Ocean energy essentially is either kinetic or thermal, but those forms of energy can be tapped in various ways.

The Roosevelt Island project uses tidal in-stream energy conversion, or TISEC. It generates electricity with a turbine generator driven by the kinetic energy of ebbing and flowing tides. Ideal conditions for TISEC are currents running between 3.6 and 4.9 knots. The density of the water flow means that a TISEC turbine of a given diameter can generate as much energy as a wind turbine four times as large. TISEC currently is one of the two most common types of ocean energy conversion.

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    Rising Tide

    Wave energy conversion (WEC), the other most common type at present, is the subject of extensive research in Europe. Most are offshore systems, using the bobbing motion of waves to operate pumps that generate electricity. Pelamis, for example, a string of railcar-sized articulated tubes tethered at one end, oscillates with the waves. Rotation at the joints compresses hydraulic pistons, powering generators. PowerBuoy, anchored to the ocean floor, pumps hydraulic fluid to a motor driving a generator as it rides up and down on the waves. Offshore WEC systems must be sited where waves have not yet begun to lose their energy on the continental shelf, but not so far from shore that transmission cables become long and costly. Onshore WEC systems, less common, exploit the energy in breaking waves.

    Ocean thermal energy conversion (OTEC) exploits temperature differences between cold deep seawater and warm surface water in tropical oceans to drive turbines. One method uses a heat exchanger to boil ammonia, which drives a turbine generator. A second heat exchanger condenses the vapor and recycles the ammonia. OTEC’s reliance on large temperature differences restricts its application to the tropics. The Natural Energy Laboratory of Hawaii Authority is leading research on OTEC.