11. Tidal turbines

Designed along the same lines as wind turbines, they harness the energy of river or sea currents, with speeds of 1 to 3 m/s.

In China, hundreds of thousands of small tidal turbines are operated in series on rivers, at very low cost per kW. Unit power is in the order of kW or a few kW; they have been used mainly for isolated villages, with the great disadvantage of a long dry season with no production. They account for a few percent of China's hydroelectric production, but seem to have little economic interest in industrialized countries, which receive electricity at 10 ce/kWh from the grid.

Marine turbines have a unit power of the order of one MW; they require a current speed that can only be found in tidal currents, which in places can exceed 4 m/s on spring tides. Power is proportional to the cube of the speed, and the most favorable locations...