HydroelectricityHydroelectricity is electricity produced by hydropower. Hydroelectricity now supplies about 715,000 MW or 19% of world electricity (16% in 2003). It is also the world's leading form of renewable energy, accounting for over 63% of the total in 2005.
Although large hydroelectric installations generate most of the world's hydroelectricity, small hydro schemes are particularly popular in China, which has over 50% of world small hydro capacity.
Electricity generation
Most hydroelectric power comes from the potential energy of dammed water driving a water turbine and generator. In this case the energy extracted from the water depends on the volume and on the difference in height between the source and the water's outflow. This height difference is called the head. The amount of potential energy in water is proportional to the head.
Pumped storage hydroelectricity produces electricity to supply high peak demands by moving water between reservoirs at different elevations. At times of low electrical demand, excess generation capacity is used to pump water into the higher reservoir. When there is higher demand, water is released back into the lower reservoir through a turbine. Pumped storage schemes currently provide the only commercially important means of grid energy storage and improve the daily load factor of the generation system.
Advantages
Economics
The major advantage of hydroelectricity is elimination of the cost of fuel. Hydroelectric plants tend to have longer economic lives than fuel-fired generation, with some plants now in service having been built 50 to 100 years ago. Operating labor cost is usually low since plants are automated and have few personnel on site during normal operation.
Where a dam serves multiple purposes, a hydroelectric plant may be added with relatively low construction cost, providing a useful revenue stream to offset the costs of dam operation.
Related activities
Reservoirs created by hydroelectric schemes often provide facilities for water sports, and become tourist attractions in themselves. In some countries, farming fish in the reservoirs is common. Large hydro dams can control floods, which would otherwise affect people living downstream of the project. When dams create large reservoirs and eliminate rapids, boats may be used to improve transportation.
Greenhouse gas emissions
Since no fossil fuel is consumed, emission of carbon dioxide from burning fuel is eliminated. However, there may be other sources of emissions as discussed below.
Disadvantages
Environmental damage
Hydroelectric projects can be disruptive to surrounding aquatic ecosystems. For instance, studies have shown that dams along the Atlantic and Pacific coasts of North America have reduced salmon populations by preventing access to spawning grounds upstream, even though most dams in salmon habitat have fish ladders installed.
Generation of hydroelectric power changes the downstream river environment. Water exiting a turbine usually contains very little suspended sediment, which can lead to scouring of river beds and loss of riverbanks. Since turbines are often opened intermittently, rapid or even daily fluctuations in river flow are observed. Dissolved oxygen content of the water may change from pre-construction conditions. Water exiting from turbines is typically much colder than the pre-dam water, which can change aquatic faunal populations, including endangered species. Some hydroelectric projects also utilize canals, typically to divert a river at a shallower gradient to increase the head of the scheme. In some cases, the entire river may be diverted leaving a dry riverbed.
large-scale hydroelectric dams have created environmental problems both upstream and downstream.
A further concern is the impact of major schemes on birds.
Greenhouse gas emissions
The reservoirs of hydroelectric power plants in tropical regions may produce substantial amounts of methane and carbon dioxide. This is due to plant material in flooded areas decaying in an anaerobic environment, and forming methane. In certain conditions greenhouse gas emissions from the reservoir may be higher than those of a conventional oil-fired thermal generation plant.
Population relocation
Another disadvantage of hydroelectric dams is the need to relocate the people living where the reservoirs are planned. In many cases, no amount of compensation can replace ancestral and cultural attachments to places that have spiritual value to the displaced population. Additionally, historically and culturally important sites can be flooded and lost.
Dam failures
Failures of large dams, while rare, are potentially serious - the Banqiao Dam failure in China resulted in the deaths of 171,000 people and left millions homeless, more than some estimates of the death toll from the Chernobyl disaster. Dams may be subject to enemy bombardment during wartime, sabotage and terrorism. Smaller dams and micro hydro facilities are less vulnerable to these threats.
The creation of a dam in a geologically inappropriate location may cause disasters like the one of the Vajont Dam in Italy, where almost 2000 people died, in 1963.
Comparison with other methods of power generation
Hydroelectricity eliminates the flue gas emissions from fossil fuel combustion, including pollutants.
Compared to the nuclear power plant, hydroelectricity generates no nuclear waste, nor nuclear leaks.
Compared to wind farms, hydroelectricity power plants have a more predictable load factor. If the project has a storage reservoir, it can be dispatched to generate power when needed. Hydroelectric plants can be easily regulated to follow variations in power demand.
Unlike fossil-fueled combustion turbines, construction of a hydroelectric plant requires a long lead-time for site studies, hydrological studies, and environmental impact assessment. Unlike plants operated by fuel, such as fossil or nuclear energy, the number of sites that can be economically developed for hydroelectric production is limited. New hydro sites tend to be far from population centers and require extensive transmission lines. Hydroelectric generation depends on rainfall in the watershed, and may be significantly reduced in years of low rainfall or snowmelt. Utilities that primarily use hydroelectric power may spend additional capital to build extra capacity to ensure sufficient power is available in low water years.
HYDROPOWER
Hydropower is the capture of the energy of moving water for some useful purpose. Prior to the widespread availability of commercial electric power, hydropower was used for irrigation, milling of grain, textile manufacture, and the operation of sawmills.
The energy of moving water has been exploited for centuries; in India, water wheels and watermills were built; in Imperial Rome, water powered mills produced flour from grain, and in China and the rest of the Far East, hydraulically operated "pot wheel" pumps raised water into irrigation canals. In the 1830s, at the peak of the canal-building era, hydropower was used to transport barge traffic up and down steep hills using inclined plane railroads. Direct mechanical power transmission required that industries using hydropower had to locate near the waterfall. For example, during the last half of the 19th century, many grist mills were built at Saint Anthony Falls, utilizing the 15 metres drop in the Mississippi River. The mills contributed to the growth of Minneapolis. Today the largest use of hydropower is for electric power generation, which allows low cost energy to be used at long distances from the watercourse.
Types of water power
There are many forms of water power:
* Water wheels , used for hundreds of years to power mills and machinery
* Hydroelectric energy, usually referring to hydroelectric dams or run-of-the-river setups.
* Tidal power, which captures energy from the tides in horizontal direction
* Tidal stream power, which does the same vertically
* Wave power, which uses the energy in waves
Water wheels
A water wheel is a hydropower system; a machine for extracting power from the flow of water. Water wheels and hydropower was widely used in the Middle Ages, powering most industry in Europe, along with the windmill. The most common use of the water wheel was to mill flour in gristmills, but other uses included foundry work and machining, and pounding linen for use in paper.
A water wheel consists of a large wooden or metal wheel, with a number of blades or buckets arranged on the outside rim forming the driving surface. Most commonly, the wheel is mounted vertically on a horizontal axle, but the tub or Norse wheel is mounted horizontally on a vertical shaft. Vertical wheels can transmit power either through the axle or via a ring gear and typically drive belts or gears; horizontal wheels usually directly drive their load. A channel created for the water to follow after leaving the wheel is commonly referred to as a "tailrace."
Types
Undershot wheel
Overshot wheel
Backshot wheel: an overshot wheel is backshot by introducing the intake water from the same direction as the flow of the output water.
Hydroelectric power (Pumped-Storage Plant)
Hydroelectric power now supplies about 715,000 MWe or 19% of world electricity (16% in 2003). Large dams are still being designed. Apart from a few countries with an abundance of it, hydro power is normally applied to peak load demand because it is readily stopped and started. Nevertheless, hydroelectric power is probably not a major option for the future of energy production in the developed nations because most major sites within these nations are either already being exploited or are unavailable for other reasons, such as environmental considerations.
Hydropower produces essentially no carbon dioxide or other harmful emissions, in contrast to burning fossil fuels, and is not a significant contributor to global warming through CO2.
Hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy. Areas with abundant hydroelectric power attract industry. Environmental concerns about the effects of reservoirs may prohibit development of economic hydropower sources.
The chief advantage of hydroelectric dams is their ability to handle seasonal (as well as daily) high peak loads. When the electricity demands drop, the dam simply stores more water (which provides more flow when it releases). Some electricity generators use water dams to store excess energy (often during the night), by using the electricity to pump water up into a basin. Electricity can be generated when demand increases. In practice the utilization of stored water in river dams is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.
Tidal power
Tidal power, sometimes called tidal energy, is a form of hydropower that exploits the rise and fall in sea levels due to the tides, or the movement of water caused by the tidal flow. Because the tidal forces are caused by interaction between the gravity of the Earth, Moon and Sun, tidal power is essentially inexhaustible and classified as a renewable energy source.
Although not yet widely used, tidal power has great potential for future electricity generation and is more predictable than wind energy and solar power.
Tidal power can be classified into two types,
* Tidal stream systems make use of the kinetic energy from the moving water currents to power turbines, in a similar way to underwater wind turbines. This method is gaining in popularity because of the lower ecological impact compared to the second type of system, the barrage.
* Barrages make use of the potential energy from the difference in height (or head) between high and low tides, and their use is better established. These suffer from the dual problems of very high civil infrastructure costs and environmental issues.
Tidal stream power
A relatively new technology tidal stream generators draw energy from currents in much the same way as wind turbines. The higher density of water, some 832 times the density of air, means that a single generator can provide significant power.
Even more so than with wind power, selection of location is critical for a tidal stream power generator. Tidal stream systems need to be located in areas with fast currents where natural flows are concentrated between obstructions, for example at the entrances to bays and rivers, around rocky points, headlands, or between islands or other land masses.
Barrage tidal power
The barrage method of extracting tidal energy involves building a barrage and creating a tidal lagoon. The barrage traps a water level inside a basin. Head (a height of water pressure) is created when the water level outside of the basin or lagoon changes relative to the water level inside. The head is used to drive turbines.
The basic elements of a barrage are caissons, embankments, sluices, turbines and ship locks. Sluices, turbines and ship locks are housed in caisson (very large concrete blocks). Embankments seal a basin where it is not sealed by caissons. The sluice gates applicable to tidal power are the flap gate, vertical rising gate, radial gate and rising sector.
Barrage systems are sometimes affected by problems of high civil infrastructure costs associated with what is in effect a dam being placed across two estuarine systems, and the environmental problems associated with changing a large ecosystem.
Wave power
Wave power refers to the energy of ocean surface waves and the capture of that energy to do useful work - including electricity generation, desalination, and the pumping of water (into reservoirs). Wave power is a form of renewable energy. Though often co-mingled, wave power is distinct from the diurnal flux of tidal power and the steady gyre of ocean currents. Wave power generation is not a widely employed technology, with the world's first commercial wave farm, the Aguçadora Wave Park in Portugal, being established in 2006.
Wave power devices are generally categorized by the method used to capture the energy of the waves. They can also be categorized by location and power take-off system. Method types are point absorber or buoy; surfacing following or attenuator; terminator, lining perpendicular to wave propagation; oscillating water column; and overtopping. Locations are shoreline, nearshore and offshore. Types of power take-off include: hydraulic ram, elastomeric hose pump, pump-to-shore, hydroelectric turbine, air turbine, and linear electrical generator. Some of these designs incorporate parabolic reflectors as a means of increasing the wave energy at the point of capture.
Wave power could yield much more energy than tidal power. The energy potential of waves is certainly greater and wave power can be exploited in many more locations. Countries with large coastlines and strong prevailing winds could produce five percent or more of their electricity from wave power. Excess capacity (a problem common with variable energy sources) could be used to produce hydrogen or smelt aluminum.
Small hydro
Small hydro is the application of hydroelectric power on a commercial scale serving a small community or medium sized industry. During 2005 small hydro installations grew by 8% to raise the total world small hydro capacity to 66 GW. Over 50% of this was in China (with 38.5 GW), followed by Japan (3.5 GW) and the United States (3 GW). China plans to electrify a further 10,000 villages by 2010 under their China Village Electrification Program using renewable energy, including further investments in small hydro and photovoltaics.
Hydroelectric power is the technology of generating electric power from the movement of water through rivers, streams, and tides. Water is fed via a channel to a turbine where it strikes the turbine blades and causes the shaft to rotate. To generate electricity the rotating shaft is connected to a generator which converts the motion of the shaft into electrical energy.
Small hydro is often developed using existing dams or through development of new dams whose primary purpose is river and lake water-level control, or irrigation. A small-scale hydroelectric facility requires a sizeable flow of water and a reasonable height of fall of water, called the head.
A generating capacity of up to 10 megawatts (MW) is becoming generally accepted as the upper limit of what can be termed small hydro. Small hydro can be further subdivided into mini hydro, usually defined as less than 1,000 kW, and micro hydro which is less than 100 kW. Micro hydro is usually the application of hydroelectric power sized for small communities, single families or small enterprise.
Hydroelectric power (Pumped-Storage Plant)
Hydroelectric power now supplies about 715,000 MWe or 19% of world electricity (16% in 2003). Large dams are still being designed. Apart from a few countries with an abundance of it, hydro power is normally applied to peak load demand because it is readily stopped and started. Nevertheless, hydroelectric power is probably not a major option for the future of energy production in the developed nations because most major sites within these nations are either already being exploited or are unavailable for other reasons, such as environmental considerations.
Hydropower produces essentially no carbon dioxide or other harmful emissions, in contrast to burning fossil fuels, and is not a significant contributor to global warming through CO2.
Hydroelectric power can be far less expensive than electricity generated from fossil fuels or nuclear energy. Areas with abundant hydroelectric power attract industry. Environmental concerns about the effects of reservoirs may prohibit development of economic hydropower sources.
The chief advantage of hydroelectric dams is their ability to handle seasonal (as well as daily) high peak loads. When the electricity demands drop, the dam simply stores more water (which provides more flow when it releases). Some electricity generators use water dams to store excess energy (often during the night), by using the electricity to pump water up into a basin. Electricity can be generated when demand increases. In practice the utilization of stored water in river dams is sometimes complicated by demands for irrigation which may occur out of phase with peak electrical demands.
