[RE-wrenches] DC to AC derate

[Ray Walters]

I have a related question to this. Does setting up a pair of Outback 
GVFXs with the balancing transformer and allowing one inverter to go to 
sleep at lower AC output improve efficiency?  It seems we would be 
reducing electronic loads, but adding transformer losses.   Any 
comments? I have a recently commissioned double Outback system with 200 
AH @48v battery bank.  He's reporting efficiency at 77% DC in to AC 
out.  I'd love to improve that, as his aging array of Sharp modules 
seems to be under performing as well, (4.08 KW STC rated,  3 Kw actual 
out)  I haven't gone back and done testing with my insolation meter, so 
I'm not sure exactly what is happening, but his overall efficiency (STC 
rated to AC out) is only 56% .

Ray

On 1/4/2012 7:55 PM, Maverick Brown [Maverick Solar] wrote:
> Please use the 90% of the CEC or PTC rating of the PV module at most.
> Among several systems that I monitor daily, I have an example 11.760kw 
> (STC) system that has a 500Ah battery bank and it peaks at 77-80% of 
> the STC rating each day.
> Peak is defined as the top of  the bell curve that forms from the 
> graphing watts output versus nameplate watts.
> If I use the CEC rating (10.54kw), it peaks at 86-90% of the CEC 
> rating each day.
> If I go to the CEC list and copy & paste into a spreadsheet and enter 
> the values for the STC versus PTC, I /might/  find I would loose 10% 
> (or more) in the PV rating.
> http://www.gosolarcalifornia.org/equipment/pv_modules.php (go ahead, 
> find your modules and divide PTC by Nameplate).
> I also have a grid-interactive system that has a very large battery 
> bank (2600Ah @ 48V, with 9kw STC Array) and it only has a peak 
> efficiency rating of 69% because of the load of the batteries. The 
> larger the battery bank, the happier customers are for outage 
> situations, but the larger the battery bank, the more current goes 
> into "Floating" the batteries during selling. For that system, I may 
> lower "Grid Support/SellRE" and use Auto EQ to keep the batteries fresh...
> Anyway, keep in mind that you cannot simply assume:
> 12kw Array x 6 hours of sun =  72kwh of generation each day. The brand 
> of PV, the size of the battery bank and even the charge controller's 
> Absorb & Float settings can greatly reduce what the inverter thinks it 
> can sell. With multiple inverters and charge controllers it gets even 
> more complicated (worse).
> Good luck,
> Maverick
>
> ------------------------------------------------------------------------
> *From:* re-wrenches-bounces at lists.re-wrenches.org 
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] *On Behalf Of 
> *boB at midnitesolar.com
> *Sent:* Wednesday, January 04, 2012 5:14 PM
> *To:* RE-wrenches
> *Subject:* Re: [RE-wrenches] DC to AC derate
>
> On 1/4/2012 3:01 PM, All Solar, Inc. wrote:
>> Wrenches,
>> Once again I am trying to size a grid tie w/battery system and I 
>> struggle a little with the efficiencies of them.
>> With PV Watts (v.1), is changing the DC to AC derate an accurate 
>> method for getting the output?
>> Thanks in advance!
>> Jeremy
>
>
> If you keep the batteries charged  to a voltage just above their 
> resting voltage,
> say, 52 volts for a 48V system, and the battery bank isn't too large, 
> (~225 A-hours,
> maybe a bit more), about 90%  PV to grid AC efficiency is in the ball 
> park.
>
> That's what we figured in the early days of GFX inverters at OB.  
> That's giving
> 5% for the electronics and 5% for keeping up the batteries.
> Different and/or more accurate information may have come along since
> that time.
>
> boB
>
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