If an experimental system becomes a reality, residents of San Francisco might soon need scuba gear to watch their city's source of electric power in action.

West-coast utility company PG&E ( PCG), along with Oceana Energy Company and the city of San Francisco, is currently conducting a study to see if the Bay Area's notoriously strong tidal currents can be harnessed to generate energy.

The technology for the project is being developed by Oceana and the U.S. Navy. Although the specifications are protected by a confidentiality agreement, it is likely to involve a system that uses what's known as hydrokinetic energy.

Tidal power turbines use ocean and river currents to generate electricity, much the same way that wind turbines do. Tidal energy is generated in two ways. Hydrokinetic energy is produced when spinning turbines are placed directly in a water current or stream. Barrage energy takes advantage of the ebb and flow of tides to create a potential energy differential. Gravity ultimately pulls water from its level at high tide down to its level at low tide, generating electricity in the process.

The scale of the project in San Francisco, as well as the potential amount of electricity that could be generated, is not yet known.

Although not widely used, tidal energy infrastructure could provide a number of benefits over other renewable energy sources, according to PG&E. For one, tidal energy is highly predictable and reliable, unlike solar power, which only works during the daytime, and wind power, which functions poorly without a steady supply of wind.

Also, hydrokinetic technology is not visible when placed below the surface of the water on the seabed or river bottom, as opposed to traditional hydroelectric power, which can a monstrous eyesore. Dan Power, the aptly named president of Oceana Energy, says any subsurface turbines deployed in the San Francisco project must be at least 15 meters deep.

"The bay is the heart of San Francisco. Its residents receive much enjoyment from the bay, and it is also an important channel for commerce," he said.

"The ocean is rife with options for generating renewable energy," Power adds. "Tidal power could be a real solution to the difficult position the world has now found itself in concerning limited and dangerous sources of energy."

The first operational test of a hydrokinetic energy system in the U.S. is currently being conducted by a small company called Verdant Power in New York City's East River. The company recently planted six turbines in the river at the southern end of Governor's Island.

However, the technology still needs to be improved. The current in the East River was so strong that it sheared off several blades from the turbines weeks after they were installed. Verdant's turbines are now reportedly shut down for repairs.

Meanwhile, PG&E has become an industry leader in the exploitation of renewable energy sources, which account for 13% of its energy. It's increasing that figure at a rate of 300 kilowatts a year. PG&E now purchases solar, wind, geothermal and wave-generated energy.

The joint study of San Francisco Bay will likely cost roughly $2 million and run through the summer of 2008. The directors of the study intend to run a cost-benefit study and an environmental impact analysis of potential tidal power installations.

If the findings of the feasibility study are positive, the project would still face several regulatory hurdles. Thus, PG&E says that 2011 is the earliest that an operational tidal energy plant could come online in San Francisco.