Sources of Energy

What to remember

• Energy sources are conventional (coal, petroleum, natural gas, hydro, firewood) or non-conventional (solar, wind, biogas, tidal, geothermal, nuclear). Fossil fuels are non-renewable and release carbon dioxide when burnt.
• A good fuel has high calorific value, burns without smoke, is cheap and easy to store. Hydrogen has the highest calorific value of common fuels (about 150,000 kJ/kg).
• Nuclear energy comes from the mass defect, using Einstein's equation E = mc². Fission splits a heavy nucleus (uranium-235). Fusion joins light nuclei (hydrogen) and powers the Sun.

Classification of energy sources

A renewable source is naturally replaced: sun, wind, water, biomass, tides. A non-renewable source is used up faster than it forms: coal, petroleum, natural gas. Forests and forest produce matter here: firewood and charcoal are biomass fuels, and trees also take carbon dioxide from the air.

Fossil fuels

Coal formed from buried plants and forests millions of years ago, under heat and pressure. Petroleum and natural gas formed from tiny marine organisms buried under sediments. The main component of natural gas is methane.

Coal ranks (from low to high carbon): peat, lignite, bituminous, anthracite. Anthracite has the highest carbon content and calorific value.

Petroleum products come by fractional distillation: LPG, petrol, kerosene, diesel, lubricating oil, paraffin wax, bitumen.

LPG is mainly butane and propane. A strong-smelling gas, ethyl mercaptan, is added so that leaks can be noticed. CNG is mainly methane. It is a cleaner vehicle fuel than petrol or diesel.

Calorific value is the heat produced when 1 kg of fuel burns completely. Unit: kJ/kg.

Harm from fossil fuels: carbon dioxide (global warming), sulphur dioxide and nitrogen oxides (acid rain), carbon monoxide (poisonous), and particles (smog). Incomplete burning gives carbon monoxide.

Hydro and thermal power

Hydro power: water stored in a dam falls through pipes (penstocks) and turns turbines. Potential energy → kinetic energy → mechanical energy → electrical energy. It is clean and renewable, but dams submerge land and forest and affect people and wildlife. Srisailam and Nagarjuna Sagar are well-known hydroelectric projects in the Telugu states.

Thermal power: coal is burnt to make steam, the steam turns a turbine, and the turbine drives a generator. Examples in Andhra Pradesh include the plants at Vijayawada and near Visakhapatnam (Simhadri). Ash and carbon dioxide are the drawbacks.

Solar energy

The Sun's energy comes from nuclear fusion. The solar constant is about 1.4 kJ per second per square metre (about 1.4 kW/m²) at the top of the atmosphere.

• Solar cell (photovoltaic cell): converts sunlight directly into electricity. It is made of silicon (a semiconductor). Many cells joined give a solar panel.
• Solar cooker: a box with a mirror reflector, a glass cover and a black inner surface. Black absorbs heat best. The glass cover traps heat (greenhouse effect).
• Solar water heater and solar street lights are other uses.
• Limits: works only in the day, depends on weather, and has a high set-up cost.

Large solar parks have come up in Andhra Pradesh, in Rayalaseema, including the Kurnool and Anantapur regions. Treat project names and capacities as "check the latest official release".

Wind energy

Moving air turns the blades of a wind turbine, which drives a generator. A wind farm needs steady wind (usually above about 15 km/h). It is clean but needs a large area. Rayalaseema and coastal districts have wind sites. India's wind belt is strong in the south and west.

Biomass and biogas

Biomass is plant and animal material used as fuel: wood, agricultural waste, dung. It gives energy but, if burnt unsafely, causes smoke.

Biogas is made by anaerobic decomposition (no oxygen) of dung and plant waste by bacteria in a digester. It is mainly methane (about 55–75 per cent) with carbon dioxide. It burns without smoke. The leftover slurry is a good manure. The plant is also called a gobar gas plant.

Forest-based biofuel crops such as pongamia (Indian beech) and jatropha give oil seeds used for biodiesel. Improved cookstoves reduce firewood use and forest pressure.

Nuclear energy

Mass defect: the mass of a nucleus is slightly less than the sum of its parts. The missing mass becomes energy: E = mc².

Reactor parts: fuel (enriched uranium), moderator (heavy water or graphite, slows neutrons), control rods (cadmium or boron, absorb neutrons), coolant, and shielding. A chain reaction occurs when released neutrons split more nuclei.

India's three-stage nuclear programme: stage 1 uses natural uranium in pressurised heavy water reactors, stage 2 uses fast breeder reactors with plutonium, and stage 3 aims to use thorium. India has large thorium resources. Nuclear waste is radioactive and needs safe storage. Kovvada in Andhra Pradesh has been proposed for a nuclear power project; check the latest official release for status.

Ocean, geothermal and hydrogen

• Tidal energy: the rise and fall of the sea level turns turbines at a barrage. It needs a narrow opening to the sea.
• Wave energy: moving surface waves drive a turbine.
• OTEC (ocean thermal energy conversion): uses the temperature difference between warm surface water and cold deep water.
• Geothermal energy: hot rocks heat underground water to give steam, which turns a turbine.
• Hydrogen: burns to give water only, so it is clean. Storage and cost are problems.

Worked examples and uses

Heat from a fuel: heat released = mass × calorific value. Burning 2 kg of a fuel of 45,000 kJ/kg gives 90,000 kJ. To get the same heat from wood of 18,000 kJ/kg you need 90,000 ÷ 18,000 = 5 kg. So a fuel with a higher calorific value needs less mass for the same heat.

Efficiency: efficiency = useful output energy ÷ input energy × 100 per cent. A device that converts 400 J input into 100 J of useful work is 25 per cent efficient.

Why an ideal fuel matters: it should have a high calorific value, a suitable ignition temperature (not too low, which is dangerous, and not too high, which makes it hard to light), burn at a moderate rate, leave little ash, produce no harmful gases and be easy to transport.

Greenhouse effect and energy: carbon dioxide from burning fuels traps heat. Planting and protecting forests helps take carbon back from the air, so energy planning and forest cover are linked. Using biogas, solar cookers and improved stoves in villages cuts the cutting of trees for firewood.

Energy in daily life of forest staff: solar lanterns and solar fencing power for crop-protection fences near forest edges, solar pumps for water holes for wildlife, and biogas units in forest-fringe villages are practical uses of non-conventional energy.

Advantages and limits at a glance

Chemistry of burning: combustion is a reaction with oxygen that gives heat and light. Carbon + oxygen gives carbon dioxide. Hydrogen + oxygen gives water. A substance catches fire only above its ignition temperature. Fuels should be stored away from heat sources.

Exam traps

• 1. Biogas vs CNG: both mainly methane, but biogas comes from dung, and CNG from natural gas.
• 2. Fission vs fusion: fission splits, fusion joins. The Sun runs on fusion.
• 3. Moderator vs control rod: moderator slows neutrons; control rod absorbs them.
• 4. Solar cell material is silicon, not carbon or copper.
• 5. Highest calorific value is hydrogen, not LPG or petrol.
• 6. Ethyl mercaptan is added to LPG for smell; it is not the main gas.
• 7. Renewable ≠ pollution-free: large dams cause forest loss.
• 8. Anthracite is the best coal; peat is the lowest rank.

One-liners

• 1. Coal, petroleum and natural gas are fossil fuels.
• 2. The main component of natural gas is methane.
• 3. LPG is mainly butane and propane.
• 4. Biogas is about 55–75 per cent methane.
• 5. Calorific value unit is kJ/kg.
• 6. E = mc² gives nuclear energy.
• 7. Uranium-235 is the common fission fuel.
• 8. Heavy water and graphite are moderators.
• 9. Cadmium and boron rods control a reactor.
• 10. Solar cells are made of silicon.
• 11. The Sun's energy is from nuclear fusion.
• 12. Tidal energy needs a barrage across a narrow sea inlet.

Practice questions

1. Which of the following is a non-renewable source of energy?

   1. Wind
   2. Tides
   3. Coal
   4. Firewood from planted trees
   Answer

   C. Coal

   Coal is a fossil fuel formed over millions of years and is used up faster than it forms.

2. The main component of natural gas and CNG is

   1. ethane
   2. hydrogen
   3. butane
   4. methane
   Answer

   D. methane

   CNG is mainly methane.

3. LPG is mainly a mixture of

   1. carbon monoxide and methane
   2. butane and propane
   3. methane and ethane
   4. hydrogen and oxygen
   Answer

   B. butane and propane

   LPG contains butane and propane under pressure.

4. A strong-smelling gas is added to LPG so that leaks can be detected. It is

   1. ethyl mercaptan
   2. sulphur trioxide
   3. methane
   4. ammonia
   Answer

   A. ethyl mercaptan

   Ethyl mercaptan gives the typical smell.

5. Which common fuel has the highest calorific value?

   1. Petrol
   2. Hydrogen
   3. Coal
   4. LPG
   Answer

   B. Hydrogen

   Hydrogen has about 150,000 kJ/kg.

6. The unit of calorific value is

   1. kJ/kg
   2. kW
   3. J/s
   4. kJ/m²
   Answer

   A. kJ/kg

   Calorific value is heat per kilogram of fuel burnt completely.

7. Which coal has the highest carbon content?

   1. Bituminous
   2. Peat
   3. Lignite
   4. Anthracite
   Answer

   D. Anthracite

   Rank: peat < lignite < bituminous < anthracite.

8. Biogas is mainly composed of

   1. carbon monoxide
   2. hydrogen sulphide
   3. methane
   4. propane
   Answer

   C. methane

   Biogas is about 55–75 per cent methane.

9. The leftover slurry from a biogas plant is useful as

   1. fuel for vehicles
   2. a coolant
   3. manure
   4. a moderator
   Answer

   C. manure

   Slurry is rich in nutrients and is used as manure.

10. Biogas is produced from dung by bacteria in the

    1. absence of oxygen
    2. presence of oxygen
    3. presence of strong sunlight
    4. presence of high pressure only
    Answer

    A. absence of oxygen

    Anaerobic decomposition forms biogas.

11. Solar cells are generally made of

    1. iron
    2. silicon
    3. carbon
    4. copper
    Answer

    B. silicon

    Silicon is the semiconductor used in photovoltaic cells.

12. The Sun produces energy by

    1. chemical burning
    2. radioactive decay of uranium
    3. nuclear fission
    4. nuclear fusion
    Answer

    D. nuclear fusion

    Hydrogen nuclei fuse into helium in the Sun.

13. In a solar cooker, the inner surface is painted black because black

    1. reflects heat best
    2. produces light
    3. absorbs heat best
    4. is a good insulator
    Answer

    C. absorbs heat best

    Black surfaces absorb the most radiant heat.

14. The equation that relates mass and energy in nuclear reactions is

    1. E = mc²
    2. P = VI
    3. F = ma
    4. V = IR
    Answer

    A. E = mc²

    Einstein's mass-energy relation.

15. The common fuel used for fission in reactors is

    1. carbon-12
    2. hydrogen-1
    3. sodium-23
    4. uranium-235
    Answer

    D. uranium-235

    U-235 splits on capturing a neutron.

16. In a nuclear reactor, the moderator is used to

    1. shield from radiation
    2. slow down neutrons
    3. absorb neutrons
    4. cool the walls
    Answer

    B. slow down neutrons

    Heavy water or graphite slows fast neutrons.

17. Control rods in a nuclear reactor are usually made of

    1. cadmium or boron
    2. iron or copper
    3. graphite or heavy water
    4. lead or zinc
    Answer

    A. cadmium or boron

    Cadmium and boron absorb neutrons.

18. Which of the following is used as a moderator?

    1. Cadmium
    2. Boron
    3. Uranium
    4. Heavy water
    Answer

    D. Heavy water

    Heavy water and graphite are moderators.

19. Which of these is a fusion reaction?

    1. Splitting of water by current
    2. Uranium splitting into two parts
    3. Hydrogen nuclei joining to form helium
    4. Burning of coal
    Answer

    C. Hydrogen nuclei joining to form helium

    Fusion joins light nuclei.

20. Tidal energy needs

    1. a coal-fired boiler
    2. a barrage across a narrow sea opening
    3. a large solar panel
    4. a tall chimney
    Answer

    B. a barrage across a narrow sea opening

    The rise and fall of water turns the turbines.

21. Ocean thermal energy conversion (OTEC) uses

    1. the speed of waves
    2. tides
    3. the salt content
    4. the temperature difference between surface and deep water
    Answer

    D. the temperature difference between surface and deep water

    A heat engine runs between warm and cold water layers.

22. In a hydroelectric plant, the energy change is

    1. chemical → heat → electrical
    2. nuclear → heat → electrical
    3. potential → kinetic → mechanical → electrical
    4. light → chemical → electrical
    Answer

    C. potential → kinetic → mechanical → electrical

    Stored water falls and turns the turbine.

23. Which gas, released by burning fossil fuels, mainly causes acid rain along with nitrogen oxides?

    1. Sulphur dioxide
    2. Oxygen
    3. Nitrogen
    4. Helium
    Answer

    A. Sulphur dioxide

    SO₂ and NOₓ form acids in rain.

24. Incomplete burning of fuel releases the poisonous gas

    1. helium
    2. carbon monoxide
    3. nitrogen
    4. carbon dioxide
    Answer

    B. carbon monoxide

    Lack of oxygen produces CO.

25. A wind turbine converts

    1. nuclear energy into sound
    2. kinetic energy of air into electrical energy
    3. heat of air into light
    4. chemical energy of air into heat
    Answer

    B. kinetic energy of air into electrical energy

    Moving air turns blades that drive a generator.

26. Which energy source is obtained from hot rocks below the earth's surface?

    1. Solar energy
    2. Tidal energy
    3. Wave energy
    4. Geothermal energy
    Answer

    D. Geothermal energy

    Hot rocks heat underground water into steam.

27. Which fuel gives only water when burnt?

    1. Hydrogen
    2. Methane
    3. Petrol
    4. Coal
    Answer

    A. Hydrogen

    2H₂ + O₂ → 2H₂O.

28. Which tree oil seeds are used in biodiesel production?

    1. Sandalwood and rosewood
    2. Teak and sal
    3. Pongamia and jatropha
    4. Neem and bamboo
    Answer

    C. Pongamia and jatropha

    Pongamia (Indian beech) and jatropha seeds give oil for biodiesel.

29. The third stage of India's nuclear programme aims to use

    1. natural uranium only
    2. plutonium only
    3. thorium
    4. coal gas
    Answer

    C. thorium

    India has large thorium resources.

30. Which of these is a thermal power plant energy conversion?

    1. Chemical energy of coal → heat → electricity
    2. Wind → electricity
    3. Water flow → electricity
    4. Sunlight → electricity
    Answer

    A. Chemical energy of coal → heat → electricity

    Coal is burnt to make steam which turns a turbine.

31. The heat released when 2 kg of fuel with a calorific value of 45,000 kJ/kg burns completely is

    1. 45,000 kJ
    2. 90,000 kJ
    3. 135,000 kJ
    4. 22,500 kJ
    Answer

    B. 90,000 kJ

    2 × 45,000 = 90,000 kJ.

32. A fuel releases 150,000 kJ when 1 kg burns. How much will 0.5 kg release?

    1. 50,000 kJ
    2. 300,000 kJ
    3. 150,000 kJ
    4. 75,000 kJ
    Answer

    D. 75,000 kJ

    Heat is proportional to mass: 0.5 × 150,000 = 75,000 kJ.

33. A biogas fuel of calorific value 36,000 kJ/kg is burnt. What mass gives 108,000 kJ?

    1. 4 kg
    2. 1.5 kg
    3. 2 kg
    4. 3 kg
    Answer

    D. 3 kg

    Mass = 108,000 ÷ 36,000 = 3 kg.

34. Statements: 1. Fusion needs extremely high temperatures. 2. Fission is the source of the Sun's energy. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    The Sun runs on fusion.

35. Statements: 1. Biogas burns without smoke. 2. Biogas is a fossil fuel. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Biogas is made from dung and plant waste, so it is renewable.

36. Statements: 1. Control rods slow down neutrons. 2. Heavy water acts as a moderator. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Control rods absorb neutrons; moderators slow them.

37. Statements: 1. Coal and petroleum are fossil fuels. 2. Fossil fuels are renewable. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    They are non-renewable.

38. Statements: 1. Hydro power projects can submerge forest land. 2. Hydro power is a renewable source. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are true.

39. Statements: 1. Solar energy can be used day and night without storage. 2. Solar cells convert light directly into electricity. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Solar cells need sunlight or storage.

40. Statements: 1. Wood has a higher calorific value than hydrogen. 2. LPG has a higher calorific value than coal. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    B. 2 only

    Hydrogen has by far the highest value.

41. Statements: 1. Petroleum is separated into products by fractional distillation. 2. CNG is mainly methane. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are true.

42. Statements: 1. Tidal energy depends on the pull of the Moon and Sun. 2. Wave energy depends on the movement of surface waves. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are true.

43. Statements: 1. Mass defect is converted into energy in nuclear reactions. 2. A chain reaction in fission is started by neutrons. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both statements are correct.

44. Statements: 1. Improved cookstoves reduce pressure on forests. 2. Burning firewood indoors produces smoke. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    C. Both 1 and 2

    Both are true.

45. Statements: 1. Heavy water and graphite are moderators. 2. Boron and cadmium are fuels in a reactor. Which is/are correct?

    1. 1 only
    2. 2 only
    3. Both 1 and 2
    4. Neither 1 nor 2
    Answer

    A. 1 only

    Boron and cadmium are control-rod materials.