The principal sources of energy from the sea are oil and gas produced from beneath the seabed. Basically, these are non-renewable resources. Offshore oil and gas production began on the Gulf Coast of the United States after World War II, with the first well out of sight of land constructed in 1957. Today, 35% of U.S. oil and 27% of our natural gas are produced offshore. Worldwide, 33% of known oil reserves are estimated to be beneath the sea, with more than 8,300 platforms engaged in production.
Clearly, the production of hydrocarbons will be the dominant source of "sea power" for many decades to come. It is important, however, that research and development be conducted rigorously on renewable energies from the sea.
These alternative sources can be divided into wind, motions of the oceans, and differences in seawater temperature. With varying degrees of success each of these areas has progressed to operational stages ranging from proof of concept to viable production of energy.
"Wind farms," which use groups of giant wind-driven rotors on towers that can be up to 80 feet high, already are producing energy on a commercial basis. Most of these installations are on land, but prevailing winds in offshore areas are attracting attention. The largest offshore complex of wind machines is in Denmark. Recently, there has been a proposal to set up a large wind farm off Cape Cod. Many local residents have objected to the adverse visual impact while agreeing that this form of energy can reduce atmospheric emissions produced by fossil-fuel power plants. Will the "not in my backyard" forces win? This remains an issue in progress.
Some scientists claim that wind-farm structures can interfere with bird migrations and that collisions can result in many bird deaths. Each year tens of millions of birds die as a result of mankind's activities. The question is, how much would be added to the total by the extensive use of wind farms?
The motions of the oceans that generate energy consist of tidal ranges, wave forces, and current flow. The best-known tidal-energy power system is in France. The Ranee River feeding into the Bay of Biscay has a tidal range of 35 feet. Thirty years ago a dam was put in place that lets the tidal flow pass through turbines when the tide is coming in. This water then is stored behind the dam and the gates shut. When the sea level in the bay is near its lowest level the impounded water is released through the gates to generate more power. Several other sites in the world have been considered for tidal energy; perhaps the most well known is the Bay of Fundy in Canada, where the tidal range is up to 50 feet.
Wave power is more elusive and offers a good example of the considerable distance between theory and practice. Theory tells us that along a shoreline a three-foot-high wave six feet wide produces the equivalent of 35 horsepower. The problem is, how do you capture that energy? Various devices have been tested, with most of them being anchored platforms through which the waves flow to operate a turbine or where the rocking motion of the platform is converted to mechanical energy much as in a self-winding wristwatch. In many places where waves crash into rocky cliffs there is a natural phenomenon called a blowhole, where the water has cut a natural tunnel through the rock and jets up in an impressive display. In Norway, experimental man-made tunnels have been cut to direct water through turbines to generate power.
Ocean temperature differences between warm surface water and deep cold water can be used to generate power. This is ocean thermal energy conversion, and it has been around in theory for many decades. It has yet to become operational in any large scale. Basically, the power system in this case is a steam plant. The working fluid is a substance with a low boiling point such as ammonia. The hot surface water boils the fluid into steam, which passes through a huge low-pressure turbine. The exiting steam is condensed by the cold deep-ocean water. Because of inherent low efficiencies, this type of power system would need to be quite large and require enormous volumes of water to pass through it to be worthwhile. There also must be at least a 30[degrees]F temperature difference between the warm and cold water supplies. This means that ocean thermal energy conversion will be useful mostly in tropical regions.
To move from theory to commercial practice generally requires long-term research-and-development investments. Continuous technological progress ensures that laboratory curiosities some day may become operational. In a world of growing populations and finite petroleum resources, looking for renewable sea power is a good national investment.