[BETA] Fortnightly

Tidal energy technology improves, but is it enough?

Could ocean energy be the next big thing in renewables?

The San Francisco Board of Supervisors in early May approved a pilot project, estimated at $2 million, to test technology that produces electricity from the tides in the San Francisco Bay.

The city hopes to produce 1 MW with the project and add it to the San Francisco grid by Jan. 1, 2006.

The idea of using the ocean to produce power is hardly new-it goes back as far as 1200 A.D., when farmers in Great Britain and France trapped sea water in ponds to power mills as the tide dropped. But in the modern age of pulling electricity from the grid, ocean energy has not been economic.

Improvement in technology and rising natural gas costs may change the economics of ocean power during the next decade, though, making ocean energy a viable-not to mention pollution-free-option for some coastal communities.

Joseph Neil, CEO of HydroVenturi, certainly hopes so. His company has been one of the driving forces behind the San Francisco project, and it is a leading contender to land the pilot there.

While tidal energy projects are new to the United States, Europe has been pursuing them for decades. In 1966, France commissioned a 240-MW tidal plant in Brittany on the River Rance. Although the plant still operates, its owner, Electricité de France, has not expressed interest in building more tidal projects.

More recently, the Philippines has embarked on a $2.8 billion tidal energy project, the Dalupiri Ocean Power Plant. Phase one of the project is expected to produce in the neighborhood of 100 MW. Project developer Blue Energy, of Canada, projects initial construction costs to be comparable to those of conventional power projects, possibly less as the scale of the project increases.

Advantage: Predictability

One advantage tidal energy has over both wind and solar is predictability. High and low tides are known months, years, even decades in advance. The twice-daily high and low tides produce four peak wave velocity events every day. Of course, the downside is that these peak wave periods don't necessarily coincide with peak electricity demand periods. But since the peak tidal electricity times are known in advance, adding tidal energy to the mix would allow generators to plan better than they can with solar and wind.

There are three main types of technologies that convert ocean energy into electricity:

• makes use of rising tides or currents through a relatively narrow channel to drive a turbine; in some installations, water is trapped behind a gate and released to spin the turbine at low tide.
• harnesses the movement of surface waves to pump a working fluid through a turbine.
• n uses the temperature gradient between varying layers of the ocean to create steam, which then drives turbines.

The San Francisco project will use the tidal power of its bay, which has several potential sites, including the bottleneck at the point where the Golden Gate Bridge crosses.

Disadvantage: Cost

The utility industry has been highly skeptical of tidal energy, and rightly