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

Joel Davidson joeldavidson at earthlink.net
Fri Mar 26 19:35:13 PST 2004


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KC120 modules from yours truly.

----- Original Message ----- 
From: "Bill Brooks" <billb at endecon.com>
To: <RE-wrenches at topica.com>
Sent: Friday, March 26, 2004 6:27 PM
Subject: RE: RE: inverter shoot-out at high noon [RE-wrenches]


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**** Bounces like rubber! Shatters like ceramic! ****
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handfuls. It's the creativity unleashing, mood enhancing
desk toy!
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Chris and Joel,

I can't remember what modules Mr. Cutler had on his house, but a 5% increase
is a little hard to believe even with the worst mismatch in modules. I'm
sure some manufacturers are worse than others, but I would be careful about
throwing around those kinds of numbers without a little better
substantiation.

In the old days of high voltage grid connected systems, mismatch was a huge
issue because of the manufacturing tolerances were so bad. Now things are
much better. Mismatch is still an issue and current matching helps (Mr.
Cutler did not invent the concept--although he may claim to have discovered
a huge issue).

The point Chris was making is that other things can affect system
performance other than inverter operation and there is no doubt about that.
System installation, orientation, SHADING, wire losses, dirt, bad
modules--any one of these things can impact performance much more than the
efficiency of an inverter. But when all those things are perfect and the
system still doesn't work right--all eyes turn to the inverter as the
logical place to find the problem.

Bill.

-----Original Message-----
From: Joel Davidson [mailto:joeldavidson at earthlink.net]
Sent: Friday, March 26, 2004 6:02 PM
To: RE-wrenches at topica.com
Subject: Re: RE: inverter shoot-out at high noon [RE-wrenches]


===========================================================
**** Bounces like rubber! Shatters like ceramic! ****
Discover Crazy Aaron's Thinking Putty in grown up
handfuls. It's the creativity unleashing, mood enhancing
desk toy!
http://click.topica.com/caab4CEbz8QcsbaaVcJa/ Crazy Aaron Enterprises
===========================================================

Bill, Will the results of the side-by-side testing that you are doing for
the CEC be made public?

----- Original Message -----
From: "Bill Brooks" <billb at endecon.com>
To: <cfreitas at outbackpower.com>; <RE-wrenches at topica.com>
Sent: Friday, March 26, 2004 9:16 AM
Subject: RE: RE: inverter shoot-out at high noon [RE-wrenches]


===========================================================
**** Bounces like rubber! Shatters like ceramic! ****
Discover Crazy Aaron's Thinking Putty in grown up
handfuls. It's the creativity unleashing, mood enhancing
desk toy!
http://click.topica.com/caab4CEbz8Qcsbc0WJAa/ Crazy Aaron Enterprises
===========================================================

Chris,

Although I certainly support side-by-side testing--and am currently doing
just that on 12 systems for the CEC, the information that is gleaned from
this testing process is incredibly valuable. I would love to see both--let's
get everyone together and propose it to someone who can fund it. That being
said, I still want to challenge you to participate in the development and
refinement of the test procedure--you're viewpoint is very valuable.

I have learned things about the inverters we have tested so far that would
never have been learned from the manufacturer because they either 1.) didn't
know there was a problem; or, 2.) didn't think it mattered to a system
designer. We make huge assumptions about how an inverter is supposed to do
its job, and then are shocked when it doesn't meet our expectations. If
Outback has not performed this test on their inverters, I am sure there are
some things about your inverter's performance that you are unaware of.

Our industry has been starving for a metric to provide a baseline for the
performance of an inverter. Your message below supports that notion. The
inverter development efforts at Sandia are a totally separate issue, and I'm
not sure that is the best program either. But the need for a performance
test is unquestionable. Don't throw the baby out with the bathwater.

My hope is that all these new inverters, including Outback, will succeed as
high-quality, efficient new options for system designers and integrators. I,
for one, wouldn't put one on a customer's facility until I learned a lot
more about the unit. This test procedure is just one of several hoops that
any sane installer would force a prospective product to jump through before
being willing to install it at a customer's facility. I think every PV
installer in California that has been working for more than 3 years can
personally attest to that truth.

Bill.

-----Original Message-----
From: Christopher Freitas / cfreitas at outbackpower.com
[mailto:cfreitas at outbackpower.com]
Sent: Friday, March 26, 2004 7:13 AM
To: billb at endecon.com; RE-wrenches at topica.com
Subject: RE: RE: inverter shoot-out at high noon [RE-wrenches]



Gee - I actually find myself agreeing here with Marco...

Although there is nothing technically wrong about the Sandia/Endecon
test proceedure - I think there is a lot more value in real world
side-by-side comparison testing of similar (or even not so similar)
inverter systems.  OutBack, together with help from SMA, has
been completing just such a test in Grass Valley with an OutBack
GVFX and a SMA Sunny Boy inverters.

The bottom line for a grid-tie system is the number of KWH back
into the grid - how the inverters achieve that is not the point
- the end result (KWHs) is what matters.

I think each inverter manufacturer should be allowed to optimize
the test system's configuration - including PV array configuration
and total size - although it should be done with the same brand
of PV module.  Then the total KWH produced should be divided
by the PV array's size.

There are still some potential potholes even in this path - for
example, Henry Cutler (Solar Guppy guy / Suntie re-engineer)
found that by simply curve tracing each of his PV module individually
and then grouping the PV modules into series strings by their
amperage output levels (high with high / low with low etc.) he
was able to increase the KWH output of his suntie system by more
than 5%.

This type of mismatch loss would not be have been noticed if
just the entire PV array was curved traced for DC wattage - you
need to curve trace each module individually - and so is not
usually seen as a loss in the performance of a high voltage series
string inverter system where the PV modules are connected in
just one or two strings and cannot be re-arranged to eliminate
these types of losses. Since we usually talk about the inverter
efficiency only (DC watts into the inverter versus the AC watts
out) and not the actual system performance, these types of losses
are not typically accounted for. A similar issue can be said
for the wire losses in a low voltage type PV system when the
PV array is curve traced at the terminals of the inverter.


I also have concerns about having government agencies like Sandia
and NREL messing with what should be a competitive marketplace
- they have provided significant funding to some companies (include
a few I worked for previously) which can influence the market
in not-so-positive ways. I would rather see them provide more
support on a industry wide basis (like this type of performance
benchmarking) and not funding R&D of an individual manufacturer
which seems to me to be nothing more than them placing bets on
what they think the next big technical advance will be.

Christopher

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