On 7/29/20 5:04 PM, Haudy Kazemi via EV wrote:
On Wed, Jul 29, 2020, 18:36 Willie via EV <ev@lists.evdl.org> wrote:
Perhaps you can comment on some trouble I've run into.
After observing that some microinverters worked on battery input and
that string inverters worked well with combined strings of both east and
west facing panels, I became convinced that a single microinverter could
serve one east facing panel and one west facing panel. The two panels
connected in parallel to the microinverter. Tests were disappointing
with generally a microinverter, over a day, producing less energy than
had it been attached to either a single east facing panel or a single
west facing panel. Slopes on the panels are 15-20 deg from vertical so
that panels are shaded around 30% of the day and both E and W receive
sun about 30% of the day. It is semi-clear that even a shaded panel
interferes with production from the not shaded panel.
String inverters can do "double duty", producing from east facing panels
early in the day and west facing panels late in the day. With a smooth
change over at mid-day. Microinverters seem unwilling to do that
"double duty".
Which microinverter model did you use?
The ones that will be useful to me for double duty are Enphase M215. I
have hundreds of them and only a few of other types.
My cost for one inverter per panel has been about $100; $50 for the
panel and $50 for inverter plus cabling. If I could get double duty to
work, the cost per panel would drop to about $78. A pair of MC4
splitters is about $5, about $2.50/panel.
Were your solar modules identical?
Yes.
Which solar module model did you use?
I don't recall the brand name. 250w.
Were the solar modules facing completely opposite directions, true east and
true west, i.e. truly 180 degrees apart?
Yes. With a vertical angle between of 30-40 deg; a pair of panels make
a near equilateral triangle with the ground. I'm wondering if I might
see better results by reducing that angle between panels.
MPPT algorithms can get stuck at a local maximum point point instead of the
global maximum point point along the complex I-V curve created by 2
parallel modules. Shading can also create complex I-V curves. Some string
inverter brands (SMA) talk about how their products handling this situation
better than the competition.
*https://www.solarquotes.com.au/blog/sma-shadefix/
*
https://www.sma-america.com/fileadmin/content/www.sma-america.com/Partners/Images/Knowledgebase/OptiTrac_Global_Peak/OPTITRACGPWP-UUS131010W.pdf
I have had good luck with several SMA/SunnyBoy 4kw old transformer type,
single tracker. Typically, two strings of 10E combined with 10W in the
field plus another pair of 10E plus 10W combined in field, then combined
at the inverter with the other pair. BUT, I've also had good luck with
smaller samples of other transformer type string inverters configured
similarly.
With independent modules and independent MPPT, things are easy. When
paralleled, the two modules will need to operate on a common voltage. The
best power point voltage for one won't be the best for the other, except
when both modules have identical illumination. At other times one module
will be no longer at its personal peak. The question is whether or not the
microinverter can find the highest peak overall.
When I monitor the power from a single pair of panels (one inverter) at
1 second intervals, I see what Lee described for one of his inverters.
Good power briefly, going to zero, then back and forth. The end result
being not useful energy.
Thanks for the response!
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