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If you really want to throw a monkey wrench into the mix, bring up
partial shading...<br>
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Or, maybe that's just not allowed in that debate ?? Just a
thought.<br>
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boB<br>
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On 11/29/2011 8:39 PM, Kent Osterberg wrote:
<blockquote cite="mid:4ED5B371.3070705@coveoregon.com" type="cite">
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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 moz-do-not-send="true" 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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