[Spam] RE: proper inverter [RE-wrenches]

[Bill Brooks]

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Bob-O,

I would contend that the only way to get the Fronius inverter to operate in
its "sweet spot" is to run the Voc up to the maximum voltage. I had a chat a
good while back with my friend, Brian Farhi, at Fronius on this very topic.
It was related to the fact that the Fronius inverters like to operated at
about 280 Vdc. The only way to get that inverter to consistently operate
near that range is to put as many modules in series as possible. At the time
the Fronius maximum voltage was 450 Vdc and there was only one module on the
market at the time that would operate that inverter at the proper voltage by
allowing 9, 72-cell modules in series. Mysteriously, they increased their
maximum voltage to 500 Vdc soon after that so that now most 72-cell modules
can be placed 9 in series and operate that inverter near the 280-Volt sweet
spot.

With all due respect to the folks at Fronius and other manufacturers, the
inverter manufacturer is not the best resource to be telling installers how
to design PV arrays. They have a conflict of interest to tell you that their
inverter can take the widest variety of PV modules in order to make the
sale. That is why you can look at a string sizing chart and the 5 sets of 5
series modules option is considered nearly numerically equivalent to 3 sets
of 8 modules in series (in fact 5x5 gives a slightly better rebate) when
these array configurations are far from equal. 

The module manufacturers should have string sizing charts telling you which
inverters best match their modules and what configurations they recommend
for various inverters. Of course, that will never happen in today's
supply-constrained market, so the inverter manufacturers are trying to fill
the gap created by the module manufacturers.

It is always better to put as many modules in series as the inverter will
allow for the following reasons:

1.) The voltage fluctuations of a PV array are always more than most people
predict.
2.) The voltage of a PV module starts out as much as 5-10% below rating at
max power due to inaccuracies in rating.
3.) The voltage of a PV module degrades over time by as much as 10-20% below
its initial rating over the life of the PV system.
4.) During a shading condition the voltage of a PV array can vary
dramatically so that the greater the available operating voltage range, the
more likely the inverter will be able to track the array during shading.

I would gladly give up a 1-2% loss in efficiency to be able to stay on the
maximum power point of the array. That is a no-brainer decision. However,
inverter manufacturers are so focused on hitting a single dc voltage number
(that is impossible to hit consistently anyway) to achieve some perfect
efficiency number, that they are missing the point of operating the PV array
as well as possible over the life of the system.

Many string sizing programs have become more conservative over the past year
or two, but I believe they need to become even more conservative on the low
voltage end if they want to be truly useful for longterm system performance.
A good rule of thumb is to not use the lowest one or two series string
options of any inverter string sizer. That's not always possible with an
inverter with a small voltage window, but it is clearly possible with a
Fronius. The good news is that if you load it up on voltage you will be
living near the "sweet spot" all summer long with plenty of voltage head
room for array degradation and shading.

Can you tell you pushed one of my buttons, Bob-O. I'm not passionate at all
about this stuff ;-).

Bill.


-----Original Message-----
From: Bob-O Schultze, Electron Connection [mailto:econnect at snowcrest.net] 
Sent: Wednesday, March 22, 2006 7:16 AM
To: RE-wrenches at topica.com
Subject: Re: proper inverter [RE-wrenches]

Bill,
while I certainly agree with you to stay away from the minimum input  
voltage, I also try to say away from the Max input voltage as well.  
That's got little to do with giving the inverter an over voltage  
condition. All inverters have a "sweet" spot range where they  
operated most efficiently. For anyone who has taken the Fronius  
installer training, for example, they explain that in detail. Other  
manus, not so much. A competent, conscientious installer will press a  
manu for that data and try to hit it as a service for his/her clients. Bob-O
On Mar 21, 2006, at 10:48 AM, Bill Brooks wrote:
>

> Darryl and William,
>
> As those of you know who have been to my courses, I strongly caution 
> installers when using the string sizing programs from inverter
> companies.

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