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 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 a simple generation system for tidal plants involves a dam, known as a barrage, across an inlet. Sluice gates (gates commonly used to control water levels and flow rates) on the barrage allow the tidal basin to fill on the incoming high tides and to empty through the turbine system on the outgoing tide, also known as the ebb tide, 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 water pressure 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.
Wave Energy Technology
The concentrated power of breaking waves shapes shorelines and erodes beaches. However, it is the kinetic energy associated with up-and-down oscillations within the water column that is transformed into electricity by most wave energy technologies.
Many regions of the world have commercially viable wave resources, including the northeastern and northwestern coasts of the United States. For shoreline and nearshore applications, most technologies include semi-enclosed steel or concrete structures anchored to the ground or integrated within a breakwater.
Oscillating water column (OWC) devices are partially submerged boxes with an opening to the sea at the bottom and an opening to the air at the top. Waves cause the enclosed water column and the air on top of it to rise and fall. This motion alternately forces and draws air through a turbine located in the skyward outlet.
Pendulum or flap devices employ a box with an opening on the seaward side. Incident waves cause the pendulum or flap within the opening to move back and forth, driving a hydraulic motor that powers a generator.
Tapered channel devices have walls that rise several meters above water level. A wave grows progressively higher as it travels through the narrowing channel, spilling water into a storage area. The stored water returns to the sea through an outlet that contains a turbine.
Both seafloor-tethered and seafloor-anchored wave technologies are being examined for higher-energy offshore environments, where water depths are more than 40 m. Many concepts exploit the differential motion of floats attached to one another, while others employ floats that move relative to the seafloor. For these technologies, up-and-down motions are used to drive hydraulic pumps or to force water through a turbine. Candidate offshore technologies also include tapered channel and OWC devices constructed on floats.
More than 1,000 wave energy patents exist, and numerous prototype or pre-commercial systems have been tested in shoreline, near shore, and offshore environments. Only one commercial wave power station has been developed, however. This 500-kW plant, sited on the shoreline in Islay, Scotland, employs OWC technology. It began operation in 2000, and a second commercial installation is being developed for the Faroe Islands, an island group situated between the Norwegian Sea and the North Atlantic Ocean.
Dual-use applications of wave energy technology may prove cost-effective in the near term. For example, power generation technologies could be incorporated into breakwaters, harbor walls, or other structures, or they could be integrated with other commercial activities, acting as artificial reefs for Mari culture operations or as platforms for desalination facilities.
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