Installing A Group Net-Metered Solar Array
June 12, 2013
The 18-kW photovoltaic array on our barn roof is nearing completion
When we started planning the rebuild of our house and the rest of the farm in West Dummerston, Vermont my wife and I knew that we wanted to produce all of our energy onsite. That meant a solar photovoltaic (PV) system that would generate as much electricity as the house and barn are consuming—net-zero energy.
We also wanted to protect as much of the ten acres of agricultural land as possible. That meant we wanted to avoid a ground-mounted PV system. Wherever land can be used for farming—now or in the future—I prefer to install PV arrays on buildings, keeping the land open for agricultural uses.
Fortunately, the 1812 barn has a long roof facing almost due south. That would be the perfect location for the solar array. Our builder, Eli Gould, spent several months restoring the barn, which involved replacing damaged posts, adding sturdy granite supports under those posts, rebuilding several dry-stone walls to support the barn sills, lowering and leveling the floor, replacing some timber framing elements (including about a dozen round-log joists that we cut on the land), and reinforcing the roof to hold the solar modules.
To maximize durability, we wanted the roof to be sturdy and not flex with wind or snow loads, so after stripping the layers of metal and asphalt roofing, we added a layer of 2x6 and 2x4 framing to the roof structure, flattening the roof plane at the same time. Zip sheathing went on over that, and then the roofing.
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Standing-seam metal roofing
One of our goals for the whole project has been to maximize durability, so we spent quite a while debating different roofing materials. We wanted the solar panels to be able to attach to the roof without any penetrations, so that meant standing-seam metal roofing. S-5 brackets for the solar array tracks clamp on to the raised seams of the roofing with absolutely no penetrations of the roof. If panels have to be removed down-the-road for some reason, that’s relatively easy to do.
For the roofing itself, we chose 24-gauge Englert galvalume 1301 roofing with the company’s low-gloss Ultra Cool coating. According to James Hazen of the company, Englert’s paint line is one of the cleanest operations of its kind in the world, with 100% of solvent fumes from painting, drying and curing operations captured. The captured paint fumes are burned with all the recovered heat used in manufacturing. The company expects a 150-year life for the roofing. Roofing contractor Travis Slade, of River Valley Roofing in Putney, Vermont, has done an incredible job installing the standing seam roofing.
Group net-metered system
We have a great location that can hold an 18-kilowatt (kW) PV array, but we don’t need a system that large. So last fall we began investigating community solar options, and we found a neighbor who wanted to buy 6 kW out of the 18 kW system. In other words, this neighbor will actually own a third of the PV system that’s on our barn roof.
This option for someone else to own part of a PV system in a different place is referred to as group net metering, and Vermont is one of the few places where this can be done. Green Mountain Power bends over backwards to facilitate such systems, which is wonderful. Through this option, someone without a south-facing roof where PV modules can be installed can look elsewhere for a good south exposure.
Because the 12-kW system that we will own is still larger than we will need for our house and barn (at least until our farm needs expand), we will plan to sell our excess capacity to another Green Mountain Power customer.
Selection of the PV modules
At the recommendation of our solar installer, Integrated Solar Applications, in Brattleboro, we are installing highly rated REC 250PE modules. The modules are rated at 250 watts, have 15.1% module efficiency, and come with a 10-year product warranty and 25-year “linear power output warranty” (guaranteeing that the degradation of power output will not exceed a 0.7% per year). REC is a Norwegian company with the silicon raw materials produced in the U.S. and silicon wafer, PV cell, and PV module manufacturing being done in Singapore.
Poor reliability and early failure of PV modules has been in the news lately, so I’m relieved that ours aren’t simply commodity Chinese-made modules (though some Chinese products are no-doubt fine).
In future columns I will address other features of our PV system, including “islanding” capability that will provide us with some electricity even when the electric grid is down.
Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.