Marine Energy

Introduction

Fig. 1: SeaGen: The World's First Major Commercial Tidal Stream Generator. (Source: Wikimedia Commons)

Marine energy, also known as tidal energy, is the energy that is carried by waves, tides, salinity, and temperature differences in the ocean. Some of these stores of kinetic energy can be used for the generation of electricity. Proponents of tidal energy advocate for its potential on account of the relative predictability of swells, particularly during the winter, when energy demand is at its highest. [1] Difficulties, however, include issues such as fishing rights and shipping byways, issues consequential enough to require a significant degree of international cooperation and political will. Moreover, it is unclear whether even significant investments in marine energy will result in a dent being made in the United States' overall energy consumption, which currently stands at 9.7 × 10¹⁶ Btu/year (1.02 × 10²⁰ joules/year). [2]

Thermal and Mechanical Energy

There are two kinds of energy in the ocean that we can harness: thermal energy, which is derived from the heat of the sun, and mechanical energy, which is derived from the waves and tides. [3]

The general way to exploit thermal energy is through ocean thermal energy conversion. This process produces energy by exploiting the thermal gradients that exist between warmer ocean surface waters and ocean depths. [4] As over 70% of the world's surface is sea, oceans serve as collectors of solar energy. Theoretically, much of this heat can be used to power our energy grid. [3]

There are four general ways to exploit mechanical energy in the ocean. First, there is tidal power, often harnessed through the use of a dam, which takes advantage of the difference in water levels between low and high tide. Second, there are tidal currents, which can be harnessed by underwater turbines, operating in a similar fashion to wind turbines on the surface; the SeaGen (see Fig. 1) is the world's first large scale commercial tidal stream generator, with a potential annual energy production of 5038 MWh (adjustments for expected periods of maintenance, however, reduce this to 4736 MWh/year). [5] Third, there is wave power; there are several current projects that seek to harness the power of waves into electricity mechanically. Fourth, there are salinity gradients, whereby energy is harnessed by exploiting the difference in salt concentration that exists between freshwater and seawater. [3]

© Cyrus Reza. 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.

References

[1] J. Kanter, "Harnessing the Power of Ocean Waves for Energy," New York Times, 7 Nov 07.

[2] "Monthly Energy Review, February 2019," U.S. Energy Information Administration, DOE/EIA-0035(2019/2), February 2019

[3] K. Galbraith, "Generating Energy From the Deep," New York Times, 29 Apr 09.

[4] L. Greenemeier, "It Came From the Sea - Renewable Energy, That Is," Scientific American, 10 Mar 08.

[5] C. A. Douglas, G. P. Harrison, and J.P. Chick, "Life Cycle Assessment of the Seagen Marine Current Turbine," Proc. Inst. Mech. Eng. M 222, 2 (2008).