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Kent (continuing an older discussion thread),<br>
You addressed the particulars of the specific application below
correctly; thanks. But the question I was asking was about he effect
if the example had been the other way around.<br>
<br>
You write that "the combined array will be operating at the Vmp of
the largest
subarray" - right. In the case below the larger subarray operates at
a lower Vmp, and thus simply pulls the VMP of the weaker subarray
down to it, resulting in an insignificant performance loss. <br>
<br>
But what if the larger subarray had the higher Vmp? It would pull
the Vmp of the weaker subarray up, rather than down. It seems to me
that this would pull the weaker subarray past the knee of its I/V
curve, resulting in a substantially reduced current output for that
string. <br>
<br>
That's what I was asking about. Your thoughts?<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 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 href="http://www.positiveenergysolar.com/"
target="_blank">www.positiveenergysolar.com</a></span></p>
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<br>
On 1/16/2011 5:30 PM, Kent Osterberg wrote:
<blockquote cite="mid:4D338DA5.3020204@coveoregon.com" type="cite">
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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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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
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<a moz-do-not-send="true"
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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/"
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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>
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