inverter shoot-out at high noon [RE-wrenches]

[Bill Brooks]

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Graham,

The test of an inverter has some parallels to a vehicle, but I think it is
actually simpler in many ways. A PV module is probably a more complex thing
to characterize and model, and David King at Sandia and others have done a
pretty good job of nailing that down (with 500 coefficients). There are
basic operational issues that anyone designing a PV system needs to know
about a PV inverter. Those issues include:
1. the dc and ac voltage range of the inverter at full power
2. the temperature/power foldback characteristics of the inverter at
elevated temperatures
3. the power foldback at enhanced irradiance conditions
4. the efficiency curve of the inverter at several different dc and ac
voltages
5. the maximum power point tracking effectiveness on partially cloudy,
sunny, and cloudy days

Using one array or even a dozen sites to provide this characterization will
probably still leave some of these parameters untested. Much of this
information is now fairly well understood about PV-series Xantrex inverters
and SMA SunnyBoy inverters because we have a large experience base with
these inverters. The key now is how do we learn about all the other new
grid-connected inverters entering the market when it is unlikely that any
one of these inverters will enjoy the market share that Xantrex and SMA now
enjoy. The learning curve is far too long to sit back and wait for guiney
pig installers to run out there, try an  inverter, and then report back the
Wrenches list whether they like it or not. There has to be a more
straightforward method of getting to the correct answer. This protocol does
a nice job of gathering a lot of the data that is needed.

Does it solve all the information needs on inverters--of course not. And no
one thing ever will. But it is tremendously better than flying blind--which
anyone who tries a new inverter is absolutely doing. As I tell my students
all the time, if you are putting a new, unfielded inverter on a customer's
site, they better be understanding folks, and the site better be close to
your shop so you can babysit it. Otherwise, put it on your own shop or house
and live with it for six months before making any decisions on whether to
start installing it.

Bill.


-----Original Message-----
From: Graham Owen [mailto:graham at solarexpert.com]
Sent: Friday, March 26, 2004 5:13 PM
To: RE-wrenches at topica.com
Subject: RE: RE: inverter shoot-out at high noon [RE-wrenches]

Bill,

Seems to me each individual PV installation has so many site-specific
variables that it would be difficult to construct a perfect protocol for
judging and pre-judging PV systems and components for losses and
inefficiencies.  This testing needs to be done and it will be
challenging. The auto industry struggled with this issue decades ago
when establishing a benchmark for predicting vehicle MPG.  The auto
industry developed a standard using dynamometers.  Automobiles were
placed on a dyno, run the engine, spin the wheels and count the MPG.
Well, this did not take into account hills, aerodynamics and other
losses.  Customers were often displeased when actual MPG was
significantly lower than projected. Industry standard test tracks had to
be designed upon which automobiles ran through their paces.  The result
was sleeker more efficient vehicles from manufacturers who marketed the
increased MPG their vehicles could provide, when compared to
competition.  Optimum performance seems to be the direction the PV
industry is moving and I applaud and congratulate you for your efforts
in overseeing and helping drive this momentum. That said I believe a
number of utilities have been collecting and analyzing installed PV data
and I don’t completely understand why they are so loath to share it.
The solar industry is hungry for this data and it needs to be digested.
This data is our industries equivalent of a test track.  Hopefully,
eventually this utility collected data can be combined with data from
your testing protocols and will allow for the most accurate and
comprehensive means of simulating performance including new products
introduced to the PV market.  Thanks again.

Graham Owen
GO Solar Company

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