Marine energy generation is a nascent industry that holds the potential to provide carbon-free energy resources to the world. Using much of the extant technology that drives wind energy conversion, marine power stations convert wave energy, tidal current energy, and tidal stream energy into electricity utilizable for human consumption. Although these sources appear promising, careful analysis of potential benefits and limitations must first be done before significant investment in infrastructure can occur.
Oceanic waves are indirectly generated by solar power, which creates atmospheric pressure differences from which winds arise. Wave power stations can be located on the shoreline, nearshore (seabed mounted), or offshore (generally floating).  They utilize various technologies to convert the waves' potential energy or forward kinetic energy into electricity. Devices include attenuators, pitching devices, oscillating wave surge converters, overtopping devices, point absorbers, submerged pressure differential devices, and oscillating water columns (OWC). [2,3] The most common technology used is the oscillating water column, in which the wave, while cresting and toughing in a fixed housing chamber, displaces air at a regular pace. This air flow is directed to a turbine (generally a Wells turbine) to generate electricity, in a manner quite similar to how wind turbines extract energy. [1,3] A number of OWC devices have been constructed in countries including China, India, Japan, Norway, and the UK. 
Tidal power is generated by variations created by Earth's gravitational interactions with celestial bodies, predominantly the moon. Tidal barrages are dams that rely on the twice daily cycle of the tides. They impound water at high tide, and let the water ebb out through turbines at low tide. This type of ebb generation is generally reliable, but cannot occur continuously (power is generated for only about 12 hours every day). 
In addition to harvesting energy from tidal barrages, we can also derive energy from marine currents. The tidal stream is driven by global oceanic circulation and seawater density variations. Current technologies to extract tidal stream power include horizontal and vertical axis turbines, venture devices, and oscillating hydrofoil devices. 
Figures cited for marine power's potential vary widely. Extracting meaningful data regarding the tides is difficult, as scientists currently lack the technology and data to predict the long-term behavior of marine systems. Deep-sea systems are especially unclear, as some of these regions have never been explored.
However, certain basic calculations can provide an upper limit to how much energy the world can expect to extract from tidal power. Earth's length of day (period T) is slowly increasing due to tidal friction, at a rate of 32.184 sec/cy2 as measured by eclipse records and atomic clocks.  This indicates that amount of energy contained in tidal currents is decreasing Earth's kinetic energy of rotation, E, which is a function of the moment of inertia, I, and period.
From this we can see that the Earth loses roughly 4 ∼ 1019 J/year (∼104 TWh/year) to its tides. Much of this energy is lost as friction or is held within inaccessible currents (i.e., deep-sea currents) but this calculation provides an upper limit to the amount of energy tidal streams contain. There are only a small number of possible sites in the world where coastal topography, tide amplitude, or current speed are adequate enough to justify the expense of building a station. 
Moving away from pure theory, one study estimates that only 155 TWh/year of this energy is extractable, worldwide.  Limiting factors include financial c onsiderations, the infeasibility of power stations in certain areas, electricity transmission issues, and efficiency concerns.
Wave power is also difficult to predict. Although the descriptors for wave energy are well-known to science, oceanic waves are extremely variable. Power density, amplitude, direction, period, spectral content, and groupiness vary based on location and local weather conditions.  Using satellite technology, surface wave characteristics can be estimated. The mean annual wave power density of developed waves could range from 27 to 206 kW/m.  A U.S. estimate puts the amount of extractable wave and current energy around the U.S. continental shelf at 10% of America"s electrical power demand.  Feasible locations for tidal power stations in the United States include the Gulf Stream in Florida and possibly the Gulf of Mexico.  Worldwide, wave power has been estimated at 2000 TWh/year if wave devices perform as predicted.  In perspective, the world energy consumption in 2010 was approximately 5 × 1020 J, or 105 TWh/year. 
The long-term impact of marine power stations on local ecology has yet to be studied, but researchers have noticed certain immediate impacts on aquatic flora and fauna, which raise concerns. Most marine plants and animals live in the epipelagic zone- from the ocean surface to a depth of 200m - where the majority of the energy conversion devices would be installed. Engineers would have to address entanglement and noise pollution issues.  Other factors, including coastal erosion and sedimentary flow patterns would have to be considered on a case-by-case basis. 
Transmitting the generated marine power to population centers would prove a major financial and logistical concern. Electrical infrastructure is expensive to build and maintain (subsea cabling and installation is estimated at $5 million/mile).  Furthermore, many sites with high marine activity are distant from large population centers. Considering the cost, commercialization is most probable in high-value areas where long-range electrical transmission is unneeded. But wave and tidal power could be utilized by offshore petroleum operations, island communities, military naval projects, and remote scientific projects, in which case the cost of power transmission would prove less of a hindrance. In high-potential areas, wave electricity costs about 8 cents/kWh, which makes it competitive in niche markets situated away from conventional power generation. 
The extraction equipment must operate in a harsh, marine environment, with all that it entails- high stress during typhoons and hurricanes, and the continual effects of seawater.
Marine power continues to be a fertile, emerging alternative- energy industry. Right now, it is perhaps 10-15 years behind the wind industry.  However, the limitations and relative scarcity of wave and tidal energy will probably prevent them from ever dominating the world"s energy market. Optimistic predictions for wave and tidal energy resources put available energy at about 1% of current world energy consumption, and energy consumption continues to grow year by year. The major disadvantage of marine energy, even as compared to a niche energy market such as wind, is that the energy is only conveniently available on the ocean. The private sector, as of now, is unlikely to make the uncertain leap to marine power without substantial government support. 
© Zoe Yan. 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.
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