Editing Tidal power (section)

== Principle ==
[[File:Tide type.svg|thumb|upright=1.25|Variation of tides over a day]]
{{Main|Tide|Tidal acceleration}}

Tidal energy is taken from the Earth's oceanic [[tide]]s. [[Tidal force]]s result from periodic variations in gravitational attraction exerted by celestial bodies. These forces create corresponding motions or currents in the world's oceans. This results in periodic changes in sea levels, varying as the Earth rotates. These changes are highly regular and predictable, due to the consistent pattern of the Earth's rotation and the Moon's orbit around the Earth.<ref>{{cite book|last=DiCerto|first=JJ|title=The Electric Wishing Well: The Solution to the Energy Crisis|url=https://archive.org/details/electricwishingw0000dice|url-access=registration|date=1976|publisher=Macmillan|location=New York|isbn=9780025313200 }}</ref>  The magnitude and variations of this motion reflect the changing positions of the Moon and Sun relative to the Earth, the [[Coriolis effect|effects of Earth's rotation]], and local [[bathymetry|geography of the seafloor and coastlines]].

Tidal power is the only technology that draws on energy inherent in the orbital characteristics of the Earth–Moon system, and to a lesser extent in the Earth–Sun system. Other natural energies exploited by human technology originate directly or indirectly from the Sun, including [[fossil fuel]], [[Hydroelectricity|conventional hydroelectric]], [[Wind power|wind]], [[biofuel]], [[wave power|wave]] and [[solar energy]]. [[Nuclear power|Nuclear energy]] makes use of Earth's mineral deposits of [[Fissile#Fissile vs fissionable|fissionable]] elements, while [[geothermal power]] utilizes the Earth's [[internal heat]], which comes from a combination of residual heat from [[planetary accretion]] (about 20%) and heat produced through [[radioactive decay]] (80%).<ref name="turcotte">{{cite book| last=Turcotte| first=D. L.|author2=Schubert, G. | title=Geodynamics | publisher=Cambridge University Press| location=Cambridge, England, UK| date=2002 | edition=2nd| pages=136–137 | chapter=Chapter 4 | isbn=978-0-521-66624-4 }}</ref>

A tidal generator converts the energy of tidal flows into electricity. Greater tidal variation and higher tidal current velocities can dramatically increase the potential of a site for tidal electricity generation. On the other hand, tidal energy has high reliability, excellent energy density, and high durability.<ref>{{Cite journal |last1=Chowdhury |first1=M. S. |last2=Rahman |first2=Kazi Sajedur |last3=Selvanathan |first3=Vidhya |last4=Nuthammachot |first4=Narissara |last5=Suklueng |first5=Montri |last6=Mostafaeipour |first6=Ali |last7=Habib |first7=Asiful |last8=Akhtaruzzaman |first8=Md. |last9=Amin |first9=Nowshad |last10=Techato |first10=Kuaanan |date=2021-06-01 |title=Current trends and prospects of tidal energy technology |url=https://doi.org/10.1007/s10668-020-01013-4 |journal=Environment, Development and Sustainability |language=en |volume=23 |issue=6 |pages=8179–8194 |doi=10.1007/s10668-020-01013-4 |issn=1573-2975 |pmc=7538194 |pmid=33041645|bibcode=2021EDSus..23.8179C }}</ref>

Because the Earth's tides are ultimately due to gravitational interaction with the Moon and Sun and the Earth's rotation, tidal power is practically inexhaustible, and is thus classified as a [[renewable energy]] resource. Movement of tides causes a [[tidal acceleration|loss of mechanical energy]] in the Earth-Moon system: this results from pumping of water through natural restrictions around coastlines and consequent [[viscosity|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&nbsp;billion years since its formation. During the last 620&nbsp;million years the period of rotation of the Earth (length of a day) has increased from 21.9&nbsp;hours to 24&nbsp;hours;<ref>{{cite journal | author=George E. Williams | bibcode=2000RvGeo..38...37W | title=Geological constraints on the Precambrian history of Earth's rotation and the Moon's orbit | journal=Reviews of Geophysics | volume=38 | issue=1 |date=2000 | pages=37–60 | doi=10.1029/1999RG900016 | citeseerx=10.1.1.597.6421 | s2cid=51948507 }}</ref> in this period the Earth-Moon system has lost 17% of its rotational energy. While tidal power will take additional energy from the system, the effect is negligible and would not be noticeable in the foreseeable future.