Hydro Energy The ocean is a body of water occupying about two-thirds of a world made for man - who has no gills. ~ Ambrose Bierce (1842 - 1914) CHAPTER 4 Oceans cover more than 70% of the earth’s surface, making them the world’s largest source of hydro energy. There are many different ways to extract energy from water. Seawater is the source of deuterium, the ideal fuel for nuclear fusion. Surface water stores a massive amount of solar energy that can be exploited to design thermal power plants. In addition, water contains mechanical energy that can be converted to useful work in the form of the kinetic energy of river streams, or the potential energy of waterfalls, tides, and ocean waves. According to some estimates, these resources have the potential to produce 1-2 terawatts of electricity. This amount is enough to cover the energy demands of the entire globe, but tapping into most of that potential is not yet economically feasible. In this chapter, we will investigate how the mechanical energy stored in water can be used to design practical devices. Thermal and nuclear conversion options are discussed in Chapters 5 and 11 respectively. Hydroelectric Energy Egyptians harnessed energy from flowing water about 2,000 years ago by turning waterwheels to grind their grain. These primitive devices allowed the force of falling water to act on a waterwheel and provide rotational energy or shaft power. Through the centuries, mechanisms were designed to facilitate many other applications beyond the simple grain mills of the Egyptians. By the time of the Industrial Revolution, water power was driving tens of thousands of waterwheels. Today, hydropower is the most widely available renewable energy. It is exploited almost exclusively for generating electric power, providing about 20% of all electricity used around the world and 10% of the US electrical capacity. Question: Two medieval varieties of waterwheels were undershot and overshot wheels1,2 (Figure 4-1). Undershot refers to a paddle wheel fixed to the bank of a river or hung from an overhead bridge. It is turned by the impulse of the water current. Overshot water mills work by bringing a stream of water through a pipe or canal and pouring it onto the wheel from above. Which of the designs seems to have a higher efficiency? Answer: Overshot wheels. The weight of the water falling on the blades (gravity) forces the wheel to turn at a faster speed. Furthermore, 1 2 Waterwheel Force Water (a) Waterwheel Water Force (b) Figure 4-1 Vertical water mills; (a) undershot, and (b) overshot Hodges, H., “Technology in the Ancient World,” Barnes & Noble, N.Y., 1992. R eynolds, J. W indmills and Watermills, Praeger, New York, 1970. undershot wheels require a steady wave current, so their operation could be interrupted when water levels drop during the summer months. Question: Watermills come in vertical and horizontal configurations (Figure 4-2). What are the advantages and disadvantages of each configuration? Answer: Most early watermills were used solely for grinding wheat, barley and other seeds using a flat rock (stone mill). Horizontal mills were easiest to operate as they could directly turn the stone mill. The vertical designs required gears to transfer power to the stone mill and thus were not very convenient. (a) Millstones Vertical Waterwheel Vertical Gear Power Generation Currently there are about 680,000 MW of hydroelectric capacity installed throughout the world, which fulfills about a quarter of the world’s electricity demand and supplies more than one billion people with power. Worldwide, the amount of hydroelectric power that can be developed is about five times greater than current capacity. With 332 billion kWh and 12.5% of the world’s production, Canada is the leading hydropower producer; it satisfies 60% of its electricity needs by hydropower. After Canada, Brazil, and China, the United States is the fourth largest producer of hydroelectricity in the world.3 Norway relies on its vast hydropower resources for 99% of its electricity, making it the cleanest country in the world in terms of energy use.4 Hydroelectric plants range in scale from large falling water plants used in developed nations to small river runoff plants with no dams or water storages used for rural electrification in less-developed countries. China has the most hydropower resources and has installed more than 40% of the entire world’s small hydro (50 kWe-10 MWe) capacity.5 For now, the largest hydroelectric plant in the world is located at Itaipú along the Parana River between Paraguay and Brazil. It has a power output of 12,600 MW and supplies 80% of the electricity consumed in Paraguay and a quarter of Brazil’s total supply (Figure 4-3). The Itaipu plant will, however, concede its ranking to the Three Gorges plant being built over the Yangtze River in China. Slated for completion by 2010, it will produce 18,200 MW. The tallest waterfall in the world is Angel Falls in Venezuela, with a total drop of 980 m. The largest hydroelectric plant in the United States (and the third largest in the world after Itaipu in Brazil and Guri in Venezuela) is the Grand Coulee on the Columbia River in Washington State. Its three power plants can collectively produce 6,800 MWe. The power plant in Niagara, at 1,950 MWe capacity, is only of modest size. (b) Figure 4-2 Water wheel configurations: (a) horizontal and (b) vertical 2 002 data can be found at http://www.eia.doe.gov/pub/international/iealf/table15.xls. E nergy Information Agency (http://www.eia.doe.gov) 5 Please note the difference between MWe denoting the electrical power output and MW, which refers to thermal power output. As we will see later, only 30-35% of the thermal energy c an be converted to electricity. For hydroelectric plants we can use kW and kWe i nterchangeably. 3 4 66 Chapter 4 - Hydro Energy Although the majority of large hydroelectric plants (>10 MWe) have already been constructed where it made economic sense, small-scale hydro plants have been largely ignored. Figure 4-4 shows the exploited and potential hydroelectric resources for various continents.6 As this data indicates, most of the potential hydro sources for industrial nations have been used up. However, a huge hydro potential remains to be utilized by developing countries. Plant Types (a) There are three kinds of hydropower plants: impounded plants, pumped storage plants, and run-of-the-river plants. Impounded plants, in which water is impounded in a reservoir behind a dam, are the most common. The water storage and release cycles can be relatively short (storing water at night for daytime power generation), or long (storing spring runoff for power generation in the summer). In these plants, water always flows downward from a storage reservoir behind a dam to run a turbine. Turbines are devices that convert the energy of a moving fluid (usually water, steam, or air) into the rotational energy of a shaft. During peak demands, where sufficient electricity cannot be generated by conventional means, enough water is released from the reservoir to meet additional power requirements. The primary issue with these plants is that the water flow rate downstream from the dam can change greatly, causing a sudden power surge. This often involves dramatic environmental consequences including soil erosion, degrading shorelines, crop damage, disruption of fisheries and other wildlife, and even flooding or droughts. Pumped storage plants (PSP) reuse water after its initial use to generate electricity. This is accomplished by pumping water back into a storage tank at a higher elevation during off-peak hours when the need for electric power is low. During peak demands or when there is an unexpected spike in the electrical load, water is allowed to flow back into the lower reservoir to produce more electricity. An important advantage of PSPs is the quick delivery of power during emergencies and power surges. In comparison, a typical coal- or natural gas-fired power plant takes many hours to start. In the United States, about one quarter of all hydropower generated is from pumped storage plants. Instead of water being pumped to the storage reservoir at a higher elevation, during the peak demand, water can be allowed to fall in underground reservoirs dug out in hard rocks. When demand falls, the base load can pump the water back to the reservoir. Since there is smaller fluctuation in the level of water in the storage reservoir, the system is more stable. In modern pumped storage plants, the same turbine-generator that 6 (b) Figure 4-3 The hydroelectric generating station in Itaipu with a capacity of 12,600 MWe boasts the largest facility of this kind in the world (a). The tallest waterfall in the world is the Angel Falls in Venezuela (b). 8000 Output TWh/year 7000 6000 5000 4000 3000 2000 1000 0 Africa Asia Technical potential Economic potential Exploited potential Australia/ Europe Oceania North South & America Central America Figure 4-4. Exploited hydro potential by continent. Paish, O., “Small hydro power: technology and current status”, R enewable & Sustainable Energy Reviews, 6 (2002) 537-556. 67 A Mathematical Interlude ... E = mgH Power from Falling Water certain mass of water, m, has a certain amount of potential energy given by (i) where H is the effective pressure head of water across the turbine (roughly equal to the height of the waterfall) and g is the gravitational acceleration equal to 9.81 m/s2 at sea level. The power produced is calculated by replacing the mass, m, by mass flow rate m[kg/s] in the above equation: (ii) Pe = mgH Not all the power can be converted to electricity, as there are losses in the turbine and in the generator. If ηT and ηG are the efficiencies of the turbine and the generator, r is the water density and Q is the volumetric flow rate of water, then electric power generated is written as: (iii) P = η η ρgQH e T G Modern hydroelectric plants employ turbines and generators with efficiencies as high as 90%. This gives an overall plant efficiency of η = ηT.ηG= 0.90x0.90 ~ 80%. Example: A hydroelectric plant is supplied with water from a dam located 50 meters above the inlet to a turbine. As the water falls, its potential energy is converted into kinetic energy before it flows through the turbine at the rate of 10 cubic meters per second. Assume water has a density of 1,000 kg/m3 and that both the turbine and generator efficiencies are ηT = ηG= 1.0. Estimate the electrical power in megawatts that this plant can produce. Solution: Power produced is calculated from equation (iii) as: Pe = (1.0)(10.0 m 3 /s)(1000 kg/m 3 )(10.0 m/s2 )(50m) = 5x10 We =50 MWe 6 generates electricity from falling water can also be used to pump the water back into the storage tank. In this case, the generator changes the direction of the electric field, forcing the turbine to rotate in the reverse direction and act as a motor which runs the pump. Run-of-River Plants are typically low dams where the amount of water running through the turbine varies with the flow rate of water in the river. The flow rate of water and the amount of electricity that is generated in the run-of-river plants is generally smaller than in pumped storage plants, and changes continuously with seasons and weather conditions. Since these plants do not block water in a reservoir, their environmental impact is minimal. Plant Design Water used by a hydroelectric plant is stored behind a dam at a certain elevation (head) above a turbine. The water flows through a penstock and through the blades of the turbine, causing the turbine to rotate. Depending on amount of electric energy required, the flow through the turbine is adjusted by series of gates and valves. The turbine shaft is normally coupled to the generator shaft to produce electricity. In a typical small hydro scheme, a portion of the water is diverted from a river or stream through an intake valve. It is then passed through a metal screen into a settling chamber where stones, timbers, and other debris 68 Chapter 4 - Hydro Energy are removed and suspended particles of dirt settle before water enters the turbine. Since no reservoir is blocking the flow of water, the impact on the river and its habitat is minimized. Depending on its application, either an impulse or a reaction turbine is used. In an impulse turbine, the blades are fixed to a rotating wheel and available head is converted into kinetic energy by a contracting nozzle. The high velocity jet then impinges on the blades and turns the turbine. The most common impulse turbines are of the Pelton type, where a series of cupped buckets are set around its rim. A high-speed jet of water enters the wheel tangentially (Figure 4-5a) and is deflected 180 degrees by the cups. Nearly the entire momentum of the water is used to impart an impulse that forces the wheel to turn. Impulse turbines are used most often with falling mountain streams with large heads and relatively small flow rates. A reaction turbine uses the pressure difference across the blades for producing a hydrodynamic lift that propels the blades. To produce the highest lift, blades must be highly aerodynamic and shaped like airfoils. Unlike the impulse turbine, the rotors (also called runners) of reaction turbines are fully immersed in water. Two common reaction turbines are the Kaplan and Francis turbines. In Kaplan (propeller) turbines, water impinges on a propeller -- similar to a ship’s propeller-- fitted inside the penstock (Figure 4-5b); in Francis turbines, water flows radially inward to the runner housed in a spiral casing and is exited through a diffuser (Figure 4-5c). Kaplan turbines are common for the run-of-river projects with low head and large flow rates. Francis turbines are suitable in dams where high flow rates and relatively large heads are available. The main advantage of reaction turbines over impulse turbines is that the blades can be adjusted to match the rotor speed closely to the generator speed. Recent advances in power electronics allow turbines and generators to run at varying speeds. This allows simpler and cheaper propeller turbines like Pelton to be used instead of the more advanced and expensive Kaplan and Francis. (a) (b) (c) Figure 4-5 a) Pelton; b) Kaplan; and c) Francis Turbines. Photo Courtesy of Gugler Hydro Energy GMBH. 69 Tidal Energy Tides are the vertical rise and fall of the sea level as a result of gravitational attraction between the earth, the moon and, to a lesser degree the sun. The gravitational attraction of the moon causes the water facing toward the moon to bulge. On the other side of the earth, the moon pulls the earth from the water, and we have a second bulge and another high tide. At the same time, low tides occur at the right angle to the moon’s pull. Other factors that affect the force, shape, and height of the tides are the time of the year (position of the sun), size, depth, and shape of the ocean basin. The highest highs and the lowest lows are called the spring tides. Spring tides (they come from the Saxon word springen meaning the swelling of water and have nothing to do with the spring season) occur a few days after the full and the new moons, when sun, moon, and earth are aligned. Spring tides take place about twice a month. When the sun-earth line is at the right angle to the earth-moon line (i.e during half moons), the gravitational effects of the sun and the moon tend to cancel each other, tides are lower than average range and we have the neap tides (neap means scanty or lacking). Since with respect to the moon, the earth rotates about its axis once every 24 hours 50 minutes, we expect an interval of 12 hours and 25 minutes between two high or two low tides; each tidal day is therefore 1.035 times a solar day. Accompanying the vertical rise and fall of water, there are also horizontal or lateral movements. As the tides channel between islands or into bays and estuaries tidal currents occur. During the high tide, the tidal current known as the flood tide flows towards the shore. About six hours later, the current reverses and flows away from shore. This is the ebb tide. Using tides for power generation is not a new concept. Tidal mills operated off the western coasts of Europe for many centuries before being replaced by cheaper methods of producing energy. These mills took advantage of the natural rise and fall of coastal tides by allowing water to fill up a pond during flood tides and then emptying that water over a watermill during the ebb tide.7 Modern versions of tide mills are similarly designed. A barrier is built across an estuary and is allowed to be filled by water during the flood tide. The barrier is equipped with gates that are closed as the basin fills with water. As the tide recedes into the open sea during its ebb, the released water drives a number of turbines which in turn drive generators and produce electricity. Upon emptying, the basin gates are closed to allow the water in the sea to rise. This creates a head against the basin during the flood which can be emptied for a second time, turning the turbines in the opposite direction and generating power (Figure 4-6). 7 Gies, F. J., Cathedral, Forge, and Waterwheel: Technology and Invention in the Middle Ages , Harper Perennial, New York, 1994. 70 Chapter 4 - Hydro Energy Land Ocean Gate closed Gate opened Gate closed Gate opened Gate closed (a) (b) (c) (d) (e) Figure 4-6 Operation of a tidal plant (a) During period of high-tides water fills the basin. (b) At low tides the gate is opened and water is allowed to empty into the open sea while turning a turbine. (c) Once all the potential energy is exhausted gates are closed again, allowing water rise in the open sea. (d) The flow is from the open sea to the basin and turbine runs in the opposite direction. (e) The gate is closed until water rises again and repeats the operation. The largest tidal power station that was ever constructed, a 240-MW plant built in 1966, is still operating today in La Rance estuary in Brittany, France (Figure 4-7). The annual generation is some 640 million kWh. Two other plants are operating commercially. The first, with an electrical generating capacity of 18-MW, is the Annapolis Royal Station in Nova Scotia off the Bay of Fundy in Canada; the second is a small 1.8 MW tidal plant in Kislaya Bay near Murmansk in the Russian Arctic.8 Currently, there are no tidal power plants in the United States and none are planned. The potential energy of tidal power has been shown to increase with the basin area and the square of the average range between the low and high tides.9,10 To be practical, tidal ranges of at least 5 meters (16.4 feet) are recommended. Oceans have tidal ranges between 5 to 10 feet; in narrow passages and estuaries ranges as high as 50 feet are common. Unfortunately, there are only about 40 sites on the planet that meet this requirement.11 Tides are clean, entirely predictable and renewable sources of energy. The major drawbacks are the large distances of most suitable sites from the population centers and the high capital cost of building the barrage across the estuaries. In addition, tidal power can be used only a few hours a day when the tide is moving in or out of the barrage. Figure 4-7 Tidal power plant at the Bay of Rance estuary in France. Waves A wave is a disturbance that travels through space and time. Unlike electromagnetic (optics) waves that can propagate through vacuum, mechanical waves (acoustics, ocean surface waves) need a medium to travel. Waves are characterized by their crests (highs) and troughs (lows), and may be classified as traverse waves with vibrations perpendicular to the direction of propagation (violin strings, light) or longitudinal waves with vibrations parallel to the direction of the propagation (horn, radar). Ripples on the surface of water are a combination of both traverse and C harlier, R. H., “Sustainable co-generation from the tides: A review,” Renewable & Sustainable Energy Reviews, 7, 187-213, 2003. Bernstein, L. B., “Central tidal-power stations in contemporary energy production,” State Publishing House, Moscow, 1961. Frau, J. P., “Tidal energy: promising projects: La Rance, a successful industrial-scale experiment,” I EEE Transactions on Energy Conversion, Volume: 8, Issue: 3, September 1993. pp. 552-558. 11 See the Department of Energy website at http://www.eere.energy.gov/consumerinfo/factsheets/nb1.html . 8 9 10 71 Digging Deeper ... Tides T ides are vertical displacement of water. The action of tides is best understood by examining the law of gravity as stated by the Newton’s Law of Universal Gravitation. According to Newton, any two masses m1 and m2, separated by a distance r, are attracted to each other with a force of m 1 . m2 (iv) F=G where G is the Universal Gravitation constant equal to 6.67x10-11 N.m2/kg2. According to this law, as the moon orbits the earth it is pulled by the earth as though it were connected by a rope to the earth’s center. The pull of gravity becomes weaker at distances farther away from the moon. As result of progressively declining gravity, the earth is stretched along the earth/moon line. Liquids have weaker internal bonds than solids, so ocean water stretches more than land masses. The points on the surface of the earth perpendicular to a line extending between the earth and moon feel less force than the earth’s center. The result is that water is pulled along the moon-earth line and is pushed in the perpendicular direction. The net effect is that water accumulates at both ends along the moonearth line (high tides). A little more than six hours later the earth has rotated by 90 degrees relative to the moon and the situation reverses (low tides). But what about the pull of the sun? It just so happens that, although the gravitational pull of the sun is much stronger than that of the moon, because the sun is so much farther than the moon, its pull does not change very much from one side of the earth to the other, and its effect is only half that of the moon. The sun, however, gives a helping hand during the new and full moons when it is in line with the earth and the moon. Under these conditions, we have very strong tides which are called spring tides. When the sun, earth and the moon forming a right angle, as is the case during quarter moons, the effect is smallest and we have neap tides.Because the positions of the earth, sun, and moon change every day, tides vary from day to day, and from one location to another. Question: Why do celestial bodies tend to be round? Answer: Gravity pulls all the mass toward the center. Example: Which system experiences a stronger attraction force, the earth-sun or the earth-moon? The sun, earth, and moon have masses of 2x1030, 6x1024, and 7.4x1022 kilograms respectively. The mean distance between the earth and the sun is 1.5x1011 m while the distance between the earth and the moon is 3.84x108 m. Solution: Substitute into equation (iv), we get: Fsun-Earth = G MS . ME rS-E2 = 6.67x10-11 (2x1030) (6x1024) (1.5x1011)2 = 3.56x1022 N r2 FMoon-Earth = G MM . ME rM-E2 = 6.67x10 -11 (7.4x1022) (6x1024) (3.84x108)2 = 2x1020 N This calculation shows that the gravitational force of attraction of the sun on the earth is over 177 times that of the moon. As we mentioned above, it is not the absolute force of gravity, but the relative strength of gravitational forces across opposite points on the earth that determines the size of tidal range. longitudinal waves. Waves are mathematically represented by their wavelength (l), the distance between two consecutive crests (or troughs), and amplitude, the maximum height between crests and troughs. Period, T, is the time for one complete cycle of a wave oscillation. 72 Chapter 4 - Hydro Energy Wave Energy As wind blows across the ocean, some of its energy is transferred to the water in the form of waves. Since wind itself is a result of non-uniform heating of the earth’s surface by the sun, waves can be considered a stored form of solar energy (Figure 4-8). The rougher the water, the larger the ripples and the easier it is for the wind to transfer its energy. As wind loses its energy, waves become calmer and eventually reach their steady-state oscillations. As waves get closer to the coast and enter shallower water, they grow taller, their lengths decrease, and water particles move forward. The particles closer to the bottom fall behind until the wave eventually breaks on to the shore. Waves have form (potential energy) and motion (kinetic energy). Potential energy is that which is expended to distort a flat sea surface into the shape of the wave. Kinetic energy is the energy put into wave by water motion in and beneath the wave, causing molecules of water to rotate in a circular fashion, higher in diameter at the surface and decreasing exponentially with depth.12 Contrary to popular belief, waves do not move along the water surface – the water simply cycles around in small circles without any mass being transported forward. These are called stationary or transverse waves and are very different than longitudinal waves such as sound waves and waves generated by compressing a spring. Surface Waves Figure 4-8 As wind blows over large bodies of water it continuously supplies energy to waves. Surface waves result from the superposition of many waves of varying wavelengths and heights. The wavelengths are, however, limited to about twice the average depth of the ocean. Ocean floors are, at maximum, 4 kilometers deep, so waves can be as long as 8 kilometers. Longer surface waves travel faster and further than shorter ones, but the energy content of a wave is related solely to its height. The surface currents flow clockwise in the northern and counter-clockwise in the southern hemispheres. Wave power is measured in kW/m of wave crest (the highest point on a wave). The maximum power a wave can carry is about 100 kW per meter of wave front in the high seas, about 30-60 kW/m around the Atlantic coasts of Europe, and 10 kW/m off the western coast of the United States (See box “Power of a Wave”). Due to their greatest potential, the west coast of Europe is the most suitable for developing wave energy schemes (Table 4-1).13 For example, it is estimated that waves in the British Isles could be tapped for more than 20% of British and 75% of Irish electricity needs.14 At this time, most efforts remain in the research stage, although the few demonstration projects promise to play a significant role for wave technology in the near future. The European Union projects that by 2010 up to 1,000 megawatts of electricity could come from ocean waves. 12 13 14 Table 4-1. Potential for wave energy for major European countries Country UK Ireland Portugal Spain France Near Shore (TWh/year) 14-21 7-11 4-6 3-5 3-5 Offshore (TWh/year) 43-64 21-32 12-18 10-16 12-18 M ayo, N., “Ocean Waves -- Their Energy and Power,” Physics Teacher, Vol. 35, September 1997. Thorpe, T. W., “A Brief Review of Wave Energy,” Report no. ETSU-R120, The UK Department of Trade and Industry, May 1999. E dwards, R.“The Big Break,” New Scientist, pp. 30-34, October 1998. 73 Mathematical Interlude ... Power of a Wave T he power delivered by a wave is obtained by dividing the total energy transported by its period: P= ρg 2TH 2 32π (v) (v) In this equation: P is power of wave front [W/m] g is the local gravitational acceleration [m/s2] r is the water density (kg/m3) T is period (time it takes two successive crests or troughs pass the same point [s], and; H is the wave height (amplitude) [m] Example: A deep-water wave with a wavelength of 20 m and amplitude of 3 m travels at a speed of 5 m/s. Calculate the maximum power that can be exploited from such a wave. Solution: Taking water density r = 1030 kg/m3, g=9.81 m/s2, and a period of T = 20/5 = 4 s, we have: 2 2 P = (1030)(9.81) (4)(3 ) = 35,500 W/m = 35.5 kW/m of wave front 32π Underwater (Marine) Currents Just as wind is affected by local terrain, coastal waves are affected by underwater topography. Narrow channels between islands and around the coastal edges of oceans are best suited for power generation and can provide as much as one megawatt of power per square kilometer of seabed. To increase the velocity of the water and thus its kinetic energy, some propose building barriers across channels and narrow straights. The best locations are near the shore at depths of between 20 to 30 meters, where wave velocities are approximately 2-3 meters per second. Power Generation Wave power generation plants are of either the fixed or floating types. Fixed generating devices are located along the shore or are fastened to the seabed and are generally simpler to maintain and operate. Floating devices are installed on floating platforms. Examples of fixed energy conversion systems are oscillating water columns, tapered channel systems, and underwater turbines. Examples of floating systems are the Archimedes Wave Swing and the Salter’s Duck. These devices work directly by activating a generator or pushing a working fluid (water or air) to drive a turbine and generator. No large-scale commercial wave power plants have been built yet, although major research is underway and several prototype systems have been built in Norway, Japan, India, and Scotland. Onshore Systems Figure 4-9 Schematic of an Oscillating Water Column. Image courtesy of Fujita Research Most wave power machines use the Oscillating Water Column. They work by trapping air over the surface of water in a chamber that then moves a piston up and down. This can be achieved by the up and down motion of water in open seas, or by the back and forth motion of waves as they slam on the shoreline. In one design, waves coming toward the shore 74 Chapter 4 - Hydro Energy Power from Underwater Currents Mathematical Interlude ... T he power of a current follows the same formulation that was given for wind power: π Pe = ρ d 2V 3η (vi) 8 where: P is delivered power in watts, r is the density of seawater, d is the turbine diameter, and V is the current speed. For tidal currents close to the shoreline in estuaries is a sinusoidal function in time, and η is the turbine efficiency. push water through a channel and act as a pump to compress a column of air that is then passed through a turbine-generator system to produce electricity. As the waves recede, air is sucked back, causing the turbine to continue its operation (Figure 4-9). Wells turbines are the key to the successful operation of the oscillating wave column system (Figure 4-10). These turbines always turn in one direction, independent of the direction of flow. This feature allows them to provide continuous operation whether waves are moving toward the coastlines or moving away from them. The first operational wave power station, called LIMPET (for Land-Installed Marine-Powered Energy Transformer), was installed on the Scottish island of Islay and generates 500 kW of electric power (Figure 4-11).15 Another onshore wave technology device is the Tapered Channel, which consists of a tapered channel and a reservoir constructed on a cliff a few meters above sea level (Figure 4-12). Due to the narrowing of the channel, waves rise and water pours into and fills a reservoir. The reservoir provides the necessary head to run a turbine and generate power. Tapered channel systems are especially suitable during peak demands since they store energy in the reservoir until it is needed. Offshore Systems Figure 4-10 The Wells Turbine continues to rotate in one direction even as the direction of flow reverses Photo courtesy of Wavegen Corporation (http://www.wavegen.co.uk) In an effort to develop practical viable floating wave power devices for offshore applications, a prototype floating-type version of the oscillating water column called the Mighty Whale was designed by the Japan Marine Science and Technology Center. The 50 x 50 m platform was anchored to the bottom of the sea near Japan, operated from 1998-2002, and produced about 110 kW of electricity (Figure 4-13).16 Mighty Whale also acts as a wave breaker to calm water for the fisheries. The Archimedes Wave Swing (AWS) consists of a number of air-filled chambers submerged below the sea surface and is connected by movable floats that oscillate up and down as waves move over them. A series of linkages convert the vertical oscillation of the platform into rotational motion, which is in turn used to generate electricity. A pilot project off 15 16 Figure 4-11 The Limpet 500. Image Courtesy of Wavegen Corporation Converging inclined channel Cli face Turbine House Raised reservoir Return to the sea Figure 4-12 TapChan wave energy device. A demonstration plant has been installed off a remote Norwegian island and operating since 1985. I slay LIMPET Wave Power Plant, Company website at http://www.wavegen.co.uk. Japan Marine Science and Technology Center, http://www.jamstec.go.jp/jamstec/MTD/Whale/. 75 the coast of Portugal is constructed and produces 8 MW of electricity. Salter’s Duck (Figure 4-14) operates on a similar principle with the exception that it is installed on a floating chamber connected to a fixed platform that swings up and down as a wave passes. The bobbing motion is converted to rotational motion (for example, using it to pump a hydraulic fluid through the blades of a turbine) which then runs a generator.17 Underwater turbines are similar to wind turbines, except that the kinetic energy in water is converted into rotational energy by underwater axial turbines (Figure 4-15). In contrast to wind velocity, currents in deep waters have relatively steady speeds, and therefore no energy storage system is necessary. Siting Figure 4-13 The Mighty Whale Image courtesy of The Marine Science & Technology Centre of Japan (http://www.jamstec.go.jp/) Like wind turbines, wave energy devices can be clustered in “wave farms” near the shorelines or moved off-shore. The main advantages of nearshore plants are that they are much cheaper to construct and there is less distance for electrical cables to be laid on the ocean floors. Their main disadvantage is that waves break up and lose much of their energy as they approach the coastlines. Shore-based stations are mostly considered for small-scale electricity generation. Offshore devices have the advantage of being more efficient, and they are not as unsightly to the public as the near-shore facilities. No matter which approach is used there are always some navigational hazards to shipping and other marine transportation. Economical Concerns Figure 4-14 Salter’s Duck Power output from a hydroelectric station increases with the flow rate of the water and its available head. For a given power output, less water is required for the higher head so the plant size would be smaller and the cost of construction and equipment would be lower. Unfortunately, taller waterfalls tend to be in mountainous regions and areas with lower population density. Therefore, transmission costs of electricity may be higher. Thus, before selecting the plant site, it is important to consider not only the total power but also the initial cost, operating cost, and cost of transmission. Typically the installed cost of hydro electrification projects is about $1,000-1,500 per kW for large, $2,500-3,000/kW for small (2.5-25 MW), and $10,000/kW for micro (below 500 kW) hydro projects.18 Environmental Concerns Figure 4-15 Underwater turbines Image courtesy of TidalStream Ltd., UK. 17 18 Hydropower technologies have many of the same advantages of other renewable energy sources. They are clean, do not leave any waste, do not contribute to global warming, nor do they produce any of the air Thorpe, T. W,b”A Brief Review of Wave Energy,” ETSU Report No. ETbSU-R-12, May 1999. See for example, Mayo, N., “Ocean Waves - Their Energy and Power,” The Physics Teacher, 35, pp. 352-356, Sep. 1997. 76 Chapter 4 - Hydro Energy pollutants that result from the combustion of fossil fuels.19 The large reservoirs formed behind hydroelectric dams or tidal plants can provide a barrier against storm surges and benefit the public by providing beaches for swimming, fishing, and other recreational activities. Hydropower has an additional advantage over other renewable sources such as solar and wind in that it is a lot more reliable. This does not mean that hydropower is without any environmental impact, however. Some of the concerns specific to hydroelectric and tidal projects are: a. Interrupting the natural flow of a river – Change in river condition and adjacent lands may impact fish and aquatic population, threaten wildlife natural habitats, affect vegetation, and degrade shorelines. b. Affecting water quality – Impounding water flow raises the surface water temperature which in turn affects the concentration of dissolved oxygen and other nutrients, and allows certain bacteria to grow in hydro rivers and reservoirs, which may potentially pose a health hazard. c. Causing floods – Although dams can act as a flood control measure, there are also concerns for the flooding of lakes and rivers upstream of the plant. In addition, a dam failure in the event of an earthquake or other disaster could cause immediate flooding of the area below the dam, costing society countless lives and great economical loss. d. Impeding the natural transport of sediments – Restricting the amount of water flow results in a reduction in sedimentation in the river downstream of the dam and an accumulation of silt in lakes behind the dam. As the storage volume becomes filled with silt, the generation capacity of the plant is reduced and the stored water may require periodic dredging and flushing of sediments at a considerable cost. 5. Displacing Population – Large hydropower projects will have a huge impact on the surrounding population. Many people have to be relocated which in turn may have considerable impacts on the local economy. The major concerns raised against wave power are their vulnerability to violent storms, aesthetics, noise, and the additional costs if electricity is brought to land. Summary The ocean alone can fulfill all the energy needs of humans for a long time. Water is a clean source of energy, and environmental impacts are relatively limited. Among the drawbacks are the potential disturbances or destruction of some marine life and the interference with marine transport and shipping. For many, the installation of large wave-energy W hat differentiates hydroelectric and wind plants from thermal plants is that in hydroelectric and wind plants water or air flow directly through the turbine, whereas in thermal power plants, fossil or nuclear fuels heat water to steam before it flows through the steam turbine. 19 77 devices, overhead transmission lines, and their supporting facilities may not be aesthetically pleasing. Except for the wave technologies, much of the hydropower potential in the developing countries has already been exploited. It appears that in the future, much of the hydropower development will be in developing countries especially in Asia and Africa where much of the world’s small scale and low head hydro capacity exists. Additional Information Books 1. Bose, N. and Brooke, J., Wave Energy Conversion, Elsevier, 2003. 2. Ross, D., Energy from the Waves, Oxford University Press, 1995. 3. Cruz, J., Ocean Wave Energy: Current Status and Future Perspectives, Springer Series in Green Energy and Technology, Springer-Verlag, Berlin, 2008 Periodicals 1. International Journal of Wave Motion, Elsevier Science Publishing Company. 2. International Journal of Renewable Energy, Elsevier Science Publishing Company. Government Agencies and Websites 1. National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center (http://www.csc.noaa.gov). 2. European Commission on Tidal Energy (http://europa.eu.int/comm/ energy_transport/atlas/htmlu/tidal.html). 3. OTEC, U.S. DoE, Energy Efficiency and Renewable Energy (http:// www.eere.energy.gov/RE/ocean.html). Non-Government Organizations and Websites 1. Wave Energy Council: Survey of Energy Resources (http://www. worldenergy.org/wec-geis/publications/reports/ser/wave/wave.asp). 78 Chapter 4 - Hydro Energy Exercises I. Problems 1. What is the gravitational force of attraction between Lisa Carr and a Ferrari parked in a showroom 100-m away? Assume Lisa and the Ferrari have masses of 50 and 2000 kilograms, respectively. 2. What is the force of attraction between an astronaut and a 500 kg console in a spacecraft? Is it zero? The astronaut has a mass of 75 kg and is standing 80 cm from the console. 3. Use data given in Appendix A to calculate the gravitational force of attraction between a. The earth and the sun b. The electron and nuclei of a hydrogen atom 4. An underwater turbine with rotor diameter of 2 meters is facing a strong wave at 10 m/s. Assuming rotor spins at 300 rpm, and for the water density of 1030 kg/m3, calculate the maximum power output (kW) and torque delivered. 5. Calculate power delivered by a hydroelectric generating station, when water at a rate of 15 cubic meters per second is falling on a turbine located at a bottom of a 70-m waterfall. Assume water density of 1,000 kg/m3. 6. What is the maximum power that can be exploited from a deep-water wave with a wavelength of 15 m and height of 5 m travelling at a speed of 4 m/s. Assume the combined turbine-generator efficiency of 85%. II. Essay Questions 1. What are different methods of utilizing water as an energy source? Describe the advantages and disadvantages of each. 2. What were the earliest uses of hydropower? How did hydropower applications change throughout the centuries? 3. Which country houses the largest hydroelectric power station? Which waterfall is the tallest in the world? 4. What are the different types of hydro plants? What are their applications? 5. What is the difference between an impulse turbine and a reaction turbine? 6. Describe the principal of operation of an oscillating water column, a tapered channel, an Archimedes Wave Swing, and a Salter’s Duck. 7. Which country is the largest producer of hydroelectric power? Which countries have the highest hydroelectric potentials that have not yet been exploited? Which countries have the highest potential for producing power from surface waves? From underwater currents? 8. How do tides form? What causes spring and neap tides? How can tides be predicted? How can we use tides to produce power? 9. Name three technologies which are best suited for near shore power production. Name three ways of producing power for offshore applications. 10. How much hydropower is generated for your country of birth? Is there a potential for exploiting waves, tides, and underwater currents for generating electricity? Discuss their environmental and economical impacts. III. Multiple Choice Questions 1. The force of attraction between all masses is a. Electrical b. Magnetic c. Chemical d. Mechanical e. Gravitational 2. The best location for placing a water turbine is a. Close to the top of a waterfall b. Halfway down the waterfall c. At the bottom of the waterfall d. Does not make any difference e. Depends on the topography of the area 79 3. Energy can be extracted from water by a. Exploiting its potential energy as is done in hydroelectric and tidal plants b. Exploiting the temperature gradients in oceans as is done in OTEC plants c. Exploiting its mechanical energy as is done in ocean currents and surface waves d. Exploiting the energy in its nucleus as is done in fusion power plants e. All of the above 4. A pumped storage plant is a. The location where all pumps are stored b. A special plant for storing hydropower during the peak demand c. Used during emergencies, when quick delivery of power is demanded d. The auxiliary pump used to deliver water downstream of the hydro plant e. Used to act as a turbine when turbines are being serviced 5. In addition to using hydropower to produce electricity, hydro projects can also be used to support a. Recreation b. Irrigation c. Flood control d. Municipal water supply e. All of the above 6. The best way to control the amount of electricity generation is by a. Changing the head by moving the turbine b. Controlling the speed of the turbine c. Controlling the speed of the generator d. Controlling the flow of water e. Controlling the frequency of the transformer 7. Hydro energy accounts for about what percentage of global energy consumption? a. 1-2% b. 6-7% c. 10-15% d. 20-25% e. More than half 8. Which of the following is often cited as a major potential concern regarding the use of hydropower as a source of electricity generation? a. It contributes to global warming. b. It is a source of water and possibly air pollutions. c. Hydroelectric facilities can block fish passage. d. Reduces amount of water available for drinking and agriculture. e. All of the above. 9. What is (are) the main advantage(s) of hydropower over other fuel sources? a. It is economical to produce. b. It is a “clean” energy resource. c. It is a renewable fuel source. d. It provides additional benefits such as flood control and irrigation. e. All of the above. 10. A hydropower project that utilizes the river’s natural flow for generating electricity during peak demand is called a a. Run-of-river project b. Storage project c. Waterfall project d. Overflow project e. None of the above 11. The electric generation capacity from a hydropower plant will depend on a. The flow rate of water b. The turbine’s efficiency c. The head of the turbine d. The generator’s efficiency e. All of the above 12. The largest hydroelectric plant in the world is in a. The United States b. Russia c. Brazil d. Tajikistan e. Canada 13. The leading producer of hydropower in the world is a. The United States 80 Chapter 4 - Hydro Energy b. c. d. e. Russia Brazil France Canada d. When the moon is farthest away from the earth (at its apogee) e. When the moon, the earth, and the sun are along a line 19. The maximum power generated from tidal energy a. Increases with the area of the basin and the square of average tidal range b. Is independent of the average tidal range c. Is independent of the basin surface area d. Occurs during the high tides e. Occurs during the low tides 20. Tidal power operates by building a barrier across: a. River estuaries b. Oceans c. Creeks d. Lakes e. Ponds 21. What percentage of the US electrical power comes from tidal plants? a. None b. Roughly 1% c. About 2-3% d. About 5-7% e. None of the above 22. The tidal interval is a. Time between two high tides b. Time between high tides and low tides c. Vertical difference between high and low tides d. Vertical difference between the highest high and lowest low tides e. The period when strongest tides occur during the year. 23. Tidal electricity generation does not require: a. Maintenance b. Storage system c. Waste disposal d. High capital cost e. None of the above 24. Power produced by hydro energy is often expressed in a. Gigavolts 81 14. Which of the following statements is correct? a. There are two high tides and two low tides every 24 hours. b. When the sun is along the line extending from the earth to the moon, we have higher-than normal high tides and lower-than-normal low tides (spring tides). c. The effect of the moon’s gravity on the earth is the main cause of tide formation. d. The gravitational pull of the sun is much greater than that of the moon. e. All of the above. 15. High spring tides and low neap tides occur a. Twice a year following the solar cycle b. Twice a month following the lunar cycle c. Twice a day, roughly every 12 hours d. Once a month e. Once a day 16. Tidal energy has its origin in a. The phase of the moon b. The gravitational potential energy of the earthmoon-sun system c. Storms d. The rotation of the earth e. The change in surface temperature of the ocean 17. Spring tides occur a. During a full or a new moon b. When the moon is in the first and last quarter c. Mostly in spring d. When the moon is closest to the earth (at its perigee) e. When the gravitational forces of the moon and the sun are perpendicular to one another 18. Neap tides occur a. During a full or new moon b. When the moon is in the first and last quarter c. Mostly during falls and winters b. c. d. e. Megawatts Terajoules Kilowatt-hours None of the above. b. c. d. e. A tidal day is exactly equal to a solar day. A tidal day is slightly longer than a solar day. A tidal day is the same as a lunar day. None of the above. 25. Today, which renewable energy source provides the US with the most energy? a. Fossil b. Wind c. Solar d. Hydroelectric e. Nuclear 26. Which of the following statements is not correct? a. A major environmental concern regarding hydro and tidal plants is that the change in river condition and surrounding beaches may impact fish and marine life. b. Dams act mostly to control floods, but may also be the cause for flooding the lakes and rivers upstream of the plant. c. To prevent accumulation of silt, lakes behind dams must be occasionally cleaned at great costs. d. Unlike hydroelectric and tidal plants, wave plants have no environmental impact. e. Impounding water flow may cause certain bacteria grow in hydro rivers which could potentially pose a health hazard. 27. Which of the following statements is not correct? a. Since gravitational force decreases with square of the distance, the side facing the moon experiences the strongest pull. b. Tidal effects cause the earth to stretch along the earth-moon line. c. The oceans rise relative to land at both sides of the earth-moon line. d. The oceans rise relative to land at the point closest to the moon and falls relative to land at the point farthest away from the moon. e. Although the sun exerts a higher gravitational pull on earth than the moon, its tidal effect is smaller than the moon. 28. Which of the following statements is correct? a. A tidal day is slightly shorter than a solar day. 82 29. The major drawback to tidal power plants is that they a. Pollute the water b. Cause ozone depletion c. Heat the water d. Are too expensive e. Alter the tidal currents 30. The highest high tides and lowest low tides occur a. When the moon and the sun are nearly at right angles to each other b. When the moon and the sun are nearly in line with earth c. During summers when earth is closest to the sun d. During winters when earth is closest to the sun e. When sky is clear and moon is clearly visible III. True or False? 1. Ocean waves have the potential to satisfy all of our energy needs. 2. The primary mover of surface waters is the wind. 3. The main application of the pumped storage plant is when demand is high and quick delivery of power (such as during emergencies) is needed. 4. There are no environmental concerns associated with hydroelectric plants because they are clean and no fuel is burned. 5. Tidal energy exploits the available head between the flood and ebb tides. 6. The tidal energy from the sun is negligible compared to that from the moon because the large distances between the sun and the earth make the pull of gravity by the sun very small. 7. Power outputs of most hydro plants are usually Chapter 4 - Hydro Energy expressed in megawatts. 8. Two common types of impulse turbines are Kaplan and Francis turbines. with seasons and weather. 22. Underwater turbines are similar to wind turbines. 23. There exists a tremendous potential for exploiting the hydroelectric power in the United States. 9. The power delivered by waves is inversely proportional to the period between two successive crests. 24. Hydroelectric, tidal, waves, and other technologies that exploit mechanical energy of water do not have 10. Ocean waves are the result of the non-uniform any adverse environmental impacts. heating of the ocean’s surface. 25. The impact of hydropower on global warming is 11. During the night, water can be pumped from a minimal and not of major concern. lower lake to a higher lake in a hydroelectric power station. The water can be reused during peak power IV. Fill-in the Blanks when additional energy is needed. 1. The best means of achieving the quick delivery of power during emergencies and power surges is 12. Wave size is affected by wind velocity, wind duration, through _____________ plants. and distance from the shore. 13. The potential energy of tidal power increases with the basin area and the square of the average range between the low and high tides. 14. Horizontal movement of the water accompanying tides is called a tidal current. 15. Each day, the high tides occur about fifty minutes later than the day before. 2. The largest hydroelectric project is being built in _________. 3. _________ turbines are most suitable for small hydro applications and heads of a few tenths to hundredths of meters. 4. The most suitable place for installing wave power stations is off the coasts of the _______________ in the Atlantic Ocean. 16. The speeds of currents in deep waters vary 5. ___________ devices have the advantage of being significantly from one location to another, and more efficient, and they are not as unsightly to the therefore an energy storage system is necessary. public as the near-shore facilities would surely be. 17. The onshore wave plants are advantageous because they cost less and waves are stronger at the beaches. 18. There are currently no working tidal power plants in operation. 19. The power generated from a waterfall increases with the height of waterfall and flow rate of the water. 20. Waves have potential energy but no kinetic energy. 6. ____________________ between islands and around the coastal edges of oceans is best suited for exploiting wave’s energy. 7. ________ tides form when the sun is at the right angle to the earth-moon line. 8. The leading producer of electricity by tidal power is ____________. 9. The power produced by underwater currents varies with the _________ of turbine diameter and cube 21. Power output from run-of-river plants vary widely of water ____________. 83 10. Wave power is usually measured in _______ of wave front. 11. The cost per kilowatt of electricity generation is __________ for smaller projects. 12. Two promising technologies for exploiting wave energy for onshore applications are oscillating water column and ____________. 13. The time between two successive waves is called the wave __________ . 14. Tides are primarily __________________. caused by 1985. 17. _____________________ is a device located onshore, installed on a floating buoy or fixed to the seabed; as wave surges into or recedes from it, air is pushed into or sucked out of a housing driving a turbine that is coupled to a generator that produces electricity. 18. An example of a floating wave energy convertor is a ____________. 19. We can extract power from moving water by installing ____________ anchored to the bottom of ocean floors; power can be extracted from water in the same way that it is extracted from wind. 20. In addition to the potential and kinetic energies contained in water waves, the temperature difference between the surface and deep ocean waters can be exploited in a process called ______________. 15. When water moves in toward shore, it’s called _________ tide. 16. One way to harness wave energy is to channel the waves into a narrow funnel into a catch basin. A demonstration project called ___________ has been installed off a remote Norwegian island since 84