BharatNotes Overview
Energy — the capacity to do work — is the foundation of modern civilisation. From burning coal to splitting atoms to harnessing sunlight, the physics of energy conversion determines how societies develop, how economies grow, and how the climate changes. For UPSC, energy science spans GS3 (Science & Technology, Energy Security, Environment) and is frequently tested in both Prelims and Mains.
Forms of Energy
| Form | Description | Example |
|---|---|---|
| Kinetic Energy | Energy of motion; KE = ½mv² | Moving vehicle, flowing water, wind |
| Potential Energy | Energy stored by position or configuration | Water behind a dam (gravitational PE), compressed spring (elastic PE) |
| Thermal (Heat) Energy | Energy due to random motion of molecules | Hot water, steam in a boiler |
| Chemical Energy | Energy stored in chemical bonds | Fossil fuels, food, batteries |
| Nuclear Energy | Energy stored in the nucleus of an atom | Uranium-235 fission, hydrogen fusion |
| Electrical Energy | Energy of moving electric charges | Current in a wire, lightning |
| Radiant (Electromagnetic) Energy | Energy carried by electromagnetic waves | Sunlight, radio waves, X-rays |
Law of Conservation of Energy
Energy can neither be created nor destroyed — it can only be converted from one form to another. The total energy of an isolated system remains constant. This is the First Law of Thermodynamics.
For Prelims: Every energy technology is fundamentally an energy CONVERSION device — solar cells convert radiant → electrical; wind turbines convert kinetic → electrical; thermal power plants convert chemical → thermal → mechanical → electrical.
Conventional Energy Sources
Coal (Thermal Power)
| Aspect | Details |
|---|---|
| Principle | Chemical energy in coal → thermal energy (combustion) → steam → mechanical energy (turbine) → electrical energy (generator) |
| Efficiency | ~33–40% (subcritical); ~42–45% (supercritical); ~46–49% (ultra-supercritical) |
| India's coal capacity | ~49% of India's installed power capacity (2025); coal-fired generation fell ~3% in 2025 — first significant decline |
| Environmental impact | Largest source of CO₂ emissions; SO₂ and NOₓ (acid rain); particulate matter; fly ash disposal; mercury emissions |
Petroleum and Natural Gas
| Aspect | Details |
|---|---|
| Petroleum | Used primarily for transport (petrol, diesel, aviation fuel) and petrochemicals; India imports ~85% of its crude oil needs |
| Natural Gas | Cleaner than coal (~50% less CO₂ per unit energy); used in Combined Cycle Gas Turbine (CCGT) plants with efficiency ~55–60% |
| CCGT Principle | Gas turbine (Brayton cycle) + steam turbine (Rankine cycle) in series — waste heat from gas turbine generates steam for a second turbine, boosting overall efficiency |
Nuclear Energy
Nuclear Fission
| Aspect | Details |
|---|---|
| Principle | A heavy nucleus (e.g., Uranium-235) absorbs a thermal neutron and splits into two lighter nuclei + 2–3 neutrons + energy (~200 MeV per fission event) |
| Chain Reaction | Released neutrons cause further fissions → self-sustaining chain reaction; controlled in a reactor by moderators (slow neutrons) and control rods (absorb excess neutrons) |
| Fuel | U-235 (only 0.7% of natural uranium — the rest is U-238); enriched to 3–5% for most reactors; India also uses Thorium-232 (3-stage programme) |
| Energy Density | 1 kg of U-235 releases energy equivalent to ~2,500 tonnes of coal |
Nuclear Reactor Types
| Type | Moderator | Coolant | Fuel | Global Share | India's Fleet |
|---|---|---|---|---|---|
| PWR (Pressurised Water Reactor) | Light water (H₂O) | Pressurised light water | Enriched uranium (3–5%) | ~70% of global reactors | Kudankulam (2 units operational, Tamil Nadu — with Russian collaboration) |
| BWR (Boiling Water Reactor) | Light water | Boiling light water (direct steam to turbine) | Enriched uranium | ~15% globally | Tarapur (2 units, Maharashtra — India's oldest nuclear plant) |
| PHWR (Pressurised Heavy Water Reactor) | Heavy water (D₂O) | Pressurised heavy water | Natural uranium (0.7% U-235) | ~11% globally | India's workhorse — 19 PHWRs operational; NPCIL design based on CANDU technology |
| FBTR/FBR (Fast Breeder Reactor) | No moderator (fast neutrons) | Liquid sodium | Plutonium-239 + U-238 (or Th-232) | Limited globally | PFBR at Kalpakkam (500 MWe, under commissioning); Stage 2 of India's 3-stage programme |
For Prelims: India has 19 PHWRs + 2 PWRs + 2 BWRs. India's 3-stage nuclear programme: Stage 1 = PHWR (natural uranium), Stage 2 = FBR (plutonium), Stage 3 = thorium-based reactors. India has the world's largest thorium reserves (~25% of global).
India's Three-Stage Nuclear Programme
| Stage | Reactor Type | Fuel | Purpose |
|---|---|---|---|
| Stage 1 | PHWR | Natural uranium → Plutonium-239 (bred from U-238) | Currently operational; breeds Pu-239 for Stage 2 |
| Stage 2 | Fast Breeder Reactor (FBR) | Pu-239 + U-238 (blanket of Th-232) | Breeds U-233 from thorium for Stage 3; PFBR at Kalpakkam |
| Stage 3 | Advanced Heavy Water Reactor (AHWR) | U-233 + Thorium-232 | Exploits India's vast thorium reserves (~963,000 tonnes); self-sustaining thorium cycle |
Nuclear Fusion
| Aspect | Details |
|---|---|
| Principle | Light nuclei (hydrogen isotopes — deuterium and tritium) fuse at extreme temperatures (~150 million °C) to form helium + neutron + enormous energy (17.6 MeV per D-T fusion) |
| Fuel | Deuterium (from seawater — virtually unlimited) + Tritium (bred from lithium) |
| Advantages | No CO₂ emissions; no long-lived radioactive waste; fuel is abundant; no meltdown risk |
| Challenges | Achieving and sustaining plasma at 150+ million °C; plasma containment; energy break-even not yet achieved at commercial scale |
| ITER (International Thermonuclear Experimental Reactor) | World's largest fusion experiment; located in Cadarache, France; 35 nations including India; first plasma expected 2033–2034; D-T operations by 2039; tokamak design — uses powerful magnetic fields to confine plasma in a doughnut-shaped chamber |
For Prelims: Fusion = light nuclei combine (opposite of fission). ITER: Cadarache, France; 35 countries including India; tokamak design. Fusion fuel = deuterium (seawater) + tritium (lithium). No CO₂, no long-lived waste. First plasma now expected 2033–34.
Solar Energy
Photovoltaic (PV) Effect
| Aspect | Details |
|---|---|
| Principle | Photons from sunlight strike a semiconductor (silicon) and knock electrons free, creating a flow of electric current — this is the photovoltaic effect, discovered by Edmond Becquerel in 1839 |
| Solar Cell | A p-n junction diode — p-type silicon (boron-doped, excess "holes") + n-type silicon (phosphorus-doped, excess electrons); sunlight creates electron-hole pairs at the junction → current flows |
| Efficiency | Commercial monocrystalline silicon cells: ~20–24%; multi-junction cells (space applications): ~47%; perovskite solar cells (emerging): ~25–33% in lab |
| India's solar capacity | ~123 GW installed (as of mid-2025); target 280 GW by 2030; flagship schemes — PM-KUSUM (farmers), PM Surya Ghar (rooftop) |
Concentrated Solar Power (CSP)
| Aspect | Details |
|---|---|
| Principle | Mirrors/lenses concentrate sunlight to heat a fluid → steam → turbine → electricity |
| Types | Parabolic trough, solar tower (power tower), Fresnel reflectors, dish-Stirling |
| Advantage over PV | Can store thermal energy (molten salt) for generation after sunset |
| India | Limited deployment; Bikaner (Rajasthan) has CSP projects |
Wind Energy
| Aspect | Details |
|---|---|
| Principle | Kinetic energy of wind → mechanical rotation of turbine blades → electrical energy (generator) |
| Betz Limit | Theoretical maximum efficiency of a wind turbine = 59.3% (16/27) — derived by German physicist Albert Betz in 1919; no turbine can extract more than 59.3% of the wind's kinetic energy |
| Practical efficiency | Real turbines achieve ~35–45% overall efficiency (75–80% of the Betz limit at peak) |
| HAWT vs VAWT | Horizontal Axis Wind Turbine (HAWT) — blades rotate around a horizontal axis; most common; higher efficiency. Vertical Axis Wind Turbine (VAWT) — blades rotate around a vertical axis; works in any wind direction; lower efficiency |
| India's wind capacity | ~54 GW installed (end 2025); 4th largest globally; major states — Tamil Nadu, Gujarat, Rajasthan, Karnataka, Maharashtra |
| Offshore wind | India's target: 37 GW by 2030; first auction completed for Gujarat coast (2023) |
For Prelims: Betz Limit = 59.3% = maximum theoretical efficiency of a wind turbine. HAWT is most common. India = 4th largest wind energy producer globally.
Hydropower
| Type | Principle | Example |
|---|---|---|
| Conventional (Dam-based) | Water stored behind a dam → gravitational PE converted to KE as water falls → turbine → generator | Tehri Dam (Uttarakhand, 2,400 MW); Sardar Sarovar (Gujarat/MP, 1,450 MW) |
| Run-of-River | Diverts a portion of river flow through a turbine without large reservoir; minimal storage | Most Himalayan projects; Nathpa Jhakri (Himachal Pradesh, 1,500 MW) |
| Pumped Storage | Water pumped uphill to a reservoir during low demand (using surplus electricity); released downhill during peak demand — acts as a giant "battery" | Purulia PSP (West Bengal, 900 MW); Kadamparai (Tamil Nadu) |
| Small/Micro Hydro | <25 MW (small) / <100 kW (micro); no large dam; decentralised power for remote areas | Widely used in Himalayan and NE India states |
| Key Fact | Detail |
|---|---|
| India's hydropower capacity | ~47 GW (including large hydro); ~5% of electricity generation |
| Advantage | Zero emissions during operation; base-load + peaking capability; long lifespan (50–100 years) |
| Concerns | Displacement (Tehri, Sardar Sarovar); seismicity risk in Himalayas; downstream ecological impact; siltation reducing dam life |
Hydrogen Energy
Colour Coding of Hydrogen
| Type | Production Method | CO₂ Emissions | Cost (approx.) |
|---|---|---|---|
| Grey Hydrogen | Steam methane reforming (SMR) from natural gas | 10–19 tonnes CO₂ per tonne H₂; ~95% of global hydrogen is grey | Cheapest (~$1–2/kg) |
| Blue Hydrogen | SMR + Carbon Capture and Storage (CCS) | Lower than grey, but CCS does NOT capture 100% | $1.5–3/kg |
| Green Hydrogen | Electrolysis of water using renewable energy (solar/wind) | Zero CO₂ | $3–6/kg (declining rapidly) |
| Pink Hydrogen | Electrolysis using nuclear electricity | Zero CO₂ (if nuclear is considered clean) | Similar to green |
Fuel Cells
| Aspect | Details |
|---|---|
| Principle | Electrochemical device that converts hydrogen + oxygen → electricity + water + heat; reverse of electrolysis |
| PEM (Proton Exchange Membrane) Fuel Cell | Low operating temperature (~80°C); fast start-up; used in vehicles (Toyota Mirai, Hyundai Nexo) and portable applications |
| SOFC (Solid Oxide Fuel Cell) | High operating temperature (~600–1,000°C); high efficiency (~60%); used for stationary power generation |
| Advantages | Zero tailpipe emissions (only water); high efficiency; quiet operation |
| Challenges | Hydrogen storage (high pressure/low temperature); lack of refuelling infrastructure; cost of green hydrogen production |
For Prelims: Green hydrogen = electrolysis + renewable energy = zero CO₂. PEM fuel cell = hydrogen + oxygen → electricity + water. India's National Green Hydrogen Mission (2023) targets 5 million tonnes/year of green hydrogen by 2030.
Other Renewable Sources
| Source | Principle | India's Status |
|---|---|---|
| Geothermal | Heat from Earth's interior drives turbines; best in volcanic/tectonic regions | ~10,000 MW potential; Puga Valley (Ladakh) most promising; no commercial plant yet |
| Tidal Energy | KE and PE of tides drives turbines | Gulf of Khambhat (Gujarat) — ~7,000 MW potential; Gulf of Kutch; no commercial plant |
| Biomass | Chemical energy in organic matter (crop residue, wood, dung) → combustion/gasification → electricity/heat | ~10 GW installed; significant in rural India; biomass-based co-generation in sugar mills |
| Wave Energy | KE of ocean surface waves → mechanical/electrical energy | Vizhinjam (Kerala) — experimental oscillating water column plant |
India's Energy Mix (2025)
| Source | Installed Capacity | Share |
|---|---|---|
| Coal (Thermal) | ~250 GW | ~49% |
| Solar | ~133 GW | ~26% |
| Wind | ~54 GW | ~10% |
| Large Hydro | ~47 GW | ~9% |
| Nuclear | ~8.8 GW | ~2% |
| Biomass + Small Hydro + Others | ~30 GW | ~6% |
| Total | ~505 GW (October 2025) | — |
For Prelims: India's total installed capacity crossed 500 GW in 2025. Non-fossil fuel sources = ~51.5% of installed capacity (as of November 2025) — India has achieved the milestone of 50% non-fossil fuel capacity ahead of its 2030 target. Solar leads renewable additions.
Energy Efficiency Comparison
| Source | Overall Efficiency | Capacity Factor |
|---|---|---|
| Coal (Subcritical) | ~33–37% | ~55–60% (India) |
| Coal (Supercritical) | ~42–45% | ~60–65% |
| CCGT (Gas) | ~55–60% | ~25–30% (India — gas shortage) |
| Nuclear | ~33–37% (thermal efficiency) | ~70–90% globally; ~80% in India |
| Solar PV | ~20–24% (cell efficiency) | ~18–22% (India; depends on irradiance) |
| Wind | ~35–45% (turbine efficiency, Betz limit applies) | ~25–35% |
| Hydropower | ~80–90% (turbine efficiency) | ~30–40% (India; seasonal variation) |
Recent Developments (2024–2026)
India's Record Renewable Energy Expansion (2024–25)
India added 24.5 GW of solar capacity and 3.4 GW of wind capacity in 2024 — making India the world's third-largest solar producer, overtaking Japan. India's total non-fossil fuel installed capacity reached 217.62 GW by January 2025 (target: 500 GW by 2030). The PM Surya Ghar: Muft Bijli Yojana (launched 2024) facilitated 7 lakh rooftop solar installations within 10 months. India's solar module manufacturing capacity more than doubled from 38 GW to 74 GW in FY 2024–25.
UPSC angle: India's renewable energy data (solar 24.5 GW added in 2024, third globally, 217.62 GW total non-fossil capacity) are among the most frequently tested energy policy figures for GS3 — use these specific numbers.
Nuclear Energy Mission and SMRs — India's Third Energy Pillar (2025)
Union Budget 2025–26 launched a Nuclear Energy Mission with ₹20,000 crore for SMR R&D, targeting five indigenously built SMRs by 2033. The PFBR at Kalpakkam achieved first criticality on 6 April 2026, advancing India's three-stage programme. India's nuclear capacity is 8,880 MW from 25 reactors. The Nuclear Energy Mission also supports long-term fusion research, with India a partner in the ITER (International Thermonuclear Experimental Reactor) project in France.
UPSC angle: Nuclear Energy Mission (Budget FY26) and PFBR criticality are the most current nuclear energy developments — connects nuclear fission/fusion physics to India's energy mix and clean energy policy.
Exam Strategy
Prelims Focus Areas
- Forms of energy: kinetic, potential, thermal, chemical, nuclear, electrical, radiant
- Conservation of energy = First Law of Thermodynamics
- Nuclear fission: U-235; chain reaction; moderator slows neutrons; control rods absorb neutrons
- India's 3-stage nuclear programme: PHWR → FBR → Thorium; Kalpakkam PFBR
- ITER: Cadarache, France; tokamak; 35 nations; first plasma 2033–34
- Solar PV: photovoltaic effect; p-n junction; silicon
- Betz Limit: 59.3% maximum efficiency for wind turbines
- Green hydrogen: electrolysis + renewable energy; National Green Hydrogen Mission (2023)
- India's energy mix: ~505 GW total; ~51.5% non-fossil installed capacity (2025)
Mains Focus Areas
- Nuclear energy debate: safety (Fukushima), waste disposal, India's thorium advantage, 3-stage programme viability
- Solar energy economics: declining costs, PM-KUSUM, grid integration challenges, manufacturing (PLI for solar modules)
- Hydrogen as future fuel: green vs grey; infrastructure challenges; India's National Green Hydrogen Mission
- Energy transition: coal phase-down vs energy security; just transition for coal-dependent communities
- Fusion energy: ITER's significance; timeline; why fusion is the "holy grail" of energy
- India's energy security: import dependence (oil ~85%, gas ~50%); diversification strategy
Vocabulary
Photovoltaic Effect
- Pronunciation: /ˌfoʊtoʊvɒlˈteɪɪk ɪˈfɛkt/
- Definition: The generation of an electric current when a semiconductor material (typically silicon) is exposed to light — photons with sufficient energy liberate electrons from their atomic bonds, creating electron-hole pairs at a p-n junction that produce a voltage and drive a current through an external circuit.
- Origin: From Greek phōs (φῶς, "light") + Italian physicist Alessandro Volta (1745–1827, pioneer of electrical science) + English -ic; the effect was first observed by French physicist Edmond Becquerel in 1839 while experimenting with an electrolytic cell.
Chain Reaction
- Pronunciation: /tʃeɪn riˈækʃən/
- Definition: A self-sustaining sequence of nuclear fission events in which the neutrons released by one fission event cause additional fissions in nearby fissile nuclei, releasing more neutrons and energy in an exponentially growing cascade — controlled in a nuclear reactor (steady chain reaction) or uncontrolled in a nuclear weapon (supercritical chain reaction).
- Origin: The term was coined in the context of nuclear physics in the 1930s–40s; Enrico Fermi achieved the first controlled nuclear chain reaction on 2 December 1942 at the University of Chicago (Chicago Pile-1), demonstrating that fission could be sustained and regulated.
Betz Limit
- Pronunciation: /bɛts ˈlɪmɪt/
- Definition: The theoretical maximum fraction of kinetic energy that can be extracted from wind by a wind turbine, equal to 16/27 or approximately 59.3% — derived by German physicist Albert Betz in 1919 from the principles of conservation of mass and momentum, assuming an ideal frictionless rotor in an open flow.
- Origin: Named after Albert Betz (1885–1968), who published the derivation in his 1920 paper and later in his 1926 book Wind-Energie und ihre Ausnutzung durch Windmühlen; the limit arises because if all wind energy were extracted, the air behind the turbine would have zero velocity and would block incoming flow.
Key Terms
Green Hydrogen
- Pronunciation: /ɡriːn ˈhaɪdrədʒən/
- Definition: Hydrogen produced by the electrolysis of water using electricity generated from renewable energy sources (solar, wind, or hydropower), resulting in zero carbon dioxide emissions during production — considered the cleanest form of hydrogen and a key enabler of the global energy transition, particularly for decarbonising hard-to-abate sectors such as steel, cement, shipping, and heavy transport.
- Context: India launched the National Green Hydrogen Mission in January 2023 with an outlay of ₹19,744 crore, targeting production of 5 million tonnes of green hydrogen per year by 2030. Currently, ~95% of global hydrogen is grey (fossil-fuel-based), making the transition to green hydrogen a massive industrial challenge.
- UPSC Relevance: GS3 (Science & Technology, Energy Security, Environment). Prelims: definition; colour coding (green/blue/grey); electrolysis; National Green Hydrogen Mission (2023). Mains: hydrogen economy — challenges of cost, storage, and infrastructure; India's green hydrogen strategy; comparison with battery electric vehicles.
Tokamak
- Pronunciation: /ˈtoʊkəmæk/
- Definition: A device that uses powerful magnetic fields to confine a hot plasma (ionised gas at temperatures exceeding 150 million °C) in a doughnut-shaped (toroidal) chamber for the purpose of achieving controlled nuclear fusion — the most advanced and widely pursued approach to fusion energy, with ITER being the world's largest tokamak under construction.
- Context: The tokamak concept was invented by Soviet physicists Igor Tamm and Andrei Sakharov in the 1950s. The name is a Russian acronym: тороидальная камера с магнитными катушками (toroidal chamber with magnetic coils). India's Institute for Plasma Research (IPR) in Gandhinagar operates the SST-1 tokamak and is a partner in ITER.
- UPSC Relevance: GS3 (Science & Technology). Prelims: definition; ITER location (Cadarache, France); 35 partner nations; first plasma target (2033–34). Mains: fusion as the future of clean energy; ITER challenges; India's contribution to ITER; fusion vs fission comparison.
Sources: World Nuclear Association (world-nuclear.org — reactor types, nuclear power), U.S. EIA (energy data), ITER Organization (iter.org), NITI Aayog / CEA (India power sector data), pib.gov.in (India energy capacity, National Green Hydrogen Mission), Battery University (batteryuniversity.com), Energy Education (Betz Limit), Britannica (photovoltaic effect)
