SAN FRANCISCO -- While giant wind farms create electricity in the hills high above the San Francisco Bay, Mayor Gavin Newsom foresees a time when similar turbines would be placed deep under the Golden Gate Bridge or off-shore at Ocean Beach to produce power using the surging wave and tides.

Newsom told KTVU Tuesday that the idea was "not some crazy mayor from San Francisco talking about a tomorrow that will never come."

"It's simple," Newsom said of the plan. "Sustainable underwater power plants -- 100 percent renewable energy harnessing the energy of wave power as well as tidal power. Just imagine a giant tube being placed at the base of the San Francisco Bay underneath the Golden Gate Bridge."

Newsom said such a generating device would be 190-200 feet below the surface and would not impact marine life or freighter traffic. It could create enough power to handle the electric needs of 8-10 percent of all the homes in the city.

The mayor said he was meeting with manufacturers and others -- including officials from Pacific Gas & Electric -- at a meeting on Wednesday to discuss the plans.

The idea of such generating plants comes from a report by the Electric Power Research Institute.

Its study concluded that tidal power could produce electricity for 4.2 to 6.5 cents per kilowatt-hour at three locations in Maine. For comparison's sake, the standard offer for Central Maine Power customers is 8.4 cents per kilowatt hour.

"I was shocked when my team came up with those cost-of-electricity numbers. I thought, 'It can't be,'" said Roger Bedard, the project leader in Palo Alto.

Bedard's team provided a detailed analysis of the technological and economic feasibility of tidal resources at one site apiece in Maine, Massachusetts, San Francisco, Washington and Alaska, and in two Canadian provinces, New Brunswick and Nova Scotia.

In Maine, the study focused on the Western Passage in Passamaquoddy Bay, where twice a day the tide rises and falls 20 feet, the greatest tide change in the continental United States. The other two sites with the greatest potential were at San Francisco's Golden Gate Bridge, and in the Bay of Fundy at Minas Pass, Nova Scotia, Bedard said.

Tidal power appeals in several ways.

Unlike the wind, tides are predictable, for starters. And water's greater density means fewer turbines are needed to produce the same amount of electricity as wind turbines. Since they're underwater, there aren't aesthetic issues associated with wind farms.

The study focused on locations like rivers and other narrow passages where the tidal flow is concentrated, creating the greatest potential for affordable electricity.

The tidal-powered turbines are much like wind turbines, but their blades turn more slowly. There are several types, some of which look like two- or three-bladed wind turbines. Others look like a cylinder surrounding multiple blades. All are attached to the ocean floor. An underground cable would connect to a substation on shore.

The EPRI study focused on large, commercial turbines that make one revolution every six seconds. That should be slow enough for fish to escape harm, and the theory will be tested later this year in New York's East River and in the United Kingdom, Bedard said.

For all its promise, the obvious problem for developers is the regulatory process, since no one has ever obtained a permit for a commercial-scale project and the current regulatory scheme was designed for conventional hydroelectric projects, Bedard said. But that isn't stopping companies from rushing to file proposals with state and federal regulators to get their foot in the door.

So far, the Federal Energy Regulatory Commission has received 21 preliminary permit applications for tidal power projects, said spokeswoman Celeste Miller. Ten have been granted and the others are pending, Miller said.

The technology is in its infancy, similar to wind power 20 or 25 years ago. And the permitting process will have to evolve, as well.