User Tools

Site Tools


electrical:solar:shading

This is an old revision of the document!


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.

It works this way:

  1. partial shade causes voltage differences between cells
  2. electricity flows from higher-voltage areas to lower-voltage ones, so voltage differences would cause power to rush into the lower-voltage (shaded) cells
  3. overheating the shaded cell and possibly damaging it

To prevent damage panel manufacturer's insert diodes between cells or strings of cells. Partial shading trips the diodes and effectively takes the shaded cells/strings offline for their own protection.

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:

  • 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 power points (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

FIXME

electrical/solar/shading.1578458196.txt.gz · Last modified: 2020/10/11 19:48 (external edit)