[RE-wrenches] calculating low string voltage

Tue Nov 29 22:18:19 PST 2011

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 <kirk at vtsolar.com>
>> *To:* RE-wrenches <re-wrenches at 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
>>
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