Hydro Electric Power Station

Generation of Electrical Energy

The conversion of energy available in different forms in nature into electrical energy is known as generation of electrical energy. Energy available in various forms from different natural sources such as pressure head of water, chemical energy of fuels, nuclear energy of radioactive substances etc. All the forms of energy can be converted into electrical energy by the use of an alternator coupled to a prime mover. The prime mover is driven by the energy obtained from various sources such as burning of fuel, pressure of water, force of wind ,etc.

For example: Chemical energy of a fuel can be used to produce steam at high temperature and pressure. The steam is fed to a prime mover which may be a steam engine or a steam turbine. The turbine converts heat energy of steam into mechanical energy which is further converted into electrical energy by the alternator.

Sources of Energy

There are various sources of energy produced from energy available in various forms in nature. Due to the number of limitations The energy due to sun and wind is not utilized on large scale due to a number of limitations. But at present context ,water, fuels and nuclear energy are primarily used for the generation of electrical energy.

The sun
The wind
water
Fuels
Nuclear energy

Generating Station

Generating stations is the power stations where bulk electric power is produced .A generating station essentially employs a prime mover coupled to an alternator for the production of electric power. The prime mover converts energy from some other form into mechanical energy. The alternator converts mechanical energy of the prime mover into electrical energy. The electrical energy produced by the generating station is transmitted and distributed with the help of conductors to various consumers.

Hydro-Electric Station

A generating station which utilizes the potential energy of water at a high level for the generation of electrical energy is known as a hydro-electric power station. Hydro electric power stations are generally located in hilly area where dams can be built conveniently and large water reservoirs can be obtained. In a hydro-electric power station, water head is created by constructing dam across a river or lake. From the dam, water is led to a water turbine. The water turbine captures the energy in the falling water and changes the hydraulic energy into mechanical energy at the turbine shaft. The turbine drives the alternator which converts mechanical energy into electrical energy. Hydro-electric power stations are becoming very popular because the reserves of fuels are depleting day by day.

Schematic Arrangement of Hydro-electric power station

Hydro-electric power station simply involves the conversion of hydraulic energy into electrical energy. The dam is constructed across a river or lake and water from the catchment area collects at the back of the dam to form a reservoir. A pressure tunnel is taken off from the reservoir and water brought to the valve house at the start of the penstock. The value house contains main sluice valves and automatic isolating valves. The former controls the water flow to the power house and the latter cuts off supply of water when the penstock bursts. From the valve house, water is taken to water turbine through a huge steel pipe known as penstock. The water turbine converts hydraulic energy into mechanical energy. The turbine drives the alternator which converts mechanical energy into electrical energy. A surge tank is built just before the valve house and protects the penstock from bursting in case the turbine gates suddenly close due to electrical load being thrown off. When the gate close, there is a sudden stopping of water at the lower end of the penstock and consequently the penstock can burst like a paper log. The surge tank absorbs this pressure swing by increase in its level of water.

water turbines: Water turbines are used to convert the energy of falling water into mechanical energy. The pricniple of water turbines are:

Impulses turbines
Reaction turbines

Impulses turbines:

Impulses turbine are basically used for high heads. In an impulses turbine, the entire pressure of water is converted into kinetic energy in a nozzle and the velocity of the jet drives the wheel. Pelton wheel is the impulse turbine. It consists of a wheel fitted with elliptical buckets along its periphery. The force of water jet striking the buckets on the wheel drives the turbine. The quantity of water jet falling on the turbine is controlled by means of a needle or spear placed in the tip of the nozzle. The movement of the needle is controlled by the governor. If the load on the turbine decreases, the governor pushes the needle into the nozzle, thereby reducing the quantity of water striking the buckets .Reverse action takes place if the load on the turbine increases.

Reaction turbines:

Reaction turbines are used for low and medium heads. In a reaction turbine water enters the runner partly with pressure energy and partly with velocity head. There are two types of reaction turbine:

Francis turbines
Kaplan turbines

A Francis turbine is used for low to medium heads. It consists of an outer ring of stationary guide blades fixed to the turbine casing and an inner ring of rotating blades forming the runner. The guide blades control the flow of water to the turbine. water flows radially inwards and changes to a downward direction while passing through the runner. As the water passes over the “rotating blades” of the runner, both pressure and velocity of water are reduced. This causes a reaction force which drives the turbine.

A Kaplan turbine is used for low heads and large quantities of water. It is similar to Francis turbine except that the number of Kaplan turbine receives water axially. Water flows radially inwards through regulating gates all around the sides, changing direction in the runner to axial flow. This causes a reaction force which drives the turbine.

Selection of site for Hydroelectric power plants

For selecting the suitable site for a hydroelectric power plant, lakes at high altitudes and with large catchment areas is more economical. There are various essential characteristics of a good site:

Large catchment area
High average rainfall
steep gradients
Favorable place for constructing the storage or reservoir and
Also geological, geographical and meteorological conditions of sites.

The following are the factors for selection of site for hydroelectric power plants:

1.Availability of water: since in hydroelectric power plants potential energy of waterfall or kinetic energy of flowing stream is utilized for generation of electric power so such stations should be built where there is adequate water available at good head or huge quantity of water if flowing across a given point.

2.Water storage: Since storage of water in a suitable reservoir at a height or buildings of dam across the river is essential in order to have continuous and perennial supply during the dry seasons therefore convenient accommodation for the erection of a dam or reservoir must be available.

3.Water Head: The available water head depends upon the topography of the area. Increase in effective heads reduces the quantity of water to be stored and handled by penstocks, screens and turbines and therefore capital cost of the plant is reduced.

4.Distance from the load centers: Hydroelectric plant is located far away from the load center .For economical transmission of electric power, the routes and the distances need active considerations.

5.Accessibility of the site: Adequate transportation facilities must be available or there should be possibility of providing the same so that the necessary equipment and chinery could be easily transported.

6.Water pollution: Polluted water may cause excessive corrosion and damage to the metallic structures. Hence the availability of good quantity of water is essential.

7.Sedimentation:Gradual deposition of slit may reduce the capacity of the storage reservoir and may also cause damage to the turbine blades. Silting from forest areas is negligible but the regions subject to violent storms and not protected by vegetation contribute lot of slit to the run-off.

8.Large catchment areas: The reservoir must have a large catchments area so that level of water in the reservoir may not fall below the minimum required in dry seasons.

9.Availability of Land: The land available should be cheap in cost and rocky in order to withstand the weight of the large buildings and heavy machinery.

10.There should be possibility of stream diversion during period of construction.

Merits and demerits of hydroelectric power plants

Advantages

No fuel is required by such plants as water is the source of energy.
The plant is highly reliable and it is cheapest in operation and maintenance.
The plant can be run up and synchronized in a few minutes.
Very accurate governing is possible with water turbines so such plants have constant speed and hence constant frequency.
The load can be varied quickly and the rapidly changing load demands can be met without any difficulty.
There are no standby losses in such plants.
Has longer life(50 years)
Efficiency doesn’t fall with the age.
It is very neat and clean plant because no smoke or ash is produced.

Disadvantages

It requires large area.
Its construction cost is enormously high and takes a long time for erection.
Long transmission lines are required as the plants are located in hilly areas which are quite away from the load center.
The output of such plants is never constant owing to vagaries of monsoon and their dependence on the rate of water flow in the river. Dry season affect the power supply.
The firm capacity of hydroelectric plants is low and so backup by steam plants is essential.