|Fig. 1: U.S. energy consumption by energy source. 2013.  (Courtesy of the U.S. Department of Energy)|
Renewable energy is something that scientists around the world are constantly trying to develop and make more efficient. As our world of finite resources continues to age and we continue to deplete those resources, making improvements and developments that help us reduce out dependency on coal and oil is critical. In 2013, renewable energy made up about 11% of the total production budget for the U.S. Currently, the U.S. can only meet about 84% of its energy demands with domestically produced energy, the rest is covered by imported petroleum. [1,2] (See Fig. 1.)
Renewable energy accounted for around 9 quadrillion Btu of energy in the U.S. in 2013, which was actually a record high. Seeing that about 70% of the earth is covered by water, and about 97% of that water is located in the ocean, it seems both logical and promising that we should take advantage of this incredible resource to help produce power. In 2013, hydropower made up about 6% of total U.S. electricity generation and 52% of generation from renewable energy overall. One of the biggest advantages of using tidal movement to generate power is its predictability. Because the tides rely on the moon and sun in combination with rotation of the earth, it is easier to predict how they will act when compared to wind or solar energy. In order to efficiently produce energy using tidal power a tidal range of about 10 feet is needed, although in some places, tidal ranges can vary up to about 40 feet. [2,3]
This is a two-way system that can produce electricity with the ebb of the tide as it comes in and goes out. This system involves the use of a dam that is placed between inlets. Sluice gates that run across the barrage fill with water, and empty through the turbine system as the tide rises and falls which then turns the turbine and creates electricity. There are currently no tidal barrages in the United States. Other European countries harness this power much more effectively including France, England and Russia. [2,4] See Conceptual Tidal Barrage in Fig. 2.
|Fig. 2: Conceptual Tidal Barrage. (Source: Wikimedia Commons)|
Tidal fences are cheaper to install than barrages and can also harness the incoming and outgoing tide. A fence is installed between two landmasses and has vertical turbines that rotate as water passes by them. 
This design follows a similar format to a wind turbine, except it is placed under water. Tidal turbines are the heaviest and most expensive design to build of these three, but they also produce the most energy. The turbine uses the incoming and outgoing water current to turn the turbine. [4,5] See Tidal Turbine Design in Fig 3.
|Fig. 3: Tidal Turbine. (Courtesy of the U.S. Department of Energy.)|
There is an almost endless potential for harnessing this energy as tidal power uses as much usable energy as is contained in the Earth's water currents, accessibility is the biggest problem. The total energy contained in tides worldwide is 3,000 GW, but it is estimated that somewhere between 120 and 400 GW are available for use. Tidal power in shallow water areas is estimated to be able to produce 3,800 terawatt-hours per year if fully harnessed and applied to the 120-400 GW of available energy. This could account for 20% of global electricity production. [3,6]
Two of the main challenges that face tidal energy production is cost and environmental impact. It is currently not the most cost effective option for energy production, which is a problem that plagues many potential renewable energy sources. The second and more troubling problem is the effect that many of these tidal turbines have on marine life. The turbines that harness the energy of the currents as water passes through them can kill fish and marine mammals if they are trapped. Some tidal turbines can also affect plant and animal life within estuaries by affecting the natural flow of water. [4,6]
While there is promise for this technology to produce clean, renewable energy, the challenges of cost and environmental impact have kept this from being a primary energy producing method for the U.S. and countries abroad. That being said, the potential for tidal energy to create power for a large part of the earth's needs is there. Until the money and resources needed to make clean energy a focus are put into the project it will not be realized. A current map of projects that are functioning, planned, and under development worldwide can be found at the Marine and Hydrokinetic Technology Database.
© Alex Blandino. 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.
 "Monthly Energy Review," U.S. Energy Information Administration, DOE/EIA-0035(2014/05), May 2014.
 "How Much Electricity Comes From Renewable Sources" New York Times, 3 Mar 14.
 "Electric Power Monthly," U.S. Energy Information Administration, February 2014.
 M. Robinson, "Ocean Energy Technology Development," U.S. National Renewable Energy Laboratory, NREL/PR-500-40461, October 2006.
 S. Reed, "Going Under the Sea for Clean Energy," New York Times, 2 Apr 2014.
 J. Pyper, "The Quest to Make Hydrokinetic Power a Source of U.S. Energy and Jobs," New York Times, 17 Aug 2011.