Generation of tidal energyTidal power is the only form of energy which derives directly from the relative motions of the Earth–Moonsystem, and to a lesser extent from the Earth–Sunsystem. The tidal forcesproduced by the Moon and Sun, in combination with Earth's rotation, are responsible for the generation of the tides. Other sources of energy originate directly or indirectly from the Sun, including fossil fuels, conventionalhydroelectric, wind, biofuels, wave powerand solar. Nuclearis derived using radioactivematerial from the Earth, geothermal poweruses the Earth's internal heatwhich comes from a combination of residualheat from planetary accretion(about 20%) and heat produced through radioactive decay(80%).Variation of tides over a dayTidal energy is generated by the relative motion of the water which interact via gravitational forces. Periodic changes of water levels, and associated tidal currents, are due to the gravitational attraction by the Sun and Moon. The magnitude of the tide at a location is the result of the changing positions of the Moon and Sun relative to the Earth, the effects of Earth rotation, and the local shape of the sea floor andcoastlines.Because the Earth's tides are caused by the tidal forces due to gravitational interaction with the Moon and Sun, and the Earth's rotation, tidal power is practically inexhaustible and classified as a renewable energysource.A tidal generator uses this phenomenon to generate electricity. The stronger the tide, either in water level height or tidal current velocities, the greater the potential for tidal electricity generation.
Tidal movement causes a continual loss of mechanical energyin the Earth–Moon system due to pumping of water through the natural restrictions around coastlines, and due to viscousdissipation at the seabedand 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 hourswe see now; in this period the Earth has lost 17% of its rotational energy. While tidal power may take additional energy from the system, increasing the rate of slowdown, the effect would be noticeable over millions of years only, thus being negligible.Categories of tidal powerTidal power can be classified into three main types:•Tidal stream systemsmake use of the kinetic energyof moving water to power turbines, in a similar way to windmills that use moving air. This method is gaining in popularity because of the lower cost and lower ecological impact compared to barrages.•Barragesmake use of the potential energyin the difference in height (or head) between high and low tides. Barrages are essentially damsacross the full width of a tidal estuary, and suffer from very high civil infrastructure costs, a worldwide shortage of viable sites, and environmental issues.