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<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'>This brings up some interesting questions I had yesterday on
this subject.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'>1. If the percentage change for Vmp is greater than Voc but the
manufacturer only lists the temperature coefficient for Voc, what would be
considered the best practice to calculate the Vmp loss? Use the temperature
coefficient for power (typically ~-0.5%/C for crystalline)? Evergreen is the
only manufacturer I have found that lists all these coefficients, and as much
as I would like it to, one data point doesn’t constitute a trend.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'>2. If I have a module with power tolerance of +-3%, and I
presume the power will be at or near the -3%, should I apply -1.5% to the
voltage and -1.5% to the current out of the box? Or will one be weighted more
than the other?<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'>To me it makes sense to do what have heard Bill Brooks
advocate for, determine the minimum string length and then add one module. That
would take the issue of dropping out of the inverter’s window out of the
equation. But, as Jeff has brought up, we don’t always have the liberty
to do that.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'><br>
Ryan<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-family:"Arial Narrow","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
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<p class=MsoNormal><b><span style='font-size:10.0pt;font-family:"Tahoma","sans-serif";
color:windowtext'>From:</span></b><span style='font-size:10.0pt;font-family:
"Tahoma","sans-serif";color:windowtext'>
re-wrenches-bounces@lists.re-wrenches.org
[mailto:re-wrenches-bounces@lists.re-wrenches.org] <b>On Behalf Of </b>Kent
Osterberg<br>
<b>Sent:</b> Monday, December 07, 2009 10:25 PM<br>
<b>To:</b> jryago@netscape.com; RE-wrenches<br>
<b>Subject:</b> Re: [RE-wrenches] large array string sizing<o:p></o:p></span></p>
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<p class=MsoNormal><o:p> </o:p></p>
<p class=MsoNormal>Jeff,<br>
<br>
The maximum voltage of the PV modules will probably be a little less than the
spec'd value and it will decrease slightly as the modules age. But going
over the inverter's voltage rating can (probably will) be catastrophic. It
looks like your leaning towards avoiding that possibility by using 12 modules
and I think that is the right approach. While inverters do have a minimum
operating voltage, the more likely circumstance is that the array voltage drops
outside the MPPT window. The inverter will still operate and the output power
will drop off but by less than the percentage by which the maximum array
voltage drops outside the MPPT window. So if you are loosing 5% of your
potential output for 60 hours a year, that's not a serious problem. But
you still need to make this decision carefully. In 10 to 20 years the
system voltage will drop 5 or maybe 10%. How far are you going to be
outside the MPPT window then? Is the inverter still going to
operate? <br>
<br>
Another potential gotcha working against you is that some inverter
manufacturers string calculators assume that the peak power voltage coefficient
is essentially the same as the open circuit voltage coefficient; that is to say
the percentage change applies to both numbers. It doesn't! In fact
the peak power voltage changes almost as many volts as the open circuit
voltage. Take a look at the voltage/temperature coefficients for the
Evergreen modules; Evergreen publishes both coefficients. While most
manufacturers only publish one value, the numbers are similar for all poly or
mono crystalline cells. <br>
<br>
Sounds like this module/inverter combination may not be a good match.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
<br>
<br>
<span style='color:windowtext'><o:p></o:p></span></p>
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