[RE-wrenches] calculating low string voltage

Wed Nov 30 06:38:53 PST 2011

Here¹s a link to an Excel version of our c-Si PV module specifications
table:

https://solarprofessional.com/webexclusive/

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.)

David Brearley, Senior Technical Editor
SolarPro magazine 
NABCEP Certified PV Installer 
david.brearley at solarprofessional.com
Direct: 541.261.6545

On 11/30/11 12:18 AM, "Ray Walters" <ray at solarray.com> wrote:

>    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> <mailto:kirk at vtsolar.com>
>>>  To: RE-wrenches <re-wrenches at lists.re-wrenches.org>
>>> <mailto: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
>>>  
>>> NABCEPTM Certified installer Charter Member
>>>  
>>> NYSERDA-eligible  Installer
>>>  
>>> VT RE Incentive Program Partner
>>>  
>>>   
>>>  
>>>  
>>>  
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