Power Plant Engineering (ME-702 (C)) - Unit 1 Short Notes

UNIT 1: Power Plant Engineering - Comprehensive Short Notes

I. Fundamentals of Energy Sources

Classification of Energy Sources

Energy sources are classified based on their origin, renewability, and form.

[!TIP] Exam Focus: Primary sources are natural; secondary are derived. Interconvertibility is governed by energy conservation but with efficiency losses (2nd Law).

II. Renewable and Alternative Energy Conversion Systems

Solar Energy Systems

• Factors Affecting Solar Intensity:

  • Geographical location: Latitude (higher intensity near equator).

  • Season & Time: Maximum at solar noon, varies with seasons.

  • Atmospheric conditions: Cloud cover, humidity, aerosols.

  • Surface orientation & tracking: Tilt angle, tracking systems.

• Indian Regional Data (Approximate Annual Global Insolation):

  • High Potential: Rajasthan (Jaisalmer, Bikaner): 5.5–7.0 kWh/m²/day.

  • Moderate: Gujarat, Maharashtra, Andhra Pradesh, Tamil Nadu: 4.5–5.5 kWh/m²/day.

  • Lower: Northeastern states, Kerala: <4.0 kWh/m²/day (due to cloud cover).

• Advantages (India): Abundant, decentralized, low operating cost, no fuel cost, suitable for remote areas.

• Limitations: Intermittency (day/night, weather), low efficiency (~15-22% PV), high initial cost, large land area requirement, storage challenge.

Wind Energy Systems

• Factors Affecting Wind Velocity:

  • Geographical & Topographical: Coastal areas, mountain passes, open plains.

  • Altitude: Velocity increases with height above ground.

  • Surface roughness: Urban/forested areas reduce wind speed.

  • Atmospheric stability & pressure gradients.

• Indian Regional Data (Wind Power Density at 80m hub height):

  • Excellent: Tamil Nadu (Coastal, Palghat Gap), Gujarat (Saurashtra), Maharashtra (Sahyadri), Rajasthan (Jaisalmer), Karnataka.

  • Good: Parts of Andhra Pradesh, Odisha, Madhya Pradesh.

  • Wind Zones: Onshore (4-6 m/s avg), Offshore (6-8 m/s avg).

• Advantages (India): Clean, renewable, good potential in specific zones, declining cost, creates local jobs.

• Limitations: Intermittent & variable, noise pollution, visual impact, threat to birds, requires storage/grid backup, site-specific.

Comparative Analysis: Solar vs. Wind Energy for Indian Conditions

[!TIP] Key Insight: Solar and wind are complementary in India—solar peaks in summer afternoons, wind peaks in monsoon nights. This enables hybridization.

Hybrid Energy Systems

• Definition & Concept: Integration of two or more renewable energy sources (e.g., solar-wind, solar-biomass) with or without storage (batteries, diesel backup) to improve reliability, reduce intermittency, and optimize cost.

• Feasible Hybrid Options in Indian Context:

  1. Solar-Wind-Battery: Most common. Solar for day, wind for night/monsoon, battery for smoothing.

  2. Solar-Wind-Diesel: For remote off-grid areas (islands, telecom towers).

  3. Solar-Biomass: Biomass provides baseload, solar peaks during day.

  4. Solar-Hydro (Pumped Storage): Solar powers pumping, hydro provides dispatchable power.

  5. Wind-Solar-Hydro: Optimal mix for grid stability.

MagnetoHydroDynamic (MHD) Converters

• Working Principle: Direct conversion of thermal energy (from hot, ionized combustion gases) into electrical energy without moving parts.

  1. Seeding: Alkali metal vapor (Cs, K) added to combustion gases → creates plasma (ionized gas).

  2. Acceleration: Hot plasma expelled at high velocity through a duct.

  3. Power Extraction: Duct placed across perpendicular magnetic field (from superconducting magnets). Lorentz force ($$\displaystyle \vec{F} = q(\vec{v} \times \vec{B}) $$) drives positive/negative ions to electrodes → Direct Current (DC) generated.

  • Schematic:
    DiagramSEARCH: "MHD generator schematic diagram open cycle"

• Factors Limiting Commercial Utilization:

  1. Extreme Temperatures: ~2000°C required for sufficient ionization → severe material challenges.

  2. Seed Recovery & Corrosion: Alkali metals are corrosive; recovery systems are complex/expensive.

  3. Magnet Cost: Superconducting magnets require cryogenic cooling (liquid helium).

  4. Low Demonstrated Efficiency: Practical efficiencies (~25%) not significantly higher than combined cycles.

  5. High Capital Cost & Complexity.

  6. Environmental Issues: Seed disposal, NOx formation.

Fuel Cells

• Definition: Electrochemical devices that convert chemical energy of a fuel (H₂, hydrocarbons) and an oxidant (O₂/air) directly into electricity and heat, with high efficiency and low emissions.

• Classification:

  • By Electrolyte: PEMFC, AFC, PAFC, MCFC, SOFC, DMFC.

  • By Operating Temperature: Low (<100°C), Medium (100-400°C), High (>400°C).

  • By Fuel: Hydrogen, methanol, natural gas.

• Working Principle (Example: PEMFC - Polymer Electrolyte Membrane Fuel Cell):

  1. Anode: H₂ → 2H⁺ + 2e⁻ (oxidation).

  2. Electrolyte: Proton exchange membrane (Nafion) allows only H⁺ ions to pass.

  3. Cathode: ½O₂ + 2H⁺ + 2e⁻ → H₂O (reduction).

  4. Overall: H₂ + ½O₂ → H₂O + Electricity + Heat.

  • Thermodynamic Equations:

    • Reversible Cell Voltage (Nernst Equation):

$$E = E^0 - \frac{RT}{2F} \ln \left( \frac{P_{H_2O}}{P_{H_2} \sqrt{P_{O_2}}} \right)$$

    *   **Maximum Theoretical Efficiency:** 

$$\eta_{max} = \frac{\Delta G}{\Delta H} \times 100\%$$

(For H₂, ~83% at 298K).

    *   **Actual Efficiency:** $$\displaystyle \eta = \frac{V_{cell}}{1.48} \times 100\% $$ (1.48V is reversible voltage at standard conditions).

*   **Schematic:** `
DiagramSEARCH: "PEM fuel cell diagram anode cathode membrane"
`

• Applications: Transportation (vehicles), stationary power (backup, remote), portable electronics, space missions (historically).

III. Thermal Power Plants

Coal Handling and Preparation Systems

• Elements with Schematic Flow:

  1. Unloading: Wagon tippler/arm/unloader → Coal received from rail/road.

  2. Crushing: Primary crusher (jaw/impact) → reduces size to ~300mm.

  3. Screening & Separation: Vibrating screens → separates fine/coarse coal.

  4. Secondary Crushing: For coarse coal → ~25mm size.

  5. Pulverizing: Pulverizers (ball mill, bowl mill, hammer mill) → fine powder (~70% < 75µm).

  6. Storage & Feeding: Coal bunkers → feeders → boiler furnace.

  • Schematic:
    DiagramSEARCH: "coal handling plant thermal power plant schematic"

Combustion Technologies: Fluidized Bed Combustion (FBC)

• Working Principle:

  1. Air blown at high velocity through a bed of inert material (sand, limestone) + fuel.

  2. At critical velocity, particles suspend → fluidized state (like boiling liquid).

  3. Excellent mixing, high heat transfer, uniform temperature (~850-950°C).

  4. Limestone (CaCO₃) added for in-situ desulfurization: CaCO₃ → CaO + CO₂; CaO + SO₂ → CaSO₄.

  5. Types: Bubbling Fluidized Bed Combustion (BFBC) & Circulating Fluidized Bed Combustion (CFBC) (solids recirculated).

  • Schematic:
    DiagramSEARCH: "CFBC boiler diagram furnace cyclone"

• Advantages over Conventional (Pulverized Fuel) Systems:

  • Fuel Flexibility: Can burn low-grade coals, biomass, waste.

  • In-situ SO₂/NOₓ Control: Lower combustion temperature (~900°C) reduces thermal NOₓ; limestone captures SO₂.

  • Compact Size & Lower Capital Cost for small/medium capacity.

  • Higher Combustion Efficiency for low-grade fuels.

  • Reduced Ash-related Problems (lower temp, less slagging).

Fuel Burning Principles

• Overfeed Principle: Fuel fed above the air inlet. Air flows upward through the fuel bed. Common in traveling grate stokers. Fuel moves horizontally on grate; combustion air from below.

• Underfeed Principle: Fuel fed below the air inlet. Air flows upward through the fuel. Used in underfeed stokers and some spreader stokers. Fuel pushed up from below grate.

Steam and Gas Cycle Improvements: Reheating in Gas Turbines

• Simple Open Cycle Gas Turbine: Compressor → Combustor → Turbine → Exhaust.

  • Problem: High exhaust temperature (~500°C) → waste heat.

• Reheating: Exhaust from high-pressure (HP) turbine is sent back to combustor (reheater), reheated, and expanded in intermediate-pressure (IP) / low-pressure (LP) turbine(s).

• Impact on Thermal Efficiency:

  1. Increases average temperature of heat addition → improves cycle efficiency (Carnot principle).

  2. Reduces moisture content at final turbine stage (if steam turbine) → reduces blade erosion.

  3. Increases work output for same turbine inlet temperature.

  4. Trade-off: Slight increase in complexity and pressure losses.

  • Schematic (T-s Diagram):
    DiagramCANVAS: "T-s diagram showing Brayton cycle with reheating. Show two turbines (HP, LP), reheater between them. Highlight area increase (work output) and shift of mean T_add to higher value."

Boiler Technology: Recent Trends

• Size: Trend towards supercritical (SC) and ultra-supercritical (USC) units (500-1000+ MW). Higher pressure/temperature → higher efficiency (45-48% vs 35-38% subcritical).

• Selection: Based on fuel type (coal quality), load requirement, emission norms (NOₓ, SO₂, particulates), site constraints.

• Operation: Digital control systems (DCS), online cleaning (sootblowers), low-NOₓ burners, fabric filters/ESP for pollution control, flexible operation for grid support (ramp rates, part-load efficiency).

Plant Heat Balance

• Concept: Quantitative accounting of all energy inputs (fuel LHV) and outputs (electricity, heat losses) in a power plant to determine efficiency.

• Example (Fossil Fuel Plant - Simple Steam Cycle):

  • Input: Fuel energy = $$\displaystyle m_f \times LHV $$

  • Outputs:

    1. Net Electrical Output = $$\displaystyle W_{net} $$ (turbine work - pump work - auxiliaries).

    2. Heat Loss in Flue Gases = $$\displaystyle m_{fg} \times C_p \times (T_{exit} - T_{amb}) $$.

    3. Heat Loss in Cooling Water = $$\displaystyle m_{cw} \times C_p \times (T_{out} - T_{in}) $$.

    4. Radiation & Unaccounted Losses (~0.5-1%).

  • Thermal Efficiency:

$$\eta_{thermal} = \frac{W_{net}}{m_f \times LHV} \times 100\%$$

*   **Heat Rate:** $$\displaystyle HR = \frac{3600}{\eta_{thermal}} $$ (kJ/kWh). Lower HR = better.

*   **Schematic (Heat Balance Diagram):** `
DiagramSEARCH: "power plant heat balance diagram Sankey diagram"
`

Feed Water Treatment

• Elements of Feed Water Treatment Plant:

  1. Screening & Filtration: Remove suspended solids.

  2. ** softening:** Remove hardness (Ca²⁺, Mg²⁺) via lime-soda process or ion exchange.

  3. Deaeration: Remove dissolved gases (O₂, CO₂) using deaerator (stripping with steam).

  4. Condensate Polishing: Mixed-bed ion exchange for high-purity condensate return.

  5. Chemical Dosing: Oxygen scavengers (hydrazine, sulfite) to remove traces of O₂; pH control (ammonia, morpholine) to prevent corrosion.

  6. Filtration: Cartridge/multimedia filters.

  • Goal: Achieve ultra-pure water (conductivity < 0.1 µS/cm) to prevent scale & corrosion in boiler tubes.

Cooling Systems: Cooling Towers

• Principle: Reject waste heat from condenser cooling water to atmosphere via evaporative cooling.

  • Warm water from condenser → sprayed at top → falls through fill media (increases air-water contact) → air blown by fans (or natural draft) → evaporation cools water → cold water collected at basin → recirculated.

• Types:

  1. Natural Draft: Hyperbolic concrete structure (like giant cooling tower). Uses chimney effect. Very large capacity, low operating cost, high capital cost.

  2. Mechanical Draft: Uses fans to force/draw air.

     • Forced Draft: Fan at air inlet (positive pressure).

     • Induced Draft: Fan at air outlet (negative pressure) – most common.

     • Fan-assisted natural draft: Hybrid.

  • Schematic:
    DiagramSEARCH: "induced draft cooling tower diagram cross section"

IV. Nuclear Power Plants

Nuclear Fission Phenomenon

• Definition: Splitting of a heavy nucleus (U-235, Pu-239) into two lighter nuclei (fission fragments) upon absorption of a thermal (slow) neutron, releasing large energy (~200 MeV/fission), 2-3 fast neutrons, and gamma radiation.

• Chain Reaction: Fast neutrons are slowed (moderated) to thermal energies to cause further fissions. Multiplication factor (k):

  • k < 1: Subcritical (dies out).

  • k = 1: Critical (steady).

  • k > 1: Supercritical (increases).

• Controlled Chain Reaction is the basis of nuclear reactors.

Pressurized Heavy Water Reactor (PHWR) - Detailed Working

• Fuel: Natural Uranium (0.7% U-235) – no enrichment required.

• Moderator & Coolant: Heavy Water (D₂O). Excellent moderator (low neutron absorption). Also acts as primary coolant.

• Pressure: High pressure (~100 bar) to keep D₂O liquid at ~300°C.

• Core: Horizontal cylindrical pressure vessel (calandria) containing fuel channels. Each channel has:

  • Fuel Bundles: Natural U metal or UO₂ pellets in Zr alloy cladding.

  • Coolant: Pressurized D₂O flows inside pressure tubes surrounding fuel.

  • Moderator: Cool, low-pressure D₂O fills annulus between pressure tubes in calandria.

• Heat Transfer: Heat from fission → coolant D₂O in pressure tubes → steam generators (D₂O/light water) → steam to turbine.

• Control: Adjuster rods (liquid neutron absorber - Cd solution) inserted in calandria for coarse power control. Shut-off rods (fall by gravity for scram).

• Advantages: Uses natural U, good neutron economy, online refueling possible.

• Schematic:

  DiagramSEARCH: "PHWR pressurized heavy water reactor diagram calandria pressure tube"

Advanced Gas Cooled Reactor (AGR)

• Fuel: Enriched Uranium (2.5-3.5% U-235).

• Moderator: Graphite (blocks in core).

• Coolant: Carbon Dioxide (CO₂) at high pressure (~40 bar), inlet ~300°C, outlet ~650°C.

• Core: Graphite moderator blocks with vertical fuel channels. Fuel is UO₂ pellets in stainless steel cladding.

• Heat Transfer: CO₂ coolant circulates through core → steam generators (once-through) → steam to turbine.

• Generation: Second generation of British gas-cooled reactors (after Magnox). Higher temperature & efficiency than Magnox.

Comparison: Thermal vs. Fast Breeder Reactors

Moderators

• Function: Slow down fast fission neutrons to thermal energies to increase probability of fission in U-235 (or Pu-239).

• Types & Characteristics:

  | Moderator | Characteristics | Used In | | :--- | :--- | :--- | | Light Water (H₂O) | Cheap, good moderator, high neutron absorption → requires enriched fuel. | PWR, BWR | | Heavy Water (D₂O) | Excellent moderator, very low neutron absorption → can use natural uranium. Expensive. | PHWR, CANDU | | Graphite | Good moderator, very low absorption, stable at high temp. Requires high purity. | RBMK, AGR, Magnox, HTGR | | Beryllium | Good moderator & reflector, low absorption, toxic, expensive. | Some research/reactors (e.g., as reflector). |

Reactor Control and Safety

• Principles of Reactor Control:

  1. Control Rods: Neutron-absorbing materials (B₄C, Ag-In-Cd, Hf). Inserted/withdrawn to control reactivity.

  2. Chemical Shim: Soluble neutron absorber (e.g., boric acid in PWR coolant) for fine power shaping.

  3. Burnable Absorbers: Gadolinium, Erbium in fuel pellets → initially high absorption, burns away over time.

  4. Moderator Temperature Coefficient: In PWRs, hotter moderator → less dense → fewer thermal neutrons → negative feedback (inherently safe).

• Reliability Features for Nuclear Power Plants:

  • Multiple, Independent Safety Systems: Redundant & diverse (e.g., 3 emergency core cooling systems).

  • Defense-in-Depth: Multiple physical barriers (fuel cladding, reactor pressure vessel, containment building).

  • Containment Structure: Massive reinforced concrete/steel dome → prevents radioactive release even during accidents.

  • Emergency Core Cooling System (ECCS): Provides water to core if LOCA occurs.

  • Emergency Power Supply: Diesel generators, batteries.

  • Passive Safety Systems: Gravity-driven water injection, natural circulation cooling (no active pumps needed).

  • Rigorous Quality Assurance & Regulatory Oversight.

V. Hydropower Plants

Hydraulic Turbines - Types & Characteristics

Factors for Turbine Selection in Hydel Plants

1. Available Net Head (H): Primary determinant (Pelton: High, Francis: Medium, Kaplan: Low).

2. Design Discharge (Q): Flow rate capacity.

3. Load Characteristics: Base load vs peak load (Kaplan good for part-load).

4. Specific Speed (Ns): $$\displaystyle \boxed{N_s = N \sqrt{P} / H^{5/4}} $$ (N=rpm, P=kW, H=m). Higher Ns → higher flow, lower head turbine.

5. Site Constraints: Size, orientation, cavitation risk (Kaplan sensitive).

6. Cost & Efficiency: Francis most versatile & efficient for medium head.

7. Environmental: Fish passage, sediment handling.

Site Selection for Hydropower

• Key Parameters:

  1. Hydrological: Reliable, high annual rainfall, catchment area, river flow (avg, monsoon, dry season), sediment load.

  2. Topographical & Geological: Narrow gorge for dam (reduces cost), sound rock foundation for dam & powerhouse, steep gradients.

  3. Environmental & Social: Minimal displacement, forest/eco-sensitive areas, impact on fisheries/downstream flow.

  4. Economic: Proximity to load center (reduces T&D loss/cost), accessibility for construction.

  5. Technical: Storage vs Run-of-river potential, head availability, seismic stability.

  6. Political & Legal: Inter-state/river disputes, clearances (environmental, forest).

Small Hydro Power (SHP)

• Definition: Hydro plants with installed capacity ≤ 25 MW (India's definition).

• Micro vs. Pico Hydro Machines:

  | Feature | Micro Hydro | Pico Hydro | | :--- | :--- | :--- | | Capacity | 5 kW – 100 kW | < 5 kW (often 0.5-5 kW) | | Head | Medium to High (10-100m) | Low to Very Low (1-10m) | | Application | Village/mini-grid, small communities, remote areas. | Single home, small workshop, very remote hamlet. | | Turbine Type | Pelton, Crossflow, Small Francis. | ** propeller, water wheel, very small Pelton.** | | Civil Works | Small dam/weir, channel, forebay, penstock. | Minimal (weir, small canal, direct drive). | | Grid Connection | Possible (mini-grid). | Almost always standalone/off-grid. | | Cost & Complexity | Moderate, requires some civil works. | Very low, simple, often locally manufactured. |

Spillways

• Purpose: Safely pass flood flows (design flood, probable maximum flood) without overtopping the dam, protecting dam structural integrity.

• Types:

  1. Ogee Spillway: Overflow section with ogee-shaped crest (matches trajectory of water nappe). Most common for gravity/arch dams. Controlled (gated) or uncontrolled.

  2. Chute (Open Channel) Spillway: Steep lined channel from crest to riverbed. Used when valley sides are steep.

  3. Side Channel Spillway: Spillway channel runs parallel to dam axis. Used when downstream valley is wide.

  4. Shaft (Morning Glory) Spillway: Circular crest, vertical shaft, horizontal tunnel. For narrow gorges where space limited.

  5. Siphon Spillway: Operates on siphon principle (automatically starts when water level rises). Used for auxiliary spillway.

  • Schematic:
    DiagramSEARCH: "ogee spillway diagram dam crest"

VI. Power Plant Economics and Operation

Performance Metrics - Definitions

1. Maximum Demand (MD): The greatest demand of load on the power station during a given period (usually a year). Expressed in kW/MW.

   • $$\displaystyle \boxed{MD = \text{Peak Load}} $$

2. Load Factor (LF): Ratio of average load to maximum demand over a specified period.

   • $$\displaystyle \boxed{LF = \frac{\text{Average Load}}{MD} = \frac{\text{Total Energy Produced in period}}{MD \times \text{Period Hours}}} $$ (Always < 1).

3. Diversity Factor (DF): Ratio of sum of individual maximum demands of consumers to the maximum demand of the system.

   • $$\displaystyle \boxed{DF = \frac{\sum MD_i}{MD_{system}}} $$ (Always > 1).

4. Plant Factor (PF) / Capacity Factor: Ratio of actual energy produced to the maximum possible energy if run at full capacity all the time.

   • $$\displaystyle \boxed{PF = \frac{\text{Annual Energy Output (kWh)}}{MD \times 8760 \text{ hrs}}}} $$ (Always < 1).

[!TIP] Common Pitfall: Diversity Factor > 1 (loads don't peak simultaneously). Load Factor & Plant Factor < 1 (plant doesn't run at peak continuously).

Load Analysis - Numerical Example (From Past Paper)

Problem: A power station supplies 4 regions with peak loads: 10 MW, 5 MW, 8 MW, 7 MW. Diversity Factor = 1.5, Annual Load Factor = 0.6.

• (i) Maximum Demand on Station:

  • Sum of individual MD = 10 + 5 + 8 + 7 = 30 MW.

  • $$\displaystyle DF = \frac{\sum MD_i}{MD_{station}} \Rightarrow 1.5 = \frac{30}{MD_{station}} $$

  • $$\displaystyle \boxed{MD_{station} = \frac{30}{1.5} = 20 \text{ MW}} $$

• (ii) Annual Energy Supplied:

  • Average Load = LF × MD = 0.6 × 20 MW = 12 MW.

  • Annual Energy = Average Load × 8760 hrs = 12 × 8760 = 105,120 MWh = 105.12 GWh.

  • $$\displaystyle \boxed{\text{Annual Energy} = 105.12 \text{ GWh}} $$

Depreciation and Financial Evaluation

• Sinking Fund Method: Annual payment ($A$) is deposited in a fund that accumulates with interest ($i$) to recover initial cost ($P$) minus salvage value ($S$) over life ($n$ years).

  • Formula: $$\displaystyle A = \frac{P - S}{A_{F,i,n}} = (P - S) \times \frac{i}{(1+i)^n - 1} $$

  • Example (From Past Paper): P=Rs 90,000, S=Rs 5,000, n=15 yrs, i=6%.

    • $$\displaystyle A = (90,000 - 5,000) \times \frac{0.06}{(1.06)^{15} - 1} = 85,000 \times \frac{0.06}{2.3966 - 1} = 85,000 \times 0.0430 \approx \boxed{\text{Rs. 3,655}} $$

• Straight Line Method: Equal annual depreciation of (Cost - Salvage Value).

  • Formula: $$\displaystyle A = \frac{P - S}{n} $$

  • Example: $$\displaystyle A = \frac{90,000 - 5,000}{15} = \frac{85,000}{15} \approx \boxed{\text{Rs. 5,667}} $$

Load Curves

• Load Duration Curve: Load values arranged in descending order vs. time (percentage of period). Shows percentage of time a given load is exceeded. Useful for evaluating base, intermediate, peak load plant requirements.

• Power Duration Curve: Same as load duration curve but for power (MW). Directly shows capacity factor and utilization.

  • Schematic:
    DiagramSEARCH: "load duration curve power plant economics"

Tariff Structures

• Types of Tariffs (Electricity Pricing):

  1. Flat Rate Tariff: Fixed charge per unit of energy consumed. Simple, but no incentive for off-peak use.

  2. Block Rate Tariff: Different rates for different consumption blocks (slab system). Progressive for higher consumption.

  3. Two-Part Tariff: Fixed Charge (based on MD or connected load) + Variable Charge (per kWh). Most common for industrial/commercial.

  4. Time-of-Day (TOD) Tariff: Different rates for peak, normal, off-peak hours. Encourages load shifting.

  5. Power Factor Tariff: Incentive/penalty based on power factor (for industrial consumers with inductive loads).

VII. Comparative Assessment of Power Plant Types

Comparison: Fossil Fuel, Hydro, and Renewable Energy Plants

[!TIP] Exam Conclusion: No single source is perfect. Optimal mix depends on national resource endowment, grid stability needs, environmental commitments, and economics. India's mix: Coal (baseload), Hydro (flexible baseload/peak), Renewables (growing share), Gas (peak).