Thanks to a power purchase agreement with a solar energy company and incentives from the local electricity provider, the City of Sacramento (Calif.) Department of Utilities expects to save up to $57,000 a year and prevent 54 million pounds of carbon dioxide emissions over the next 20 years by using solar energy.

In 2007, the city council adopted its first sustainability master plan, setting greenhouse gas emission reduction goals. The city manager identified solar power projects as one way to achieve those goals. That same year, project rebates with Sacramento Municipal Utility District (SMUD) started a revolution in solar energy in the city that continues today. Once the city saw success with four solar arrays on buildings, the drinking water utility jumped in.

“There was a push in our department to find green energy sources,” says James Christensen, senior electrical engineer with the Department of Utilities, which provides water to the city. “But we had to identify projects that wouldn’t have a negative impact on the overall operations and maintenance of the drinking water treatment system.”

Cooperative contract

Sacramento signed a power purchase agreement with SolarCity, leasing the land to the company, which designs, constructs and maintains the solar arrays for 20 years. The city buys the generated power at an agreed-upon rate with a 2 percent annual escalator, expected to cost less than buying that same power from SMUD. The utility also expects to pay lower demand charges to SMUD over time.

“The exact amount of savings depends on future rate increases by SMUD, but even if grid power rates remained unchanged for the next 20 years, the water utility would still pay less for the electricity generated by the solar arrays,” Christensen says. At an estimated annual rate increase of 3.5 percent on grid power, the water utility could save up to $57,000 annually.

To serve a population of 466,500, the Sacramento water utility operates two conventional drinking water treatment plants (Sacramento and Fairbairn) that produce a combined average of 120 mgd. Twenty-seven wells produce an additional 25 mgd. Surface water from the Sacramento and American Rivers is combined with groundwater before being treated and distributed. The staff of 141 includes operations, distribution and maintenance. The annual operations and maintenance budget is $100 million.

Ample space

Ample space was available for the solar arrays, which ultimately were placed on the open areas above two underground storage tanks next to the treatment plants. There were initial concerns that the weight of the arrays might be dangerous to the tanks below. “Our structural engineers evaluated the storage tanks to make sure the weight above wouldn’t cause a failure,” says Dave Hansen, supervising engineer. “Even the Department of Health was involved to ensure there were no water-quality issues with having the array in that location.”

In April 2013, two solar arrays of fixed, ground-mounted panels, went online to provide a total of 1,485 kW DC power. One array includes 1,428 Yingli Green Energy solar panels and a 300 kW Solectria Renewables inverter; it generates up to 371 kW DC, or 3 percent of the Sacramento treatment plant’s energy needs. The second array includes 4,284 Yingli panels and two 500 kW Xantrex inverters (Schneider Electric) that generate up to 1,114 kW DC, 12 percent of the Fairbairn plant’s needs.

Operators as stakeholders

Operations and maintenance staff were active in every step of planning and execution for the solar arrays. “Operators are part of our key stakeholders because they have to live with the project and operate the plant,” Hansen says. “We definitely wanted their input as early as possible.”

There were preliminary meetings with maintenance, operations and engineering staff to review the expectations and rationale for the project. Once construction began, the maintenance team supported the SolarCity staff, such as when tying into medium-voltage switchgear. As part of the power purchase agreement, SolarCity paid Sacramento’s staff costs to help manage the project, get the plans through city council and coordinate construction.
Project managers worked with operators daily to make sure the solar array installation wasn’t affecting day-to-day water treatment operations. At times, the installers had to shut off power and do cutover connections, and that had to be coordinated with maintenance staff. Throughout the project, operators were eager to learn how the construction would affect their responsibility to provide a reliable water supply to the community.

“The operators saw the advantage to saving energy and doing things that benefit the environment. They are always open to new ideas,” says Hansen. “But they wanted to make sure they could still operate the plant, which is their primary concern. Everyone was aware of that goal and worked hard to make sure that happened. It was a big team effort.”