Miller Bros.: Salisbury Project

Miller Bros. (MB) is a leading utility-scale solar project construction, operations and maintenance business based in Pennsylvania, serving south-central states through the eastern seaboard. In April 2015, it employed Solvida Design + Engineering (SD+E) to provide electrical and structural design engineering services for a project expected to be the largest in the state—a challenging 17-plus MW solar power generation plant located on the Delmarva Peninsula near Salisbury, Maryland. SD+E, led by Stephen Smith, is a team of solar design experts with hundreds of megawatts of operational projects under their belts.

The project team’s first challenge was to determine how to maximize system MWh production given the limited acreage and oddly shaped tree-lined site. To determine the most appropriate solution among a matrix of design possibilities, SD+E designers calculated each option’s annual energy production forecast, construction cost, annual O&M costs and land utilizations. The team’s final recommendation was a single-axis tracking system with a tight ground-cover ratio (GCR). This design fit the desired capacity within the space limitations and delivered the increased energy yield of a tracking system versus fixed-tilt mounting.

SD+E’s next challenge was designing a cost-effective tracker foundation, given the site’s poor soils and high-water table. The site was previously used for agriculture, and the subcontracted geotechnical report revealed relatively loose soils as far below grade as 4 feet in some locations. It also exposed seasonal and variable high-water table conditions. The geotechnical report recommended cast-in-concrete foundations for the trackers, which would likely have cost MB two times the labor and materials outlay, as well as longer installation time compared to a driven-steel pile. The SD+E team worked closely with the Array Technologies (ATI) services team to study other foundation options. The project team looked at worst-case scenario wind and structural loading, which was important given the site's close proximity to the Chesapeake Bay and Atlantic Ocean. With this data and the use of ATI’s wind tunnel test data, SD+E’s structural engineer was able to design and certify a standard 13-foot minimum embedment for a driven-steel pile.

Due to commissioning and interconnection deadlines, the team set the project build plan to an aggressive timeline, with construction beginning on June 15 and completion expected within 4 months. MB created carefully detailed construction schedules with additional crews and hours to accommodate the contracted build schedule. MB tasked SD+E to provide a site architecture and layout that allowed maximum labor crew access. SD+E’s electrical design strategically placed dc combiners and located dc/ac feeders along east-west main access roads. This helped the MB installation teams erect tracker mechanical blocks and install dc systems rapidly. At maximum utilization, the site hosted 30 tracker and module assembly crews and six civil installation crews working harmoniously.

“The most difficult part of these larger arrays is getting around the site and getting out of the way of other aspects of concurrent construction.”

—Joe Marrone, Miller Bros.

“Miller Bros. are resourceful partners who pay attention to the details important to investors and project owners, including controlling costs, adhering to timelines and producing reliable, high-performance projects in spite of on-site challenges.”

—Stephen Smith, COO, Solvida Design + Engineering

Overview

DESIGN FIRM: Solvida Design + Engineering, solvidadesign.com

LEAD INSTALLER: Joe Marrone, project lead, Miller Bros., millerbros.us

DATE COMMISSIONED: December 2015

INSTALLATION TIME FRAME: 110 days

LOCATION: Salisbury, MD, 38.4°N

SOLAR RESOURCE: 4.12 kWh/m2/day

ASHRAE DESIGN TEMPS: 91.4°F 2% average high, 6.8°F extreme minimum

ARRAY CAPACITY: 17.8 MWdc

ANNUAL AC PRODUCTION: 22.6 GWh

Equipment Specifications

MODULES: 57,546 Renesola JC310M-24/Ab Virtus II, 310 W STC, +5/-0 W, 8.38 Imp, 37 Vmp, 8.8 Isc, 45 Voc

INVERTERS: 25 kV service; 20 Schneider Electric Conext Core XC 680-NA inverters, 680 kW rated output, 1,000 Vdc maximum input, 550–800 Vdc MPPT range, 380 Vac rated grid voltage; inverters integrated with five 2.72 MW Schneider Electric PV Skids, each with four Conext Core XC 680-NA inverters and two 1,360 kVA 25 kV/380 V ANSI C57 transformers in daisy-chain configuration to central point of interconnection

ARRAY: 19 modules per source circuit (5,890 W, 8.38 Imp, 703 Vmp, 8.8 Isc, 855 Voc), 30–36 source circuits per combiner, 131–156 source circuits per inverter, 3,440 kW–3,605 kW per power block, 17.8 MWdc array total

ARRAY INSTALLATION: Array Technologies DuraTrack HZ v2.5 single-axis tracking system, 41% ground cover ratio, 0° azimuth, ±45° tracking range

ARRAY SOURCE CIRCUIT COMBINERS: SolarBOS CSK350C-32A-15-N3 32-circuit Disconnect Combiners, 15 A fuses

ARRAY RECOMBINERS: Factory-integrated with Schneider Electric inverter package

SYSTEM MONITORING: AlsoEnergy performance monitoring and portfolio management

Article Discussion