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 ------------------------------------------------------------------------------ _______________________________________________ List sponsored by Home Power magazine List Address: RE-wrenches@lists.re-wrenches.org Options & settings: http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org List rules & etiquette: www.re-wrenches.org/etiquette.htm Check out participant bios: www.members.re-wrenches.org
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