In the search for sources of electric power to replace fossil fuels, technologists have explored all the natural phenomena that might be exploited to generate electricity. As a consequence, wind power, solar power, and biomass-based generation are today becoming mature technologies alongside hydropower. How- ever, one potential source of energy has yet to exploited commercially—the energy contained in the seas and oceans.

Part of the reason for the slow advance of marine power generation is the hos- tile environment in which marine energy conversion devices have to operate. The development of power generation technologies that can withstand these conditions requires great tenacity and it has not always been clear that the seas can yield an economical means of generating power. In spite of this technologists have persevered, and while marine power generation remains at the development and demonstration stage, there is greater confidence today than there has ever been that it will produce results in the near future.

There are four ways in which energy can be extracted from the seas to pro- vide power generation. Perhaps the most significant today, because it is the easiest to exploit, is the energy provided by marine currents. These currents are generated by tidal movements and are normally found in estuaries and around rock formations close to shore. Energy can be extracted from them by using underwater turbines that are conceptually similar to wind turbines.

Sea and ocean waves offer the second source of energy. Waves are generated by the wind and like the wind they are erratic, but it is possible to exploit the energy they contain with devices of various kinds, some mounted close to the shore, some in deep water, some floating, and some anchored to the seabed. This is potentially the most difficult marine energy source to tap but there is a significant resource available if it can be mastered.

The third source of marine energy is provided by the sun. The world’s seas, particularly those in the tropics, are massive solar collectors absorbing heat energy throughout the day. This absorption creates a layer of hot water at the sea’s surface, while below the surface the water temperature is much lower, creating a temperature gradient that can be exploited to drive a heat engine. The technique is known as ocean thermal energy technology (OTEC).

The fourth source of energy is probably the most unexpected. When fresh water from a river reaches the sea and mixes with salt water, energy is released. This energy of dilution is predicted by classic thermodynamics, and in the past decade one or two pilot projects have sought, with some success, to take advantage of this as a way of producing electricity. Today this resource is referred to as salinity gradient, and while it is immature, it does offer future promise.

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