[RE-wrenches] DC to AC derate

[Kent Osterberg]

Ray,

The balancing transformer isn't allowed with the GVFX inverters. In a 
dual FX inverter system, using the balancing transformer or not using it 
is nearly a wash. I think you save 15 watts when the inverter is asleep 
but the transformer losses are 10 or 12 watts.

Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com
t: 541-568-4882


On 1/5/2012 1:33 PM, Ray Walters wrote:
> 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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