[RE-wrenches] array overloading of an inverter

[Kirk Herander]

Of the many Enphase systems I've done, clipping is a regular issue in
wintertime Vermont, especially with the older micros whose output wattage
ran way below the max. panel wattage. I've seen clipping for 3 hours or more
on super-cold,  crystal clear days, with customer complaints about not being
able to harness the peak . The DC:AC ratio of greater than 1 may also not be
advantageous when using module based electronics, ala Solaredge, since the
claim of 5 -7 % greater dc to ac conversion over a string inverter appears
to hold true in my experience. You may be already getting the gains which
overloading will accomplish, making overloading unfeasible economically. 

-----Original Message-----
From: RE-wrenches [mailto:re-wrenches-bounces at lists.re-wrenches.org] On
Behalf Of Matt Partymiller
Sent: Wednesday, July 13, 2016 4:16 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] array overloading of an inverter

Kirk,

I would recommend you consider using a tool like PVsyst or Helioscope to
model DC:AC ratios.  A quick modeling of roughly 200 kW on three Solectria
50 kW inverters shows about 1.3% clipping here.

It then comes down to analysis of your costs for additional modules, dc
wiring, and inverters.

Admittedly I sit on the opposite side of the fence.  I try to provide at
least a 1.2 ratio to minimize cost per kWh.  I am not aware of any large
scale studies comparing PVsyst or Helioscope clipping projections to real
world performance.  I can tell you that our arrays with monitoring and a
~1.2 DC:AC ratio perform as well as our older 1:1 systems.  Not that I use
Enphase often but they have some research/whitepapers on the topic since
their micros tend to undershoot 60 cell module power ratings.

Good luck!


Matt

Matthew Partymiller
Solar Energy Solutions LLC
(877) 312-7456
matt at solar-energy-solutions.com


On Wed, July 13, 2016 3:02 pm, Kirk Herander wrote:
> Hello folks,
>
>
> I am composing a response to a question a potential customer asked me. 
> It  seems a competitor is trying to talk him into a 20% larger KW 
> array than the inverter AC max output rating is. The idea of course is 
> to generate more power on either side of peak output, but at a cost I 
> don't feel is justified. My opinion of this particular big-box 
> installer I will keep to  myself. My response to the customer, trying to
keep it simple:
>
>
>
> "On the DC array input side, most inverters do allow an overload factor.
> For
> instance, a 10kw AC inverter may allow for 12 kw of DC array as an input.
> Whether or not this is a good idea boils down to economics and 
> technical reasons.
>
> On a sunny day, the inverter generates power as a typical bell curve.
> Power
> output rises in the morning, peaks at noon, declines in the afternoon. 
> In my example, the inverter can't output more than 10 kw AC. What 
> overloading the input will do will widen the bell curve, i.e. 
> generating more power in the morning and afternoon, BUT clipping the 
> peak at 10kw on either side of noon. So there is power to be gained in 
> morning and afternoon, but peak power is lost(if conditions allow the 
> peak output to be reached), since the 12 kw array can never be converted
to more than 10 kw of AC power.
> Depending upon
> time of year(ambient / cell temperature) and weather conditions, the 
> peak may be clipped at 10 kw for several hours a day. So you are both 
> gaining and losing power using this method. And typically the 
> inverters are only overloaded in this manner on large-scale farms 
> where the economics are favorable.
>
> In your case, if you could actually put 200kw of DC array into 150 Kw 
> of inverter, the economics would never justify it. That extra 50 kw of 
> array would cost you $100k of more, and the dollar payback for the 
> power that extra 50Kw would generate will take 2 - 3x the time that 
> the array size does that stays inside the output limit of the 
> inverters. This is why I'm not a big fan of dramatically overloading 
> the inverters, if at all,  in your case. Any KW portion of the array 
> which is above the nameplate kw rating of the inverter is going to 
> have a longer payback for these reasons. "
>
> I feel my reasoning is sound, But I don't want to be too loose with 
> the facts. Comments are appreciated. Thanks.
>
>
>
> Kirk Herander
>
>
> Owner|Principal, VT Solar, LLC
>
>
> Celebrating our 25th Anniversary 1991-2016
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