In sheer size and wattage, Exelon City Solar, a 10-megawatt installation in Chicago's West Pullman neighborhood, eclipses every urban solar power plant in the nation.
Spanning some 41 acres, Exelon features no fewer than 32,000 solar photovoltaic panels, which collectively capture and convert radiant energy in amounts sufficient to produce 14,000 megawatt-hours of electricity per year. The panels do so with solar tracking sensors and motors that tilt the panels to follow the sun's path from dawn to dusk.
The clean energy generated by the installation displaces more than 30 million pounds of greenhouse gas emissions per year – the equivalent of removing more than 2,500 vehicles from the road. The energy is also sufficient to power up to 1,500 homes per year.
Completed last year, the project was the largest of its kind ever undertaken by Turner Construction Company, Chicago, construction manager for Exelon. Although San Jose, California-based SunPower Corporation, supplier and designer of the panels and tracking system, normally self performs construction work, "Here they were confronted a large urban project, which they weren't accustomed to," says Turner Senior Project Manager Dino Sartori. "They also weren't accustomed to contracting and sub-contracting in a union environment such as Chicago's"
The project's brownfield site, former home to a rail yard, further prompted SunPower to seek local expertise. Turner, says Sartori, needed to do its homework before bidding on the project.
So, too, did Libertyville, Illinois-based Aldridge Electric, an electrical contractor more accustomed to projects involving wind-based energy, according to senior project manager Terry Hardiman. While Skokie-based contractor Maron Electric Company assumed responsibility for installing the project's panels and support system, including a network of torque tubes, Aldridge presided over installation of its substation, cable trays and distribution cabling.
"We located all cable trays in their proper inverter boxes, mounted all combiner boxes, pulled all string wire from panel strings to the combiners, and pulled combiner cable from combiner to inverter, which took care of the DC side," Hardiman recalls. "We also performed work on the AC side, which called for routing more traditional 600 volt cabling from the inverter to transformer and 12,470 volt cabling from the transformer to a substation."
In all, he says, the project contains nine prefabricated inverter houses, with two inverters per house. To integrate power with the utility ComEd's grid, the transformers "kicked up the voltage" from 265 volts to 12.470 volts.
Project team members were faced with a tight six-month construction schedule, from September 2009 to March 2010, with three-fourths of power scheduled to come on line Dec. 31, the project's midpoint.
Due to weather and site conditions, there were delays. "When we drilled piers we discovered shallow basements below slabs on grade," says Sartori. "As a result, we had to demolish the slabs and backfill. Structural engineers were on site full time to help us determine how to engineer those spaces. You could say we were engineering on the spot."
Logistics posed additional challenges. "Every inch of those 41 acres was accounted for," Sartori says, "either by the arrays or their supports," Delivery and staging were relegated to an adjacent site provided by the city.
During peak periods, more than 200 workers were on site.
Despite tight quarters, and an even tighter schedule, the project was completed on time. "I attribute much of that to a union market," says Sartori. "Here in Chicago, we have some of the best union craftsmen, be they electricians or excavators. They have good ideas and they're committed to meeting deadlines."