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Ryan,<br>
<br>
SANDIA report SAND2004-3535 (<a
href="http://prod.sandia.gov/techlib/access-control.cgi/2004/043535.pdf">http://prod.sandia.gov/techlib/access-control.cgi/2004/043535.pdf</a>)
discusses a model for PV module performance. Their work includes a
database (<a href="http://photovoltaics.sandia.gov/docs/Database.htm">http://photovoltaics.sandia.gov/docs/Database.htm</a>)
that has the temperature coefficients for Voc and Vmp for a wide
variety of (older) PV modules. It's an Access database which
unfortunately I no longer have the tools to open. But you'll find that
all monocrystalline and all polycrystalline modules have a temperature
coefficient for Voc that is near -0.35%/°C and a temperature
coefficient for Vmp that is near -0.46%/°C and that for any module the
two values are nearly the same if expressed in units of volts/°C. It's
the nature of the material. The % numbers are different for thin film
and I think there is more variability between manufacturers. The %
numbers are different for Sanyo HIP modules too, but if my memory is
any good, Voc and Vmp changed about the same amount of V/°C. <br>
<br>
As far as how the power tolerance gets reflected into the values of
voltage and current, I'm not certain. Maybe the SANDIA database can
give you the answer to that question too.<br>
<br>
Kent<br>
<br>
<br>
Ryan Mayfield wrote:
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<p class="MsoNormal"><span
style="font-family: "Arial Narrow","sans-serif"; color: rgb(31, 73, 125);">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: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-family: "Arial Narrow","sans-serif"; color: rgb(31, 73, 125);">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: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-family: "Arial Narrow","sans-serif"; color: rgb(31, 73, 125);">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: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-family: "Arial Narrow","sans-serif"; color: rgb(31, 73, 125);">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: rgb(31, 73, 125);"><br>
Ryan<o:p></o:p></span></p>
<p class="MsoNormal"><span
style="font-family: "Arial Narrow","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-family: "Arial Narrow","sans-serif"; color: rgb(31, 73, 125);"><o:p> </o:p></span></p>
<p class="MsoNormal"><span
style="font-family: "Arial Narrow","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>Kent
Osterberg<br>
<b>Sent:</b> Monday, December 07, 2009 10:25 PM<br>
<b>To:</b> <a class="moz-txt-link-abbreviated" href="mailto:jryago@netscape.com">jryago@netscape.com</a>; 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>
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