Generation of Electrical Energy
The conversion of energy available in different forms in nature into electrical energy is known as the generation of electrical energy. Energy is available in various forms from different natural sources, such as the pressure of water, the chemical energy of fuels, the nuclear energy of radioactive substances, etc. All forms of energy can be converted into electrical energy by using an alternator coupled to a prime mover. The prime mover is driven by the energy obtained from various sources such as the burning of fuel, the pressure of water, the force of wind, etc.
For example, The chemical energy of a fuel can be used to produce steam at high temperatures and pressure. The steam is fed to a prime mover which may be a steam engine or a steam turbine. The turbine converts the 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 a large scale due to a number of limitations. But in the present context, water, fuels, and nuclear energy are primarily used for the generation of electrical energy.
- The sun
- The wind
- Nuclear energy
Generating stations are the power stations where bulk electric power is produced. A generating station essentially employs a prime mover coupled with an alternator for the production of electric power. The prime mover converts energy from some other form into mechanical energy. The alternator converts the 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.
A generating station that utilizes the potential energy of water at a high level for the generation of electrical energy is known as a hydroelectric power station. Hydroelectric power stations are generally located in hilly areas where dams can be built conveniently and large water reservoirs can be obtained. In a hydroelectric power station, a water head is created by constructing a 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. Hydroelectric power stations are becoming very popular because the reserves of fuel are depleting day by day.
Schematic Arrangement of Hydroelectric Power Station
Hydroelectric power stations simply involve 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 is 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 powerhouse, and the latter cut off the supply of water when the penstock bursts. From the valve house, water is taken to the water turbine through a huge steel pipe known as a 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 an electrical load being thrown off. When the gate closes, there is a sudden stoppage 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 increasing its level of water.
Water turbines are used to convert the energy of falling water into mechanical energy. The principles of water turbines are:
- Impulses turbines
- Reaction turbines
Impulse turbines are basically used for high heads. In an impulse turbine, the entire pressure of water is converted into kinetic energy in a nozzle, and the velocity of the jet drives the wheel. The Pelton wheel is the impulse turbine. It consists of a wheel fitted with elliptical buckets along its periphery. The force of the 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 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 turbines:
- 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 the pressure and velocity of water are reduced. This causes a reaction force that drives the turbine.
A Kaplan turbine is used for low heads and large quantities of water. It is similar to the 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 that drives the turbine.
Selection of site for Hydroelectric power plants
For selecting a suitable site for a hydroelectric power plant, lakes at high altitudes and with large catchment areas are more economical. There are various essential characteristics of a good site:
- Large catchment area
- High average rainfall
- steep gradients
- A favorable place for constructing the storage or reservoir and
- Also geological, geographical, and meteorological conditions of sites.
The following are the factors for the selection of sites for hydroelectric power plants:
- Availability of water: since in hydroelectric power plants the potential energy of a waterfall or kinetic energy of a flowing stream is utilized for the generation of electric power so such stations should be built where there is adequate water available at a good head or huge quantity of water if flowing across a given point.
- Water storage: Since storage of water in a suitable reservoir at a height or buildings of a dam across the river is essential in order to have a continuous and perennial supply during the dry seasons therefore convenient accommodation for the erection of a dam or reservoir must be available.
- Water Head: The available water head depends upon the topography of the area. An increase in effective heads reduces the quantity of water to be stored and handled by penstocks, screens, and turbines and therefore the capital cost of the plant is reduced.
- Distance from the load centers: Hydroelectric plant is located far away from the load center. For the economical transmission of electric power, the routes and the distances need active consideration.
- Accessibility of the site: Adequate transportation facilities must be available or there should be the possibility of providing the same so that the necessary equipment and machinery could be easily transported.
- Water pollution: Polluted water may cause excessive corrosion and damage to metallic structures. Hence the availability of a good quantity of water is essential.
- Sedimentation: Gradual deposition of the 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 a lot of slit to the run-off.
- Large catchment areas: The reservoir must have a large catchment area so that level of water in the reservoir may not fall below the minimum required in dry seasons.
- Availability of Land: The land available should be cheap in cost and rocky in order to withstand the weight of large buildings and heavy machinery.
- There should be the possibility of stream diversion during the period of construction.
Merits and demerits of Hydroelectric Power Station
- 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 a 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 age.
- It is a very neat and clean plant because no smoke or ash is produced.
- It requires a large area.
- Its construction cost is enormously high and takes a long time to erect.
- 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 the vagaries of monsoon and their dependence on the rate of water flow in the river. The dry season affects the power supply.
- The firm capacity of hydroelectric plants is low and so backup by steam plants is essential.