Solar systems are predominantly installed on rooftops in urban settings, where open land is scarce but multi-level buildings are aplenty. Often, the roof space on these structures isn’t large enough to host solar systems that can cover a building’s entire energy footprint, so they’re supplemented with renewable energy credits (RECs) generated by out-of-city solar systems.
Onion Flats, an architecture firm, wanted to buck that trend and maximize PV output without having to subscribe to RECs on its Front Flats apartment building project in the Kensington neighborhood of Philadelphia, Pennsylvania. The solution was installing solar panels on the roof, as well as on the majority of the east-, west- and south-facing walls of the building. In total, the solar system generates 176 kW, which is more than the Front Flats needs — and that’s by design.
Like an onion, Front Flats has layers
Being in a state and city not known for solar incentives, the Front Flats project is an anomaly, both in appearance and size.
“I wouldn’t call this a solar city,” said Tim McDonald, CEO of Onion Flats. “The renewable energy credits in Pennsylvania suck, but you go across the bridge to New Jersey and it’s a different world. There’s not a real incentive financially to push it. We push it because we think that’s where we need to be.”
From the second story up, the 28-unit apartment building’s windows are slightly obscured by bifacial solar panels protruding from its exterior walls. Those vertical panels on the east, west and south façades meet the horizontal modules held by canopy supports on the roof. Tenants have rooftop access, where shade gardens are planted, and McDonald said the bifacial module shell still allows natural light into the building and offers additional visual privacy.
Onion Flats’ leadership is composed of brothers Tim, Patrick and Johnny McDonald and architect Howard Steinberg. The company handles development, design and construction of its projects, working alongside subcontractors, and owns the service GRASS (Green Roofs And Solar Systems) that installs PV and gardens on rooftops.
The design-build firm has made solar part of its new construction projects, previously only including functional spaces around PV like the Front Flats roof canopy, but never at this scale or panel orientation. To ensure a project of this scope was installed correctly, Onion Flats brought on longtime solar industry member and collaborator Ron Celentano of Celentano Energy Services as a consultant.
“These guys are crazy — I love them,” Celentano said. “They’re techs and they’re out there doing their own building. They just go wherever and just wait for the ramifications.”
The vertically-mounted solar panels are attached to rails that are on angled aluminum brackets — both specifically fabricated for the project — bolted into the exterior walls, extending the building’s façade by 24 inches. The sub-structure holding the array in place is bolted into Boral composite planks, a cementitious material.
“The façade that holds all the rails away from the wall, you’re getting in and climbing around this complete stratus of aluminum, climbing between the walls and the modules,” Celentano said.
Depending on the obstacles presented by the city block, installers used scissor and articulating boom lifts to move along and fasten the brackets to the building’s exterior. A high-tension electrical line is 6 ft from the west-facing panels, and an elevated train track runs along the east side of the building.
“Learning curves were definitely happening as we went,” Celentano said. “We were using suction cups initially to lift the glass up, but after having a few of those things down, it was amazing how fast it moved.”
The greater issue for Onion Flats was finding a solar panel supplier. The system was designed with Germany’s AE Solar bifacial modules in mind, but when that transaction fell through, the firm had to find an alternative. The Front Flats array instead uses Prism Solar Technologies bifacial modules, which are made in America.
The frameless panels are held in place vertically with two clamps on both the top and bottom edges, and each clamp is backed by a square support on the racking. Mounting panels in succession vertically means the bottom clamp is taking all the weight, Celentano said, so to prevent the glass from resting directly on metal, weather-protected gaskets were added.
The canopy portion of the array stands 10 ft above the roof surface, the panels at a 5° angle. There is a 3/4-in. gap between each panel and several openings in the middle of the rooftop panels for additional light and airflow.
Prior to installing any mounting structure, inverter and optimizer layout had to be mapped out. The system uses five SolarEdge inverters and optimizers attached to each module, meeting rapid shutdown requirements. Each string was directed onto a single plane.
“You’re putting optimizers in places and you can’t get them easily afterward, Celentano said. “But I always felt pretty confident about the Front Flats project. Tim’s group has great support.”
Generating more power than needed
Since the Front Flats array layout isn’t typical, its generating capacity isn’t typical either. The east-facing wall produces well in the morning, but by the second half of the day, it drops down to 1 kW, Celentano said. But taking a cut from vertical angles and maximizing the number of panels on the building compensates for that loss, allowing the array to potentially produce more solar power than it will need and net-meter the excess energy. Thanks to the solar project, Onion Flats charges its tenants a flat rate of $40 a month for utilities.
The Front Flats started accepting tenants on January 1, 2020, with half of the rooms filled by March. Its solar system also came online at that time.
Onion Flats won’t be able to gather accurate annual solar generation numbers until the summer of 2021. However, the architects and Celentano still expect the system to produce more energy than needed.
Celentano said he understands those who might question why one would design a system like Onion Flats did. “But on the flipside, that’s what you need to do if you want to get enough solar to offset your usage,” he said.
In addition to its massive solar system, the Front Flats was built to meet the Passive House Institute’s design standards — an airtight building envelope, insulation, double or triple-paned windows, balanced-heat and moisture-recovery ventilation and a low-impact space conditioning system — to encourage greater energy efficiency.
“I’m getting exhausted doing demonstration projects for other people to say, ‘Yeah, I can do this too,’” McDonald said. “If we don’t take buildings on and get them to a carbon-neutral standard in 20 years, we’re all screwed.”