Tidal Current Technology
All flowing water carries kinetic energy. Moving water carries much more energy than the moving air used in wind power because water is much denser than air. From a practical perspective, this means that even slow tidal currents may represent an economical energy source. Electricity can be produced from tidal energy using several existing and emerging technologies.
For applications in tidal basins with intermittent flows and other marine areas with persistent currents, many emerging technologies are identical to those being pursued for hydropower applications. These free-flow (low-head) devices may be anchored by an underwater foundation, or they may be attached to moorings. In either instance, the energy conversion system is located in (but does not impede or direct) the flow of water. Some designs are similar in form to modern, three-blade wind turbines, while others rely on more elaborate blade and device configurations. Additional concepts are being engineered for use in deeper waters.
Tidal current systems may be deployed individually or in multi-unit arrays. Tidal fences, for example, incorporate many turnstile-like turbines. Pre-commercial installations of these technologies are operating elsewhere in the United States and around the world, and a demonstration project is under way in Massachusetts. Commercial technologies are expected to be suitable for off-grid power on islands and in other remote regions, as well as in grid-connected applications of all scales. They have the potential to provide a predictable, near-continuous source of power in the many areas of the world where moderate tidal currents exist.
Tidal barrages are applicable only in areas where physical features permit the installation of barriers capable of storing water on the incoming tide. These barriers incorporate gates that are closed at high tide to create an impoundment basin. They act like a temporary hydro dam, creating artificial head by delaying the ebb tide. Eventually, they direct the outgoing water through turbines, converting the potential energy of stored water into the kinetic energy of flowing water and the positive energy of clean electricity.
Only two large-scale tidal barrage facilities have been constructed to date. A barrage with a capacity of 240 MW has been in operation in La Rance, France, since 1966, and a 20-MW plant has been generating electricity in Annapolis, Nova Scotia, since 1984.