The ocean holds vast amounts of energy. The endless motion of tides and waves, and the warmth of the upper layer of water can all be utilized to generate electricity. This makes the ocean potentially a limitless source of power. And with seawater as fuel, power produced by the ocean is clean. There are no emissions of carbon dioxide or any other gases that add to global warming. The challenge: taking all that potential energy and turning it into something usable.

Click for animation. Tidal barrage The tidal barrage is a dam that divides a tidal basin into two sections. The barrage has a series of openings with gates that can let water pass through or block the flow. Inside the opening are turbines. When the tidal flow reaches a certain level, the gates open and water flows from one basin to the other, depending on the direction of the tide. The turbines spin as water passes

The tides, the waves, and the thermal content of the ocean each require very different and specific technology to convert the existing energy to electricity. Only tidal power plants are proven energy providers. Tidal plants in France and Nova Scotia have operated for years, with similar plants planned in other suitable locations. Methods to utilize the energy in ocean waves are under development. There are prototype wave-energy plants operating in Scotland and Norway. The other technologies are still experimental.

The biggest limitation on ocean energy is the cost of building a plant. Construction costs are very high. Even with free fuel, it takes a very long time to pay off those costs. This means that electricity from fossil fuels is cheaper than power from the ocean, in most locations. The need for clean electricity may change that, and may make ocean energy more cost-effective, in the right locations.

Tidal Energy

The idea of harnessing the flow of the tide dates back to the Middle Ages in Europe. Tides powered waterwheels that turned grindstones for mills on the coast of Brittany. Colonists in New England also used tidal power. Along Long Island Sound in Connecticut, tidal coves became mill ponds, with sluice gates controlling the flow of water in and out of the pond. Eventually, other sources of energy surpassed tidal power to run mills. Tidal energy is the only ocean-energy source currently in regular use.

To understand how tidal energy works, you need to know a little about tides. Usually twice a day, but not always, the sea reaches a high point on a shore—high tide—and a low point on a shore—low tide. High and low tides occur at regular and predictable times. Gravity is the force behind tides. The pull of the Moon and the Sun on the Earth, as well as the rotation of the Earth, causes tidal currents. Weather also has a slight influence on tides: A powerful storm can cause higher than normal tides.

The simplest tidal-energy plant takes advantage of the landscape of a bay or an estuary. This is a place where the tide moves in and out of an area that is surrounded by land on two or three sides. A dam called a barrage is built across the bay or estuary. The barrage separates the tidal area into upper and lower basins. Turbines sit within the barrage. When there is enough of a difference in the water level on each side of the barrage, gates are opened to allow the water to flow past the turbines. The turbines spin as the water moves from one basin to the other, depending on the flow of the tide. The turning of the turbines powers a generator, which then makes electricity.

The oldest tidal-energy plant is La Rance Station in Brittany, France. This plant has operated since 1966 and generates 240 megawatts of power. The Annapolis Royal plant, in Nova Scotia, Canada, which opened in 1984, makes 20 megawatts of power. Tidal-energy plants have been built on the Barents Sea in Russia, in eight locations around China, and in India and Wales.

Tidal energy sounds simple, but if that were so, lots of bays and estuaries would have tidal power plants. Installing a tidal plant is expensive, so the plant must be able to generate enough electricity to make the investment worthwhile. That happens when there is a difference of at least 5 m (16 ft) between high and low tide in an estuary or a bay. Tides with a smaller difference than that do not generate enough electricity to pay for building the power plant. Such tides occur in only about 40 locations around the world.

The tidal fence operates on principles similar to the tidal barrage. Instead of a solid dam, the tidal fence consists of a series of turbines spaced apart in an open fence, more like gates across the water. These turbines spin on a vertical axis. One advantage of the tidal fence is that it does not need to completely block a bay to work. The tidal fence can be used in tidal streams or currents that flow through straits and channels.

Tidal currents can generate as much energy as winds of higher speeds. This is because ocean water has a higher density than air, and so carries much more energy than wind. A tidal fence requires currents of 5–8 knots (6–9 mph) to produce enough electricity to make the project worthwhile financially. However, a tidal fence is less expensive to install than a barrage. A tidal-fence project is in the works for the San Bernardino Strait, in the Philippines.

Tidal turbines, which resemble wind turbines, can be installed anywhere there is a strong tidal flow. The blades look like giant propeller blades. However, tidal-turbine blades are not as large as wind-turbine blades—often 15 m (50 ft) in diameter, as opposed to wind turbines, which can be 60 m (197 ft) in diameter. The turbines are anchored to the bottom in water about 20 to 30 m (66 to 99 ft) deep, in currents between 3.6 and 4.9 knots (4 and 5.5 mph). At those speeds, a tidal turbine can make as much electricity as a much larger wind turbine. There are plans to test tidal turbines in a number of locations, including New York City’s East River.

Tidal-Energy Issues

Tidal energy does not produce emissions. However, the impact of the barrages and turbines on sea life is unknown. We do know that, like dams, barrages affect migration, as well as the flow of sediment in and out of the estuary or bay. Therefore a barrage must certainly have an impact on the local ecosystem. Tidal fences resolve some of these issues. The open structure permits the movement of silt, sand, and smaller sea animals. However, larger fish and sea mammals may not be able to pass through the turbines without injury, which may affect migration patterns. Freestanding tidal turbines are likely to have the least effects on the ecosystem. Tidal-fence and -turbine projects have yet to be completed, though, and the effects remain unknown.

Right now, the biggest barrier to more common use of tidal energy is cost. Tidal plants are inexpensive to operate, especially since the seawater fuel is free. However, installation of these plants is costly, although tidal turbines are less expensive to build than barrages. The building costs make energy generated from these plants more costly than fossil-fuel energy, at this time.

Wave Energy

It’s easy to see the energy in waves that break on the shore. Whether they crash in a spray of white salty foam or smoothly run up the beach, waves are a relentless force. But wave energy is more than crashing waves. Offshore, there is even greater energy in the movement of the ocean. The steady bobbing of the waves never stops. Wave-energy researchers are looking at using both the energy of the breaking waves in onshore plants and the continuous movement of waves in offshore devices.

A number of different types of onshore plants are being tested. The oscillating water column is one approach to capturing wave energy. A hollow, partially submerged concrete or steel structure opens to the sea under the water. The interior of the structure contains a column of air above a column of water. Waves enter the structure and cause the water column to rise and fall. The movement of the water alternately compresses and depressurizes the air column. The compressed air created when a wave enters the structure is forced through a turbine attached to a generator. The wave retreats and air moves back through the turbine and into the structure, because of the reduced air pressure on the ocean side of the turbine. The oscillating water column uses a Wells turbine, which has uniquely shaped blades that permit it to continue to spin in the same direction no matter the direction of the air. A prototype of this was built on the coast of Scotland. It produces about 500 kilowatts of electricity.

The tapered-channel system, also known as TAPCHAN, uses the seawater to collect the wave energy. A reservoir is built into the shoreline cliffs at a height slightly above sea level. Leading into the reservoir is a tapered channel, which is wider at the ocean side and narrows as it goes into the reservoir. Waves enter the wide part of the channel and increase in height as the channel narrows. At some point the water spills over the sides of the channel and into the reservoir. The water returns to the ocean through a pipe. While in the pipe the water passes through a turbine attached to a generator. A prototype plant has been operating in Norway since 1985, with other projects under consideration.

The pendulor wave-power device reflects its name. A large rectangular box is installed at the shore. One end of this box opens to the water. The entrance of the open end has a flap that swings back and forth with the movement of the waves, like a pendulum. The back-and-forth motion powers a hydraulic pump attached to a generator. The pendulor wave-power device remains in the testing stages.

Offshore systems are usually set up in deeper water of 40 m (131 ft) or more. The Salter Duck uses the bobbing motion of the waves to move a pendulum back and forth. The pendulum connects to a generator. A series of Ducks can be set up in rows or fields to collect as much energy as needed.

The hosepump collects wave energy with a water-filled hose attached to a float. The hose stretches and relaxes with the motion of the waves, pressurizing the water inside the hose. The water is forced out of a one-way valve at the bottom of the hose and is sent to a turbine attached to a generator. Like the Salter Duck, hosepumps can be set up in rows or fields.

The pelamis, named for a sea snake, is a segmented floating tube that moves with the motion of the waves. The joints that connect the segments contain hydraulic motors. The motion of the water activates the motors, creating electricity. The electricity is then transmitted to shore. A pelamis prototype is being tested in Scotland.

Wave-Energy Issues

Selecting a location to build a wave-energy plant is tricky. Like wind, waves are not the same strength all the time: they increase in size with storms and decrease during calmer weather. A successful wave-energy plant requires productive, relatively consistent waves. There are a number of regions that are rich in wave power, including the western coasts of Scotland, northern Canada, southern Africa, and Australia; parts of the Hawaiian Islands; and the northeastern and northwestern coasts of the United States.

A location’s visual appeal also comes into play. Local residents would not be pleased to have a large wave-energy plant ruin a scenic area. Environmental impact presents other concerns. Plant builders do not want to significantly alter existing sediment movement patterns, as doing so could can have consequences for the whole region. In addition, the equipment must be able to stand up in rough weather.

Costs present the biggest hurdle for wave energy. Wave-power plants are expensive to build. Despite free fuel and the potential to produce lots of energy, it is likely to be difficult for a wave-power plant to provide cheaper energy than a fossil-fuel plant under current conditions.

Ocean Thermal Energy Conversion

In the ocean, the water temperature decreases with depth. Ocean thermal energy conversion (OTEC) takes advantage of this difference in temperature to create energy. The best locations for this kind of operation are in the tropics and subtropics, in spots where deep waters are relatively close to the land. In these waters, there can be a temperature difference of as much as 20 degrees C (36 degrees F) between the upper layer of water and deeper water.

French physicist Jacques Arsene d’Arsonval conceived of this idea in 1881. The first actual OTEC plant was built in 1930 by one of his students, Georges Claude, in Cuba. Claude built another on a vessel off the coast of Brazil in 1935. However, weather and waves demolished both plants before they were able to produce enough electricity to be useful. Other attempts have been made since then, but OTEC remains an experimental technology.

An OTEC plant is set up in tropical coastal waters. A pipe brings warm surface water into the plant. This water is used to create steam, which turns a turbine. The turbine powers a generator, which makes electricity. Cold deeper water is pulled up from about 1.6 km (1 mi) down by a large-diameter pipe. This cold water condenses the steam, and the cooled seawater returns to the ocean. Three different systems have been tested. An open-cycle system uses the seawater directly, with a pressurized boiler helping to create the steam (flashed steam). In a closed-cycle system, the warm water comes in contact with pipes containing a heat-exchanging fluid with a lower boiling point. The fluid powers the generator. A hybrid system uses the flashed steam of an open cycle to warm a heat-exchanging fluid to steam, which then powers the turbine.  

OTEC faces the same problems as all the other ocean-based energy systems. The building costs are high, making ocean energy more expensive than fossil-fuel energy. The environmental impact of an OTEC plant remains unknown.

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