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

wes kennedy hathasolar at yahoo.com
Tue Apr 20 08:29:09 PDT 2010


Hi Jim,

I like using one of the design emulators for just this reason. PVSYST and PVSOL.  You can model each scenario and see how the numbers crunch. You import historical weather data for the region to get as accurate as possible. 

You mention the silver painted roof, these emulators allow you to input albedo affects, too.

Both packages offer free trial periods, so you could run just this case for free.  If you like them then you spend the money for the license. 



-Wes Kennedy
NABCEP certified PV Installer

303-653-3073

--- On Mon, 4/19/10, Jim MacDonald <jmac at solaresystems.com> wrote:

From: Jim MacDonald <jmac at solaresystems.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, 5:40 PM




 
 






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 
    
  No
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