Kent, The graphs aren¹t mislabeled. The one-minute average data has more granularity, which shows up at the upper limits of the data. It reveals more peaks over 1,000 W/m2, even in excess of 1,200 W/m2. These values get compressed into the lower bins in the hourly data.
The results from the Florianopolis site in Brazil, gives a pretty good idea of how significantly different the results can be based on monitoring frequency: ³Considerable differences emerge when looking at the high end of the radiation level Distribution, which shows for the one-minute averages that some 9% of the daytime hours present radiation levels 1000W/m2, with a corresponding energy content of some 23%; hourly averages for the same range correspond to around 6% of daytime hours, and below 11% of the total energy content. Radiation levels above 900W/m2 occur some 16% of the time when looking at one-minute averages, and below 13% of daytime hours when using hourly averages, with corresponding energy fractions respectively above 38% and 25%.² >From the conclusion: ³we have demonstrated that the estimation of the actual losses due to inverter undersizing increases with increased time resolution of the radiation measurements, revealing that hourly averages hide important irradiation peaks. In fact, results with hourly averages are an experimental artifact, and lead to an estimation of the solar energy resource distribution that does not correspond to reality. Hourly averages of irradiation values lead to inverter undersizing and the associated energy losses.² Figures 8 & 9 are interesting. You could imagine what these results would look like if overlaid onto your charts. Best, david On 1/19/11 8:14 PM, "Kent Osterberg" <[email protected]> wrote: > David, > > Thanks for sharing that paper. The labeling the graphs for the 10-second and > 1-minute data in Freiburg appears to be reversed - the one minute averaging > seems to have more data in all of the bins above 1000 W/sq m. Basically, > these graphs show that irradiance observations above 1100 watts per square > meter are fleeting and disappear in hourly averages. Such occurrences are > also masked to a small extent by 1-minute averages. > > Kent Osterberg > Blue Mountain Solar, Inc. > > > > David Brearley wrote: >> Re: [RE-wrenches] Inverter oversizing This reminds me of a scholarly article >> I came across about a year ago while doing some research. Here¹s a link to it >> if anyone is interested: >> >> www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf >> <http://www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.p >> df> >> >> David Brearley, Senior Technical Editor >> SolarPro magazine >> NABCEP Certified PV Installer >> [email protected] >> Direct: 541.261.6545 >> >> On 1/19/11 12:29 PM, "Bill Brooks" <[email protected]> wrote: >> >> >>> Kent, >>> >>> How often were your data records? To capture edge of cloud effects, you need >>> one-second data. Not many people gather that fast or that much data on >>> inverters. I don¹t think there is that much energy in these spikes, but they >>> are real and make some difference. 15-minute average data will completely >>> wash out this data. >>> >>> This is also a deficiency in modeling software since most models are using >>> hourly data. >>> >>> Bill. >>> >>> >>> From: [email protected] >>> [mailto:[email protected]] On Behalf Of Kent >>> Osterberg >>> Sent: Tuesday, January 18, 2011 1:18 PM >>> To: Wrenches; Marco Mangelsdorf >>> Subject: [RE-wrenches] Inverter oversizing >>> >>> >>> Attached is a graph that I produced to document the effect of various ratios >>> between the PV array size and the inverter size. I extracted output power >>> data for a 1020-watt system located in NE Oregon that is on the Sunny Portal >>> <http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a73- >>> a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&splan >>> g=en-US> . The data are publicly accessible so feel free to run your own >>> analysis. Better yet, analyze the data for a system near you. >>> >>> Using 2009 data, I looked at how much energy would have been lost if the >>> output was clipped at 800W, 810W, .... 1020W. I used 2009 data because >>> there was a period in 2010 when the Sunny Webbox didn't have internet >>> access. At 800 watts, power clipping would have happened on about 25% of >>> the days. Yet the energy that would have been lost was only 0.38% of the >>> annual total. >>> >>> The results shown on this graph aren't universal, results would be a little >>> different in 2010, it would be different in some other climate, it would >>> have been different at another elevation, it would be different with a >>> different array angle, ..., and the module tolerance and inverter efficiency >>> also effect the results. Modules in this system are Suntech 170-watt +/-3%. >>> The inverter is Sunnyboy 1800 that should be operating at close to 93% >>> efficiency. >>> >>> Kent Osterberg >>> Blue Mountain Solar. Inc. >>> >>> >> >> >> >> >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: [email protected] >> >> 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 <http://www.re-wrenches.org/etiquette.htm> >> >> Check out participant bios: >> www.members.re-wrenches.org <http://www.members.re-wrenches.org> >> >> > > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: [email protected] > > 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 > David Brearley, Senior Technical Editor SolarPro magazine NABCEP Certified PV Installer [email protected] Direct: 541.261.6545 Fax: 541.512.0343 Visit our Web site at solarprofessional.com (Sample copy available for download at: solarprofessional.com/sample)
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