|Fig. 1: A Tidal Stream Generator (Source: Wikimedia Commons)|
Tidal energy is a form of renewable energy that utilizes the rise and fall of ocean tides to generate power. Tidal power is the only form of energy which derives directly from the relative motions of the Earth/Moon system.  Tidal forces produced by the Moon and Sun, in combination with Earth's rotation, are responsible for the generation of the tides.  Tidal energy derives from the relative motion of large bodies of water. 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 classified as a renewable energy resource. [1,2]
There are three main methods used to generate tidal power: tidal stream generator, tidal barrage, and dynamic tidal power. A tidal stream generator is a machine that extracts energy from moving masses of water. These machines function very much like underwater wind turbines, and are sometimes referred to as tidal turbines.  A tidal barrage is essentially a large dam built across a bay or river. Most barrages consist of turbines, sluice gates, embankments and ship locks, and allow tides to flow in both directions. Fig. 1 shows an example.  The sluice gates open as the tide rises, and then close at high tide to create a tidal lagoon. The water behind the gates is then released through the turbines, generating electricity.  Dynamic tidal power (DTP) is a newer, untested method of tidal power generation. DTP involves creating a large dam-like structure extending from the coast straight to the ocean, with a perpendicular barrier at the far end, forming a large 'T' shape. This long T-dam would interfere with coast-parallel oscillating tidal waves which run along the coasts of continental shelves, containing powerful hydraulic currents.  One DTP dam can produce an estimated annual power production of about 23 billion kWh (8.28 × 1016 Joules). To put this number in perspective, an average European person consumes about 6800 kWh per year, so one DTP dam could supply energy for about 3.4 million Europeans. 
Tidal energy is still relatively new, and many of these tidal power technologies used to harness the power of the seas are still at an early stage of development.  Further development of these technologies is paramount to prove reliability and robustness and to reduce costs but also deployment and risk reduction.  However, it is hard to achieve this further development due to an overall lack of funding in the industry.  Additionally, there is resistance to tidal energy because of the possible negative environmental impacts. In general, benthic habitats will be affected by tidal current energy technologies due to the change of water flows, composition of substrate and sediment dynamics. Potential other effects include mortality of fish passing through turbines (blade-strike) and the collision risk of marine mammals with tidal stream farms. 
Renewable energy is a constant theme in the world today. Tidal energy, a relatively new and underdeveloped form of renewable energy provides many opportunities for large scale power. Perhaps we should put more effort and resources into developing and implementing tidal energy technologies to harness the oceans power and provide energy for the world. With increased development, tidal energy can become a viable alternative to fossil fuels.
© Collin Riccitelli. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 M. Shaikh et al., "Tidal Power: An Effective Method of Generating Power," Int. J. Sci. Eng. Res 2, 5, (2011), pg 241-245.
 Z. White, "Tidal Energy," Physics 240, Stanford University, Fall 2015.
 A. Uihlein and D. Magagna, "Wave and Tidal Current Energy - A Review of the Current State of Research Beyond Technology," Renew. Sust. Energy Rev. 58, 1070 (2016).