After success with a small photovoltaic installation, officials in Gaston County, N.C., looked to the roof of their largest building, the York-Chester plaza in downtown Gastonia. It seemed a prime spot to further their green-energy ambitions.
The county, however, could not take advantage of state and federal tax savings that often make projects financially viable, so officials sought to lease the rooftop. Charlotte-based National Renewable Energy Corporation, or NARENCO, won the contract and started planning for installation.
An aerial view of the sprawling 100,000 square-foot building, which houses a police station and other government offices, suggests plenty of space to install solar panels. But structural analysis showed that some parts of the roof were unfit for an array’s weight because inside it supported hanging equipment.
In all, NARENCO lost 12% of the installation space because of the equipment and another portion to a solar hot water system. Keith Davis, vice president of construction, was responsible for the installation and determining how to make up for all that lost space.
“If I start losing panels, my lease doesn’t decrease, but my panels and kilowatt hours do — and therefore my revenue,” Davis says. “I needed to put more panels on this roof without negatively affecting generating capacity.”
The optimum tilt angle for solar panels is the latitude of their installation — in Gastonia, that’s 35 degrees— but such a steep angle is unlikely for flat rooftops. The massive ballast blocks and wind shielding required to keep an array in place would put too much strain on the roof. In addition, steeply angled solar panels require more space between rows — seven feet at 35 degrees.
In an open field, it’s easy to achieve the optimum, Davis says. But on roofs, developers trade efficiency for lower angles that require lighter ballast blocks and less distance between rows.
“It’s a balancing act,” Davis says. “You weigh this factor and you weigh that factor, and you make a decision about what’s the best return for your money.”
Davis reached out to PanelClaw, a Massachusetts-based solar racking company, for a ballasted system with low tilt angles. He and PanelClaw regional account manager Chris Amsbary decided to use two systems together.
The company’s Grizzly Bear system supports 178 kW of the array, holding panels at 10 degrees. Amsbary says this angle is standard for flat roofs.
A larger number of the panels — 571 kW worth — is supported by the company’s polar Bear 5-degree system, installed using its reduced inter-row spacing option. (Polar Bear can also hold panels at 10-degrees.)
“It’s often the case when designing a PV system that the integrator is trying to get a certain power output, but they are limited by factors including roof space,” says Amsbary. “With a 5-degree system, we were able to get more panels on the roof and meet the production targets.”
Though actual spacing varies from project to project, the roof in Gastonia needed only eight inches of spacing between panels on polar Bear racks. The Grizzly Bear racks, using standard spacing, required about 21 inches between panels.
The change in angle from 10 to 5 degrees affected panel output by 2.5%, Davis says. They compensated for the loss by adding more solar panels, the cost of which was negligible, he says.
“We could generate that much energy to cover that cost pretty easily,” Davis says.
But there was another hitch.
The system needed to be operational by Jan. 1, 2012, to be eligible for the government incentives, and crews didn’t take to the roof until mid-October. That left 11 weeks to install a 3,122-panel system that typically takes 12 weeks to build, all amid the holidays.
To make matters worse, a string of inclement weather was headed their way.
Davis says crews unloaded panels in the rain. They waited for frost to thaw and then worked into the evening. They worked on combiner boxes inside pop-up tents under downpours. Crews even worked Christmas eve and for overtime.
“When you get down to that point, you do what you have to do,” Davis says. “It was worth those tax credits.”
But the racking again contributed to the array’s success. The PanelClaw systems are relatively light on components, consisting of three major pieces and two nuts and bolts. A mechanically attached roof-protection pad and integrated UL 2703-certified grounding come standard, says Amsbary, all reducing installation time.
The system first generated power on Dec. 28, feeding into the grid. It was a double-win for Gaston County.
“[The system] generates a small revenue off that formerly empty rooftop, and renewable energy is being sold into the grid,” says Dan Ziehm, assistant director of public works. “In 15 or 16 years, Gaston County will be able to purchase the York-Chester solar array at a significantly depreciated cost, and the power-purchase agreement can be transferred to the county.”
It’s going well for NARENCO, too. The system created about 107 MW of energy from July 1 to 30, averaging about 4 MW per day. “We are exceeding our expectations pretty handily on generation output,” Davis says. SPW