HI Allan; I found the 175s play very well when mixed with the 165s on an MX 60. We've done it several times. What's interesting is that the Solarworlds often outperform their nameplate rating, I've seen the 165s close to the 175 numbers. Essentially they're all the same module, just flash tests got them sorted into different categories. The voltage difference is slight enough to be totally inconsequential in the real world. I wouldn't worry about mixing them at all. The customer will be satisfied with the overall performance, guaranteed.
R. Walters r...@solarray.com Solar Engineer On Sep 17, 2010, at 10:04 AM, Allan Sindelar wrote: > Fellow Wrenches, > 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. > > 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. > > 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? > > 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. > > 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. > -- > Allan Sindelar > al...@positiveenergysolar.com > NABCEP Certified Photovoltaic Installer > EE98J Journeyman Electrician > Positive Energy, Inc. > 3201 Calle Marie > Santa Fe, New Mexico 87507 > 505 424-1112 > www.positiveenergysolar.com > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Options & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org >
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