East versus West [RE-wrenches]

Sat Nov 22 19:37:42 PST 2003

All:

John B beat me to it... And he's right, speaking from a monthly recorded
AC KWH output perspective since TOU Net-Metering is not applicable in
this case.  Thank you, John B!  I didn't want to speak until I compared
the data of known sites.  

I concur with John B's comments (With an ever so-slight deference to the
statement of "same temp at Solar Noon"... My testing shows East to be
slightly warmer at this time due to time of direct sun exposure and the
amount of power passing through the glass which in itself generates
additional heat... Not big enough a factor to even try to make a
conclusive statement about regarding power output, but measurable.).

Put the Parallel across the Ridge!  Bottom line.  We learned this early!
(For once!)

My words of caution regarding this application:  Be sure that your
Customer is made aware, in writing, that their system will NEVER see a
"peak power output" value in the display!  (No matter how well you think
they understand what you have honestly told them, they want to look for
that reading!  Scars to prove it!)  Also, if you intend to do any "KWH"
or "Cost-Return" types of projections, I would hit a single-orientation,
East or West, projection with about an 85% factor.  (More scars there!)

Aloha to all... My Lotto ticket will pay off tomorrow!

Just kidding... If I really believed that, I'd already have left and you
wouldn't have gotten this message.

Best to all... Congrats to you NABCEPers!

Praying for Sun on the Glass in Sacramento and beyond,

Matt Lafferty
Universal Energies Institute
mlafferty at universalenergies.com
(916) 422-9772
(916) 628-7694 Cell
(916) 914-2247 Fax
www.universalenergies.com

-----Original Message-----
From: John Berdner [mailto:jberdner at sma-america.com] 
Sent: Saturday, November 22, 2003 5:59 PM
To: RE-wrenches at topica.com
Subject: East versus West - Was: S&M in America/Sunny Boys on
parade[RE-wrenches]

Marco/ Wrenches:

Here in CA I would go for the all West solution if the customer could
use time-of-day net metering 
With a TODNM and a summer peaking utility load profile, you get more
credit for power produced in the afternoon. The downside of this is you
get reamed if you use power on weekday summer afternoons so make SURE
the customer will produce more than they use during the peak period. If
TODNM is not a consideration then the East or all West decision would
probably depend on local weather conditions.

Weather patterns with morning fog or afternoon showers would probably
make it go one way or the other. All things being equal, I would lean
towards all East because the array should be producing more power in the
cooler part of the day. That pesky temperature coefficient of power
(-0.5%/C typical for
crystalline) should give all East a slight edge.

If you split the array half and half you would ideally want a separate
inverter for each array orientation. Since this is not an option for
this site, the next best thing is to wire all the modules in one series
string in the same orientation. In this case, wire one full series
string on the West roof and the other full series string on the East
roof. Make your parallel connection across the ridge line. DO NOT wire
series strings across the ridge line! The latter will give you the
"worst of both worlds" and low energy production.

Slightly different orientations, i.e. on each side of a ridge line, is
actually not too bad in terms of energy production as long as all the
modules in each series string have the same orientation. The obvious
question is how much energy production will be lost compared to the
ideal. This is difficult to quantify since it will vary throughout the
day but, IMHO, the total energy "lost" is not that much. The reduced
energy production is caused by the two sub-arrays having different max
power voltages. The inverter with two combined sub-arrays will operate
between the ideal max power points for each sub-array. The max power
point of an array is largely determined by it's temperature and is
largely independent of irradiance. You can see how this is true by
looking at the temperature coefficients for a module. The voltage
coefficient is a negative coefficient and is MUCH larger than the
slightly positive current coefficient. This means the temperature has a
much greater effect on the MPP voltage that does irradiance.

In the morning the East facing array will have higher irradiance and
hence more current than the West facing array. It will also run hotter
(due to the higher irradiance) so it's max power point will be lower
than the West facing array. The array with the higher irradiance (let's
say the East facing array) will produce more current so the inverter
will be tracking closer to this MPP voltage.  
The lower irradiance array (let's say the West facing array) will be
cooler so it's MPP voltage will be higher. From the West array's
perspective, the inverter is loading the array at a slightly lower than
optimal voltage. This sounds bad but remember the West facing array has
low irradiance in the morning and so it can not contribute that much
power anyway. 
As the sun gets higher in the sky, the irradiance becomes more balanced
on the two arrays. This means the temperature and MPP voltage of the two
sub-arrays also get closer together. At solar noon both arrays should
have the same irradiance, temperature, and MPP voltage As a result, the
energy production "losses" should be near zero at solar noon. Because
most of the power is produced during the middle of the day I believe
(intuitively and not based on any studies) that the overall energy
production will be pretty close to that of two independently tracked
arrays. You could validate this theory, or prove me wrong, by picking
some nice stable days and then, on consecutive days, record the output
for:
1) Both arrays together, 2) The East array only, and 3) The West array
only.

I hope this helps.
Let me know what you come up with!

Best Regards,

John Berdner

>>> marco at pvthawaii.com 11/21/2003 11:41:29 PM >>>
John B. and other invited guests,

Opinions, please. An east-west roof at a shallow pitch, say 20 degrees
or less. Two series strings of modules on a SB 2500. Neither east or
west roof is favored in terms of better sun-hours. The choices: both
series strings on the same plane or split 'em--one on the east and one
on the west pitch? And no, getting two smaller Sunny Boys is not an
option.

tanks,
marco

Marco Mangelsdorf, Ph.D.
President
ProVision Technologies, Inc.
69 Railroad Avenue, A-7
Hilo, Hawai'i 96720
(808) 969-3281, 934-7462 facsimile www.provisiontechnologies.com 

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