My company, Mile Hi Solar, was hired to install a grid-tied PV system at an old homestead near Loveland, Colorado. It is one of the original turn-of-the-century farms in the area—its few acres now largely surrounded by suburban houses.
The only catch was that large cottonwood trees made just about all areas subject to too much shading for the size of the specified array. Only two suitably sunny locations existed—right in the middle of the front yard and on the roof of an old barn, built around 1910. The front yard was quickly ruled out, since it would block views from the house. So the question became whether the old barn structure could support the weight of a large PV system.
A structural engineer found the structure to be sound. Its rough-cut beams of high-quality wood even foiled some of our attempts to drive nails into them to build interior scaffolding to work from. The steep slope of the barn roof—36° on the shallow topmost section and 55° on the steeper section—also reduced the snow load engineering requirements.
With the engineer’s stamp in hand, the second hurdle was how to work on a steep corrugated metal roof. Since we would be connecting our safety harnesses to the racks and walking on them a lot while building the array, they needed to be stronger than typical aluminum PV mounting rails. We designed a system using steel Unistrut bolted through the roof to another piece of Unistrut on the inside of the rafters, sandwiching the roof between them. For the topmost section, where the slope was not as steep and the force on the rails wouldn’t be as great, Spax engineered lag screws secured the Unistrut Rails to the rafters. Where each bolt penetrates the metal roof, a steel spacer holds the rail off the roof surface; a neoprene washer, sealed with silicon sealant, is used against the metal roof. This is not suitable for a residential roof (we always use fully flashed feet rather than relying on sealant), but for the roof on this barn, which already had hundreds of random nail holes, our lack of flashing wasn’t too concerning.
The three rows of modules each feed an SMA America SB2500 inverter. Since one row has a different slope (and thus will also experience a different insolation and temperature), having individual multiple power point tracking was advantageous. The three inverters tie into a subpanel, which is fed via a new, 400-foot-long, underground feeder that ties in at the utility meter at the pole. The existing feeder at the barn was at least 30 years old, and making it Code-compliant was more difficult than adding a new feeder solely for the PV.
—Zeke Yewdall • Mile Hi Solar