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Power optimizers

Power optimizers are electronic devices, which aim to draw the maximum energy possible from the PV modules within an array (i.e. they try to ensure that each module is working at its MPP at any time).

There is usually one device per PV module, often directly mounted on the module rear side.

As is well known, the current in a string of modules is limited by the current of the weakest module. Optimizers are DC-DC converters, which convert the (Vmpp, Impp) maximum operating point of the PV module into a (Vout, Iout) representing the input power, but with the constraint that the output currents are the same for all optimizers within a string.

If we consider the I/V curve at the output of the optimizer, this condition is equivalent to extending the voltage range for which the device can provide the maximum power Pmpp of the considered device, so that one can find a common current for several optimizer power outputs. See the Optimizer's definition dialog, page "Output I/V behavior", for some examples.

Benefits

Regarding power enhancement, the optimizers cannot create energy, of course. The benefit we can expect from Optimizers is the recovery of electrical mismatch losses. The mismatch may be of two different kinds:

  • Current differences: all modules of a real PV array cannot be rigorously identical. There may be differences expressed by a tolerance range, especially in current, within a given string. During the PVsyst simulation, the mismatch between modules is characterized by a constant derate factor. But this loss may significantly increase along the years, as the degradation of the modules is not homogeneous.
    • Current differences may also be the result of uneven soiling of the modules.
    • Another cause of mismatch may be due to different orientations of modules within a string. This situation is not allowed for PVsyst simulations presently.
  • Partial shadings of some sub-modules: when one or several cells are shaded, the whole sub-module becomes reverse-biased, the by-pass diode is activated above the current corresponding to the residual power of the diffuse part. This results in a quite different situation as the previous one: the true Pmpp maximum is decreased in voltage (voltage of each sub-module, plus the diode reverse voltage), but the current remains the same Impp.
    • When you have one only string per MPPT input, the Vmpp of the string is reduced but the Impp current is identical for all modules, so that the Pmpp of the string remains the sum of the Pmpp of all modules. However the voltage of this Pmpp may be (often) below the voltage threshold (VmppMin) of the inverter.
    • With several strings in parallel on a single MPP input, this reduced Vmpp induces a severe mismatch. As a consequence the optimizers don't have anything to recover with partial shadings on a single string in parallel with unshaded ones.
    • When you open the optimizer's dialog, page "Output I/V behavior", you can play with parameters for a deeper understanding of these phenomenons.

Additional advantages are sometimes provided by the optimizers:

  • Some of them work with a centralized control, allowing for a continuous supervision of the system running at the module level.
  • Most of them may be turned off in a centralized way. This is very important for firemen. In a usual PV installation the array voltage cannot be switched off and may be a danger during emergency interventions.
  • In some cases the maximum voltage is reduced with respect to the Voc of the module, allowing to connect more modules in one string, which reduces the wiring resistive loss and the mounting costs, intermediate boxes, etc.

Counterparts

The Optimizers cannot have a perfect efficiency. There is an unavoidable loss of around 1%, which is usually balanced by the module mismatch benefit.

Mounting on the module may locally perturb the thermal behavior, slightly increasing the temperature of the nearby cell. However this will affect its Vmpp, not its current.

Each manufacturer has its own technology, which behaves in a particular way. In PVsyst several optimizers have already been integrated:

A generic optimizer is also available, with no restriction regarding the compatible inverters and PV cells.