Sunsense Solar: Garfield County Airport Solar Array (Phase 2)

The Clean Energy Collective (CEC) developed the Garfield County Airport Solar Array project as an 858 kW community solar array (GCASA Phase 1) and commissioned it in 2011. CEC has its roots in Carbondale, Colorado, and has a long-standing and continuing partnership with Sunsense Solar, also based in Carbondale. Equally integral to the project was Holy Cross Energy (HCE), the local cooperative utility. Since the early 2000s and at the request of its customers, HCE has been incentivizing and interconnecting PV systems within its service territory in western Colorado.

After completing this first phase, CEC wanted to expand its community solar reach within HCE territory and approached Sunsense with the possibility of expanding the GCASA site. The size of the expansion, fixed at 826 kW, required developing two sites. The solution (GCASA Phase 2) was to straddle the original array by splitting the new array into Site A (599.4 kW) to the west and Site B (226.8 kW) to the east. This approach proved challenging on several levels. 

From a product perspective, GCASA 1 and GCASA 2 deploy markedly different components that required different design and installation approaches. GCASA 1 consists of helical piers and racking from Solar FlexRack, polycrystalline modules from Hanwha Solar and central inverters from Advanced Energy. In contrast, GCASA 2 includes TerraSmart ground screws and racking, First Solar thin-film modules and a decentralized design that uses SMA America string inverters.

Site A simply extended the original side-of-runway location and featured the same 20° south-facing slope. A portion of Site B has the same south-sloping topography, with the balance relatively flat. While this sloped surface assisted Sunsense in achieving the system design tilt, it resulted in significant issues with construction, particularly through the winter months when ice and snow made things tricky. When the weather warmed up, the mud was equally problematic. The solution was to utilize tracks, chains and other traction methods on all equipment and, during some phases, to wait for drier conditions. Upon completion, the site required significant restoration.

The utility interconnection was an additional challenge. The project team brought utility power to the GCASA 1 site via an overhead line. The team determined that construction of Site B would require excavating a 700-foot trench from the switchgear to the existing transformer at GCASA 1. With the trenching, conduit and conductor installation, and backfilling scheduled to begin during the winter months, the thawing of the ground collapsed the trench in the spring to the point where it required further restoration.

Sunsense considered several strategies, including overhead lines and extensive trenching, for utility interconnection at Site A. HCE proved to be particularly adept at running service lines in a variety of geotechnical conditions throughout the mountains of western Colorado. Ultimately, the utility determined that the best and safest route for the interconnection would be through the adjacent restored Dry Creek drainage.

“With the guidance of our developer partners at CEC, the operations crew at Holy Cross Energy and our subcontractors from Expert Electric and Lyons Fencing, Sunsense Solar was able to address challenges and provide solutions leading to an extremely successful expansion of the Garfield County Airport Solar Array.”

—Mark Item, Sunsense Solar


DESIGNER: Jeff Lauckhart, lead PV designer, Sunsense Solar,

LEAD INSTALLER: Mark Item, commercial site supervisor, Sunsense Solar



LOCATION: Rifle, CO, 39.5°N

SOLAR RESOURCE: 5.8 kWh/m2/day

ASHRAE DESIGN TEMPS: 93.2°F 2% average high, -7.6°F extreme minimum



Equipment Specifications

MODULES: 7,344 First Solar FS4112A-2, 112.5 W STC, +5/-5%, 1.60 Imp, 70.2 Vmp, 1.75 Isc, 87.7 Voc

INVERTERS: 3-phase 277/480 Vac service, 26 SMA America Sunny Tripower 24000TL-US, 24 kW; nine Sunny Tripower 20000TL-US, 20 kW; two Sunny Tripower 15000TL-US, 15 kW; 1,000 Vdc maximum input, 150–1,000 Vdc operating voltage range

ARRAY: 5,328 modules Site A, 2,016 modules Site B; nine modules per source circuit (1,012.5 W, 1.6 Imp, 631.8 Vmp, 1.75 Isc, 789.3 Voc); 28 source circuits (typical) per SMA Sunny Tripower 24000TL-US (28.35 kW, 44.8 Imp, 631.8 Vmp, 49 Isc, 789.3 Voc); 826.2 kWdc array total

ARRAY INSTALLATION: Ground-mount, TerraSmart TerraFarm racking system, 180° azimuth, 30° tilt

SOURCE CIRCUIT COMBINERS: 37 SMA America Connection Unit CU1000-US-10; First Solar four-string fused harnesses combine eight source circuits on each array table into two sets of PV output circuits; each four-string PV output circuit is landed on a 20 A fused input in an SMA Connection Unit

SYSTEM MONITORING: Accuenergy AcuPanel 9100 Series revenue grade meter, AlsoEnergy performance monitoring and portfolio management

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