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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<br>
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?<br>
<br>
Now I have to correct that coefficient for my next class......but
that's why I asked for everybody to look it over.<br>
<br>
Thanks Everyone,<br>
<br>
Ray<br>
<br>
<br>
On 11/29/2011 8:08 PM, Doug Wells wrote:
<blockquote
cite="mid:EA73FC62-B72A-4D85-855B-358EFF82C279@thesolarspecialists.com"
type="cite">Ray,
<div><br>
</div>
<div>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. </div>
<div><br>
<div>
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<div>
<div style="font-size: 12px; ">Doug Wells</div>
<div style="font-size: 12px; ">The Solar Specialists</div>
<div style="font-size: 12px; ">Morrisville, VT 05661</div>
<div style="font-size: 12px; ">(p) 802-223-7014</div>
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<br>
<div>
<div>On Nov 29, 2011, at 5:55 PM, Ray Walters wrote:</div>
<br class="Apple-interchange-newline">
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http-equiv="Content-Type">
<div bgcolor="#FFFFFF" text="#000000"> I'm prepping for a
class on this very subject tonight, so I'll just wrap up
everybody's good points:<br>
V min = Vmpp x (1- temp diff x Temp coefficient)<br>
<br>
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<br>
then, 65C - 25C(STC) = a differential of 40 deg C from
standard test conditions<br>
<br>
The Aug/ Sept 2010 edition of Solar Pro shows the
coefficients for numerous modules, <br>
Solar world for example is -0.33 %/degC<br>
<br>
With a 40 degC differential then, the voltage would change
by 40 x .33, or 13.2%<br>
<br>
So just assuming Vmpp = 35v, the adjustment would be 35v x
(100%-13.2%), or 86.8% of 35v, <br>
which gives a Volt min of 30.4 v<br>
<br>
Somebody please correct my math if I got that wrong.<br>
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.<br>
<br>
Ray<br>
<br>
On 11/29/2011 2:07 PM, Darryl Thayer wrote:
<blockquote
cite="mid:1322600875.70711.YahooMailNeo@web39313.mail.mud.yahoo.com"
type="cite">
<div style="color:#000; background-color:#fff;
font-family:arial, helvetica, sans-serif;font-size:12pt">
<div><span>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. </span></div>
<div><span></span> </div>
<div><span>Best I can do from memory.</span></div>
<div><span>DT</span></div>
<div><br>
</div>
<div style="font-family: arial, helvetica, sans-serif;
font-size: 12pt;">
<div style="font-family: times new roman, new york,
times, serif; font-size: 12pt;"> <font face="Arial"
size="2"> <b><span style="font-weight: bold;">From:</span></b>
Kirk Herander <a moz-do-not-send="true"
class="moz-txt-link-rfc2396E"
href="mailto:kirk@vtsolar.com"><kirk@vtsolar.com></a><br>
<b><span style="font-weight: bold;">To:</span></b>
RE-wrenches <a moz-do-not-send="true"
class="moz-txt-link-rfc2396E"
href="mailto:re-wrenches@lists.re-wrenches.org"><re-wrenches@lists.re-wrenches.org></a>
<br>
<b><span style="font-weight: bold;">Sent:</span></b>
Tuesday, November 29, 2011 1:48 PM<br>
<b><span style="font-weight: bold;">Subject:</span></b>
[RE-wrenches] calculating low string voltage<br>
</font> <br>
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<div class="yiv1276106914WordSection1">
<div class="yiv1276106914MsoNormal">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).</div>
<div class="yiv1276106914MsoNormal"> </div>
<div class="yiv1276106914MsoNormal">Kirk
Herander</div>
<div class="yiv1276106914MsoNormal">VT Solar,
LLC</div>
<div class="yiv1276106914MsoNormal">dba
Vermont Solar Engineering</div>
<div class="yiv1276106914MsoNormal">NABCEP<sup>TM
</sup>Certified installer Charter Member</div>
<div class="yiv1276106914MsoNormal">NYSERDA-eligible
Installer</div>
<div class="yiv1276106914MsoNormal">VT RE
Incentive Program Partner</div>
<div class="yiv1276106914MsoNormal"> </div>
</div>
</div>
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