David,
Take a close look at the upper and lowers portions of the graphs of the
10-second data and the 1-minute data for Freiburg.
To make it easier to see, attached is reproduction of their graphs on
one page. Just as fleeting peaks in irradiance (edge of cloud effect)
should be fewer in the 1 minute averages than in the 10 second raw
data; fleeting lows (caused by passing clouds) should be fewer in the 1
minute averages than in the 10-second data. Inspect the graph and label
which you think is 10-second data and which you think is 1-minute data.
Then compare to the original. The two graphs appear to be mislabeled.
Kent Osterberg
Blue Mountain Solar, Inc.
David Brearley wrote:
Re: [RE-wrenches] Inverter oversizing
Kent,
The graphs aren’t 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:
“Considerable 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%.”
>From the conclusion:
“we 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.”
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. Here’s 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.pdf>
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 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:
[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&splang=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.
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Burger_Ruther Graphs.pdf
