Like solar radiation, the ocean is another widely available and distributed energy source. The energy in the ocean is in part kinetic energy in the form of tides, currents, and waves, and part thermal energy from the sun stored as heat. The quantity of energy stored in the ocean is enormous but can be quite diffuse; in some cases, it is about the same as from solar radiation and, as a result, it is difficult to harness. Marine energy systems can be classified into four main groups:
-Tidal-barrage energy
-Wave energy
-Tidal/marine currents
-Ocean thermal-energy conversion.
The following Sections briefly summarize some of the interesting aspects of these technologies.
Tidal-Barrage Energy Systems
This type of energy system is equivalent to a low head hydrosystem and makes use of the rise and fall of the tides. A dam is built to separate the open water from a reservoir and the changing tides will drive water through the turbines. Electricity can be generated by water flowing either way. Also, the system can be used for pumped storage for cheap off-peak electricity. A 240-MW electric-generation power plant was built in France in the 1960s. There have been plans to design and build GW size plants in the Bay of Fundy in Canada and in the Severn estuary in Great Britain, but the costs are expected to be extremely high. Though such a system does not emit environmental pollutants, the effects on a tidal estuary and on the complex and often fragile ecosystem are poorly understood.
Wave Energy Systems
Waves are a source of kinetic energy, which can be converted via a generator to electrical energy. Technologies have been developed to exploit wave energy offshore, as well as along the shoreline, but most demonstration units have been built along the shoreline because of the relative simplicity. There are several types of shoreline wave energy units but each relies on the periodic nature of waves. The oscillating water column device uses the vertical motion of the wave to compress an air column and drive a turbine. A tapered channel device uses wave energy to move water into an elevated reservoir, where it can be used to drive a turbine. These systems can be used to drive electrical generators.
Tidal Marine Currents
Tidal power energy from ocean currents can be converted to electrical energy using large submerged turbines. Since the motion of sea water is rather slow in most places, the amount of power/energy, which can be extracted, is small; this is due to the cubic relationship between velocity and power. Grid connected systems are being investigated. The turbine designs are similar to those for wind turbines. Marine-current turbines should operate near to the surface, where the velocity is highest.
Ocean Thermal- Energy Converters (OTEC)
OTEC systems are heat engines that rely on the temperature difference of the ocean at different depths to drive a Rankine (steam/vapor) cycle. Warm seawater is heat exchanged with a low-boiling temperature fluid such as ammonia. There are several challenges in developing large systems:
-the need to circulate large volumes of sea water through pipes,
-the small temperature change between source and sink, limiting the theoretical efficiency for thermal to electric energy conversion which at best is of the order of 7% in a practical system and less than half of this value, and
-the need to lower the cost of transmission of electricity to the land.
