2.            STATE OF THE ART TECHNOLOGY

Tidal barrage energy

The rise and fall of the tides creates, in effect, a low-head hydropower system. The modern version of a tide mill is a semi-permeable barrage built across an estuary, allowing floodwaters to fill an impounded basin via a series of sluices. At high water the sluice gates are closed, creating a head of water on the ebb tide. Electricity is generated by releasing the water through a series of conventional bulb turbines. In future schemes the energy yield would be enhanced by pumping water into the estuary on the flood tide ('flood pumping'), thereby increasing the volume of water released through the turbines on the ebb tide.

Wave energy conversion systems

Wave energy conversion systems can be classified as:

Shoreline devices

Shoreline devices have the advantage of relatively easier maintenance and installation and do not require deep-water moorings and long underwater electrical cables. The less energetic wave climate at the shoreline can be partly compensated by the concentration of wave energy that occurs naturally at some locations by refraction and/or diffraction. The three major classes of shoreline devices are the oscillating water column (OWC), the convergent channel (Tapchan) and the Pendulor.

 

The OWC comprises a partly submerged concrete or steel structure, which has an opening to the sea below the water line, thereby enclosing a column of air above a column of water. As waves impinge on the device, they cause the water column to rise and fall, which alternately compresses and depressurises the air column. This air is allowed to flow to and from the atmosphere through a turbine, which drives an electric generator. Both conventional (i.e. unidirectional) and self-rectifying air turbines have been proposed. The axial-flow Wells turbine, invented in the 1970s, is the best-known turbine for this kind of application and has the advantage of not requiring rectifying air valves. A number of OWC devices have been installed worldwide, with several of them being built into a breakwater to lower overall construction costs.

 

 

The Tapchan comprises a gradually narrowing channel with wall heights typically 3 to 5 m above mean water level. The waves enter the wide end of the channel and, as they propagate down the narrowing channel, the wave height is amplified until the wave crests spill over the walls to a reservoir, which provides a stable water supply to a conventional low head turbine. The requirements of low tidal range and suitable shoreline limit the worldwide replicability of this device.

The Pendulor device consists of a rectangular box, which is open to the sea at one end. A pendulum flap is hinged over this opening, so that the action of the waves causes it to swing back and forth. This motion is then used to power a hydraulic pump and generator. Worldwide, only small devices have been deployed.