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</o:shapelayout></xml><![endif]--></head><body bgcolor=white lang=EN-US link=blue vlink=purple><div class=WordSection1><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Folks,<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>One factor often overlooked when calculating minimum system voltage is degradation of the array and rating variations of the modules.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>If you assume 0.5% loss in voltage per year, you need to factor a 10% higher voltage at year one.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Manufacturers tolerance on voltage and current can be 5-10% (rare in real life). It is healthy to add another 5% for voltage tolerance. Voltage tolerance can be much larger than power tolerance that may only be +/- 3%.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>The voltage tolerance numbers used by Ray are for Voc not Vmp. Vmp numbers are closer to 0.45% to 0.5%. This why the power loss due to temperature is about 0.5%/C—all of it is in voltage, not current. Therefore, for a 40C rise in temperature, there will be a 20% loss in voltage. Factoring another 15% loss for degradation and module tolerance, the total loss in voltage would be:<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Vmin = 35V x 0.8 x 0.9 x 0.95 = 23.94V => 24 Volts, not 30 Volts. It is always lower than you think unless you are Eeyore. This is one reason why the most common string sizing mistake is not enough modules in series.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'>Bill.<o:p></o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D'><o:p> </o:p></span></p><div><div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in'><p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext'>From:</span></b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif";color:windowtext'> re-wrenches-bounces@lists.re-wrenches.org [mailto:re-wrenches-bounces@lists.re-wrenches.org] <b>On Behalf Of </b>Ray Walters<br><b>Sent:</b> Tuesday, November 29, 2011 2:55 PM<br><b>To:</b> RE-wrenches<br><b>Subject:</b> Re: [RE-wrenches] calculating low string voltage<o:p></o:p></span></p></div></div><p class=MsoNormal><o:p> </o:p></p><p class=MsoNormal>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: <o:p></o:p></p><div><div><p class=MsoNormal style='background:white'><span style='font-family:"Arial","sans-serif"'>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. <o:p></o:p></span></p></div><div><p class=MsoNormal style='background:white'><span style='font-family:"Arial","sans-serif"'> <o:p></o:p></span></p></div><div><p class=MsoNormal style='background:white'><span style='font-family:"Arial","sans-serif"'>Best I can do from memory.<o:p></o:p></span></p></div><div><p class=MsoNormal style='background:white'><span style='font-family:"Arial","sans-serif"'>DT<o:p></o:p></span></p></div><div><p class=MsoNormal style='background:white'><span style='font-family:"Arial","sans-serif"'><o:p> </o:p></span></p></div><div><div><p class=MsoNormal style='margin-bottom:12.0pt;background:white'><b><span style='font-size:10.0pt;font-family:"Arial","sans-serif"'>From:</span></b><span style='font-size:10.0pt;font-family:"Arial","sans-serif"'> Kirk Herander <a href="mailto:kirk@vtsolar.com"><kirk@vtsolar.com></a><br><b>To:</b> RE-wrenches <a href="mailto:re-wrenches@lists.re-wrenches.org"><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</span><o:p></o:p></p><div id=yiv1276106914><div><div><div><p class=MsoNormal style='background:white'>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).<o:p></o:p></p></div><div><p class=MsoNormal style='background:white'> <o:p></o:p></p></div><div><p class=MsoNormal style='background:white'>Kirk Herander<o:p></o:p></p></div><div><p class=MsoNormal style='background:white'>VT Solar, LLC<o:p></o:p></p></div><div><p class=MsoNormal style='background:white'>dba Vermont Solar Engineering<o:p></o:p></p></div><div><p class=MsoNormal style='background:white'>NABCEP<sup>TM </sup>Certified installer Charter Member<o:p></o:p></p></div><div><p class=MsoNormal style='background:white'>NYSERDA-eligible Installer<o:p></o:p></p></div><div><p class=MsoNormal style='background:white'>VT RE Incentive Program Partner<o:p></o:p></p></div><div><p class=MsoNormal style='background:white'> <o:p></o:p></p></div></div></div></div><p class=MsoNormal style='margin-bottom:12.0pt;background:white'><br>_______________________________________________<br>List sponsored by Home Power magazine<br><br>List Address: <a href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a><br><br>Options & settings:<br><a href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org" target="_blank">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a><br><br>List-Archive: <a href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org" target="_blank">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a><br><br>List rules & etiquette:<br><a href="http://www.re-wrenches.org/etiquette.htm">www.re-wrenches.org/etiquette.htm</a><br><br>Check out participant bios:<br><a href="http://www.members.re-wrenches.org">www.members.re-wrenches.org</a><br><br><br><o:p></o:p></p></div></div></div><p class=MsoNormal><br><br><br><o:p></o:p></p><pre>_______________________________________________<o:p></o:p></pre><pre>List sponsored by Home Power magazine<o:p></o:p></pre><pre><o:p> </o:p></pre><pre>List Address: <a href="mailto:RE-wrenches@lists.re-wrenches.org">RE-wrenches@lists.re-wrenches.org</a><o:p></o:p></pre><pre><o:p> </o:p></pre><pre>Options & settings:<o:p></o:p></pre><pre><a href="http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org</a><o:p></o:p></pre><pre><o:p> </o:p></pre><pre>List-Archive: <a href="http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org">http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org</a><o:p></o:p></pre><pre><o:p> </o:p></pre><pre>List rules & etiquette:<o:p></o:p></pre><pre><a href="http://www.re-wrenches.org/etiquette.htm">www.re-wrenches.org/etiquette.htm</a><o:p></o:p></pre><pre><o:p> </o:p></pre><pre>Check out participant bios:<o:p></o:p></pre><pre><a href="http://www.members.re-wrenches.org">www.members.re-wrenches.org</a><o:p></o:p></pre><pre><o:p> </o:p></pre></div></body></html>