HYDROELECTRICITY and DAMS

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1 Definition Girotte lake, France Hydroelectricity or hydroelectric energy uses the potential energy of water flows (rivers, waterfalls, ocean currents, etc.). The kinetic energy of the water stream is converted into mechanical energy by a turbine, and then into electrical energy by a generator. Hydropower represents today 16.2% of total electricity production in the world. This is the renewable energy source most used. It is the third general source of electricity generation in the world behind coal (40.6%) and gas (22.2%). Each year about 3600 TWh of hydroelectric energy are produced worldwide In addition, the facilities can be distinguished according to their power. In Europe, the threshold power of 10 megawatts (MW) which delimits the small from the large hydroelectric plant. These two facility categories are complementary. If large dams produce the bulk in hydropower that will be distributed by the national transportation grid, small power plants, highly decentralized, are closer to consumers. They thus avoid the transport of electricity over long distances, this helps to limit losses on the grid. Categories of power plants A hydroelectric power station consists of a water retention (taken "run-of-river" or created thanks to a dam) as well as a production facility. There is a wide diversity of hydropower plants: 1) The converted gravity power plants take advantage of the flow of water along a slope of the ground. They consist of three main elements: A dam more or less important: its role is, first, to create a waterfall to turn the turbines, on the other hand, to store water to supply the powerhouse; A bypass channel: it withdraws water from its natural environment to supply the dam's reservoir. It can be an existing open channel, an underground gallery or a conduct; A powerhouse: it includes turbines, which rotate thanks to the waterfall and drive the electricity generator. Operating principle of a gravity plant (see the movie functioning of a dam ) They can be classified into three different operating modes in order to meet the electricity consumption fluctuations This ranking is based on the hydroelectric storage capacity, which is the theoretical time it would take to empty the reserve with turbining at full power. 1

2 1.1) The run-of-river plants have a hydroelectric storage capacity generally lower than 2 hours. Mainly installed in lowland areas, they have low-height water retentions. They use the river flow as it stands, with no significant capacity of storage modulation. They provide a very inexpensive basic energy. They are typical of the facilities made on major rivers. 1.2) The sluiceway power plants have a hydroelectric storage capacity comprised between 2 and 200 hours. They present some largest lakes, allowing them a modulation in the day or the week. Their management allows tracking the variation of consumption in these time periods (consumption peaks in the morning and evening, difference between business days and weekends...). They are typical of the facilities made in the uplands (Middle Mountain) Francis turbine 1.3) The lake power plants (or reservoir power plants) have a hydroelectric storage capacity higher at 200 hours. They correspond to the engineering structures with the most important reservoirs. These allow a water seasonal storage and modulation of production to erase the peak consumption as in winter during very cold days. These power plants are typical of facilities done in middle and high mountains. New words Penstock: Headwater level: Tailwater level : Sluice gates: Silt: Tailrace: Surge tank: Gross head: Outlet: Intake: Turbine runner Wicket gate Scroll Bucket conduct upstream level downstream level canal lock doors, gates commonly used to control water levels and flow rates. slime, mud water channel below a dam Expansion chamber (or head of water) Vertical distance between the headwater and the tailwater. exit, output input, inlet, admission turbine rotor water inlet slot volute, swirl like a spoon 2

3 2) The pumped-storage power plants (PSP), in addition to produce energy from the natural flow, have a pumping mode. Electricity cannot be stored; any surplus of supply relative to demand is lost. In contrast, water can be stored in retention basins or reservoir lakes; this property is used by these hydro power plants. They are therefore particularly useful for balancing the transport and distribution grid. These power plants have two basins, an upper basin and a lower basin between which is placed a reversible hydroelectric machine: The hydraulic part can work as well like a pump or like a turbine and the electrical part both like an engine and like an alternator (synchronous machine). In accumulation mode, the machine converts electricity produced by other power plants to raise the water from the lower basin to the upper basin. In production mode, the machine converts gravitational potential energy of water into electricity. The yield between the energy produced and the energy consumed is in the range of 70% to 80%. The interest of PSP also depends on the energy mix of local electricity generation. If electricity production is mainly provided by energy sources not enough flexible and adjustable (nuclear, hydroelectric run-of-water), the PSP are a great way to use amounts of electricity that could be lost. When there is no accessible fresh water, it is possible to use some sea water to operate the PSP. (See seawater pumped-storage hydro system) Operating principle of a Pumped-Storage Power plant (see the movie Pumped Storage Station ) Revin PSP, Ardennes, France Seawater PSP, Okinawa Island 3

4 3 Tidal Power: Tides are caused by the gravitational pull of the moon and sun, and the rotation of the Earth. Near shore, water levels can vary up to 12 metres due to tides. Tidal power is more predictable than wind energy and solar power. A large enough tidal range (3 metres) is needed to produce tidal energy economically. Tidal barrages A tidal power plant, at the mouth of a river is composed of a dam erected across the opening of a tidal basin. Sluice gates, on the barrage, allow filling the basin with the arrival of high tide and its emptying with the reflux of ebb tide. This is a bidirectional system that generates electricity on both the incoming and outgoing tides through the bulb turbine system To further increase the operating time of the plant, the bulb groups were designed to be used as pump. Thus, when the sea is close to the basin level, the filling of the latter is accelerated by pumping. Operating principle of a tidal power plant The Rance dam, France The Rance station, in France, generates 240 megawatts of power. A potential disadvantage of tidal power is the effect a tidal station can have on plants and animals in the estuaries. Tidal barrages can change the tidal level in the basin and increase turbidity (the amount of matter in suspension in the water). They can also affect navigation and recreation. The tidal power plants at large use the energy of motion of the sea, whether it is the alternation of flow of the tides, permanent marine currents or movement of the waves. (See Marine hydropower) 4

5 4) Turbines (see the movie comparison between Pelton, Francis and Kaplan turbines ) The power plants are equipped with turbines which convert the energy of the water flow in a mechanical rotation to actuate alternators. The type of turbine used depends on the height of the waterfall: According to the flow rate and velocity of the water, the turbine will be different. For low water heights with high flow rates (e.g. an alluvial plain river), we will use vertical-axis turbines (type Kaplan or Francis) but, also horizontal-axis bulb turbines. For the waterfalls of large heights with low flow (cascade or torrent deflected in penstocks), the horizontal-axis turbines (type Pelton or Francis) give the best results. For lowest falls (1-30 metres) and Bulb (Tidal dams) tidal barrages, the bulb turbine is privileged. The bulb group is a Kaplan type turbine, which consists of an axial turbine directly driving an alternator operating inside a bulbous housing All the parts of the bulb group are immersed in a gallery. The propeller blades are orientable. This allows adjusting the power of the turbine but also the functioning in both directions. Kaplan (Low falls) For low falls (5-50 metres) and high flow rates, the Kaplan turbine is privileged: the blades are orientable, which allows adjusting the power of the turbine to the drop height maintaining a good yield. The Francis turbine is used for Francis (Average falls) average falls (40 to 600 metres) and a mean throughput. The water, which is coming from the penstock, runs through the inlet scroll and finds itself equally distributed all around the wicket gates by cyclonic effect. These adjustable wicket gates control the water amount passing through the turbine, before reaching the runner. This has the effect to control the power output of the dam. The Francis turbine can also be used as water pump. This makes it ideal for pumped storage systems. Pelton (High falls) The Pelton turbine is suitable for high falls (200 to 1800 metres) and low flow. It receives water under very high pressure through an injector (dynamic water impact on the bucket). 5

6 5) Dam design: materials and shapes The most common are the earth embankment dams. Concrete or masonry dams have shapes that depend on the structure and the width of the valley. Gravity dams: Their own mass is sufficient to resist the pressure exerted by the water. They may be in masonry or concrete. They are relatively thick and their shape is usually simple (in angled triangle). The concrete gravity dam is chosen when the rock of the chosen site is enough resistant to support such structure (wide valley with an own bedrock). Arch dams: We can build less heavy dams provided they are in the form of vault. Indeed, if the dam is straight, the water force will end up destroying it. Thanks to the vault shape, the water force (which is channeled sideways) is applied outwardly of the dam, in the mountain. The technique of arch dam requires a rather narrow valley. Buttress Dams (or multi-arches): This type of dam is used when the supports are too remote or when the local material is so compact that extraction is almost impossible. These dams are thinner than gravity dams and have a much lower mass. They can however withstand the force of the water through a series of parallel walls, often triangular shaped more or less spaced that support them. Rockfill or embankment dams: Made from an earth or rock backfill, these dams rely on their intense weight and sloped shape to retain water. There may be an impervious layer of concrete, plastic or other material on the upstream face if the particle sizes in the earth are big enough for water to seep through. Earth-filled dams can be made completely from one type of material, but may need a layer that collects and drains seepwater to ensure the structure stay intact. 6

7 Zadorra river, Spain Gravity dam Photo Credit Girotte dam Revin dam La Rance dam Zadorra dam Monteynard dam Granval dam Assouan dam John Day dam Beaufortain Tourisme Le monde.fr EDF.fr.Hypothese.be.Hypothese.be cc-paysdesaintflour.fr French.memphistour.com ecologismos.com Monteynard, France Arch dam Granval, France Buttress dam Assouan, Egypt 7 Embankment dam

8 Pros and cons Benefits of dams Dams provide a range of economic, environmental, and social benefits, including hydroelectric power, recreation, water supply, flood control, waste management and wildlife habitat. Electrical Generation: Dams produce renewable electricity. Hydropower is considered clean because it does not contribute to global warming, air pollution, acid rain, or ozone depletion. Recreation: An artificial lake increases the tourism and provides prime recreational facilities: Boating, skiing, camping, picnic areas. Irrigation: 18% of world's arable land is irrigated with water stored behind dams. They produce 40% of the harvests and employ 30% of the population dispersed in rural areas. Flood Control: In addition to helping farmers, dams help prevent loss of life and damage caused by flooding. The dams store floodwaters and then reject them under control, downstream of the river or divert them to other uses. Debris Control: Dams provide enhanced environmental protection, such as the intercept of floating tree trunks, hazardous materials and detrimental sedimentation. Disadvantages: The construction of a dam to create a reservoir may obstruct migration of fish to their upstream spawning areas, change the water temperature, change the natural environment and produce greenhouse gas emissions. Impact on fish: Hydroelectric turbines kill and injure most of the fish that pass through. The construction of fish ladders, near the run-of-river dams, can allow to a certain number of fish achieve their spawning grounds The fish ladders (or fishways, fish pass) could reduce fish deaths to less than 2%, in comparison to fish kills of 5 to 10% for the best existing turbines. Rise of water temperature: The retention basin and operation of Fish ladder the dam can also change the natural water temperatures, its chemistry, its flow characteristics, and silt loads, all of which can lead to significant changes in the ecology (living organisms and environment). Changes Natural Habitats: The dam creation can change landforms of the river upstream and downstream. These changes may have negative impacts on native plants and animals in and next to the river, and in the deltas that form where rivers empty into the ocean. The reservoir-lakes may cover important natural areas, agricultural land, and archeological sites, and cause the relocation of people. Greenhouse gases: Greenhouse gases, carbon dioxide and methane, may also form in reservoirs and be emitted to the atmosphere. The exact amount of greenhouse gases produced from hydropower plant reservoirs is uncertain. The emissions from reservoirs in tropical and temperate regions may be equal to or greater than the greenhouse effect of the carbon dioxide emissions from an equivalent amount of electricity generated with fossil fuels. [Return to Renewable energies ] John Day dam, Columbia River, USA 8