Holiday greetings from the Southeast National Marine Renewable Energy Center!
Ocean energy, while renewable, clean, and plentiful, must be converted to electricity before it can replace more traditional forms of energy. To do this requires technology—machines of one kind or another.
Ocean tides, currents, and waves represent marine hydrokinetic energy—the energy of moving seawater. Tidal and open-ocean currents can be converted just as the wind is converted, using windmills or wind turbines. Of course, the turbines must be able to operate under water, but the principle is the same: the moving water turns a rotor, the rotor turns a generator, and electricity is the result. Waves can be converted in various ways, many involving devices that ride up and down on the crests and troughs and, in the process, drive a mechanical linkage that turns a generator.
Ocean thermal energy, although not so obvious as marine hydrokinetic energy, is conceptually quite simple, because it works just like traditional electrical power plants. A heat source (such as burning coal) is used to boil a working fluid (water), creating high-pressure steam. The high-pressure steam is used to turn a turbine and a generator, and electricity is produced. Once past the turbine, the steam is cooled back to liquid water using a “cold” source—generally air, in the case of traditional power plants. This process is called a Rankine Cycle. Because the tropical oceans are quite warm at the surface and much colder at depth, the temperature difference can be used to drive a Rankine Cycle, only with a different working fluid. It turns out that ammonia boils at the warm ocean surface temperatures and re-condenses at the cold ones. So the temperature difference between the ocean surface and the deep water becomes a source of energy—ocean thermal energy.