Shading (was MX60 question) [RE-wrenches]

[Joel Davidson]

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Has anyone done a similar shaded modules comparison with SMA or other
non-battery inverters?

----- Original Message ----- 
From: "Christopher Freitas --- OutBack Power"
<cfreitas at outbackpower.com>
To: <RE-wrenches at topica.com>
Sent: Tuesday, September 21, 2004 5:11 PM
Subject: RE: MX60 question [RE-wrenches]

sunwise wrote:
> Can you explain the partial shading a little better?

Performance of a partially shaded PV array is extremely variable - it
depends on the specific wiring configuration and even the orientation of
the modules themselves.

The paritally shaded condition I was referring to as being improved by
having a higher than normal PV array voltage being stepped down by a
MX60 is one in which the multiple sets of series connected panels are
getting similar shading on each individual panel or series set.

The situation most of us are familar with is one were you shade 20% or
so of the PV panel and the output drops 80% - this is caused by the
panels Vmp (voltage at max power) being lower than the battery's voltage
it is charging - you "fall" off the power curve due to the shading of a
few cells.  When you start with a peak power point 2 or three times that
of the battery voltage and a MPPT type controller which can also do
step-down, then shading 20% of the PV array reduces the output by around
20% instead.

Of course things in the real world are never that simple... We are
planning to use our matched PV arrays here at OutBack to do some testing
of different shading conditions to see what the impacts are... stay
tuned again!

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas @ outbackpower.com
www.outbackpower.com
Arlington WA USA
Tel 360 435 6030

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