Water Energy

INTRODUCTION: WHAT IS WATER ENERGY?

Water energy is energy derived from the power of water, most often its motion. Energy sources using water have been around for thousands of years in the form of water clocks and waterwheels. A more recent innovation has been hydroelectricity, or the electricity produced by the flow of water over dams. In the twenty-first century scientists are developing water-based applications ranging from tidal power to thermal power.

Historical overview

The history of water energy is almost as old as the history of human civilization itself, making it the first form of "alternative energy" people employed. Many centuries ago the ancient Egyptians devised water clocks, whose wheels were turned by the flow of water. The Egyptians and Syrians also used a device called a noria, a waterwheel with buckets attached, that was used to raise water out of the Nile River for use on their crops. Two thousand years ago the ancient Greeks built waterwheels to crush grapes and grind grains. At roughly the same time, the Chinese were using waterwheels to operate bellows used in the casting of iron tools such as farm implements.

The ancient Romans were especially skilled at managing water. In fact, the English word plumber comes from the Latin word plumbum, meaning "lead," referring to the lead pipes used in plumbing and reflected in the symbol for lead in the periodic table of elements, Pb. The Romans built water-carrying structures called aqueducts to channel water from natural sources to canals, where the water's energy could be harnessed by waterwheels. Near Arles in what is now southern France, for example, the Romans built a massive grain mill powered by sixteen waterwheels.

In the centuries that followed, until fossil fuels became the preferred power source during the industrial revolution of the nineteenth century, farmers continued to take advantage of the currents in rivers and streams for a variety of agricultural purposes, including grinding grain and pumping water for irrigation (watering crops). An English manuscript called the Domesday Book, written in 1086, listed 5,624 waterwheel-driven mills south of the Trent River in England, one mill for every four hundred people.

Words to Know

Flow

The volume of water in a river or stream, usually expressed as gallons or cubic meters per unit of time, such as a minute or second.

Hydraulic energy

The kinetic energy contained in water.

Hydropower

Any form of power derived from water.

Kinetic energy

The energy contained in any fluid mass, such as water, that is in motion.

Mechanical energy

The energy output of tools or machinery.

Ocean thermal energy conversion (OTEC)

The process of converting the heat contained in the oceans' water into electrical energy.

Stored energy

The energy contained in water that is stored in a tank or held back behind a dam in a reservoir.

Thermal energy

Any form of energy in the form of heat; used in reference to heat in the oceans' waters.

Thermal gradient

The differences in temperature between different layers of the oceans.

Work

The conversion of one form of energy into another, such as the conversion of the kinetic energy of water into mechanical energy used to perform a task.

Farmers, though, were not the only ones to use waterwheels. Early factories, especially in Great Britain and in the American Northeast, relied heavily on water power as well because of the large number of rivers and streams in the British Isles and in such states as Massachusetts, Connecticut, and New York. In these examples, rivers often powered such enterprises as sawmills, but the textile industry, in particular, used water to power the "Spinning Jenny," a cotton-spinning machine for making cloth. In 1769 English inventor and industrialist Richard Arkwright (1732–1792) patented a water-powered textile loom for spinning cotton (originally meant to be powered by horses) that revolutionized the textile industry.

The result over the next half-century was a boom in the textile industry, both in Britain and, later, in the United States. One of the pioneers in this effort was a New England businessman, Francis Cabot Lowell (1775–1817). In the early nineteenth century Lowell imported British technology to the Charles River in Waltham, Massachusetts, where he and other business owners built textile mills powered by the river. Later, Waltham's mill owners, needing more power than the Charles could supply, moved to an area north of Boston. Here they created the industrial town of Lowell, Massachusetts, almost entirely around water power. Soon, textile mills were able to produce millions of yards of cloth, thanks largely to water power.

The major problem with early waterwheels, though, was that they could not store power for later use, nor could they easily distribute power to several users. This disadvantage was overcome by the development of hydroelectricity (though modern waterwheels can also produce electricity). Hydroelectric dams, unlike waterwheels, do not depend entirely on the rate of flow of the water in a river or stream. Moreover, by producing electricity, power can be stored and distributed to more than one user in a community.

Hydroelectricity was first used in 1880, when the Wolverine Chair Factory began producing hydroelectric power for its own use in its Grand Rapids, Michigan, plant (perhaps it is no accident that the city had the word Rapids in its name). The first hydroelectric plant whose power went to multiple customers began operation on September 30, 1882, on the Fox River near Appleton, Wisconsin. Major improvements in hydroelectric power generation were made by Lester Allan Pelton (1829–1908), an inventor who is sometimes called the "father of hydroelectric energy." Sometime in the late 1870s Pelton developed the Pelton wheel, a new, more efficient design for turbines that powered hydroelectric plants. A later design, developed by Eric Crewdson in 1920 and called the turgo impulse wheel, improved on the efficiency of Pelton's design. Because of these improvements, more and more electrical needs in the United States were being met by hydroelectric power.

The water in rivers and streams, though, is not the only water in motion. The oceans move too, and in the late twentieth and early twenty-first century, efforts have been launched to tap the power contained in the oceans' tides, waves, and currents. Fundamentally, though, these sources of power are little different from the power provided by rivers and streams. The water is moving, so the challenge for engineers is to devise ways to convert that motion into electricity. While strides have been made, the practical use of these power sources is still in the beginning stages.

Tidal power for electrical generation is relatively new. Currently, only one tidal power-generating station has been built and is in use. This plant is located at the mouth of the La Rance River along France's northern coast. The plant was built in 1966 and provides 240 megawatts, or 240 million watts, of electricity. There is a 20-megawatt experimental station in Nova Scotia, Canada, and Russia has a 0.4-megawatt station near the city of Murmansk. Other promising sites include the Severn River in western England, Cook Inlet in Alaska, and the White Sea in Russia.

Richard Arkwright

Richard Arkwright, the youngest of thirteen children, began his career as a barber's apprentice. He wanted to run his own company, so he decided to become a wig maker. He spent the early part of his career traveling through England collecting discarded hair he could use to make wigs.

After Arkwright became involved in the textile industry in the 1760s, he built many profitable mills in England, Wales, and Scotland. When he died, he was worth nearly a million dollars, an enormous fortune in the late eighteenth century. In 1786 he was knighted by England's King George III.

But like many industrialists of the time, Arkwright built his fortune on the backs of his workers, who toiled from 6:00 in the morning to 7:00 in the evening. Among his 1,900 employees, two-thirds were children. While many other mill owners employed children as young as five, Arkwright was slightly enlightened for his time: he did not hire children under the age of six. Nor would he hire anyone over the age of forty.

Waves and ocean currents, like the tides, contain enormous amounts of energy, as any swimmer who has been pelted by a wave or swept along on an ocean current knows. The first patent for a wave power machine that would function much like a waterwheel in powering grain mills and sawmills was filed in France in 1799, although there is no evidence that the device was ever built. One of the first important developments for harnessing this power took place in 1974, when a British engineer named Stephen Salter invented a device called a "duck." This was a hydraulic mechanism that converted wave power into electricity, but this is only one of many ingenious innovations that scientists and engineers have developed. In the years that followed, scientists and engineers sought ways to transform innovations like the duck into a working wave power-generating station. Their efforts were finally successful in 2000, when the United Kingdom opened the first such station on the island of Islay, off the coast of Scotland. This station is called the Limpet 500, which stands for Land-Installed Marine-Powered Energy Transformer. The number 500 refers to the 500 kilowatts of electricity it feeds into the United Kingdom's power grid.

The Pelton Wheel

Lester Allan Pelton (1829–1908) was born in Ohio but migrated to California during the gold rush of the late 1840s. In the 1870s he conceived the design for the Pelton wheel. He tested a prototype in 1879 and received a patent for the design in 1889.

Before the Pelton wheel, the most common type of turbine was the reaction turbine, which came equipped either with flat paddles or with cups or buckets. In either case, the water came straight at the paddle or bucket. As the water struck it, it pushed the paddle or bucket, thus turning the wheel. The Pelton wheel was the result of an accident. Pelton was watching a spinning water turbine. The key that held the wheel onto the shaft slipped out of place so that the wheel tilted. Instead of hitting the paddles on the waterwheel directly in the center, the water hit near the edge and was diverted to flow in a half-circle. To Pelton's surprise, the wheel actually began to spin faster.

The turgo turbine was developed in 1919 and represented an improvement in the Pelton wheel. It is less expensive to make and can handle a greater flow of water, so a smaller turgo turbine can generate the same amount of power as a larger Pelton wheel.

The world's oceans are also the source of thermal energy, or the heat that oceans absorb from the sun. The word thermal comes from a Greek word, therme, meaning "heat," and is related to another Greek word, thermos, meaning "hot."

The first scientist to propose that the thermal energy of the oceans could be tapped for human needs was a French physicist named Jacques Arsene d'Arsonval (1851–1940) in 1881. D'Arsonval may very well have gotten the idea, though, from author Jules Verne (1828–1905), who imagined the use of ocean temperature differences to produce electricity in his novel Twenty Thousand Leagues under the Sea in 1870. In 1930 one of d'Arsonval's students, Georges Claude, built the first-ever system for doing so off the coast of Cuba. The system he built generated 22 kilowatts, or 22,000 watts, of electricity. However, this it represented a net power loss, because it actually took more power to run the system than it was able to generate. Then in 1974 the Natural Energy Laboratory of Hawaii Authority (NELHA) was formed. In 1979 NELHA successfully demonstrated a plant that produced more energy than it consumed (50-kilowatts gross; 15-kilowatts net). In 1981 Japan built a system that produced 31.5 kilowatts of net power. In 1993 NELHA set a record when it produced a net power of 50 kilowatts in a demonstration.

Georges Claude

Georges Claude (1870–1960) may have built the first system for harnessing the thermal energy in oceans, but his impact as a scientist was probably much greater in a way that is glaringly obvious every day (or every night) in just about every city and town throughout the developed world.

As a young engineer, chemist, and inventor, Claude turned his attention to the inert gases. He discovered that passing an electrical current through cylinders filled with inert gases such as neon produces colored light. In other words, Claude was the inventor of the neon sign, which he first demonstrated in Paris in 1910. The first neon signs arrived in the United States when he sold two of them to a Packard automobile dealership in Los Angeles in 1923.

How water energy works

To understand fully the nature of water energy, two terms have to be defined more precisely: energy and work. In everyday use, the word energy often refers to a substance, such as gasoline, coal, or natural gas. Strictly speaking, though, these substances are not energy; they are just chemical substances. Their energy is locked inside their chemical bonds, and it has to be released by burning them. What makes these substances useful is that they contain a lot of energy that can easily be released through combustion (burning).

Put differently, these substances can do a great deal of work, but scientists define work in their own peculiar way. To most people, "work" means something like a chore or job, such as mowing the lawn. To a scientist, though, "work" refers to the process of converting one form of energy into another, such as converting the chemical energy of natural gas into heat used to boil water or heat a house. Scientists usually measure energy output in terms of the amount of work that can be done with it. For example, the calorie, used most often in discussions of diet, exercise, and weight, is actually a unit that measures a form of work. A more commonly used unit of work among scientists is the joule. The joule is part of the metric system units, and it is used to measure heat, electric energy, and the energy of motion.

To produce energy, though, it is not always necessary to burn something. When cleaning up after dinner, a family's first task is to rinse off the dishes, pots, and pans, using water from the kitchen faucet. What rinses the dishes, though, is not the water from the faucet by itself so much as it is the energy contained in the running water. This type of energy is called kinetic energy. The word kinetic comes from a Greek word, kinesis, which means "motion," so kinetic energy is the energy contained in a body of water when it is in motion. In discussions of water energy, sometimes the term hydraulic energy is used instead of kinetic energy. The word hydraulic is derived from hydro, the Greek word for "water." In this context, kinetic energy and hydraulic energy refer to the same thing.

To put water to work, then, the water has to be in motion. The best way to put large amounts of water in motion is to let gravity do the work. Streams and rivers, for example, flow because the water in them is moving downhill, even if only slightly, following the downward pull of gravity. In a home, water flows "downhill" because a city's water is stored in large elevated tanks, where it contains stored energy. When a homeowner opens a faucet, the water flows in a downward direction from the tank through the city's water pipes and out the faucet, where it carries enough kinetic energy to knock food remnants off dirty dinner dishes. Helping out is the sheer weight of the water, which pushes it down through the city's water pipes.

Scientists measure how much work a body of water can do using flow, which is simply the volume of water measured in, for example, gallons or liters per second or minute. This is just common sense. A homeowner who wants to rinse off a dirty porch uses a hose, not a squirt gun, because the flow from the hose is much greater than the flow from a squirt gun, so the water can do more work in a given period of time. A squirt gun might work, but the job would take a very long time.

This, then, is the basic science behind kinetic energy. Water flowing downhill, pulled by gravity, contains kinetic energy. A tool such as a waterwheel can be used to convert this kinetic energy into mechanical energy, which can then be harnessed to perform a task, such as grinding grain, sawing lumber, or running a textile loom. Or the kinetic energy can be transformed into electricity, which can be stored and distributed to many different users.

Current and future technology

The moon in large part is responsible for another type of energy that water can provide: tidal power. Every day, the moon (and, to a lesser extent, the sun), exerts gravitational pull on the Earth, causing the Earth's oceans to bulge outward. At the same time, the Earth rotates beneath this water, so twice each day, the Earth's coastlines experience high and low tides. These tides, just like rivers and streams, are water in motion. This motion, driven by the pull of gravity, imparts kinetic energy to the oceans. The ebb and flow of the tides along a coast, or perhaps into and out of an inlet or bay, are little different from the flow of water in a river, and they can be harnessed using technology similar to that used on rivers. Because the water flows in two directions, though, the system can generate power when water is flowing in and when it is ebbing out. However, a tidal power-generating station can operate only about ten hours a day, during the times when the tides are in motion.

The oceans' waves are yet another potential source of kinetic energy. Waves, which average about 12 feet (almost 4 meters) in height in the oceans, are caused by wind blowing across the surface of the water, just as tiny ripples are created when a person blows across the surface of a cup of hot chocolate to cool it. The height of a wave—from its peak, or crest, to its bottom, or trough—is determined by how fast the wind is blowing, the length of time it has blown in the same direction, and the width of the open water over which it is blowing. The steepest and most powerful waves are caused by winds that blow strongly in the same direction across oceans, such as the trade winds.

Waves move across the waters of the open ocean with little change. But as they approach the shore and the water gets shallower, they begin to release their enormous energy. First, the ocean's floor causes the wave to slow and to increase in height. Then, the front of the wave "breaks," or collapses, hurling tons of water at the coastline. The force of this wave power is so great that it continues to wash away the coastlines. It is estimated, for example, that parts of Cape Cod are eroding at a rate of 3 feet (0.9 meter) per year. Like the water in rivers and streams, these waves could potentially be used for their kinetic energy.

A final source of kinetic energy in the oceans is their currents. Currents, like waves, are usually propelled by the wind blowing across the surface. The wind has to be strong and consistent. But other currents are formed by differences in water temperature and salinity (salt content) and even by slight differences in the elevation of the sea's surface. The currents follow paths determined by the Coriolis effect, or the effect of the Earth's rotation. In the Northern Hemisphere, the Earth's rotation deflects the currents into a clockwise rotation; in the Southern Hemisphere, the currents flow counterclockwise.

One of the most studied and well-known ocean currents is the Gulf Stream, which originates near Florida, crosses the Atlantic Ocean, and warms much of northern Europe. The Gulf Stream is 50 miles (80 kilometers) wide, and an estimated 10 cubic miles (16 cubic kilometers) of water move through it every hour. It moves so fast that its warm waters do not mix with the colder water that surrounds it. The Gulf Stream is, in effect, a river. The water is in motion, so it contains vast amounts of kinetic energy that could be tapped for human use.

There is also thermal energy, or the heat contained in the world's oceans. Tapping the oceans' thermal energy, though, is not just a matter of somehow going out and piping in the heat. The process, called ocean thermal energy conversion (OTEC), is driven by the ocean's thermal gradient, which refers to the differences in temperature between the ocean's layers of water. Power can be produced when the difference between the warmer surface waters and the colder deep waters is at least 36°F (20°C). Energy-producing systems for tapping the ocean's thermal energy rely on a system of condensers, evaporators, and turbines to generate electricity. OTEC could provide electricity, especially to many tropical nations that currently have to import all their fuel.

Benefits of water energy

The major benefit that all forms of water energy have is that they provide power without burning fossil fuels. Energy can be provided for human use without having to tear up the land to mine coal or disrupt ecosystems to drill for oil. The power they provide is clean—it does not release particulate matter, carbon dioxide, or sulfur dioxide into the air, contributing to smog and the ill health effects that smog can cause, such as lung disease. Also, because water energy does not depend on the burning of fossil fuels, it does not contribute to global warming, caused by the buildup of gases such as carbon dioxide in the atmosphere. Nor does it contribute to acid rain, or precipitation that is more acidic than normal because it contains such substances as sulfur dioxide. Acid rain, like any acidic substance, can have harmful effects on forests, wildlife, and even structures built by people.

Another major benefit of water energy is that it is virtually inexhaustible. Once fossil fuels run out, they are gone. There is no way to somehow manufacture more oil or natural gas. However, the energy provided by water will be there as long as the sun shines and as long as the Earth contains oceans and rivers. Further, the energy provided by water is essentially free—once, of course, the technology is put in place to extract the energy. While money would continue to have to be spent to build plants, maintain them, and distribute the power they produce, a major benefit is that power providers would not have to buy fuel for them. The potential savings is huge. As of mid-2005 the cost of a barrel of oil was hovering around $60. The United States uses about twenty million barrels of oil each day. That means about $1.2 billion per day is spent for just that one form of fuel. Replacing that fuel with water energy would result in enormous savings for consumers.

Drawbacks of water energy

These energy sources, though, are not without their drawbacks. While hydroelectric dams have been around for well over a century, stations for harvesting tidal, wave, ocean current, and ocean thermal power are still in the developmental stages. Exploiting these forms of power would require a huge investment. The cost of building a tidal power-generating station, for example, could run as high as $15 billion.

A second drawback is that water energy is not totally reliable. In an energy plant that burns fossil fuels, the fuel can be fed into the system at a constant rate. As a result, the energy output of the system can be predicted and maintained at a steady pace. Water energy can be a little more variable. In a dry season, the water in a river may not run as fast. The level of the water in the reservoir behind a hydroelectric dam may fall so far that the dam's operators have to slow the flow of water over the dam, cutting power output. In the case of ocean energy, plant operators have no control over the water. Tidal power, for example, can vary from day to day, depending on the alignment of the Earth with the sun and the moon. Wave power could be highly variable, depending on prevailing winds. While the power in ocean currents and in the ocean's thermal gradient is more predictable, the chief obstacle is getting to it. Creating a power plant in the middle of the Gulf Stream would be no easy feat.

A related problem is that water energy is not evenly distributed across the Earth. Providing tidal power to the residents of Nebraska would be impractical because Nebraska is nowhere near an ocean. While tides operate throughout the world, not every coastal region can produce tidal power very efficiently. Some coastal regions have higher tides than others, usually because of some geographical feature, such as bays and inlets that push the water to a higher level than it would otherwise reach. To be practical, efforts to harness tidal power require a difference of about 16 feet (5 meters) between high and low tide. This difference can be found at only about forty places around the world. As the water flows in, and then as it flows out, it can be harnessed in much the same way that the water in any river can be harnessed. However, tidal power stations would be possible only in a limited number of locations.

The use of river power, too, is highly variable. While hydroelectric power provides 24 percent of the electricity used worldwide and 9 to 10 percent used in the United States, much of that hydroelectric power is concentrated in regions with several rivers. In the United States, for example, 14 percent of the power used in the Rocky Mountain states comes from hydroelectric dams; in the Pacific Northwest, in contrast, some 65 percent of power demand is filled by 58 hydroelectric dams. While hydroelectric dams provide almost all of the electricity in Norway, 83 percent in Iceland, 67 percent in Austria, and 60 percent in Canada, they can provide little or none in the desert countries of the Middle East or in most of Africa. This suggests that no one source can magically solve any nation's energy problems.

A final drawback is that a fossil fuel-fired plant can be built essentially anywhere because the fuel is brought to the plant. With water energy, the plant has to be brought to the fuel, meaning that plants have to be built on rivers, along shorelines, and in bays, where they disrupt the natural environment.

Environmental impacts of water energy

A major drawback to the use of water energy is the potential environmental impact. On one level, using water energy would have benefits for the environment, including cleaner air and reduced global warming, compared to the use of fossil fuels. However, the power plants themselves could potentially have a devastating effect on local ecosystems.

Hydroelectric dams are a good example. Throughout the world, about 40,000 large dams are in use to provide hydroelectric power. Most of these dams were built with little regard to the environmental impact they would have. Dams, for example, require reservoirs. In effect, they turn a river ecosystem into a lake ecosystem, at the same time gobbling up large tracts of land. Moreover, they block the migration of fish, such as salmon in the Pacific Northwest. They also prevent the downstream movement of silt, which is often rich in nutrients.

Such facilities as tidal power-generating stations could have similar environmental impacts. The construction and operation of such facilities could have a serious impact on marine and coastal ecosystems, fisheries, and the like. They could disturb the silt on the ocean bed, with unintended consequences. Further, they could convert beautiful natural areas into eyesores.

Another potential drawback to hydroelectric dams—or any water energy project—concerns ownership rights. Rivers usually flow through more than one country. In Southeast Asia, for example, six countries make up the Mekong River's watershed. During rainy seasons this would not be a problem, for the Mekong flows at a rate of 31 cubic miles (50,000 cubic meters) per second. During the dry season, however, the river flows at a rate of only about 1.2 cubic miles (2,000 cubic meters) per second, seriously reducing the amount of power that could be produced. This would provide an upriver country with an incentive to block the flow of the river, denying water and power to the downriver countries. The result could be serious regional conflict over water rights. A similar problem could occur in the oceans. It is an established principle that no country owns the oceans in its vicinity, other than a narrow strip along the coastline. Any type of power-generating station that lies outside of a nation's coastal waters would run into serious legal difficulties if it used international seas to provide power for just one nation.

Economic impact of water energy

The economic impact of water energy has always been great, but new forms have the potential to dwarf the impact that has been felt throughout human history. While water power has been used throughout much of history, its economic impact began to be felt more fully in the late eighteenth and early nineteenth centuries. The town of Lowell, Massachusetts, which grew as textile firms built up around the availability of water power, by the mid-1830s boasted 20 textile mills employing 8,000 people and producing 50 million yards (46 million meters) of cloth per year.

Hydroelectricity had an even larger impact. In the early twenty-first century hydroelectric dams provide about 9 to 10 percent of the electricity used in the United States. Worldwide, though, hydroelectric plants provide about 24 percent of electricity, serving a billion people. Together, they annually produce about 675,000 megawatts (mega -, meaning "million"), the equivalent of about 3.6 billion barrels of oil. That represents a savings of about $180 billion that might otherwise be spent on oil. These hydroelectric plants are the world's single largest source of renewable energy.

Other sources of water energy hold even greater promise. Just over 70 percent of the Earth's surface is covered by oceans. The amount of water they contain is staggering: 328 million cubic miles (527 million cubic kilometers), or 361.2 quintillion gallons (1,367.3 quintillion liters). (A quintillion is 1,000,000,000,000,000,000.) Every day the sun shines on these oceans, and every day they absorb a great deal of thermal energy. In fact, the oceans can be thought of as the world's single largest solar panel. It is estimated that on a typical day, about 23 million square miles (60 million square kilometers) of the world's tropical oceans absorb an amount of energy from the sun equal to about 250 billion barrels of oil.

To put that figure in perspective, the total amount of oil produced in the world each day in 2005 was about 76 million barrels. That means that each day, the tropical oceans absorb three thousand times more energy than that provided by oil. This is an enormous amount of energy. Some experts estimate that the amount of power that could potentially be produced from heat in the oceans is 10 trillion watts. Just 1/200th of one percent of this thermal energy—absorbed by the tropical oceans in just one day—could provide all the electricity consumed in the entire United States. This energy would be clean and endlessly renewable. The problem, of course, is finding ways to capture that energy.

Societal impact of water energy

The societal impact of water energy is essentially the same as the impact of any alternative energy. Clean, renewable energy would lessen the adverse health effects of fossil fuel burning. Because the fuel itself is essentially free, more reliance on water power would free up billions of dollars that could be used for other human needs. Using water power would also benefit the environment, reducing the need for environmentally disruptive coal mining and oil drilling, along with the regular oil spills that spoil many nations' coastlines. Water power could also have a major impact on poorer nations, which lack the resources to import fossil fuels for economic development. Water energy could provide these nations with a clean, relatively inexpensive way to develop and provide a richer economic, social, educational, and cultural future for their peoples.

HYDROPOWER

The term hydropower is a general one that can be used to refer to any type of water energy. Here, though, the term will be used to refer to the earliest form of hydropower, the kind used in primitive waterwheels, though modern waterwheels are not as primitive as those of the past. In the early 2000s waterwheels continue to be used for low-level electrical power generation.

A waterwheel is a paddlewheel attached to a fixed rotor, or axle, and placed in the current of a river or stream. The wheel is actually a pair of parallel wheels connected to the rotor by radial spokes. Between the two wheels is an arrangement of paddles. As the water passes, the kinetic energy of the water pushes against the paddles, turning the wheel and producing mechanical energy, which in turn is transferred through gears to machinery that accomplishes the task at hand. In the past this machinery was very often a large stone used to grind grain, but could also consist of saws in a sawmill, bellows in a foundry, looms in a textile mill, abrasive tools for polishing metal, pumps for removing water from a mine, and many other applications. Some wheels, rather than using paddles, used buckets. The weight of the water in the buckets helped to propel the wheel around.

Early waterwheel users were creative with the placement of waterwheels. While the wheels were often inserted directly into a stream or river and connected to a facility on the riverbank, often they were placed on barges and boats (called ship mills), sometimes suspended between two barges or boats. Others were attached to the abutments of stone bridges over rivers.

Historically, three different types of waterwheels were used. The first was the horizontal waterwheel. This type of wheel was lowered horizontally into the water, where it was totally submerged. Attached to the wheel were veins, which were somewhat like the veins on a pinwheel that turns when air blows over it. This type of wheel was attached to a rotor that protruded up out of the water and connected directly to something like a millstone. Horizontal waterwheels are still in use in India and Nepal.

A more efficient and powerful design is the vertical waterwheel. Vertical waterwheels came in two types, the undershot and the overshot, both of which required a system of gears to turn the machinery. An undershot wheel was lowered vertically into the water of a river. The water passed by the lower portion of the wheel, pushing on the paddles to turn it. A major disadvantage of this type of wheel was the variability in the river's water level. During dry spells, the water level in the river would fall, diminishing the wheel's power. Sometimes the water level would fall so much that the wheel was entirely out of the water, making it useless.

With an overshot wheel, the water flowed from above. These types of wheels were sometimes positioned underneath waterfalls so that the water struck the paddles as it fell, or alternatively poured into buckets so that the weight propelled the bucket forward, turning the wheel. More commonly, the source of the water was an artificial channel that flowed to a position above the waterwheel.

Current uses of hydropower

Although waterwheels are thought of as a feature of earlier societies, in fact they are still widely used for irrigation, pumping water, and even occasionally still to power machinery such as sawmills. These types of wheels can be found in many areas of the world. In Turkey and Afghanistan, waterwheels are still used to grind grain. In the United States, a company called Equality Mills in West Virginia still manufactures waterwheels, and one of the first wheels the company ever produced, in 1852 (under earlier owners), is still in operation at the Tuscorora Iron Works just across the creek.

Companies in the United States and Germany also manufacture waterwheels for electrical power generation, and the British Hydropower Association provides detailed information about building small waterwheel power plants. Typically, such a plant would involve the following:

• A water intake from a river or stream
• A small canal to channel the water
• A forebay tank, where the water is slowed so that debris can settle out, along with a trash rack to filter out debris
• A penstock, which shoots the water downward to the turbine
• A powerhouse, which contains a turbine where the power is actually generated
• A tailrace, which channels the water back into the river or stream

Benefits of hydropower

Prior to the industrial revolution, waterwheels were essentially the only form of alternative energy available. In Europe, the rapid spread of waterwheels may have been a function of the Black Death, the plague that wiped out large portions of the population in the late Middle Ages. Waterwheel use expanded rapidly in England, France, and other European nations as a way to replace lost labor.

In modern times waterwheels are used primarily for low-level electrical power generation. The British Hydropower Association notes that small-scale hydropower generation is highly efficient, between 70 and 90 percent (meaning that 70 to 90 percent of the available power can actually be generated).

Drawbacks of hydropower

Historically, waterwheels had two primary drawbacks. The first was that they required a great deal of maintenance. Because they were constructed mostly of wood, they tended to break down over time. Further, water is not very friendly to wood, causing it to deteriorate and rot. The second problem was that in northern climates, waterwheels were of limited usefulness in cold weather, when the water froze.

The primary drawback of modern waterwheels is that building such a power plant is expensive for the amount of energy it can produce. The bulk of the expense lies in the turbines needed to generate the power, gearboxes needed to convert kinetic energy into mechanical energy, and generators needed to convert mechanical energy into electrical energy. The extent to which this is a drawback depends on the amount of available energy. When flow is high, the amount of power generated is more likely to justify the cost; when it is low, the amount of power generated may not be worth the cost. The British Hydropower Association estimates that the total cost of building a 100-kilowatt (kW) power plant could range from roughly $150,000 to $470,000. Adding to the cost is the need to acquire rights to use the land.

Another potential drawback of waterwheel power plants is safety. Such plants, including the wheel itself, have to be fenced off so that they do not injure curious people who get too close. This fencing, combined with the plant itself, has the potential to become an eyesore, though manufacturers attempt to make the equipment as visually attractive as possible.

A final drawback stems from the variability of water flow. During spring runoff, when snow is melting and rivers run rapidly, the amount of power generated is much higher than in, say, August, when rivers are running low, providing less flow.

Issues, challenges, and obstacles of hydropower

The primary issue surrounding the use of waterwheels is ownership rights. Any stream or river almost certainly flows through property owned by many people. The river itself is common property; no one individual owns it. If one property owner builds a waterwheel, other property owners along the river might object, particularly if they are uncertain about the effects the wheel might have downstream.

Another challenge concerns distribution of the power. One property owner might build a waterwheel for personal use, but larger waterwheels in high flow streams might generate enough electricity for multiple users. The questions then become how that power is going to be distributed and how its users will divide the cost of constructing the waterwheel.

HYDROELECTRICITY

Hydroelectricity is any electricity generated by the energy contained in water, but most often the word is used to refer to the electricity generated by hydroelectric dams. These dams harness the kinetic energy contained in the moving water of a river and convert it to mechanical energy by means of a turbine. In turn, the turbine converts the energy into electrical energy that can be distributed to thousands, even millions, of users.

One of the most prominent hydroelectric dams in the United States is the Hoover Dam on the Colorado River along the border between Arizona and Nevada. Construction on the dam began in 1931; it was completed five years later, under budget, for $165 million. Behind the dam is a reservoir, Lake Mead, containing about 1.24 trillion gallons of water. The dam is 726 feet (221 meters) tall, and at its base is 660 feet (201 meters) thick. Its 4.5 million cubic yards of concrete would be enough to build a two-lane highway from Seattle, Washington, to Miami, Florida. Each year, the dam produces 4 billion kilowatt-hours of electricity, enough to serve 1.3 million people.

The largest hydroelectric dam in the United States is the Grand Coulee Dam on the Columbia River in Washington State. Construction began on the dam in 1933 and was completed in 1942. The original purpose of the dam, however, was not to generate electricity but to irrigate one-half million acres of agricultural land. From 1966 to 1974 the power-producing ability of the dam was expanded with the addition of six new electrical generators. The scope of the Grand Coulee Dam amazes visitors. It is the largest concrete structure in the United States, at 11,975,521 cubic yards. At its widest point, it is almost exactly a mile (1.6 kilometers) long. At 550 feet (167 meters) tall, it is twice the height of the Statue of Liberty and more than twice the height of Niagara Falls. Its reservoir, Roosevelt Lake, contains up to 421 billion cubic feet of water. Its four power plants and 33 generators produce 6,809 megawatts of power annually.

A hydroelectric dam consists of the following components:

• Dam: The dam is built to hold back water, which is contained in a reservoir. This water is regarded as stored energy, which is then released as kinetic energy when the dam operators allow water to flow. Sometimes these reservoirs, such as Lake Mead, are used as recreational lakes.
• Intake: Gates open to allow the water in the reservoir to flow into a penstock, which is a pipeline that leads to the turbine. The water gathers kinetic energy as it flows downward through the penstock, which serves to "shoot" the water at the turbine.
• Turbine: A turbine is in many ways like the blades of a windmill or the veins of a pinwheel. The water flows past the turbine, striking its blades and turning it. The most common turbine design used in large, modern hydroelectric power plants is the Francis turbine, which is a disc with curved blades. The Francis turbine was developed by British-American engineer James B. Francis (1815–1892), who began and ended his professional career in the United States as an engineer at the Locks and Canal Company in Lowell, Massachusetts. In the largest hydroelectric plants, these turbines are enormous, weighing up to 170 tons or more. The largest ones turn at a rate of about 90 revolutions per minute.
• Generator: The turbine is attached by a shaft to the generator, which actually produces the electricity. Generators are based on the principle of electromagnetic induction, discovered by British scientist Michael Faraday (1791–1867) in 1831. Faraday discovered that as a metal that conducts electricity, such as copper wire, moves through a magnetic field, an electrical current can be induced, or created, in the wire from the flow of electrons. The mechanical energy of the moving wire is therefore converted into electrical energy. In a hydroelectric plant, the mechanical energy is supplied by the turbine, which in turn is powered by the kinetic energy of moving water.
• Transformer: A transformer converts the alternating current produced by the generator and converts it into a higher voltage current.
• Power lines: Power lines transmit the power out of the power plant to the electrical grid, where it can be used by consumers.
• Outflow: Pipes called tailraces channel the water back into the river downstream.

Roll on, Columbia

In the 1940s folk singer Woodie Guthrie (1912–1967) was hired by the Bonneville Power Administration to write folk songs about the dams being built on the Columbia River. Over a period of about a month, Guthrie wrote twenty-six folk songs under the general title Columbia River Ballads. One of the most popular of these songs was "Roll on, Columbia," which the state of Washington adopted as its official folk song in 1987.

Hydroelectric power plants come in three basic types:

• High head: "Head" refers to the difference in level between the source of the water and the point at which energy is extracted from it. Assuming other things are equal, the higher the head, the more power is generated. A high head hydroelectric plant is one that uses a dam and a reservoir to provide the kinetic energy that powers the plant. Most major hydroelectric plants are of this type.
• Run-of-the-river: In contrast, a run-of-the-river plant requires either no dam or a very low dam. It operates entirely, or almost entirely, from the flow of the river's current. No energy is stored in a reservoir. These hydroelectric plants are generally small, producing less than about 25 kilowatts.

• Pumped-storage: Some hydroelectric plants rely on a system of two reservoirs. The upper reservoir operates exactly as the reservoir does in a high head plant: Water from the reservoir flows through the plant to turn the turbines, then exits the plant and reenters the river downstream. In a pumped-storage plant, the water exiting the plant is stored in a lower reservoir rather than reentering the river. Using a reversible turbine, normally during off-peak hours (or hours when power usage is low, usually at night), water is then pumped from the lower to the higher reservoir to refill it. This gives the plant more water to use to generate electricity.

The World's Biggest Hydroelectric Power Plant

The world's biggest hydroelectric power plant is in South America. From 1975 to 1991 the Itaipú Dam was built across the Paraná River as a joint project by Brazil and Paraguay. The plant has eighteen generating units that can provide 12,600 megawatts of power, or 75 million megawatt-hours per year, enough wattage to power most of California. By 1995 the dam was providing 25 percent of Brazil's energy and 78 percent of Paraguay's.

The dam, called one of the "Seven Wonders of the Modern World" by the American Society of Civil Engineers, is enormous. The amount of iron and steel used in its construction could have built 380 Eiffel Towers (the famous landmark in Paris). The volume of concrete used to construct it is equal to fifteen times the volume used to construct the tunnel under the English Channel that connects France and England. To build the dam, workers had to rechannel the seventh largest river in the world and remove 50 million tons of earth and rock.

Current uses of hydroelectricity

During the 1930s a large number of hydroelectric dams were built on the waterways of the United States. Many of these dam projects were the result of that decade's Great Depression. During the depression, the U.S. government sponsored public-works projects designed to put people to work and recharge the economy. These dams, such as Hoover Dam and the 192 dams that were built along the Columbia River in the Northwest, produced hydroelectric power, and by the end of the 1930s they were meeting about 40 percent of the nation's electricity needs.

Many dams were also built in a seven-state region around the Tennessee River Valley under the guidance of the Tennessee Valley Authority (TVA). In the early twenty-first century about two thousand hydroelectric dams in the United States provide about 9 to 10 percent of the nation's electricity. They have not kept pace with U.S. demand for power simply because most of the best sites for hydroelectric dams already have one. Worldwide, about 40,000 hydroelectric dams provide a total of 675,000 megawatts of power to a billion users.

The World's Smallest Hydroelectric Power Plant

Someday soon the world's smallest hydroelectric power plants may appear—in people's shoes. On file at the U.S. Patent and Trademark Office is patent number 6,239,501. The patent is held by Canadian inventor Robert Komarechka, who conceived the idea that a tiny hydroelectric power plant embedded in the soles of shoes could provide power to run cell phones, compact-disc players, laptop computers, and other modern electronic gadgets.

The design is based on the way people walk. When a person takes a step, force is exerted downward on the heel. The foot then rolls forward, so that force is exerted on the toe. Komarechka found a way to harness this power by inserting sacs of fluid in the soles of shoes, one at the heel end and one at the toe end. Connecting the sacs is a conduit through which the fluid, a gel-like substance, can flow. As it flows, it turns a tiny turbine that is attached to a microgenerator, which in turn produces electrical power. A tiny socket allows the user to connect an electronic gadget to the power source, either directly at the shoe or at a power pack attached to, perhaps, a person's belt.

Benefits of hydroelectricity

The chief benefit of hydroelectric power, like the power provided by waterwheels, is that fossil fuels do not have to be burned, releasing particulate matter and greenhouse gases (such as carbon dioxide and sulfur dioxide) into the atmosphere, where they produce smog and contribute to global warming and acid rain. Hydroelectric power is also free in the sense that fuel does not have to be purchased to produce it, although of course money has to be spent to build and maintain the power plant and to distribute power to consumers.

Another major benefit of hydroelectric energy is that it is renewable. Over time, it will become more and more expensive to extract fossil fuels from the earth until eventually these fuels will be entirely depleted. Hydroelectric power will remain available as long as there are rivers. Hydroelectric energy, in contrast to oil, is not dependent on imported fuels from other countries, which could be cut off by one or more of those countries and make a nation vulnerable to political pressures from them. Hydroelectric dams can also have secondary benefits. They provide flood control on rivers, and their reservoirs often serve as lakes for recreational activities such as boating and swimming.

Drawbacks of hydroelectricity

Hydroelectric energy has always been thought of as clean energy, but scientists and engineers have started to understand that it has significant drawbacks as well. One drawback is that damming rivers floods large areas of land. When the water fully rises behind the new Three Gorges Dam on China's Yangtze River (under construction in 2005), for example, it will wipe out 13 cities, 140 small towns, and over 1,300 small villages, forcing over two million people to leave some of China's richest farmland. In Quebec, Canada, the first phase of a major hydroelectric project on the watershed flowing into the James Bay flooded nearly 3,900 square miles (10,000 square kilometers); the second phase of the project more than doubled that figure. A third phase of the project was still in the planning stages in 2005, but if the entire project is carried out as planned, the size of the flooded regions would be greater than the size of the country of Switzerland. Flooding vast amounts of land like this often has a disproportionate effect on native peoples, whose way of life can be destroyed.

Constructing hydroelectric dams, converting a free-flowing river of fresh water into a lake, also has a profound effect on ecosystems. Dams and reservoirs affect such factors as water quality, the amount and kinds of bacteria in the water, bank erosion, nutrient transport, the salt content of soil, and water temperature. Some dams have been implicated in the spread of waterborne diseases such as malaria. When a large dam fails, the results can be catastrophic, wiping out wildlife, vegetation, houses, roads, even whole towns downstream.

Dams also affect the amount of water in rivers downstream, with effects on wildlife that are only beginning to be understood. They also block the flow of silt downstream, affecting the flow of nutrients through a river system. In Egypt, the Aswan Dam along the Nile River, which provides 10 billion kilowatt-hours of electricity every year (and has a reservoir of nearly 6 trillion cubic feet [170 million cubic meters], four times that of the Hoover Dam), blocked the flow of nutrient-rich silt to the nation's agricultural floodplains. Farmers have had to replace those nutrients with a million tons of artificial fertilizer each year. Meanwhile, the silt can build up at the dams over time, causing them to be less efficient.

Some scientists estimate that 93 percent of the declines in freshwater marine life are caused by hydroelectric dams. The dams in the U.S. Pacific Northwest are regarded as a major cause in the decline of the salmon population because the dams prevent salmon from migrating upriver to spawn. Although "fish ladders" are installed to lessen this impact, they are by no means 100 percent effective.

Another drawback is that hydroelectric energy may not be as clean as once thought. Decaying vegetation in reservoirs may give off quantities of greenhouse gases equal to those emitted by burning fossil fuels. This can be an ongoing problem because when the water level in a reservoir falls during an extended dry period, vegetation grows on the banks. This vegetation, then, is covered by water when the reservoir refills during wet periods, causing the vegetation to rot again and emit gases such as methane and carbon dioxide, contributing to global warming. Finally, this decaying vegetation can alter the form of mercury contained in rocks into a form that is soluble in water. Mercury, a heavy metal like lead, can accumulate in the tissues of fish. It thus poses a health hazard to people who consume the fish.

Economic impact of hydroelectricity

As of 2005 there are about 40,000 large hydroelectric dams in operation worldwide (a large dam is defined as one that is taller than a four-story building, or more than 49 feet [15 meters]). The country with the greatest number of large dams is China, with 19,000. The United States is second, with 5,500. Major dams are defined as those more than 492 feet (150 meters) in height. The United States leads the world with 50 major dams.

The economic impact of hydroelectric power can be considerable. In some countries, such as Norway, hydroelectric dams provide virtually all of the nation's electrical needs. In Canada, about 60 percent of the nation's electricity is provided by hydroelectricity. Canada, and especially the province of Quebec, provides a good example of the economic impact of hydroelectricity. In the 1960s Quebec launched a program to foster economic development. One of the centerpieces of this program was the development of hydroelectric power in the James Bay region of northwestern Quebec. The first phase of the project began in 1972, when three rivers—the Caniapiscau, Eastmain, and Opinaca—were diverted into reservoirs. These reservoirs, along with a system of 215 dikes and dams and four power stations, nearly doubled Quebec's hydropower production. Construction employed 12,000 people and required 203 million cubic yards (0.9 million cubic meters) of fill dirt and rock, 138,000 tons of steel, 550,000 tons of cement, and 70,000 tons of explosives—all of which provided economic opportunities for Canadians. This first phase of the project, completed in 1985, provided 10,300 megawatts of electricity at a total cost of $14 billion.

Construction on the second phase of the project began in 1989, but it was suspended in 1994, when the project was nearly complete, because of environmental concerns, as well as objections raised by the Cree, a native community that lived in the James Bay region. These problems were resolved, and construction was completed in 2002. Combined, the two phases of the project produce 15,000 megawatts of electricity, or three times the amount of power produced by Niagara Falls. A third phase of the project was scheduled to begin in 1989, but that phase was put on indefinite hold because of environmental concerns. In large part because of the James Bay project, Quebec's electrical output increased from 3,000 megawatts in the early 1960s to 33,000 megawatts in 2002. Further, in 1997 Canada sold about $600 million in electrical power to the United States; by 2002 that figure had climbed to $3.5 billion. Ninety-three percent of this electricity is hydroelectric power.

Societal impact of hydroelectricity

The negative societal impact of hydroelectric power development is often felt most by native peoples. In northern Quebec, the Cree, an Algonquin-speaking people, were profoundly affected by the James Bay project. In 1975 the Cree were awarded $225 million in compensation for the disruption that the project caused in the Cree way of life, which revolved around fishing, hunting, and fur trapping in the watershed around James Bay. That money, however, could not compensate the Cree for the immense changes the project caused in Cree society. One Cree band, or tribe, was forced entirely off the land. Among the two remaining bands, the hydroelectric project (along with other enterprises such as mining and lumber) virtually destroyed hunting and trapping grounds, threatening the economic and cultural survival of the Cree.

This type of social problem is not limited to Quebec. In the United States, the construction of the Grand Coulee Dam in Washington State forced the Colville Indian tribe off their traditional hunting and fishing grounds. The Colville tribe sued the federal government and in the 1990s was awarded a $52 million lump-sum settlement. An organization called the International Rivers Network estimates that worldwide, between thirty and sixty million people, about two million a year, have been displaced (driven off their land) by hydroelectric dams. In most cases, the displaced people are small farmers and native peoples.

Issues, challenges, and obstacles of hydroelectricity

Hydroelectric power faces many obstacles. It is estimated that the amount of hydroelectricity available is about four times the amount being used. The United States has over 5,000 sites that have been identified as possible sites for hydroelectric dams. Many other sites have been identified in Asia and Africa. However, hydroelectric projects often meet with much resistance from environmental groups and others who are concerned about the effects of hydroelectric dams. In the past, the World Bank was willing to loan money to countries to build dams. In more recent years, largely because of environmental concerns and the effect of dams on native peoples, the World Bank has provided less money for these projects.

Research continues on the impact such dams have on fish populations, along with ways to minimize this impact. Research also continues on ways to improve water quality and dam safety, as well as ways to improve the efficiency of hydroelectric dams. In the United States, numerous efforts have been made to "uprate," or improve the efficiency, of older dams. The result since the late 1970s has been to add about 1.6 million kilowatts to the nation's power supply without building new dams. This power costs less than one-fifth of the cost of electricity produced by new oil-fired generators.

OCEAN THERMAL ENERGY CONVERSION

Ocean thermal energy conversion, or OTEC, is the primary means of extracting thermal energy from the world's oceans. It is based on the thermal gradient, which refers to the difference in temperature between the ocean's surface waters, which are warmed by the sun, and its deeper waters, which originate in polar latitudes and are therefore much colder. The concept of using the thermal gradient to produce electricity was first proposed by French biophysicist Jacques Arsene d'Arsonval (1851–1940) in 1881. D'Arsonval proposed the basic form of a system that is still used.

OTEC is based on two different technologies, closed cycle and open cycle, which can be combined into a hybrid system as well:

• Closed cycle: The system that d'Arsonval envisioned was a closed-cycle system. The working fluid was ammonia, which boils at a low temperature, 28°F (33°C). Heat transferred from the warm surface waters of the ocean boils the ammonia. As the vapors expand, they turn a turbine, which is connected to a generator that produces electricity. Cold seawater pumped up from depths of 2,625 to 3,280 feet (800 to 1,000 meters) is used to condense the ammonia vapor in a condenser. The ammonia is then recycled back through the system.
• Open cycle: In an open-cycle system, the working fluid is the warm surface water itself. In a near vacuum, the warm water vaporizes at the surface-water temperature. Like the ammonia vapor in the closed-cycle system, the expanding water vapor drives a turbine, which is attached to a generator that produces electricity. The open-cycle system has the added advantage of producing desalinized water, or water from which the ocean's salt has been removed. Thus, when the water is condensed by the cold water pumped from the depths, it can be siphoned off and used as drinking water. The underlying process is little different from the condensation that forms on a glass of iced tea on a humid summer day. Unlike the closed-cycle system, in which the ammonia is recycled again and again, the open-cycle system operates with a continuous supply of warm seawater.

• Hybrid systems: Hybrid systems employ both closed- and open-cycle systems, getting the benefits of each. The closed-cycle system produces more electricity than the open-cycle system, but the open-cycle system produces fresh water as well as electricity.

Current uses of ocean thermal energy conversion

Most research on OTEC is conducted by the Natural Energy Laboratory of Hawaii Authority (NELHA), formed in 1974. NELHA conducted the first at-sea test of a closed-cycle plant in 1979. The project was called Mini-OTEC, and it took place on a converted navy barge off the coast of Keahole Point, Hawaii. For three months the plant generated 50 kilowatts of gross power. The plant pumped 2,700 gallons (10,220 liters) per minute of cold (42°F [5.5°C]) seawater up from a depth of 2,200 feet (670 meters). The plant pumped an equal amount of warm (79°F [26°C]) surface water. Some of the plant's power had to be used to run the pumps, so the net power output of the plant ranged from 10 to 15 kilowatts.

From 1992 to 1998 NELHA conducted a major demonstration project at its Keahole Point facility. It designed and built a 210-kilowatt open-cycle plant. At its peak the plant produced about 255 kilowatts of power. However, it generally used about 200 kilowatts to pump 6,500 gallons (24,605 liters) per minute of 43°F (6°C) water from a depth of 2,700 feet (823 meters) and 9,600 gallons (36,340 liters) per minute of 76° to 81°F (24° to 27°C) surface water, for net power of some 50 to 55 kilowatts. Its highest net power output was 103 kilowatts, along with production of about six gallons (22 liters) per minute of desalinated fresh water. Designs were drawn for a 1.4-megawatt plant with the potential to produce about 400 net kilowatts, but funding was unavailable, so the project put on hold. As of the early 2000s no OTEC plant is operating anywhere in the world.

Benefits of ocean thermal energy conversion

OTEC draws on natural resources that are renewable, abundant, and clean. Rather than burning fossil fuels, OTEC power plants rely on warm seawater on the oceans' surfaces and cold seawater from their depths. By replacing such fuels as coal and oil, they can help eliminate the need for mines and oil-drilling platforms, which are not only unsightly but also are potential sources of pollution. Further, the amount of solar energy absorbed by the oceans, particularly in tropical climates, is far in excess of current human energy needs. Unlike wind and tidal energy, thermal energy is always present at consistent levels, which would make it an extremely reliable source of energy.

A second benefit is that OTEC plants do not release greenhouse gases such as carbon dioxide that contribute to global warming, nor do they release sulfur dioxide, a chief cause of acid rain. Further, scientists have concluded that discharging water back into the oceans has only minimal environmental drawbacks. A third benefit is that OTEC can reduce dependence on imported fuel. A state such as Hawaii, as well as many nations around the world, has to import most or all of its fuel. This need to import fuel both drains cash from the economy and makes the state or country dependent on other countries for its energy needs.

Finally, OTEC has a number of secondary benefits. It produces fresh water as well as electricity, a potentially major benefit for countries in which the amount of fresh water is limited. The amount of fresh water created can be up to 1.3 gallons for every 264 gallons (5 liters for every 1,000 liters) of cold seawater in an open-cycle plant. The cold seawater in OTEC can also be used to air-condition buildings, and contribute to mariculture, the cultivation of fish, shellfish, kelp, and other plants that grow abundantly in cold water. Also, eighty-four of the Earth's elements are in solution in the oceans' waters in trace amounts. Some of these elements, such as magnesium and bromine, have commercial value and could be efficiently extracted from the water used in OTEC.

Drawbacks of ocean thermal energy conversion

The major drawbacks to OTEC are geographical and economic. OTEC plants have to be located in places where the difference in temperature between the warm surface waters and cold deep-sea waters is great enough—at least 36°F (2°C); 40°F (4°C) would make the plant even more efficient. For shore-based plants, this difference would have to be present fairly close to the shore, although floating OTEC ships could expand the range of plants' geographic locations.

OTEC faces a number of economic obstacles. The cost of producing electricity through OTEC is higher than the cost of producing it from fossil fuels. Presently, there is not enough economic incentive for nations to invest billions of dollars in OTEC plants. Scientists and engineers estimate that after the high initial construction costs, the electricity produced over a long period, perhaps thirty years, would be economical, but no one knows how long these types of plants could function without requiring a major overhaul. Scientists and engineers are continuing to work on the development of major OTEC components to make them more durable, more efficient, and less costly.

Environmental impact of ocean thermal energy conversion

OTEC has very little in the way of environmental impact. The only hazardous substance is the working fluid, which in the case of closed-cycle plants is ammonia. However, the ammonia is recycled through the system, so an OTEC plant does not release any noxious substances into the water or atmosphere. An open-cycle plant releases some carbon dioxide, but the amount is 1 percent of the amount released by fuel-oil plants per kilowatt-hour.

What needs to be tested in a large commercial or experimental station is the effect of an OTEC plant on water temperatures and on marine life in the upper layer of the water. An OTEC plant pumps cold, nutrient-rich water from the depths up to the surface. This mixing of different temperatures of water could have effects on marine life that are currently not well understood. OTEC engineers are also concerned about the potential effects on fish populations. The discharge of nutrient-rich water could increase fish populations in the vicinity of a plant. On the other hand, the plant itself could also disrupt spawning patterns or result in the loss of fish eggs and tiny young fish. Again, these potential environmental impacts are not known.

Economic impact of ocean thermal energy conversion

Given current technology and the cost of fossil fuels, the economic impact of OTEC would most likely be greatest for small island nations that have to import all their fuel. Such a country, for example, Nauru in the South Pacific, would be able to benefit from a 1-megawatt plant. Such a plant could produce electricity for pennies per kilowatt-hour. It has been estimated that a 100-megawatt OTEC plant could produce electricity for about $0.07 per kilowatt-hour. The chief problem, however, is the initial cost of construction. That same 100-megawatt plant would cost about $4,200 per kilowatt capacity, or about $420 million. It is unlikely with the cost of fossil fuels relatively low that nations will make this type of investment. However, as of 2005 the cost of fuel oil was rising and reached $60 per barrel. If fuel oil continues to become more expensive, OTEC may become more of an option, and organizations such as the World Bank may become more willing to loan funds for construction.

Issues, challenges, and obstacles of ocean thermal energy conversion

The chief obstacle to OTEC development is the high initial construction cost of such a plant. Researchers continue to find ways to bring down the construction costs, particularly to reduce the cost of condensers and other components of the system. Research is also being conducted to find ways to boost the net power output of the system—that is, the amount of power left over after a portion of the power is used to pump water through the system. As of 2005, governments and international organizations remained reluctant to provide funds for the development of OTEC plants, whose long-run benefits are not entirely clear.

TIDAL POWER

Tidal power refers to the use of the oceans' tides to generate electricity. Sir Isaac Newton (1642–1727) pointed out in the seventeenth century that every day, the gravity of the moon exerts a pull on the Earth. This gravitational pull has little effect on the Earth's solid landmasses. But the oceans' waters are fluid, so as the moon's gravity pulls on them, they bulge outward. These bulges, which place along an axis (an imaginary line) that points toward the moon, are called lunar tides; on the other side of the Earth, the side away from the moon, the waters bulge out away from the gravitational pull of the center of the Earth.

While the moon does most of this work, the sun helps out, but to a lesser extent. This is because the gravitational attraction one body has on another is the result of two factors: its size and its distance. Although the sun is much bigger than the moon, the moon is much closer to the Earth, so it exerts a greater gravitational pull. Nonetheless, the sun's gravitational pull also creates tides, called solar tides.

When the Earth, moon, and sun are aligned in a straight line during a full or new moon, both the sun and moon are pulling together in the same direction, like two teammates in a tug-of-war. During a full moon the pull is greatest, creating large tides called spring tides. During half-moon periods, when the moon and sun are at right angles, or 90 degrees, to each other, the tides created, called neap tides, are lower, simply because the lunar tides are being pulled out along one axis and the solar tides along a perpendicular axis. During these times the coasts have two low and two high tides over a period of less than twenty-four hours.

At the same time, the Earth rotates beneath these bulges, passing under each one during a twenty-four-hour period. The result is that tides rise and fall rhythmically along the world's coastlines approximately twice each day in predictable patterns. These flows of water are very like the flows of rivers, and their energy can be harnessed in much the same way that a river's energy is by a hydroelectric dam.

There are two ways to harness energy in tidal power-generating stations: the tidal barrage and tidal streams. A tidal barrage, also called an ebb generating system, is very similar to a dam. The barrage is constructed at the mouth of a bay or estuary (a water passage where the tide meets the lower end of a river). For a barrage to be workable, the difference in water elevation between low tide and high tide has to be at least 16 feet (5 meters).

Tidal Power Forever?

In most discussions of tidal power, one of the chief advantages cited is that tidal power is endlessly renewable—that the Earth will never run out of it because the tides will always be there. Technically, this claim is not entirely true. The bulging oceans exert friction on the Earth, gradually slowing down the speed of the Earth's rotation. This means that in time, tidal power will no longer exist.

As a practical matter, though, this is no cause for concern. This slowing of the Earth's rotation will not have any significant effect for billions of years! By that time humankind will no doubt have harnessed a form of power that cannot be imagined today. In the meantime, scientists have calculated that harnessing all of the tidal power of the oceans would slow the Earth's rotation by twenty-four hours every two thousand years.

When the tide flows in, the water moves through moveable gates in the barrage called sluice gates, similar to a "doggy door" a family pet can use to enter the house just by pushing on it. When the tide stops flowing in, the gates are closed, trapping the water in a basin. The water now represents stored energy, in much the same way that the reservoir behind a hydroelectric dam does. As the tide then flows out (ebb tide), the gates in the barrage are opened. This allows the water to turn turbines as it flows back out to sea. Just as in hydroelectric plants, the turbines are connected to a generator, which produces electricity. It is possible to have flood-generating systems, where the water turns the turbines as it flows in rather than out, but hydrologists and engineers believe that these systems are less efficient. It is also possible to have systems that work in both directions, but these kinds of systems would be difficult and more expensive to build because the turbines that would have to be used would have to work in both directions. Consequently, the best design for most sites is the ebb-generating system.

Other technologies exist for harnessing tidal power, but all these technologies are in early stages of development. In each case, the goal is to tap the energy contained in tidal streams. A tidal stream is a fast-flowing current of water caused by the movement of the tides. These streams can occur wherever a natural barrier constricts the flow of water, which then speeds up after it passes the constriction. Thus, a tidal stream might flow between two islands, or between the mainland and an offshore island. The chief advantage of these technologies is that a tidal basin does not have to be constructed.

Current use of tidal power

Currently, only one major tidal power generating station is in operation. This station is located on the estuary of the La Rance River in France. Construction of the barrage began in 1960 and was completed in 1966. The barrage is almost 1,100 feet (330 meters) long with a 13.7-square-mile (22-square-kilometer) basin. The station uses twenty-four turbines, each 17.7 feet (5.4 meters) in diameter. Each turbine is rated to produce about 10 megawatts of power, so the station can produce a maximum of 240 megawatts. (To put that figure in perspective, the average coal- or oil-fired power plant produces about 1,000 megawatts.) There are 8,760 hours in a year, so the system can produce 2,102,400,000 kilowatt-hours per year, enough to supply most of the electricity needs of the Brittany region of France.

Other nations have explored the possibility of harnessing tidal power. Since the 1960s tidal power has been proposed in the Kimberley region of western Australia. There, it was estimated that tidal power could provide 3,000 megawatts of electricity. Australia's Renewable Energy Commercialisation Program awarded a grant to develop a 50-megawatt plant in the Derby region of Australia. Scotland, too, has explored tidal energy, and proposals have been made for the construction of a tidal station on Solway Firth in southwest Scotland; in the 1970s Scotland built a 15-kilowatt experimental tidal turbine on Loch Linnhe. In England, the Severn River has been identified as a promising site for a tidal power station. The most promising site in the world is the Bay of Fundy in Canada, which, at up to 56 feet (17 meters), has the highest tides in the world.

Benefits and drawbacks of tidal power

The chief benefits of tidal power, as of most forms of alternative energy, are that it is clean, renewable, and does not consume resources such as coal or oil. It does not discharge pollutants into the water or atmosphere, so it does not contribute to acid rain or global warming. Further, the energy source is free. Tidal power barrages have a secondary benefit, for they can function as bridges linking communities on opposite sides of an estuary, making travel quicker.

The chief drawback of tidal power stations is their expense. It has been estimated, for example, that construction of a tidal power station on the Severn River in England would cost about $15 billion. A second drawback is that not every coastal region is suitable for tidal power. Generally, a difference between high and low tides of about 16 feet (5 meters) is necessary for a tidal power station to be cost-effective. Only about forty such sites in the world have been identified. A third drawback is that the tides are in motion only about ten hours per day. This means that tidal power cannot be provided consistently throughout the day and would have to be supplemented with other forms of power.

Environmental impact of tidal power

The environmental impact of tidal power stations has not been fully explored for the simple reason that only one major power station exists. Although the potential environmental impacts would be specific to the individual site, a few generalizations can be made. A tidal power station would change the water level in an estuary, affecting patterns of vegetation growth. It would have an impact on the ecosystems of the shoreline and of the water. It would likely have an impact on the quality of the water in an estuary; for example, it could change the cloudiness of the water, which in turn could affect the types of fish that could live in the water. This which would in turn have an effect on birds that feed off the fish. Fish life would also be affected by a barrage unless a way was found to allow the fish to pass through. Further, a tidal station could change patterns of bird migration and reproduction.

Economic impact of tidal power

Because of the limited availability of suitable sites, only about 2 percent of potential tidal power can currently be harvested. The potential amounts to 3,000 gigawatts (giga -, meaning billion) of electricity, so roughly 60 gigawatts could actually be produced with current technology. The economic impact to tidal electricity would likely be local. For instance, it is estimated that a tidal power station on England's Severn River could produce up to ten percent of England's electricity.

Issues, challenges, and obstacles of tidal power

The chief issues facing tidal power are economic. The cost of building such a plant is high. However, once the plant is built, the energy it generates is essentially free, although the costs of maintaining the plant and distributing the power have to be included in cost estimates. The cost of such a plant would therefore be spread out over a period of thirty years or more, but finding initial funding is difficult. Also, because of limited experience with tidal power stations, their environmental impacts are not well understood. A final challenge is developing equipment that can withstand the harsh marine environment.

OCEAN WAVE POWER

Wave power is actually another form of solar power. As the sun's rays strike the Earth's atmosphere, they warm it. Differences in the temperature of air masses cause the air to move, resulting in winds. As the wind passes over the surface of the oceans, a portion of the wind's kinetic energy is transferred to the water, producing waves. These waves can travel essentially unchanged for enormous distances. But as they approach a shoreline and the water becomes shallower, their speed slows and they become higher. Finally, the wave collapses near shore, releasing an enormous amount of energy. It has been estimated that the amount of kinetic energy contained in a wave is up to 110 kilowatts per meter.

Capturing wave energy means that the kinetic energy of waves is converted into electrical power. In many respects, the technology is the same as it is with tidal and hydroelectric power. The kinetic energy turns a turbine attached to a generator, which produces electricity.

Current uses of ocean wave power

Scientists and engineers have devised hundreds of ways to capture wave power. The first, developed by a company called Wavegen, is being used at the world's only major wave power station in operation, the 500-kilowatt Land-Installed Marine-Powered Energy Transformer (Limpet) on the island of Islay off Scotland's western coast. The basic design is called an oscillating water column (OWC). The water from a wave flows into a funnel and down into a cylindrical shaft. The rise and fall of the water in the shaft drives air into and out of the top of the shaft, where it blows past turbines, causing them to turn. In a sense, then, an OWC is a combination of hydropower and a windmill, with the "wind" consisting of air pressurized by the power of the wave. As with most other forms of hydropower, the turbines are attached to a generator, which produces electricity. In the case of Limpet, two turbines are in place. A chief advantage of this design is that the generators are not submerged in the water, making maintenance easier. Wavegen has built and tested a number of prototypes and in the early 2000s was constructing an OWC station on Pico Island in the Azores. It was anticipated that the plant would provide ten percent of the island's power requirement for its 15,000 people.

A second design is generally referred to as a wave-surge or focusing device. With these systems, sometimes called tapered channel or "tapchan" systems, a structure mounted on shore, which looks a little like a skateboard ramp, channels the waves and drives them into an elevated reservoir. As water flows out of the reservoir, it generates electricity in much the same way a hydroelectric dam does. A variation of this design was developed by a Norwegian company called WaveEnergy. This design consists of a series of reservoirs layered into a slope. WaveEnergy has also proposed attaching its design to old deep-sea oil-drilling platforms.

Engineers continue to work on other designs. One example that can be cited is the hosepump, which makes use of a type of hose called an elastomeric hose, the volume of which decreases as the hose is stretched in length. The hose is attached to a float that rides the waves on the ocean's surface, pulling it and relaxing it. This movement pressurizes seawater in the hose, which is then fed through a valve past a turbine attached to a generator. This is one example of the many ingenious devices with which scientists are experimenting. Many of these devices have fanciful names: the Mighty Whale, the Wave Dragon, Archimedes Wave Swing, WavePlane, Pendulor, and the Nodding Duck.

Benefits and drawbacks of ocean wave power

Like other forms of hydropower, wave power does not require the burning of fossil fuels, which can pollute the air, contributing to acid rain and global warming. The energy is entirely clean and endlessly renewable. Further, in contrast to tidal power and thermal energy stations, which can be built in only a limited number of locations, wave power stations could be built along virtually any seacoast. Some of these devices could provide artificial habitats for marine life. They could also serve a secondary function as breakwaters.

The chief drawback of any onshore wave power station is the disruption caused to the natural environment by the presence of the station itself. OWC stations could potentially be noisy, although engineers continue to work on ways to dampen the noise they produce. A further drawback is that many of the technologies are new and untried, making it difficult to find funding to build the plants. In addition, these types of devices could cause navigational hazards for the shipping and fishing industries. Because of their location by the open ocean, these power stations could sustain severe damage from storms affecting the coastline, such as hurricanes.

Impact of ocean wave power

Wave power stations could impact the environment in a number of ways. Offshore or near-shore devices could change the flow of sediment, affecting marine life in unpredictable ways. Onshore devices could have an impact on, for example, turtle populations or other shoreline creatures that use the shorelines for nesting and breeding.

The economic impact of wave power is hard to calculate, but the potential impact is enormous. It is estimated that the total amount of wave energy that strikes the world's coastlines is about 2 to 3 million megawatts. In many locations throughout the world, the waves along one mile of coast contain the equivalent of 65 megawatts of power, or about 35,000 horsepower. Some experts say that if existing technologies were widely adopted, wave power could provide about 16 percent of the world's electricity needs. A large wave power station (100 megawatts) could provide power for as little as three to four cents per kilowatt-hour; a smaller station (1 megawatt) could provide power for seven to ten cents per kilowatt-hour. Both of these ranges include the cost of the plant's construction divided out over a period of years.

Issues, challenges, and obstacles of ocean wave power

As with other forms of water power, the chief obstacle is funding. Many wave-power technologies are unproven, particularly on a large scale, so it is difficult for developers to attract funding from private and governmental organizations. Another challenge is building equipment that is sturdy enough to withstand the harsh marine environment over long periods of time.

For More Information

Books

Avery, William H., and Chih Wu. Renewable Energy from the Ocean. New York: Oxford University Press, 1994.

Berinstein, Paula. Alternative Energy: Facts, Statistics, and Issues. Phoenix, AZ: Oryx Press, 2001.

Boyle, Godfrey. Renewable Energy, 2nd ed. New York: Oxford University Press, 2004.

Cuff, David J., and William J. Young. The United States Energy Atlas, 2nd ed. New York: Macmillan, 1986.

Howes, Ruth, and Anthony Fainberg. The Energy Sourcebook: A Guide to Technology, Resources and Policy. College Park, MD: American Institute of Physics, 1991.

Periodicals

Freeman, Kris. "Tidal Turbines: Wave of the Future?" Environmental Health Sciences (January 1, 2004): 26.

Valenti, Michael. "Storing Hydroelectricity to Meet Peak-Hour Demand." Mechanical Engineering (April 1, 1992): 46.

Web sites

O'Mara, Katrina, and Mark Rayner. "Tidal Power Systems." http://reslab.com.au/resfiles/tidal/text.html (accessed on September 13, 2005).

"Tidal Power." University of Strathclyde. http://www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/Tidal%20Power.htm (accessed on September 13, 2005).

Vega, L. A. "Ocean Thermal Energy Conversion (OTEC)." http://www.hawaii.gov/dbedt/ert/otec/index.html (accessed on September 13, 2005).

Weiss, Peter. "Oceans of Electricity." Science News Online (April 14, 2001). http://www.sciencenews.org/articles/20010414/bob12.asp (accessed on September 13, 2005).