Chapter 4

Water

4.1 Introduction 4-1

4.2 World Resource 4-3

4.3 Hydroelectric 4-4

Large (≥30 MW) • Small Hydro (100 kW to 30 MW, 10 MW in Europe) • Microhydro (<100 kW)

4.4 Turbines 4-10

Impulse Turbines • Reaction Turbines

4.5 Water Flow 4-12

4.6 Tides 4-15

4.7 Ocean 4-17

Currents • Waves • Ocean Thermal Energy Conversion • Salinity Gradient

4.8 Other 4-24

References4-25

Recommended Resources 4-26

Vaughn Nelson

West Texas A&M University

4.1 Introduction

Energy from water is one of the oldest sources of energy, as paddle wheels were used to rotate a millstone to grind grain. A large number of watermills, 200–500 W, for grinding grain are still in use in remote mountains and hilly regions in the developing world. There are an estimated 500,000 in the Himalayas, with around 200,000 in India [1,2]. Of the 25,000–30,000 watermills in Nepal, 2,767 mills were upgraded between 2003 and 2007 [3]. Paddle wheels and buckets powered by moving water were and are still used in some parts of the world for lifting water for irrigation. Water provided mechanical power for the textile and industrial mills of the 1800s as small dams were built, and mill buildings are found along the edges of rivers throughout the United States and Europe. Then, starting in the late 1800s, water stored behind dams was used for the generation of electricity. For example, in Switzerland in the 1920s there were nearly 7000 small-scale hydropower plants.

The energy in water can be potential energy from a height difference, which is what most people think of in terms of hydro; the most common example is the generation of electricity (hydroelectric) from water stored in dams. However, there is also kinetic energy due to water flow in rivers and ocean currents. Finally, there is energy due to tides, which is due to gravitational attraction of the moon and the sun, and energy from waves, which is due to wind. In the final analysis, water energy is just another transformation from solar energy, except for tides.

The energy or work is force * distance, so potential energy due to gravitation is

$W=F\text{*}d=m\text{*}g\text{*}H(\text{J)}$ (4.1)

The force due to gravity is mass * acceleration, where the acceleration of gravity g = 9.8 m/s2 and H = height in meters of the water. For estimations, you may use g = 10 m/s2.

For water, generally what is used is the volume, so the mass is obtained from density and volume.

$\rho =\frac{m}{V}\text{or}m=\rho \text{*}V,\text{where}\rho =1000{\text{kg/m}}^{\text{3}}\text{for}\text{water}$

Then, for water Equation 4.1 becomes

$PE=\rho \text{*}g\text{*}H\text{*}V=10,000\text{*}H\text{*}V$ (4.2)

Example 4.1

Find the potential energy for 2000 m3 of water at a height of 20 m.

$PE=10,000\text{*}20\text{*}2,000=4\text{*}{10}^{\text{9}}\text{J}=4\text{GJ}$

If a mass of water is converted to kinetic energy after falling from a height H, then the velocity can be calculated.

$KE=PE$

or

$0.5m\text{*}{v}^2=m\text{*}g\text{*}H$

Then, the velocity of the water is

$v={(2\text{*}g\text{*}H)}^{0.5}\text{(m/s)}$ (4.3)

Example 4.2

For data in Example 4.1, find the velocity of that water after falling through 20 m.

$v={(2\text{*}10\text{*}20)}^{\text{0}\text{.5}}=20\text{m/s}$

Instead of water at some height, there is a flow of water in a river or an ocean current, such as the Gulf Stream. The analysis for energy and power for moving water is similar to wind energy, except there is a large difference in density between water and air. Therefore, for the same amount of power, capture areas for water flow will be a lot smaller.

$\frac{P}{A}=0.5\text{*}\rho \text{*}{v}^3{\text{(W/m}}^{\text{2}})$ (4.4)

Example 4.3

Find the power/area for an ocean current that is moving at 1.5 m/s.

$\frac{P}{A}=0.5\text{*}{(1.5)}^3=1.7{\text{kW/m}}^{\text{2}}$

Power is energy/time, and hydraulic power from water or for pumping water from some depth is generally defined in terms of water flow Q and the height. Of course, if you know the time and have either power or energy, then the energy or power can be calculated.

$P=10,000\text{*}H\text{*}\frac{V}{t}=10\text{*}Q\text{*}H(\text{kW)}$ (4.5)

where Q is the flow rate (m3/s). In terms of pumping smaller volumes of water for residences, livestock, and villages, Q is generally noted as cubic meters per day, so be sure to note what units are used. There will be friction and other losses, so with efficiency ε the power is

$P=10,000\text{*}\epsilon \text{*}Q\text{*}H$ (4.6)

Efficiencies from input to output (generally electric) range from 0.5 to 0.85. Small water turbines have efficiencies up to 80%, so when other losses are included (friction and generator), the overall efficiency is approximately 50%. Maximum efficiency is at the rated design flow and load, which is not always possible as the river flow fluctuates throughout the year or where daily load patterns vary.

The output from the turbine shaft can be used directly as mechanical power, or the turbine can be connected to an electric generator. For many rural industrial applications, shaft power is suitable for grinding grain or oil extraction, sawing wood, small-scale mining equipment, and so on.

4.2 World Resource

Around one-quarter of the solar energy incident on the Earth goes to the evaporation of water; however, as this water vapor condenses, most of the energy goes into the atmosphere as heat. Only 0.06% is rain and snow, and that power and energy of the water flow is the world resource, estimated at around 40,000 TWh/year. The technical potential (Table 4.1) is 15,000 TWh/year, and economic and environmental considerations reduce that potential.

The classification of hydropower differs by country, authors, and even over time. One classification is large (>30 MW), small (100–30 MW), and micro (≤100 kW). Some examples are as follows: In China, small hydro refers to capacities up to 25 MW, in India up to 15 MW, and in Sweden up to 1.5 MW. Now, in Europe, small hydro means a capacity of up to 10 MW. Today in China, the classifications are large (>30 MW), small (5–30 MW), mini (100 kW to 5 MW), micro (5–100 kW), and pico (<5 kW). Others classify microhydro as 10–100 kW, so be sure to note the range when data are given for capacity and energy for hydropower.

4.3 Hydroelectric

4.3.1 Large (≥30 MW)

In terms of renewable energy, large-scale hydropower (Figure 4.1) is a major contributor to electric generation in the world, over 3000 TWh/year. The world installed capacity for large-scale hydroelectricity has increased around 2% per year, from 462 GW in 1980 to around 850 GW in 2009. However, the hydroelectric percentage of electric power has decreased from 21.5% in 1980 to 16% in 2008 as other sources of electrical energy have increased faster. China is now the leader in installed capacity and generation of electricity (Figure 4.2), with about 14% of their electricity from hydroelectric sources. However, coal in China is the major energy source for the production of electricity, and more coal power has been added than hydroelectric power. In Norway, 98% of the electrical energy is from hydro; Paraguay sells most of its share of electricity from the Itaipu Dam to Argentina. In the United States, the hydroelectric contribution is around 6%. The contribution from small or micro-hydro plants is difficult to estimate but could represent another 5%–10% in terms of world capacity. The capacity factor for hydroelectric power in the world has been fairly consistent at 40%–44% from 1980 to 2008. The capacity factor for hydroelectric power in the United States was 37% in 2008.

Large-scale hydroelectric plants have been constructed all across the world (Table 4.2). The Three Gorges Dam (Figure 4.3) on the Yangtze River is the largest power hydro plant in the world with 18.3 GW and will have a power of 22.5 GW when the rest of the generators are installed in 2011. Previously, the largest project was the Itaipu Dam on the Paraná River between Paraguay and Brazil. The series of dams is 7744 m long and was built from 1975 to 1991. The Aswan High Dam, Egypt (2100 MW), was completed in 1967 and produces more than 10 TWh/year, provides irrigation water for 3.2 million ha, and produces 20,000 ton of fish per year. The entire Temple of Abu Simel had to be moved to higher ground, a major feat in archeology. One of the problems of the Aswan Dam was that farming practices on the banks of the river downstream had to be changed since no yearly floods meant no deposition of fertile silt.

The benefits or advantages of hydropower are as follows:

1. Renewable source, power on demand with reservoirs
2. Long life, 100 years
3. Flood control, water for irrigation and metropolitan areas
4. Low greenhouse gas emission
5. Reservoir for fishing, recreation

Some disadvantages or problems are the following:

1. There is a large initial cost and long construction time.
2. Displacement of population due to reservoir may occur. For example, 1.24 million people were relocated due to the Three Georges Dam.
3. On land downstream, there is loss of nutrients from floods.
4. Drought by season or year may restrict power output due to low water.
5. Lack of passage for fish to spawning areas, for example, salmon.
6. Rivers with high silt content may limit dam life.
7. Dam collapse means many problems downstream. There have been over 200 dam failures in the twentieth century, and it is estimated that 250,000 people died in a series of hydroelectric dam failures in China in 1975.
8. Resource allocation between countries can be a problem [4], especially if a series of dams that use a lot of water for irrigation are built upstream of a country.

In the United States, the first commercial hydroelectric plant (12.5 kW) was built in 1882 on the Fox River in Appleton, Wisconsin. Then, commercial power companies began to install a large number of small hydroelectric plants in mountainous regions near metropolitan areas. The creation of the Federal Power Commission in 1920 increased development of hydroelectric power with regulation and monetary support. The government supported projects for hydroelectric power and for flood control, navigation, and irrigation. The Tennessee Valley Authority was created in 1933 [5], and the Bonneville Power Administration was created in 1937 [6]. Construction of the Hoover Dam (Figure 4.4) started in 1931, and when completed in 1936, it was the largest hydroelectric project in the world at 2 GW [7]. Hoover Dam was then surpassed in 1941 by the Grand Coulee Dam (Figure 4.5) (6.8 GW) on the Columbia River [8]. The larger power output is due to the higher volume of water available.

A geographic information system (GIS) application, the Virtual Hydropower Prospector, provides maps and information for the United States [9]. The application allows the user to view the plants in the context of hydrography, power and transportation infrastructure, cities and populated areas, and federal land use. Most of the possible sites will be for small hydro.

In the developed countries with significant hydroelectric capacity, many of the best sites for hydroelectric potential already have dams. Many more reservoirs have been built for irrigation, water supply, and flood control than for hydropower as only 3% of the 78,000 dams in the United States have hydropower. Also, the construction of dams has declined in the United States since 1980 (Figure 4.6). So, there is a potential for hydropower by repowering some defunct hydroelectric installations or by new installations of small or medium hydropower at existing dams.

4.3.2 Small Hydro (100 kW to 30 MW, 10 MW in Europe)

The definition of small hydro differs by country, so the range in Europe is 100 kW to 10 MW and in other countries is up to 25 or 30 MW. The World Energy Council estimated small hydro (up to 10 MW) was around 25,500 MW in 2006, with the major capacity in Europe, nearly 17,000 MW, and the energy production was estimated at 66 TWh. Now, the World Energy Council estimates the installed capacity of small hydro was around 55 GW in 2010, with China having the largest capacity. The current small hydro electricity generation in Europe (European Union-25, the candidate countries, and Switzerland) is around 47 TWh/year, and the remaining potential is estimated at another 49 TWh/year. This potential consists mainly of low-head sites (below 30 m).

Hydroelectric plants in the United States are predominantly private (69%); however, 75% of the capacity is owned by federal and nonfederal public owners [10], primarily from large power plants. The percentage of low and small hydropower plants in terms of numbers is 86%. This indicates future expansion for hydroelectric power in the United States will be from distributed generation.

A resource assessment of hydropower for 49 states (no resource in Delaware) identified 5667 sites (Figure 4.7) with a potential of around 30,000 MW [11]. The criteria were low power (<1 MW) or small hydro (≥1 MW and ≤30 MW), and the working flow was restricted to half the stream flow rate at the site or sufficient flow to produce 30 MWa (megawatts average), whichever was less. Penstock lengths were limited by the lengths of penstocks of a majority of existing low-power or small hydroelectric plants in the region. The optimum penstock length and location on the stream was determined for the maximum hydraulic head with the minimum length. The number of sites studied was 500,000, with approximately 130,000 sites meeting the feasibility criteria. Then, application of the development model with the limits on working flow and penstock length resulted in a total hydropower potential of 30,000 MWa. The approximately 5,400 sites that could potentially be developed as small hydro plants have a total hydropower potential of 18,000 MWa. Idaho National Lab also developed a probability factor model, Hydropower Evaluation Software, to standardize the environmental assessment.

4.3.3 Microhydro (<100 kW)

Estimation of the number of installations and capacity for microhydro is even more difficult. In general, microhydro does not need dams and a reservoir as water is diverted and then conducted in a penstock to a lower elevation and the water turbine. In most cases, the end production is the generation of electricity.

There are thought to be tens of thousands of microhydro plants in China and significant numbers in Nepal, Sri Lanka, Pakistan, Vietnam, and Peru. The estimate for China was about 500 MW at the end of 2008. China started a program, SDDX [12], in 2003 that installed 146 hydro systems with a capacity of 113.8 MW in remote villages in the Western Provinces and Tibet. Hydropower was the predominant system in terms of capacity compared to wind and photovoltaics (PV), with 721 installations (15.5 MW) for villages and 15,458 installations (1.1 MW) for single households. The average size of the hydropower systems was 780 kW, which is much larger than average for the wind and PV systems (22 kW). Case studies are available for a number of countries [13], and software is available from Microhydro [14].

The advantages of microhydro are the following:

1. Efficient energy source. A small amount of flow (0.5 L/min) with a head of 1 m generates electricity with micro hydro. Electricity can be delivered up to 1.5 km.
2. Reliable. Hydro produces a continuous supply of electrical energy in comparison to other small-scale renewable technologies. Also, backup, whether diesel or batteries (which causes operation and maintenance and cost problems), is not needed.
3. No reservoir required. The water passing through the generator is directed back into the stream with relatively little impact on the surrounding ecology.
4. It is a cost-effective energy solution for remote locations.
5. Power for developing countries. Besides providing power, developing countries can manufacture and implement the technology.

The disadvantages or problems are as follows:

1. Suitable site characteristics are required, including distance from the power source to the load and stream size (flow rate, output, and head).
2. Energy expansion may not be possible.
3. There is low power in the summer months. In many locations, stream size will fluctuate seasonally.
4. Environmental impact is minimal; however, environmental effects must be considered before construction begins.

Impulse turbines are generally more suitable for microhydro applications compared with reaction turbines because of

Greater tolerance of sand and other particles in the water

Better access to working parts

Lack of pressure seals around the shaft

Ease of fabrication and maintenance

Better part-flow efficiency

The major disadvantage of impulse turbines is that they are generally unsuitable for low-head sites. Pelton turbines can be used at heads down to about 10 m; however, they are not used at lower heads because their rotational speeds are too slow, and the runner required is too large. The cross-flow turbine is the best machine for construction by a user.

4.4 Turbines

The two main types of hydro turbines are impulse and reaction. The type selected is based on the head and the flow, or volume of water, at the site (Table 4.3). Other deciding factors include how deep the turbine must be set, efficiency, and cost. Many images are available on the Internet for the different types of turbines.

4.4.1 Impulse Turbines

The impulse turbine uses the velocity of the water to move the runner (rotating part) and discharges to atmospheric pressure. The water stream hits each bucket on the runner, and the water flows out the bottom of the turbine housing. An impulse turbine is generally used for high-head, low-flow applications.

A Pelton turbine (Figure 4.8) has one or more free jets of water impinging on the buckets of a runner. The jet is directed at the centerline of the two buckets. Draft tubes are not required for the impulse turbine since the runner must be located above the maximum tail water to permit operation at atmospheric pressure.

A cross-flow turbine (Figure 4.9) resembles a squirrel cage blower and uses an elongated, rectangular-section nozzle to direct a sheet of water to a limited portion of the runner, about midway on one side. The flow of water crosses through the empty center of the turbine and exits just below the center on the opposite side. A guide vane at the entrance to the turbine directs the flow to a limited portion of the runner. The cross flow was developed to accommodate larger water flows and lower heads than the Pelton turbine.

4.4.2 Reaction Turbines

A reaction turbine develops power from the combined action of pressure and moving water, as the pressure drop across the runner produces power. The runner is in the water stream flowing over the blades rather than striking each individually. Reaction turbines are generally used for sites with lower head and higher flows.

Francis turbines (Figure 4.10) are the most common for hydropower. They are an inward flow turbine that combines radial and axial components. The runner has fixed vanes, usually nine or more. The inlet is spiral shaped with guide vanes to direct the water tangentially to the runner. The guide vanes (or wicket gate) may be adjustable to allow efficient turbine operation for a range of water flow conditions. The other major components are the scroll case, wicket gates, and draft tube (as water speed is reduced, a larger area for the outflow is needed). However, the Francis turbine can be used for heads to 800 m.

A propeller turbine (Figure 4.11) generally has a runner with three to six blades running in a pipe, where the pressure is constant. The pitch of the blades may be fixed or adjustable. The major components besides the runner are a scroll case, wicket gates, and a draft tube.

There are several different types of propeller turbines:

Bulb: The turbine and generator are a sealed unit placed directly in the water stream.

Straflo: The generator is attached directly to the perimeter of the turbine.

Tube: The penstock bends just before or after the runner, allowing a straight-line connection to the generator.

Kaplan: Both the blades and the wicket gates are adjustable, allowing for a wider range of operation.

4.5 Water Flow

Kinetic energy turbines, also called free-flow turbines, generate electricity from the kinetic energy of the flowing water, similar to wind turbines, which generate energy from the flowing air. Systems are also referred to as hydrokinetic, tidal in-stream energy conversion (TISEC), or river in-stream energy conversion (RISEC). The systems may operate in rivers, tides, ocean currents, or even channels or conduits for water. Kinetic systems do not require large civil works, and they can be placed near existing structures such as bridges, tailraces, and channels that increase the natural flow of water. For tidal currents, unidirectional turbines are available; rotation is the same, even though current is from opposite directions. One hydrokinetic system has a hydraulic pump to drive an onshore electric generator. Kinetic energy turbines would have less environmental impact than dams, and like wind turbines, they are modular and can be installed in a short time compared to large civil structures.

The power/area is proportional to the cube of the velocity (Equation 4.4). Large rivers have large flows, and the Amazon, with an average flow of 210,000 m3/s, has around 20% of the river flow of the world. At the narrows of Óbidos, 600 km from the sea, the Amazon narrows to a single stream that is 1.6 km wide and over 60 m deep and has a speed of 1.8–2.2 m/s. At New Orleans, the speed of the Mississippi is 1.3 m/s, and some sections of the river have flows of 2.2 m/s. At Hastings, Minnesota, a 250-kW hydrokinetic unit located below a dam (4.4 MW hydroelectric) was placed in operation in 2008. The ducted rotor is suspended from a barge with the generator on the barge (Figure 4.12).

The United States could produce 13,000 MW of power from hydrokinetic energy by 2025. As of March 2010, the Federal Energy Regulatory Commission (FERC) had issued 134 preliminary permits for hydrokinetic projects (Figure 4.13) with a total capacity of 9864 MW. Notice that many of the permits are on the Mississippi River.

The FERC requires consideration of any environmental effects of the proposed construction, installation, operation, and removal of the project. The description should include

1. Any physical disturbance (vessel collision or other project-related risks for fish, marine mammals, seabirds, and other wildlife as applicable)
2. Species-specific habitat creation or displacement
3. Increased vessel traffic
4. Exclusion or disturbance of recreational, commercial, industrial, or other uses of the waterway and changes in navigational safety
5. Any above- or below-water noise disturbance, including estimated decibel levels during project construction, installation, operation, and removal
6. Any electromagnetic field disturbance
7. Any changes in river or tidal flow, wave regime, or coastal or other geomorphic processes
8. Any accidental contamination from device failures, vessel collisions, and storm damage
9. Chemical toxicity of any component of, or biofouling coating on, the project devices or transmission line
10. Any socioeconomic effects on the commercial fishing industry from potential loss of harvest or effect on access routes to fishing grounds

An important factor for water flow is that, at good locations, power will not vary like that of wind turbines, especially for in-river locations, so capacity factors can be much higher. One manufacturer stated that capacity factors should be at least 30% for tides and 50% for in-river systems. As always, the final result for comparison is the cost per kilowatt hour, which should be life-cycle costs.

4.6 Tides

Tides are due to the gravitational attraction of the moon and the sun at the surface of the Earth. The effect of the moon on the Earth in terms of tides is larger than the effect of the sun, even through the gravitational force of the sun is larger. To find how the gravitational force of the moon distorts any volume of the material body of the Earth, the gradient of the gravitational force of the moon on that volume must be found (a gradient is how force changes with distance; in calculus, it is the differentiation with respect to length). The tidal effects (Figure 4.14) are superimposed on the near-spherical Earth, and there will be two tides per day due to the spin of Earth. When the tidal effects of the sun and moon are aligned, the tides are higher, spring tides. When the continents are added, the ocean bulges reflect from shorelines, which causes currents, resonant motions, and standing waves, so there are some places in the oceans where the tidal variations are nearly zero. In other locations, the coastal topography can intensify water heights with respect to the land. The largest tidal ranges in the world are the Bay of Fundy (11.7 m), Ungava Bay (9.75 m), Bristol Channel (9.6 m), and the Turnagain Arm of Cook Inlet, Alaska (9.2 m). The potential world tidal current energy is on about 2200 TWh/year.

Small mills were used on tidal sections of rivers in the Middle Ages for grinding grain. Today, there are only a few tidal systems installed in the world: the French installed a tidal system on the Rance Estuary (constructed from 1961 to 1967) with a power of 240 MW; an 18-MW rim generator at Annapolis Royal, Nova Scotia, Canada (1984); a 400-kW unit in the Bay of Kislaya, Russia (1968); and a 500-kW unit at Jangxia Creek, East China Sea.

The simplest system for generation of electricity is an ebb system, which involves a dam, known as a barrage, across an estuary. Barrages make use of the potential energy in the difference in height between high and low tides. Sluice gates on the barrage allow the tidal basin to fill on high tides (flood tide) and to generate power on the outgoing tide (ebb tide). Flood generating systems generate power from both tides but are less favored than ebb generating systems. Barrages across the full width of a tidal estuary have high civil infrastructure costs, there is a worldwide shortage of viable sites, and there are more environmental issues.

Tidal lagoons are similar to barrages but can be constructed as self-contained structures, not fully across an estuary, and generally have much lower cost and environmental impact. Furthermore, they can be configured to generate continuously, which is not the case with barrages. Different tidal systems, installed and proposed plants, and prototype and demonstration projects are given in Ref. [15].

The potential for tidal in-stream systems was estimated at 692 MW for five states in the United States [16]. A kinetic energy demonstration project (Figure 4.15) is installed in the East River, New York City, and consists of two 35-kW turbines, 5-m diameter, with passive yaw. In 9000 h of operation, the system generated 70 MWh. Another prototype, SeaGen, is installed in Strangford Narrows, Northern Ireland, with rated power of 1.2 MW at a current velocity of 2.4 m/s and with twin 16-m diameter rotors (Figure 4.16). The rotor blades can be pitched through 180° to generate power on both ebb and food tides. The twin power units are mounted on winglike extensions on a tubular steel monopole, and the system can be raised above sea level for maintenance. Kinetic energy systems are being considered because of the lower cost, lower ecological impact, increased availability of sites compared to barrages, and shorter time for installation.

Advantages for tidal systems are as follows:

1. Renewable
2. Predictable

Disadvantages or problems are as follows:

1. A barrage across an estuary is expensive and affects a wide area.
2. The environment is changed upstream and downstream for some distance. Many birds rely on the tide uncovering the mudflats so that they can feed. Fish ladders are needed.
3. There is intermittent power as power is available for around 10 h each day when the tide is moving in or out.
4. There are few suitable sites for tidal barrages.

4.7 Ocean

As with other renewable resources, the ocean energy is large [17]. The global technical resource exploitable with today’s technology is estimated to be 20,000 TWh/year for ocean currents, 45,000 TWh/year for wave energy, 33,000 TWh/year for ocean thermal energy conversion (OTEC), and 20,000 TWh/year for salinity gradient energy. Of course, economics and other factors will greatly reduce the potential production, and future actual energy production will be even smaller.

Besides the environmental considerations mentioned, there are a number of technical challenges for ocean energy to be utilized at a commercial scale:

Avoidance of cavitations (bubble formation)

Prevention of marine growth buildup

Reliability (since maintenance costs may be high)

Corrosion resistance

4.7.1 Currents

There are large currents in the ocean (Figure 4.17), and detailed information on surface currents by ocean is available [18, only Atlantic and Polar at this time]. For example, the Gulf Stream transports a significant amount of warm water toward the North Atlantic and the coast of Europe. The core of the Gulf Stream current is about 90 km wide and has peak velocities greater than 2 m/s. The relatively constant extractable energy density near the surface of the Gulf Stream, the Florida Straits Current, is about 1 kW/m2. Although the volume and velocity are adequate for in-stream hydro-kinetic systems, an ocean current would need to be close to the shore.

The total world power in ocean currents has been estimated to be about 5000 GW, with power densities of up to 15 kW/m2 [19]. The European Union, Japan, and China are interested in and pursuing the application of ocean current energy systems.

4.7.2 Waves

Waves are created by the progressive transfer of energy from the wind as it blows over the surface of the water. Once created, waves can travel large distances without much reduction in energy. The energy in a wave is proportional to the height squared. In data for wave heights, be sure to note that height is for crest to trough, and amplitude is midpoint to crest.

$E=0.5\text{*}\rho \text{*}g\text{*}\frac{H^2}{16}(\text{J)}$ (4.7)

where H is wave height. This is for a single wave, but in the ocean, there is superposition of waves, and the energy transported is by group velocity. The speed of the wave, wave length, and frequency (or period, which is 1/frequency) are related by

$\text{Speed}=\text{Wavelength}\text{(}\lambda \text{)}\text{*}\text{Frequency}\text{(}f\text{)}$

In deep water where the water depth is larger than half the wavelength, the power per length (meter) of the wave front is given by

$\frac{P}{L}=\rho \text{*}{g}^{\text{2}}\text{*}{H}^2\text{*}\frac{T}{(64\text{*}\pi )}~0.5\text{*}{A}^2\text{*}T(\text{kW/m)}$ (4.8)

where T is the period of the wave (time it takes for successive crests to pass one point). In major storms, the largest waves offshore are about 15 m high and have a period of about 15 s, so the power is large, around 1.7 MW/m.

Example 4.4

Calculate power/length for waves off New Zealand if the average wave height is 7 m with a wave period of 8 s. The power/length is

$\frac{P}{L}=0.5\text{*}{7}^2\text{*}8=196\text{(kW/m)}$

An effective wave energy system should capture as much energy as possible of the wave energy. As a result, the waves will be of lower height in the region behind the system. Offshore sites with water 25–40 m deep have more energy because waves have less energy as the depth of the ocean decreases toward the coast. Losses become significant as the depth becomes less than half a wavelength, and at 20 m deep, the wave energy is around one-third of that in deep water (depth greater than one-half wavelength). The North Atlantic west of Ireland has wavelengths of around 180 m, and off the West Coast of the United States, the wavelengths can be 300 m.

The potential for wave energy (per meter of wave front) for the world is much larger than ocean currents due to the length of coastline (Figure 4.18). The potential for the United States is 240 GW, with an extractable energy of 2100 TWh/year based on average wave power density of 10 kW/m. The technically and economically recoverable resource for the United Kingdom has been estimated to be 50–90 TWh of electricity per year or 15%–25% of total U.K. demand in 2010. The western coast of Europe and the Pacific coastlines of South America, Southern Africa, Australia, and New Zealand are also highly energetic. Any area with yearly averages of 15 kW/m has the potential to generate wave energy. Note that this threshold excludes areas such as the Mediterranean Sea and the Great Lakes of North America.

The resource or wave climate can be obtained from recorded data, and satellites now provide current worldwide data and are used for prediction of wave heights. For wave energy systems, it is also important to determine the statistical occurrence of the extreme waves that can be expected at the site over the lifetime of the system since the system should be designed to survive peak waves.

Once the general area of the wave farm site has been determined, more analysis is needed to pick the best site within that area, for example, by examining the mean wave direction, variability, and the possibility of local focusing of waves. Another essential task includes the calculation of calm periods that allow sufficient time for maintenance and other operations. However, as noted, large waves have lots of power and could damage or destroy the system, so design and construction must take these large waves into account.

The mechanisms for capture of wave energy are point absorber, reservoir, attenuator, oscillating water column, and other mechanisms. There are a number of prototypes and demonstration projects but few commercial projects. A point absorber has a small dimension in relation to the wavelength (Figure 4.19).

The reservoir system is where the waves are forced to higher heights by channels or ramps, and the water is captured in a reservoir (Figure 4.20). Locations for land installations for reservoir and oscillating water column systems will be much more limited than offshore systems; however, land installations are easier to construct and maintain. The Wave Dragon is a floating offshore platform (Figure 4.21 and Figure 4.22).

The Pelamis Wave Energy Converter [20], an attenuator, is a semisubmerged, articulated cylindrical attenuator linked by hinged joints (Figure 4.23). The wave-induced motion of these joints drives hydraulic rams, which pump high-pressure fluid through hydraulic motors via smoothing accumulators. The hydraulic motors drive an electrical generator, and the power from all the joints is fed down a single cable to a junction on the seabed. Several devices (Figure 4.24) can then be linked to shore through a single seabed cable. Current production machines have four power conversion modules: 750-kW rated power, 180 m long, 4-m diameter. The power table and the wave climate are combined to give the electrical power response over time and, from that, its average level and its variability. Depending on the wave resource, the capacity factor is 25%–40%.

In an oscillating water column, as a wave enters the column, the air pressure within the column is increased, and as the wave retreats, air pressure is reduced (Figure 4.25). The Wells turbine turns in the same direction irrespective of the airflow direction. The land-installed marine power energy transmitter (LIMPET) unit on Isle of Islay, Scotland [21], has an inclined oscillating water column, with an inlet width of 21 m (Figure 4.26) with the mean water depth at the entrance at 6 m. The system (rated power is 500 kW) has three water columns contained within concrete tubes, 6 m by 6 m, inclined at 40° to the horizontal, giving a total water surface area of 169 m2. The upper parts of the tubes are connected to a single tube, which contains a Wells generator.

The design of the air chamber is important to maximize the conversion of wave energy to pneumatic power, and the turbines need to be matched to the air chamber. The performance has been optimized for annual average wave intensities of between 15 and 25 kW/m.

In another system, waves drive a hinged flap connected (Figure 4.27) to the seabed at around 10 m depth, which then drives hydraulic pistons to deliver high-pressure water via a pipeline to an onshore electrical turbine.

4.7.3 Ocean Thermal Energy Conversion

OTEC for producing electricity is the same as solar ponds, for which the thermal difference between surface water and deep water drives a Rankine cycle. There is one major difference: The deep ocean water is rich in nutrients, which can be used for mariculture. In both systems, there is the production of freshwater.

An OTEC system needs a temperature difference of 20°C from cold water within 1000 m of the surface, which occurs across vast areas of the world (Figure 4.28). The systems can be on or near the shore. The three general types of OTEC processes are closed cycle, open cycle, and hybrid cycle.

In the closed-cycle system, heat transferred from the warm surface seawater causes a working fluid to turn to vapor, and the expanding vapor drives a turbine attached to an electric generator. Cold seawater passing through a condenser containing the vaporized working fluid turns the vapor back into a liquid, which is then recycled through the system.

An open-cycle system uses the warm surface water itself as the working fluid. The water vaporizes in a near vacuum at surface water temperatures. The expanding vapor drives a low-pressure turbine attached to an electrical generator. The vapor, which is almost pure freshwater, is condensed into a liquid by exposure to cold temperatures from deep ocean water. If the condenser keeps the vapor from direct contact with seawater, the water can be used for drinking water, irrigation, or aquaculture. A direct contact condenser produces more electricity, but the vapor is mixed with cold seawater, and the mixture is discharged to the ocean. Hybrid systems use parts of both open- and closed-cycle systems to optimize production of electricity and freshwater.

The first prototype OTEC project (22 kW) was installed at Matanzas Bay, Cuba, in 1930 [22]. Then, in the latter part of the twentieth century, experimental systems were installed in Hawaii and Japan. An experimental, open-cycle, onshore system was operated intermittently between 1992 and 1998 at the Keahole Point Facility, National Energy Laboratory, Hawaii. Surface water is 26°C, and the deep-water temperature is 6°C (depth of 823 m); the system produced a maximum power of 250 kW. However, the power requirements for pumping the surface (36.3 m3/min) and deep (24.6 m3/min) seawater were around 200 kW. A small fraction (10%) of the steam produced was diverted to a surface condenser for the production of freshwater, about 22 L/min. In 1981, Japan demonstrated a shore-based, 100-kWe closed-cycle plant in the Republic of Nauru in the Pacific Ocean. The cold-water pipe was laid on the seabed at a depth of 580 m. The plant produced 31.5 kWe of net power during continuous operating tests.

4.7.4 Salinity Gradient

Salinity gradient energy is derived from the difference in the salt concentration between seawater and river water. Two practical methods for this are reverse electrodialysis and pressure-retarded osmosis; both rely on osmosis with ion-specific membranes. A small prototype (4 kW) started operation in 2009 in Tofte, Norway. The pressure generated is equal to a water column of 120 m, which is used to drive a turbine to generate electricity.

4.8 Other

Another application for water flow is ram pumps, where the pressure from water over a drop of a few meters is used to lift a small percentage of that water through a much greater height for water for people or irrigation. Ram pumps were developed over 200 year ago and can be made locally [23–25]. The operation of a ram pump (Figure 4.29) is as follows:

1. Water from a stream flows down the drive pipe and out of the waste valve.
2. As the flow of water accelerates, the waste valve is forced shut, causing a pressure surge (or water hammer) as the moving water is brought to a halt.
3. The pressure surge causes the check valve to open, allowing high-pressure water to enter the air chamber and delivery pipe. The pressurized air in the air chamber helps to smooth out the pressure surges to give a continuous flow through the delivery pipe.
4. As the pressure surge subsides, the pressurized air in the air chamber causes the check valve to close. The sudden closure of the check valve reduces the pressure in the drive pipe so that the waste valve opens, and the pump is returned to start the cycle again. Most ram pumps operate at 30–100 cycles a minute.

The Alternative Indigenous Development Foundation in the Philippines developed durable ram pumps, and the maintenance is done locally on the moving parts that need regular replacement. The five different size ram pumps can deliver between 1,500 and 72,000 L/day up to a height of 200 m. The 98 ram pumps installed by 2007 were delivering over 900 m3/day of water, serving over 15,000 people and irrigating large areas of land.

References

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3. Centre for Rural Technology, Nepal. http://crtnepal.org (accessed on 2/13/2012), see gallery photos of watermills.

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Recommended Resources

Links

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Bonneville Power Administration. 2008. Renewable energy technology roadmap (wind, ocean wave, in-stream tidal and solar).

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European Ocean Energy Association. http://www.eu-oea.com (accessed on 2/13/2012).

Hydropower Research Foundation. http://www.hydrofoundation.org/index.html (accessed on 2/13/2012).

International Energy Agency, Ocean Energy Systems. http://www.iea-oceans.org (accessed on 2/13/2012).

International Hydropower Association. http://www.hydropower.org (accessed on 2/13/2012).

International Network on Small Hydro Power. http://www.inshp.org/main.asp (accessed on 2/13/2012).

International Small-Hydro Atlas. http://www.small-hydro.com (accessed on 2/13/2012).

Microhydro Power. http://practicalaction.org/energy/micro_hydro_expertise (accessed on 2/13/2012).

Microhydro Power. Links to case studies. http://www.microhydropower.net/index.php (accessed on 2/13/2012).

Micro hydro Solomon Islands. http://www.pelena.com.au/pelton_turbine.htm (accessed on 2/13/2012).

National Hydropower Association. http://www.hydro.org (accessed on 2/13/2012).

Oceanweather, current significant wave height and direction by regions of the world. http://www.oceanweather.com/data (accessed on 2/13/2012).

United States, Water Power Technology Program. http://www1.eere.energy.gov/water/about.html (accessed on 2/13/2012).

Wavebob. http://www.wavebob.com (accessed on 2/13/2012).

Wave Dragon. http://www.wavedragon.net (accessed on 2/13/2012).