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<font face="Verdana">Great, Bill. Now let's think about wind blowing
across the peak of the structure. The upwind side receives different
cooling than the downwind string. Of course this also depends on
module tilt, module spacing, presence or absence of flashing around the
skirt of the sub-array . . . This could go on and on, with no
definitive answer. <br>
<br>
It's very interesting how we push so hard on inverter and module
efficiency, then are willing to give up some (or all) of that energy
harvest with orientation, string/module matching, wire size, etc. Not
to mention those arrays that never get washed. Once again it
demonstrates how each and every installation is unique, and requires
fairly comprehensive analysis to predict performance. And then it
requires monitoring and active participation to maintain performance.<br>
<br>
</font>
<pre class="moz-signature" cols="72">See ya!
Marv
707 763-4784 x7016
</pre>
<br>
<br>
Bill Brooks wrote:
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<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Marv,<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">I
thought I might catch the attention of a few folks with my “ambient
temperature” comment. It was not a typo. While cell temperature
dominates
Vmp, irradiance also has a small impact (less than most people think).
Since
there is a counterbalancing effect of temperature and irrandiance in a
two
orientation scenario, the only consistent condition is ambient
temperature.
This is for the people that make the argument that Vmp can’t possibly
be
the same for two arrays in two different orientations. For a given cell
temperature,
irradiance does not change Vmp much. However, Irradiance does change
cell
temperature relative to the ambient. Since both strings are in the same
ambient
temperature conditions, that is one way to differentiate that strings
on the same
roof will be similar, whereas strings in two different states will have
different operating voltages because the ambient temperatures are
likely to be
different. Just a brain teaser—it was fun teasing your brain.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);">Bill.<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-size: 11pt; font-family: "Calibri","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif"; color: windowtext;">From:</span></b><span
style="font-size: 10pt; font-family: "Tahoma","sans-serif"; color: windowtext;">
<a class="moz-txt-link-abbreviated" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">re-wrenches-bounces@lists.re-wrenches.org</a>
[<a class="moz-txt-link-freetext" href="mailto:re-wrenches-bounces@lists.re-wrenches.org">mailto:re-wrenches-bounces@lists.re-wrenches.org</a>] <b>On Behalf Of </b>Marv
Dargatz<br>
<b>Sent:</b> Friday, May 21, 2010 7:26 AM<br>
<b>To:</b> RE-wrenches<br>
<b>Subject:</b> Re: [RE-wrenches] Two strings of Different Orientation<o:p></o:p></span></p>
</div>
</div>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span
style="font-family: "Verdana","sans-serif";">Bill,<br>
<br>
I agree with everything you're saying, if you substitute "cell
temperature" for "ambient temperature".<br>
<br>
<br>
</span><o:p></o:p></p>
<pre>See ya!<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>Marv<o:p></o:p></pre>
<pre>707 763-4784 x7016<o:p></o:p></pre>
<p class="MsoNormal"><br>
<br>
Bill Brooks wrote: <o:p></o:p></p>
<pre>Peter,<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>Not to belabor the issue, but your understanding of the I-V curve and how it<o:p></o:p></pre>
<pre>responds to changes in irradiance and temperature are flawed. I'm sorry you<o:p></o:p></pre>
<pre>believe that I am misleading you, but the truth is that the I-V curve moves<o:p></o:p></pre>
<pre>up and down with changing irradiance (current changes with irradiance), and<o:p></o:p></pre>
<pre>the I-V curve moves left and right with temperature (voltage changes with<o:p></o:p></pre>
<pre>temperature). Without these fundamental understandings, we do not have an<o:p></o:p></pre>
<pre>understanding of the I-V characteristics of a PV device. <o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>At lower irradiances, some PV modules will slightly reduce in max power<o:p></o:p></pre>
<pre>voltage for a given temperature, but the temperature is not constant. As the<o:p></o:p></pre>
<pre>irradiance reduces, so does the temperature, causing a slight increase in<o:p></o:p></pre>
<pre>the max power voltage. These two counteracting aspects of the I-V curve make<o:p></o:p></pre>
<pre>the max power voltage of a PV array nearly identical throughout the<o:p></o:p></pre>
<pre>temperature and irradiance range for a given ambient temperature. Thus, the<o:p></o:p></pre>
<pre>ambient temperature is the ultimate deciding process on voltage in a round<o:p></o:p></pre>
<pre>about way. I hope we can come to a meeting of the minds on this some day.<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>Bill.<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre><o:p> </o:p></pre>
<pre>Peter,<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
<pre>The issue is the same as it was last year. <o:p></o:p></pre>
<pre>[#] But it hasn't been resolved to my satisfaction, yet<o:p></o:p></pre>
<pre>The operating voltage of each<o:p></o:p></pre>
<pre>string will be nearly identical throughout the day if the strings are the<o:p></o:p></pre>
<pre>same length and not shaded. <o:p></o:p></pre>
<pre>[#] This can't be the case. After all, the SE sub-array will have greater<o:p></o:p></pre>
<pre>irradiance than the SW sub-array in the AM and the converse will be true in<o:p></o:p></pre>
<pre>the PM <o:p></o:p></pre>
<pre>Put them on a single inverter.<o:p></o:p></pre>
<pre>[#] I'd dearly like to.<o:p></o:p></pre>
<pre><o:p> </o:p></pre>
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