Environmental Engineering Reference
In-Depth Information
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Ocean
Almost three-fourths of the surface of the earth is covered by water, most of it in oceans. Technolo-
gies for deriving electrical power from the ocean include tidal power, wave power, ocean thermal
energy conversion, and ocean currents. Of these, the three most well-developed technologies are
tidal power, wave power, and ocean thermal energy conversion. All of them require construction
of onshore substations, transmission lines, and submersible electric cables to transport electrical
current to shore for incorporation into an electrical grid.
Some of the oldest ocean energy technologies use tidal power. All coastal areas experience two
high tides and two low tides over a period of slightly more than twenty-four hours. For those tidal
differences to be harnessed as electricity, the difference between high and low tides must be more
than sixteen feet (or at least five meters). However, there are only about forty sites on earth with
tidal ranges of this magnitude, so the utility of tidal power may be geographically limited. Cur-
rently, there are no tidal power plants in the United States, but conditions are favorable for tidal
power generation in the Pacific Northwest and the Atlantic Northeast regions (USDOI 2011a).
Tidal energy technologies include barrages or dams, tidal fences, and tidal turbines.
Barrages or Dams
A barrage or dam is typically used to convert tidal energy into electricity by capturing tidal flows
behind a stationary structure and forcing water through turbines, which drive a generator. Gates
and turbines are installed along a dam. When tides produce an adequate difference in the level of
water on opposite sides of the dam, the gates are opened and water flows through the turbines,
turning an electric generator to produce electricity (USDOI 2011a).
Tidal Fences
Tidal fences look like giant turnstiles. They can reach across channels between small islands or
across straits between the mainland and an island. The turnstiles turn and pivot slowly around a
central point containing a generator using tidal currents typical of coastal waters. Some of these
currents run at five to eight knots (5.6 to 9 miles per hour) and generate as much energy as winds
of much higher velocity (USDOI 2011a).
Tidal Turbines
Tidal turbines usually look like wind turbines. They may be moored to the bottom and arrayed
underwater in rows, similar to some wind farms. The turbines function best where coastal currents
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