E-W Roof Tops [RE-wrenches]

[jberdner at sma-america.com]

Peter:

Yes, I think you have it!

Just to be thorough:

If you have multiple strings and multiple inverters, e.g. 4 total
strings with 2 strings East and 2 strings West, the best configuration
is both East strings to one inverter and both West to the other
inverter.  This gives you the maximum from each array independently of
what is happening on the other array.  As you noted, this is one of the
big reasons for string inverters - the best from each array, all the
time.

I am a little unclear on your last question.  The best way to estimate
power output for the teeter totter is to look at it as 2 separate
systems - one facing East and one facing West.  You take the "ideal"
output estimate for each system and add the two together.  B. Brooks
mentioned some of these estimating tools in one of his posts.  Compare
this to the other possible array configurations and make a decision on
that first.  Now the question remaining is how much less power will the
single inverter solution produce compared to the ideal dual inverter
solution.  Given the ratio of installed inverter cost to installed array
cost it better be less than a 10% penalty or you should be thinking hard
about a second inverter.  Frankly I do not >> think << it will be that
much.

This will all get easier one we have the 2500, the 1800, the 1100 and
the 700.  Right now it is hard to justify 2 2500's when one will do.  2
1800's or 1100's may be a completely different story.

Best Regards,

John Berdner

-----Original Message-----
From: asap at podnine.com [mailto:asap at podnine.com] 
Sent: Monday, July 15, 2002 4:04 PM
To: RE-wrenches at topica.com
Subject: RE: E-W Roof Tops [RE-wrenches]

Thanks John,
Whew!  But yeah, makes sense.  Renaming it the teeter-totter array.  And
now
I want to consider the US116's instead of KC120's...maybe.  I can use
the
PVL116 specs for that, correct?

The one array orientation per inverter rule should be such I would think
and
not just your humble opinion!  With the choices we have today that
should be
an easy one in most cases.  The one crossover string mentioned
before--with
what was a PV shingle install (tricky enough)--has been the only
exception.

Your explanation is brilliant.  I'm assuming you're discussing one
inverter
with two differently oriented sub-arrays with the temperature vs.
orientation issues.  So, with TWO inverters and two strings in the
overall
array, that is, one string per inverter per orientation then, there
should
be no problems at all with the teeter-totter array.  Each inverter
tracking
and pulling as much as it can from its respective orientation, as the
sun's
"weight" is toggled over the rooftop each day.  Diurnal phasing and all,
just as Hugh had said.

And that means we can assume there would be no problem at all with one
inverter in a multiple inverter system not delivering anything to the
grid
with its sub-array, while another inverter in the same system is still
pushing its arrays peak.  This is another value of a string inverter
system
is it not?

So is the two string two inverter teeter-totter array going to have
production that is less, about the same, or more, than flat mounting
both
strings on either an E or W side?   I think it should be the same 11-12%
less overall output (from perfect orientation for both strings) with
either
setup.

Thanks again.
Peter @ ASAP POWER!
asap at podnine.com


-----Original Message-----
From: jberdner at sma-america.com [mailto:jberdner at sma-america.com]
Sent: Monday, July 15, 2002 1:02 PM
To: RE-wrenches at topica.com
Subject: RE: E-W Roof Tops [RE-wrenches]


Peter:

IMHO, the ideal situation is one array orientation per inverter.
Unfortunately, this is not always possible given the layout of the roof,
module choice, and inverter choice. A less optimal approach is to split
the arrays into two or more sub-arrays.  With crystalline modules it is
ESSENTIAL that all of the modules in the series string have the same
orientation.  Example: You have a N/S ridge with two strings with 10
modules in each string.  You should put all 10 modules in the same
series string (sub-array) on the West facing roof and the other 10
module string on the East facing roof.  Parallel the outputs of the two
sub-arrays with different orientation.  You should definitely avoid
putting modules facing East in series with modules facing West.

In a PV system the current is proportional to sunlight intensity (first
order approximation) so all the modules should have the same irradiance
to optimize the power from that sub-array.  If you split the string onto
multiple orientations then you will get the lowest of the two output
currents.  With an East and West orientation you will get low output all
day.  This is a bad thing.

On the other hand the voltage in a PV array is proportional to
temperature.  For most of the peak period the day (when all the power is
produced) the temperature of the two E/W arrays will be pretty close.
This, coupled with the I/V characteristics of the module, means that the
voltage will be similar so it is probably not too bad if you parallel
them.  Before I get flamed... Yes, the module temperature will be
somewhat different in the early morning and late afternoon due to
differing sunlight intensity but... The array with more irradiance will
be hotter so it's MPP voltage will be lower and it's current will be
higher.  The array with less irradiance will be cooler so it's MPP
voltage will be higher and it's current lower.  Now, what we are worried
about is reverse biasing the array (driving current backwards through
the array and bypass diodes) but his probably won't happen because the
hotter array (higher current) will largely determine the MPP of the
inverter.  The cooler array (lower current) can't overcome the higher
current array so it's voltage will tend to be pulled off of it's ideal
max power point.  Since it is cooler, this means it will operate closer
to Isc (since it has more voltage to begin with) which has a much lower
impact on power production than if you push it towards Voc and the
current crashes. Sorry if this is about a clear as mud - it is much
easier to see on an IV curve than it is to explain with words.

I do not know how to quantify the losses of an array with multiple
orientations on one inverter compared to the same number of modules
split onto two inverters.  If you can wire the array as noted above, my
experience tells me the differences should not be all that that great.

One last point - all of this applies to both crystalline and thin film
but the lower fill factor and smaller temperature coefficient of thin
film should make it more forgiving than crystalline.

Best Regards,

John Berdner



-----Original Message-----
From: asap at podnine.com [mailto:asap at podnine.com]
Sent: Monday, July 15, 2002 11:19 AM
To: RE-wrenches at topica.com
Subject: RE: E-W Roof Tops [RE-wrenches]

Wrenches,

What is the consensus on north-south ridged roof tops with perfect east
and
west roof faces?  I keep running into folks that want to go solar, they
just
don't have very cooperative rooftops.   I believe we've determined the
pitch
question (in this list, you all remember the ASCII chart from Bill?) and
that
*********
Bill said
This chart tells us the impact of fixed orientation on annual energy
production for a
grid-connected, net-metered PV system. Do not use this for off-grid
since we
cannot predict array utilization nearly as well. The interesting thing
about
this chart is that it clearly shows that annual energy producting is not
very sensitive to fairly wide variations in orientation (4:12 to 12:12,
SE
to SW only has about 5% loss from ideal). Moral of the story is don't
tilt
your modules on rooftop PV systems because it looks like hell, costs
more to
install, and often doesn't buy much as far as performance.
*********
and
*********
Hugh once said:
Tilting east and west is interesting because it creates a 'phase
shift' in the diurnal power cycle, plus a small amount of effective
shading, evening or morning. If you have a building with pitched
roof whose ridge runs north-south, you can spread the peak slightly,
by splitting the pv panels between the two roof orientations. This
leads to a more constant power production with a slightly lower
overall power peak.
*********
I've talked with John Berdner on the subject last year concerning how
SB's
would react, but we had 3j/a-Si and S and W faces, and had concerns
about
shading as well as having strings "crossing over," etc.  This two
inverter
system performs fairly well even with one of 5 strings crossing over
orientations, although I have not done a detailed analysis of the
customer's
"data" he emails to me from time to time.
*********
But now I'm wanting to know specifically about homes with 4:12
north-south
ridge lines and designing for one string to be on each E and W side,
each
c-Si string feeding its own inverter, grid-tied, net-metered systems.
Both
sides of the roof would have a string and their modules at roof pitch
(about
18 degrees). Is this a perfect way to do it or just a great way...an OK
way?
What do y'all think?

Seems like many a suburban street grid and row housing are oriented in
one
solid direction or the other, so half the homes in the area have e-w
ridges
and half the homes have n-s!  Before today, I was considering having one
such home with a long n-s ridge having a 5.5kw array being flat,
"hinging"
the unshaded long array up to horizontal, at just below ridge line, but
only
on the East side.  This was to consider potential permitting hassle I
may
have for "things being able to be seen from the street (W face seen in
this
case) and to still get some later afternoon sun.  Does anyone think this
would be better than splitting strings with modules flush on both E and
W
sides?

This is in Southern CA.  The aforementioned chart (for all of CA I
believe)
shows that modules pitched flat OR at 4:12, for E and W facing roofs, to
have about the same loss (11-12% impact; vs. 7:12/South being 100%).
Does
anyone think tilting modules S on E and W faces is worth it?

My guess, only if you can handle the aesthetics and the extra wrenching,
but
then 10% more is 10% more?  But we'd still have the E and W face
splitting
question.  I like what Hugh has said but didn't see any comment after
that
last year.  I understand that this arrangement would have slightly lower
peak and/or shorter duration of peak but can it be overall more output
in
the course of a day?  Could we call it a "fixed-tracking array"? ;)

Also, does anyone know of PV software that can configure/compare
multiple
strings/faces out for me?

Thanks!
Peter @ ASAP POWER!
asap at podnine.com

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