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
Water in the nature
Parts of a Hydropower Installation
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
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
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
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
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
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
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:
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 energy source.
· 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
· 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
· 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
The Earth Report: The Essential Guide to Global
Ecological Issues ed. Edward Goldsmith and Nicholas Hildyard, Mitchell Beazley
Publishers, London, 1988.
Ideas for Action:
A Technology Information Kit, November 23 - 28,