1. Introduction
We are familiar with the term energy. Everyday, we hear about the scarcity of energy and a looming crisis caused by the rapid depletion of energy reserves. Energy can neither be created nor destroyed; hence, the need to conserve energy.

Energy is the ability or the capacity of a physical system to do work.
Are there different forms of energy? Can each form of energy be changed into another form?
Yes, energy exists in various forms such as kinetic energy, heat energy, chemical energy, etc. We can change energy from one form to another. For example, when a candle is burned, it produces heat and light. Here, we can see that the chemical energy in the candle is converted to light and heat energy.
Thus, we can say that a candle is a source of both light and heat. On the other hand, burning of coal produces heat. Does it also produce light?
What makes some forms of energy good while others bad?
Let us answer the question using light and coal as examples. Which of the two will you use as a source of heat energy to cook food? 
You will use coal as a source of heat energy to cook food. But, why coal and not candle? After all, both sources of energy produce heat.
This is because the heat from the candle is not sufficient to cook.
This implies that specific sources of energy are used for specific purposes, which we refer to as good sources of energy for that particular task.

Characteristics of a Good Source of Energy
Good sources of energy exhibit some special characteristics that are listed below.

  • It should possess a high calorific value, i.e., the amount of energy obtained by burning one kilogram of the fuel should be high
  • It should not leave residue after burning, i.e., it should burn completely
  • It should burn without producing too many pollutants
  • It should be easily available and accessible
  • It should be economical
  • It should be easy to store and transport

Coming back to our earlier discussion,When we compare coal and candle as sources of heat energy, we find that both are easily available, economical, and easy to store. However, coal is more efficient than candle, i.e., coal has a higher calorific value. One kilogram of coal will provide more energy as compared to one kilogram of candle-wax. Therefore, we prefer coal to candle when we require heat energy.
Let us further understand some characteristics of a good fuel.
In addition to a high calorific value, a good fuel must also have a fairly low ignition temperature. What will happen if the ignition temperature of a substance is lower than the normal room temperature? 
The fuel will be very difficult to store and transport. Any thermal contact with the atmosphere will ignite the fuel and it may result in an explosion.
Calorific value is defined as the amount of heat energy obtained by burning one gram of a substance. The unit of calorific value is kJ/g.
The ignition temperature of a substance is defined as the temperature at which the substance starts burning. It is measured in °C, °F, or K.

Calorific values and ignition temperatures of some common fuels.


Ignition temperature (°C)

Calorific value (in kJ/g)















35 – 40






25 – 30




2. Renewable and Non-Renewable Sources of Energy
Based on whether a source of energy can be replenished, it can be classified as a

  • Renewable source of energy
  • Non-renewable source of energy

Renewable sources of energy
Renewable sources of energy are those that are replenished at a rate faster than that at which they are consumed. About 13 percent of the primary energy comes from renewable resources. Renewable resources of energy are known as inexhaustible sources of energy as they can be easily regenerated at a constant rate.
Sunlight, wind, tides, and geothermal energy.
Non-Renewable sources of energy
Non-renewable sources of energy are those that are consumed at a rate faster than that at which they are replenished. Non-renewable resources of energy are known as exhaustible sources of energy as they can be easily exhausted.
Fossil fuels, which include coal, petroleum, and natural gas. These resources are widely used. In addition to being an exhaustible source of energy, fossil fuels also release polluting emissions on burning.
Fossil Fuels
Fossil fuels are the most commonly used sources of energy.Essentially, fossil fuels are hydrocarbons found within the top layer of the Earth’s crust. These include liquid petroleum, coal, natural gas, etc. The development of civilization has increased the requirement and demand for energy.
Fossil fuels are formed by biogenic processes from the fossils of dead plants and animals in the Earth’s crust. These fuels take over millions of years to form. The dead plants on the surface of the Earth were deposited under sediments of sand and clay in low lying swamp areas. In the course of time, as the thickness of this sediment increased, it compressed the dead plants under conditions of high pressure and temperature. This led to the formation of coal.
Petroleum, on the other hand, is formed from dead organisms found deep at the bottom of oceans. These organisms were covered by a layer of sediment, which, over the course of time, increased in thickness, thereby causing the dead organisms to compress. This compression of dead organisms under high pressure and temperature, in the absence of oxygen, converted these ancient sea organisms into petroleum.
Consequences of using fossil fuels
Combustion of fossil fuels produces air pollutants such as major greenhouse gases, nitrogen oxides, sulphur dioxide, volatile organic compounds, and heavy metals. Burning of such fuels increases the concentrations of sulphuric, carbonic, and nitric acids in the atmosphere. This results in acid rain.
Fossil fuels also contain radioactive materials, mainly uranium and thorium, which are released into the atmosphere.
The greenhouse effect and global warming
Before humans started using fossil fuels, there was a uniform recycling of carbon dioxide and oxygen in the atmosphere.
Plants absorb carbon dioxide and emit oxygen during the process of photosynthesis. Animals inhale oxygen and exhale carbon dioxide, thus maintaining a balance.
However, as humans began burning fossil fuels, more carbon dioxide was emitted into the atmosphere and the delicate balance between oxygen and carbon dioxide in the atmosphere was destroyed.
Greenhouse effect is caused by the accumulation of carbon dioxide in the Earth’s atmosphere.
A potential danger of global warming is the melting of the polar ice caps, which is causing the ocean levels to rise. This may lead to flooding of many coastal cities.
Disadvantages of using Fossil fules

  • They are exhaustible sources of energy
  • They are non-renewable sources of energy
  • They take millions of years to form. Hence, their availability is limited
  • They release pollutants such as carbon dioxide, which contributes to global warming
  • They release ash and unburned particles of carbon; they also release oxides of nitrogen and sulphur, which cause acid rain

In spite of all these disadvantages, fossil fuels are the most commonly used form of energy. As a result of their continuous use, there is an increase in such phenomena as global warming, air pollution, acid rain, and other problems related to the environment.

3. Thermal Power Plants
Power plants generate power. You may have seen smoke coming out of the chimneys of power stations. These power stations use coal or petroleum as fuel to produce steam by heating water. The steam is then used to rotate a turbine, which drives a generator. Electric energy thus generated is known as thermal power, and such power stations are known as thermal power plants.
Steam is passed through the turbine and is allowed to condense in a condenser. Since a thermal power plants uses coal or petroleum as fuel, it releases huge amounts of smoke from its chimneys.
Make your own power station
You can make your own, miniature thermal power plant at home using a bicycle dynamo, pressure cooker, turbine, and bulb as illustrated in the figure.

Heat is used to make steam from water in the pressure cooker. Steam spins the turbine, which in turn spins the dynamo. The dynamo generates electricity and this lights the bulb.
Since it is more convenient to transport electricity rather than fossil fuels such as coal, many thermal power plants are set up near coal or oil fields.
4. Hydro power plants
Instead of using steam to spin a turbine, hydro power plants use the potential energy of water accumulated at a height to spin a turbine.
In India, most of the hydropower plants are associated with dams. This is because the natural flow of water in our water bodies is not such that it can spin a turbine. Construction of dams over rivers helps to accumulate water and increase its force. A quarter of our energy requirement is met by hydroelectricity.
Major hydropower plants in India

Hydro power plant







Uttar Pradesh




Jammu & Kashmir

Sardar Sarovar



Andhra Pradesh



Advantages of hydropower plants

  • Clean and cheap
  • Do not produce pollutants
  • No waste by-products are produced
  • They do not require transportation of fuels
  • Hydro power is a renewable source of energy

Disadvantages of hydropower plants 

    • Limited geographic potential
    • Causes a change in the course of rivers
    • Sediment accumulates in a dam and eventually reduces its water storage potential
    • Lot of land is submerged under water
    • Fish and wildlife are affected drastically
    • Large dams release methane, which is a greenhouse gas that contributes to global warming

5. Biomass
Biomass refers to those living and non living organic materials that can be used as sources of energy in the form of fuel. Some examples of biomass fuels are wood, crops, and organic garbage. The chemical energy in biomass is released as heat on burning.
Biomass can also be converted to other usable forms of energy such as methane gas or transportation fuels such as ethanol and biodiesel. It is a renewable source of energy.
Wood as biomass 
The most common form of biomass is wood. For thousands of years people have burned wood for heating and cooking. Until the mid-1800s, wood was the main source of energy in the world. It still continues to be a major source of energy in most of the developing countries. Many manufacturing plants in the wood and paper products industry use wood waste to produce their own steam and electricity.
Municipal solid waste as biomass
Garbage is another source of biomass. This is also called municipal solid waste (MSW). MSW includes food scraps, lawn clippings, leaves, etc. These are also called biomass trash.
6. Biogas
Gas made from the anaerobic digestion of agricultural and animal waste is called biogas. When this biomass (agriculture and animal waste) is decomposed in the absence of oxygen, it releases biogas.
In India, biogas is commonly known as “gobar gas”.Biogas is a mixture of methane (75%) and CO2. It is used for cooking, lighting of lamps, generating electricity, etc. In “landfills”, biomass rots and releases methane gas through the process of biodegradation.
Biodegradation is the process of biological degradation of organic matter by bacteria and fungi.
Biogas production
Biogas technology provides an alternate source of energy in rural India. It is a more efficient and developed form of fuel.
It uses a thick mixture of cattle waste and kitchen garbage. This is fed into an inlet chamber. The micro organisms present in the digester tank are mainly methanogens.

These methanogens decompose the organic waste (slurry) anaerobically to produce methane and some carbon dioxide. This process is called biodegradation.
Biofuels, such as ethanol and biodiesel, are fuels that are made from biomass materials. These are used in transport vehicles. These fuels are usually blended with petroleum fuels such as gasoline and diesel.
Biodiesel is a domestic renewable fuel for diesel engines derived from natural oils such as soybean oil. It is typically produced by the reaction of a vegetable oil, or animal fat with an alcohol such as methanol or ethanol in the presence of a catalyst to yield mono-alkyl esters and glycerine. Glycerine is removed from the biodiesel.
Biodiesel fuel can be used in diesel engines without making any modifications to the engine. It is a safe, renewable, and biodegradable fuel that produces fewer polluting emissions.

7. Wind Energy
Wind energy is the energy harnessed from wind. The kinetic energy of wind is converted into mechanical energy. If this energy is used directly by a machine (such as a grinding stone or a water pump), it is called a windmill. If the mechanical energy is first converted into electricity, then the machine is called a wind turbine.
Working of wind energy

  • The wind blows on the blades and rotates them
  • The blades turn a shaft inside the nacelle (the box at the top of the turbine)
  • The shaft goes into a gear box and starts producing electrical energy
  • The national grid transmits the power around the country

Wind turbines operate at wind speeds of about 10 miles an hour. They reach a maximum power output at about 33 miles per hour. When a number of windmills are erected over a large area, in the same location, it is known as a wind farm. One such farm in India is near Muppandal, Tamil Nadu.
Windmills are pollution free. The energy harnessed by windmills can be used in pumps to lift water from wells or for grinding grains. Most of these windmills can be found at high altitudes where there is a continuous wind speed of over 10 mph.
The advantages and limitations of wind energy are listed in the following table.



Wind is freely available in nature

Wind farms can be established only at places where there is a continuous wind speed of over 10 mph

Air is a renewable and inexhaustible source of energy

A wind farm requires a large area (about 2 hectares) to facilitate a 1 MW generation

It reduces the cost of electricity produced

Lack of energy storage facilities to provide energy back-up in the absence of wind

Wind energy does not cause any pollution and is eco-friendly

The initial cost of setting up a farm is quite high


A wind farm requires high quality maintenance facilities

8. Devices That Use Solar Energy
We have studied about photosynthesis, a process in which plants absorb solar energy and convert it into chemical energy. Scientists too have developed technologies that enable us to trap solar energy to produce heat energy or electricity.
Solar Energy
Let us understand the uses of solar energy through an animation.
You know, solar electricity is mainly produced by using photovoltaic cells or solar cells. Such cells are made up of semiconductors that convert solar energy directly into electricity.
How does a photovoltaic solar cell work?
Photovoltaic (PV) cells are made of special materials called semiconductors, such as silicon. When light strikes the solar cell, a certain portion of it is absorbed by the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor.
This energy loosens the electrons from the atoms in the semiconductor material and allows them to flow freely. This produces direct current (DC) or electricity.

The advantages and disadvantages of solar energy are listed in the following table.



It is a renewable source of energy.

Photovoltaic cells are not cost effective.

Solar ra diations are abundantly availabl e.

A typical cell develops a voltage of 0.5 to 1

V and can produce about 0.7 W of

electricity when exposed to the sun.

Solar cell panels have no moving parts and

hence require little mai ntenance.

Availability of special grade of si licon used

to produce solar cells is limited.

A solar panel can be set up in any pla ce and

requires little or no space on the ground.

Present applications of solar energy

Solar energy is currently being used in the following ways:

  • Some water heaters use solar panels.
  • Calculators and many toys are powered by solar cells.
  • Traffic signals in most cities are powered by solar energy.
  • In some remote areas, the electricity requirements of households are fulfilled by generating electricity
    using solar energy.

9. Sources Of Energy From Sea
Sea water is a highly potential resource of energy. Some forms of energy that can be obtained from sea water are tidal energy, wave energy, and ocean thermal energy.
Tidal Energy
Tides are the daily rise and fall of ocean levels relative to coastlines. They are a result of the gravitational forces of the moon and the sun on Earth, and also the revolution of the Earth. A large amount of energy is stored in tides. They can be used as renewable sources of energy to generate electricity.
Tidal electricity generation involves the construction of a barrage across deltas, estuaries, beaches, or other places that witness increased tidal action.
Working of Tidal energy
The barrage (dam) allows incoming tidal water during high tides, but does not allow it to flow out. This causes water to accumulate at a higher level in the coastal basin. When the tide flows out, the level of water in the sea decreases, and the water which was blocked using the dam is released. The head of this water is then used to spin the turbine of the electric generator.
The barrage is equipped with sluices and turbines (similar to hydropower plants). Turbines generate electricity as the water flows out.
The vast potential energy of the seas and oceans can make a significant contribution toward meeting the increasing energy needs. Commercially viable technologies to harness tidal power, wave power, and ocean thermal energy are therefore being developed.
The advantages and disadvantages of tidal energy are listed below.

  • It does not need any fuel
  • Waves are predictable
  • It is available free of cost
  • It is relatively cheap and easy to maintain
  • It produces no greenhouse gases or other polluting wastes


  • There are only limited sites that are suitable for constructing tidal barrages
  • Shipping facility is affected due to the construction of barrages
  • Construction of barrages causes changes in the level of tides, thereby resulting in floods
  • Building a barrage across an estuary is a very expensive affair. It also affects a very wide area. This results in the loss of biodiversity in the particular area. Many birds rely on the uncovering of mud flats by tides for feeding

Wave energy
Ocean waves are caused by winds as they blow across the sea. Waves are a powerful source of energy. Electricity can also be produced from wave energy.

Production of electricity from wave energy
At a wave power station, waves cause the water in the chamber to rise and fall, which means that air is forced in and out of the hole located at the top of the chamber. A turbine placed in this hole is turned by the movement of air rushing in and out. The turbine turns a generator to produce electricity.

  • The energy is free – no fuel is needed, no waste is produced
  • It is not expensive to operate and maintain
  • A wave power station can produce a lot of energy


  • It needs a suitable site where waves are consistently strong
  • It is less viable for commercial exploitation in comparison to other traditional forms of energy
  • We are able to harness only a minuscule part of a potentially huge energy resource

Ocean thermal energy
Sunlight falls on oceans and seas. This causes the temperature of water on the surface to rise, while the temperature at the bottom remains comparatively cooler. Ocean thermal energy conversion plants use the warm surface-water to boil volatile liquids such as ammonia. Ammonia gas, thus produced, creates pressure and runs the turbine of the generator. This produces electricity. Cold water is pumped up to liquefy the gas. This creates a cycle for generating electricity.

  • Power plants can be operated only if there is a temperature difference of 20°C between the surface sea water and the sea water at a depth of up to 2 km
  • Commercial exploitation is difficult

10. Geothermal And Nuclear Energy
Geothermal energy is the heat energy in the Earth’s interiors. The temperature at the centre of the Earth is about 6000°C. In some areas, molten rock (magma) is found very close to the surface of the Earth. This hot rock meets underground water and heats it up. This produces steam. Such a place is known as a hot spot. Sometimes, this heated underground water finds an outlet through which it comes out on the surface of the Earth. Such outlets are known as hot springs.
Holes are drilled down to the regions through which steam comes up. This flow of steam is used to drive the turbines of generators to produce electricity. One important use of geothermal energy is to heat buildings.
The advantages and disadvantages of geothermal energy are listed below.

  • No fuel is needed
  • Once a geothermal power station is built, the energy is almost free
  • It does not produce any polluting emissions and does not contribute to the greenhouse effect
  • Since geothermal power stations are not very large, they have a minimal impact on the environment


  • Geothermal energy is not commercially viable
  • Sometimes, a geothermal site may undergo depletion and may lose its heat. These geothermal sites then have to be left alone to help them recover their lost heat
  • Hot spots or regions that can generate this energy are limited
  • Hazardous gases and minerals may emerge, which may be difficult to dispose off

Nuclear energy
In order to understand nuclear energy we should first understand nuclear fission reaction.
What is nuclear fission?
Do you know that the difference in mass (m) between the original nucleus and the product nuclei is converted into energy, E? The amount of energy produced is governed by Einstein’s equation,
E = mc2
Where ‘c’ is the speed of light

How is electricity produced in nuclear power plants?
Nuclear power plants consist of nuclear reactors. These reactors use uranium rods as fuel and heat is generated by the process of nuclear fission.
Neutrons smash into the nucleus of the uranium atoms, which roughly split into half and release energy in the form of heat.
Carbon dioxide gas is pumped through the reactor to take the heat away. The hot gas then heats water to form steam. This steam drives the turbines of generators to produce electricity.

Disadvantages of nuclear energy

  • Construction of nuclear power plants needs huge investments
  • Radioactive wastes such as used uranium are a dangerous hazard to the environment
  • Nuclear energy can be used for negative purposes. Therefore, there is always a fear of misuse
  • There is always a danger of leakage of radioactive material and radiations from nuclear power plants

Did you know that during World War II nuclear bombs were dropped on the twin cities of Hiroshima and Nagasaki in Japan by America?

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