|Ideas for Action : Save, Recycle and Do Not Pollute (International Institute for Rural Reconstruction (IIRR), 1992)|
What is water power?
Waterpower or hydropower is the energy contained in moving water as it falls downhill towards the sea. This energy can be traced to the water cycle lowered by the sun, which causes water to evaporate from lakes and oceans, carries it as clouds over land masses and deposits it as rains.
The pulling force of gravity gives water above sea level stored or potential energy. As the water rushes downhill towards the sea, this potential energy is converted into moving or kinetic energy. The tremendous power of moving water pushes against anything in its path, displacing huge boulders and tree trunks and, over the course of centuries, carving valleys between mountains. This power can be harnessed to do useful work.
Moving water can be used to spin a wheel or turbine by falling from an intake or head pond to a lower reservoir or tail water. The resulting movement can either provide direct mechanical energy or drive an electrical generator. The vertical height between the upper reservoir and the tailwater is known as the head and can be used with water flow rate to calculate the expected power output.
Parts of a Hydropower Installation
There are three elements in a hydroelectric scheme:
· the dam;
· the penstock or channel down which the water flows to the hydropower machine; and,
· the hydropower machine.
In any hydropower scheme, water must be diverted from a river or stream to the hydropower device. One way to do this is to completely dam the stream and feed the water to the hydropower device via a penstock. This is called a regulated system. Building a dam across a large river is a major construction job. Another way, called a run-of-the-river method, is to divert only part of the stream with a small dam or weir, conveying the water to the hydropower machine by flumes, canals and/or penstocks. This method is often the best choice for small-scale hydro installations.
Dams fulfill several functions:
1. By storing power behind them, they ensure the regular flow of water to the turbine or waterwheel even during times when the flow of water is low. Stored water in a dam is stored (potential) energy. Stored water in a dam can also be used for fish farming, irrigation and recreational purposes.
2. Dams raise the head of a stream, thereby increasing the easily obtained power. In a small stream, the head can be raised several meters by choosing an appropriate site for building a dam.
3. Dams enable easy diversion of water to the turbine or waterwheel. In a run- of-the-river system, a partial dam simply diverts some of the flow into a canal (called a headrace) or flume which carries it to the hydropower device. In a regulated installation, the water flow is completely blocked and fed directly into the hydropower machine located below the dam.
Depending upon the size of the hydropower installation, dams are built of concrete, stone, wood or even earth. Partial dams are usually easier to build than full dams. Because a dam burst will cause considerable damage downstream, structures called spillways are always part of the scheme, allowing excess water to escape from the dam. The smaller the dam, the safer it is and the easier and cheaper it is to build.
Flumes, headrace canals and penstocks
Every hydropower installation requires some means of conveying water from the dammed stream to the hydropower device. The water should be conveyed with as little head loss and expense as possible.
Headrace canals and flumes are open channels built with very slight gradient so that little energy is lost from the water as it travels through them. Penstocks are pipes used to carry water from the dam or forebay to turbines.
Long penstocks are quite expensive. Often, the penstock is anchored into the ground to prevent it from shifting position. Water rubbing against the inside of the pipe and against bends in the penstock cause head loss which reduces available power.
Control and screening of water flow
Water flowing into a hydropower installation should be controlled to prevent damage to the hydropower device during floods and to allow for occasional maintenance. Penstocks have control valves and/or gates in the forebay which can limit flow. Waterwheel installations have sluice gates (wood or metal panels which can be closed or opened) for controlling water flow.
Water entering hydropower installations must be free from foreign matters, such as leaves and sand.
Trashracks (grates or screens which prevent the passage of solids) are used in turbine and waterwheel installations. Trashracks must be cleaned on a regular basis to prevent materials from clogging the screen or grates.
Types of hydropower devices
1. Waterwheels are large, slow-spinning wheels with attached paddles or buckets which are turned by the force of moying water. They are mainly used for mechanical work such as grinding.
2. Turbines have fast-spinning blades (called runners) that are turned by the pressure or impact forces of moving water. The most common application of turbines is electrical generation, though they can be used for mechanical work.
3. Hydraulic rams use the momentum of water moving through an inclined drive pipe to push part of that water uphill through a delivery pipe to a place where it is required.
Large and small hydropower schemes
Developments in hydropower are taking place at the two extremes of project size.
Huge dams can make a substantial contribution to economic development. On the face of it, hydroelectricity is cheap, renewable and nonpolluting and, thus, is seen es a major source of energy. (In 1987, almost 20 percept of the world's electricity was supplied by hydropower.) Dams are also seen es haying an important pert to play in the battle against world hunger, by providing water for irrigation projects. Large dams also provide water storage and flood control. But they are not unreservedly good:
· Reservoirs inundate forests, farmland and wildlife habitats
and uproot entire communities of indigenous peoples. If China proceeds with its
Three Gorges project -- the world's largest at 13,000 megawatts -several million
people will be displaced.
· Impounding a river severely disrupts the surrounding ecosystems. Dams permanently change the flow of rivers and streams. They cause the water table to be raised upstream and lowered downstream, the altered flow and lowered downstream and the altered flow of water affect the downstream inhabitants and ecosystems. A dam traps silt and, thus, valuable nutrients are neither deposited on floodplain farmland nor provide food for downstream fish. Before the Aswan Dam was built, sardine catches in the eastern Mediterranean totalled 18,000 tonnes a year: by 1969, the catch was down to 500 tonnes a year. The 100 million tonnes of sediments deposited on farmland fell to just a few tonnes and to compensate for the loss, Egypt must apply artificial fertilizer at a cost of about $100 million a year.
· Reservoirs in tropical environments expand the breeding grounds for the carriers of malaria, bilharzia (schistosomiasis) and river blindness. In Ghana, before the Volta Dam was built, the rate of infection for bilharzia was 2 percent; now, it is 80 percent.
· The reservoirs behind many large dams, especially those downstream from deforested watersheds, have silted up much faster than anticipated. This shortens the working life of the projects, sometimes by decades.
Micro-hydroelectric power: its strengths and weaknesses
· Micro-hydro power provides a renewable, non-polluting
· Micro-hydro power can meet the needs of many small industrial processes, such as the milling of flour. rice hulling, coffee processing, sugar cane crushing, sawmills, bakeries and other small workshops.
· Micro-hydro power creates employment through encouraging the creation of small-scale industries.
· A low-cost alternative to diesel, micro-hydro power can serve areas where national electric supplies cannot reach.
· Most of the needed equipment, such as turbines, can be manufactured locally.
· Micro-hydro power provides power for domestic use, such as lighting and cooking, thereby reducing the burden on dwindling forest cover.
· The initial cost for installing a micro-hydro plant may be
too expensive for a community.
· A wide range of technical skills is required to survey, design, manufacture and install micro-hydro systems and also, where applicable, to identity and adapt equipment for industrial activities that can be powered by the plant. In many developing countries, the skills required for such a project are not generally available at a local level.
· Micro-hydro power requires land areas of adequate annual rainfall and hilly terrain to work efficiently.
The Earth Report: The Essential Guide to Global Ecological Issues ed. Edward Goldsmith and Nicholas Hildyard, Mitchell Beazley Publishers, London, 1988.
Ideas for Act/on:
A Technology Information Kit, November 23 - 28, 1992