[RE-wrenches] Micro-Inverter Vs Central Inverters $150 DesignChallenge

Thu Sep 2 11:07:34 PDT 2010

I don’t have the time either, but on the other hand I suspect that Ryan
isn’t making an honest offer if he is the sole judge “that I can't redesign
using something else [or] with equal or better cost, [or] time or
performance results”. 

As for microinverteres, I am an early adopter so I have to moderate my
impulses with reason. 

As an aside, does the new two-module microinverter cost the same $/W as a
central inverter?

Back to Ryan. We have used the microinverter twice and in both cases the PV
modules consisted of two physically parallel strings on an E-W axis, and the
reference design had one inverter.

In my “winning submission”, there was a big pine tree in the front yard (to
the East) and a neighbor’s tree to the West. What made the microinverter
approach a superior one was the fact that the retreating shadow in the AM
and the advancing shadow in the PM had an effect on only those modules that
were shaded, but a string inverter would have been begun to lose efficiency
and then shut down completely over significant (energy wise) portions of the
day. Interestingly, this case does not really have a strong seasonal factor;
it’s an AM/PM thing (without the burrito).

The client is experiencing about 20% greater energy production from his
system compared with a PVWatts simulation using actual shading numbers from
our trusty SunEye – and has now for about a year.

He is happy as a clam as far as the design and performance goes, but
believes his utility (LADWP) should give him a rebate based on performance
and stop charging him an obscene amount to be a "net metering customer"
(more on that later on the Marketing Forum).

Purchase price was a tad higher, the design time was a tad longer (but not
any more because we have designed two already), but the performance was and
the RoI will be superior.

Ryan, make sure you spell my name with “two r’s” on the check.

- Peter 

Peter T. Parrish, Ph.D., President
California Solar Engineering, Inc.
820 Cynthia Ave., Los Angeles, CA 90065
CA Lic. 854779, NABCEP Cert. 031806-26
peter.parrish at calsolareng.com  
Ph 323-258-8883, Mobile 323-839-6108, Fax 323-258-8885

________________________________________
From: Ryan LeBlanc <ryan at naturalenergyworks.com>
To: re-wrenches at lists.re-wrenches.org
Sent: Tue, August 31, 2010 9:47:34 PM
Subject: [RE-wrenches] Micro-Inverter Vs Central Inverters $150 Design
Challenge

Ok just got off the phone with a former student, now a designer at a
significant module manufacturer and I've almost hit my limit discussing
this, which now I'll make worse I'm sure.  

... I can't believe that Enphase has received as much attention as they
have. 

In my humble opinion, I'm asked way too much about this relatively
insignificant industry product. I wish I never discussed the ups and downs
of micro-inverters with Raghu and Martin before Enphase was Enphase. 

Too many downsides to list, there are few niche applications that they can
work well within. Small residential is absolutely as far as it goes. Large
scale, no way. Their marketing department deserves the MVP for the company,
and the psychology of regular people and mostly solar newbie's today, plays
right into the "New & Shiny" toy thing.  We all know conductor runs benefit
from higher voltages, at lower voltages for the same power, systems require
larger more expensive conductors, overcurrent protection devices, Yadda
Yadda Yadda.  

They are not less expensive.
They are not easier to design with.  
They are not more reliable.  
The proof will surely come out.

Don't be afraid of string inverters, you can do very short strings, and
don't choose to go with the bite size, but still un-satisfying experience of
working with them.

OK, yes, it may be true that, central inverters are no more reliable than
micro-inverters, by the same logic micro-inverters are no more reliable than
central inverters. There will always be a small percentage of bad units, a
known reality of electronics. 

So I ask, as you are installing these inverters, do you know if they are
good until you have installed the modules, plugged them in and run them? I
know I don't, in which case I don't know if I'm burying 1,2,5,10,20, etc of
bad units under the modules...  in which case, you have to remove the
modules, often involving a lot of labor on the roof, requiring getting the
harnesses back on, setting up the ladder, replacing the "one-time-only"
WEEBS (if you use them) or undoing the equipment ground, cutting and
re-bundling wires and zip-ties, etc, etc. 

The reason they won't be able to get much cheaper will be because of quality
control and testing each one extensively before leaving the factory, and
support, service and warranty response teams will be busy, busy. 

If there's a project bigger than one string of 3 typical cSi modules,
shortest length I know the Sunny Boy 700U can work on, and you believe
Enphase is required due to shading or whatever, then A.) You probably
shouldn't be installing PV there, or B) You may need to learn about the
existing options and brush up on some other design techniques. 

A good question recently asked about performance is, "Are the claims of
shading performance gains that Enphase provides only good for panels that
don't have any bi-pass diodes or just one diode?" - I'm not sure and haven't
looked at their literature in a while, nor do I care to, so I would
appreciate an education on this one.

All I know is that customers don't like to see me back too many times, and I
know I've had to replace the same inverter a few times... with micro, it's
multiplied. 

The beauty of central is that it doesn't take much, if any work on the roof,
to trade out a Xantrex for an SMA, Fronius, or whoever, but say you get to a
point where you do want out of the Enphase family... you'll have to
reinstall every single module...  I'm nervous for the cool-aid drinkers.

Oh, and as for the future, ACPV... none for me thanks either... not for a
professional, for similar reasons mentioned above, and not for homeowners.
We really don't need a homeowner installing their own ELECTRICAL systems.
This can only fuel the counter argument to solar with a likely exacerbated
blanket statement of solar being a dangerous fire-hazard on rooftops.

Here's a challenge, I will pay someone $150 dollars (same as Enphase
inverter replacement reimbursement) to show me a project designed using
them, greater than three modules, that I can't redesign using something
else, with equal or better cost, time or performance results. Seriously,
send me an old or new Enphase design, I'll look it over, and propose an
alternative. If a reasonable person won't agree in that alternative, I will
send a check.

Ryan J. LeBlanc
NABCEPT Certified Solar PV Installer
Cell: 707.591.1950
Direct: 707.536.9839
ryan at NaturalEnergyWorks.com
http://www.NaturalEnergyWorks.com

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