CONVENTIONAL SOURCES OF ENERGY
The main conventional sources of energy are:
(a) Fossil fuel
(b) Large hydro
(c) Nuclear fuel
Fossil Fuel
Petroleum, coal and natural gas are called fossil fuels because these are formed by decomposition of plants and animals buried under the earth for a very long time. These fuels cannot be formed in a short span of time and are called non-renewable sources of energy. Petroleum is refined in huge refineries to produce various fuels like petrol and diesel.
1. Petroleum
It is a dark coloured, viscous and foul smelling crude oil. This oil is lifeline of global economy. Thirteen countries of OPEC (Organisation of Petroleum Exporting Countries) have 67% of world oil reserves. USA is the world's largest consumer of oil using 30% of global total. Maximum use of oil is in transportation (63%), followed by industries (24%), residential and commercial building (8%) and electric utilities (8%). At the present rate of consumption, the world's crude oil reserves are estimated to be depleted in 40 years. Undiscovered oil deposits could meet world demand for another 30-40 years.
India has total reserves of 1201 million metric tonnes of crude oil. India has a total installed capacity of 1200 MW of oil based power plant and price per kWh is about 8. So these power plants are mainly used during peak hours.
Some of the crude oil producing locations in India are:
1. Ankleshwar and Kalol in Gujarat
2. Rudrasagar and Lakwa in Assam
3. Bombay high (off-shore area)
The oil wells of Bombay high are producing about 22 million tons of crude petroleum oil every year which is about 50% of country's total oil requirement. The crude petroleum is refined by the process of fractional distillation to obtain useful products as listed below:
(iii) Kerosene (170°C to 250°C) for household and industrial use
(iv) Diesel oil (250°C to 350°C) for heavy vehicles.
lubrication oils • paraffin wax • asphalt
2. Coal
Coal is the most common and main source of energy. It is a complex mixture of carbon, hydrogen and oxygen. Small amount of nitrogen and sulphur compounds are also present in coal.
Coal is the most abundant fossil fuel of the world. About 50% of coal reserves are located in USA, Russia and China alone. At the present rate of consumption, world coal reserves will be exhausted in 154 years. India has about 5% of world's coal reserve and coal India is available mainly in Bihar, Jharkhand, West Bengal, Odisha, Madhya Pradesh and Chhatisgarh. The big coal mines India are at Jharia and Bokaro in Jharkhand and at Raniganj in West Bengal. Coal is main energy source of power production in India, generating approximately 57% of total electricity by coal based thermal power plants. These power plants are generally installed near coal mine areas to reduce transportation cost of coal. Total installed capacity of coal powered thermal power plants is 1,64,636 MW by year 2015.
Depending upon the carbon contents of coal, it is classified as follows:
| S No. | Type of coal | Carbon content (%) |
|---|---|---|
| 1 | Peat | 60 |
| 2 | Lignite (soft coal) | 70 |
| 3 | Bituminus (household coal) | 80 |
| 4 | Anthracite | 90 |
From the above table it is clear that peat is poor quality of coal and anthracite is best quality of coal.
Coal is used as fuel in thermal power plant. Coal occurs in coal mines and is brought to power plant by means of railways and road transportation where it stored in coal yard or coal storage. Then it is sent to coal handling plant for different processes. Coal is pulverized by crushers and magnetic material present are separated by big magnets before coal is being fed to boiler furnace by conveyor belt. Combustion of coal takes place at furnace to produce hot gases called flue gas. Air is fed to furnace bed by Forced Drought (FD) fan as sufficient air is required for proper burning of coal. Water from nearby pond or river is processed by water treatment plant and clean water is fed to boiler where it is heated by flue gas to produce steam. High pressure and temperature steam is fed to steam turbine through a set of stationary nozzles and steam is expanded in the turbine causing turbine to rotate. Thus heat energy of steam is converted into mechanical energy the turbine. Steam turbine is coupled with alternator which converts mechanical energy into electrical energy. Alternator is synchronized with grid and electrical energy can be transmitted to load centre by transmission lines. Generally thermal power plant is located near load centre to minimise transmission cost.
After doing useful work over turbine, steam is cooled into hot water at turbine outlet by condenser and this hot water is again fed back into boiler and the process is repeated. Sufficient water is required for steam production and for cooling, so this type of plant is installed near river or other water bodies.
Efficiency of thermal power plant is low and is of the order of 26-30% due to heat losses at various stages. Heat energy of flue gases is recovered at different points by super heater, economizer and air pre-heater to improve efficiency before the gas is allowed to atmosphere via chimney. This gas pollutes air as it contains suspended micro solid particles. Pollution is minimized by Electro Static Precipitator (ESP) where particles are charged and settled down. Left out of burned coal appears as hot ash which is cooled and treated in ash handling plant and is then disposed of.
Main drawback is maximum global warming caused by coal due to carbon dioxide (CO₂) gas emission to the atmosphere.
3. Natural Gas
Methane, ethane and propane are main constituents of natural gas. It is found deep under the earth crust above petroleum deposits. It is a product of petroleum mining and is used as industrial and domestic fuel.
Large Hydro
Potential energy of water is used to produce electricity. Water from river or lake is stored in a reservoir at a sufficient height or head to get a considerable amount of energy. Water head is created by constructing a dam across the river or lake. This type of power mainly depends on rain fall of catchment area. Also the dam is to be built in narrow canyon so that capital cost and gestation period are reduced and it is economical. In view of this, hydropower plants are generally constructed in hilly areas where there is high head and narrow canyon between two hills. Hydro potential of India is around 89,000 MW at 60% load factor which is equivalent to installed capacity of 1,48,700 MW. At present we are using about 25% of hydropower in our country.
The main parts of a hydroelectric power plant are a dam to store water in a reservoir, a penstock for delivering water, electric generators, a valve house which contains the main sluice valve, automatic isolating valves and control equipments A surge tank is located just before the valve house to protect the penstock from pressure surge, called water hammering, in case the turbine gates are suddenly closed.
When gates of the dam are opened, water from reservoir flows down through. large pipes called penstock where potential energy of water is converted into kinetic energy. This water is led to the water turbine through penstock. Penstock is a big pipe made of concrete or steel and its other end opens at a spacing at the bottom called spiral casing where guide vanes are mounted to control flow of water from penstock to turbine. Flow of water can be regulated by changing the angle of opening of guide vanes. Water is impinged on runner blades of hydraulic turbine after guide vanes are opened at spiral casing. The turbine starts to rotate and converts kinetic energy of water into mechanical energy. The turbine is coupled with alternator which converts mechanical energy into electrical energy. After doing useful work, water at the runner outlet of turbine is collected in draft tube and discharged to tail race of river at the downstream side of the dam. Generation of electric power is controlled in control room. Control of turbine and its auxiliaries are facilitated at turbine floor.
Flowing water of river carries grass, debris etc., along with it. This will reduce the passage of water in penstock and decrease the pressure in it. Reduction of pressure in penstock effect generation drastically. Trash racks are placed in water inlet at forebay which filter out grass, debris, etc., present in water. Also trash racks are choked many times. In this case, diving teams are called upon to clean these racks. The most significant operating characteristics of hydropower plant are quick starting and loading, long life and low operating and maintenance cost. Hydraulic turbine operates at low pressure and low speed. Therefore generators are usually salient type rotor with large no of poles. To maintain the generator voltage at constant frequency, turbine must rotate the generator at constant speed given by the formula
n = 120 f/P
where f is frequency and P is number of poles of generator. A number of control schemes are used to regulate the flow of water in order to make the turbine speed constant.
Hydropower
The potential energy (PE) of water in the reservoir is proportional to the mass (m) of water and difference in height between the water impoundment and water outflow. This height difference is called head (h).
mass of water m = volume × density
PE = mgh = volume × pgh
Power is rate of energy, therefore available hydropower is given by
P= Energy/time = volume/t × pgh
=Q ρgh, watt
where,
Q = rate of flow of water in m³/s
h = head in m
ρ = density of water = 1000kg/m³ (approximately)
g= acceleration due to gravity = 9.81 m/s²
P= 9.81 Qh x 1000 watt = 9.81 Qh, kW
If n is overall efficiency of hydropower plant, the electrical power output is
P= 9.81 Qhη, kW
where,
η = ηP . ηt . ηg
ηP = penstock efficiency
ηt = turbine efficiency 11 generator efficiency
ηg = generator efficiency
Design of penstock depends on head. From the expression of power of hydro-plant, it is evident that the length of penstock is large and its diameter is less if the head is high. The Pelton wheel turbine is an impulse hydraulic turbine and is normally used for high head hydro-plant. Penstock of low head plant has large diameter and less length as it requires more quantity of water to be discharged for same power. A Kaplan turbine is a propeller or axial type hydraulic turbine and is used for low head plant. The Francis turbine is a radial hydraulic turbine commonly used for medium head plant. In general, efficiency of hydraulic turbine lies in between 80 to 94 percent during normal operation and generator efficiency is from 95 to 99 percent.
Generation in hydro-plant depends on storage of water. During rainy season there is sufficient water in reservoir. Power is generated through out the day and plants are operated as base load plant. When there is less water in dam, particularly during low flow period, plants are operated in peak hours as peak load plants. Pumped storage plant also supply peak load. Large hydro plant is a multi-utility project. Besides producing power, it also helps in flood control, irrigation and supply of drinking water.
Nuclear Fuel
Heavier atoms like Uranium and Thorium are unstable and they produce huge amount of heat energy by nuclear fission process. The heat enrgy liberated by complete fission of one kg of Uranium is equivalent to heat energy produced by burning 4,500 tonnes of good quality coal. Uranium reserves in world is small at present.
Nuclear power is the fourth largest source of electricity in India after thermal, hydroelectric and renewable sources. By the year 2015, India has 21 nuclear reactors in operation in six nuclear power plants, generating 5,780 MW. India is involved in the development of nuclear fusion reactor through its participation in ITER project. India has small reserve of uranium. Available reserve supply fuel for 10000 MW. The cost of nuclear energy is about 4 per kWH as compared to solar energy at 20 per kWH. About 2.25% of energy produced in India is obtained from nuclear power plants.
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