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Allan,<br>
<br>
As a first estimate, the mismatch losses can evaluated by assuming that
the combined array will be operating at the Vmp of the largest
subarray. In this case that would mean the SW175s are operating at a
voltage that 3% lower than their Vmp. Since the power curve is nearly
second order near Vmp, the power output of the SW175s will be about
1.5% off Pmax and the combined array will suffer from about 0.6%
mismatch.<br>
<br>
In reality the maximum power point will occur at a point someplace in
between the Vmp values and the mismatch loss will be a even less.<br>
<br>
Kent Osterberg<br>
Blue Mountain Solar, Inc.<br>
<br>
<br>
<br>
Allan Sindelar wrote:
<blockquote cite="mid:4D324708.8010809@positiveenergysolar.com"
type="cite">
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http-equiv="Content-Type">
A little more musing here...and I'm looking for others' opinions on
this theoretical piece.<br>
<br>
If you have two unmatched strings, won't the string with the larger Imp
define the MPP of the inverter? <br>
<br>
If so, I would expect that the weaker string would be forced to operate
at the Imp of the stronger string.<br>
<br>
If the stronger string operates at a higher Vmp, it will pull the
weaker string's operating voltage above its own MPP, which could pull
the current down substantially as voltage moved from Vmp toward Voc.<br>
<br>
Conversely, if the stronger string operates at a lower Vmp, it will
pull the weaker string's operating voltage back toward its own MPP,
which would maintain approximately the same current at a slightly lower
voltage. This would reduce output slightly, but not nearly as much as
in the other direction.<br>
<br>
Does this make sense? And does real-world experience bear this out?<br>
<br>
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<p class="MsoNormal" style=""><span class="il"><b style="">Allan</b></span><b
style=""> Sindelar</b><span style="font-size: 10pt;"><br>
<a moz-do-not-send="true" href="mailto:Allan@positiveenergysolar.com"
target="_blank"><span class="il">Allan</span>@positiveenergysolar.com</a><br>
NABCEP Certified Photovoltaic Installer<br>
EE98J Journeyman Electrician<br>
<b style="">Positive Energy, Inc.</b><br>
3201 Calle Marie<br>
Santa Fe, New Mexico 87507<br>
<b style="">505 424-1112</b><br>
<a moz-do-not-send="true" href="http://www.positiveenergysolar.com/"
target="_blank">www.positiveenergysolar.com</a></span></p>
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<br>
On 1/15/2011 11:50 AM, William Miller wrote:
<blockquote
cite="mid:6.0.1.1.2.20110115104240.032e9918@millersolar.com"
type="cite"> Friends:<br>
<br>
I have a client with a Xantrex GT2.8 inverter. Feeding it are 8 Solar
World SW175 modules. This is half the modules that this inverter can
handle. The client wants to realize the full potential of this
inverter. Adding 8 additional SW175 modules would be ideal, but they
are no longer manufactured.<br>
<br>
I need to find a group of modules to match the 175s. I am
contemplating 9 SW240s. The specifications for this match up are
below. I imagine the IV curves are similar for these two modules as
they are the same chemistry and the same manufacturer.<br>
<br>
<font size="3"><x-tab> </x-tab>8-SW175<x-tab> </x-tab>9-SW240<br>
Voc<x-tab> </x-tab>403<x-tab> </x-tab><x-tab> </x-tab>382<br>
Vmp<x-tab> </x-tab>283<x-tab> </x-tab><x-tab> </x-tab>275<br>
Imp<x-tab> </x-tab>4.95<x-tab> </x-tab><x-tab> </x-tab>7.87<br>
Pcsi<x-tab> </x-tab>1252<x-tab> </x-tab><x-tab> </x-tab>1715<br>
Ptotal<x-tab> </x-tab>2.967<br>
<br>
Might this match be close enough? I know this over-powers the inverter
slightly. I imagine that the Xantrex will self regulate this very rare
over wattage condition.<br>
<br>
Comments?<br>
<br>
William Miller<br>
<br>
<br>
<br>
<x-sigsep></x-sigsep></font>
<p> <font size="3"><u>Please note new e-mail address and domain:<br>
<br>
</u>William Miller <br>
Miller Solar<br>
Voice :805-438-5600<br>
email: <a moz-do-not-send="true" class="moz-txt-link-abbreviated"
href="mailto:william@millersolar.com">william@millersolar.com</a><br>
<a moz-do-not-send="true" href="http://millersolar.com/"
eudora="autourl">http://millersolar.com<br>
</a>License No. C-10-773985<br>
</font></p>
</blockquote>
</blockquote>
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