Riding the waves of hydropower
› firstname.lastname@example.org 
GREEN CITY Turning the tides that flow through the Golden Gate into a source of clean, renewable energy was contemplated long before Mayor Gavin Newsom partnered up with Pacific Gas and Electric Co. to announce the latest study (see "Turning the Tides," page 11), even before Matt Gonzalez proposed the idea in his 2003 race against Newsom. Tidal power is an old concept now getting a new push, thanks to the climate change threat and the unique dynamics of San Francisco.
An independent study by the nonprofit Electric Power Research Institute was conducted last year to assess the feasibility of tidal energy in North America and concluded that the Golden Gate is "the second largest tidal in stream energy resource" on the continent. A combination of the Golden Gate's powerful currents and its proximity to existing power infrastructure makes San Francisco the most promising site for a tidal energy pilot project in the lower 48 states.
However, the EPRI's analysis revealed the Golden Gate's tidal power potential to be far less than the 1,000 megawatts first mentioned by Gonzalez, which would have more than covered the city's annual energy needs. The EPRI estimates that the 440 billion gallons of water in the Golden Gate's tidal stream hold a total of 237 megawatts of energy. The study also suggests that a tidal program in San Francisco could only safely extract 35 megawatts of that available energy without negatively affecting the surrounding environment.
At 35 megawatts, tidal power would meet roughly 4 percent of the city's energy demands. Internal San Francisco Public Utilities Commission documents obtained by the Guardian revealed that SFPUC officials lack confidence in those numbers and place the estimate at only 1 percent of the city's energy needs.
Regardless of the potential output, the major challenge is still establishing the proper technology to safely harness the power of the tides.
Tidal power, much like hydropower, harnesses the energy of water currents to create electricity. In the case of tidal power, the force of the ocean currents generated by the rise and fall of the tides spins turbines placed underwater.
La Rance Tidal Power Plant in France, operating since 1966, is the oldest such system in the world. It generates 240 megawatts of power a day, which is enough to cover 90 percent of Brittany's demand. At 3.7 cents per kilowatt hour, the electricity generated by La Rance is among the most affordable in France, which relies heavily on nuclear power.
However, La Rance like Canada's Annapolis Royal Generating Station, built in 1984 is essentially a hydroelectric dam that spans a river, capturing and releasing the tides, so it's not a viable design for San Francisco. A tidal power project at the Golden Gate would have to be largely submerged to leave vital shipping lanes unobstructed. So far, there is no existing tidal power program similar to the one being proposed for San Francisco. There are many tidal technology projects under development around the world that use partial and completely submerged systems that could be compatible with the Golden Gate. None has a model that's seen commercial use, except Verdant Power, which has a single test turbine submersed in New York City's East River that powers a nearby parking garage and supermarket.
The EPRI study evaluated eight possible turbine designs for San Francisco. Among these designs, the maximum output per turbine is two megawatts. The installation and maintenance of a project using several of these turbines would not only be inherently expensive but also require the heavy lifting of barges, cranes, drills, and derricks as well as ongoing activity that likely would affect what went on above and below the surface of the sea.
Many of these turbine designs involve spinning blades, which can threaten marine life. The tides are also essential for transportation and the distribution of silt. A pilot project would address these challenges, perhaps demonstrating whether the planet's natural flows can offer another key to slowing its warming trend.*
Comments, ideas, and submissions for Green City, the Guardian's weekly environmental column, can be sent to email@example.com.