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Kirk,<br>
<br>
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.<br>
<br>
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.<br>
<pre class="moz-signature" cols="72">Kent Osterberg
Blue Mountain Solar, Inc.
<a class="moz-txt-link-abbreviated" href="http://www.bluemountainsolar.com">www.bluemountainsolar.com</a>
t: 541-568-4882</pre>
<br>
On 11/29/2011 11:48 AM, Kirk Herander wrote:
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<p class="MsoNormal">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).<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Kirk Herander<o:p></o:p></p>
<p class="MsoNormal">VT Solar, LLC<o:p></o:p></p>
<p class="MsoNormal">dba Vermont Solar Engineering<o:p></o:p></p>
<p class="MsoNormal">NABCEP<sup>TM </sup>Certified installer
Charter Member<o:p></o:p></p>
<p class="MsoNormal">NYSERDA-eligible Installer<o:p></o:p></p>
<p class="MsoNormal">VT RE Incentive Program Partner<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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