Environmental Engineering Reference
In-Depth Information
DID YOU KNOW?
Spring tides have a range about twice that of neap tides; the other cycles can
cause further variations of up to 15%. The tidal range is amplified in estuar-
ies, and in some situations the shape of the estuary is such that near resonance
occurs.
T
idal
e
nergy
T
eChnologies
Some of the oldest ocean energy technologies use tidal power. Tidal power is more
predictable than solar power and wind energy. All coastal areas consistently experi-
ence two high and two low tides over a period of slightly greater than 24 hours. For
those tidal differences to be harnessed into electricity, the difference between high
and low tide must be at least 5 meters, or more than 16 feet. There only about 40 sites
on the Earth with tidal ranges of this magnitude. Currently, there are no tidal power
plants in the United States, although conditions are good for tidal power generation
in both the Pacific Northwest and the Atlantic Northeast regions of the country. Tidal
energy technologies include the following:
•
Tidal barrages
—A barrage is a simple generation system for tidal plants
that involves installing a dam, or barrage, across an inlet. Sluice gates
(gates commonly used to control water levels and flow rates) on the bar-
rage allow the tidal basin to fill on the incoming high tides and to empty
through the turbine system on the outgoing tide, also known as the ebb
tide. Two-way systems generate electricity on both the incoming and out-
going tides. A potential disadvantage of a barrage tidal power system is
the effect a tidal station can have on plants and animals in estuaries. Tidal
barrages can change the tidal level in the basin and increase the amount of
matter in suspension in the water (turbidity). They can also affect naviga-
tion and recreation.
•
Tidal fences
—These look like giant turnstiles. A tidal fence has vertical-
axis turbines mounted in a fence. All the water that passes is forced through
the turbines. Some of these currents run at 5 to 8 knots (5.6 to 9 miles per
hour) and generate as much energy as winds of much higher velocity. Tidal
fences can be used in areas such as channels between two land masses.
Tidal fences are less expensive to install than tidal barrages and have less
impact on the environment tidal barrages, although they can disrupt the
movement of large marine animals.
•
Tidal turbines
—These are basically wind turbines in the water that can be
located anywhere with strong tidal flow; they function best where coastal
currents run at between 3.6 and 4.9 knots (4 to 5.5 mph). Because water is
about 800 times more dense than air, tidal turbines have to be much sturdier
than wind turbines. Tidal turbines are heavier and more expensive to build
but capture more energy.
