7: 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 21st 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. It was used to raise water out of the Nile River for use on the crops. Some 2,000 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 16 waterwheels.
In the centuries that followed, until fossil fuels became the preferred power source during the Industrial Revolution that begin in the 18th
century, farmers continued to take advantage of the currents in rivers and streams for a variety of agricultural purposes. These included grinding grain and pumping water for irrigation (watering crops). A British 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 400 people.
Farmers, though, were not the only ones to use waterwheels. Early factories, especially in Great Britain and in the Northeastern United States, 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 British 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. (See sidebar on page 306.)
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 19th 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.” During 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 (1888–1966) 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 20th and early 21st
centuries, efforts were 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. Although strides have been made, the practical use of these power sources is still in the early stages.
Tidal power for electrical generation is relatively rare in the world when compared to other sources of power generation. By the beginning of the 2010s, only a handful of tidal power-generating stations were in service. The first tidal power-generating station was built in 1966 at the mouth of the La Rance River along France's northern coast. The Rance tidal power plant provides 240 megawatts, or 240 million watts, of electrical power.
In 2011, the largest tidal power station in North America was the 20-megawatt Annapolis Royal Generating Station in Nova Scotia, Canada, which first started generating power in 1984. There are many other promising sites for tidal power throughout the world, such as Severn River in western Great Britain, Cook Inlet in Alaska, and the White Sea in Russia.
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 Limpet 500 consists of a large blockhouse on the shoreline into which waves can flow in and out. When seawater flows into the blockhouse, air is trapped in the upper part of the structure and is subsequently compressed. The compressed air is vented to the atmosphere through a turbine. The high pressure of the compressed air turns the turbine, which then generates electricity. By 2011, the Limpet 500 had produced electricity from wave power for some 11 years.
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 have borrowed 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 electrical power. However, this 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.
By 1998 the ocean-thermal power plant located at NEHLA was closed. But escalating energy costs in the early 2000s, as well as ongoing concern about climate change, eventually prompted renewed interest in ocean thermal energy. In 2011 a new, large facility for testing heat exchanger
technology was dedicated at the NELHA site. The test facility, standing 40 feet (12 meters) high, was constructed to test various materials for the heat exchangers that form a necessary part of ocean thermal power generation devices, with the goal of making the heat exchangers more affordable.
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 of 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 (or any solid, liquid, or gaseous body) 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, most energy-harvesting technologies require that 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 Earth, causing its oceans to bulge outward. At the same time, Earth rotates
beneath this water, so twice each day, 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. As such, 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 10 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 meters) 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 Earth's rotation. In the Northern Hemisphere, 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 (41.7 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 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 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. Although people would have to continue to spend money to build plants, maintain them, and distribute the power that is produced, a major benefit is that power providers would not have to buy fuel for them. The potential savings are huge. Also, the prices of fossil fuels, especially petroleum, can sometimes change dramatically, leading to uncertainty for consumers and businesses regarding how much money they will have to spend on energy. In contrast, energy from water power is usually very reliable, and the prices charged to consumers are not as volatile as are the prices associated with fossil fuels.
During the first few years of the 2000s, the cost of a barrel of oil was typically well under $50, and at times under $20 per barrel. But increasing global demand for oil, coupled with instabilities in some oil-producing regions of the world, led to considerable increases in petroleum prices in 2008 and again in 2011. For instance, in 2008 the price rose to nearly $150 per barrel, before falling to under $50 by the end of that year. In 2011, world oil prices surged again to over $100 per barrel. Replacing that fuel with water energy would result in enormous savings for consumers.
The price of electricity produced by water power is much more stable. Large hydroelectric dams use reservoirs, or human-made lakes, that are capable of releasing water through electricity-generating turbines on a steady basis. Energy from the tides, and from river and ocean currents, is even more reliable. This reliability means that the electrical power supplies and prices from water-derived power are fairly stable over time.
Drawbacks of water energy Water-based energy sources, though, are not without their drawbacks. Although hydroelectric dams have been around for well over a century, stations for harvesting tidal, wave, ocean current, and ocean thermal power are still fairly low in number. Exploiting these forms of power would require a huge investment. The cost of building a large tidal power-generating station, for example, could run as high as $15 billion.
A second drawback is that various forms of water energy are either somewhat variable, or are difficult or expensive to exploit on a large scale. 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 Earth with the sun and the moon. Wave power could be highly variable, depending on prevailing winds. Although 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 Earth. Providing tidal power to the residents of Nebraska would be impractical because Nebraska is nowhere near an ocean. Although 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. 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. In 2010, hydroelectric power provided nearly one fifth (20 percent) of the electricity used worldwide, and 6 percent of the electricity used in the United States. However, much of that hydroelectric power is concentrated in regions with several rivers. In the United States, for example, in 2011, more than half of all hydroelectric power capacity was located in just the three states of California, Oregon, and Washington. And almost one third of all U.S. hydroelectric power was produced in Washington state alone.
At the beginning of the 2010s, hydroelectric dams provided almost all of the electricity in Norway, over 70 percent in Iceland and Austria, and about 60 percent in Canada. However, hydroelectric dams can provide little or no power in the desert countries of the Middle East or in much 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, tens of thousands of dams are used to provide hydroelectric power. In 2010, in fact, there were around 58,000 large dams used to produce electricity. 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 as high as 52,000 cubic meters (13.7 million gallons) per second. During the dry season, however, the river flows at a rate of only about 2,000 cubic meters (nearly 530,000 gallons) 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. Although water power has been used throughout much of history, its economic impact began to be felt more fully in the late 18th and early 19th centuries. By the mid-1830s, the town of Lowell, Massachusetts, which grew as textile firms built up around the availability of water power, 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 2010, hydroelectric dams provided about 6 percent of the electricity used in the United States. Worldwide, though, hydroelectric plants provided almost 20 percent of electrical output, serving over a billion people. Together, in 2010, worldwide hydroelectric plants had an electrical generating capacity of approximately 800,000 megawatts (mega-, meaning “million”). Hydroelectric power 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 Earth's surface is covered by oceans. The amount of water they contain is staggering: 328 million cubic miles (1.37 billion 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 the energy contained in about 250 billion barrels of oil.
To put that figure in perspective, the average amount of oil produced in the world each day in 2010 was about 82 million barrels. That means that each day, the tropical oceans absorb over 3,000 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 1 percent of the 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 economical 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 burning fossil fuels. 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. However, energy derived from water, such as from hydroelectric dams, can have negative impacts on the environment as well, such as by altering river ecosystems.
Water power could have a major positive 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.
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 21st century, 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. Although 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.
Companies in the United States and Germany also manufacture waterwheels for electrical power generation. 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 result of the Black Death, the plague that wiped out large portions of the population in the late Middle Ages. Waterwheel use expanded rapidly in Great Britain, 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 power available from the water can actually be converted into useful power).
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 convert the kinetic energy of the water into mechanical energy, the electric generators needed to convert mechanical energy into electrical energy, and any gearboxes needed to convert the rotation rate of the waterwheel (turbine) to the acceptable rotation rate of the electric generator. 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 $500,000. 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 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 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 continues to amaze visitors. It is the largest concrete structure in the United States, at 11,975,521 cubic yards (9,155,943 cubic meters). At its widest point, it is almost exactly 1 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 have a combined capacity of 6,809 megawatts of electrical power.
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 Page 326 | Top of Articlescientist 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.
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.
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 the Great Depression, which occurred during that decade. 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 many dams that were built along the Columbia River in the Pacific Northwest, produced hydroelectric power. 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). During much of the first decade of the 2000s, around 2,000 hydroelectric dams in the United States provided about 9 to 10 percent of the nation's electricity. By 2010 that percentage was down
to 6 percent. The share of electricity produced by hydroelectric dams decreased largely because most of the best sites for hydroelectric dams already had one, plus there were serious concerns about the environmental impact of building new dams.
As a result, practically no new large dams were being built to keep up with America's appetite for ever-more power. However, developing countries such as China—in search of greatly expanded electricity production, and with far fewer environmental rules in place—were constructing large hydroelectric dams at a frenetic pace. Worldwide in
2010, hydroelectric dams provided a total of around 800,000 megawatts of power to over a billion users.
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. Fossil fuels release 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 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. However, scientists and engineers have started to understand that hydroelectric power has significant drawbacks as well. One drawback is that damming rivers floods large areas of land. Construction of the Three Gorges Dam on China's Yangtze River, completed in 2008, resulted in the forced evacuation of 1.4 million people from the reservoir area. Thirteen cities, 140 small towns, and over 1,350 small villages had to be abandoned because of the dam.
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. 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 electrical energy every year (and has a reservoir of nearly 6 trillion cubic feet [170 billion 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.
In a 2007 report, researchers with Brazil's National Institute for Space Research asserted that 104 million tons of methane were released by vegetation rotting within dam reservoirs that year. Moreover, the researchers attributed 4 percent of global warming to the emissions from reservoirs of hydroelectric dams. However, another study published in 2011 by a different group of researchers significantly lowered the estimated amount of methane released by hydroelectric reservoirs. It appears that more research needs to be performed to assess the actual impact of hydroelectric dams on the environment.
Another problem caused by the decaying vegetation within reservoirs is that it 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 In 2010 there were about 58,000 large hydroelectric dams in operation worldwide (a large dam is defined
as one that is taller than a four-story building, or more than about 50 feet [15 meters]). China has the greatest hydroelectric power capacity at 200 gigawatts (200,000 megawatts), followed by Canada with about 90 gigawatts installed capacity, with the United States coming in third at 80 gigawatts of hydroelectric power capacity.
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 4 power stations, nearly doubled Quebec's hydropower production. Construction employed 12,000 people and required 203 million cubic yards (155.2 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. The work stopped due to environmental concerns, as well as objections raised by the Cree, a native community that lived in the James Bay region. These problems were eventually 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. In large part because of the James Bay project, Quebec's hydroelectric power plants produced 34,490 megawatts in 2010. In 1997 Canada sold about $600 million worth of electrical energy to the United States; by 2009 that figure had climbed to $2.38 billion. Nearly all of this exported electricity is derived from hydroelectric power.
Societal impact of hydroelectricity The negative societal impact of hydroelectric power development is often felt most by people native to the region. 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 its 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 its 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 30 and 60 million people, about 2 million a year, have been displaced 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 locations 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.
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 envisioned 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 liquid 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 back into a liquid. 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 Page 338 | Top of Articleday. 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 6 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 late 2011 there were no OTEC power plants operating anywhere in the world. However, new funding for OTEC technology was allocated through the U.S. Department of the Navy beginning in 2009. The Office of Naval Research funded a new test facility to investigate improvements to OTEC heat exchangers. In April 2011, the new heat exchanger test facility was officially opened at NELHA as part of a Navy-funded $5.8 million research project. The research will test the use of lower-cost aluminum alloys in place of titanium for OTEC heat exchangers.
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. The risks of ocean-based oil drilling—both in terms of human life and environmental damage—was starkly demonstrated by the April 20, 2010 explosion of the Deepwater Horizon oil platform. The explosion killed 11 crewmen and ultimately released almost 5 million barrels (a little over 200 million gallons) of crude oil into the Gulf of Mexico over a period of nearly three months.
Utilizing the thermal energy present in the oceans is an extremely reliable method of generating renewable energy. Unlike wind and tidal energy, thermal energy is always present at consistent levels. Moreover, the amount of solar energy absorbed by the oceans, particularly in tropical climates, is far in excess of current human energy needs, so there would not be the risk of having only a limited supply of energy available.
A second benefit of developing ocean thermal energy 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 can produce freshwater as well as electricity, a potentially major benefit for countries in which the amount of freshwater is limited. The amount of freshwater 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 it can contribute to mariculture, which is the cultivation of fish, shellfish, kelp, and other plants that grow abundantly in cold water. Also, 84 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); a 40°F (4°C) temperature gradient 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 power 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 or from several other kinds of renewable energy, such as wind or geothermal. 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 30 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—such as testing new materials for heat exchangers—to make them more durable, more efficient, and less costly. A new heat exchanger test facility standing 40 feet (12 meters) in height was opened at Hawaii's NELHA research center in 2011.
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 of energy produced.
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. However, 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, in
2008, and again in 2011, the cost of oil exceeded $100 per barrel. If 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, heat exchangers, 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.
In the early 2010s, governments and international organizations generally remained reluctant to provide funds for the development of OTEC plants, whose long-run benefits are not entirely clear.
Tidal power refers to the use of the oceans' tides to generate electricity. Sir Isaac Newton (1642–1727) pointed out in the 17th century that every day, the gravity of the moon exerts a pull on Earth. This gravitational pull has little effect on 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 are placed along an axis (an imaginary line) that points toward the moon, are called lunar tides. On the other side of Earth, the side away from the moon, the waters bulge out away from the gravitational pull of the center of Earth.
Although the moon does most of this work, the sun also 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 mass and its distance. Although the sun is much bigger with a much higher mass than the moon, the moon is much closer to 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 more-or-less straight line during a full or new moon, both the sun and moon are pulling along 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 24 hours.
At the same time, Earth rotates beneath these bulges, passing under each one during a 24-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 much like the flows of rivers, and their energy can be harnessed in much the same way that a river's energy is harnessed 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).
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. However,
these types of systems would be difficult and more expensive to build because the turbines 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. 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 off-shore island. The chief advantage of these technologies is that a tidal basin does not have to be constructed.
Current use of tidal power By 2012, there were only a handful of tidal power-generating stations operating in the world, but many more were in the planning or pilot demonstration phases. The very first tidal power-generating station—the Rance tidal plant—was based on the barrage concept. It was constructed at the mouth of the La Rance River along France's northern coast.
Construction of the Rance barrage began in 1960 and was completed in 1966. The barrage is almost 1,100 feet (330 meters) long with an 8.5-square-mile (22-square-kilometer) basin. The station uses 24 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 kilo-watt-hours per year, enough to supply most of the electricity needs of the Brittany region of France.
In Canada, the Annapolis Royal Generating Station (also called the Annapolis Tidal Power Plant) is situated at the Annapolis basin in Nova Scotia. It began generating electricity in 1984. Like the big Rance tidal power plant in France, the Annapolis plant uses a barrage system to capture the incoming tidal water. Power is then generated by spinning turbines as the captured water is allowed to flow back to the sea. The Annapolis tidal plant annually produces around 30 million kilowatt-hours of electrical energy, enough to supply the needs of 4,500 Canadian homes.
One of the only tidal power plants operating in the United States in 2011 was the Roosevelt Island Tidal Energy (RITE) project. Unlike the barrage systems at Annapolis and La Rance, the RITE project uses the freely flowing waters within the tidal stream. The RITE tidal power system uses multiple turbines affixed to the riverbed of the East River next to New York City's Roosevelt Island. The turbines are owned by a company called Verdant Power, with public financing from agencies of both New York State and New York City.
The RITE project began in 2002 and was planned to consist of three phases. The first phase operated from 2002 to 2006 and was a proof-of-concept pilot project to demonstrate the feasibility of harvesting the East River's tidal stream. The second phase ran from 2006 through 2008 and deployed an array of six turbines, each with a rotor diameter of 16.4 feet (5 meters). The second phase of the RITE project accumulated 9,000 hours with turbines generating power from the East River's tidal stream, which delivered a total of 70 megawatt-hours of electrical energy to two customers.
Completion of the RITE project was planned for its third phase, scheduled to run from 2009 to 2012. The goal of phase three was to build a system of 30 turbines in the East River to generate 1 megawatt of electrical power from the river's tidal stream. In late-2011, Verdant Power was working with the U.S. Environmental Protection Agency and the Federal Energy Regulatory Commission to obtain final approval for the RITE project's phase 3 completion.
In the early 2010s, many other projects designed to tap into tidal power were either already operational or underway around the world. For instance, near the Annapolis tidal barrage power plant in Canada, two new projects were being developed in 2011 to harvest energy from the tidal stream in the nearby Minas Basin. Like the RITE project in New York City, the new projects in Minas Basin would use turbines placed on the seabed for generating power. Both the Annapolis and Minas basins are inlets from the Bay of Fundy, which possesses some of the greatest tidal ranges (the difference in height between the high and low tides) on Earth.
Also in 2011, the Scottish government gave its consent for the construction of a 10-turbine array to be situated between the islands of Islay and Jura off the western coast of Scotland. The turbines would use the tidal stream to generate power. Other countries with tidal power plants in operation in 2011 included South Korea, China, and Russia.
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 large tidal power station on the Severn River in Great Britain 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 barrage system to be cost-effective. Only about 40 such sites in the world have been identified. However,
more recently developed systems using the flow of water in tidal streams open up many more coastal areas to the practical development of tidal power.
A third drawback is that the tides are in motion only about 10 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 As of 2012, the environmental impact of tidal power stations has not been fully explored for the simple reason that only a few major power stations exist. 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.
In addition, 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 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 According to the World Offshore Renewable Energy Report released in 2009, a staggering 3,000 gigawatts (giga-, meaning billion) of electrical power is available from the energy of tides. However, only about 3 percent of that potential tidal power can realistically be harvested. By developing the 3 percent of potential tidal energy that is cited in the World Offshore Renewable Energy Report, roughly 90 gigawatts of electrical power could be produced worldwide.
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 10 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 30 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 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 of the water is used to produce 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 first 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. In 1999 the construction of an OWC station was completed on Pico Island in the Azores. Throughout the first decade of the 2000s, the Pico Island power plant was upgraded and modified in order to increase its reliability and performance, with autonomous operation of the plant being achieved in 2010.
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 that 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 that scientists are developing. Many of these devices have fanciful names: the Mighty Whale, the Wave Dragon, Archimedes Wave Swing, WavePlane, Pendulor, and the Nodding Duck.
A wave energy device designed by a company called Pelamis Wave Power has been used to produce power off the coasts of Great Britain and Portugal. The device (called the Pelamis Wave Energy Converter) floats on top of the water, and consists of long, tubular sections that are attached to one another by pivoting joints. As the device bobs up and down with the passing waves, the joints connecting each section push and pull on hydraulic cylinders. The pushing and pulling of a piston within the cylinders pump up hydraulic oil to a high pressure, which ultimately is used to turn generators that produce electricity. The electricity is passed through electrical cables over the seafloor and onto land.
In 2004, a Pelamis Converter became the first wave power machine in the world to feed electricity into a nation's (Great Britain's) power
grid. And in 2008, three Pelamis wave converters began operation off the coast of Portugal. The three wave converters measured 466 feet (142 meters) long. Together, they could generate up to 2.25 megawatts of electrical power. However, the company that owned the Pelamis wave converters off the Portuguese coast encountered financial difficulties and the machines were removed after only two months of operation. Despite such setbacks, in 2011 the Pelamis Wave Power company was marketing its next generation wave energy converter known as the P2.
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, such as the oscillating water column technology, 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 lessen the noise they produce. A further drawback is that many of the
wave energy converter 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 nearshore 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 coastal areas 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 1 mile (1.6 kilometers) 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 10 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.
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