From Solar Cells to Solar Panels

A single crystalline solar cell supplies a no-load voltage of about 0.6 V, independent of its size. A cell made from amorphous silicon produces a slightly higher voltage of about 0.8 V. Under normal circumstances, i.e., assuming a normal cell size of 10x19 cm, the power output is relatively low at 1.2 to 1.4 Watt. Consequently, cells have to be joined into solar panels (or modules) before usable currents and/or voltages become available. As with batteries in a torchlight, cells are connected in series to obtain a higher output voltage. Conventional solar modules supply a no-load voltage of between 15 V and 22 V, which indicates that they consist of up to 40 series-connected solar cells. To raise their output voltage, solar cells may be connected in series. Similarly, parallel connection may be used to raise the output current. In many modules, cells are connected in parallel as well as in series. The size of the cell surface determines the maximum output current, which is usually indicated as the short-circuit current (i.e., at an output voltage of 0 V). Available versions range from small amorphous cells with an output current capacity in the micro-amps range, right up to square-metre size modules from monocrystalline silicon with an output short-circuit current rating of more than 5 A. Several identical modules may be connected in parallel to obtain a higher output current. The output voltage then equals that of a single cell. Finally, it is also possible to resort to a combined parallel-serial configuration. Strictly speaking, 20 cells connected in series should be sufficient to charge a 12 V battery. In practice, however, a solid margin should be designed into such a system. Unfortunately, the output voltage of a solar cell is not constant. In fact, it drops with increasing temperature, and decreasing brightness of sunlight. This effect is far more pronounced with polycrystalline cells than with monocrystalline types. Because of this, the voltage characteristics of the relevant cells or modules should be studied before a solar power system is planned and built. To achieve the highest possible output power, the cell or module should be operated at the so-called maximum power point, MPP, at which the electrical output power reaches its maximum. The MPP shifts with light intensity and cell temperature. Inside a module, the individual cells are connected in such a way that the lower part of a solar cell is always connected to the upper part of another cell. Professional modules constitute a symmetrical glass assembly with a layer structure: melting adhesive foil, solar cells, melting adhesive foil, glass. High-end frames consist of stainless V4A steel.

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