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.