Joel,
Thanks for the information. 35% loss due to to smog or soiling is a
big hit and, I'm sure, an exception. Still, if the loss is even 20% it
supports the concept that an array here on the cool, cloudy west side
of the WA Cascades can perform on par with an equivalent array in a
hot, dusty/smoggy location in CA.
I'd still like to hear of actual less-than-stellar production numbers
recorded in such situations.
-Kelly
On Jun 10, 2010, at 9:02 AM, Joel Davidson wrote:
PVWatts is a good general estimator when you fine-tune the derate
factor. For single crystal and multicrystalline arrays, I generally
use 0.65 for battery-based PV and inverter systems and 0.82 for
batteryless inverter systems. PVWatts annual results are l5% low for
Unisolar arrays because PVWatts uses the crystalline silicon
temperature coefficient.
Even though PVWatts2 may seem more accurate, it does not factor in
unique local climate conditions like California coastal morning and
afternoon fog or inland persistent winter Tule fog. However, NREL's
climate data does include LA's "June gloom" see http://en.wikipedia.org/wiki/June_Gloom
For flat roofs in snowy climates like the Unisolar project in
Rochester NY, I deduct 30% from PVWatt's November through March
estimates for a fairly accurate annual estimate. Powerlight (now
SunPower) also uses an additional 30% monthly derating for flat
roofs in snowy locations.
PVWatts does not account for really dusty and dirty air locations.
Bill Brooks worked at PVUSA and is very knowledgeable about power
loss from soiling from agricultural dust in central California. Bill
also measured 35% power loss at the Long Beach CA harbor waste-to-
energy powerplant PV systems. That location and most LA county
freeways experience particulate pollution that not only reduces PV
production but causes permanent respiratory damage to children and
shorten the lives of elderly people, sort of like the canary in the
mine.
Joel Davidson
----- Original Message -----
From: Kelly Keilwitz, Whidbey Sun & Wind
To: RE-wrenches
Sent: Thursday, June 10, 2010 7:57 AM
Subject: Re: [RE-wrenches] Real world PV production
Thanks, Don,
Performance predictions such as from PV Watts VASTLY underestimate
PV production in our area. For example, PV Watts estimates 940 kWh/
kW for a 4:12 pitch at 180˚, whereas such systems are actually
producing up to 1300 kWh/kW (as measured by a renenue-grade
production meter). We have to set the derating factor to nearly 100%
in order for the predictions to match.
I assume that is partly due to coarse irradiance data (not
accounting for higher irradiance for our location in the rain shadow
of the Olympic Mountains), but also due to our clear, cool, windy
summers, good natural washing, and (perhaps) dispersed distribution
of irradiance.
I want to know if the opposite is true: Do performance predictions
OVERESTIMATE PV production in areas with historically high
irradiance, but significant soiling and temperature issues.
Thanks,
-Kelly
Kelly Keilwitz, P.E.
Whidbey Sun & Wind
Renewable Energy Systems
ke...@whidbeysunwind.com
360-678-7131
On Jun 9, 2010, at 8:59 AM, i2p wrote:
On Jun 9, 2010, at 8:22:12 AM, "Kelly Keilwitz, Whidbey Sun & Wind"
<ke...@whidbeysunwind.com> wrote:
Thanks, Joel
How about PV systems away from the coast, in a hotter, dustier
location, like Bakersfield, Fresno, Sacramento, Palm Springs,
etc........?
In central CA we do a little better. I casually monitor several
systems in this area and expect around 1500-1600 kwh/kw. per year.
Don Loweburg
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