HYDROPOWER, GEOTHERMAL POWER AND OCEAN ENERGY

GEOTHERMAL ENERGY

The Earth's heat---called geothermal energy---can be harnessed to produce electricity.Many technologies have been developed to take advantage of geothermal energy---the heat from the earth. This heat can be drawn from several sources: hot water or steam reservoirs deep in the earth that are accessed by drilling; geothermal reservoirs located near the earth's surface; and the shallow ground near the Earth's surface that maintains a relatively constant temperature of 50̊-60̊F. [Source: U.S. Department of Energy]

This variety of geothermal resources allows them to be used on both large and small scales. A utility can use the hot water and steam from reservoirs to drive generators and produce electricity for its customers. Other applications apply the heat produced from geothermal directly to various uses in buildings, roads, agriculture, and industrial plants. Still others use the heat directly from the ground to provide heating and cooling in homes and other buildings. [Ibid]

Technologies with geothermal applications include: 1) Direct use---Producing heat directly from hot water within the earth. 2) Electricity production---Generating electricity from the earth's heat. 3) Heat pumps---Using the Earth's shallow ground temperature for heating and cooling. [Ibid]

Geothermal Direct Use

Geothermal reservoirs of hot water, which are found a few miles or more beneath the Earth's surface, can be used to provide heat directly. This is called the direct use of geothermal energy. Geothermal direct use has a long history, going back to when people began using hot springs for bathing, cooking food, and loosening feathers and skin from game. Today, hot springs are still used as spas. But there are now more sophisticated ways of using this geothermal resource. In modern direct-use systems, a well is drilled into a geothermal reservoir to provide a steady stream of hot water. [Source: U.S. Department of Energy]

The water is brought up through the well, and a mechanical system---piping, a heat exchanger, and controls---delivers the heat directly for its intended use. A disposal system then either injects the cooled water underground or disposes of it on the surface. Geothermal hot water can be used for many applications that require heat. Its current uses include heating buildings (either individually or whole towns), raising plants in greenhouses, drying crops, heating water at fish farms, and several industrial processes, such as pasteurizing milk. [Ibid]

Geothermal heat pumps take advantage of the nearly constant temperature of the Earth to heat and cool buildings. The shallow ground, or the upper 10 feet of the Earth, maintains a temperature between 50̊ and 60̊F (10̊--16̊C). This temperature is warmer than the air above it in the winter and cooler in the summer. Geothermal heat pump systems consist of three parts: the ground heat exchanger, the heat pump unit, and the air delivery system (ductwork). The heat exchanger is a system of pipes called a loop, which is buried in the shallow ground near the building. A fluid (usually water or a mixture of water and antifreeze) circulates through the pipes to absorb or relinquish heat within the ground. [Ibid]

Heat pumps work much like refrigerators, which make a cool place (the inside of the refrigerator) cooler by transferring heat to a relatively warm place (the surrounding room), making it warmer. In the winter, the heat pump removes heat from the heat exchanger and pumps it into the indoor air delivery system, moving heat from the ground to the building's interior. In the summer, the process is reversed, and the heat pump moves heat from the indoor air into the heat exchanger, effectively moving the heat from indoors to the ground. The heat removed from the indoor air during the summer can also be used to heat water, providing a free source of hot water. [Ibid]

Geothermal heat pumps use much less energy than conventional heating systems, since they draw heat from the ground. They are also more efficient when cooling your home. Not only does this save energy and money, it reduces air pollution. [Ibid]

Geothermal Electricity Production

Geothermal power plants use steam produced from reservoirs of hot water found a few miles or more below the Earth's surface to produce electricity. The steam rotates a turbine that activates a generator, which produces electricity.There are three types of geothermal power plants: dry steam, flash steam, and binary cycle. [Source: U.S. Department of Energy]

Dry steam power plants draw from underground resources of steam. The steam is piped directly from underground wells to the power plant where it is directed into a turbine/generator unit. There are only two known underground resources of steam in the United States: The Geysers in northern California and Yellowstone National Park in Wyoming, where there's a well-known geyser called Old Faithful. Since Yellowstone is protected from development, the only dry steam plants in the country are at The Geysers. [Ibid]

Flash steam power plants are the most common and use geothermal reservoirs of water with temperatures greater than 360̊F (182̊C). This very hot water flows up through wells in the ground under its own pressure. As it flows upward, the pressure decreases and some of the hot water boils into steam. The steam is then separated from the water and used to power a turbine/generator. Any leftover water and condensed steam are injected back into the reservoir, making this a sustainable resource. [Ibid]

Binary Geothermal Systems

With conventional geothermal power generation, vapor reaching 200 degrees C is pumped from deep underground to a power-generating turbine. Japan and other countries are now studying binary systems in which liquids with lower boiling points than water---such as ammonia or pentane, which has a boiling point of only 36 degrees C are sent to a heat exchanger within which they are transformed to vapor using hot water. The vapor is then directed to a turbine for power generation. In this way power can be generated with sources whose temperatures are as low as 100 degrees C. [Source: U.S. Department of Energy]

Binary cycle power plants operate on water at lower temperatures of about 225̊--360̊F (107̊ --182̊C). Binary cycle plants use the heat from the hot water to boil a working fluid, usually an organic compound with a low boiling point. The working fluid is vaporized in a heat exchanger and used to turn a turbine. The water is then injected back into the ground to be reheated. The water and the working fluid are kept separated during the whole process, so there are little or no air emissions. Currently, two types of geothermal resources can be used in binary cycle power plants to generate electricity: enhanced geothermal systems (EGS) and low-temperature or co-produced resources. [Ibid]

Enhanced Geothermal Systems (EGS) provide geothermal power by tapping into the Earth's deep geothermal resources that are otherwise not economical due to lack of water, location, or rock type. The U.S. Geological Survey estimates that potentially 500,000 megawatts of EGS resource is available in the western U.S.---about half of the current installed electric power generating capacity in the United States. [Ibid]

Low-temperature and co-produced geothermal resources are typically found at temperatures of 300̊F (150̊C) or less. Some low-temperature resources can be harnessed to generate electricity using binary cycle technology. Co-produced hot water is a byproduct of oil and gas wells in the United States. This hot water is being examined for its potential to produce electricity, helping to lower greenhouse gas emissions and extend the life of oil and gas fields. [Ibid]

Hydropower

Flowing water creates energy that can be captured and turned into electricity. This is called hydropower or hydroelectric power. If you have access to flowing water on your property, you can use a microhydropower system to generate your own electricity. Hydropower is currently the largest and least expensive source of renewable electricity available. Large and small-scale hydropower projects are most commonly used by clean-power generators to produce electricity. A photo of water flowing through a hydropower plant. Hydropower produces 10 percent of the electricity in the United States. [Source: U.S. Department of Energy]

Most hydropower projects use a dam and a reservoir to retain water from a river. When the stored water is released, it passes through and rotates turbines, which spin generators to produce electricity. Water stored in a reservoir can be accessed quickly for use during times when the demand for electricity is high. [Ibid]

Dammed hydropower projects can also be built as power storage facilities. During periods of peak electricity demand, these facilities operate much like a traditional hydropower plant---water released from the upper reservoir passes through turbines, which spins generators to produce electricity. However, during periods of low electricity use, electricity from the grid is used to spin the turbines backward, which causes the turbines to pump water from a river or lower reservoir to an upper reservoir, where the water can be stored until the demand for electricity is high again. [Ibid]

A third type of hydropower project, called "run of the river," does not require large impoundment dams (although it may require a small, less obtrusive dam). Instead, a portion of a river's water is diverted into a canal or pipe to spin turbines. [Ibid]

Dams have tamed floods, watered crops and generate 16 percent of the world’s electricity. They have also displaced 40 million to 80 million people and destroyed river ecosystems. More than half the world’s major rivers are now dammed. Many large-scale dam projects have been criticized for altering wildlife habitats, impeding fish migration, and affecting water quality and flow patterns. As a result of increased environmental regulation, the National Hydropower Association forecasts a decline in large-scale hydropower use in the United States through 2020. [Ibid]

Research and development efforts have succeeded in reducing many of these environmental impacts through the use of fish ladders (to aid fish migration), fish screens, new turbine designs, and reservoir aeration. Although funding has been limited, current research focuses on the development of a "next generation turbine," which is expected to further increase fish survival rates and improve environmental conditions. [Ibid]

Microhydropower Systems

Microhydropower systems usually generate up to 100 kilowatts (kW) of electricity. Most of the hydropower systems used by homeowners and small business owners, including farmers and ranchers, would qualify as microhydropower systems. In fact, a 10-kilowatt microhydropower system generally can provide enough power for a large home, a small resort, or a hobby farm. [Source: U.S. Department of Energy]

Hydropower systems use the energy in flowing water to produce electricity or mechanical energy. Although there are several ways to harness the moving water to produce energy, run-of-the-river systems, which do not require large storage reservoirs, are often used for microhydropower systems. A microhydropower system can be connected to an electric distribution system (grid-connected), or it can stand alone (off-grid). [Ibid]

For run-of-the-river microhydropower systems, a portion of a river's water is diverted to a water conveyance---channel, pipeline, or pressurized pipeline (penstock)---that delivers it to a turbine or waterwheel. The moving water rotates the wheel or turbine, which spins a shaft. The motion of the shaft can be used for mechanical processes, such as pumping water, or it can be used to power an alternator or generator to generate electricity. [Ibid]

In a typical microhydropower system a river flows first flows through an intake. The intake diverts water to a canal. From the canal, the water travels to a forebay, which looks like a white, rectangular, aboveground pool. A pipeline, called a penstock, extends from the forebay to a building, called the powerhouse, contains a turbine and other electric generation equipment. The water flows in and out of the powerhouse, returning to the river. Power lines also extend from the powerhouse, along and through transmission towers, to a house that uses electricity generated by the system. [Ibid]

Ocean Energy

Oceans cover more than 70 percent of the Earth's surface. As the world's largest solar collectors, oceans generate thermal energy from the sun. They 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. The main types of ocean energy that are being explored for: 1) Ocean thermal energy conversion; 2) Tidal power; and 3) Wave power. [Source: U.S. Department of Energy]

Turbines have been installed in rivers and maritime areas with strong tides. Some use dams to funnel and focus water to increase its power to turn turbines. The Dutch have developed a means of deriving energy by mixing sea water with river water. When river water runs into sea water a huge amount of energy is released because of the difference of salt concentrations. The system essentially takes energy from this natural process, with thin film membranes to produce electric current by chemical separation sort of like a water battery. [Ibid]

Methane hydrate, also called combustible ice, resembles mica and can be mined and used a source of natural gas. About 164 cubic meters of natural gas can be released from one cubic meter of methane hydrate. Most deposits lie on the world’s sea beds. A number of countries, including Japan, China and Germany are looking into mining it as an energy source. [Ibid]

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. [Source: U.S. Department of Energy]

Tidal power technologies include the following: 1) 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. 2) 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). 3) 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). [Ibid]

Harnessing tidal power presents a host of 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. [Ibid]

Ocean Wave Energy

Elizabeth Rusch wrote in Smithsonian magazine: Unlike wind and solar power, wave energy is always available. Even when the ocean seems calm, swells are moving water up and down sufficiently to generate electricity. And an apparatus to generate kilowatts of power from a wave can be much smaller than what's needed to harness kilowatts from wind or sunshine because water is dense and the energy it imparts is concentrated. All that energy is also, of course, destructive, and for decades the challenge has been to build a device that can withstand monster waves and gale-force winds, not to mention corrosive saltwater, seaweed, floating debris and curious marine mammals. And the device must also be efficient and require little maintenance. [Source: Elizabeth Rusch, Smithsonian magazine, July 2009]

“Still, the allure is irresistible. A machine that could harness an inexhaustible, nonpolluting source of energy and be deployed economically in sufficient numbers to generate significant amounts of electricity---that would be a feat for the ages. In the United States, waves could fuel about 6.5 percent of today's electricity needs, says Roger Bedard of the Electric Power Research Institute, an energy think tank in Palo Alto, California. That's the equivalent of the energy in 150 million barrels of oil---about the same amount of power that is produced by all U.S. hydroelectric dams combined---enough to power 23 million typical American homes. The most powerful waves occur on western coasts, because of strong west-to-east global winds, so Great Britain, Portugal and the West Coast of the United States are among the sites where wave energy is being developed. [Ibid]

Ocean Wave Power Systems

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 instantaneous electricity (1 terawatt = 1 trillion watts), which is twice the electric generating capacity currently available throughout the world. 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. [Source: U.S. Department of Energy] Technologies

Wave energy can be converted into electricity through both offshore and onshore 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. [Ibid]

Built along shorelines, onshore wave power systems extract the energy in breaking waves. Onshore system technologies include the following: 1) 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. 2) 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. 3) 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. [Ibid]

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. [Ibid]

Wave Energy Machines

Elizabeth Rusch wrote in Smithsonian magazine: Engineers have built dozens of the machines, called wave-energy converters, and tested some on a small scale.” Research has been going on for more than a century. Some have resembled windmills, animal cages or ship propellers. A modern one looked like a huge whale. The gadgets all had one problem in common: they were too complicated. [Source: Elizabeth Rusch, Smithsonian magazine, July 2009]

---Ocean Power Delivery of Edinburgh has developed snake-like machines that bob with the waves and use hydraulic equipment to convert wave energy into electricity. The machines, Pelmais wave energy convertors, are 492 feet long and 11½ feet in diameter and are composed of three power conversion modules connected by weighted tubes. The module are joints that resist some motions but move with others allowing hydraulic rams to pumps high-pressure fluid into chambers that feed the fluid to a motor that drives a generators that creates electricity. [Ibid]

On a device called the Pelamis Attenuator, which was recently deployed for four months off the coast of Portugal by Pelamis Wave Power, Rusch wrote:---It looks like a 500-foot-long red snake. As waves travel its length, the machine bends up and down. The bending pumps hydraulic fluid through a motor, which generates electricity. Complex machines like this are riddled with valves, filters, tubes, hoses, couplings, bearings, switches, gauges, meters and sensors. The intermediate stages reduce efficiency, and if one component breaks, the whole device goes kaput. [Ibid]

In February 2012, Discover magazine reported: “Michigan State mechanical engineer Norbert Müller aims to do far better with his wave disk engine, in which a rotating wheel sucks fuel and air into small internal channels. As the wheel spins, ports on the outer rim of the engine block the fuel-air mixture from flowing out of the channels. The blockage creates shock waves, and the resulting pressure helps the fuel to ignite, pushing against curved blades on the disk and causing it to spin. Müller says his engine has the potential to be 60 percent efficient. In contrast a car’s engine converts only about 15 percent of it fuel energy into propulsion. Muller hopes to finish a prototype large enough to power an SUV by 2013. [Source: Discover, February 16, 2012]

Energy-Generating Buoys

Annette von Jouanne, a professor at Oregon State University and expert on wave energy, has developed an electricity-generating buoy. Elizabeth Rusch wrote in Smithsonian magazine: Her breakthrough was to conceive of a device that has just two main components. In the most recent prototypes, a thick coil of copper wire is inside the first component, which is anchored to the seafloor. The second component is a magnet attached to a float that moves up and down freely with the waves. As the magnet is heaved by the waves, its magnetic field moves along the stationary coil of copper wire. This motion induces a current in the wire---electricity. It's that simple. [Source: Elizabeth Rusch, Smithsonian magazine, July 2009]

“Early tests” didn't go as planned." Von Jouanne and co-workers plopped the buoy in the 15-foot-deep channel and buffeted it with two-, three- and four-foot waves. The first five-foot wave tipped it over. "We had a ballast problem," von Jouanne told Smithsonian magazine. "We're electrical engineers, and we really needed more help from ocean engineers, but to get them we needed more funding, and to get more funding we needed to show some success.” [Ibid]

“Von Jouanne kept refining her buoys. A small group watched as a five-foot wave headed for one of her latest versions. As the buoy lifted with the surge, a 40-watt light bulb on top of it, powered by wave energy, lighted up. "We all cheered," Cox recalls. Von Jouanne recently towed her best-performing buoy---her 11th prototype---out through Yaquina Bay and one and a half miles offshore. The buoy, which resembles a giant yellow flying saucer with a black tube sticking through the middle, was anchored in 140 feet of water. For five days it rose and fell with swells and generated around 10 kilowatts of power. In the next two to three years, Columbia Power Technologies, a renewable energy company that has supported von Jouanne's research, plans to install a buoy generating between 100 and 500 kilowatts of electricity in the test berth off the coast of Oregon. [Ibid]

---George Boehlert, a marine scientist at Oregon State's Hatfield Marine Science Center, told Smithsonian magazine. "Ocean energy is a fast-moving field and environmental researchers have a lot of questions. For instance, the buoys absorb energy from waves, reducing their size and power. Would shrunken swells affect sand movement and currents near shore, perhaps contributing to erosion? Buoys, as well as the power cables that would connect to the electrical grid on-shore, emit electromagnetic fields. And mooring cables would thrum in the currents, like a guitar string. Might these disturbances confuse whales, sharks, dolphins, salmon, rays, crabs and other marine animals that use electromagnetism and sound for feeding, mating or navigation? Would birds collide with the buoys or turtles become entangled in the cables?

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 20̊C (36̊F). 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. [Source: U.S. Department of Energy]

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. [Ibid]

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. [Ibid]

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. [Ibid]

Ocean Thermal Energy Conversion Systems and Technologies

The main types of OTEC systems are 1) the Closed-Cycle; 2) the Open-Cycle; and 3) the Hybrid. Closed-cycle 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. [Source: U.S. Department of Energy]

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. [Ibid]

Open-cycle 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 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. [Ibid]

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 produce electricity. [Ibid]

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. [Ibid]

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. [Ibid]

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. [Ibid]

Image Sources: U.S. Department of Energy; Wikimedia Commons

Text Sources: New York Times, Washington Post, Los Angeles Times, Times of London, The Guardian, National Geographic, The New Yorker, Time, Newsweek, Reuters, AP, AFP, Wall Street Journal, The Atlantic Monthly, The Economist, Global Viewpoint (Christian Science Monitor), Foreign Policy, U.S. Department of Energy, Wikipedia, BBC, CNN, NBC News, Fox News and various books and other publications.

Last updated August 2012


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