A California City Tries Different Approaches to Renewable Energy. Here Are the Two That Produced.

Sonoma Water has tried a variety of renewable and carbon-free energy sources. Solar energy and hydropower have proven successful.

A California City Tries Different Approaches to Renewable Energy. Here Are the Two That Produced.

Sonoma Water tested a 10-kW floating solar array on a recycled water pond to test the feasibility of a larger floating array at the site.

As Sonoma Water developed renewable and other carbon-free energy sources, the utility learned what does and doesn’t work in the wine country north of San Francisco.

The Sonoma County Water Agency treats wastewater for more than 75,000 people with eight sanitation zones and districts, including seven small plants, and is a water wholesaler to water retailers serving more than 600,000 people. It achieved its goal of using entirely carbon-free energy sources in 2014.

Dale Roberts, principal engineer, says the agency has numerous renewable energy projects and another one in development, but not all have been successful. A geothermal project, for example, was explored but never developed. A chicken manure digesting plant that could produce biogas never got off the drawing board. A small wind turbine is still working, but not well enough to justify putting up more of them.

Pooling resources

The two things that have undeniably worked are solar projects and the pooling of hydroelectric resources with other utilities. In 2004, Sonoma Water and utilities such as the Santa Clara Valley Water District and irrigation districts in California’s Central Valley created the Power and Water Resources Pooling Authority.

“We pooled all of our power resources together because we all have an allocation of Sierra Nevada hydropower, as many water districts do,” Roberts says. “It allows us to buy and sell that power among ourselves.

“The irrigation districts mainly have a power load in the summer, when they are delivering water to irrigators, whereas we and other municipal water districts have power demands year-round. This gives the irrigation districts a secure buyer in the winter, and it gives the rest of us an inexpensive and carbon-free power source the rest of the year.”

Power from large hydroprojects is not considered renewable, but it is carbon-free. Sonoma Water gets renewable power from a variety of solar projects and a small hydroplant and through a community choice aggregation entity known as Sonoma Clean Power, which offers the option to buy a renewable power portfolio.

Varied sources

Sonoma Water also generates power from a 1.4-MW hydroplant at Lake Sonoma. It has 2 MW of solar power in operation and another 1 MW under development. One solar array is at Geyserville, where Sonoma Water has a 92,000-gpd (average) wastewater treatment plant.

Geyserville also has a wind turbine. “We put up a very small 5-kW wind turbine as part of a California Energy Commission study to evaluate local renewable resources,” Roberts says. “It doesn’t produce very much, maybe 1 percent of the treatment plant’s power. The turbine was in a valley that gets an upstream wind in the afternoon, but we needed the wind to get to 9 mph, and we only got 7 or 8.”

Solar power did work well at Geyserville. A 40-kW array on the embankment of the plant’s lagoon provides almost all the power the plant needs. The agency has three other solar plants, which it owns. “That seemed to be the cheapest life cycle cost,” Roberts says. “We had the capital in our power fund, so we purchased them.”

Panels on a pond

A new project, a 1-MW floating solar array on a recycled water pond in Oceanview near Santa Rosa, is in development. If it is built, Sonoma Water will buy the power under a power purchase agreement. The developer, Cratus Energy, plans to lease the surface of the pond, which stores the effluent of a 900,000-gpd wastewater treatment plant. Treated water is stored in the pond until it is used to irrigate vineyards and hayfields.

Floating solar power is more complicated to develop than land-based system, Roberts says. For the Oceanview project, a 10-kW pilot system operated from September 2016 until March 2017, which turned out to be the wettest winter on record in Sonoma County. The pilot survived, so the project is still a contender for the agency’s renewable power portfolio.

No-go digester

Cost issues have derailed some energy projects. Sonoma Water serves the city of Petaluma, once known as the world’s egg basket because of its poultry industry. A developer proposed a chicken manure digester that could produce biogas — an appealing idea since using local resources is a hallmark of sustainability.

“The developer was going to aggregate manure from the chicken farms and haul it to a central location,” Roberts says. “They couldn’t get it off the ground to make it cost-effective. With solar and natural gas prices dropping, they couldn’t compete, so they walked away from it.”

Sonoma Water continues to look for ways to produce cost-effective carbon-free power.

Electric rates are becoming more variable depending on the time of day, and the low-cost times aren’t necessarily what people would expect. “In California, there’s so much solar that the middle of the day, which used to be the most expensive time, is now the cheap time. The evening, when all the solar power is going offline, is becoming the expensive time.

“We don’t want to get hit hard by that, so we’ve been looking at energy storage and load shifting. Instead of just pumping steadily throughout the day, we could just pump a ton in the middle of the day when power is cheap and stop pumping in the evening when prices go back up. We’ve experimented with that. We’re ready for it.”


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