Folks,
One factor often overlooked when calculating minimum system voltage is degradation of the array and rating variations of the modules. If you assume 0.5% loss in voltage per year, you need to factor a 10% higher voltage at year one. 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%. 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: 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. Bill. From: [email protected] [mailto:[email protected]] On Behalf Of Ray Walters Sent: Tuesday, November 29, 2011 2:55 PM To: RE-wrenches Subject: Re: [RE-wrenches] calculating low string voltage 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 <mailto:[email protected]> <[email protected]> To: RE-wrenches <mailto:[email protected]> <[email protected]> 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 NABCEPTM Certified installer Charter Member NYSERDA-eligible Installer VT RE Incentive Program Partner _______________________________________________ List sponsored by Home Power magazine List Address: [email protected] 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: [email protected] 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
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