In May, Gonzalez persuaded the Board of Supervisors to green-light a $2 million pilot project to investigate how well electricity could be extracted from the 400 billion gallons of water that rush in and out of the bay each day. The city's Department of the Environment is expected to begin soliciting proposals for the project later this year.
Though it's sometimes criticized as a dreamy approach to meeting San Francisco's energy needs, tidal power has been exploited by humans since the eighth century, when ebbing and flowing seawater was used to grind grain along the coasts of Britain, France, and Spain. Flood tides filled man-made ponds; ebb tides turned water wheels, producing mechanical power to mill grain.
In more recent years, turbines have been submerged in bays, estuaries, and rivers, where moving water turns their blades and generates electricity. But such devices have serious technical problems. Among them: Fish and other marine creatures get chewed up by spinning blades, and salt water erodes metal parts, causing expensive maintenance headaches.
A British company, HydroVenturi Inc., believes it has found a neat way around such difficulties, and it intends to bid for the San Francisco tidal project. The firm has designed a concrete box, 30 feet by 30 feet by 40 feet, filled with non moving vertical blades that resemble airplane wings. In airplanes, the curved side of the wing faces up, and the movement of air over it creates lift. In the bay, water washing past the curved vanes would create a vacuum that sucked air through concrete pipes connected to turbines on land. The airflow would turn the turbines' blades and produce electrical power. (Much of the air drawn underwater would be compressed and fed back into the turbines to give them an extra kick.)
Each HydroVenturi turbine generates only two megawatts of electricity, enough to fuel 1,500 average homes. But since they are about the size of an office desk, many of the machines can be clustered together in a small space, says HydroVenturi CEO Joseph Neil. A turbine-stuffed building the size of an ordinary house could yield 500 megawatts, and two such buildings, he contends, could more than supply San Francisco's daily consumption of about 800 megawatts.
Barges would sink the concrete boxes in areas of the bay where tides move the fastest. Likely spots include both sides of the Golden Gate Bridge, off the Presidio and Alcatraz, in Raccoon Strait between Angel Island and Tiburon, and off the Brother Islands near Point Richmond. The boxes are small enough that they wouldn't disrupt shipping or fishing. And since they have no moving parts, marine creatures shouldn't be endangered or normal tidal patterns altered, Neil says.
He compares the blade-filled boxes to a "big 3-D comb" or the baleen through which certain whales filter their food. Their vanes, he says, are about two feet apart, so fish can swim through them unharmed. He doesn't believe the boxes will disrupt plankton or change the topography of the bay floor, since water and silt will flow through and around them.
HydroVenturi already has a tidal demonstration project operating at Grimsby on England's northeastern coast. U.K. environmental regulators issued permits for the plant and so far, Neil says, "we haven't seen any impact at all" in tests of how it's affecting marine life.
Unlike solar or wind power, tidal power is as reliable and predictable as, well, the rise and fall of the tides every day. And unlike imported Mideast oil, it's not subject to interruptions caused by wars, boycotts, or other international conflicts.
How much would tidally generated juice cost? Neil says the price tag will be roughly equivalent to that of electricity derived from coal, nuclear fission, and natural gas. "We're very competitive because we don't have fuel costs, and we have much lower maintenance costs," he says, adding that costs should fall over time as the system is expanded.
Neil estimates it would take about 20 years to build out a tidal system big enough to supply San Francisco's current needs. (The precise timeline, he says, depends on how much political will there is to expand the system and how much monitoring is needed to determine that expanding it won't harm the marine environment.) The cost: about $1.5 billion, he says, which could be financed with municipal bonds.
A bond issue that big, however, would trigger serious questions among voters, notes Ann Marie Harmony, CEO of Practical Ocean Energy Management Systems Inc., a San Diegobased nonprofit that promotes tidal power. "Voters are going to go, 'What's the precedent?'" she says.
The short answer: There isn't one.
Most tidal generators involve damming a river or bay or placing turbines, levers, or other moving devices in a waterway. The world's oldest and largest tidal plant -- a 240-megawatt station in northern France -- draws power from water passing through a barrier at the mouth of the La Rance River. Submerged turbines are in use in New York's East River and off the coast of Norway.
But there's nothing like what HydroVenturi wants to build, with turbines on land.
"HydroVenturi so far is a unique design," says Harmony. "Being a pioneer, you get arrows in the back, not the front. When you call for a bond issue of $1 billion, that's pretty pioneering."
Neil says studies of the bay's water volume and speed indicate it contains an enormous amount of kinetic energy -- enough to convert into as many as 30 gigawatts a day. (The entire nine-county Bay Area consumes about two gigawatts daily.) Given technical limitations and environmental concerns, he contends, HydroVenturi could produce one gigawatt, if enough boxes and turbines were clustered together. Theoretically, that power could be shared among Bay Area cities, but Neil says he's committed to supplying San Francisco first.
But can San Francisco unilaterally exploit the waters of the bay? Could it be required to share its electricity harvest with other Bay Area cities?
Department of the Environment spokesman Mark Westlund notes that installing a tidal energy system would require a blizzard of permits and environmental impact reports. No fewer than 17 local, state, and federal regulatory agencies, he says, have jurisdiction over various aspects of the bay.
Among them is the U.S. Army Corps of Engineers, which oversees construction projects on all navigable waters in the United States. Corps spokesman Doug MaKitten says HydroVenturi's boxes would require "extensive" review of their impact on water quality and endangered marine species. Such reviews and public hearings could take months, if not years, he says.
MaKitten says a project like HydroVenturi's would probably require wide agreement among government regulators, as well as congressional approval. "Anything of this magnitude would involve consensus-building," he says. "I don't know how it could unilaterally happen."
In any event, the Department of the Environment strongly supports tidal energy. Westlund says it would help meet San Francisco's goal of cutting greenhouse gases 20 percent below 1990 levels by the year 2012. And it meshes well with the city's drive to reduce its use of nonrenewable energy sources, symbolized by its program to place solar panels on 20,000 rooftops.
Neil's company is likely to face competition from other firms for the contract to build San Francisco's pilot project. But he told SF Weekly that HydroVenturi is willing to pick up the tab for it.
What happens to the whole lovely scheme if Gonzalez is defeated in the mayor's race? He's closely identified with the notion of tidal power, and he's consistently ranked only fourth in recent polls. Would his political ideas -- including tidal energy -- be discredited in the eyes of voters if he loses? Would a tidal bond issue be doomed as a result?
Neil doesn't think so. While he appreciates Gonzalez's leadership, he believes that tidal power's future doesn't hinge on the career fortunes of any particular politician. It is simply an idea whose time has come.
"I don't think it matters who's in power," he says. "They're going to have to come to terms with the fact that we need alternative sources of energy.
"I think you'll find that any serious candidate will look at what we're doing and think he should be there as well."