I pulled the voltage temp coefficient off of an article from Solar Pro
Magazine. It is indeed listed for Voc. I have never seen a separate
coefficient for Vmpp. Maybe we should use the Power mpp coefficient and
then factor the slight rise in current by temp (which would make Vmpp
even worse). My Solar world example shows a Pmpp coeff. of -0.47, while
the current coeff. (short circuit though) is positive 0.04
so correcting just for voltage, that would be at or slightly worse than
Bill's suggested -0.5 coefficient for Vmpp. Which would pull the Vmpp
from 35 to 28 in my example, not 30.4 v. The degradation and mismatch
Bill points out make sense, but is it really another 15% combined?
Would it be fair to say different modules and manufacturers would
degrade at different rates? Finally, the ambient temp I picked was the
opposite of the NEC suggested ASHRAE low temp. Is that really the best
number for calculating this? Or is that too extreme a temperature, that
wouldn't occur most of the time?
Now I have to correct that coefficient for my next class......but that's
why I asked for everybody to look it over.
Thanks Everyone,
Ray
On 11/29/2011 8:08 PM, Doug Wells wrote:
Ray,
I agree with what you have calculated, but the temp. coef. listed is
for a Voc. For voltage sag, the temp. value would be a Vmp value and
I don't know how much of a difference that makes. I have seen some
manufacturers offer a temp. coefficient for Vmp as well as Voc. Not
sure what the difference would be, but if it gets down to a few volts
with the PE, it is would be worth checking.
Doug Wells
The Solar Specialists
Morrisville, VT 05661
(p) 802-223-7014
(c) 802-498-5856
www.thesolarspecialists.com <http://www.thesolarspecialists.com>
On Nov 29, 2011, at 5:55 PM, Ray Walters wrote:
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
<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
