[RE-wrenches] PV Basics - Sting Shading

[markf at berkeleysolar.com]

Friends,

Help disabuse this old timer of his outdate notions:

crystalline modules
MPPT tracking inverter

a sting of modules operate at max power in full sun

shading causes enough cells in the first substring in the first module to
go into resistance to the point that the substring "turns off" and the
current of the full string is shunted through the substring bypass diode

the following power losses occur:

1 - the power of the substring is lost completely
2 - power is lost in the bypass diode

AND

3 - power is lost becasue the remaining cells in the system can no longer
operate at their max power point

progressively shading more and more substrings eventually causes the
output power to collapes when the operating voltage of the string drops
below the MPPT tracking window.

I have always thought that #3 above was very significant such that power
loss with progressivley increasing shading was on the order of the square
of the percentage of shading ( you loss "x" percent of your productive
cells and the remaining cells operate "x" percent below their max power)

a collegue pulled out a PVSyst simulation that stacked the I/V curves for
this scenario and it basic showed a linear reduction in power relattive to
the amount of shading

Have I been wrong in my thinking all along?

Thanks,

Mark Frye
Berkeley Solar Electric Systems