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Bill,<br>
<br>
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.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
<br>
<br>
<br>
Bill Brooks wrote:
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<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Kent,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">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.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">This
is also a deficiency in modeling software since most models are using
hourly data.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Bill.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif"; color: windowtext;">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif"; color: windowtext;">
<a class="moz-txt-link-abbreviated" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>
[<a class="moz-txt-link-freetext" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On Behalf Of </b>Kent
Osterberg<br>
<b>Sent:</b> Tuesday, January 18, 2011 1:18 PM<br>
<b>To:</b> Wrenches; Marco Mangelsdorf<br>
<b>Subject:</b> [RE-wrenches] Inverter oversizing<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-bottom: 12pt;"><br>
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 <a moz-do-not-send="true"
href="http://www.sunnyportal.com/Templates/PublicPageOverview.aspx?page=85820a73-a347-48fb-b8d1-92e5f9b78ab3&plant=608681a7-ef60-4edb-84ff-07110db0ab6a&splang=en-US">Sunny
Portal</a>. The data are publicly accessible so feel free to run your
own analysis. Better yet, analyze the data for a system near you. <br>
<br>
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. <br>
<br>
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.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar. Inc.<br>
<br>
<o:p></o:p></p>
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