Tidal Energy

 With tidal range energy, constructions are needed to convert the potential energy into kinetic energy. This is always done by creating a water level difference, the head, by restricting the water flow in or out of a storage basin (often an estuary). Dams or (natural) barriers usually form the boundary of these storage basins. The turbines that extract the potential energy are placed in the barrier that encloses the lagoon. The head over the basin boundary or dam creates a current through the installed turbines, which generates electricity. The storage basin can be a natural part of the water system, such as an estuary or an inland sea arm, or can be a man-made lagoon. In the first case ‘simply’ a barrier has to be constructed, while in the second case a lagoon must be created. These lagoons can be connected to the coast or detached at sea, depending on local conditions. Energy can be extracted whenever there is a head over the basin boundary, either when the outside water level is higher than inside, or vice versa. However, the flow characteristics of the basin determine if it is possible to generate electricity by filling or emptying the basin, or in both situations. Two-directional generation requires special turbines, which are more expensive than one-way turbines, but generate more energy.

 The other way of extracting energy from tides is to use the tidal current energy directly. Energy in currents can be harvested by free-flow driven turbines. Tidal currents are variable, both in velocity and direction. The energy production depends on the water velocity, direction and the type of turbine that is used. When turbines are used that rotate in a plane perpendicular to the flow direction, like windmills in the wind, a construction is needed to adjust their orientation to the direction of flow. Generally, this type of turbine will be placed in barriers, under bridges or in tidal flow channels where flow directions are more or less constant. A two-directional-flow turbine can then generate electricity during ebb and flood tides. Turbines with a vertical axis (the Darrieus type) are not dependent on the direction of the flow.