Chapter 5: Sources of Energy

Solution

Coal is a non-renewable source of energy. It was formed from dead plant matter over millions of years and exists in limited quantities. Once used up, it cannot be replenished in a human lifetime. Solar, wind, and tidal energy are all renewable — they are continuously available from natural processes.

Solution

When fossil fuels (especially coal and diesel) burn, they release sulphur dioxide (SO₂) and nitrogen oxides (NOₓ) along with other gases. These gases dissolve in atmospheric moisture to form sulphuric acid and nitric acid, which fall as acid rain. Acid rain damages buildings, corrodes metals, harms aquatic life, and damages vegetation.

Solution

In a hydroelectric power plant, water stored at a height behind a dam has gravitational potential energy. When the water is released, it falls and gains kinetic energy. This kinetic energy rotates turbines, which are connected to generators that convert the mechanical energy into electrical energy.

Energy flow: Potential energy → Kinetic energy → Mechanical energy (turbine) → Electrical energy (generator)

Solution

Solar cells (photovoltaic cells) are made from semiconductors, most commonly silicon (both monocrystalline and polycrystalline). When sunlight falls on the semiconductor junction, it knocks electrons loose, creating an electric current. Other semiconductor materials used include gallium arsenide and cadmium telluride.

Solution

The Sun produces energy through nuclear fusion. In its core, where temperatures reach about 15 million °C, hydrogen nuclei fuse together to form helium, releasing enormous amounts of energy according to Einstein's equation E = mc².

4 hydrogen nuclei → 1 helium nucleus + 2 positrons + energy

Nuclear fission is the splitting of heavy nuclei and is used in nuclear power plants. Fusion releases even more energy per unit mass than fission.

Solution

Advantages:

• Renewable: Wind is a natural phenomenon driven by solar heating of the Earth. It will never run out.
• Clean energy: No air pollution, no greenhouse gas emissions during operation, no fuel cost.

Limitations:

• Wind dependency: Wind speed must be consistently above 15 km/h. Calm days mean no power generation.
• Large area required: Wind farms need vast open spaces. A single wind turbine cannot generate much power, so many turbines are needed.

Solution

Biogas is produced by anaerobic decomposition of organic waste. Its composition is approximately:

• Methane (CH₄): ~75% — the combustible component that provides the energy
• Carbon dioxide (CO₂): ~20-25%
• Traces of hydrogen, hydrogen sulphide, and other gases

Methane is an excellent fuel with a high calorific value. Biogas burns with a clean blue flame and produces minimal smoke.

Solution

A good source of energy should have the following characteristics:

• High calorific value: It should produce a large amount of energy per unit mass or volume.
• Easy availability and accessibility: It should be easily obtainable and not require complex infrastructure to access.
• Easy to store and transport: The fuel should be safe and convenient to store and move from one place to another.
• Economical: It should be affordable and cost-effective for widespread use.
• Environment-friendly: It should cause minimum pollution and environmental damage during use.

Solution

Geothermal energy is the heat energy from the interior of the Earth. In certain regions (called hot spots), due to geological activity, hot molten rocks (magma) are relatively close to the Earth's surface.

How it is harnessed:

• Underground water comes in contact with these hot rocks and gets heated, sometimes forming steam.
• This steam rises through natural vents or drilled wells.
• The steam is channelled through pipes to drive turbines connected to generators.
• The electricity produced is fed into the power grid.

Examples: Iceland, New Zealand, and parts of the USA (Yellowstone region) are well-known for geothermal energy. In India, Manikaran in Himachal Pradesh and Puga Valley in Ladakh have geothermal potential.

Solution

A solar cell (photovoltaic cell) converts solar energy directly into electrical energy using the photovoltaic effect in semiconductors. Solar cookers, water heaters, and furnaces convert solar energy into heat energy, not electrical energy directly.

Solution

Harnessing tidal energy:

• A dam (barrage) is built across a narrow opening to the sea (an estuary or bay).
• During high tide, seawater flows in and is trapped behind the dam.
• During low tide, the stored water is released through turbines, which rotate and drive generators to produce electricity.

Why not widely used:

• Very few suitable locations: Requires a large tidal range (difference between high and low tide levels) and a suitable geography for dam construction.
• Relatively small energy output: Tides occur only twice a day, limiting generation time.
• High construction cost of the barrage.
• Environmental impact on marine ecosystems and coastal habitats.

Solution

Hydrogen is considered a clean fuel because when it burns, the only product is water:

2H₂ + O₂ → 2H₂O + energy

There are no carbon emissions, no SO₂, no particulate matter — just water. However, hydrogen is not easily available as a free gas (it must be produced, usually from water using electricity). It is also difficult to store and transport safely due to its highly flammable nature.

Solution

Wind energy is an indirect form of solar energy. The Sun heats the Earth's surface unevenly — land heats up faster than water, equatorial regions receive more heat than poles. This uneven heating creates pressure differences in the atmosphere, which cause air to move — that's wind.

So the energy in wind ultimately comes from the Sun, making it an indirect form of solar energy. Other indirect forms include hydroelectric (water cycle driven by solar evaporation), biomass (photosynthesis powered by sunlight), and wave energy.

Solar cells and cookers use sunlight directly. Nuclear energy comes from nuclear reactions, not from the Sun.

Solution

Thermal power plants are set up near coal or oil fields primarily to reduce transportation costs. Coal is bulky and heavy — transporting it over long distances is expensive and energy-consuming. On the other hand, electricity can be transmitted efficiently over long distances through power lines (especially using high-voltage AC, which minimises transmission losses). So it is more economical to generate electricity near the fuel source and transmit the electricity.

Solution

Black-painted interior:

• Black surfaces are the best absorbers of heat radiation. A black interior absorbs maximum sunlight and converts it to heat, raising the temperature inside the cooker.

Glass cover:

• Glass allows sunlight (short-wavelength infrared and visible light) to enter the box.
• The heated interior emits longer-wavelength infrared radiation, which cannot pass through the glass easily.
• This traps heat inside — a greenhouse effect — maintaining high temperatures for cooking.

Plane mirror (reflector):

• The mirror reflects additional sunlight into the box, increasing the intensity of solar radiation falling on the food.
• This raises the temperature further (up to 100-140°C), improving cooking efficiency.

Solution

Control rods (made of boron or cadmium) are used to absorb excess neutrons in a nuclear reactor. By absorbing neutrons, they regulate the rate of the chain reaction.

• Inserting control rods deeper → absorb more neutrons → slow down the reaction.
• Withdrawing control rods → fewer neutrons absorbed → speed up the reaction.

The moderator (like heavy water or graphite) slows down the neutrons, and the shield contains the radiation. The control rods specifically control the chain reaction rate.

Solution

Nuclear Fission:

• A heavy nucleus (like U-235) is split into two lighter nuclei by bombarding with a neutron.
• Releases energy (~200 MeV per event) and additional neutrons.
• Can be controlled using control rods.
• Used in nuclear power plants and atomic bombs.

Nuclear Fusion:

• Two light nuclei (like hydrogen isotopes) combine to form a heavier nucleus.
• Releases even more energy per unit mass than fission.
• Requires extremely high temperature (~10⁷ K) and pressure to overcome electrostatic repulsion.
• Powers the Sun and stars. Not yet achieved in a controlled manner for power generation.

Nuclear power plants use fission because we can control fission reactions with current technology. Controlled fusion remains a research challenge.

Solution

OTEC requires a temperature difference of at least 20°C between the warm surface water and the cold deep water (at depths of about 1 km). The warm surface water is used to vaporise a volatile working fluid (like ammonia), which drives a turbine. The cold deep water is used to condense the vapour. A temperature difference of less than 20°C would not provide enough thermodynamic efficiency to be practical.

Solution

While some energy sources are cleaner than others, no source is completely pollution-free when we consider the full lifecycle:

• Fossil fuels: Release CO₂, SO₂, NOₓ, and particulate matter during combustion. Cause global warming and acid rain.
• Nuclear energy: Generates radioactive waste that remains hazardous for thousands of years. Mining and processing uranium also cause environmental damage.
• Solar energy: Manufacturing solar cells involves toxic chemicals and energy-intensive processes. Disposal of old panels creates electronic waste.
• Wind energy: Manufacturing turbines requires energy and materials. Turbines cause noise pollution and can kill birds and bats.
• Hydroelectric: Large dams submerge forests and displace communities. Decomposing vegetation in reservoirs releases methane (a greenhouse gas).
• Biomass/biogas: Produces CO₂ when burned (though this is partially offset by the CO₂ absorbed during plant growth).

Therefore, we should focus on using the least polluting sources and on energy conservation to minimise overall environmental impact.

Solution

Why non-renewable:

• Fossil fuels (coal, petroleum, natural gas) were formed from the remains of organisms that lived millions of years ago.
• The process of formation takes millions of years under specific geological conditions.
• We are consuming them at a rate far faster than they can be formed.
• Once exhausted, they cannot be replaced in any foreseeable timeframe.

If we continue at the current rate:

• Resource depletion: Known petroleum reserves may last only 40-50 years; coal may last 100-200 years.
• Climate change: Continued CO₂ emissions will worsen global warming, leading to rising sea levels, extreme weather events, and ecosystem destruction.
• Air quality: Increased air pollution and associated health problems.
• Energy crisis: As supplies dwindle, prices will rise sharply, causing economic disruption.

This is why the transition to renewable energy sources is urgent and essential.

Solution

Given: 1 kg uranium ≡ 25,000 tonnes of coal

For 10 kg uranium:

Coal equivalent = 10 × 25,000 = 250,000 tonnes of coal

This illustrates the enormous energy density of nuclear fuel. Transporting 250,000 tonnes of coal per day would require about 2,500 railway wagons — this is why nuclear power is so space-efficient compared to coal power, despite its other challenges.

Solution

Despite the Sun providing abundant energy, solar cells face several practical challenges:

• High manufacturing cost: The silicon used in solar cells must be extremely pure (99.9999%). The processing and manufacturing is expensive, making the initial investment high.
• Low efficiency: Current commercial solar cells convert only about 15-20% of incident solar energy into electricity. Most of the energy is lost as heat or reflection.
• Intermittent availability: Solar energy is available only during daytime and is affected by clouds, rain, and seasonal variations. Expensive batteries are needed to store energy for night use.
• Large area requirement: To generate a significant amount of power, very large areas of solar panels are needed. A 1 MW solar plant requires about 4-5 acres of land.

As technology improves and costs decrease (which is happening rapidly), solar energy is becoming increasingly competitive with fossil fuels.

Solution

Construction:

• A biogas plant consists of a dome-shaped structure built with bricks.
• It has an inlet (mixing tank) where biomass (like cattle dung) is mixed with water to form slurry and fed into the digester.
• The digester is an underground sealed chamber where anaerobic bacteria decompose the slurry.
• A gas outlet at the top of the dome leads to a pipe for gas supply.
• An outlet (overflow tank) allows the spent slurry to exit.

Working:

• Anaerobic bacteria decompose the biomass in the absence of oxygen.
• This produces biogas (mainly methane ~75%, CO₂ ~25%).
• The gas collects in the dome, building up pressure, and is piped to kitchens for cooking or to generators for electricity.

Why biogas is better than cow dung cakes:

• No smoke: Biogas burns with a clean flame; dung cakes produce a lot of smoke, causing respiratory problems.
• Higher calorific value: Methane has much higher energy content than raw dung.
• Manure as byproduct: The spent slurry is an excellent fertiliser, unlike dung cakes whose ash has no nutrient value.
• Hygienic: Biogas production sanitises the waste, killing pathogens.

Solution

For a remote hilly village without grid connectivity, micro-hydro power combined with solar panels is the most suitable option because:

• Micro-hydro: Hilly areas often have streams and rivers with flowing water. Small hydroelectric setups (micro-hydro) can generate reliable electricity without massive dams.
• Solar panels: Provide additional power during daytime and can supplement hydro during dry seasons.

Thermal and nuclear power plants are massive installations — impractical for a small village. Tidal energy requires coastal locations, not hilly terrain. The combination of micro-hydro and solar provides both reliability and suitability for the geographic conditions.

Solution

Solar Energy:

Advantages:

• India receives abundant sunlight (~300 sunny days/year in many regions) — enormous potential.
• Clean, renewable, and increasingly affordable (costs have dropped over 80% in the last decade).

Disadvantages:

• Intermittent — no power at night or on cloudy days; needs expensive battery storage.
• Requires large land areas, competing with agriculture in a densely populated country.

Nuclear Energy:

Advantages:

• Very high energy output from small fuel quantity — ideal for baseload (24/7) power.
• India has thorium reserves (one of the world's largest) that can be used in advanced reactors.

Disadvantages:

• Radioactive waste disposal is a long-term challenge with no perfect solution yet.
• High capital costs and risk of accidents (though safety technology has improved significantly).

Recommendation: India should pursue both as part of a diversified energy mix. Solar for decentralised, rapidly deployable power; nuclear for reliable baseload generation. Over-reliance on any single source creates vulnerability.