East versus West - Was: S&M in America/Sunny Boys on parade [RE-wrenches]

John Berdner jberdner at sma-america.com
Sat Nov 22 17:59:16 PST 2003


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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