|Fig. 1: Tidal barrage in La Rance, France. (Source: Wikimedia Commons)|
Tidal Energy is a renewable form of energy that harnesses the power generated by the rise and fall of ocean tides. Ocean tides are controlled by the Earth's rotation within the gravitational fields of the sun and the moon. [1,2] The ocean's tidal phenomenon occurs twice every 24 hours, 50 minutes and 28 seconds, making it an incredibly predictable source of energy compared to other renewable energy sources like wind and solar power.  Although the wind and solar industries are largely developed markets for renewable energy, their yield can be unreliable in terms of energy intensity and availability (due to weather and other variables). [3,4] The world oceans are a largely unused source of renewable energy, especially in the United States, yet they hold potential to help reduce our global dependence on fossil fuels to meet energy requirements. 
Tidal energy can generate potential energy through changes in water levels, as well as kinetic energy from undersea currents. [2,3] Tidal barrages are one way of harnessing the potential energy of ocean tides. A tidal barrage is similar to a large dam, and is usually built across a bay or estuary. Most barrages consist of turbines, sluice gates, embankments and ship locks, and allow tides to flow in both directions.  The sluice gates open as the tide rises, and close at high tide to create a tidal lagoon. The water behind the gates is then released through the turbines, generating electricity.  La Rance, France is home to the world's first and largest operating tidal barrage power plant, which has a generating capacity of 240 MW.  Currently, there are no tidal barrages in the United States and there are very few practical locations for such structures. 
The main challenges that have kept tidal energy, and ocean energy in general, from reaching the magnitude of wind and solar energy are financial and environmental concerns. [4,5] Although most technical and maintenance issues have been resolved with utilizing tidal barrages, these structures are still extremely costly to construct because of the large amounts of materials needed to create a dam that can resist large loads. Depending on the site, tidal barrages can have large impacts on the local environment and marine life. Behind a barrage, there can be a change in water quality and sediment deposits as the result of changes in water turbidity. Increased water salinity may cause changes in maritime traffic as freshwater species attempt to move seawards. [1,3] Tidal current turbines pose some similar challenges to tidal barrages. Like barrages, these turbines are expensive because they require heavy-duty materials that can resist the strong thrust of dense seawater. And although the environmental impacts of tidal turbines seem negligible in comparison to those of tidal barrages, turbines could negatively impact the environment by capturing the kinetic energy of the tidal flow. There are also challenges that come along with installation and maintenance of tidal turbines because they are not always easily accessible. 
Because the world still relies so heavily on fossil fuels to meet global energy needs, it is clear that advancements must be made in the development of sustainable and renewable energy sources. Although ocean energy is currently costly, both financially and environmentally, it is important to remember that this type of energy extraction is about 15 years behind the wind energy industry. With increasing technological advancements and a global focus on ocean energy, we can work to make ocean energy a sustainable option for our Earth. 
© Zoe White. 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.
 C. Baker, "Tidal Power," Energy Policy 19, 792 (1991).
 W. Chen, "Tidal Energy," Physics 240, Stanford University, Fall 2010.
 F. O. Rourke, F. Boyle, and A. Reynolds, "Tidal Energy Update 2009," Appl. Energy 87, 398 (2010).
 S. Reed, "Going Under the Sea for Clean Energy," New York Times, 2 Apr 14.
 A. Blandino, "Tidal Power," Physics 240, Stanford University, Fall 2014.