Ocean Energy
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Introduction
Oceans cover more than 70 percent of the Earth's surface. As the world's largest solar collectors, they generate thermal energy from the sun. Oceans also produce mechanical energy from the tides and waves. Even though the sun affects all ocean activity, the gravitational pull of the moon primarily drives the tides. And the wind powers the ocean waves.
Most people have been witness to the awesome power of the world's oceans. For at least a thousand years, scientists and inventors have watched ocean waves explode against coastal shores, felt the pull of ocean tides, and dreamed of harnessing these forces. As early as the 11th century, millers in Britain figured out how to use tidal power to grind their grain into flour. But it's only been in the last century that scientists and engineers have begun to look at capturing ocean energy to make electricity.
Because ocean energy is abundant and non-polluting, today's researchers are exploring ways to make ocean energy economically competitive with fossil fuels and nuclear energy. European Union (EU) officials estimate that by 2010 ocean energy sources will generate more than 950 megawatts (MW) of electricity—enough to power almost a million homes in the industrialized world.
This publication will give you a brief overview of the technologies that harness the ocean's power, including the prospects and challenges for this vast renewable energy source.
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. But 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 planet with tidal ranges of this magnitude.
Where tidal power generation is possible, the most prevalent technology is one similar to that used in traditional hydroelectric plants. The first requirement is a dam, or barrage, across a bay or estuary. Gates and turbines are installed along the dam. Since building dams is an expensive proposition, the best tidal sites are those where a bay has a narrow opening. When the tides produce an adequate difference in the level of the water on opposite sides of the dam, the gates are opened, water flows through the turbines, and the turbines turn an electric generator to produce electricity.
France's La Rance station is the only industrial-sized tidal power station in the world. It produces 240 MW of power via a barrage across the estuary of the river Rance, near Saint Malo in Brittany. The plant went on-line in 1966. It supplies about 90 percent of Brittany's electricity. In Canada, the experimental Annapolis Royal Station in Nova Scotia produces about 20 MW of power. Near Murmansk, Russia, there is a small, 0.4-MW tidal power plant.
In the 1930s, the United States considered building a barrage-style tidal power plant at Passamaquoddy Bay, Maine, but the project was dropped because it was not considered economically viable at the time. Currently, there are no tidal power plants in the United States, and none are planned. However, conditions are good for tidal power generation in both the Pacific Northwest and the Atlantic Northeast regions of the country.
Tidal Fence
Another technology to harness tidal energy is the 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 to 8 knots (5.6 to 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).
There are no large-scale commercial tidal fences currently in operation anywhere in the world. However, there are plans to construct a fence across the Dalupiri Passage between the islands of Dalpiri and Samar in the Philippines.
Tidal Turbine
A third technology is the 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 to 30 meters (65.5 to 98.5 feet).
Currently, there are no operational tidal turbine farms. But European Union officials had identified 106 sites in Europe as suitable locations for such farms. The Philippines, Indonesia, China, and Japan also have underwater turbine farm sites that could be developed in the future.
Wave Power
Harnessing the power in ocean waves is another way to extract energy from the seas. 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 to 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.
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 bobbing mechanisms, such as the Salter Duck, created by Scottish physicist Stephen Salter, 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.
In the United States, at least one company is developing buoys, which dangle specialized plastic streamers that create electricity when deformed by an outside force such as water. The buoys generate 20 kW of electricity each and have been developed to recharge Navy robot submarines. However, its inventors say the devices could supply electricity for offshore desalination plants. Arrays of buoys could even provide enough electricity for a small community.
Another way to capture the energy of offshore waves is via specially built seagoing vessels. These floating platforms create electricity by funneling waves through internal turbines and then back into the sea. The Japan Marine Technology Center has developed a prototype wave power vessel that carries three air turbine generator units. Called the Mighty Whale, the vessel's designed to be anchored to the seabed but can be remotely controlled from shore. Beyond producing electricity, Japanese researchers have found that the calm seas created astern of the Mighty Whale can be used for fish farming or watersports.
Onshore Systems
Onshore wave power systems, as their name suggests, are those built along shorelines to extract the energy in breaking waves. There are three general varieties of onshore system technologies: the oscillating water column, the Tapchan, and the pendulor device.
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.
Several oscillating water column devices have been built worldwide; some have been constructed in existing breakwaters. For instance, India has an oscillating water column wave-energy plant under testing at Vizhinjam, Kerala. A 500-kW oscillating water column plant is being built on the island of Pico in the Azores. It is expected to generate enough electricity to power several hundred island homes. In November 2000, the world's first commercial wave-power plant, the Limpit, was commissioned on the rocky west coast of the Scottish island Islay. The Limpit, which uses oscillating water column technology, also generates 500 kW.
The tapchan, or tapered channel system, is an onshore wave-energy power generation device that has its roots in traditional hydroelectric power plant technology. The 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. Because the site requirements for tapchan systems are daunting—low tidal ranges and cliff-like shoreline characteristics—tapchan systems have yet to be constructed for commercial purposes. However, a demonstration system was built in Toftesfallen, Norway, during the 1980s. It functioned successfully until it was damaged during maintenance operations.
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. Worldwide, only small pendulor devices have been constructed.
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.
Even though it sounds technologically sophisticated, OTEC technology is not new. It has progressed in fits and starts since the late 1800s. 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 actually built the first OTEC plant. Claude built his plant in Cuba in 1930. The system produced 22 kW of electricity with a low-pressure turbine. In 1935, Claude constructed another plant, this time 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-MW 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, when the Natural Energy Laboratory of Hawaii Authority was established at Keahole Pointe on the Kona coast of Hawaii. The Laboratory has become one of the world's leading test facilities for OTEC technology. The Japanese government also continues to fund research and development in OTEC technology.
Some energy experts believe that if it could become cost-competitive with conventional power technologies, OTEC could produce billions of watts of electrical power. Bringing costs into line is still a huge challenge, however. All OTEC plants require an expensive, large diameter intake pipe, which is submerged a mile or more into the ocean's depths, to bring very cold water to the surface. This cold seawater is an integral part of each of the three types of OTEC systems: closed-cycle, open-cycle, and hybrid.
Closed-Cycle
Closed-cycle systems use fluid with a low-boiling point, such as ammonia, to rotate a turbine to generate electricity. Here's how it works. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Then, 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.
Then, the Natural Energy Laboratory in 1999 tested a 250-kW pilot OTEC closed-cycle plant, the largest such plant ever put into operation. Since then, there have been no tests of OTEC technology in the United States, largely because the economics of energy production today have delayed the financing of a permanent, continuously operating plant.
Outside the United States, the government of India has taken an active interest in OTEC technology. India has built and plans to test a 1-MW closed-cycle, floating OTEC plant.
Open-Cycle
Open-cycle OTEC uses 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 open-cycle plants. Energy conversion efficiencies as high as 97 percent were achieved. In May 1993, an open-cycle OTEC plant at Keahole Point, Hawaii, produced 50,000 watts of electricity during a net power-producing experiment. This broke the record of 40,000 watts set by a Japanese system in 1982.
Hybrid
Hybrid 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 produces electricity.
Some Proposed Projects
OTEC projects on the drawing board include a small plant for the U.S. Navy base on the British island of Diego Garcia in the Indian Ocean. There, a proposed 8-MW OTEC plant, backed up by a 2-MW gas turbine, would replace an existing 15-MW gas turbine power plant. A private U.S. company also has proposed building at 10-MW OTEC plant on Guam.
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 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 because so much energy is required to pump the large volume of water needed and because of the expense involved in 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.
A New Source of Natural Gas?
Researchers now are beginning to imagine a new frontier in ocean energy development: methane gas harvesting. Methane is the main ingredient of natural gas, which is widely used for electric power generation, and for heating homes and commercial buildings.
The idea that ocean-based energy farms could grow kelp crops to produce methane first appeared 25 years ago, but it didn't amount to much because kelp couldn't produce enough methane to make the farms economical. Recently, however, researchers began looking into a little known microbe called Methanococcus jannaschii in their quest to harvest methane from the sea.
M. jannaschii was first discovered at a Pacific Ocean thermal vent in 1983. This unique single-celled organism is one of the oldest life forms on Earth and is able to live and grow, while producing methane as a byproduct, in the complete absence of sunlight. Researchers think they may, with further understanding of the organism, be able to genetically engineer it to produce methane in sufficient quantities that it could become another source of renewable energy from the ocean.
Economic and Environmental Challenges
Although ocean energy is renewable and clean, it is not without environmental challenges. For instance, 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.
In general, careful site selection is the key to keeping the environmental impacts of OTEC and wave energy systems 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. Similarly, wave energy system planners can choose sites that preserve scenic shorefronts and avoid areas where wave energy systems are likely to significantly alter flow patterns of sediment on the ocean floor.
Another challenge with ocean energy systems is economics. It doesn't cost much to operate ocean energy facilities, but they are very expensive to build. For example, construction costs for tidal power plants are high, and payback periods are long. The cost of a proposed tidal power plant across the Severn River in the United Kingdom is estimated at about $12 billion, far more expensive than even the largest fossil fuel power plants. As a result, the cost per kilowatt-hour of tidal power is not competitive with conventional fossil fuel power.
Wave energy systems also cannot compete economically with traditional power sources. However, the costs to produce wave energy are coming down, and some European experts predict that wave power devices will find lucrative niche markets soon. Once built, however, wave energy systems (and other ocean energy plants) should have low operation and maintenance costs because the fuel they use—seawater—is free.
Like tidal power plants, 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.
Once these challenges can be overcome through technological advancements, ocean energy will gain more leverage as a viable renewable energy option.
