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<FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>Here’s a link to an Excel version of our c-Si PV module specifications table:<BR>
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<a href="https://solarprofessional.com/webexclusive/">https://solarprofessional.com/webexclusive/</a><BR>
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It includes coefficients for Voc, Pmp and Isc. For Vmp calculations, use the coefficient of Pmp, which is primarily voltage driven. (Temperature coefficients of current are VERY small, as seem in the Isc values.)<BR>
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David Brearley, Senior Technical Editor<BR>
<I>SolarPro</I> magazine <BR>
NABCEP Certified PV Installer ™<BR>
<a href="david.brearley@solarprofessional.com">david.brearley@solarprofessional.com</a><BR>
Direct: 541.261.6545<BR>
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On 11/30/11 12:18 AM, "Ray Walters" <<a href="ray@solarray.com">ray@solarray.com</a>> wrote:<BR>
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</SPAN></FONT><BLOCKQUOTE><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'> 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>
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Now I have to correct that coefficient for my next class......but that's why I asked for everybody to look it over.<BR>
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Thanks Everyone,<BR>
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Ray<BR>
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On 11/29/2011 8:08 PM, Doug Wells wrote: <BR>
</SPAN></FONT><BLOCKQUOTE><FONT FACE="Calibri, Verdana, Helvetica, Arial"><SPAN STYLE='font-size:11pt'>Ray, <BR>
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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. <BR>
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</SPAN><FONT SIZE="1"><SPAN STYLE='font-size:9pt'>Doug Wells<BR>
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</SPAN><FONT SIZE="1"><SPAN STYLE='font-size:9pt'>www.thesolarspecialists.com <<a href="http://www.thesolarspecialists.com">http://www.thesolarspecialists.com</a>> <BR>
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On Nov 29, 2011, at 5:55 PM, Ray Walters wrote:<BR>
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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>
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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>
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The Aug/ Sept 2010 edition of Solar Pro shows the coefficients for numerous modules, <BR>
Solar world for example is -0.33 %/degC<BR>
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With a 40 degC differential then, the voltage would change by 40 x .33, or 13.2%<BR>
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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>
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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>
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Ray<BR>
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On 11/29/2011 2:07 PM, Darryl Thayer wrote: <BR>
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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. <BR>
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Best I can do from memory.<BR>
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DT<BR>
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</SPAN></FONT><SPAN STYLE='font-size:12pt'><FONT FACE="Times New Roman"> </FONT></SPAN><FONT FACE="Arial"><SPAN STYLE='font-size:11pt'> <B>From:</B></SPAN><SPAN STYLE='font-size:12pt'> Kirk Herander <<a href="kirk@vtsolar.com">kirk@vtsolar.com</a>> <<a href="mailto:kirk@vtsolar.com">mailto:kirk@vtsolar.com</a>> <BR>
<B>To:</B> RE-wrenches <<a href="re-wrenches@lists.re-wrenches.org">re-wrenches@lists.re-wrenches.org</a>> <<a href="mailto:re-wrenches@lists.re-wrenches.org">mailto:re-wrenches@lists.re-wrenches.org</a>> <BR>
<B>Sent:</B> Tuesday, November 29, 2011 1:48 PM<BR>
<B>Subject:</B> [RE-wrenches] calculating low string voltage<BR>
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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).<BR>
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Kirk Herander<BR>
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VT Solar, LLC<BR>
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dba Vermont Solar Engineering<BR>
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NABCEPTM Certified installer Charter Member<BR>
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VT RE Incentive Program Partner<BR>
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