Round house [RE-wrenches]

[William Miller]

Colleagues:

I have a unique design challenge on my desk right now.  I'd like to
describe it and solicit any input any of you may have.

I have a potential grid tie client with a 12 sided, passive solar house.
He designed and built it himself and it is quite nice.  He wants to install
a grid tie system that will supply 100% of his power needs.  He has a large
lot, but it is heavily forested and there are no ground areas that are not
shaded.  His roof is large (the home has a radius of 40 plus feet), but is
has many skylights and has some shading.

I determined where we could shoehorn 36 panels onto the roof in areas with
minimum shading.  These panels will by necessity be facing various
directions, from 75 through 275 degrees true, in 30 degree increments.

I performed an analysis on each group of panels.  I also analyzed the same
number of panels facing due south.  The panels arranged around his roof
will produce 93% of the power that the control group will.  This is great
news.

The supplier raised a point that has to do with the MPPT circuitry in the
available grid tie inverters.  The MPPT values for a given panel change
drastically if the orientation changes significantly in a given moment.
Therefore, the panels facing 75 degrees will have different power point
values than the panels facing 275 degrees.  Will this cause a problem with
the MPPT electronics on a Sunny Boy or an ST inverter?  I suspect it will.

One work-around would be to provide 2 or 3 inverters and wire them to
similarly oriented panels.  This, by the way, is much more easily done with
the ST series inverters as they require only 4 12 volt modules in a string.
 This is indeed the advantage of the ST 4 module string wiring, a point I
have been meaning to make since a recent posting to Wrenches asking about
the lower voltage strings.  I might also add that the ST inverters have the
metering, ground fault interrupter and input and output breakers built in.

Please let me know if any of you have any experience with any scenarios
such as the one described here.

Thanks.

William Miller

__________________________________________________________________
William Miller
SLO Communications: Communications and Power Systems Consulting
PO Box 50, Santa Margarita, CA 93453
Voice :805-438-5600		Fax: 805-438-4607	VMail: 805-546-4875
email: wrmiller at slonet.org
License No. C-10-773985
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