[RE-wrenches] Inverter oversizing
Nick Soleil
nicksoleilsolar at yahoo.com
Thu Jan 20 09:02:08 PST 2011
This is very interesting. I think a lot of us already oversize inverters to
reduce wear and tear, and to operate the inverter closer to its peak
efficiency. This is another good reason.
What will it take to get more data on this subject. What percentage of the
annual radiation is occuring over 1000 w/m^2 in our local regions? Who is
collecting 1 minute data?
If we could capture .5% of the annual power on a 5 KW AC system, and the
system cost $42,000. That would be valued about $210 upfront, but even more
over the life of the system. That increased value would just pay for the
increased cost of the inverter. It is probably a smart decision, as long as the
increased inverter size doesn't increase the system cost significantly, by
requiring a supply side tap, a service panel upgrade, or other major upgrades to
the system..
Nick Soleil
Project Manager
Advanced Alternative Energy Solutions, LLC
PO Box 657
Petaluma, CA 94953
Cell: 707-321-2937
Office: 707-789-9537
Fax: 707-769-9037
________________________________
From: Bill Brooks <billbrooks7 at yahoo.com>
To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
Sent: Wed, January 19, 2011 4:33:00 PM
Subject: Re: [RE-wrenches] Inverter oversizing
David,
Good question, but difficult to answer. The basic rule in electronics is that
every 10C hotter that we run something continuously, the overall life is cut in
half. Conversely, if we run inverters cooler, they should last longer. The
difficult question is how much to oversize the inverter to get a 10C temperature
drop continuously. Not easy to answer. Also, heat is not the only killer of an
inverter. Line surges and lightning surges are probably bigger inverter killers.
And then there is engineering design. More inverters have died prematurely for
design and quality control reasons than any other failure type.
The final answer is 42.
Bill.
From:re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Dave Palumbo
Sent: Wednesday, January 19, 2011 3:55 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Inverter oversizing
Bill,
In addition to power production losses due to arrays that are oversized for the
inverter (although within manufacturers specs) how concerned should we be with
shorter inverter service life because the inverter is working at its maximum?
David Palumbo
Independent Power LLC
462 Solar Way Drive
Hyde Park, VT 05655
www.independentpowerllc.com
NABCEP Certified PV Installer
Vermont Solar Partner
23 Years Experience, (802) 888-7194
From:re-wrenches-bounces at lists.re-wrenches.org
[mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Bill Brooks
Sent: Wednesday, January 19, 2011 6:20 PM
To: 'RE-wrenches'
Subject: Re: [RE-wrenches] Inverter oversizing
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. 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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