Tidal Energy
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
24 hours. For those tidal differences to be harnessed into electricity, the
difference between high and low tides must be more than 16 feet (or at least 5
meters). However, there are only about 40 sites on Earth with tidal ranges of
this magnitude.
Tidal Energy Technologies
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
forcing water through turbines, which activate a generator. Gates and turbines
are installed along the dam. When the tides produce an adequate difference in
the level of water on opposite sides of the dam, the gates are opened. The water
then flows through the turbines. The turbines turn an electric generator to
produce electricity.
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 spin via tidal currents typical of coastal waters. Some of these
currents run at 5–8 knots (5.6–9 miles per hour) and generate as much energy
as winds of much higher velocity. Because seawater has a much higher density
than air, ocean currents carry significantly more energy than air currents
(wind).
Tidal Turbines
Tidal turbines look like wind turbines. They are arrayed underwater in rows, as
in some wind farms. The turbines function best where coastal currents run
between 3.6 and 4.9 knots (4 and 5.5 mph). In currents of that speed, a
49.2-foot (15-meter) diameter tidal turbine can generate as much energy as a
197-foot (60-meter) diameter wind turbine. Ideal locations for tidal turbine
farms are close to shore in water 65.5–98.5 feet (20–30 meters) deep.
Wave Energy
Wave energy technologies extract energy directly from surface waves or from
pressure fluctuations below the surface. Renewable energy analysts believe there
is enough energy in ocean waves to provide up to 2 terawatts of electricity. (A
terawatt is equal to a trillion watts.)
However, wave energy cannot be harnessed everywhere. Wave power-rich areas of
the world include the western coasts of Scotland, northern Canada, southern
Africa, and Australia as well as the northeastern and northwestern coasts of the
United States. In the Pacific Northwest alone, it is feasible that wave energy
could produce 40–70 kilowatts (kW) per 3.3 feet (1 meter) of western
coastline.
Wave Energy Technologies
Wave energy can be converted into electricity by offshore or onshore systems.
The
Pelamis device is a semi-submerged, articulated structure, 120m long and 3.5m in
diameter, that is composed of cylindrical sections linked by hinged joints. The
wave-induced motion of these joints is resisted by hydraulic rams, which pump
high-pressure fluid through hydraulic motors via smoothing accumulators. The
hydraulic motors drive electrical generators to produce electricity. Power from
all the joints is fed down a single umbilical cable to a junction on the sea
bed. Several devices can be connected together and linked to shore through a
single seabed cable. Depending on the wave resource, machines will on average
produce 25-40% of the full rated output.Source: Wind & Hydropower
Technologies Program, U.S. Department of Energy, Energy Efficiency and Renewable
Energy
Offshore Systems
Offshore systems are situated in deep water, typically of more than 131 feet (40
meters). Sophisticated mechanisms—such as the Salter Duck—use the bobbing
motion of the waves to power a pump that creates electricity. Other offshore
devices use hoses connected to floats that ride the waves. The rise and fall of
the float stretches and relaxes the hose, which pressurizes the water, which, in
turn, rotates a turbine.
Specially built seagoing vessels can also capture the energy of offshore
waves. These floating platforms create electricity by funneling waves through
internal turbines and then back into the sea.
Onshore Systems
Built along shorelines, onshore wave power systems extract the energy of
breaking waves. Onshore system technologies include:
•Oscillating Water Columns: Oscillating water columns consist of a
partially submerged concrete or steel structure that has an opening to the sea
below the waterline. It encloses a column of air above a column of water. As
waves enter the air column, they cause the water column to rise and fall. This
alternately compresses and depressurizes the air column. As the wave retreats,
the air is drawn back through the turbine as a result of the reduced air
pressure on the ocean side of the turbine.
•Tapchans: Tapchans, or tapered channel systems, consist of a tapered channel
that feeds into a reservoir constructed on cliffs above sea level. The narrowing
of the channel causes the waves to increase in height as they move toward the
cliff face. The waves spill over the walls of the channel into the reservoir,
and the stored water is then fed through a turbine.
•Pendulor Devices: Pendulor wave-power devices consist of a rectangular box
that is open to the sea at one end. A flap is hinged over the opening, and the
action of the waves causes the flap to swing back and forth. The motion powers a
hydraulic pump and a generator.
credit: NOAA, U.S.
DOE, American Wind Energy Association, Bureau of Land Management, Sandia
National Labooratory Australian CRC for Renewable Energy,Fujista
Data
compiled from The British Antarctic Study, NASA, Environment Canada,
UNEP, EPA and other sources as stated and credited Researched by Charles
Welch-Updated daily This Website is a project of the The Ozone Hole Inc.
a 501(c)(3) Nonprofit Organization http://www.theozonehole.com
