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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 |
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