Power generation: thermal, hydro, nuclear and renewable

In short

Power generation is the conversion of a primary energy source — coal, gas, falling water, nuclear fission, sunlight, wind, biomass — into bulk electrical energy at a generating station. The topic covers plant layout and working, the thermodynamic or fluid-mechanical basis, site selection, performance factors (heat rate, plant load factor, CUF) and the Indian institutional frame. It is examined in GATE EE (power systems), university Power Systems I and Power Plant Engineering papers, and is the standard opening area in interviews at NTPC, NHPC, NPCIL, SJVN, THDC, Tata Power, Adani Power, JSW Energy, Torrent, ReNew, Suzlon, BHEL, state genco/transco boards and PSU recruitment through GATE. It also underpins energy-audit, O&M, planning and EPC roles where coal, water and fuel costs per kWh decide commercial viability.

Explained

India's generation mix and who runs it

• Installed capacity is roughly 490 GW (mid/late 2025 order of magnitude; it changes monthly — quote CEA's latest monthly report in an exam or interview rather than a fixed number).
• Indicative split: coal + lignite ≈ 220 GW, gas ≈ 25 GW, large hydro ≈ 48 GW, nuclear ≈ 8.9 GW, solar ≈ 110-125 GW, wind ≈ 52 GW, biomass/small hydro/waste ≈ 15 GW.
• India crossed 50% of installed capacity from non-fossil sources in 2025, ahead of the 2030 Panchamrit/NDC target of 500 GW non-fossil capacity. Note the distinction: coal still supplies roughly 70% of the energy (kWh) because of its far higher plant load factor.
• Institutions: CEA (technical/planning, under Ministry of Power), CERC and SERCs (tariff and regulation), NTPC/NHPC/NLC/SJVN/THDC/DVC (central gencos), NPCIL and BHAVINI (nuclear), MNRE with SECI, IREDA and NIWE (renewables), AERB (nuclear safety), POSOCO/Grid-India (dispatch).
• Legal frame: Electricity Act 2003 (generation delicensed, except that a hydro scheme above a notified capital-cost limit needs CEA concurrence; captive generation freely allowed; open access), National Electricity Policy 2005, Tariff Policy 2016, Energy Conservation Act 2001 as amended in 2022 (carbon credit trading, non-fossil consumption obligation), the SHANTI Act 2025 (which replaced the Atomic Energy Act 1962 and the Civil Liability for Nuclear Damage Act 2010 and allows limited private participation in nuclear power), IEGC 2023 and CEA (Technical Standards for Connectivity) Regulations.

Thermal (steam) power plants

• Work on the Rankine cycle: feed pump → boiler/superheater (heat addition at constant pressure) → turbine (expansion) → condenser (heat rejection) → pump. Efficiency improved by reheat, regenerative feed heating (bled steam to LP/HP heaters and deaerator), high steam pressure/temperature and a good vacuum in the condenser.
• Circuits: (1) coal and ash, (2) air and flue gas, (3) feed water and steam, (4) cooling water.
• Main equipment: coal handling plant and pulverising mills, water-tube boiler with economiser, superheater, reheater and air preheater, ID/FD/PA fans and chimney (balanced draft), steam turbine (HP-IP-LP), surface condenser, cooling tower or once-through cooling, DM water plant, ESP and ash-handling system, turbo-alternator (2-pole, 3,000 rpm at 50 Hz).
• Boiler types: pulverised-coal tangential/corner-fired; CFBC/AFBC fluidised bed for high-ash, low-grade coal and lignite; once-through (Benson) boilers for supercritical units.
• Steam parameters: subcritical ≈ 170 bar/540 °C (η ≈ 33-36%), supercritical ≈ 247 bar/565/593 °C (η ≈ 38-40%), ultra-supercritical/advanced USC targeting 45%+ (AUSC is an Indian R&D programme by BHEL-IGCAR-NTPC). All new large Indian units are supercritical 660/800 MW.
• Gas-based: open-cycle gas turbine (Brayton, η ≈ 30-35%) and combined cycle (CCGT) where exhaust feeds an HRSG and a steam turbine, η ≈ 55-60%; fast-starting, used for peaking — constrained in India by gas price and availability.
• Diesel plants: small capacity, high running cost; used for standby, islands, telecom towers and industry.
• Site selection: coal availability (pithead vs load-centre station), huge cooling-water supply, rail/road access, ash-disposal land, away from dense habitation, stable soil.
• Environment: MoEFCC 2015 norms tightened SPM, SO₂, NOₓ and mercury limits and water use; FGD (flue gas desulphurisation) deadlines have been extended several times and the 2025 notification narrowed the requirement by plant category/location — state the direction and say the schedule has been revised, rather than quoting a date. Fly-ash utilisation is mandated (Fly Ash Notification 2021) for cement, bricks, road embankments and mine filling.
• Limitations: low efficiency, large CO₂ output (≈0.9-1.0 kg/kWh), slow start-up (hours), high ash with Indian coal (30-45%), water intensive; but high PLF, dispatchable, and provides inertia and reactive support.

Hydroelectric plants

• Convert potential energy of stored or flowing water: dam/barrage → intake and trash rack → penstock (with surge tank) → turbine → draft tube → tailrace, with a spillway, gates and a governor controlling speed.
• Classification by head: low (<30 m), medium (30-300 m), high (>300 m). By storage: run-of-river (no pondage), run-of-river with pondage, storage/reservoir, pumped storage (PSP). By capacity: micro (<100 kW), mini, small hydro (≤25 MW, under MNRE), large.
• Turbines: Pelton — impulse, high head, low discharge; Francis — mixed-flow reaction, medium head; Kaplan/propeller — axial reaction, low head, high discharge; bulb/tubular for very low head. Choice follows specific speed.
• Surge tank absorbs water-hammer pressure rise on sudden load rejection; the draft tube converts exit kinetic energy into pressure and allows the runner to sit above tailwater (recovering head).
• Hydrology tools: hydrograph, flow duration curve (gives firm/secondary power), mass curve (reservoir sizing). Firm power is available 100% of the time; secondary power only in the wet season.
• Pumped storage stores energy by pumping to an upper reservoir off-peak; cycle efficiency ≈ 70-80%. India's PSP push (Tehri 1,000 MW stage, Purulia 900 MW, Kadamparai, Srisailam LBPH, plus large pipelines in Maharashtra and Andhra) is the main grid-scale storage route for balancing solar.
• Indian examples: Tehri (2,400 MW with PSP), Koyna (1,960 MW), Srisailam, Nathpa Jhakri (1,500 MW), Bhakra (1,325 MW), Sardar Sarovar (1,450 MW), Subansiri Lower (2,000 MW, under construction).
• Advantages: no fuel cost, very long life (50-100 years), start/stop in minutes (ideal for peaking, spinning reserve and black start), high efficiency (85-92%), low O&M. Limitations: very high capital cost and long gestation, site-specific, submergence, rehabilitation and forest clearances, seasonal and silt problems. Statutory sweeteners: 12% free power to the home state plus 1% towards a local area development fund. Large hydro was classified as renewable energy in March 2019.

Nuclear power plants

• Fission: a thermal neutron absorbed by U-235 splits it into fission fragments + 2-3 neutrons + ≈200 MeV. A self-sustaining chain reaction needs multiplication factor k = 1 (critical); k>1 supercritical, k<1 subcritical. Delayed neutrons (≈0.65%) make control practicable.
• Reactor components: fuel (natural UO₂ for PHWR; 3-5% enriched for BWR/PWR), moderator (heavy water in PHWR; light water in LWR; graphite historically) to slow neutrons, coolant to carry heat, control rods (boron, cadmium, hafnium) plus liquid poison, reflector, pressure vessel/calandria, thermal and biological shield, containment.
• Types in India: PHWR (indigenous workhorse — 220 MW, 540 MW Tarapur 3-4, 700 MW at Kakrapar 3-4 and Rajasthan 7-8), BWR (Tarapur 1-2, 160 MW each), PWR/VVER-1000 (Kudankulam, Russian design, units under construction beyond 1-2), FBR (PFBR 500 MW sodium-cooled at Kalpakkam, in commissioning stages).
• Three-stage programme (Bhabha), designed around India's modest uranium and large thorium reserves:
  1. PHWRs on natural uranium, producing Pu-239 in spent fuel.
  2. Fast breeder reactors on Pu-239 with a thorium blanket, breeding ratio >1, converting Th-232 to U-233.
  3. Thorium-U-233 reactors (AHWR design) for long-term energy security.
• Fuel cycle: mining (Jaduguda, Tummalapalle) → milling → fuel fabrication → reactor → spent-fuel pool → reprocessing (India follows a closed cycle) → waste vitrification and deep geological disposal.
• Thermal efficiency is modest (≈29-33% for PHWR) because of lower steam temperature; fuel cost per kWh is very low and energy density is enormous (~1 g of U-235 per MWd thermal).
• Advantages: near-zero operational CO₂, baseload, tiny fuel logistics. Limitations: very high capital cost, long construction, radioactive waste, public acceptance, strict AERB licensing. Operator liability was capped at ₹1,500 crore under the Civil Liability for Nuclear Damage Act, 2010; the SHANTI Act, 2025 replaced that Act and the Atomic Energy Act, 1962, opened nuclear generation to limited private participation and recast operator liability - check the current caps and rules before quoting figures.

Renewable generation

• Solar PV: photon absorption in a p-n junction creates electron-hole pairs; cells are rated at STC — 1,000 W/m², 25 °C, AM 1.5. Cell → module → string → inverter → transformer. Technologies: mono/poly crystalline silicon, mono PERC and TOPCon/HJT now dominant, thin film (CdTe), bifacial modules with trackers. Key terms: Isc, Voc, fill factor, MPPT, temperature coefficient (≈ −0.35%/°C), degradation (~0.5%/yr), performance ratio 0.75-0.82, CUF 17-22% (higher with trackers). Roughly 4 acres/MW of land. Indian examples: Bhadla (2,245 MW), Pavagada, Kurnool; PM Surya Ghar for rooftop, PM-KUSUM for agricultural pumps; ALMM and domestic content rules support local manufacture.
• Solar thermal (CSP): parabolic trough, central tower, dish-Stirling; concentrates heat to run a Rankine cycle, can store heat in molten salt. Very limited deployment in India.
• Wind: a turbine extracts kinetic energy; power varies as V³, so site wind speed dominates economics. Modern Indian machines: 2-4.5 MW, 100-140 m hub height, pitch-regulated variable-speed with DFIG or full-converter PMSG, cut-in ≈3 m/s, rated ≈12 m/s, cut-out ≈25 m/s. CUF 25-35% onshore. Resource states: Tamil Nadu (Muppandal), Gujarat, Karnataka, Maharashtra, Rajasthan, Andhra Pradesh. NIWE publishes the wind atlas (potential assessed at 100/120 m); offshore wind policy exists for Gujarat and Tamil Nadu but capacity is yet to be built.
• Biomass and bagasse cogeneration: direct combustion or gasification of agro-residue; sugar mills export bagasse-based power in the crushing season. Waste-to-energy (RDF combustion, biomethanation) and compressed biogas under SATAT.
• Small hydro, geothermal, tidal, ocean: small hydro is mature (canal-drop and hill schemes); geothermal in India is at exploration stage (Puga, Ladakh); tidal (Gulf of Kutch/Khambhat) and OTEC remain non-commercial.
• Green hydrogen: National Green Hydrogen Mission targets 5 MMT/yr by 2030 using RE-powered electrolysers — an emerging generation-linked load.
• Common limitations: variability and non-dispatchability, low capacity factors, land and evacuation needs (Green Energy Corridors), no inherent inertia; they get must-run status and are supported by RPO under Section 86(1)(e) of the Electricity Act.

Performance factors, load curves and choice of plant

• Connected load, maximum demand, demand factor, diversity factor describe the consumer side; the load curve (hourly) and load duration curve (sorted) describe what the plants must supply. Area under the load curve = energy; average load = area/time.
• Base load plants (coal, nuclear, run-of-river hydro) run at high PLF and low running cost; peak load plants (hydro, PSP, gas turbines, diesel, batteries) start fast and run few hours. Base-load plants should have low variable cost; peakers may have high variable but low fixed cost.
• Cost structure: fixed cost (capital recovery, interest, depreciation, insurance, fixed O&M, salaries) + variable cost (fuel, water, variable O&M). Indicative capital costs: solar ≈ ₹4-4.5 crore/MW, wind ≈ ₹6.5-7 crore/MW, coal (supercritical) ≈ ₹8.5-9.5 crore/MW, hydro ≈ ₹8-12 crore/MW, nuclear ≈ ₹15-20 crore/MW — all indicative and moving, so quote as ranges.
• Recent competitive-bid tariffs: solar ≈ ₹2.4-2.8/kWh, wind ≈ ₹3.0-3.4/kWh, new coal ≈ ₹4.5-5.5/kWh landed; hydro and nuclear tariffs are regulated on a cost-plus basis. Treat all as indicative of 2024-25 auctions.
• Generator speed: N = 120f/P — hence 2-pole 3,000 rpm turbo-alternators for steam/gas, and low-speed salient-pole machines with many poles (e.g. 24 poles → 250 rpm) for hydro.

Key points

• Rankine cycle - the steam power cycle; reheat and regeneration raise its efficiency, condenser vacuum raises output.
• Station heat rate - kcal or kJ of heat per kWh sent out; 860/HR(kcal/kWh) = efficiency.
• Supercritical - steam above 221 bar/374 °C with no distinct boiling; standard for new 660/800 MW Indian units.
• Plant load factor (PLF) - actual energy generated ÷ (installed capacity × hours); coal ≈ 65-70%, nuclear ≈ 75-85%, hydro ≈ 30-45%, wind 25-35%, solar 17-22%.
• Load factor - average load ÷ peak load; always ≤ 1, and higher load factor means cheaper energy.
• Utilisation factor - maximum demand ÷ installed capacity; reserve capacity = installed − maximum demand.
• Specific speed - identifies turbine type: Pelton (low), Francis (medium), Kaplan (high).
• Surge tank - protects the penstock from water hammer during sudden load change.
• Pumped storage - the only large-scale storage in Indian hydro; 70-80% round-trip efficiency.
• Firm power - hydro output guaranteed throughout the year from the flow duration curve; the rest is secondary power.
• PHWR - natural-uranium, heavy-water moderated and cooled; India's indigenous mainstay, now at 700 MW.
• Three-stage programme - PHWR → fast breeder (Pu-239) → thorium/U-233.
• 200 MeV per fission - about 1 g of U-235 fissioned per MWd of thermal energy.
• Betz limit - maximum 16/27 ≈ 59.3% of wind kinetic energy can be extracted; practical Cp ≈ 0.40-0.48.
• Cube law - wind power ∝ V³, so a 10% higher wind speed gives ~33% more power.
• STC and performance ratio - PV rating conditions (1,000 W/m², 25 °C, AM1.5) and the 0.75-0.82 factor converting ideal to actual yield.
• Must-run and RPO - renewable plants are dispatched first; obligated entities must buy a mandated renewable share.
• Delicensed generation - under the Electricity Act 2003 anyone may build a thermal/renewable plant; hydro above a notified cost needs CEA concurrence.

Formulas and examples

Example 1 — Thermal unit: efficiency, coal and ash. A 500 MW unit has a station heat rate of 2,400 kcal/kWh, burns coal of GCV 3,500 kcal/kg with 35% ash, and runs at 85% PLF.

• Efficiency = 860 / 2,400 = 35.8%
• Specific coal consumption = 2,400 / 3,500 = 0.686 kg/kWh
• Annual generation = 500 × 8,760 × 0.85 = 3,723,000 MWh = 3.723 × 10⁹ kWh
• Annual coal = 0.686 × 3.723 × 10⁹ ≈ 2.553 × 10⁹ kg ≈ 2.55 million tonnes (≈7,000 t/day)
• Ash produced = 0.35 × 2.55 Mt ≈ 0.89 million tonnes/year — which is why ash ponds and 100% utilisation matter.

Example 2 — Hydro station output and energy. Design discharge Q = 30 m³/s, net head 60 m, turbine-generator overall efficiency 88%.

• Gross water power = 1,000 × 9.81 × 30 × 60 = 17,658,000 W = 17.66 MW
• Output P = 0.88 × 17.66 = 15.54 MW
• At 45% capacity factor, annual energy = 15.54 × 8,760 × 0.45 ≈ 61,250 MWh (61.3 GWh)
• If the machine runs at 250 rpm: Ns = 250 × √15,540 / 60^1.25 = 250 × 124.7 / 167.0 ≈ 187 → a Francis turbine, consistent with a 60 m head.

Example 3 — Nuclear fuel consumption. A 220 MWe PHWR has 29% thermal efficiency.

• Thermal power = 220 / 0.29 = 758.6 MWth
• Fissions per second = 758.6 × 10⁶ / 3.2 × 10⁻¹¹ ≈ 2.37 × 10¹⁹ /s
• Per day: 2.37 × 10¹⁹ × 86,400 ≈ 2.05 × 10²⁴ atoms = 2.05 × 10²⁴ / 6.022 × 10²³ ≈ 3.40 mol
• Mass of U-235 fissioned = 3.40 × 235 ≈ 798 g/day (~0.8 kg) — i.e. ≈1 g per MWd thermal. The same energy from coal (GCV 3,500 kcal/kg) at 29% efficiency would need about 4,500 tonnes per day.

Example 4 — Wind turbine power. Rotor diameter 120 m, hub-height wind 9 m/s, Cp = 0.42, ρ = 1.225 kg/m³.

• A = π × 120² / 4 = 11,310 m²
• Wind kinetic power = 0.5 × 1.225 × 11,310 × 9³ = 0.6125 × 11,310 × 729 ≈ 5.05 MW
• Extracted P = 0.42 × 5.05 ≈ 2.12 MW
• At 11 m/s the same turbine would give 2.12 × (11/9)³ = 2.12 × 1.826 ≈ 3.87 MW — the cube law in action.
• A 3 MW machine at 30% CUF generates 3 × 8,760 × 0.30 = 7,884 MWh/year.

Example 5 — 1 MWp solar plant. Plane-of-array insolation 1,900 kWh/m²/yr, PR = 0.78, module efficiency 20%, tariff ₹2.60/kWh.

• Annual energy = 1,000 kWp × 1,900 / 1,000 × 0.78 = 1,482 MWh (specific yield 1,482 kWh/kWp)
• CUF = 1,482,000 / (1,000 × 8,760) = 16.9%
• Module area = 1,000,000 W / (1,000 × 0.20) = 5,000 m² of module (land ≈ 16,000 m², ~4 acres)
• Annual revenue = 1,482,000 × 2.60 ≈ ₹38.5 lakh; at ₹4.2 crore capital cost the simple payback on revenue alone is ≈11 years before O&M and financing.

Where it is used

• Central and state generation utilities: NTPC, NHPC, SJVN, THDC, NLC, DVC, NPCIL/BHAVINI and state gencos (MAHAGENCO, APGENCO, KPCL, TNPGCL) — operations, efficiency/heat-rate improvement, performance testing and C&I engineering roles.
• Private IPPs and RE developers: Tata Power, Adani Power/Green, JSW Energy, Torrent, ReNew, Sembcorp, Vena/Juniper — resource assessment, DPR preparation, PPA bidding and asset-management analytics.
• Equipment manufacture and EPC: BHEL (boilers, turbines, generators), L&T, Thermax, Suzlon, Siemens Energy, GE Vernova, Waaree/Adani Solar/Tata Power Solar (modules, ALMM), Andritz and Voith (hydro turbines).
• Energy audit and efficiency practice: BEE-certified energy auditors computing station heat rate, auxiliary power consumption (6-9% in coal plants), condenser performance and specific oil consumption under the PAT scheme.
• Grid planning and regulation: CEA National Electricity Plan, CERC tariff filings and norms (target availability, station heat rate, auxiliary consumption), state resource-adequacy studies and PSP siting.
• Captive and distributed generation: sugar-mill bagasse cogeneration, cement and steel waste-heat recovery, rooftop solar under PM Surya Ghar, DG sets and solar pumps under PM-KUSUM.

Interview questions

• Q: Why is the overall efficiency of a coal-fired plant only about 35-40%? A: The Rankine cycle is limited by metallurgy at the top temperature and by condenser temperature at the bottom (Carnot limit), and further losses occur in the boiler, turbine, condenser, auxiliaries and stack gases.
• Q: What does a station heat rate of 2,400 kcal/kWh mean? A: The plant consumes 2,400 kcal of fuel heat to send out one kWh, which corresponds to an efficiency of 860/2,400 ≈ 35.8%.
• Q: Difference between load factor and plant capacity factor? A: Load factor is average load divided by peak load on the system; plant capacity factor is actual energy generated divided by what the installed capacity could generate — they are equal only when peak load equals installed capacity.
• Q: Why are hydro plants preferred for peak load? A: They can be started, loaded and shut down within minutes with no fuel cost and little wear, and they can provide spinning reserve, frequency support and black start.
• Q: Which turbine would you choose for a 400 m head and why? A: A Pelton wheel — high head with low discharge gives a low specific speed, and an impulse turbine handles the high jet velocity efficiently.
• Q: What is the function of a surge tank? A: It absorbs the pressure surge (water hammer) when flow in the penstock changes suddenly on load rejection, and supplies water on load increase.
• Q: Why does India use heavy water as moderator and natural uranium as fuel? A: Heavy water absorbs very few neutrons, so a chain reaction is sustainable with natural uranium without enrichment — matching India's limited enrichment infrastructure and supporting the three-stage programme.
• Q: What is a breeder reactor? A: A fast reactor that produces more fissile material than it consumes (breeding ratio > 1), converting U-238 to Pu-239 or Th-232 to U-233; India's PFBR at Kalpakkam is stage two of the three-stage plan.
• Q: Why is wind power proportional to the cube of velocity? A: Kinetic energy per unit mass goes as V² and the mass flow through the rotor goes as V, so power goes as V³.
• Q: A solar plant is rated 1 MWp but its meter never reads 1 MW — why? A: The rating is at STC (1,000 W/m², 25 °C); in the field irradiance is lower, cell temperature is 20-30 °C higher, and inverter, cable, soiling and mismatch losses apply, giving a performance ratio of about 0.78.
• Q: Why do solar and wind not reduce the need for firm capacity one-for-one? A: Their output is variable and uncorrelated with the evening peak, so the system still needs dispatchable capacity or storage; this is why pumped storage and batteries are being added rapidly.
• Q: Is generation licensed in India? A: No — the Electricity Act 2003 delicensed generation, subject to grid-connectivity and environmental clearances, with hydro above a notified capital cost requiring CEA concurrence.

Common mistakes

• Mixing up installed capacity share with energy share — non-fossil capacity above 50% does not mean half the kWh are non-fossil.
• Using gross head instead of net head in P = ρgQHη, or forgetting the factor 9.81 and 1,000.
• Writing efficiency = 1,000/HR or similar — the correct conversions are 3,600 kJ/kWh and 860 kcal/kWh.
• Confusing moderator with coolant and with reflector in a reactor, or claiming control rods "slow" neutrons (they absorb them).
• Taking the Betz limit as an achievable efficiency, or applying ρ = 1.225 kg/m³ at high-altitude or hot sites without correction.
• Treating CUF as if it were module efficiency, or quoting solar CUF for a tracker plant (22-25%) where a fixed-tilt figure (17-19%) applies.
• Calling a run-of-river plant a peaking station — without pondage or storage it must generate whatever the river brings.

Related topics

Power system operation, SCADA and EMS · Economic dispatch and unit commitment · Smart grid and renewable integration · Distribution systems and substations · Power system stability · Electricity markets and regulation in India