CBSE NOTES CLASS 10 SCIENCE CHAPTER 14
SOURCES OF ENERGY
Conversion of usable energy to heat is irreversible.
Energy comes in different forms and one form can be converted to another.
If we light a candle, the process is highly exothermic so that the chemical energy in the wax is converted to heat energy and light energy on burning.
Since we cannot reverse the change involved in this process, we cannot get back the original usable form of energy.
Due to this, it becomes important to think about energy shortage and the related energy crisis.
Characteristics of a good source of energy
- It should be able to do large amount of work for each unit of mass or volume.
- It should be easily accessible.
- It should be easily transported.
- It should be economical.
- It should do minimum damage to the environment.
Conventional Sources of Energy
The sources of energy which have been in use since a long time are called conventional sources of energy. Coal, petroleum, natural gas, hydel energy, wind energy and nuclear energy are considered to be the conventional sources of energy. additionally, firewood is also a conventional source of energy but its usage is now limited to kitchens in the rural parts of India.
Demand for energy has risen due to,
- Increasing population
- Increasing industrialization
- Development of technologies for use of energy sources
The growing demand for energy was largely met by the fossil fuels – coal and petroleum.
Thermal Power Plants
In a thermal power plant, coal or petroleum is used for converting water into steam. The steam is used to run the turbine for generating electricity.
Disadvantages of burning fossil fuels
Disadvantages of thermal power plants
(i) The fossil fuels are non-renewable sources of energy, so we need to conserve them.
It takes millions of years for the formation of fossil fuels. Since they cannot be replenished in the foreseeable future, they are known as non-renewable sources of energy.
To avoid running out of energy sources, alternate sources of energy need to be explored.
(ii) Burning fossil fuels also causes air pollution. The oxides of carbon, nitrogen and sulphur that are released on burning fossil fuels are acidic oxides. These lead to acid rain which affects our water and soil resources.
(iii) The CO2 also causes green-house effect and global warming.
(iv) Burning of fossil fuels also contribute to ash and suspended particulate matter, which is harmful to the environment and health.
Renewable Sources of Energy
Those sources of energy which can be replenished quickly are called renewable sources of energy. Hydel energy, wind energy and solar energy are examples of renewable sources of energy.
Hydro Power Plants
Hydel energy is produced by utilizing the kinetic energy of flowing water. Huge dams are built over a source of water. Water is collected behind the dam and released.
When the water falls on the turbine, the turbine moves because of kinetic energy of water and electricity is generated.
Water in the reservoir is replenished with rainwater and so availability of water is not a problem for hydroelectricity.
Drawbacks of Hydro Power Plant
- Building large dams is not good for the ecosystem. When a large dam is built, a vast tract of land in its vicinity gets submerged. This destroys a large part of vegetation and wildlife which does immense damage to the ecosystem.
- Moreover, when the submerged vegetation decomposes, it produces a huge amount of methane gas. Methane is a potential greenhouse gas and thus is not good for the environment.
- Building large dam also affects a huge section of population which lives in the surrounding areas. Although the government comes with some rehabilitation plan for them but the displacement of people from their roots has its own costs involved.
The plants and animals constitute the biomass. Firewood from plants is used as kitchen fuel. If large number of trees can be planted, then a continuous supply of firewood can be ensured. Farm waste such as stalks of harvested plants and dung of cattle can be used to generate methane. The decomposition of biomass produces methane which can be channelized for useful purposes.
Bio gas plant
Bio-gas plant can be very useful in solving the energy need of rural areas. A bio-gas plant is a dome-like structure which is usually built from bricks and concrete. In the mixing tank, the slurry is made from cow-dung and water. The slurry then goes to the digester; which is a closed chamber. Since oxygen is absent in the digester, the anaerobic decomposition takes place. The process of decomposition produces biogas.
Biogas has about 70% of methane and the rest is composed of other gases.
The biogas is channelized through a pipe and can be utilized as kitchen fuel and also as fuel for getting light.
The slurry left behind is removed and used as manure once it dries.
Wind energy has been in use since ages. The sail boats of the pre-industrialization era used to run on wind power. Windmills have been in use since medieval times especially in Denmark and Holland.
Now-a-days, windmills are being used to generate electricity. The kinetic energy of wind is utilized to run the turbines which generate electricity.
At present, Germany is the leading country in terms of wind energy production and India comes at number five. In India, Tamil Nadu is the largest wind energy producing state. The largest wind farm in India is near Kanyakumari in Tamil Nadu which generates 380 MW of electricity.
Limitations of Wind Energy
- Wind farms can only be established at those places where the wind speed is high enough and is more than 15 km/hr for most parts of the year.
- Wind farms need to be established on large tracts of land.
- The fan of the windmill has many moving parts so cost of maintenance and repair is quite high.
- The machines have to be in open leading to corrosion of the equipment.
- Initial cost of establishing a wind farm is very high.
Non conventional Sources of Energy
Energy sources which are relatively new are called non-conventional sources of energy, e.g. nuclear power and solar energy.
The sun is the main source of energy for all living beings on this earth. Even the energy in the fossil fuels has come from the sun. The sun has an endless reservoir of energy which would be available as long as the solar system is in existence. Technologies for harnessing the solar energy have been developed in recent times.
Solar cooker is very simple in design and mode of function.
Inside walls of the cooker are painted black. A black surface absorbs more heat as compared to a white or a reflecting surface under identical conditions.
Plain mirrors are placed inside a rectangular box. Solar cookers are covered with a glass plate to prevent the heat from escaping.
The light reflected from the plain mirrors concentrates the solar energy inside the solar cooker which generates enough heat to cook food.
Solar furnace is made like a concave mirrors. Large solar furnace has many smaller mirrors to compose a very large convex mirror. These focus the sun light. The matter to be heated is place near the focus of the mirror.
Solar cells are made from silicon. The solar panel converts solar energy into electrical energy which is stored in a battery for later use. The principle used is photo-electric effect.
Advantages of Solar Cells
- They have no moving parts, require little maintenance and work quite satisfactorily without the use of any focussing device.
- They can be set up in remote and inaccessible hamlets or very sparsely inhabited areas in which laying of a power transmission line may be expensive and not commercially viable.
- Solar energy is non-polluting.
Limitations of solar energy
- Availability of the special grade silicon for making solar cells is limited.
- The entire process of manufacture is still very expensive.
- Silver used for interconnection of the cells in the panel further makes it costly.
- At present, the cost benefit ratio for using solar energy is not conducive. Using solar energy is exhorbitantly costly.
- The actual operation of a device like the solar cell may be pollution-free, but the assembly of the device would have caused some environmental damage.
Applications of Solar Energy
- In spite of the high cost and low efficiency, solar cells are used for many scientific and technological applications.
- Artificial satellites and space probes like Mars orbiters use solar cells as the main source of energy.
- Radio or wireless transmission systems or TV relay stations in remote locations use solar cell panels.
- Traffic signals, calculators and many toys are fitted with solar cells.
- It is used in remote areas of strategic importance.
ENERGY FROM SEA
Due to the gravitational pull of mainly the moon on the spinning earth, the level of water in the sea rises and falls.
Tidal energy is harnessed by constructing a dam across a narrow opening to the sea. Water rushes up near the seashore during a high tide and goes down during a low tide. A turbine fixed at the opening of the dam converts tidal energy to electricity.
Limitations - The locations where such dams can be built are limited.
The kinetic energy possessed by huge waves near the seashore can be trapped in a similar manner to generate electricity.
The waves are generated by strong winds blowing across the sea. Wave energy would be a viable proposition only where waves are very strong.
A wide variety of devices have been developed to trap wave energy for rotation of turbine and production of electricity.
For example, a hollow tower is built near the seashore. When water gushes in the tube because of wave, it forces the air upwards. The kinetic energy of air in the tube is used to run a turbine. When the wave goes down, air from up goes down the tube which is also used in running the turbine.
Limitations – Waves with high velocity are needed for this. The locations where such dams can be built are limited.
Ocean Thermal Energy
The water at sea surface is hot during daytime, while the water at lower level is cold. The temperature differential in water levels can be utilized to generate energy. These plants can operate if the temperature difference between the water at the surface and water at depths up to 2 km is 20K (20C) or more.
For this, a volatile liquid like ammonia is boiled using the warm water at the surface. The steam of the volatile liquid is utilized to run the turbine to generate electricity. Colder water from the depth below is utilized to condense ammonia vapour which is then channelized to the surface to repeat the cycle.
Due to geological changes, molten rocks formed in the deeper hot regions of earth’s crust are pushed upward and trapped in certain regions called ‘hot spots’.
When underground water comes in contact with the hot spot, steam is generated, which finds outlets at the surface. Such outlets are known as hot springs.
The steam trapped in rocks is routed through a pipe to a turbine and used to generate electricity.
There are many power plants based on geothermal energy operational in New Zealand and United States of America.
The cost of production would not be much, but there are very few commercially viable sites where such energy can be exploited.
The nucleus of a heavy atom (such as uranium, plutonium or thorium), when bombarded with low-energy neutrons, can be split apart into lighter nuclei. This process is called nuclear fission,
During this process a large amount of energy is released if the mass of the original nucleus is just a little more than the sum of the masses of the individual products.
In a nuclear fission, the difference in mass, Δm, between the original nucleus and the product nuclei is called mass defect. This gets converted to energy E at a rate governed by the equation, first derived by Albert Einstein in 1905,
E = Δm c2,
where c is the speed of light in vacuum.
In nuclear science, energy is expressed in units of electron volts (eV): 1 eV = 1.602×10–19 joules.
One atomic mass unit (u) is equivalent to about 931 mega electron volts (MeV) of energy.
The fission of an atom of uranium produces 10 million times the energy produced by the combustion of an atom of carbon from coal !!
A nuclear reactor is a device used to initiate and control a sustained nuclear chain reaction.
Nuclear fuel is a substance that is used in nuclear power stations to produce heat.
In a nuclear reactor, the nuclear fuel can be part of a self sustaining fission chain reaction that releases energy at a controlled rate.
The released energy can be used to produce steam and further generate electricity.
Fusion means joining lighter nuclei to make a heavier nucleus, most commonly hydrogen or hydrogen isotopes to create helium, such as
2H + 2H → 3He + 1 n + energy
It releases a tremendous amount of energy, according to the Einstein equation, as the mass of the product is little less than the sum of the masses of the original individual nuclei.
Such nuclear fusion reactions are the source of energy in the Sun and other stars.
It takes considerable energy to force the nuclei to fuse. The conditions needed for this process are extreme – millions of degrees of temperature and millions of pascals of pressure.
The hydrogen bomb is based on thermonuclear fusion reaction. A nuclear bomb based on the fission of uranium or plutonium is placed at the core of the hydrogen bomb.
This nuclear bomb is embedded in a substance which contains deuterium and lithium. When the nuclear bomb (based on fission) is detonated, the temperature of this substance is raised to 107K in a few microseconds. The high temperature generates sufficient energy for the light nuclei to fuse and a devastating amount of energy is released.
Risks and Limitation of Nuclear Power
- Nuclear power is safest for the environment but the risk of contamination due to accidental leaks of radiation is pretty high. Exposure to nuclear radiation may be fatal or may damage organs.
- Storage and disposal of nuclear waste is a big problem because of potential risk of radiation involved.
- The high cost of installation of a nuclear power plant is also a limiting factor.
- Limited availability of uranium.
Despite all the risks and limitations, many countries are using nuclear power in a big way. India too has built many nuclear power plants.
Nuclear power reactors are located at
- Tarapur (Maharashtra),
- Rana Pratap Sagar (Rajasthan),
- Kalpakkam (Tamil Nadu),
- Narora (UP),
- Kakrapar (Gujarat) and
- Kaiga (Karnataka)
Accidents in the nuclear power plants in Chernobyl and Japan have forced the policymakers to rethink about the nuclear power and safety standards.
Environmental Consequences of Different Energy Sources
Exploiting any source of energy disturbs the environment in some way or the other.
The source of energy chosen depends on factors such as,
- The ease of extracting energy from that source,
- The economics of extracting energy from the source,
- The efficiency of the technology available and
- The environmental damage that will be caused by using that source
- When we talk of ‘clean’ fuels like CNG, it would be more exact to say that a particular source is cleaner than the other.