O2energies: Chocowinity Solar
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O2energies developed, owns and operates more than 80 MW of solar farms in North Carolina. Its Chocowinity Solar project is located in Beaufort County, near the Atlantic coast, in the small town of Chocowinity (population 823). The town’s name comes from the Tuscaroran Indians, who lived there several hundred years ago, and the solar farm site has been in agricultural use since 1938. The area is known for its pocosin wetlands—the raised bogs of the Atlantic coastal plain. As such, the project team performed an environmental site assessment, as well as stream and wetland verification, early in the project development process. Although the site is not located near a perennial stream, contractors protected its riparian buffers and adjacent wetlands throughout construction.
Since Chocowinity had no language pertaining to solar farms in its existing zoning ordinances, the town had to draft and adopt language for a new solar farm amendment before it could grant a conditional-use permit. That process, concurrent with the development phase, took the better part of a year. The construction phase of the project took approximately 4 months. More than 10 local contractors—including grading, equipment rental, electrical contractors, surveyors and security contractors—provided services and materials. The solar farm will continue to use a local security contractor and will contract with a local farmer for grounds maintenance.
The construction phase of the project ran from September through December 2014. Loose to medium-dense clayey sand and slightly clayey sand underlies the uppermost layer, or plow zone, at the site. This layer is interbedded with very soft to stiff clay, which necessitated a 15-foot pier embedment depth for the rack foundations. Poor weather created complications for the end-of-year completion deadline, as rain, ice, snow and freezing temperatures resulted in a knee-deep buildup of icy mud covering the site. With deadlines looming, contractors poured three concrete equipment pads, and the team received the inverters and transformers on-site. Positioning this equipment was delayed when, only 200 feet from the main entrance gate, the crane sank up to its chassis, and workers had to pull it out of the mud with an excavator. After three successive failed attempts to navigate to the pads, the project manager brought in a rough-terrain four-wheel-drive crane. Much to the construction team’s relief, the fourth positioning attempt was successful, allowing completion of construction without further delays.
The design team chose Eaton’s Power Expert 1670 inverters for the site, in part because they allow full power output while supporting ±0.91 power factor range. The close-coupled Eaton-Cooper power transformers and the Eaton Pow-R-Line recombiners with dc circuit breakers create a synergistic design. The Eaton engineering team lent its expertise to the equipment specification and system commissioning. The combination of SolarBOS disconnecting combiner boxes with the Eaton dc breaker recombiners allow technicians to easily and safely partition the arrays and subarrays for commissioning, testing and maintenance. PowerSecure Solar provides O&M for the site.
“We are proud to celebrate our tenth successful solar farm in North Carolina. It was a pleasure working with the local community and elected officials. The Chocowinity solar project is one of the best in our fleet.”
—Logan Stephens, project development manager, O2energies
DEVELOPER: O2energies, o2energies.com
DESIGNER: Jocelyn Jordan, engineer in training, PowerSecure Solar, powersecure.com
ENGINEER: James D. Ham, professional engineer, Entech Engineering, entecheng.com
PROJECT MANAGER: Vincent Zarallo, project manager, PowerSecure Solar
DATE COMMISSIONED: December 2014
INSTALLATION TIME FRAME: 120 days
LOCATION: Chocowinity, NC, 35.5°N
SOLAR RESOURCE: 4.93 kWh/m2/year
ASHRAE DESIGN TEMPERATURES: 95°F 2% avg. high, 14°F extreme min.
ARRAY CAPACITY: 6.405 MWdc, 4.998 MWac
ANNUAL AC PRODUCTION: 9,494 MWh
MODULES: 21,000 Hanwha HSL 72 305, 305 W STC, +5/-0 W, 8.42 Imp, 36.3 Vmp, 8.85 Isc, 45.1 Voc
INVERTERS: 3-phase 22.9 kV medium-voltage interconnection; three Eaton Power Xpert Solar 1670, 1,667 kW/1,850 kVA rated output, 1,000 Vdc maximum input, 550–800 Vdc MPPT range, inverter integrated dc and ac disconnects; each inverter is close-coupled to a pad-mounted 1,850 kVA Eaton-Cooper Power Systems 22.9 kVac primary/355 Vac secondary transformer
ARRAY: 20 modules per source circuit (6,100 W, 8.42 Imp, 726 Vmp, 8.85 Isc, 902 Voc), 22–24 source circuits per combiner, 15 combiners per inverter, 350 source circuits total per inverter (2,135 kW, 2,947 Imp, 726 Vmp, 3,098 Isc, 902 Voc); 6.405 MWdc array total
ARRAY INSTALLATION: Ground mount, Daetwyler Clean Energy Modu-Rack-DB-L, landscape module orientation, 180° azimuth, 20° tilt
SOURCE CIRCUIT COMBINERS: 45 SolarBOS CDKT400-24-25-N4 Disconnect Combiners, 15 A fuses
RECOMBINERS: Nine Eaton Pow-R-Line recombiners (three per inverter), five 350 A molded case breakers per recombiner
SYSTEM MONITORING: Draker PV250-120