Water on the Brain

One energy executive claims a tidal system that meets the city's electric needs could be built in over 20 years, at a cost of $1.5 billion

Buried in the avalanche of rhetoric in this fall's mayoral race is one of the more interesting ideas to emerge from local politics in some time: harnessing tidal movements in San Francisco Bay to meet the city's electricity needs. Its champion is Board of Supervisors President Matt Gonzalez, a Green Party mayoral candidate who's not afraid to raise unorthodox concepts while his opponents jaw about filling potholes, fixing schools, and ginning up more jobs in the dot-bomb economy.

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.

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