Kent,
The article that David Brearley cited is a very good discussion on this subject. It clearly shows the under-prediction of losses due to hitting the inverter peak power capability when using longer-term data. This study was done by the Fraunhofer Institute in southern Germany. They get a lot more clouds there so the results might be comparable to some of the more cloudy regions of the United States. I would expect the results to be worse in much of the U.S. that gets clouds and higher irradiation than Freiburg. However, the results will definitely be less for much of California since clouds don't happen for sections of the year. It all matters where you are. Great discussion. Bill. From: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Kent Osterberg Sent: Wednesday, January 19, 2011 1:41 PM To: RE-wrenches Subject: Re: [RE-wrenches] Inverter oversizing Bill, The data are 15 minute averages so I'm sure that some edge of cloud events are washed out in the average. The energy they contributed was in the total measurement, but wouldn't have been included in my sum of clipped output. Observing the data, you can see some edge of cloud effects despite the 15 minute averaging. And on many days I suspect there is edge of cloud effect that wouldn't have hit the clipping limit too. Obviously, missing some short interval events biases the results a little but probably not by very much. For the sake of argument assume that there was a 3 minute, spike in output 100-watts above my threshold setting. In the 15-minute average that still would have been a 20-watt bump and with 10-watt data resolution, it probably would have showed up, but say it didn't show up or that it end up just below the threshold. Say this happened once every week, not likely, but if it did the missed data would amount to 0.25 kWhr or about 5% as much as the total observed with the data clipped at 800 watts. So you have to really stretch the brief edge of cloud argument to integrate enough energy to throw my graph off by very much. Kent Osterberg Blue Mountain Solar, Inc. Bill Brooks 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 don't 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: re-wrenches-boun...@lists.re-wrenches.org [mailto:re-wrenches-boun...@lists.re-wrenches.org] 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: 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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