Commercial Grid-Direct Photovoltaic System: South San Joaquin Irrigation District

The South San Joaquin Irrigation District (SSJID) water treatment plant project is the first known solar tracking system in North America to use thin film modules. It presented an array of unique challenges that are now providing substantial rewards for the SSJID and the system owner.

The two-phased project features a durable utility grade design. Phase One comprises 6,720 Conergy 175 W crystalline modules, totaling 1,176,000 Wp. These modules were selected for their durability and value. Phase Two is composed of First Solar thin film modules because they perform at a lower cost per watt than traditional crystalline. Early indications show the output per dc capacity of First Solar thin film is about 10% higher than that of crystalline.

One of the first challenges was minimizing the number of drive motors to reduce long-term maintenance demands for the SSJID. This required designing each of the system’s 1-hp motors to effectively drive over 60,000 pounds of modules and steel as it follows the trajectory of the sun. Using a 30-ton screw jack and engineered counter balance provided the solution. Using software with origins based in military operations further optimized the system’s solar tracking capabilities.

First Solar’s thin film modules have a power capacity of 72.5 W each, thus requiring double the number of modules per subarray in Phase Two compared to the higher capacity 175 W crystalline modules in Phase One. The added labor costs for installing more thin film modules may have exceeded any realized savings over the crystalline option. Module durability was also a consideration. The tracker rack had to be extremely stable in order to keep module vibration to a minimum, allowing Conergy to meet the requisite quality assurance standards to maintain First Solar’s warranty. The conclusion, however, is that the module selection coupled with the module-torail attachment method brought First Solar thin film modules to cost parity with crystalline modules—and with enhanced energy output.

The final challenge was interconnection. California Public Utilities Commission rules mandate that only 1 MW of solar can be installed per meter, yet the final system size was nearly 1.6 MW. After deliberation with the utility, the water treatment plant was allowed a second meter. PV output was engineered to accommodate SSJID’s range of power needs while ensuring proper loading of each meter.

The project’s main objective is to deliver water at the lowest possible cost to SSJID customers by stabilizing the water treatment plant’s electrical outlay. The project achieves this goal by providing nearly all of the energy required to operate the facility. The completed PV system is expected to save the SSJID approximately $400,000 in annual electricity costs.

"Water is an increasingly precious commodity in drought-ridden California, where Governor Schwarzenegger has declared a state of water emergency. The SSJID PV plant cuts water processing and delivery costs and should serve as a bellwether for water authorities everywhere, particularly in these economic times."

David Vincent, Conergy Projects Group


DESIGNER: Rick Holz, director of engineering, Conergy Projects Group,
PROJECT DEVELOPER: David Vincent, western US project development manager, Conergy Projects Group
DATE COMMISSIONED: Phase One, May 2008; Phase Two, March 2009
INSTALLATION TIMEFRAME: Phase One, 4 months; Phase Two, 3 months
LOCATION: Oakdale, CA, 37.9°N
SOLAR RESOURCE: 7.1 kWh/m2/day
ARRAY CAPACITY: Phase One, 1.176 MW; Phase Two, 418 kW
AVERAGE ANNUAL AC PRODUCTION:Phase One, 2,100 MWh; Phase Two, 800 MWh

Equipment Specifications

MODULES, PHASE ONE: 6,720 Conergy 175MU, 175 W STC, +3%/-3%, 4.95 Imp, 35.2 Vmp, 5.2 Isc, 44.2 Voc
MODULES, PHASE TWO: 5,760 First Solar FS-272, 72.5 W STC, +5%/-5%, 1.07 Imp, 67.9 Vmp, 1.19 Isc, 90.0 Voc
INVERTERS: 3-phase, 480 Vac inverter output stepped up and interconnected at 4,160 V, seven Xantrex GT250, 250 kW, 600 Vdc maximum input, 300–600 Vdc MPPT range
ARRAY, PHASE ONE: Four 1,440 module subarrays and one 960 module subarray with 12 modules per string (2,100 W, 4.95 Imp, 422.4 Vmp, 5.2 Isc, 530.4 Voc), 120/80 circuits total (252,000/168,000 W, 422.4 Vmp, 594/396 Imp, 624/416 Isc, 508.2 Voc)
ARRAY, PHASE TWO: Two 2,880 module subarrays with six modules per string (435 W, 1.07 Imp, 407.4 Vmp, 1.19 Isc, 540 Voc), 480 circuits total (208,800 W, 513.6 Imp, 407.4 Vmp, 571.2 Isc, 540 Voc)
ARRAY INSTALLATION: Proprietary single-axis tracker by Conergy, 180° azimuth, east–west tracking from 45° to -45°

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