8.5 Sources of Energy

Why Energy Sources Matter for RRB Group D

India is among the world's largest energy consumers, and the railways are themselves a major consumer of electricity and diesel. RRB Group D usually includes 1–2 questions testing the classification of conventional vs non-conventional sources, the working of a solar cell, hydroelectric and wind power, and the basic principle of nuclear fission.

Classification of Energy Sources

Conventional (Non-Renewable) Sources

Formed over millions of years from buried plant and animal matter; available in limited quantities and cannot be replenished quickly.

1. Fossil fuels — coal, petroleum, natural gas. Burning them releases CO₂, SO₂, NOₓ, and particulates, contributing to air pollution and global warming.
2. Nuclear energy — energy released by fission of heavy nuclei such as uranium-235 or plutonium-239. The energy density is enormous: NCERT states that the fission of one uranium atom releases about ten million times the energy released by burning one atom of carbon.

Non-Conventional (Renewable) Sources

Replenished naturally and cause far less pollution.

1. Solar energy — Sunlight is converted to electricity by photovoltaic (solar) cells made of semiconductor materials such as silicon. A typical solar cell produces 0.5–0.7 V and a few milliamperes; cells are connected in series-parallel arrays to form panels.
2. Wind energy — A wind turbine's rotating blades drive a generator. Wind farms are located in coastal and high-altitude regions where wind speeds exceed 15 km/h. India has major installations in Tamil Nadu, Gujarat, and Rajasthan.
3. Hydroelectric energy — Stored water in a dam falls through a turbine, spinning a generator. It is the largest renewable contributor to India's electricity.
4. Biomass energy — Plant and animal waste (cow dung, agricultural residue, municipal waste) is converted into biogas (methane + CO₂) in a biogas plant. Biogas is a clean, smokeless fuel used for cooking and lighting.
5. Tidal energy — Harnessed by building a small dam across a narrow opening of a sea coast. Flood tide fills the reservoir; the ebb tide runs a turbine. Limited to a few sites such as the Gulf of Kutch.
6. Geothermal energy — Heat from hot rocks below the Earth's surface, used to produce steam and drive turbines. Hot springs in Manikaran (Himachal Pradesh) and Puga Valley (Ladakh) are potential sites.
7. Ocean thermal energy — Uses the temperature difference between warm surface water and cold deep water to run a heat engine.

Comparison: Conventional vs Renewable

Advantages and Limitations of Major Renewables

Environmental Consequences of Energy Use

Burning fossil fuels releases carbon dioxide (greenhouse effect), sulphur dioxide (acid rain), and nitrogen oxides (smog). The greenhouse effect traps heat and raises Earth's average temperature, causing climate change, sea-level rise, and erratic monsoons — directly relevant to India's agriculture.

Worked Example: Energy Calculation

A solar panel of area 2 m² receives an average solar energy of 1.5 × 10³ W/m². If the panel converts 12% of incident energy to electricity, find the electrical power output.

Incident power = 1.5 × 10³ × 2 = 3,000 W. Electrical power = 12% × 3,000 = 360 W.

Over 6 hours of effective sunlight, energy produced = 360 × 6 = 2,160 Wh ≈ 2.16 kWh per day.

India's Energy Mix (Indicative)

India generates the large majority of its electricity from coal. On Central Electricity Authority figures for FY 2024-25, of about 1,830 billion units generated, coal supplied roughly 73%, followed by hydro and solar at about 8% each, wind about 5%, nuclear about 3%, and gas plus other renewables making up the rest. Note the standard exam trap here: coal's share of installed capacity is far lower (about 47% as of March 2025) than its share of generation, because thermal plants run at much higher utilisation than solar and wind. The government's target of 500 GW of non-fossil capacity by 2030 makes this a current-affairs topic tested in the General Awareness section as well.

How a Solar Cell Works (Step by Step)

The solar cell is the single most-tested device in the energy-sources topic, and RRB often phrases the question around its working rather than its classification. A solar cell is a p-n junction made from a semiconductor (typically silicon) doped to create a p-side (hole-rich) and an n-side (electron-rich).

1. Light absorption. When sunlight strikes the cell, photons with energy greater than the silicon band gap transfer their energy to electrons in the p-layer, knocking them loose and creating electron-hole pairs.
2. Separation at the junction. The built-in electric field at the p-n junction sweeps the freed electrons toward the n-side and the holes toward the p-side. This separation prevents the pairs from simply recombining.
3. Current flow. When an external circuit connects the n-side (negative terminal) and the p-side (positive terminal), electrons flow through the external load, delivering electrical current.
4. Power output. A single silicon cell produces about 0.5–0.7 V and a few tens of milliamps; cells are wired in series to raise voltage and in parallel to raise current, forming a panel; panels are arrayed into a plant.

The efficiency of a commercial silicon cell sits at roughly 15–22%; the rest of the incident energy is lost as heat (thermalisation, recombination) or reflected. RRB may ask the order of voltage per cell (about 0.5 V) or why cells are connected in series-parallel arrays (to reach usable voltage and current).

Hydroelectric and Wind: The Two Workhorse Renewables

Hydroelectric power converts the potential energy of stored water into electrical energy. Water behind a dam falls through a penstock onto a turbine, spinning it; the turbine drives a generator that produces electricity. The power output depends on the flow rate and the head (the height the water falls):

$P = \eta \, \rho \, g \, Q \, h$

where $\eta$ is turbine-generator efficiency, $\rho$ the water density, $Q$ the volume flow rate, and $h$ the head. India's largest hydroelectric plants (Bhakra Nangal, Tehri, Koyna) illustrate the formula — a high head and a large catchment yield high output. The drawback is ecological: large dams submerge forests, displace communities, alter downstream flow, and are vulnerable to siltation that reduces capacity over decades.

Wind power uses the kinetic energy of moving air. A turbine's blades rotate when wind strikes them, turning a generator through a gearbox. The power scales with the cube of the wind speed ($P \propto v^{3}$), so a doubling of wind speed yields eight times the power — which is why site selection (consistent high wind) matters more than turbine count. India's installed wind capacity is concentrated in Tamil Nadu (the Muppandal cluster), Gujarat, and Rajasthan, where coastal and ridge-line wind speeds regularly exceed the 15 km/h cut-in threshold.

Nuclear Energy: Fission, Not Fusion

A nuclear reactor harnesses controlled fission of a heavy nucleus. A uranium-235 nucleus absorbs a slow (thermal) neutron, becomes unstable, and splits into two lighter fission fragments, releasing energy and 2–3 additional neutrons. Those neutrons strike other U-235 nuclei, sustaining a chain reaction; control rods (of cadmium or boron) absorb excess neutrons to keep the reaction rate steady. A moderator (heavy water, light water, or graphite) slows fast neutrons to the thermal speeds at which U-235 fission is most likely.

The energy release is enormous: the fission of one U-235 nucleus releases about 200 MeV, which is roughly ten million times the energy from burning a single carbon atom — the comparison NCERT uses. India operates pressurised heavy-water reactors (PHWRs) using natural uranium and heavy-water moderation, plus a growing fleet of light-water reactors. The persistent exam trap is fission vs fusion: commercial reactors use fission (splitting heavy nuclei); fusion (combining light nuclei, as in the Sun) is not yet a commercial power source on Earth despite research projects like ITER.

Distinguishing the Sources on the Exam

RRB frequently tests classification through a four-option question in which three sources are renewable and one is not (or vice versa). The clean rule:

• Renewable = naturally replenished on a human timescale: solar, wind, hydro, biomass, tidal, geothermal, ocean thermal.
• Non-renewable = finite stocks depleted by use: coal, petroleum, natural gas, and nuclear (uranium is a mined, finite fuel, even though nuclear plants emit no CO₂).

The subtle point is that nuclear is often misclassified as renewable because it is low-carbon. It is non-renewable but low-emission. RRB sets that confusion as a distractor, so memorise the distinction: renewable is about replenishment, not about carbon emissions.

Environmental and Climate Linkage

Burning fossil fuels releases CO₂ (the dominant greenhouse gas), SO₂ (a precursor of acid rain), and NOₓ (a smog and acid-rain precursor). The greenhouse effect traps outgoing long-wave radiation and raises Earth's average surface temperature; for India the directly relevant consequences are erratic monsoons, more intense cyclones, heatwaves, and glacial retreat in the Himalayas that feeds major rivers. Switching to renewables is therefore not merely an environmental preference but a climate adaptation imperative, which is why the 500 GW non-fossil target is a current-affairs staple that overlaps the General Awareness section.