[RE-wrenches] calculating low string voltage

[Kent Osterberg]

Kirk,

You've got some good feedback on this from Ray and Bill. I'll try to add 
a little more. Most module datasheets show a normal operating cell 
temperature, NOCT, value that's typically 47.5°C. That's 20°C ambient 
temperature, 800 W/sq m, and calm wind and nothing blocking the airflow 
on the back of the module. That 27.5°C temperature rise should be pretty 
close to the temperature rise that occurs for a pole-top mount. It's 
common to see people use 25°C for modules on a pole and 30°C or 35°C on 
a roof and there are data that support these "typical" values. With 1000 
W/sq irradiance, the temperature rise can obviously be more too. Between 
the intensity of the sun, the direction of the sun, the color of the 
roof, the spacing off the roof, and the wind speed there is a lot that 
is different from one system to the next or even one day to the next.

Most PV module spec sheets don't give you a temperature coefficient for 
Vmp. I've seen people use the the Voc coefficient, usually expressed as 
a percentage, for both Voc and Vmp. Big mistake. Data from NREL 
indicates as Bill said, the temperature coefficient for Vmp is higher 
than that the temperature coefficient for Voc. That's particularly true 
when the coefficient is expressed as a percentage per °C. Since there 
are very few manufacturer's that give both temperature coefficients, 
I'll use a value from an old Evergreen module for an example. The 
Evergreen ES-195 datasheet shows Voc = 30.5 volts with a coefficient of 
-0.34%/°C and Vmp = 27.1 volts with a coefficient of -0.47%/°C. Since 
one shouldn't add volts and percents, I'll put the temperature 
coefficients in volts/°C: Voc = 30.5 V - 0.10 V/°C and Vmp = 27.1 V - 
0.13V/°C. So Vmp is moving faster than Voc, but not a lot faster. That's 
generically true for c-Si or poly-Si.

Kent Osterberg
Blue Mountain Solar, Inc.
www.bluemountainsolar.com
t: 541-568-4882


On 11/29/2011 11:48 AM, Kirk Herander wrote:
>
> 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
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org/attachments/20111129/ba3d2f4e/attachment-0003.html>