optimized experimentally. In the present embodiment, a capacitor having a capacitance of 100 F was chosen.

(13) The Zener diode D1 here has a Zener voltage of 6.2 V. During normal operation, from the operating voltage present, 6.2 V, i.e. the Zener voltage, is dropped across the Zener diode D1. The remaining 5.8 V is dropped across the resistor R1. Consequently, 5.8 V more is present at the inverting input of the operational amplifier U1 compared with the positive input, such that the operational amplifier U1 toggles in the direction of negative operating voltage (here 0 V). Even without the use of an operational amplifier with a rail-to-rail output, significantly less than 1 V is thus present at the output of the operational amplifier. The downstream CMOS inverter interprets this voltage as a low level and pulls the output Gate in the direction of the operating voltage. The switching element T1 is turned on and conducts the string current.

(14) If a solar cell 11 is then shaded in the string, it becomes a consumer and part of the string voltage is dropped across it. As a result, the voltage at the second node N1 decreases. As soon as the voltage at the second node N1 falls below the Zener voltage of the Zener diode D1 (here 6.2 V), current can no longer flow via the Zener diode D1. Consequently, voltage is no longer dropped across the resistor R1 either. The voltage at both inputs of the operational amplifier U1 is then equal. A voltage which is greater than half the operating voltage of the operational amplifier U1 is thus established at the output of the operational amplifier U1. This means a high level for the downstream inverter, which leads to a low level at the output of the inverter, said low level being between 0 and 1 V in this embodiment. This voltage is lower than the threshold voltage of the field effect transistor T1. Consequently, the field effect transistor T1 is turned off and current no longer flows in the string. As a result, however, voltage is then no longer dropped across the shaded solar cell, and so the voltage rises again abruptly at the second node N1. The system therefore toggles again in the other direction and the field effect transistor T1 is turned on again. The cycle begins anew; the system pulsates.

(15) Since, immediately after the string has been switched off, the full voltage is available again for the supply of the control circuit SC and for charging the backup capacitor C1, the capacitance of the backup capacitor C1 can preferably be chosen to be significantly less than 100.

(16) The output resistor R2 serves to cleanly terminate the output potential from output signal Gate. The output resistor R2 should be chosen with the highest possible resistance in order not to consume current unnecessarily. The same applies to the resistor R1 in the comparator.

LIST OF REFERENCE SIGNS

(17) 1, 2, 3 String 11 Solar cell BD Bypass diode BS Bypass circuit T1 Field effect transistor, switching element R1 Resistor R2 Output resistor D1 Zener diode D3 Diode M1 NMOS field effect transistor M2 PMOS field effect transistor C1 Backup capacitor U1 Operational amplifier N0 First node N1 Second node