Tidal energy (tidal power) is a form of hydropower that converts the energy of tides into electricity or other useful forms of power. Although not yet widely used, tidal power has potential for future electricity generation. Tides are more predictable than wind energy and solar power. Among sources of renewable energy, tidal power has traditionally suffered from relatively high cost and limited availability of sites with sufficiently high tidal ranges or flow velocities. Many recent technological developments (design and turbine technology) indicate that the total availability of tidal power may be much higher than previously assumed and that economic and environmental costs may be brought down to competitive levels.
Historically tide mills have been used both in Europe and on the Atlantic coast of North America. The earliest occurrences date from the Middle Ages and Roman times. The first large-scale tidal power plant was built in France and started operating in 1966. In North America the first tidal power site was built in Nova Scotia and started operating in 1984. The first in-stream tidal current generator in North America was installed on southern Vancouver Island in 2006.
Tidal power is the only form of energy which derives directly from the relative motions of the Earth–Moon system, and to a lesser extent from the Earth–Sun system. Tidal forces produced by the Moon and Sun, in combination with Earth’s rotation, are responsible for the generation of the tides. The movement of the tides causes a continual loss of mechanical energy in the Earth–Moon system due to pumping of water through the natural restrictions around coastlines, and consequent viscous dissipation at the seabed and in turbulence. This loss of energy has caused the rotation of the Earth to slow in the 4.5 billion years since formation. During the last 620 million years the period of rotation has increased from 21.9 hours to the 24 hours, in this period the Earth has lost 17% of its rotational energy.
Tidal power is classified in three generating methods:
1. Tidal stream generator (TSG): uses the kinetic energy of moving water to power turbines. This method is gaining in popularity because of the lower cost and lower ecological impact compared to tidal barrages.
2. Tidal barrages: use the potential energy in the difference in height between high and low tides. Barrages are essentially dams across the full width of a tidal estuary and suffer from very high infrastructure costs.
3. Dynamic tidal power (DTP): is a theoretical generation technology that exploits the interaction between potential and kinetic energies in tidal flows. Long dams need to be built from coasts straight out into the sea or ocean, without enclosing an area. Tidal phase differences are introduced by the dam, leading to a significant water level differential in shallow coastal seas featuring strong coast-parallel oscillating tidal currents. Each dam would generate power at a scale of 6-15 GW.
References:
http://www.inference.phy.cam.ac.uk/sustainable/book/tex/Lagoons.pdf
