Power outages lasting minutes to hours at the Nick C. DeGroot Water Treatment Plant in Oakdale, Calif., were causing treated water storage problems — and the facility has a pair of storage tanks each holding 3 million gallons.
The board of directors of South San Joaquin Irrigation District, which operates the facility, wanted plant personnel to get comfortable with solar energy and eventually apply it across the district. A solar system seemed the ideal solution to DeGroot’s problems.
Together with Pacific Gas and Electric (PG&E) and Denver-based Conergy Americas, the district planned the Robert O. Schulz Solar Farm, a solar array estimated to produce 3 million kWh per year to offset energy costs and stabilize the power supply to the water treatment plant. The farm is believed to have the world’s first single-axis solar-tracking system using mono-crystalline and thin film and photovoltaic cells.
Zero carbon
The water treatment plant uses membrane technology to process 40 mgd for the district’s 155,000 residents. The district also allocates 230,000 acre-feet of water annually to irrigate 55,000 acres for 2,400 customers. The irrigation season is mid-March through mid-October.
Water from the Stanislaus River gravity feeds through five sequential reservoirs with three dams with capacity for 130 MW of hydroelectric power. “Between our solar farm and the dams, we have zero carbon resources,” says general manager Jeff Shields. California requires 20 percent of utility power supplies to be renewable by 2010, and 33 percent by 2020.
PG&E allows only 1 MW of energy production through one meter, forcing Conergy to build the solar farm in two stages. Phase I cost $8 million and Phase II, with a completely redundant conduit system, cost $3.5 million.
Phase I has 6,720 175-watt monocrystalline modules projected to generate 2.232 million kWh of AC power. Phase II, projected to produce 800,000 kWh, has 75.5-watt direct current (DC) thin-film modules from First Solar in Tempe, Ariz. Both arrays are mounted on single-axis tracking systems.
“Monocrystalline technology is the workhorse of the solar industry, and we bid both phases with it,” says Shields. “However, the modules generate less power in temperatures above 100 degrees or when there are reductions in irradiance, such as clouds and dust.”
Thin-film modules are reportedly less affected by these conditions, but have only a six-year history. Conergy, First Solar and the district saw an ideal situation to compare the two technologies under identical conditions.
Sunflower effect
The single-axis solar-tracking systems added 15 percent to the cost of the project, but produce 15 percent to 18 percent more energy than fixed arrays. The solar farm’s I-beam steel structures are cemented in the ground. A timer on the system moves the panels about one degree every four minutes.
The solar array occupies 14 acres of a 40-acre site, 300 feet from the water treatment plant. “As the modules rotate east to west, they cast a shadow on their neighbor if you don’t leave space between them,” says Shields. “Seven 1-hp motors drive a train serving 200 kW, and we have 1,400 kW out there.” The motors are considered parasitic load because solar energy powers them.
Five inverter boxes for Phase I and two for Phase II convert direct-current solar-generated electricity to alternating current. Inverters are rated at 250 kW each. To feed the 480-volt current into PG&E’s system, three transformers step it up to 4,160 volts. A 40 kVA transformer handles the house load: motors, lights, security system and miscellaneous equipment.
After energizing the water treatment plant, excess electricity feeds into PG&E’s grid. “Once a year, we do a true-up with them,” says Don Battles, utility systems director. “Phase I went on-line in May 2008 and Phase II late this March, so we owed some money on June 15, the end of our reconciliation period.”
Conergy didn’t size the solar farm to meet the energy demand of the water treatment plant. It was sized to offset energy payments to PG&E. When the water treatment plant opened in 2005, power cost $370,000 per year. The model for the solar farm projected electricity to cost $410,000 in 2008. “To balance our bill, we had to know how much energy to put back into the system based on the amount PG&E pays us for excess energy,” says Battles.
Coming clean
The district has a 10-year maintenance contract with Conergy. Twice per year, technicians check the motors, electrical connections and module interconnections. During the six months of summer, two district employees drive between the rows of solar arrays with a truck-mounted pressure washer and clean them using 5 gpm/40 psi. Pipes laid under the tracking system transport treated water from the plant.
“The men start at daybreak, while the panels are angled so water runs off, and work until 11 a.m.,” says Shields. “It takes 10 days to clean the arrays.” Because the district’s water source is melt water from the Sierra Mountains, there is no streaking or mineral film buildup. Rain does the job the rest of the year.
The district is experimenting with sprinkler technologies to reduce the time and cost of cleaning the modules, and is purchasing a major new thermal detection security system. The solar farm now has a fence and beam security system that sends alarms to the water treatment plant. Employees have responded to one genuine alert and several false alarms.
San Jose-based Fat Spaniel Technologies Inc., the solar farm’s qualified monitoring entity, reports net production to PG&E and the Western Renewable Energy Gener-ation Information System for green energy certificates. “We have quite a number of green tags and can market them to Oregon, Arizona and the state of Washington,” says Battles. “Their economic value is $7 to $24 per megawatt-hour.”
The district originally projected 14 years to pay off the solar farm, but escalating electricity prices, solar rebates and renewable credits have reduced that to 10 years or less for a project that will generate all necessary electricity for 25 years. “The farm is meeting or exceeding our expectations for production,” says Shields.
