Again Thanks Bill and David;
Continued excellent information.
I just jumped on Solectria'a string sizing online tool, and what are
they using for the Vmpp coefficent? just a tiny bit higher than the Voc
coefficient. SMAs new design tool just lists the Voc coeff. I think
some manus might need to be following this thread as well, and tweak
their sizing tools appropriately.
Ray
On 12/1/2011 5:54 PM, Bill Brooks wrote:
Ray,
There is an explanation on the www.solarabcs.org/permitting
<http://www.solarabcs.org/permitting> webpage for the interactive
solar map. The 2% ASHRAE data represents 14 hours in a month---meaning
temperature will be at or above that value for 14 hours in a typical
month. This is likely to correspond to at least one 3-hour timeframe
which is considered "continuous" in the NEC. The Copper Development
Association has chosen this number for ambient temperature
calculations in the NEC. The NEC Handbook (not the NEC) uses this
number in their example problems.
The 0.4% ASHRAE date represents 3 hours in a month. It is much more
conservative number. I put it in the data to provide a context for the
2% number and it allows engineers to consider a more conservative number.
Of course, the mean extreme minimum drybulb temperature is also on
that website for 690.7 max system voltage calculations. Enjoy---its
free thanks to SolarABCs.
Bill.
*From:*[email protected]
[mailto:[email protected]] *On Behalf Of *Ray
Walters
*Sent:* Thursday, December 01, 2011 3:58 PM
*To:* RE-wrenches
*Subject:* Re: [RE-wrenches] calculating low string voltage
Fantastic Info everyone, I think I've got a full grasp on this topic
now, from using the correct coefficient to the correct weather data,
to the adders for installation type, and a nice link to the applicable
data without spending $180 for the ASHRAE book.
I'm guessing the 2% number is 2% of the time year round 24/7, not 2%
of the time the sun is shining, so losses could be higher than 2% of
production? Sorry to keep asking questions, but I know my class will
probably ask the same thing.
Ray
On 12/1/2011 2:57 PM, Gary Willett wrote:
RAY:
This "Solar Reference Guide Map" web site is also linked to on the
SolarABCs site and provides the ASHRAE 0.4% & 2% high temperatures, as
well as the PV module max temp based on distance above the roof (0.5,
3.5, and 12").
http://www.solarabcs.org/about/publications/reports/expedited-permit/map/index.html
Regards,
Gary Willett, PE
On 12/1/2011 14:56, Gary Willett wrote:
Ray:
Look at Appendix F in the Expedited Permit Process for PV Systems - A
Standardized Process for the Review of Small-Scale PV Systems Prepared
by Bill Brooks, P.E. Brooks Engineering
This document provides the ASHRE temperatures for a lot of locations
around the USA.
http://www.solarabcs.org/about/publications/reports/expedited-permit/pdfs/Expermitprocess.pdf
Regards,
Gary Willett, PE
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