PETALUMA, Calif. -- Solar panels have sprouted on countless rooftops, carports, and fields in northern California.
Now, several start-up companies see potential for solar panels that float on water.
An array of 144 solar panels sits atop pontoons moored on a three-acre irrigation pond surrounded by vineyards in Petaluma in Sonoma County. About 35 miles to the north, in the heart of the Napa Valley, a 994-panel array floats on a pond at the Far Niente Winery.
"Vineyard land in this part of the Napa Valley runs somewhere between $200,000 and $300,000 an acre," said Larry Maguire, Far Niente's chief executive. "We wanted to go solar but we didn't want to pull out vines."
The company that installed the two arrays, SPG Solar of Novato, Calif., and Sunengy of Australia and Solaris Synergy of Israel are among the companies trying to develop a market for solar panels on agricultural and mining ponds, hydroelectric reservoirs, and canals. Although it is a niche market, it is potentially a large one globally.
The solar panel aqua farms have drawn interest from municipal water agencies, farmers, and mining companies, solar executives said.
Sunengy, for example, is courting markets in developing countries that are plagued by electricity shortages but have abundant water resources and intense sunshine, Philip Connor, the company's co-founder and chief technology officer, said.
Chris Robine, SPG Solar's chief executive officer, said he had heard from potential customers as far away as India, Australia, and the Middle East.
Sunengy, based in Sydney, said it had signed a deal with Tata Power, India's largest private utility, to build a small pilot project on a hydroelectric reservoir near Mumbai.
Solaris Synergy, meanwhile, said it planned to float a solar array on a reservoir in the south of France in a trial with the French utility EDF.
MDU Resources Group, a $4.3 billion mining and energy infrastructure conglomerate based in Bismarck, N.D., has been in talks with SPG Solar about installing floating photovoltaic arrays on settling ponds at one of its California gravel mines, according to Bill Connors, MDU's vice president of renewable resources.
"We don't want to put a renewable resource project in the middle of our operations that would disrupt mining," Mr. Connors said.
"The settling ponds are land we're not utilizing right now except for discharge, and if we can put that unproductive land into productive use while reducing our electric costs and our carbon footprint, that's something we're interested in."
He declined to discuss the cost of an SPG floating solar array.
But he noted, "We wouldn't be looking at systems that are not competitive."
SPG Solar's main business is installing conventional solar-power systems for homes and commercial operations.
It built Far Niente's 400-kilowatt floating array on a 1.3-acre pond in 2007 as a special project and for the last four years has been developing a commercial version called Floatovoltaics that executives say is competitive in cost with a conventional ground-mounted system.
The Floatovoltaics model being brought to market by SPG Solar is the array that bobs on the surface of the Petaluma irrigation pond.
"We have been able to utilize a seemingly very simple system, minimizing the amount of steel," said Phil Alwitt, project development manager for SPG Solar, standing on a walkway built into the 38-kilowatt array.
Long rows of standard photovoltaic panels made by Suntech, a Chinese solar manufacturer, sit tilted at an 8-degree angle on a metal lattice fitted to pontoons and anchored by tie lines to buoys to withstand wind and waves.
The array will be hooked up to a transmission line through a cable laid under the pond bed. Mr. Alwitt said that when the array is completed, 2,016 panels would cover most of the pond's surface and generate 1 megawatt of electricity at peak output.
He noted that the cooling effect of the water increased electricity production at the Far Niente winery by 1 percent over a typical ground-mounted system.
Solaris Synergy, the Israeli firm, claims that installing its floating solar arrays on the 400-mile California Aqueduct could produce up to two megawatts of electricity per mile, providing power for irrigation.
And SPG Solar executives said they held preliminary discussions with state officials about putting solar panels on the aqueduct.
Ralph Torres, deputy director of the state water project, said he had recently spoken with Solaris, the latest of many companies that he said had approached his agency over the years about installing solar panels.
"You would really have to anchor these solar arrays so they wouldn't float away," Mr. Torres said.
"If you do spring a leak and have to go in quickly these panels would be in the way and you might damage or destroy them when responding to an emergency."