SOLAR MODULE HAVING LONG SOLAR STRINGS

Abstract

The present invention relates to an electronic circuit for photovoltaic modules, which circuit ensures that the magnitude of the reverse voltage of each solar cell in a solar module does not exceed a particular value at any time. The problem solved by the invention is that of ensuring that the electrical voltage of each solar cell string within the photovoltaic module does not fall below a particular value.

Claims

1. Electronic circuit for photovoltaic modules, characterized in that it ensures that the magnitude of the reverse voltage of each single solar cell in the photovoltaic module does not exceed a value VC at any time, that is, even in the case where this solar cell is shaded and all the other solar cells are illuminated, in that it ensures that the electrical voltage at each solar cell string in the photovoltaic module does not fall below a value Vmin.

2. Electronic circuit according to claim 1, characterized in that the minimum string voltage V.sub.min is greater than 10 V, preferably, greater than 20 V.

3. Electronic circuit according to claim 1, characterized in that the magnitude of the maximum allowed reverse voltage VC per solar cell is less than 20 V, preferably, less than 12 V.

4. Electronic circuit according to claim 1, characterized in that the minimum string voltage V.sub.min is set depending on the temperature.

5. Microinverter characterized in that it contains an electronic circuit according to claim 1.

6. Photovoltaic module characterized in that it has no bypass diodes and the solar cell strings of the module are connected to an electronic circuit according to claim 1.

7. Photovoltaic module according to claim 6, which has at least one, preferably three, solar cell strings, each with at least 25, preferably at least 40, solar cells per solar cell string interconnected in series.

8. Photovoltaic module characterized in that it has no bypass diodes and the solar cell strings of the module are connected to an electronic circuit according to claim 5.

Description

DESCRIPTION OF THE INVENTION

[0016] It is object of the invention to enable a module layout in which more than 20 solar cells can be interconnected in series in a solar cell string without the reverse voltage of the individual cells exceeding a certain value, for example 12 V, in the case of shading.

[0017] The invention solves the object by means of an electronic circuit (601) which ensures that the string voltage always remains above a certain value V.sub.min.

[0018] Exemplary embodiments of the invention are described below with reference to the accompanying figures. The elements shown in the figures are not to scale. They serve to explain essential aspects of the embodiments. Complete electronic circuits and solar modules may include other elements not shown here. The features of the various embodiments can be combined with each other in any way, unless such a combination is explicitly excluded or excluded for technical reasons.

[0019] As shown in FIG. 6, for the voltage of a shaded solar cell, it applies

V.sub.c=V.sub.min−(N.sub.s−1)V.sub.oc

[0020] Vmin is the string voltage and at the same time the input voltage of the electronic control circuit (601). If the reverse voltage of a solar cell should not exceed V.sub.c=−12 V, this results in:

V.sub.min>V.sub.c+(N.sub.s−1)V.sub.oc=−12V+(N.sub.s−1),67V

[0021] In a string with 20 cells, V.sub.min=0.73 V. In a string with 60 cells, V.sub.min=27.5 V. To ensure that no power is lost in the illuminated case, V.sub.min should not be greater than the voltage V.sub.MPP at the maximum power point. For V.sub.MPP, the following applies to typical solar modules: V.sub.MPP=N.sub.s0.84 V.sub.oc. This results in a maximum number of solar cells per string as shown in FIG. 7.

[0022] Since V.sub.MPP depends logarithmically on irradiance, the dependence of the minimum irradiance required for V.sub.MPP to be greater than V.sub.min, shown in FIG. 8, results. The unit suns results from the normalized unit 1 sun=100 mW/cm.sup.2, which is approximately reached in Germany in summer at noon. From the figure it can be seen that, for example, for a string length of 60 cells, the minimum irradiation must amount to approx. 0.02 suns. The open circuit voltage of the individual cells is relatively insignificant at this point. This means that the principle can be applied well, and only very slight performance losses have to be accepted. Standard 60-cell modules or shingle modules, for example, can thus be completely interconnected in this embodiment. If the number of cells per string is reduced to 40 cells in a further embodiment of the invention, the minimum irradiance is only 0.002 suns. This is the configuration as shown in FIG. 9 for half-cell modules. It is not necessary to cut the strings in half. Each string then has 40 solar cells and is connected to a separate input of the electronic circuit.

[0023] If the irradiation is lower than the minimum irradiation, V.sub.MPP is below V.sub.min. The operating point thus shifts towards idle with decreasing irradiation. When the solar cells are no longer able to provide the voltage V.sub.min, the system shuts down. In one embodiment of the invention, the electronic circuit measures the irradiance, for example by measuring the current. This allows the input voltage to be reduced as irradiance decreases, so that when there is no irradiance V.sub.min can become greater than V.sub.MPP.

[0024] The open circuit voltage of solar cells is fairly linearly dependent on temperature. With increasing temperature the open circuit voltage decreases according to

V.sub.oc=V.sub.oc(T=25° C.)−β(T−25° C.),

wherein β for a solar cell is typically in the range of 0.27%/K or 1.8 mV/K. To ensure that the reverse voltage of the solar cells does not exceed 12V in the shading case even at low temperatures of, for example, −50° C., one embodiment of the invention measures the temperature and adjusts the minimum input voltage V.sub.min to the temperature according to

V.sub.min>V.sub.c+(N.sub.s−1)V.sub.oc=V.sub.c+(N.sub.s−1)(V.sub.oc(T.sub.oc=25° C.)−β(T−25° C.)).

[0025] In a further embodiment of the invention, the minimum input voltage V.sub.min is constant. In a further embodiment of the invention, the electronic circuit for controlling the string voltage is part of a larger electronic circuit, e.g., an inverter or microinverter.

[0026] Figures

[0027] FIG. 1: Schematic illustration of a solar module (100) consisting of solar cells (101) grouped in typically 3 solar cell strings (102). Each solar cell string is antiparallel interconnected with a bypass diode (103).

[0028] FIG. 2: Equivalent circuit diagram of N solar cells (101) interconnected in series. If a solar cell (201) is shaded, it blocks and the string current approaches zero. The illuminated solar cells are then in open circuit and supply their open circuit voltage V.sub.oc. The string voltage V, also becomes zero.

[0029] FIG. 3: Equivalent circuit diagram of a solar module (100) consisting of three solar cell strings (102), each with N.sub.s solar cells (101). Each solar cell string is interconnected in anti-parallel with a bypass diode (103). If a solar cell (201) is shaded, it blocks and the string current approaches zero. The illuminated solar cells are then in open circuit and supply their open circuit voltage V.sub.oc. The string voltage is equal to the diode voltage −V.sub.D.

[0030] FIG. 4: Schematic illustration of a solar module (100) consisting of halved solar cells (401). To prevent the solar cell strings from becoming too long, the solar module is divided into an upper and a lower half. The bypass diodes (103) are arranged in the center of the module so that one upper string and one lower string are interconnected in parallel and the bypass diode is interconnected antiparallel to these two strings.

[0031] FIG. 5: Schematic representation of a solar module (100) consisting of solar cells (501) cut into strips. Each original solar cell is typically cut into 6 strips. The module has 6 strings connected in parallel, each with 60 solar cell strips (501). To prevent the reverse voltage of a solar cell strip from becoming too large in case of shading, 3 bypass diodes (not shown here) must be interconnected in anti-parallel to 20 cells respectively.

[0032] FIG. 6: Equivalent circuit diagram of a solar cell string (102) with N.sub.s solar cells (101). The string is interconnected to an electronic control circuit (601) which ensures that the string voltage never becomes lower than a minimum voltage V.sub.min. If a solar cell (201) is shaded, it blocks and the string current approaches zero. The illuminated solar cells are then in open circuit and supply their open circuit voltage V.sub.oc. The electronic circuit prevents the magnitude of the reverse voltage V.sub.c at the shaded solar cell (201) from exceeding a critical value.

[0033] FIG. 7: Maximum number of solar cells per string as a function of the open circuit voltage of the individual solar cells, where the string voltage V.sub.MPP at the point of maximum power output is equal to the required minimum string voltage V.sub.min. The curves for allowed reverse voltages on shaded solar cells of V.sub.c=−12V and V.sub.c=−20V are shown as examples.

[0034] FIG. 8: Minimum irradiation above which the string voltage V.sub.MPP at the point of maximum power output is greater than the set required minimum string voltage V.sub.min, depending on the number of solar cells per string. The minimum string voltage V.sub.min depends on the open circuit voltage V.sub.oc of the individual solar cells and the allowed reverse voltage Vc per solar cell. The figure shows 4 curves for different combinations of V.sub.c and V.sub.c.

[0035] FIG. 9: Schematic illustration of a solar module (100) with three solar cell strings (102) consisting of at least 40 halved solar cells (401), all connected in series. No bypass diodes are used. Instead, the strings are equipped with electronic circuitry (602) that ensures that the string voltage on each string remains above a minimum string voltage V.sub.min.