[RE-wrenches] Inverter oversizing

Wed Jan 19 15:19:46 PST 2011

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-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at 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-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at 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.

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