[RE-wrenches] calculating low string voltage

[Ray Walters]

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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