PV roofing material [RE-wrenches]

[Bill Brooks]

William,

The primary issue is that all the modules in a single series string must be
in the same orientation so they have the same current output. Putting two
series strings, say--one east and one south, is not a big problem if they
are in parallel. Think about the operating voltage of an illuminated array.
If the series string is any sun at all on it, it will have an operating
voltage very close to a fully illuminated series string--just a whole lot
less current. The MPPT has a more difficult time with partial shading than
with different orientations.

If it is not adding cost, it is fine to have differently oriented sub-arrays
on separate voltage systems, but you will find that they will operate at
similar voltages for much of the day. The caveat is that in the morning or
afternoon, when shading is much more likely, the voltage output will be
affected by the shade, not the angle of the sun.

Hope that helps,

Bill.


-----Original Message-----
From: William Miller [mailto:wrmiller at slonet.org]
Sent: Monday, August 19, 2002 9:25 PM
To: RE-wrenches at topica.com
Subject: SPAM: Re: PV roofing material [RE-wrenches]


Matt:

What were your expectations, and given that the GC1000 has no display
(other than a blinking light), how were you able to tell the installation
met them?

I am looking for hard data to answer the old
"dual-different-direction-facing-arrays-into-one-MPPT" question once and
for all.  It was my understanding from this forum (or speculation,
possibly) that two arrays facing different directions would confuse one
MPPT controller, possibly significantly reducing the output.  Based on this
assumption, all of the systems we have designed with this scenario, we have
specified separate MPPT controllers for separate facing roofs.  This was
probably the conservative approach, but we would like more specific data if
you or others have it.

Thanks,

William Miller


At 10:21 AM 8/18/02 -0700, you wrote:
>William,
>
>It was a grid-tie, and I used a GC-1000 with two 15 mod strings. It has
>worked flawlessly (so far!) for about 9 months, meeting or exceeding my
>expectations. Because the roof pitch is only a 3 in 12, the system starts
>producing very early and runs very late in the day, Summer and Winter.
>
>But, according to John Berdner, if the install is with a Sunny Boy, the
>string inputs MUST be exactly balanced or something mysterious and terrible
>will occur! I have to assume (assume nothing is my motto these days)
>that an east-west array works very much like any array which has the
>possibility of being partially shaded
>because of too-tight spacing, shadows from obstructions or whatever. I
>haven't heard of any problems (beside low power output) because of this
type
>of arrangement as long as all the mods have their diodes intact.
>
>Matt
>
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