DIODES IN PV SYSTEMS

Perform two different functions- depending on where they are installed-in a PV system. These functions are often con fused and misunderstood. I will attempt to clear some of the fog with this article. A diode is a solid state device made of P type and N-type silicon. In fact, a silicon solar cell is a diode. Solar cells use the internal electrostatic field of their P/N junction to pre

Vent electrons from flowing back into the cell after they have been knocked loose by light. That is the theory anyway! Diodes use their internal field to allow electric current to flow one way in a circuit and prevent it from flowing back.

When diodes are installed in series with a string of modules, they perform a blocking function, preventing backflow down the module string. When diodes are installed in parallel with modules, they perform a bypass function allowing current to pass around a shaded area of a module.

BYPASS DIODES

When part of a PV module is shaded, the shaded cells will not be able to produce as much current as the unshaded cells.

Since all cells are connected in series, the same amount of current must flow through every cell.

The unshaded cells will force the shaded cells to pass more current that their new short circuit current. The only way the shaded cells can operate at a current higher than their short circuit current is to operate in a region of negative volt age that is to cause a net voltage loss to the system. The current times this negative voltage gives the negative power produced by the shaded cells. In other words, the shaded cells will dissipate power as heat and cause “hot spots”. And the shaded cells with drag down the overall IV curve of the group of cells. The effect of this shading is also dependent on how the module is shaded. It is far worse to shade one cell 75% than to shade three cells 25% each. So if shading cannot be avoided, try to spread the shading over the most number of cells. One way to minimize the effect shading has on a single module in a series string, is to use by pass diodes in the junction box. Bypass diodes allow current to pass around shaded cells and thereby reduce the voltage losses through the module. When a module becomes shaded its bypass diode becomes “forward biased” and begins to conduct current through itself. All the current greater than the shaded cell’s new short circuit current is “bypassed” through the diode, thus reducing drastically the amount of local heating at the shaded area. The diode also holds the entire shaded module or group of cells to a small negative voltage of approximately -0.7 volts, thus limiting the reduction in array output.

BLOCKING DIODES

Diodes placed in series with cells or modules can perform another function that of blocking reverses leakage current backwards through the modules. There are two situations where blocking diodes can help prevent the phenomenon.

• Blocking reverse flow of current from the battery through the module at night.

In battery charging systems, the module potential drops to zero at night, and the battery could discharge all night backwards through the module. This would not be harmful to the module, but would result in loss of precious energy from the battery bank. Diodes placed in the circuit between the module and the battery can block any nighttime leakage flow.

• Blocking reverse flow down damaged modules from parallel modules during the day.

Blocking diodes placed at the head of separate series wired strings in high voltage systems can perform yet another function during daylight conditions. If one string be comes severely shaded, or if there is a short circuit in one of the modules, the blocking diode prevents the other strings from loosing current backwards down the shaded or damaged string. The shaded or damaged string is “isolated” from the others, and more current is sent on to the load. In this configuration, the blocking diodes are sometimes called “isolation diodes”.