Renewable Energy Sources (CE-504 (C)) - Unit 2 Short Notes

1.0 INTRODUCTION & POLICY FRAMEWORK

India's Renewable Energy Prospects & Potential:

• Government Target: 500 GW non-fossil fuel capacity by 2030 (part of 50% cumulative electric power installed capacity from renewables).

• Resource Base: High solar insolation (4-7 kWh/m²/day), extensive wind coastline, significant biomass potential, emerging hydro (small & pumped).

• National Missions: National Solar Mission, National Wind-Solar Hybrid Mission, National Bioenergy Mission.

Global & National Role:

• Energy Security: Reduces dependence on imported fossil fuels.

• Climate Change: Mitigates GHG emissions (India's NDC commitments).

• Sustainable Development: Provides decentralized energy, rural employment.

• Grid Integration Challenges: Intermittency, variability, need for balancing power & grid modernization.

Efficient Energy Management Strategies:

• Demand-Side Management (DSM): Load shifting, peak clipping.

• Energy Conservation Policies: Perform, Achieve and Trade (PAT) scheme, Standards & Labeling.

• Fiscal Incentives: Capital subsidies, tax holidays, viability gap funding.

• International Collaborations: International Solar Alliance (ISA), technology transfer partnerships.

2.0 SOLAR ENERGY SYSTEMS

2.1 Solar Radiation Fundamentals

• Extra-terrestrial Radiation: Solar constant ($$\displaystyle I_{sc} \approx 1367 \, W/m^2 $$), radiation outside Earth's atmosphere.

• Terrestrial Radiation: Received after atmospheric attenuation. Reasons for reduction:

  1. Absorption: By ozone (UV), water vapor, CO₂ (IR).

  2. Scattering: Rayleigh (molecules), Mie (aerosols) → diffuse radiation.

  3. Clouds: Major absorber & scatterer.

• Air Mass (AM): Path length relative to vertical. AM 1.5 is standard for terrestrial PV testing.

• Variations: Diurnal (day/night), seasonal (Earth's tilt & orbit), geographical (latitude, climate).

2.2 Solar Thermal Energy Conversion

• Principle: Concentrates/solar radiation to heat a fluid → steam → turbine → electricity OR direct heat use.

• Collectors:

  • Flat Plate: Low temp (50-100°C), water heating, space heating.

  • Concentrating:

    • Parabolic Trough: Linear focus, 150-400°C, CSP.

    • Parabolic Dish: Point focus, >500°C, Stirling engine.

    • Solar Tower: Central receiver, >1000°C, large-scale CSP.

• Applications: Domestic hot water, industrial process heat, solar thermal power (CSP), space heating/cooling (solar cooling).

2.3 Solar Photovoltaic (PV) Systems

Photovoltaic Effect & Solar Cell:

• p-n Junction: Formed by joining p-type & n-type semiconductor. Built-in electric field separates electron-hole pairs generated by photons ($$\displaystyle h\nu > E_g $$).

• IV Characteristics: Key parameters: $$\displaystyle I_{sc} $$ (short-circuit current), $$\displaystyle V_{oc} $$ (open-circuit voltage), $$\displaystyle P_{max} $$ (maximum power), Fill Factor (FF).

  Maximum Power Point (MPP): Point on IV curve where $$\displaystyle P = V \times I $$ is maximum. Condition: $$\displaystyle \frac{dP}{dV} = 0 $$ or $$\displaystyle \frac{dI}{dV} = -\frac{I}{V} $$.

Solar Cell Materials (Table):

Maximum Power Point Tracking (MPPT):

• Need: PV output power varies with insolation & temperature. MPPT ensures operation at MPP for max energy harvest.

• Principle: Adjusts duty cycle of DC-DC converter to match PV impedance to load.

• Common Algorithms:

  1. Perturb & Observe (P&O): Perturbs voltage, observes power change. Simple, but oscillates at MPP.

  2. Incremental Conductance (IncCond): Uses condition $$\displaystyle \frac{dI}{dV} = -\frac{I}{V} $$. More accurate, less oscillation.

PV System Configurations:

• Stand-Alone: PV + Battery + Charge Controller + Inverter (for AC loads).

• Grid-Tied: PV + Inverter (synchronizes with grid). No battery, uses net metering.

• Hybrid: PV + Wind/Diesel/Biomass + Storage for reliability.

• Key Components:

  • Inverter: DC-AC conversion.

  • Charge Controller: Regulates battery charging (MPPT or PWM type).

  • Battery Storage: Lead-acid, Li-ion for autonomy.

3.0 WIND ENERGY SYSTEMS

3.1 Wind Energy Conversion Systems (WECS)

Windmill Specifications for Power:

• Power in Wind: $$\displaystyle P_{wind} = \frac{1}{2} \rho A v^3 $$

  • $\rho$: air density (~1.225 kg/m³), $A$: swept area ($$\displaystyle \pi R^2 $$), $v$: wind speed.

• Power Coefficient ($$\displaystyle C_p $$): $$\displaystyle P_{mech} = C_p \times P_{wind} $$.

  • Betz Limit: Theoretical max $$\displaystyle C_p = 0.593 $$ (59.3%). Practical turbines: 0.35-0.45.

• Tip Speed Ratio ($\lambda$): $$\displaystyle \lambda = \frac{\omega R}{v} $$ (ω: angular speed). Optimal $\lambda$ for max $$\displaystyle C_p $$.

• Rotor Diameter & Hub Height: Larger diameter → more power. Higher hub height → stronger, less turbulent wind.

• Wind Speed Parameters:

  • Cut-in: ~3-4 m/s (starts generation).

  • Rated: ~12-15 m/s (rated power output).

  • Cut-out: ~25 m/s (shuts down for safety).

Components:

• Rotor blades (aerofoil), gearbox (speed increase), generator (induction/synchronous), tower, yaw mechanism (faces wind), nacelle.

Types:

4.0 BIOMASS ENERGY

4.1 Biomass Resources & Conversion Technologies

Resources: Woody (forest residues), Agricultural (straw, bagasse), Energy crops (jatropha, switchgrass), Animal waste (dung), Municipal Solid Waste (MSW), Industrial waste.

Conversion Processes:

5.0 OCEAN ENERGY TECHNOLOGIES

5.1 Wave Energy Conversion

• Advantages: High energy density (5-30x wind/solar), predictable (weather-driven), vast resource.

• Limitations: Technology immaturity, harsh marine environment (corrosion, storms), high installation/maintenance cost, grid connection challenges, environmental impact on marine ecology.

• WEC Types:

  • Oscillating Water Column (OWC): Wave compresses air in chamber → turbine.

  • Point Absorber: Buoy moves with waves → linear generator/hydraulic pump.

  • Attenuator: Long multi-segment device (like Pelamis) flexes with wave → hydraulic pumps.

5.2 Ocean Thermal Energy Conversion (OTEC)

• Principle: Utilizes temperature gradient ($\Delta T$ ~20-25°C) between warm surface water (~25-30°C) and cold deep water (~5-10°C). Warm water vaporizes a working fluid → drives turbine → cold water condenses vapor.

• Closed Cycle:

  • Working fluid: Low-boiling point (e.g., ammonia).

  • Warm seawater heats ammonia → vapor → turbine → cold seawater condenses ammonia → pump back.

  • Advantages: Simpler turbine (ammonia vapor), no phase change of seawater → less scaling/corrosion, can co-produce freshwater (condensate).

• Open Cycle:

  • Working fluid: Seawater itself.

  • Warm seawater flash-evaporated in vacuum → steam → turbine → cold seawater condenses steam → produces freshwater + brine.

  • Disadvantages: Large turbine (low pressure steam), scaling issues, complex vacuum system.

• Hybrid Cycle: Combines features of both.

6.0 GEOTHERMAL ENERGY

6.1 Geothermal Resources & Classifications

By Temperature/Hydrothermal Conditions:

• High-Temp (>150°C): Vapor or liquid-dominated. Direct electricity generation (flash steam, dry steam).

• Medium-Temp (90-150°C): Liquid-dominated. Binary cycle power plants (ORC).

• Low-Temp (<90°C): Direct use (heating, greenhouse, aquaculture).

By Reservoir Type:

• Vapor-Dominated (Dry Steam): Steam-filled fractures (e.g., The Geysers, USA).

• Liquid-Dominated (Hot Water): Porous/ fractured rock with hot water (most common).

• Hot Dry Rock (HDR) / Enhanced Geothermal Systems (EGS): Artificial reservoir created by fracturing hot, dry rock → inject water → produce steam. Technology-developing stage.

7.0 HYDRO ENERGY (SMALL HYDRO)

7.1 Small Hydro Power (SHP) Development

• Definition: Generally < 25 MW (India: < 10 MW for classification as SHP under MNRE).

• Development Factors:

  • Site Selection: Head (m) & Flow (m³/s) → $$\displaystyle P_{avg} = 9.81 \times Q_{design} \times H_{net} \times \eta $$ (η: overall efficiency ~0.5-0.8).

  • Environmental Impact: Lower than large dams (no large reservoirs, less submergence, displacement).

  • Grid Connectivity: Proximity to grid reduces transmission cost.

  • Civil Works: Minimal dam/barrage, penstock (pressure pipe), surge tank, turbine house.

• Turbine Types for Low-Head (<30m):

  • Kaplan: Adjustable blades, high flow, low head.

  • Francis: Medium head (10-300m), reaction turbine.

  • Cross-Flow (Banki): Simple, good for very low head & variable flow.

8.0 ENERGY MANAGEMENT & EFFICIENCY

8.1 Energy Audit

• Concept: Systematic examination of energy use → identify quantitative & qualitative savings → recommend improvements.

• Types:

  1. Preliminary Audit (Walk-through): Quick, low-cost, identifies obvious savings.

  2. Detailed Audit (Comprehensive): In-depth measurement, data logging, detailed analysis, investment-grade proposals.

  3. Investment-Grade Audit: Focus on major projects, detailed financial analysis (IRR, NPV, payback).

• Process: Data collection → energy use analysis → identify losses & savings → report with recommendations → implementation & verification.

8.2 Energy-Efficient Motors & Systems

• Factors Affecting Efficiency:

  • Design Losses: Stator core (hysteresis, eddy current), stator/rotor winding (I²R), friction & windage, stray load losses.

  • Size: Oversized motors operate at low load → low efficiency & power factor.

  • Operating Load: Efficiency peaks at 75-100% rated load.

  • Power Factor: Low PF increases current → higher losses in supply system.

  • Maintenance: Bearing wear, misalignment increase losses.

• Improvement Measures:

  • Premium Efficiency Motors: Meet IE3/IE4 standards (lower losses).

  • Variable Speed Drives (VSDs/VFDs): Match motor speed to load → huge energy savings in fans/pumps (affinity laws: $$\displaystyle P \propto N^3 $$).

  • Power Factor Correction: Capacitor banks near motor.

  • Right-sizing & Maintenance.

9.0 POWER ELECTRONICS & CONTROL IN RE SYSTEMS

9.1 Power Electronic Interfaces

Electronic Load Controller (ELC):

• Function: Used in isolated RE systems (wind, biogas) to maintain constant power/voltage/frequency to the load/grid despite variable input.

• Principle: Diverts excess power to a dump load (resistive heater) when generation exceeds load + battery charging needs. Ensures generator operates at stable, efficient point.

• Components: Power electronic switches (thyristors/MOSFETs), controller, dump load.

Thyristor (SCR - Silicon Controlled Rectifier):

• Structure: 4-layer (p-n-p-n), 3-terminal (Anode, Cathode, Gate).

• Operation as Switch:

  • Off: Reverse biased or forward biased without gate pulse.

  • On: Forward biased + positive gate pulse → latches on until current < holding current ($$\displaystyle I_H $$).

• Use in RE: Phase control (for AC power control), rectifiers (AC-DC), inverters (DC-AC) in PV/WECS.

9.2 Emerging Conversion Technologies

Magneto-Hydrodynamic (MHD) Generation:

• Principle: Direct energy conversion. Hot ionized gas (plasma) from combustion of fossil fuel/coal passes through a magnetic field → charged particles deflected → electrodes collect DC current (Faraday's law: $$\displaystyle \varepsilon = B \cdot l \cdot v $$).

• Advantages: No moving parts → potentially higher efficiency (50-60% vs 35-40% for steam cycle), higher temperature operation possible.

• Status: Experimental, high-temperature materials challenge.

10.0 INTEGRATED APPLICATIONS & SYSTEMS

10.1 Hybrid Renewable Energy Systems

• Concept: Combine two or more RE sources (e.g., Solar-Wind, Solar-Biomass, Solar-Wind-Diesel) with energy storage (batteries, fuel cells).

• Purpose: Improve reliability & power quality, reduce storage size/cost, optimize resource utilization, provide 24x7 power in remote areas.

• Configuration: AC/DC coupled, with power management strategy (prioritize sources, charge/discharge storage).

10.2 Grid Integration Challenges

• Intermittency & Variability: Solar/wind output changes with weather → grid frequency/voltage instability.

• Forecasting Need: Essential for dispatch planning & grid balancing.

• Grid Stability: Reduced inertia from inverter-based resources → need for synthetic inertia, grid-forming inverters.

• Technical Solutions: Smart grids, advanced inverters ( Volt/VAR control), synchronous condensers, large-scale storage.

• Policy: Net Metering: Export surplus solar power to grid → credits. Feed-in Tariffs (FiT): Fixed price for RE fed into grid.

Exam Tip: For "advantages/limitations" questions (like Wave Energy), always structure as bullet points: 3-4 advantages, 3-4 limitations. For "explain principle" (OTEC, MHD), start with core physics (temp gradient, Lorentz force) before system description.