PV Series night time tare loss [RE-wrenches]

[Mark Edmunds]

Wrenches - Sorry about the 3 repeat posts last week! Our server apparently
was being choked with Mydoom spam on Tuesday and Wednesday, and was merrily
re-sending messages when it thought they weren't getting received.

Matt - thanks for your comments on the article. As all 3-phase central
inverters use an isolation transformer, core losses will be pretty much the
same while a system is energized...this isn't a PV Series specific issue.
Dealers have wanted to know what the night time losses are vs. a more
expensive central inverter that disconnects at night, so we wanted to try
and quantify that.

The following are responses to some of your specific questions, from the
report authors...

Mark Edmunds
Xantrex Technology


From: mlafferty at universalenergies.com
[mailto:mlafferty at universalenergies.com]
Sent: Wednesday, January 28, 2004 7:02 AM
To: RE-wrenches at topica.com
Subject: RE: PV Series night time tare loss [RE-wrenches]

Secondly, the calculated "losses" are NIGHTTIME only.  This is only part of
the question that needs to be answered.  The tranny is hot 24/7 in most
applications and is a 24 hour a day parasitic loss.  Not, just at night.
Just under double the 13 nighttime hours assumed in the report. Using the
example on Page 3 of the report, the nighttime tare losses are 2.85% of the
production revenue.  Multiply that by 1.846  (13 / 24 =
1.846) to correct for 24 Hour Tare and you get 5.26% for that nice optimized
system in Sunny Inland SoCal.

[response] Agreed.  The purpose of the report was to estimate nighttime core
losses only, so that a rational, engineering economics-based decision could
be made on whether nighttime disconnect would pay for itself in a particular
application and with a given return on investment criteria.

The report also assumes a 23% System Capacity Factor.  I must assume that is
annual.  OK.  I'll buy that, but let's be "apples to apples" on it.  This is
a VERY optimistic factor for all but a tracking system (up to 28 or 29%) or
a "perfectly optimized" fixed installation in a very sunny place!  Optimized
includes: Due South, Lattitude minus 10 tilt, Nil module mismatch, Nil
wiring losses, and a fully loaded inverter. Change any of these assumptions
and the effective net % tare losses grows rapidly.  For
instance:  A large percentage of PV Series systems are being installed on
flat-roof applications, for instance... Right? What kind of Annualized
Capacity Factor do you think these systems are averaging statewide in CA?
14%? 16%?  How about one in Livermore's backyard... Santa Rita, for example.

[response] The example capacity factor came from the referenced CEC
document. The capacity factor is irrelevant to the point of the report,
which is how to estimate nighttime tare loss. Use whatever capacity factor
you would like - it's the $$'s and not the %%'s that matter to a business
investing in a PV system.

Wouldn't a monthly / daily loss study / report be more useful for the
purposes of meeting the needs of those who actually have to design,
calculate production, sell, install, & warranty systems using your
equipment?  Remember, this is the very reason this question has been asked
for so long!  Who's left holding the bag when a system doesn't meet
production projections that were unwittingly based on "partial" information?
(Gotta read between all the lines to get the whole story from this
report....)  What if that report didn't use up paper trying to place $ value
on the electricity (and "present worth factor????) and just stuck to
effective %'s?  What if that report included an easy to read and interpret
chart or table for various Capacity Factors or, better yet, Slope /
Orientation / Sun-Hours / KW DC combinations?  I know that the report was
generated to show "Xantrex's good side", so it used best-case, but don't you
have a lot of existing and burgeoning markets elsewhere?

[response] Again, this is far beyond the scope of this report, which
addresses the cost of nighttime tare losses. The percentages are meaningless
to an end user business, it's capital cost and return on capital that
matters most(in $'s, not %'s). It is impossible to generalize on
percentages, because they depend on capacity factor, which is site specific.
You would do the PV industry a great favor if Universal Energies, as a PV
systems education concern, would take this inverter specific information and
integrate it into a life cycle cost model for the entire PV system that
takes into account all the other factors you mention. Xantrex can provide
you with details of all of the loss mechanisms associated with the
components it supplies for these applications.


You stated that your competitors are exaggerating the issue.  Apples to
apples, all cards on the table, this report understates the issue.

[response] The report neither understates nor overstates the issue. It
simply provides guidance on how to determine the present worth of these
losses once you have the relevant input data. You can use your own example
if you don't like the ones in the report.

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