Table of Contents

Hydroelectric power plant

Working principle

Hydroelectric power plant (Hydel plant) utilizes the potential energy of water stored in a dam built across the river. The potential energy of the stored water is converted into kinetic energy by first passing it through the penstock pipe. The kinetic energy of the water is then converted into mechanical energy in a water turbine. The turbine is coupled to the electric generator. The mechanical energy available at the shaft of the turbine is converted into electrical energy by means of the generator.
Because gravity provides the force which makes the waterfall, the energy stored in the water is called gravitational potential energy.

Layout of Hydroelectric power plant

Fig. shows the schematic representation of a Hydroelectric power plant.

The main components are
• Water reservoir
• Dam
• Spillway
• Gate
• Pressure tunnel
• Surge tank
• Penstock
• Water turbine
• Draft tube
• Tail race level
• Powerhouse

hydro electric power plant diagram — hydroelectric power plant diagram

Water reservoir:

In a reservoir the water collected from the catchment area during the rainy season is stored behind a dam. Catchment area gets its water from rains and streams. Continuous availability of water is a basic necessity for a hydroelectric power plant. The level of the water surface in the reservoir is called the Headwater level. The water head available for power generation depends on the reservoir height.

Dam:

the purpose of the dam is to store the water and to regulate the out going flow of water. The dam helps to store all the incoming water. It also helps to increase the head of the water. In order to generate a required quantity of power, it is necessary that a sufficient head is available.

Spillway:

Excess accumulation of water endangers the stability of dam construction. Also in order to avoid the overflow of water out of the dam especially during rainy seasons spillways are provided. This prevents the rise of the water level in the dam. Spillways are passages that allow the excess water to flow to a different storage area away from the dam.

Gate:

A gate is used to regulate or control the flow of water from the dam.

Pressure tunnel:

It is a passage that carries water from the reservoir to the surge tank.

Surge tank:

A surge tank is a small reservoir or tank in which the water level rises or falls due to sudden changes in pressure. There may a sudden increase of pressure in the penstock pipe due to sudden backflow of water, as the load on the turbine is reduced. This sudden rise of pressure in the penstock pipe is known as water hammer.

Penstock:

Penstock pipe is used to bring water from the dam to the hydraulic turbine. Penstock pipes are made up of steel or reinforced concrete. The turbine is installed at a lower level from the dam. Penstock is provided with a gate valve at the inlet to completely close the water supply.
It has a control valve to control the water flow rate into the turbine. Water turbine or hydraulic turbine (Prime mover): The hydraulic turbine converts the energy of water into mechanical energy. The mechanical energy (rotation) available on the turbine shaft is coupled to the shaft of an electric generator and electricity is produced. The water after performing the work on the turbine blade is discharged through the draft tube.

The prime movers which are in common use are Pelton wheel, Kaplan turbine, Francis turbine.

Draft tube:

Draft tube is connected to the outlet of the turbine. It converts the kinetic energy available in the water into pressure energy in the diverging portion. Thus, it maintains a pressure of just above the atmospheric at the end of the draft tube to move the water into a tailrace. Water from the tailrace is released for irrigation purposes.

Tailrace level:

Tailrace is a water path to lead the water discharged from the turbine to the river or canal. The water held in the tailrace is called the Tailrace water level.

Power House:

The powerhouse accommodates the water turbine, generator, transformer, and control room. As the water rushes through the turbine, it spins the turbine shaft, which is coupled to the electric generator. The generator has a rotating electromagnet called a rotor and a stationary part called a stator. The rotor creates a magnetic field that produces an electric charge in the stator. The charge is transmitted as electricity. The step-up transformer increases the voltage of the current coming from the stator. The electricity is distributed through power lines.

Classification of Hydroelectric power plant

Hydroelectric power plants are usually classified according to the available of head of water
• High head power plants
• Medium head power plants
• Low head power plants

High head power plants: When the operating head of water exceeds 70 meters, the plant is known as High head power plant. Pelton wheel turbine is the prime mover used.

Medium head power plants: When the water ranges from 15 to 70 meters, then the power plant is known as a Medium head power plant. It uses Francis Turbine.

Low head power plants: When the head is less than 15 meters, the plant is named as Low head power plant. It uses Francis or Kaplan turbine as the prime mover.

Advantages of hydroelectric power plant

1. The water source is perennially available. No fuel is required to be burnt to generate electricity. It is aptly termed as ‘the white coal’. Water passes through turbines to produce work and downstream its utility remains undiminished for irrigation of farms and quenching the thirst of people in the vicinity.
2. The running costs of hydropower installations are very low as compared to thermal or nuclear power stations. In thermal stations, besides the cost of fuel, one has to take into account the transportation cost of the fuel also.
3. There is no problem with regard to the disposal of ash as in a thermal station. The problem of emission of polluting gases and particulates to the atmosphere also does not exist. Hydropower does not produce any greenhouse effect, cause the pernicious acid rain and emit obnoxious NO.
4. The hydraulic turbine can be switched on and off in a very short time. In a thermal or nuclear power plant the steam turbine is put on turning gear for about two days during start-up and shut-down.
5. The hydraulic power plant is relatively simple in concept and self-contained in operation. Its system reliability is much greater than that of other power plants.
6. Modern hydropower equipment has a greater life expectancy and can easily last 50 years or more. This can be compared with an effective life of about 30 years of a thermal or nuclear station.

7. Due to its great ease of taking up and throwing off the load, hydropower can be used as the ideal spinning reserve in a system mix of thermal, hydro, and nuclear power stations.
8. Modern hydro-generators give high efficiency over a considerable range of load. This helps in improving the system efficiency.
9. Hydro-plants provide ancillary benefits like irrigation, flood control, afforestation, navigation, and aqua-culture.
10. Being simple in design and operation, the hydro-plants do not require highly skilled workers. Manpower requirement is also low.

Disadvantages of Water Power

1. Hydro-power projects are capital-intensive with a low rate of return. The annual interest of this capital cost is a large part of the annual cost of hydropower installations.
2. The gestation period of hydro projects is quite large. The gap between the foundation and completion of a project may extend from ten to fifteen years.
3. Power generation is dependent on the quantity of water available, which may vary from season to season and year to year. If the rainfall is in time and adequate, then only the satisfactory operation of the plant can be expected.
4. Such plants are often far away from the load center and require long transmission lines to deliver power. Thus the cost of transmission lines and losses in them are more.
5. Large hydro-plants disturb the ecology of the area, by way of deforestation, destroying vegetation and uprooting people. Strong public opinion against. The erection of such plants is a deterrent factor. The emphasis is now more on small, mini and micro hydel stations.