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Living Off-grid - The Decade Update - Part II

Posted by: chris
July 25, 2013

Rebekah Hren - SEI PV Curriculum Team Member - Nearly a year ago I started upgrading my decade-old off-grid PV system, and wrote about it for SEI's blog. The first half of the upgrade included a new 2,000 watt pure sine wave inverter, a 3,000 VA generator, and an upgrade to the AC electrical system in my cob house [] The second phase of the upgrade happened this past weekend, and included replacing the two old 155 watt modules with four 235 watt modules, and installing a charge controller that can handle the increase in current and voltage.

The old 155 watt modules are operating at somewhere between 60 to 75% of their original power output, but I don't think this is representative how all old modules will behave. These two got bashed around a bit when they shipped, and appear to have some defects beyond just power degradation, including delamination or microcracking of the cells, which could have been caused by the shipping damage, and lowers current output considerably. I probably should have refused delivery years ago when I saw the dented frames, but it took more than a month for the modules to ship, and I was so excited to receive them, sending them back was the last thing I wanted to do.

They worked well at first, meeting the warrantied power ratings, but they have been degrading in power output faster than I would have predicted. The original manufacturer has long since disappeared, and so while they still “work,” and I can certainly find a new task suited to their lower power output, I didn’t want to add new modules mixed in with the old modules to my array, and so got four new 235 watt modules. Modules prices are about 75% lower than they were a decade ago, which makes buying new modules much less painful these days. The original ground-mounted rack needed just a bit of modification, which was a fairly simple process of adding extra rails, since the original structure had sufficient pier strength for doubling the number of modules.

The original charge controller was a maximum power point tracking model ,with a 30 amp, 70 volt limit. The new array is wired with two modules in series strings, and the two strings paralleled together. The new modules have a standard test condition (STC) open circuit voltage of 35 volts, and so two in series can easily exceed the 70 volt limit of the old controller on cold days when voltage rises. Plus, when connected to a 24 volt battery bank, the 940 watt array can produce upwards of 40 amps, which also exceeded the old controller’s limit. So, a new controller was needed, and I decided on an Outback Flexmax 80, which can handle up to 150 volts open circuit from the array, and up to 80 amps output to the battery bank. While it is actually bit oversized for this array, I can add four or even five modules next year and keep the same controller!

While 940 watts is still a small array compared to many that are installed these days, I’m thrilled with the extra power!

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