Unit 3: Advanced Power Plant Engineering (Short Notes)

I. Energy Sources and Conversion Technologies

Classification of Energy Sources

• Primary Energy Sources: Obtained directly from nature in raw form (e.g., coal, crude oil, natural gas, uranium, solar radiation, wind, hydro).

• Secondary Energy Sources: Derived from primary sources after conversion (e.g., electricity, petrol, diesel, LPG, hydrogen).

• Interconvertibility: Energy can be converted from one form to another (e.g., chemical → thermal → mechanical → electrical in a thermal power plant).

Direct Energy Conversion Methods

Converts energy directly to electricity without intermediate thermal/mechanical steps.

• Classification: Electrochemical (Fuel Cells), Electromagnetic (MHD), Photovoltaic (Solar Cells), Thermoelectric.

• Magnetohydrodynamic (MHD) Converter:

  • Working Principle: Ionized hot gas (combustion products seeded with alkali metal vapour) flows through a magnetic field. Lorentz force separates positive/negative ions, inducing EMF across electrodes (Faraday's law: $$\displaystyle \mathcal{E} = B \cdot v \cdot d $$).

  • Sketch Description:

    DiagramSEARCH: "MHD generator schematic diagram"
    – Shows combustion chamber, nozzle, magnetic field, electrodes, and diffuser.

  • Limiting Factors: Material problems at high temperatures (~2000°C), ionization difficulty, seed recovery cost, low open-circuit voltage.

• Fuel Cells:

  • Definition: Electrochemical device converting chemical energy of a fuel (H₂, hydrocarbon) and oxidant (O₂) directly into electricity and heat.

  • Basic Principle: Reverse of electrolysis. Fuel oxidized at anode, oxidant reduced at cathode, ions move through electrolyte, electrons through external circuit.

  • Types: Alkaline Fuel Cell (AFC), Phosphoric Acid Fuel Cell (PAFC), Molten Carbonate Fuel Cell (MCFC), Solid Oxide Fuel Cell (SOFC), Proton Exchange Membrane Fuel Cell (PEMFC).

  • Working of PEMFC:

    • Anode: $$\displaystyle \text{H}_2 \rightarrow 2\text{H}^+ + 2\text{e}^- $$

    • Cathode: $$\displaystyle \frac{1}{2}\text{O}_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}_2\text{O} $$

    • Overall: $$\displaystyle \text{H}_2 + \frac{1}{2}\text{O}_2 \rightarrow \text{H}_2\text{O} + \text{Heat} + \text{Electrical Work} $$

    • Thermodynamic Efficiency (Ideal): $$\displaystyle \eta_{\text{max}} = \frac{\Delta G}{\Delta H} $$ (Gibbs free energy change/Enthalpy change). $$\displaystyle \Delta G = -nFE $$.

  • Applications: Spacecraft, backup power, electric vehicles, portable devices.

  • Limitations: High cost (catalysts like Pt), fuel purity requirements (CO poisoning), durability, hydrogen storage/transport.

Solar Energy

• Factors Affecting Solar Intensity: Latitude, season, time of day, cloud cover, atmospheric transparency, surface orientation.

• Indian Conditions: High potential (4-7 kWh/m²/day), abundant in Rajasthan, Gujarat, Tamil Nadu, Karnataka. Monsoon affects consistency.

• Advantages: Renewable, abundant, no pollution, low operating cost.

• Limitations: Intermittent (day/night, weather), low efficiency (~15-20% PV), high initial cost, large land area required, storage challenges.

Wind Energy

• Factors Affecting Wind Velocity: Pressure gradient, Coriolis force, surface roughness, topography, diurnal/seasonal variations.

• Indian Conditions: Good potential in coastal states (Tamil Nadu, Gujarat, Maharashtra), peninsular region, and some northern states. Monsoon winds are seasonal.

• Limitations: Intermittent and variable, noise pollution, visual impact, threat to birds, requires large area/farm, high maintenance.

Comparison: Solar vs. Wind Energy (Indian Context)

Hybrid Energy Systems

• Definition: Integration of two or more renewable energy sources (e.g., solar-wind, solar-wind-diesel) with/without storage to improve reliability and output stability.

• Types: Solar-Wind, Solar-Wind-Hydro, Solar-Wind-Biomass.

• Feasible Options in India: Solar-Wind Hybrid (complementary seasonal/diurnal patterns), Solar-Wind-Diesel for remote areas/islands.

II. Fossil Fuel Power Plants

Coal Handling System

• Elements: Wagon unloading (tippler), crushing, screening, magnetic separation, conveying (belt/chain), stacking/reclaiming (stacker-reclaimer), bunkers.

• Sketch Description:

  DiagramSEARCH: "coal handling plant thermal power plant layout"
  – Shows flow from wagon tippler → crusher → conveyor → stacker → storage → reclaiming → bunkers → boiler.

Fuel Burning Systems

• Overfeed Firing: Coal fed above the firebed. Common in chain grate stokers. Simpler, less ash carryover.

• Underfeed Firing: Coal fed below the firebed (retractable rams). Better combustion control, less excess air needed.

• Recent Trends: Larger boiler units (660 MW, 800 MW+), once-through boilers (no drums), low-NOx burners, digital control systems (DCS), biomass co-firing.

Fluidized Bed Combustion (FBC)

• Working: Air blown at high velocity through a bed of sand/limestone + fuel. Particles fluidize like liquid. Combustion at 800-900°C.

  • Sketch Description:
    DiagramSEARCH: "bubbling fluidized bed boiler diagram"
    – Shows air distributor, bed material, fuel feed, cyclone separator (for CFBC), heat exchange tubes.

• Advantages: Fuel flexibility (coal, biomass, waste), in-situ SO₂ control (limestone addition), lower NOₓ (lower temperature), compact size, efficient heat transfer.

Gas Turbine Plants

• Simple Open Cycle: Air compressor → combustion chamber → gas turbine → exhaust. Low thermal efficiency (~30%).

• Reheating: After partial expansion in high-pressure turbine, gas is reheated in a second combustion chamber before entering intermediate/low-pressure turbine(s).

  • Effect on Efficiency: Increases work output significantly. Net work $$\displaystyle W_{\text{net}} = W_{\text{HP}} + W_{\text{IP/LP}} - W_{\text{comp}} $$. Reheat raises average temperature of heat addition, improving cycle efficiency (approaching combined cycle efficiency). Reduces moisture content at final turbine stage.

Feed Water Treatment

• Elements: Aeration → filtration → softening (lime-soda process, ion exchange) → deaeration (removes O₂, CO₂) → chemical treatment (ammonia, hydrazine) → filtration → polishing (mixed bed demineralizer for ultra-pure water).

Plant Heat Balance

• Definition: Quantitative accounting of all energy inputs (fuel LHV) and outputs (steam heat, losses, auxiliary consumption) in a power plant over a period.

• Example (Fossil Fuel):

  • Input: $$\displaystyle Q_{\text{in}} = \dot{m}_{\text{fuel}} \times \text{LHV} $$

  • Useful Output: $$\displaystyle Q_{\text{steam}} = \dot{m}_{\text{steam}} (h_{\text{steam}} - h_{\text{feedwater}}) $$

  • Losses: Stack loss, radiation/convection, unburnt carbon, moisture in fuel/air, auxiliary power.

  • Thermal Efficiency: $$\displaystyle \eta_{\text{thermal}} = \frac{Q_{\text{steam}}}{Q_{\text{in}}} \times 100\% $$

Cooling Towers

• Principle: Reject waste heat from condenser cooling water to atmosphere via evaporation and convection.

• Types:

  • Natural Draft: Hyperbolic shape, large, uses chimney effect. Low operating cost, high capital.

  • Mechanical Draft: Fans force/induce air. Forced Draft (fan at inlet), Induced Draft (fan at outlet – common). Compact, controllable.

  • Dry Cooling: Air-cooled condensers (no evaporation, water saving, high cost, lower efficiency in hot climates).

III. Nuclear Power Plants

Nuclear Fission Phenomenon

Heavy nucleus (U-235, Pu-239) absorbs a thermal neutron, becomes unstable, splits into two lighter fragments (fission products), releasing ~200 MeV energy, 2-3 fast neutrons, and gamma radiation. Chain reaction sustained if multiplication factor (k) ≥ 1.

Reactor Types & Comparison

Pressurized Heavy Water Reactor (PHWR)

• Working: Natural UO₂ fuel pellets in zirconium alloy cladding. Heavy water (D₂O) acts as both moderator (in calandria at low pressure) and primary coolant (pressurized, ~100 bar). Heat transferred to secondary light water loop via steam generators. Steam drives turbine.

• Sketch Description:

  DiagramSEARCH: "PHWR reactor diagram calandria"
  – Shows pressure tubes, calandria, fuel channels, steam generators, pumps.

Advanced Gas Cooled Reactor (AGCR)

• Description: Second generation UK design. Graphite moderator, CO₂ gas coolant (pressurized). Fuel: metallic uranium (enriched) or UO₂. Helium used in some modern designs (HTGR).

• Fission Context: Graphite slows neutrons to thermal energies. CO₂ removes heat. Fission in fuel rods produces heat and neutrons. Control rods (B₄C) inserted/withdrawn to control reactivity.

Moderators

• Function: Slow down fast fission neutrons to thermal energies (0.025 eV) to increase probability of fission in U-235.

• Types & Characteristics:

  | Moderator | Absorption Cross-Section | Scattering Cross-Section | Advantages | Disadvantages | | :--- | :--- | :--- | :--- | :--- | | Light Water (H₂O) | High | High | Cheap, good heat transfer | High absorption → needs enriched fuel | | Heavy Water (D₂O) | Very Low | High | Allows natural U fuel | Very expensive | | Graphite | Very Low | Moderate | Cheap, stable, allows natural U | Low density, needs large core, fire risk (CO₂ coolant) | | Beryllium | Low | High | Good moderator & reflector | Toxic, expensive, scarce |

Reactor Control Principles

1. Control Rods: Boron/Cadmium/Hafnium rods inserted/withdrawn to absorb neutrons (primary control).

2. Chemical Shim: Dissolving soluble neutron absorber (Boric acid) in coolant/moderator (PWR, PHWR).

3. Burnable Absorbers: Gadolinium/Boron mixed in fuel pellets, burns away over time.

4. Moderator Temperature Coefficient: Increase in moderator temp reduces density → fewer neutrons thermalized → negative feedback.

Reliability Features for Nuclear Power Plants

• Multiple Physical Barriers: Fuel cladding, reactor pressure vessel, containment building (leak-tight).

• Redundant & Diverse Safety Systems: Multiple independent shutdown systems, emergency core cooling.

• Defense-in-Depth: Layers of safety (operational limits, safety systems, emergency planning).

• High-Quality Components & Rigorous QA: Stringent manufacturing, testing, and inspection.

• Seismic Design & Accident Management: Designed for safe shutdown under design basis accidents (DBA) and beyond.

IV. Hydropower Plants

Hydraulic Turbines (Types & Characteristics)

Factors for Turbine Selection

1. Available Head (H): Primary determinant.

2. Design Flow (Q): Peak/design discharge.

3. Specific Speed (Ns): $$\displaystyle N_s = N \sqrt{P} / H^{5/4} $$ (N: rpm, P: kW). Guides type.

4. Efficiency & Cost: High efficiency across operating range, capital & maintenance cost.

5. Site Constraints: Civil works, tailrace conditions, cavitation risk.

Site Selection for Hydropower

• Hydrology: Reliable, high annual flow; good catchment area; low sediment load.

• Topography: Narrow gorge for dam; steep fall (head); accessible.

• Geology: Sound rock foundation for dam & powerhouse; low seismicity; no major faults.

• Environmental & Social: Minimal displacement, forest submergence, impact on flora/fauna, cultural sites.

• Economic: Proximity to load center; transmission cost; land availability.

Comparison of Power Plant Types

Micro and Pico Hydro Machines

Spillways

• Purpose: Safely pass flood flows without overtopping dam, protecting dam integrity.

• Types:

  • Ogee Spillway: Overflow type, shaped like ogee (S-curve) profile for nappe adherence. Most common for gravity dams.

  • Side Channel Spillway: Flow enters a side channel parallel to dam, then to river. Used when valley is narrow.

  • Shaft (Morning Glory) Spillway: Circular inlet, vertical shaft, horizontal tunnel. For narrow canyons.

  • Chute Spillway: Open channel (lined) along dam abutment.

Duration Curves

• Flow Duration Curve (FDC): Plots discharge (Q) vs. percentage of time that flow is equaled or exceeded. Shows reliability/availability of water resource.

  • Use: Estimate firm power, reservoir sizing, water availability analysis.

• Power Duration Curve (PDC): Plots power output (P) vs. percentage of time that power is equaled or exceeded. Derived from FDC and plant efficiency/head.

  • Use: Determine firm power (power available 90-100% time), evaluate economic viability.

V. Power Plant Economics and Operation

Key Performance Parameters

1. Maximum Demand (MD): Highest instantaneous load during a given period (e.g., year). Units: kW/MW.

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

$$ \text{Load Factor} = \frac{\text{Average Load}}{\text{Maximum Demand}} = \frac{\text{Energy Produced in period}}{\text{MD} \times \text{Hours in period}} $$

\boxed{\text{LF} = \frac{E}{MD \times T}} \quad (E in kWh, T in hours)

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

$$ \text{Diversity Factor} = \frac{\sum \text{Individual MDs}}{\text{System MD}} $$

\boxed{\text{DF} > 1 \text{ (always)}} \quad (Indicates diversity reduces system peak)

4. Plant Factor (PF) / Capacity Factor: Ratio of actual energy produced to maximum possible energy (if run at MD continuously).

$$ \text{Plant Factor} = \frac{\text{Annual Energy Output (kWh)}}{\text{Rated Capacity (kW)} \times 8760 \text{ hrs}} $$

\boxed{\text{PF} = \frac{E_{\text{annual}}}{P_{\text{rated}} \times 8760}}

Calculation Problem (From Past Paper)

Q: 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. Calculate:

(i) Maximum demand on station.

(ii) Annual energy supplied (GWh).

Solution:

1. Sum of individual MDs = 10 + 5 + 8 + 7 = 30 MW

2. Diversity Factor (DF) = Sum of individual MDs / System MD

   \begin{align*}

   1.5 &= \frac{30}{\text{System MD}} \

   \text{System MD} &= \frac{30}{1.5} = \boxed{20 \text{ MW}} \quad \text{(Ans i)}

   \end{align*}

3. Load Factor (LF) = Average Load / MD = 0.6

   \begin{align*}

   \text{Average Load} &= 0.6 \times 20 = 12 \text{ MW} \

   \text{Annual Energy (E)} &= \text{Avg. Load} \times \text{Hours/year} \

   &= 12 \text{ MW} \times 8760 \text{ hrs} = 105,120 \text{ MWh} \

   &= \boxed{105.12 \text{ GWh}} \quad \text{(Ans ii)}

   \end{align*}

Depreciation Methods

1. Straight Line Method (SLM): Equal depreciation charge every year.

$$ \text{Annual Depreciation} = \frac{\text{Initial Cost} - \text{Salvage Value}}{\text{Useful Life}} $$

\boxed{D_{\text{SL}} = \frac{C - S}{n}}

2. Sinking Fund Method (SFM): Depreciation + Interest on accumulated fund. Annual deposit ($A$) calculated to accumulate to (C-S) in n years at interest rate (i).

$$ A = (C - S) \left[ \frac{i}{(1+i)^n - 1} \right] $$

\boxed{A = (C - S) \cdot \left( \frac{i}{(1+i)^n - 1} \right)}

*   **Example (Past Paper):** C=90,000, S=5,000, n=15, i=6%=0.06.

    *   SLM: $$\displaystyle D = (90000-5000)/15 = \boxed{5666.67 \text{ Rs/year}} $$

    *   SFM: $$\displaystyle A = 85000 \times \frac{0.06}{(1.06)^{15}-1} = 85000 \times \frac{0.06}{2.3966-1} = 85000 \times 0.0430 = \boxed{3655 \text{ Rs/year}} $$

Tariffs

• Flat Rate Tariff: Fixed charge per unit of energy consumed. Simple, but no incentive for load management.

• Block Rate Tariff: Different rates for different consumption blocks (e.g., first 100 kWh @ Rs. 5, next 200 @ Rs. 4). Encourages higher consumption.

• Two-Part Tariff: Fixed Charge (based on MD/connected load) + Running Charge (per kWh). Common for industrial/commercial consumers. Recovers fixed and variable costs.

• Power Factor Tariff: Incentive/penalty based on power factor (cos φ). Encourages improvement.

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

VI. Additional Short Note Topics (Frequently Recurring)

Overfeed and Underfeed Principle of Firing of Coal

• Overfeed Stoking: Coal fed above the firebed (e.g., chain grate). Air passes upward through grate and fuel bed. Simpler, less ash carryover, but combustion less controlled.

• Underfeed Stoking: Coal fed below the firebed through retractable rams. Air passes upward through tuyeres into fuel. Better control, less excess air, higher efficiency, suitable for low-volatile coals. More mechanical complexity.

Principles of Reactor Control

1. Control Rods: Primary method. Insertion absorbs neutrons, reducing reactivity; withdrawal increases reactivity.

2. Chemical Shim: Adjust concentration of soluble neutron absorber (e.g., boric acid in PWR coolant) for coarse, uniform control.

3. Burnable Absorbers: Temporary absorbers (Gd, B) in fuel that deplete over time, compensating for fuel burnup.

4. Moderator Temperature Coefficient: Negative feedback – as moderator temp rises, density drops, fewer neutrons thermalized → reactivity decreases automatically.

5. Coolant Temperature Coefficient: Similar negative feedback in some designs.

Flow and Power Duration Curves

• Flow Duration Curve (FDC): Graph of discharge (Q) ranked in descending order vs. percentage of time that flow is exceeded. Key Parameter: Flow available for 90% of time (Q₉₀) indicates firm capacity.

• Power Duration Curve (PDC): Derived from FDC: $$\displaystyle P = \eta \rho g H Q $$. Plots power vs. % time exceeded. Area under curve = Total annual energy. Firm Power = Power available >90% time. Used for capacity credit assessment of hydro vs. thermal.

Types of Tariffs

1. Flat Rate: Single price per kWh.

2. Block Rate: Slab system, decreasing/increasing rates.

3. Two-Part Tariff: Fixed charge (₹/kW of MD) + Variable charge (₹/kWh).

4. Power Factor Tariff: Incentive for high PF (>0.9), penalty for low PF.

5. Time-of-Day (TOD) Tariff: Peak, normal, off-peak rates.

6. Seasonal Tariff: Different rates for summer/winter.

Cooling Towers

• Principle: Heat rejection from condenser cooling water to atmosphere via evaporation (latent heat) and convection (sensible heat).

• Types:

  • Natural Draft: Hyperbolic concrete structure, uses stack effect. No fans, low operating cost, high capital.

  • Mechanical Draft: Uses fans.

    • Forced Draft: Fan at air inlet, positive pressure.

    • Induced Draft: Fan at air outlet (most common), negative pressure, better control.

  • Dry Cooling: Air-cooled condensers (ACC). No water loss, high cost, performance sensitive to ambient temp.

• Key Parameter: Approach = (Tower outlet temp - Wet-bulb temp). Lower approach = better cooling = higher capital cost.

[!TIP] Exam Focus: Be prepared to draw labeled sketches for MHD, PHWR, FBC, Coal Handling, and Turbine types. Practice numericals on Load Factor, Diversity Factor, Depreciation, and FDC/PDC conversion. Compare Solar vs Wind and Thermal vs Hydro vs Renewable in tabular form for 7-mark questions.