[RE-wrenches] overfeed 7kW gridtie inverter or drop a mod per string?

[David Brearley]

In the same issue of SolarPro as the “Optimal PV-to-Inverter” Sizing Ratio”
article (April/May 2010), the lead article is “Production Modeling for
Grid-Tied PV Systems” by Tarn Yates and Bradley Hibberd at Borrego Solar:

http://solarprofessional.com/article/?file=SP3_3_pg30_Yates

While your question below can’t be answered using PVWatts, it can be
answered using the Solar Advisor Model (SAM), which was developed by NREL
and Sandia w/ DOE funding. SAM is available for free download here:

www.nrel.gov/analysis/sam/

David Brearley, Senior Technical Editor
SolarPro magazine 
NABCEP Certified PV Installer ™
david.brearley at solarprofessional.com
Direct: 541.261.6545


On 4/19/10 6:40 PM, "Jim MacDonald" <jmac at solaresystems.com> wrote:

> SB8000US only good for 480 Vac, we’re tying in at 208.
>  
> 11 strings has been calc’d to be OK (<600vdc) for record low -4 fahrenheit
> (-20 celsius).
>  
> I think the big concern as mentioned is getting the Voc high early in the AM
> to wake the inverter up, also maintaining the Vmp later in the day to keep it
> running for as long as possible into the late afternoon/ evening.
> Will those extra kWh gleaned in the early AM/ late PM over the course of 365
> days be more than the kWh clipped during peak production times with the 9.075
> kWdc?
> NYC average summer highs between 79 and 84 fahrenheit early june-late august.?
> Sunlink rack @ 10° on a silver-painted built-up roof.
>  
> Time-of-use utility $ peak-shaving is not an issue here.
>  
> Thank you
>  
>  
>  
>  
> 
> From: re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Darryl Thayer
> Sent: Monday, April 19, 2010 5:58 PM
> To: RE-wrenches
> Subject: Re: [RE-wrenches] overfeed 7kW gridtie inverter or drop a mod per
> string?
>  
>  
>    The temperature correction is the concern.  I have heard lots of stories of
> inverters dieing when it gets to cold.  Look in the code book 690.7 at the
> temperature correction table.  Go to weather.com and find your record low
> temperature.  Find you Tc factor and will your senerios work?  I have heard
> from a sunny boy rep that it is OK to over power by 1.25%
>   darryl
>  
>  --- On Mon, 4/19/10, Dana <dana at solarwork.com> wrote:
>  
>>  
>>  From: Dana <dana at solarwork.com>
>>  Subject: Re: [RE-wrenches] overfeed 7kW gridtie inverter or drop a mod per
>> string?
>>  To: "'RE-wrenches'" <re-wrenches at lists.re-wrenches.org>
>>  Date: Monday, April 19, 2010, 4:31 PM   Many times we switch to another
>> module manufacturer as one will not fit or another inverter manufacturer.
>>  PVPowered has a more relaxed window and we have had fine success with them
>> when SMA does not fit.
>>  Cold is the dictator and the 600 VDC threshold.
>       Thanks,  Dana Orzel
>    Great Solar Works, Inc web - www.solarwork.com E - dana at solarwork.com V -
> 970.626.5253 F - 970.626.4140 C - 970.209.4076   "We have been slow to give up
> on the myth of the Earth's generosity. Rather grandly, we have overdrawn our
> accounts." Barbara Kingsolver - Water is life.     From:
> re-wrenches-bounces at lists.re-wrenches.org
> [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Jim MacDonald
>  Sent: Monday, April 19, 2010 2:37 PM
>  To: RE-wrenches at lists.re-wrenches.org
>  Subject: [RE-wrenches] overfeed 7kW gridtie inverter or drop a mod per
> string? 
>  
>    For a straight grid-tie system@ 10 degree tilt, 180 azimuth, zero/little
> shade, NYC avg 4.29 kWh/m²/day.   Feed an SMA7000 with (3) strings of 11
> Suntech STP275’s [9.075 kWdc] Or Do strings of 10, so itd be 8.250 kWdc into
> the 7 kWac inverter.   Strings of 11= 491.7 Voc; 386 Vmp Strings of 10= 447
> Voc; 351 Vmp    The tradeoff would be power clipping at/around noon in the
> sunny months in the 9.075 kW scenario; Inverter sleeping late every morning
> and retiring early in the evenings in the lowered voltage scenario.   Trying
> to figure out which would yield more kWh at the end of the year.   Any ideas
> appreciated, thanks         Jim MacDonald Solar Energy Systems, LLC Brooklyn,
> NY 11222 www.solarEsystems.com <http://www.solaresystems.com/>    No virus
> found in this incoming message.
>  Checked by AVG - www.avg.com
>  Version: 9.0.801 / Virus Database: 271.1.1/2819 - Release Date: 04/19/10
> 00:31:00 
> 






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