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Power optimizers

Power optimizers are electronic devices designed to extract the maximum possible energy from PV modules within an array (i.e., they ensure that each module operates at its MPP at all times).

Typically, there is one device per PV module, often mounted directly on the module rear.

As is well known, string current is limited by the weakest module's current. Optimizers are DC-DC converters that convert the maximum operating point (Vmpp, Impp) of a PV module to output (Vout, Iout) representing the input power, with the constraint that all optimizers' output currents within a string are identical.

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

Benefits

Regarding power enhancement, optimizers cannot create energy, of course. The expected benefit from optimizers is recovery of electrical mismatch losses. Mismatch can be of two types:

  • Current differences: all modules in a real PV array cannot be perfectly identical. Differences may exist within a given tolerance range, especially in current, within a single string. In PVsyst simulations, module-to-module mismatch is characterized by a constant derate factor. However, this loss can increase significantly over time, as module degradation is not uniform.
    • Current differences can also result from uneven soiling of modules.
    • Different module orientations within a string can also cause mismatch. This situation is not currently allowed in PVsyst simulations.
  • Partial shading of some sub-modules: when one or more cells are shaded, the entire sub-module becomes reverse-biased, and the bypass diode activates above the current corresponding to the residual power of diffuse light. This creates a different situation from the previous one: the true Pmpp maximum decreases in voltage (sub-module voltage plus diode reverse voltage), but current remains the same Impp.
    • With a single string per MPPT input, string Vmpp is reduced, but Impp is identical for all modules, so string Pmpp remains the sum of all module Pmpp values. However, this Pmpp voltage may be below the inverter's voltage threshold (VmppMin).
    • With multiple strings in parallel on a single MPPT input, this reduced Vmpp causes severe mismatch. Consequently, optimizers cannot recover much from partial shading on a single string in parallel with unshaded ones.
    • In the optimizer dialog, "Output I/V behavior" page, you can adjust parameters for deeper understanding of these phenomena.

Additional advantages sometimes provided by optimizers include:

  • Some operate with centralized control, allowing continuous system supervision at the module level.
  • Most can be turned off centrally. This is very important for emergency responders. In typical PV installations, array voltage cannot be switched off and may pose a hazard during emergency interventions.
  • In some cases, maximum voltage is reduced compared to module Voc, allowing more modules in one string. This reduces wiring resistive loss and mounting costs, intermediate boxes, etc.

Drawbacks

Optimizers cannot achieve perfect efficiency. There is an unavoidable loss of approximately 1%, which is typically offset by module mismatch benefits.

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

Each manufacturer uses their own technology with particular behavior. PVsyst has already integrated several optimizers:

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