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
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.
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.
part of a PV module is shaded, the shaded cells will not be able to produce as
much current as the unshaded cells.
all cells are connected in series, the same amount of current must flow through
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
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.
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”.
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