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U.S. Department of Energy - Energy Efficiency and Renewable Energy
Energy Savers
Ocean Tidal Power
Some of the oldest ocean energy technologies use tidal power. All coastal areas consistently experience 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 tides must be at least five meters, or more
than 16 feet. There are only about 40 sites on the Earth with tidal ranges of this magnitude.
Currently, there are no tidal power plants in the United States. However, conditions are good for tidal power
generation in both the Pacific Northwest and the Atlantic Northeast regions of the country.
Technologies
Tidal power technologies include the following:
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Barrage or dam
A barrage or dam is typically used to convert tidal energy into electricity by forcing the water
through turbines, activating a generator. Gates and turbines are installed along the dam. When the
tides produce an adequate difference in the level of the 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.
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Tidal fence
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).
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Tidal turbine
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 at between 3.6 and 4.9 knots (4 and 5.5 mph).
In currents of that speed, a 15-meter (49.2-feet) diameter tidal turbine can generate as much energy
as a 60-meter (197-feet) diameter wind turbine. Ideal locations for tidal turbine farms are close to
shore in water depths of 20–30 meters (65.5–98.5 feet).
Environmental and Economic Challenges
Tidal power plants that dam estuaries can impede sea life migration, and silt build-ups behind such facilities
can impact local ecosystems. Tidal fences may also disturb sea life migration. Newly developed tidal turbines
may prove ultimately to be the least environmentally damaging of the tidal power technologies because they
don't block migratory paths.
It doesn't cost much to operate tidal power plants, but their construction costs are high and lengthen payback
periods. As a result, the cost per kilowatt-hour of tidal power is not competitive with conventional fossil fuel
power.
U.S. Department of Energy - Energy Efficiency and Renewable Energy
Energy Savers
Ocean Wave Power
Wave power devices extract energy directly from surface waves or from pressure fluctuations below the
surface. Renewable energy analysts believe there is enough energy in the ocean waves to provide up to 2
terawatts of electricity. (A terawatt is equal to a trillion watts.)
Wave power can't be harnessed everywhere. Wave-power rich areas of the world include the western coasts
of Scotland, northern Canada, southern Africa, Australia, and the northeastern and northwestern coasts of the
United States. In the Pacific Northwest alone, it's feasible that wave energy could produce 40–70 kilowatts
(kW) per meter (3.3 feet) of western coastline. The West Coast of the United States is more than a 1,000
miles long.
Technologies
Wave energy can be converted into electricity through both offshore and onshore systems.
Offshore Systems
Offshore systems are situated in deep water, typically of more than 40 meters (131 feet). Sophisticated
mechanisms—like 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 in breaking waves. Onshore system
technologies include the following:
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Oscillating water column
The oscillating water column consists 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.
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Tapchan
The tapchan, or tapered channel system, consists of a tapered channel, which 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.
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Pendulor device
The pendulor wave-power device consists of a rectangular box, which 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.
Environmental and Economic Challenges
In general, careful site selection is the key to keeping the environmental impacts of wave power systems to a
minimum. Wave energy system planners can choose sites that preserve scenic shorefronts. They also can
avoid areas where wave energy systems can significantly alter flow patterns of sediment on the ocean floor.
Economically, wave power systems have a hard time competing with traditional power sources. However,
the costs to produce wave energy are coming down. Some European experts predict that wave power devices
will find lucrative niche markets. Once built, they have low operation and maintenance costs because the fuel
they use—seawater—is free.
Learn More
Related Links
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Introduction to Ocean Energy Systems
European Commission
Waveenergy.dk
U.S. Department of Energy - Energy Efficiency and Renewable Energy
Energy Savers
Ocean Thermal Energy Conversion
A process called Ocean Thermal Energy Conversion (OTEC) uses the heat energy stored in the Earth's
oceans to generate electricity.
OTEC works best when the temperature difference between the warmer, top layer of the ocean and the
colder, deep ocean water is about 20°C (36°F). These conditions exist in tropical coastal areas, roughly
between the Tropic of Capricorn and the Tropic of Cancer. To bring the cold water to the surface, OTEC
plants require an expensive, large diameter intake pipe, which is submerged a mile or more into the ocean's
depths.
Some energy experts believe that if it could become cost-competitive with conventional power technologies,
OTEC could produce billions of watts of electrical power.
History
OTEC technology is not new. In 1881, Jacques Arsene d'Arsonval, a French physicist, proposed tapping the
thermal energy of the ocean. But it was d'Arsonval's student, Georges Claude, who in 1930 actually built the
first OTEC plant in Cuba. The system produced 22 kilowatts of electricity with a low-pressure turbine. In
1935, Claude constructed another plant aboard a 10,000-ton cargo vessel moored off the coast of Brazil.
Weather and waves destroyed both plants before they became net power generators. (Net power is the
amount of power generated after subtracting power needed to run the system.)
In 1956, French scientists designed another 3-megawatt OTEC plant for Abidjan, Ivory Coast, West Africa.
The plant was never completed, however, because it was too expensive.
The United States became involved in OTEC research in 1974 with the establishment of the Natural Energy
Laboratory of Hawaii Authority. The Laboratory has become one of the world's leading test facilities for
OTEC technology.
Technologies
The types of OTEC systems include the following:
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Closed-Cycle
These systems use fluid with a low-boiling point, such as ammonia, to rotate a turbine to generate
electricity. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point
fluid is vaporized. The expanding vapor turns the turbo-generator. Cold deep-seawater—pumped
through a second heat exchanger—condenses the vapor back into a liquid, which is then recycled
through the system.
In 1979, the Natural Energy Laboratory and several private-sector partners developed the mini
OTEC experiment, which achieved the first successful at-sea production of net electrical power
from closed-cycle OTEC. The mini OTEC vessel was moored 1.5 miles (2.4 km) off the Hawaiian
coast and produced enough net electricity to illuminate the ship's light bulbs and run its computers
and televisions.
In 1999, the Natural Energy Laboratory tested a 250-kW pilot OTEC closed-cycle plant, the largest
such plant ever put into operation.
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Open-Cycle
These systems use the tropical oceans' warm surface water to make electricity. When warm seawater
is placed in a low-pressure container, it boils. The expanding steam drives a low-pressure turbine
attached to an electrical generator. The steam, which has left its salt behind in the low-pressure
container, is almost pure fresh water. It is condensed back into a liquid by exposure to cold
temperatures from deep-ocean water.
In 1984, the Solar Energy Research Institute (now the National Renewable Energy Laboratory)
developed a vertical-spout evaporator to convert warm seawater into low-pressure steam for opencycle plants. Energy conversion efficiencies as high as 97% were achieved. In May 1993, an opencycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net
power-producing experiment.
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Hybrid
These systems combine the features of both the closed-cycle and open-cycle systems. In a hybrid
system, warm seawater enters a vacuum chamber where it is flash-evaporated into steam, similar to
the open-cycle evaporation process. The steam vaporizes a low-boiling-point fluid (in a closed-cycle
loop) that drives a turbine to produce electricity.
Other Technologies
OTEC has important benefits other than power production. For example, air conditioning can be a byproduct.
Spent cold seawater from an OTEC plant can chill fresh water in a heat exchanger or flow directly into a
cooling system. Simple systems of this type have air conditioned buildings at the Natural Energy Laboratory
for several years.
OTEC technology also supports chilled-soil agriculture. When cold seawater flows through underground
pipes, it chills the surrounding soil. The temperature difference between plant roots in the cool soil and plant
leaves in the warm air allows many plants that evolved in temperate climates to be grown in the subtropics.
The Natural Energy Laboratory maintains a demonstration garden near its OTEC plant with more than 100
different fruits and vegetables, many of which would not normally survive in Hawaii.
Aquaculture is perhaps the most well-known byproduct of OTEC. Cold-water delicacies, such as salmon and
lobster, thrive in the nutrient-rich, deep seawater from the OTEC process. Microalgae such as Spirulina, a
health food supplement, also can be cultivated in the deep-ocean water.
As mentioned earlier, another advantage of open or hybrid-cycle OTEC plants is the production of fresh
water from seawater. Theoretically, an OTEC plant that generates 2-MW of net electricity could produce
about 4,300 cubic meters (14,118.3 cubic feet) of desalinated water each day.
OTEC also may one day provide a means to mine ocean water for 57 trace elements. Most economic
analyses have suggested that mining the ocean for dissolved substances would be unprofitable. Mining
involves pumping large volumes of water and the expense of separating the minerals from seawater. But with
OTEC plants already pumping the water, the only remaining economic challenge is to reduce the cost of the
extraction process.
Environmental and Economic Challenges
In general, careful site selection is the key to keeping the environmental impacts of OTEC to a minimum.
OTEC experts believe that appropriate spacing of plants throughout the tropical oceans can nearly eliminate
any potential negative impacts of OTEC processes on ocean temperatures and on marine life.
OTEC power plants require substantial capital investment upfront. OTEC researchers believe private sector
firms probably will be unwilling to make the enormous initial investment required to build large-scale plants
until the price of fossil fuels increases dramatically or until national governments provide financial
incentives. Another factor hindering the commercialization of OTEC is that there are only a few hundred
land-based sites in the tropics where deep-ocean water is close enough to shore to make OTEC plants
feasible.
Learn More
Department of Energy Resources

Ocean Thermal Energy Conversion
National Renewable Energy Laboratory
Federal Government Resources
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Legislative Atlas
National Oceanic and Atmospheric Administration
State & Local Resources
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Ocean Thermal Energy Conversion
State of Hawaii
Related Links
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Natural Energy Laboratory of Hawaii Authority
PROS AND CONS OF TIDAL ENERGY USE
HYDRO ENERGY FROM THE MOON
Tidal energy use harnesses the water flow created primarily by the moon orbiting the Earth. As water is pulled toward the gravity of
the moon, currents are created that can turn generator turbines.
The interplay of gravitational fields of the moon and the sun combined with the rotation of Earth, creates a twice a day ebb and flow of
the tides of our world that varies in height and strength.
Those variations in height and strength are completely predictable. As we’ll see later, that predictability is an important aspect of tidal
energy use.
Though renewable, practical tidal energy use will be limited. Tidal flows are global, but the key to using them economically is finding
either natural high tidal flow areas, or large tidal basins that can be easily dammed to channel water through turbines.
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ENVIRONMENTAL FRIENDLINESS Tidal energy use involving dams creates many of the same environmental concerns as damming rivers. Tidal dams restrict fish
migration and cause silt build up which affects tidal basin ecosystems in negative ways.
Systems that take advantage of natural narrow channels with high tidal flow rates have less negative environmental impact than
dammed systems. But they are not without environmental problems.
Both systems use turbines that can cause fish kills. But these are being replaced by new, more fish friendly turbines. The art and
science of environmentally friendly hydro engineering is well advanced and will certainly be applied to any tidal energy project.
But even with dams, the environmental impact of tidal energy projects may prove to be smaller than our use of any other energy
resource. Economics will severely limit the number of tidal energy projects.
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COST Tidal energy projects involving tidal dams are more expensive per KW of installed power than similar size systems that use river
dams. Tidal flow is intermittent. Twice a day tidal flows go through a flood stage, slow down, stop, reverse into an ebb tide, slow
down, stop, and repeat the cycle.
This constant start and stop cycle creates intermittency problems similar to wind turbines and wave generators. Though a tidal dam
might be identical to a river dam in every way including cost; the tidal dam will produce less than half the amount of electricity.
A typical average plant load factor for tidal energy generators is about 27%. Load factor defines the amount of actual power output
expected from a given capacity. Installed generating capacity of 100 MW with a load factor of 27% would produce only 27 MW per
hour when averaged over a given time, usually a year.
That makes tidal energy expensive.
This U.S. Department of Energy Tidal Energy Report concludes that tidal power costs are not competitive with fossil fuel plants.
But a private company, Blue Energy of Canada, believes that they can generate tidal electricity at rates that are highly competitive
with existing conventional power generators at rates of less than $.05 per KWh.
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AVAILABILITY The key to reliable, economic power from tidal energy involves properly engineered, economical turbine generators placed at well
researched sites with high tidal flow rates.
Because of intermittency and variable flow problems of tidal energy, it is a very limited resource. The DOE Tidal Energy link, above,
states that there are only about 40 really good sites on Earth with high enough flows to be considered economically practical.
Few studies of tidal energy resources have been done, so information is sketchy at best.
The World Energy Council Ocean Current Report states that total electrical power available from tidal energy use is about 450 GW of
installed capacity. The report is a bit confusing to read and appears to be mixing tidal information with other ocean current
information. Still, it’s worth reading.
That 450 GW figure seems to be compatible with the data on the WEC Tidal Energy page.
If we apply the .27 average load factor for tidal energy use, we can expect it to deliver about 450GW x 24 hrs x 365 days x .27 LF =
1064 TWh (Terrawatt hours) annually, or a little over 6% of global electrical demand.
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AESTHETICS Tidal energy projects involving dams would involve about the same aesthetic concerns as other dams. But many of the systems that
use natural tidal currents will be largely hidden from view.
Natural current driven tidal generators can be built into the structure of existing bridges. These generators will involve virtually no
aesthetic problems.
And, the fact that tidal energy use will be extremely limited means that any aesthetic concerns will also be limited.
Tidal energy use may not be a big player in our energy future, but it can make a contribution.
Tidal intermittency is completely predictable. Power output from tidal generators is also completely predictable. That predictability
makes tidal energy reliable and easy to integrate with the existing electrical power grid. All of that makes it valuable.
Though tidal energy use will provide only a small portion of electrical grid demand, it can be a reliable and important energy resource.
From: Biomicfuel.com
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The tidal power pros and cons should be considered carefully, as well as the benefits and drawbacks of other energy
sources
There are many sources of renewable energy in addition to tidal energy, and a combination of these may be needed to meet
the increasing energy demands of the world
Tidal power is very clean and efficient, but there are some disadvantages as well
Tidal power pros and cons must be evaluated, and compared to the pros and cons of other energy sources, to determine which ones are
the best for the earth and the environment, as well as being the most effective at meeting the global demand for power. The tides ebb
and flow, and this is caused by the gravitational pull of the moon, and to a lesser degree the sun, on the earth. Unlike the wind and sun,
the tides can be predicted, and they occur at regular intervals twice every single day. This means any shortage in supply can be
prevented, and these shortages can cause brownouts and blackouts because there is too much demand. Tidal power pros and cons
should be carefully considered against every other alternate renewable energy source, as well as all the fossil fuels, so that the best
choice can be made. Another benefit of tidal energy is the fact that this process does not emit greenhouse gases or particle pollution,
which fossil fuels like oil and coal do, which can have a devastating effect on the environment and all living things on the planet.
There is no mining or drilling involved either, which can rip up large chunks of the earth or poison the seas.
One of the tidal power pros and cons when this energy source is compared to solar power is that tidal power plants must be located in
a very specific location, one that meets all of the needed requirements. There must be a significant tidal flow of water through the area
to turn the equipment. Nova Scotia and France are the two countries which have successfully developed and built these facilities so
far. Solar power simply requires photo voltaic panels that are installed, and the sun to be shining. Tidal power pros and cons does
show that this source does outperform solar energy because it can consistently supply energy at predictable times and levels. With
solar energy, if the day is overcast or there is a storm brewing and many clouds fill the sky, it may not be possible to generate the
electricity needed to meet consumer demand. Energy from the tides is more consistent than solar or wind power, but geothermal heat
can also supply what is needed without polluting the environment or hurting the population. Geothermal energy requires very strict
specifications, and there are a limited number of places where these facilities are even possible.
Tidal power pros and cons when compared to biomass are mixed. Biomass programs can include municipal waste to energy programs,
which will clean up the garbage which litters the earth and waters, as well as all other organic material which is discarded or not used.
Algae and fungus species, as well as plants, trees, and other materials, can also be used to generate biofuels and electricity. Many of
theses processes have greenhouse gas emissions though, including carbon and methane among others, and tidal energy generation
does not have these emissions or pollution present. Some biomass, including trees and plants, absorb some carbon and help filter the
air while growing, so this does help offset any emissions from the biomass. Biomass is very flexible, with a large number of
feedstocks available that can be modified depending on the location. This is not true of renewable power that is supplied by the tides.
The future of energy does not necessarily lie in one specific alternative renewable energy source, but rather in a blend of different
sources which will allow for flexibility. Once the tidal power pros and cons are evaluated against all other possible sources, it quickly
becomes apparent that the tides will be one place where energy will be generated much more abundantly as technology advances.
Sites:
http://home.clara.net/darvill/altenerg/tidal.htm
Tidal Energy Images at:
http://www.mywindpowersystem.com/wp-content/uploads/2009/08/renewable-energy-tidal-2.gif
http://igutek.scripts.mit.edu/terrascope/tidal-energy-farm.jpg