Hydro-electric
power plant
Working principle:
Hydro-electric
power plant utilizes the potential energy of water stored in a dam built across
the river. The potential energy of the water is used to run water turbine to
which the electric generator is coupled. The mechanical energy available at the
shaft of the turbine is converted into electrical energy by means of the
generator.
General
arrangement of a hydro-electric power plant:
Image below
shows the schematic representation of the hydro-electric power plant.
Water
reservoir:
Continuous
availability of water is the basic necessity for a hydro-electric plant. Water
collected from catchment area during rainy season is stored in the reservoir.
Water surface in the storage reservoir is known as head race.
Dam:
The
function of a dam is to increase the height of water level behind it, which
ultimately increases the reservoir capacity. The dam also helps to increase the
working head of the power plant.
Spillway:
Water
in the dam after a certain level in the reservoir overflows through spillway
without allowing the increase in water level in the reservoir during rainy
season.
Pressure
tunnel:
It
carries water from the reservoir to surge tank.
Penstock:
Water
from surge tank is taken to the turbine by means of pen stocks, made up of
reinforced concrete pipe or steel.
Surge
tank:
There
is sudden increase of pressure in the penstock due to sudden backflow of water,
as load on the turbine is reduced. The sudden rise of pressure in the penstock
is known as water hammer. The surge tank is introduced between the dam and the
power house to keep in reducing the sudden rise of pressure in the penstock.
Otherwise penstock will be damaged by the water hammer.
Water
turbine:
Water
through the penstock enters into the turbine through an inlet valve. Prime
motors which are in common use are pelton turbine, francisturbine and kalpan turbine.
The potential energy of water entering the turbine is converted into mechanical
energy. The mechanical energy available at the turbine shaft is used to run the
electric generator. The water is then discharged through the draft tube.
Draft
tube:
It
is connected to the outlet of the turbine. It allows the turbine to be placed
over tail race level.
Tail
race:
Tail
race is a water way to lead the water discharged from the turbine to the river.
The water held in the tail race is called tail race water level.
Step-up
transformer:
Its
function is to rasie the voltage generated
at the generator terminal before transmitting the power consumers.
Power
house:
The
power house accommodates the turbine, generator, transformerand
control room.
Classification
of hydro-power plants
Hydro-plants
are classified according to the head of water under which they work.
When
the operating head of water exceeds 70 meters, the plant is known as “high head
power plant”. Peloton turbine is used as prime mover in such power plants.
When
the head of water range is from 15 to 70 meters then the power plant is known
as “medium head plant”. It uses francis turbine.
When
the head is less than 15 meters the plant is named as “low head plant”. It uses francis or Kaplan turbine as prime mover.
Advantages
of hydro-electric power plants
1. Water is a renewable source of energy.
Water which is the operating fluid, is neither consumed or converted into
something else..
2. Water is the cheapest source of energy
because it exists as a free gift of nature. The fuels needed for thermal,
diesel and nuclear plants are exhaustible and expensive.
3. There are no ash disposable problems as in
case of thermal power plant.
4. Hydro-plant does not pose the problem of
air pollution as in the case of thermal plant or radiation hazards as in the
case of nuclear plant.
5. Variable loads do not affect the
efficiency in the case of a hydro-plant.
6. Life of hydro-plant is very long (1 or 2
centuries) compared with thermal plant ( 3 to
4 decades). This is because the hydro-plants operate at atmospheric
temperature, whereas thermal plants operate at very high temperature (about 500
to 800’c).
7. Hydro plants provide additional benefits
like irrigation, flood control, fishery and recreation.
8. The water storage of hydro-plant can also
be used for domestic water supply.
9. Auxiliaries needed for the hydro-plant are
less compared to thermal plant of equal capacity.
10. It requires less supervising staff.
11. Maintenance cost is low.
Disadvantages
of hydro-electric power plant:
1. Hydro-plants are generally situated away
from the load centres. Hence long transmission lines are required for delivery
of power. This increases the cost of transmission lines and also transmission
losses. But a thermal plant can be located near the load centre, thereby the
transmission cost and transmission losses are considerably reduced.
2. The power produced by hydro-plant depends
upon the quantity of water which in turn is dependent upon the rainfall. The
dry year affects the hydro power generation considerably.
3. Initial cost of the plant is high.
4. Erection of hydro-plant (construction of dam) usually
takes a long period of time.