Greenwood Energy: Elizabeth Mine Solar Project
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The Elizabeth Mine Solar Project is part of a complex Superfund effort to clean up an abandoned copper mine located between the towns of Strafford and Thetford, Vermont. After 150 years of mining activity, the site left behind an old, unstable tailing dam that threatened downstream structures and up to 20 miles of Ompompanoosuc River tributaries with anoxic tailing, along with waste piles that leached acid-mine drainage into ground and surface water.
Greenwood Energy, codeveloper and now owner of the solar installation, hired Conti Solar to install the 7 MWdc project. Conti Solar’s extensive experience in working on Superfund projects influenced Greenwood Energy in selecting it as the installation partner. These types of projects generally have strict requirements and call for frequent reporting and collaboration with government and regional organizations. Elizabeth Mine was no exception.
A critical condition of the Elizabeth Mine solar site was that the team would construct the project on top of an unbreachable environmental (geosynthetic) cap. This prerequisite limited options for the solar system foundation. Conti Solar called on the Solar FlexRack engineering team to dial in a ballasted racking system that met the loading perspective for the site requirements.
Loading restraints applied across the operation, calling for minimal ground disturbance. The site is located in a remote area with challenging access for large trucks, going from the highway to single-lane winding roads. Over the course of the project, more than 1,000 trucks traveled into and out of the area. Conti Solar set mitigation plans in place to protect the roads with a survey of access road conditions prior to project initiation. Trucks entering and exiting the site could not go off road and had to stay within weight requirements of 7 psi–8 psi. Final checks of the access routes’ road conditions after completion of work showed no damage to city roads or the highway. A third party carefully monitored the site for compliance during construction, and Environmental Protection Agency officials made monthly visits to the site.
The Power Electronics inverters came on a prebuilt concrete skid containing the integrated fused recombiner, close-coupled transformer, convenience power transformer and racking for the DAS equipment. The project’s interconnection required 10 miles of upgraded fiber optic line, an upgraded regional substation and 4 miles of upgraded utility lines. As an additional part of the reliability improvement, Green Mountain Power, the local utility, also upgraded 3-phase power lines throughout Stafford. These changes benefited the local community with an improved electrical system offering greater reliability. Today the reclaimed land around the Elizabeth Mine houses a renewable energy plant that generates clean solar electricity.
“The precast foundations and intelligent design of the Solar FlexRack fixed-tilt racking made installation faster than with others in the past. Schedules in solar installations are compressed, so having both qualified people on our team and our extensive Superfund project experience made for a smooth operation.”
—Sean Harrington, Conti Solar
DESIGNER: Steven Lawrence, engineering manager, Conti Solar, contisolar.com
LEAD INSTALLERS: Alec Droussiotis, project engineer, Sean Harrington, regional manager, Conti Solar
DATE COMMISSIONED: August 2017
INSTALLATION TIME FRAME: 120 days
LOCATION: Elizabeth Mine, Mine Road, between Strafford and Thetford, VT, 43.5°N
SOLAR RESOURCE: 4 kWh/m2/day
ASHRAE DESIGN TEMPS: 89.2°F 2% average high, -17.4°F extreme minimum
ARRAY CAPACITY: 6.9993 MWdc
ANNUAL AC PRODUCTION: 9,100 MWh
MODULES: 19,998 Hyundai HiS-S350RI, 350 W STC, -0/+3%, 9 Imp, 38.7 Vmp, 9.6 Isc, 47.1 Voc
INVERTERS: Power Electronics skid-mounted inverters, three FS1801CU with 1,799 kW maximum output, one FS1201CU with 1,200 kW maximum output, one FS2000CU with 1,999 kW maximum output, 566 Vdc–900 Vdc MPPT range
ARRAY: 18 modules per source circuit (6,300 W, 9 Imp, 696.6 Vmp, 12 Isc, 983 Voc); 17–23 strings per combiner (for 23-string combiner: 144.9 kW, 153 Imp, 696.6 Vmp, 204 Isc, 983.8 Voc); 400, 266 or 445 strings per inverter; 6.9993 MWdc array total
ARRAY INSTALLATION: Ballasted ground mount, Solar FlexRack B3P-X fixed tilt racking, 180° array azimuth, 21° array tilt
SOURCE CIRCUIT COMBINERS: 50 SolarBOS CSK350C-24A-15-4XF AFCI, 15 A fuses
ARRAY RECOMBINERS: Power Electronics integrated NEC 2014– compliant recombiner per inverter, 300 A and 350 A fuses
SYSTEM MONITORING: AlsoEnergy CH-E0-03