I'm prepping for a class on this very subject tonight, so I'll just wrap
up everybody's good points:
V min = Vmpp x (1- temp diff x Temp coefficient)
So first, I'd take your ambient temp from ASHRAE Extreme Annual Mean
Maximum, then add Scott's adder (ie roof mount off 6" = +30C) so let's
say the ASHRAE # is 35C, then you have 30 + 35 = 65C cell temp
then, 65C - 25C(STC) = a differential of 40 deg C from standard test
conditions
The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for
numerous modules,
Solar world for example is -0.33 %/degC
With a 40 degC differential then, the voltage would change by 40 x .33,
or 13.2%
So just assuming Vmpp = 35v, the adjustment would be 35v x
(100%-13.2%), or 86.8% of 35v,
which gives a Volt min of 30.4 v
Somebody please correct my math if I got that wrong.
Also probably more interesting is the actual Power coefficient, which
shows the total power losses from higher temperatures, not just voltage.
The calculation would be similar.
Ray
On 11/29/2011 2:07 PM, Darryl Thayer wrote:
I am on road so I have no texts with me, but the same formula applies
about 0.4% per degree (less voltage) or use the module label voltage
coefficent. However you must use the MPPT voltage and the temperature
= (temperature ambient (2% high) plus roof temperature warming) The
roof temperature is hard to find good values because it depends upon
spacing between roof and module. I add 15 deg C for most situations.
Best I can do from memory.
DT
*From:* Kirk Herander <k...@vtsolar.com>
*To:* RE-wrenches <re-wrenches@lists.re-wrenches.org>
*Sent:* Tuesday, November 29, 2011 1:48 PM
*Subject:* [RE-wrenches] calculating low string voltage
I am in debate with a PE over calculation of low voltage of a series
string on a hot day. He insists that an arbitrary high cell temp is
factored in, not just ambient temperature. Could someone please give
an accepted formula for this calculation? Thanks. I cannot find a
clear reference to low voltage calculation on a hot day (but every
reference material is clear on how to calculate high voltage on a cold
day).
Kirk Herander
VT Solar, LLC
dba Vermont Solar Engineering
NABCEP^TM Certified installer Charter Member
NYSERDA-eligible Installer
VT RE Incentive Program Partner
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
<mailto:RE-wrenches@lists.re-wrenches.org>
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive:
http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
Options& settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules& etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
_______________________________________________
List sponsored by Home Power magazine
List Address: RE-wrenches@lists.re-wrenches.org
Options & settings:
http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org
List-Archive: http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org
List rules & etiquette:
www.re-wrenches.org/etiquette.htm
Check out participant bios:
www.members.re-wrenches.org
