Sullivan Solar Power: San Diego City College
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To adhere to its Green Building Policy standard, the San Diego Community College District designed its new Career Technology Center at San Diego City College to achieve LEED Gold certification. The architectural features of the building dictate different module types split between a roof-mounted array and two vertical wall arrays. In addition to the drastic differences in array tilt angles, each array has multiple string configurations. The number of modules per string and strings per inverter vary to maximize the physical area available and avoid dummy modules to fill spaces. The arrays fit into an area dictated by the space, shading and architectural aesthetics to match other wall facades on the west wall and walkway clearances.
While the electrical design is not completely straightforward, the structural design of the vertical arrays presented the biggest challenge. The Unirac SunFrame racking system was used because it allows for a vertical orientation, conditional to properly designed structural spacing. Working closely with Unirac and Brian Spring of Brian Spring Engineering, Sullivan Solar Power ended up with a design that requires 3/8-inch tube steel run vertically up each wall on the west and east of the building, with a maximum spacing of 4 feet to provide the racking system support. Doublewide L-feet connect to the tube using two ¼-20 high-grade stainless steel screws; the SunFrame is then attached to the L-feet for module mounting. To prevent loosening and galling and to decrease potential corrosion, the bolts and screws are treated with a thread-locking compound, and holes in the tube steel are sealed with an electrometric polyurethane sealant.
The array installation provided its own set of challenges. The tallest vertical array tops out just over 120 feet from the existing grade. Since all work was done at the array locations, drilling the holes, attaching the racking and modules and making all the electrical connections required accurate planning and installation techniques. The installation team was divided into an inside crew and a lift crew. At the west wall, the back of the modules are accessible from inside, so the inside crew had the responsibility of wiring and grounding. They installed homerun cables and attached grounding lugs prior to module installation and returned after the modules were installed to complete the array wiring. The lift crew made all the mechanical connections from boom lifts—including tapping and installing the steel tube supports, mounting the Unirac SunFrame, installing the modules and cap strip and trimming the excess rail. Due to restricted access, the lift crew accomplished the array wiring and grounding for the east wall as the modules were installed. Moving the booms even small distances added another degree of difficulty for the lift crew.
“Establishing the installation process started slowly, because it was an incredibly arduous task. Once the crews figured out best practices, by day four they increased productivity and were mounting, grounding and connecting 40 modules daily. This project’s difficult design and installation made it all the more rewarding to see it turn out as nicely as it did.”
—Quinn Laudenslager, Sullivan Solar Power
DESIGNER: Quinn Laudenslager, project manager, Sullivan Solar Power, sullivansolarpower.com
LEAD INSTALLER: Cesar Chaidez, project foreman, Sullivan Solar Power
DATE COMMISSIONED: May 2010
INSTALLATION TIME FRAME: 18 days
LOCATION: San Diego, CA, 32.7°N
SOLAR RESOURCE: Vertical arrays, 3.6 kWh/m2/day; roof array, 5.6 kWh/m2/day
RECORD LOW/AVERAGE HIGH TEMPERATURE: 29°F/78°F
ARRAY CAPACITY: 60.77 kW
ANNUAL AC PRODUCTION: 70,737 kWh projected
MODULES, VERTICAL ARRAYS: 286 Sharp ND-N2ECU, 142 W STC, +10%/-5%, 7.11 Imp, 20.0 Vmp, 7.92 Isc, 24.9 Voc
MODULES, FLAT ROOF: 90 Sharp ND-224U1F, 224 W STC, +10%/-5%, 7.66 Imp, 29.3 Vmp, 8.33 Isc, 36.6 Voc
INVERTERS: 3-phase, 277/480 Vac service; nine SMA SB 7000US total, six for the vertical arrays, three for the roof-mounted array; 7 kW, 600 Vdc maximum input, 250–480 Vdc MPPT range
ARRAY, EAST VERTICAL: 16 modules per source circuit on Inverters 1–4, (2,272 W, 7.11 Imp, 320 Vmp, 7.92 Isc, 398.4 Voc) with three circuits per inverter (6,816 W, 21.33 Imp, 320 Vmp, 23.76 Isc, 398.4 Voc); one inverter with source circuits on both east and west vertical arrays
ARRAY, WEST VERTICAL: 18 modules per source circuit on Inverter 5, (2,556 W, 7.11 Imp, 360 Vmp, 7.92 Isc, 448.2 Voc) with three circuits (7,668 W, 21.33 Imp, 360 Vmp, 23.76 Isc, 448.2 Voc); and 20 modules per source circuit on Inverter 6, (2,840 W, 7.11 Imp, 400 Vmp, 7.92 Isc, 498 Voc) with two circuits (5,680 W, 14.22 Imp, 400 Vmp, 15.84 Isc, 498 Voc)
ARRAY, FLAT ROOF: 11 modules per source circuit on Inverters 1–2 (2,464 W, 7.66 Imp, 322 Vmp, 8.33 Isc, 402.6 Voc) with three circuits per inverter (7,392 W, 22.98 Imp, 322 Vmp, 24.99 Isc, 402.6 Voc); 12 modules per source circuit on Inverter 3, (2,688 W, 7.66 Imp, 351.4 Vmp, 8.33 Isc, 439.2 Voc) with two circuits (5,376 W, 15.32 Imp, 351.4 Vmp, 16.66 Isc, 439.2 Voc)
ARRAY INSTALLATION, VERTICAL: Unirac SunFrame attached to building face via steel-tube structure anchored into concrete, 180° azimuth, 90° tilt
ARRAY INSTALLATION, FLAT ROOF: Unirac Solar Mount on TPO membrane, 170° azimuth, 19° tilt
ARRAY COMBINERS: Inverter integrated with 15 A fuses
SYSTEM MONITORING: Fat Spaniel Inverter Direct PV2Web