On a couple of Enphase systems I monitor
regularly, the modules are oversized compared to
the inverters. One system has Sharp 224 W
modules and M-190s (capable of producing 199
W). The array is a ground mount at a 30 degree tilt at 39+ degrees latitude.
When the system was new in the autumn, I noticed
two days of some clipping. Since then, none. In
the summer, when the sun angle is better, the
heat will keep the wattage down so clipping will
very seldom occur. The modules will not be close
to STC, so the inverters should not be pushed to their maximum.
The same is true of string inverters. Since real
power of the modules in generally significantly
below the nameplate, inverters will generally not
run at maximum power. When we take in all the
factors affecting the actual array power, I think
that 120% over sizing from nameplate wattage is generally safe.
Drake
At 09:14 PM 1/19/2011, you 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:
This reminds me of a scholarly article I came
across about a year ago while doing some
research. Heres a link to it if anyone is interested:
<http://www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf>www.lepten.ufsc.br/publicacoes/solar/eventos/2005/PSC/burger_ruther.pdf
David Brearley, Senior Technical Editor
SolarPro magazine
NABCEP Certified PV Installer
<david.brear...@solarprofessional.htm>david.brear...@solarprofessional.com
Direct: 541.261.6545
On 1/19/11 12:29 PM, "Bill Brooks"
<<billbroo...@yahoo.htm>billbroo...@yahoo.com> 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 dont 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.htm>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&splang=en-US>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.
_______________________________________________
List sponsored by Home Power magazine
List Address:
<mailto:RE-wrenches@lists.re-wrenches.org>RE-wrenches@lists.re-wrenches.org
Options & settings:
<http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org>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>http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
<http://www.re-wrenches.org/etiquette.htm>www.re-wrenches.org/etiquette.htm
Check out participant bios:
<http://www.members.re-wrenches.org>www.members.re-wrenches.org
_______________________________________________
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
_______________________________________________
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