Marine power can be classified into two, namely wave energy and tidal energy. Waves are created by the interaction of wind with the surface of the sea, and they have the potential to provide an unlimited source of renewable energy. Wave energy can be extracted and converted into electricity by wave power machines. These machines can be deployed either on the shoreline or in deeper waters offshore.
Tidal energy exploits the natural ebb and flow of coastal tidal waters caused principally by the interaction of the gravitational fields of the earth, moon and sun. Due to these gravitational forces, water levels follow periodic highs and lows. Associated with these water level changes, there are tidal currents. The tidal energy generator uses this phenomenon to generate energy. The stronger the tide, either in water level height or tidal current velocities, the greater the potential for tidal energy generation.
Tidal power can be extracted with two techniques, namely tidal barrage and tidal stream technology. Tidal barrages make use of the potential energy in the difference in height (or head) between high and low tides, while tidal stream technology makes use of the kinetic energy of moving water to power turbines. Tidal stream method is gaining in popularity because of the lower cost and lower ecological impact compared to barrage.
A dam or barrage is built across an estuary or bay in order to let water flow through it into the basin as the tide comes in. The barrage has gates which allow the water to pass through, thereby filling the basin. The gates are closed when the tide has stopped coming in, indicating the beginning of ebb. The water level outside the basin continues to drop during ebb while the water level inside the basin remains high. The difference of water level creates a ‘head’ or ‘fall’. The water in the basin is then released into the sea through a set of turbines. Power is thereby generated by the turbines. This takes place during ebb and continues until the tide floods and the rising water reduces the head to the minimum operating point. The water is then captured again and the process repeats. The generation of power happens during the ebbs and therefore this mode of operation is called “ebb generation”.
Though there are other modes of operation, like “flood generation” (generating power during floods) and “two-way generation” (generating power during both the floods and ebbs) but these modes of operation are not generally flavoured. “Flood generation” is in general much less efficient than “ebb generation” because the ‘head’ created is usually less than the ‘head’ created by “ebb generation’. While it is possible for “two-way generation” to generate power almost continuously, the extra length needed for the barrage contributes tremendously to the cost of construction.
The cost associate with a tidal barrage will depend on the size of the scheme and its location. Very often the building of a tidal barrage requires a high capital. It is therefore not an attractive proposition to investors due to long payback periods.
Perhaps the biggest drawback of tidal barrage is environmental and ecological. The change in water level and possible flooding would affect the ecosystems along the coast. The water quality in the basin would also be affected, and the turbidity may affect the animals that live in the water. These issues are very delicate, and need to be independently assessed.