|Fig. 1: Mexican surfers enjoy the same waves that have a lot of potential to be a source of non-polluting energy. (Source: Wikimedia Commons)|
Marine energy is a term that refers to the energy produced by wave power and tidal power.  Tidal power comes from the energy of bodies of moving water, while wave power comes from surface waves. Tidal energy comes from the motion of the Earth-Moon system.
Waves are typically classified by the source that creates them.  The most common are surface waves, which are caused by wind blowing along the air-water interface. This creates a disturbance that steadily builds as wind continues to blow and the wave crest rises. Surface waves occur constantly all over the globe. These are the waves that people generally see when they go to the beach.
Currently there is a large search for an energy source that is economic and non-polluting.  The Ocean has several different forms of energy that are potentially invaluable resources for power generation. These include thermal energy, wave energy, offshore wind energy, tidal energy, and ocean current energy.  Seas can also offer one of the largest clean energy resources in the form of waves and currents, but these can only be applied if the technology can be successfully developed to exploit such resources reliably and cost effectively.  The energy related to the change in sea level and associated currents, is a significant resource. With research currently flooding the field of renewable energy it is thought that the energy inherent in the tides could be feasibly converted into a resource that is both renewable and sustainable.
As of yet the majority of our tidal energy resources are underutilized.  However, if effectively captured using suitably engineered systems, it could be capable of making a contribution to our future energy needs.  EPRI estimates the total wave energy resource along the outer continental shelf of the United States at 2,640 TWh/yr. That is an enormous potential, considering that just 1 TWh/yr of energy will supply around 93,850 average U.S. homes with power annually. While plenty of wave energy is available, not all of it can be fully harnessed because there are many other uses of the ocean.  However, EPRI estimates that the total recoverable resource along the U.S. shelf edge is 1,170 TWh/yr, which is almost one third of the 4,000 TWh of electricity used in the United States each year.
As a general rule of thumb a tidal range of at least 7 m is required for economical operation.  Like this stipulation shows, not all ocean locations are equally valuable for marine energy.  It is thought that there are 106 potential locations in European territorial waters for extracting tidal energy which, if put into service, would provide an exploitable 48 TW/year.  Studies in Canada have shown that British Columbia could produce 50,000 MW alone. Russia, Western India, and many other locations have also shown promise. For example assessments of marine energy in the UK have shown that 20% of the energy requirement of the UK may be feasibly extracted at 51 sites around the British Isles.
The cost projections for marine energy are all speculative since the tools for extracting energy from waves have not been fully developed yet. However, over the years the cost projections have decreased rapidly and consistently.
 Currently tidal power is expected to retail at around 12 p/kWh or 18 cents/ kWh with wind energy costing only 6 p/kWh, 9 cents/kWh. The target for commercial competitors in the long term is to generate electricity for a value similar to that generated from fossil fuels - currently around 2 p/kWh, 3 cents/kWh.
At the moment marine energy is a very promising field. Unfortunately, the field is still very far away from making any sort of financial sense. In the coming years, however, there is a lot of hope that marine energy will make progress and will eventually make an impact on the clean-energy sector.
Waves are a very promising source of clean energy for the future. Research has been conducted across the world to see if ocean waves have the potential to be turned into an energy source for their area. In many cases the answers have been very positive. In almost every continent there is a location that is capable of producing energy through wave power. The opportunity is there for the taking and hopefully we will soon have developed devices to take advantage.
© Yale Goldberg. 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.
 R. F. Nicholls-Lee and S. R. Turnock, "Tidal Energy Extraction: Renewable, Sustainable and Predictable." Sci. Prog. 91, 81 (2008).
 J. VanZwieten et al., "Design of a Prototype Ocean Current Turbine - Part I: Mathematical Modelling and Dynamics Simulation," Ocean Eng. 33, 1485 (2006).
 J. Cruz, Ed., Ocean Wave Energy: Current Status and Future Perspectives (Springer, 2008).
 S. Reed, "Going Under the Sea for Clean Energy," New York Times, 2 Apr 14.