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Words of Wisdom: “The response of a series parallel array to shade will be complicated.” – sensij1)
Partial shading
Partial shading can have surprisingly dramatic effects on panel output. The weirdest part is that partial shading can have more devastating effects on output than full shading like heavy overcast or evenly shaded forest canopies.
reminder: how panels work
Before we begin: solar panels are “current sour
ces”; their voltage pops up into the normal range in any kind of meaningful light(>= 20% insolation) but current will suffer.
Partial shading in this context means:
To prevent power from rushing into the shaded cells panels have bypass diodes. Basically the shaded panels get cut off, electrically speaking, to protect them.
panel configuration for partial shade
The way to avoid the problems associated with partial shade is to avoid partial shade. Failing that, there are steps one can take to minimize the losses:
dedicated controllers for each panel is optimal
-
With PWM controllers, parallel panel configurations typically yield more power in partial shade than serial.
With MPPT controllers and low-ish voltage serial panel configurations (where the total Vmp is ⇐2x battery bank voltage) it's close but parallel will probably still yield more.
With
MPPT controllers and
higher voltage serial configs (say Vmp is >=3x bank voltage) we see
an increasing advantage of serial panel configs in partial shade. This occurs because the MPPT has a broader range of voltages to sweep and can find
other power peaks (panel voltages) that are
low enough to bring the shaded cells back online but still
high enough to charge the battery bank. It's not reality, but we can think of it as MPPT evenly “shading” the entire panel voltage-wise in order to get max juice from it in partial shade conditions.
examples