SPG Solar: Mammoth Lakes Wastewater Treatment Plant

SPG Solar was selected from an RFP process to design, engineer and install a single-axis tracking groundmount system for the Mammoth Community Water District. The 1 MW system is located at the Mammoth Lakes wastewater treatment plant and was financed through municipal bonds. Mammoth Lakes is a mountain community in California at an elevation of 7,880 feet above sea level, in a region that receives high winds, significant one-time snow events and an average yearly snowfall of 211.2 inches. Combined, these site characteristics presented some unique system design challenges.

SPG Solar’s engineering team designed an elevated PV array tracking system with support piles that extend 9 feet above grade to mitigate the impact of snow accumulation under the trackers. The array height provides ample clearance for the tracker to cycle 90° daily, from 45° east in the morning to 45° west in the evening. However, the elevated tracking system significantly increased the wind load on the array.

The tracker support’s pile foundation design required heavily reinforced 18-inch-diameter concrete columns that were set almost 9 feet deep. The reinforced columns rise 3 feet above the finished grade elevation for additional pile support. Geotechnical testing that was performed during the design phase of the project indicated that most of the site subterrain is rock. In addition to the large piles, 6-inchthick equipment pads with added spread-footings were needed to support the inverters and transformers.

The two 500 kW SMA Sunny Central inverters are power limited to 499 kW for rebate purposes. The arrays connected to each inverter are installed with different azimuth angles to conform to the site’s geometry. For each array, there are two drivelines. Each driveline’s motor utilizes a controller that identifies its location with a realtime GPS system. The GPS calculates the sun’s location throughout the year to position the PV modules for maximum harvest. The controller’s algorithm uses a high-precision inclinometer to position the tracker.

To compensate for the movement of each array over the course of a day, the conductors from each source circuit come off of the array wings through a liquid-tight cord grip into a raceway. The combiner boxes are mounted adjacent to the raceway, with the source circuits fed to them through flexible conduits. The combined circuits are then sent underground and routed to the inverter pads. The ac power from the two inverters is stepped up through a pair of 500 kVA 480:208Y/120 Vac transformers and delivered to a power panelboard that has two 800 A breakers for the inverters and two 40 A breakers for tracker motors and controls. The power output is then transmitted to the main 1,600 A PV disconnect, which couples to the line side of the facility’s existing 480 Vac switchgear.

“The project’s location presented the greatest challenges to our team. All of the site conditions—wind, snow and rocky terrain—added their own layer of complexity. We took on the challenge and engineered an innovative solution, maximizing solar production.

—Doug Haigh, SPG Solar

Overview

DESIGNER: Doug Haigh, senior designer, SPG Solar, spgsolar.com
INSTALLATION FIRM: SPG Solar
DATE COMMISSIONED: September 2011
INSTALLATION TIME FRAME: 90 days
LOCATION: Mammoth Lakes, CA, 37.4°N
SOLAR RESOURCE: 6.5 kWh/m2/day
HIGH/LOW DESIGN TEMPERATURES: per Solar ABCs solar reference map: 100°F/-9°F
ARRAY CAPACITY: 1.013 MW
ANNUAL AC PRODUCTION: 2,021 MWh

Equipment Specifications

MODULES: 4,407 Astronergy CHSM 6610M, 230 W STC, +5/-0 W, 7.93 Imp, 29.03 Vmp, 8.52 Isc, 37.48 Voc
INVERTERS: 3-phase, 480 Vac service; two SMA Sunny Central 500HE-US, 500 kW, 600 Vdc maximum input, 330–600 Vdc MPPT range
TRACKERS: four SPG Solar Sunseeker single-axis trackers, +45° to -45° tracking range
ARRAY: 13 modules per source circuit (2,990 W, 7.93 Imp, 377.4 Vmp, 8.52 Isc, 487.2 Voc); 26 source circuits per combiner typical (77.7 kW, 206.2 Imp, 377.4 Vmp, 221.5 Isc, 487.2 Voc); Inverter 1: 183 source circuits total (547.2 kW, 1,451 Imp, 377.4 Vmp, 1,559 Isc, 487.2 Voc); Inverter 2: 156 source circuits total (466.4 kW, 1,237 Imp, 377.4 Vmp, 1,329 Isc, 487.2 Voc)
ARRAY INSTALLATION: tracked ground mount, 180° and 204° azimuths, three module strings per wing (half-row) installed on each side of tracker’s drive line
ARRAY STRING COMBINERS: 13 Bentek Integrated Disconnect Combiners, 15 A fuses
SYSTEM MONITORING: proprietary SMA America monitoring with integrated ERI (Power-One) datalogger, 24/7 video surveillance

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