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A combination of factors, including President Donald Trump’s tariff on imported solar panels, have prompted cancellation of a major solar power project on six wastewater holding ponds in Sonoma County.

Sonoma Clean Power, the county’s public power supplier, also cited requirements by PG&E and the state Division of Safety of Dams as reasons for terminating a contract approved in 2015 for development of a 12.5-megawatt solar power system on the holding ponds owned by the Sonoma County Water Agency.

The developer, San Francisco-based Pristine Sun, missed its latest deadline to complete the project March 31, prompting Sonoma Clean Power to cancel the deal five days later, said Deb Emerson, the electricity provider’s director of power services.

“The costs (for Pristine) were just adding up,” she said. “It was just becoming more difficult to see how this could actually come to fruition.”

The local nonprofit agency, which provides electricity to about 225,000 accounts in Sonoma and Mendocino counties, had committed to buying power from the proposed floating power system and lost no money in canceling the deal, she said.

Emerson and Mark Landman, Sonoma Clean Power’s vice chairman, said they were still interested in the concept of placing solar panels, which some people consider unsightly, atop reservoirs of treated effluent that are generally out of sight and have no other use.

“It’s an evolving technology,” Landman said, that is “still very promising” for Sonoma County.

The mix of factors that thwarted the six-pond plan with Pristine Sun was “a perfect storm” in a negative sense, said Landman, the mayor of Cotati.

Pristine Sun could not be reached for comment Monday.

When the deal with Pristine Sun was announced in February 2015, Sonoma Clean Power hailed it as the largest floating solar project in the nation, with the capacity to power 3,000 homes. Emerson said Monday she was not able to say if that was still the case.

It was initially supposed to have been completed by the end of 2016.

Around the world, floating solar power is gaining momentum, with China in the process of starting up a 150-megawatt plant over a flooded coal mining region, supplanting a 40-megawatt plant switched on last year in the same area as the largest waterborne array in the world.

Far Niente Winery in Napa Valley installed a 400-kilowatt system on a vineyard irrigation pond in 2008, supplying enough energy to run the winery.

Windsor is pursuing a floating solar array at its largest wastewater storage pond located at the sewage treatment plant off Windsor Road, and the Sonoma County Water Agency is still engaged with Pristine Sun on development of a 1-megawatt system at its Oceanview reservoir off Mark West Station Road.

But Dale Roberts, a principal engineer with the water agency, said Pristine has encountered the same obstacles as it did with the Sonoma Clean Power project and missed the last deadline in September.

“We’ve been very forgiving,” he said, adding that the agency is negotiating a new deadline.

“We’re questioning whether they really are going to be able to finish it without time extensions or an increase in price,” Roberts said.

Trump’s tariff, announced in January, applied up to a 30 percent duty on solar equipment manufactured abroad, a move that reportedly threatens to handicap a $28 billion industry that relies on imported parts for 80 percent of its supply.

Roberts said that Pristine Sun has indicated it “can shoulder that burden for now,” noting that materials amount to a relatively small portion of solar power development costs.

The major obstacle for Sonoma Clean Power’s project was the unexpected cost of improvements PG&E said would be needed to connect the floating arrays to the power grid, Emerson said.

The state dam safety agency also required the project to meet unexpected standards for anchoring the solar arrays in the reservoirs, she said.

Sonoma Clean Power remains “interested in the concept,” she said, but is not currently looking for a developer to take Pristine Sun’s place.

County Supervisor Lynda Hopkins, a Sonoma Clean Power board member, said it was a “fantastic idea” to put solar power facilities on otherwise unusable space, like wastewater ponds, and far better than locating them on agricultural land.

You can reach Staff Writer Guy Kovner at 707-521-5457 or guy.kovner@pressdemocrat.com.