Renewable Energy Sources (CE-504 (C)) - Unit 4 Short Notes

UNIT 4: RENEWABLE ENERGY SOURCES

I. INTRODUCTION & NATIONAL CONTEXT

Prospects and Potential in India

India has abundant renewable resources (solar, wind, biomass, small hydro). The country aims for 500 GW non-fossil capacity by 2030. Key prospects include:

• Energy Security: Reducing dependence on imported fossil fuels.

• Decentralization: Power generation in remote/ rural areas.

• Employment: Job creation in manufacturing, installation, and maintenance.

• Environmental Benefits: Low greenhouse gas emissions.

Global & Indian Scenario

Strategies for Efficient Energy Management

1. Policy & Regulation: Renewable Purchase Obligations (RPOs), fiscal incentives (accelerated depreciation, GST concessions).

2. Grid Integration: Strengthening transmission, forecasting & scheduling, grid-scale storage.

3. Technology & Innovation: R&D in storage (batteries, green hydrogen), next-gen PV, offshore wind.

4. Financing: Green bonds, international climate finance, risk mitigation funds.

5. Awareness & Capacity Building: Skill development programs (Suryamitra, etc.).

[!TIP] Exam Focus: Always link India's potential to its geographical advantage (tropical climate, long coastline, wind corridors). Mention specific missions like National Solar Mission and National Wind-Solar Hybrid Policy.

II. SOLAR ENERGY

Solar Radiation & Resource Assessment

• Extraterrestrial Radiation ($$\displaystyle G_{on} $$): Solar constant (~1367 W/m²) incident on a plane perpendicular to sun's rays at top of atmosphere.

• Terrestrial Radiation ($G$): Radiation reaching Earth's surface after atmospheric interaction.

• Reasons for Variation (Atmospheric Attenuation):

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

  2. Scattering: By air molecules (Rayleigh), aerosols, dust (Mie scattering) → causes diffuse radiation.

  3. Reflection: From clouds and ground.

• Key Parameter - Air Mass (AM): Ratio of path length through atmosphere to vertical path. $$\displaystyle AM = \frac{1}{\cos \theta_z} $$ (for $$\displaystyle \theta_z $$ ≤ 75°), where $$\displaystyle \theta_z $$ is zenith angle. AM1.5 is standard test condition for solar cells.

Solar Thermal Conversion

• Principle: Solar radiation is absorbed by a collector surface, converted to heat, and transferred to a working fluid (water, air, oil).

• Types of Collectors:

  | Type | Temperature Range | Concentration? | Typical Application | | :--- | :--- | :--- | :--- | | Flat Plate Collector | Low (30-100°C) | No | Water heating, space heating | | Concentrating Collector | High (100-1000°C+) | Yes (mirrors/lenses) | Steam generation for power, industrial process heat |

Solar Photovoltaic (PV) Systems

• Solar Cell Materials:

  • Crystalline Silicon (c-Si): Monocrystalline (high efficiency, uniform black), Polycrystalline (blue speckled, lower cost).

  • Thin-Film: Amorphous Si (a-Si), Cadmium Telluride (CdTe), Copper Indium Gallium Selenide (CIGS) – flexible, lower efficiency.

  • Emerging: Perovskites (high efficiency potential, stability issues), Organic PV (OPV).

• Maximum Power Point Tracking (MPPT):

  • Objective: Operate PV array at its Maximum Power Point (MPP) which varies with irradiance & temperature.

  • Procedure (Perturb & Observe - P&O):

    1. Measure array voltage ($V$) and current ($I$), calculate power ($$\displaystyle P=VI $$).

    2. Perturb (change) duty cycle of DC-DC converter (e.g., boost converter).

    3. Observe change in $P$.

    4. If $P$ increases, continue perturbation in same direction; else, reverse direction.

    5. Iterate to track MPP.

  \boxed{P_{max} = V_{mp} \times I_{mp}} where $$\displaystyle V_{mp} $$, $$\displaystyle I_{mp} $$ are voltage & current at MPP.

• System Components:

  • Inverter: Converts DC from PV to AC. Types: Central, String, Micro-inverters.

  • Charge Controller: Regulates battery charging, prevents overcharge/over-discharge. Types: PWM (simple, inefficient), MPPT (advanced, efficient).

III. WIND ENERGY

• Wind Energy Conversion System (WECS): Converts kinetic energy of wind to electrical energy.

  • Main Components: Rotor (blades), Gearbox (increases speed), Generator (produces AC), Nacelle (housing), Tower, Yaw system (faces wind), Controller.

• Specifications of Windmills for Power Generation:

  • Turbine Type: Horizontal Axis Wind Turbine (HAWT - most common), Vertical Axis Wind Turbine (VAWT - Darrieus, Savonius).

  • Capacity: Rated power (e.g., 2 MW, 5 MW).

  • Rotor Diameter: Directly related to swept area ($$\displaystyle A = \pi (D/2)^2 $$) and power capture.

  • Hub Height: Higher hub → access to higher wind speeds (wind shear). Typically 80-150m for onshore, 100-160m+ for offshore.

  • Cut-in, Rated, Cut-out Wind Speeds: Typical: Cut-in ~3-4 m/s, Rated ~12-15 m/s, Cut-out ~25 m/s.

IV. BIOMASS ENERGY

• Biomass Conversion Technologies:

  | Conversion Type | Process | Main Products | Key Conditions | | :--- | :--- | :--- | :--- | | Thermochemical | Combustion | Heat, flue gas | Excess air, high temp | | | Gasification | Producer gas (CO, H₂, CH₄) | Limited air/oxygen, high temp | | | Pyrolysis | Bio-oil, char, syngas | No oxygen, moderate temp | | Biochemical | Anaerobic Digestion | Biogas (CH₄, CO₂) | Anaerobic microbes, wet waste | | | Fermentation | Bioethanol | Yeast, sugars (starch/sucrose) | | Chemical | Transesterification | Biodiesel, glycerol | Catalyst (NaOH/KOH), alcohol (methanol) |

V. OCEAN ENERGY

Wave Energy

• Principle: Captures energy from surface waves (caused by wind). Devices oscillate or move with wave motion to drive a generator/pump.

• Common Converters: Point Absorber, Oscillating Water Column (OWC), Attenuator (Pelamis), Overtopping device.

• Advantages: High energy density, predictable (weather-driven), abundant.

• Limitations: Harsh marine environment (corrosion, storms), high maintenance cost, intermittent, grid connection challenges, potential ecological impact.

Ocean Thermal Energy Conversion (OTEC)

• Principle: Utilizes temperature difference ($\Delta T$) between warm surface water (25-30°C) and cold deep water (5-10°C) to run a heat engine.

  • Minimum $\Delta T$: ~20°C for net power output.

• Closed Cycle OTEC System:

  • Working Fluid: Low boiling point fluid (e.g., Ammonia (NH₃), R-134a).

  • Components:

    1. Evaporator: Warm seawater vaporizes working fluid.

    2. Turbine: Vapor expands through turbine → generates power.

    3. Condenser: Cold seawater condenses vapor back to liquid.

    4. Pump: Returns liquid to evaporator.

• Advantages of Closed Cycle over Open Cycle:

  • No need for large-diameter turbines/pumps (handles vapor, not massive water flows).

  • No direct contact between working fluid and seawater → less scaling/corrosion issues.

  • Higher efficiency (smaller temperature approach in heat exchangers possible).

  • Flexibility in plant location (not necessarily on coastline).

VI. GEOTHERMAL ENERGY

• Classifications of Geothermal Sources/Resources:

  | Resource Type | Description | Temperature | Example/Status | | :--- | :--- | :--- | :--- | | Hydrothermal | Natural hot water/steam in permeable rock. | | | |   – Vapor-dominated | Steam-filled fractures (less common). | > 150°C | Larderello (Italy), The Geysers (USA) | |   – Liquid-dominated | Hot water/brine (more common). | 100-300°C | Iceland, Philippines, India (Puga Valley) | | Geopressured | Hot brine under high pressure in sedimentary basins. | 90-180°C | Gulf Coast (USA) – also contains methane | | Hot Dry Rock (HDR) | Hot, impermeable crystalline rock. Requires Enhanced Geothermal Systems (EGS) – hydraulic fracturing to create reservoir. | 150-300°C+ | Experimental (e.g., Soultz, France) | | Magma | Molten rock (very high T). Direct heat extraction extremely challenging. | > 600°C | Research stage only |

VII. HYDRO ENERGY (Small-Scale)

• Small Head Hydro Power Development:

  • Definition & Classification (by capacity):

    • Small Hydro (SHP): 2-25 MW (India's definition).

    • Mini Hydro: 101 kW - 2 MW.

    • Micro Hydro: Up to 100 kW (typically for isolated communities).

  • Schemes:

    • Run-of-River (ROR): No/very small storage. Diverts part of river flow through channel/penstock to turbine, returns to river. Most common for SHP.

    • Pondage: Small storage (pond) to store water for a few hours/days to meet peak demand.

  • Components: Intake, Headrace channel/pipe, Surge tank (protects from water hammer), Penstock, Turbine (Pelton, Francis, Kaplan depending on head), Tailrace, Generator.

  • Potential & Applications: India's potential ~5 GW (mostly in Himalayan & North-Eastern states). Used for grid-connectivity, rural electrification, irrigation.

VIII. ENERGY MANAGEMENT & EFFICIENCY

Energy Audit

• Concept: Systematic examination of energy use & flows to identify opportunities for conservation & efficiency improvement.

• Objectives: Reduce energy costs, improve productivity, lower emissions, identify energy wastage.

• Types:

  1. Preliminary Audit (Walk-through): Quick assessment, major savings areas identified.

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

  3. Investment-Grade Audit: Focuses on detailed financial analysis (ROI, NPV) for specific projects.

Energy-Efficient Systems

• Factors Affecting Performance of Energy Efficient Motors:

  • Design & Materials: High-grade steel laminations (reduce core loss), optimized winding (reduce copper loss), improved cooling.

  • Operating Load: Efficiency peaks at 75-100% of rated load. Part-load operation reduces efficiency significantly.

  • Power Quality: Voltage imbalance (>1%), harmonics increase losses and heating.

  • Maintenance: Bearing friction, rotor/stator alignment affect performance.

System Controllers & Protection

• Electronic Load Controller (ELC) for Wind/PV Systems:

  • Principle: In isolated (standalone) systems, generation must match load instantly. ELC dumps excess power (as heat in a dump load - resistor bank) when load is less than generation, preventing system over-voltage/frequency rise.

  • Application: Essential for micro-hydro and wind systems without batteries to maintain stable voltage/frequency for connected AC loads.

IX. POWER ELECTRONICS & EMERGING TECHNOLOGIES

Power Electronic Devices

• Thyristor (SCR - Silicon Controlled Rectifier):

  • Basic Structure: Four-layer (PNPN), three terminals (Anode, Cathode, Gate).

  • Operation in Power Control:

    1. Forward Blocking: Anode (+ve w.r.t Cathode), Gate open → no conduction.

    2. Forward Conduction: Apply positive gate pulse → turns ON. Remains ON until anode current drops below Holding Current ($$\displaystyle I_H $$).

    3. Reverse Blocking: Reverse voltage → blocks like a diode.

  • Use: Phase control (AC), DC-DC choppers, inverters. Key for controlling power flow in renewable systems.

Other Renewable Sources

• Magneto-Hydrodynamic (MHD) Generation:

  • Principle: Direct conversion of thermal energy of hot, ionized gas (plasma) into electricity, bypassing mechanical rotation.

  • Basic Concept: Seeded (e.g., with potassium) combustion gas at high T (~2000°C) flows through a magnetic field. Lorentz Force ($$\displaystyle \vec{F} = q(\vec{v} \times \vec{B}) $$) separates positive/negative ions → creates voltage across electrodes → DC power output.

  • Advantages: High theoretical efficiency (~50-60%), no moving parts, fast start-up.

  • Challenges: Material science (high T, corrosive plasma), seed recovery, economic viability. Mostly experimental/ demonstration stage.