[RE-wrenches] DC to AC derate

toddcory at finestplanet.com toddcory at finestplanet.com
Thu Jan 5 16:20:21 PST 2012


to my knowledge the balancing transformer cannot be used with a grid tie.
 
are you using two outback kvc2 meters for recording the ac sell kWh? (each 120 volt leg needs one.)
 
todd
 
 
 
 
On Thursday, January 5, 2012 1:33pm, "Ray Walters" <ray at solarray.com> said:


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] 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: [mailto:re-wrenches-bounces at lists.re-wrenches.org] re-wrenches-bounces at lists.re-wrenches.org [[mailto:re-wrenches-bounces at lists.re-wrenches.org] mailto:re-wrenches-bounces at lists.re-wrenches.org] On             Behalf Of [mailto:boB at midnitesolar.com] 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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