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Fellow Wrenches,<br>
Three years ago a local homeowner installed a small off-grid system
using modern hardware preassembled by a good local company. The
array consists of four Solarworld SW165s on a pole-top rack, wired
as two 48V strings feeding an MX60 controller and a 24V battery
bank. The property was sold and the new owners are not able to live
within the capacity of this small array. They have asked me to quote
an array upgrade.<br>
<br>
As modules get larger, I find it gets harder to match old arrays to
new. The SW165 is a 72-cell module with output at MPP of 4.68A at
35.3 Vdc, or 70.6 Vmp for each string. I can get some SW175s, even
though they are more-or-less discontinued by the manufacturer in
favor of the 60-cell 235s. The SW175s are rated at 4.89A at 35.8
Vdc, or one volt higher for each two-module string.<br>
<br>
The maximum power point difference between the existing and new
modules is one volt (or maybe a bit more due to light-induced
degradation and the three-year age difference). In this case, the
difference is pretty small, but I'm using this situation to ask a
bigger question, as I'm interested in what other Wrenches have
learned. An MPPT charge controller (and even more so with the MPP
tracking in a string inverter in a grid-tied system) will latch to
the voltage at which the input current is greatest. In this case, if
I add four 175s to the four existing 165s, both voltage and current
will be defined by the new array. Module I/V curves drop off current
output quickly when the MPP voltage is exceeded. Will the smaller
array be forced to operate above its max power voltage (I think
yes)? Will the output of the smaller array be reduced noticeably by
being operated off the knee of the curve? <br>
<br>
OK, this isn't the best example, as the difference is one volt, or
about 1 1/2%. But what if the original array had used SW155s (34.8
Vmp, or two volts difference)? I'm often asked to add new modules to
an older existing array, so this question comes up, and I could use
others' experience as guidance about when to pay close attention to
these differences and when to let them go.<br>
<br>
This was never an issue before MPPT charge controllers, and it never
was an issue with new systems with matching modules. But as we
regularly upgrade existing systems, and as modules quickly come and
go and specs change, these mix-n-match issues could limit our module
choices if we're seeking maximum performance. Thanks in advance for
advice.<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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