Borrego Solar: Edwards Air Force Base
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The PV design and installation at Edwards Air Force Base was executed across three separate sites. The 12,132 deployed modules were manufactured at Suntech’s Arizona factory and are compliant for procurement in American Recovery and Reinvestment Act (ARRA) and Buy American Act (BAA) projects. The individual systems are interconnected at the Base-owned distribution substations at 34.5 kV. The overall project was developed to meet the environmental compliance requirements of the National Environmental Policy Act and California’s Natural Heritage program, as well as requirements specific to the joshua tree and desert tortoise habitat conservation.
The US Air Force designated the site locations, amount of usable area and overall shape of the array footprint during the RFP process, prior to Borrego Solar’s involvement. Since the allotted land was determined prior to design, it was not optimized for the intended tracking solution. This presented a challenge when designing an optimized array configuration and layout.
In general, the most cost-effective installation for a single-axis tracker controlled by a centralized motor is to maximize the number of modules per tracker row (thus limiting the number of individual rows and racking materials), and at the same time maximize the number of tracker rows per motor (thus limiting the total number of motors and control circuits). These constraints are typically mechanical limitations of the system. However, when faced with the geometric constraints of the designated sites, Borrego Solar found it challenging to optimize both parameters.
For example, compared to the South Base site, the North Base parcel is longer in the north-south axis and narrower in the east-west direction. In addition, the site is off-azimuth, and area would be lost if the rows were installed with drive lines running directly east-west, as opposed to off-axis. To maximize rows at North Base, drive lines were installed at an angle that matched the overall site geometry. As a result, the allowable area was utilized to maximum efficiency, even though individual motors were slightly underutilized with regard to maximum mechanical capacity.
Borrego Solar maximized row lengths for further site optimization. Initial calculations from the tracker manufacturer indicated a maximum of 52 modules per row. However, with strings of 12 modules, there was significant benefit to pushing for 54 modules per row. Not only would that eliminate 4% of the total rows, it would also optimize string wiring by creating a regular stringing layout of 4.5 strings per row and optimize source-circuit combiner box sizing in relation to inverter inputs and row configurations. Working closely with Array Technologies, the tracker manufacturer, Borrego Solar was able to engineer a solution that allowed 54 modules per row, further optimizing the sites’ array capacity.
“One aspect of the design that presented a challenge was interfacing with the existing 34.5 kV electrical infrastructure on the base while still complying with Southern California Edison [the local utility] requirements for interconnection. During the design process, it was important to note which substations were fed from which utility feeders to avoid unintentional backfeed under low-load conditions.”
—Frank Haslinger, Jr., Borrego Solar
DESIGNER: Ben Walter, senior design engineer, Borrego Solar, borregosolar.com
INSTALLATION TEAM: Frank Haslinger, Jr., project manager, Borrego Solar; Phil Korycinski, construction manager, Borrego Solar; Toby Foster, project manager, Reno Contracting, renocon.com; lead subcontractor: Carl Price, project manager, HMT Electric, hmtelectric.com; racking installation: Rick Lavezzo, owner, Arraycon, arraycon.com; mediumvoltage electrical installation: Rick Redmann, project manager, Southern Contracting, southerncontracting.com; inverter skid design and MV transformer integration: Hill Phoenix Power Systems, hillphoenix.com
DATE COMMISSIONED: December 21, 2011
INSTALLATION TIME FRAME: 145 days
LOCATION: Edwards, CA, 34.9°N
SOLAR RESOURCE: 5.7 kWh/m2/day
HIGH/LOW DESIGN TEMPERATURES: per Solar ABCs solar reference map: 104°F/14°F
ARRAY CAPACITY: 3.4 MW
ANNUAL AC PRODUCTION: 7,982 MWh
MODULES: 12,132 Suntech STP280- 24/Vd, 280 W STC, +5/-0%, 7.95 Imp, 35.2 Vmp, 8.33 Isc, 44.8 Voc
INVERTERS: 3-phase, 34.5 kV service; six Satcon Equinox 500 kW UL, 600 Vdc maximum input; 320–600 Vdc input voltage range; 3-phase, 200 Vac output
TRACKERS: Array Technologies DuraTrackHZ, single axis, gear drive, algorithm with GPS input tracking control method
ARRAY: 12 modules per source circuit (3,360 W, 7.95 Imp, 422.4 Vmp, 8.33 Isc, 537.6 Voc), 18–23 source circuits per combiner, 163–174 source circuits per inverter; 3.39 MW array capacity total
ARRAY STRING COMBINERS: Custom Process Solutions, DCB-xx- 4/12-TVSS, 15 A fuses
ARRAY RECOMBINERS: Inverterintegrated recombiners, 250 A, 315 A, and 350 A fuses
SYSTEM MONITORING: PV system and environmental monitoring provided by DECK Monitoring, additional metering and interface with existing SCADA system provided by Edwards Air Force Base