Renewable Energy Technology (ME-604 (C)) - Unit 4 Short Notes

UNIT 4: RENEWABLE ENERGY TECHNOLOGY - EXAM-FOCUSED SHORT NOTES

I. SOLAR ENERGY TECHNOLOGIES

A. Fundamentals of Solar Radiation

• Sun-Earth Relationship: Earth's elliptical orbit and tilted axis (23.5°) cause seasonal variations in solar insolation. The solar constant (~1367 W/m²) is the radiation at the top of the atmosphere.

• Solar Geometry Key Angles:

  • Declination (δ): Angle between Sun-Earth line and equatorial plane. Varies ±23.45° annually.

  • Hour Angle (ω): Angular displacement of sun from local meridian (15° per hour).

  • Altitude Angle (α): Angle between sun's rays and horizontal plane.

  • Incidence Angle (θ): Angle between sun's rays and normal to the surface.

  [!TIP] Common Exam Problem: Calculate incidence angle for a tilted, south-facing collector.

• Measurement: Pyranometer (global radiation), Pyrheliometer (direct radiation), Pyrometer (diffuse).

• Estimation on Tilted Surface: $$\displaystyle H_T = H_b R_b + H_d R_d + H_g R_g $$, where $$\displaystyle R_b $$ is the tilt factor for beam radiation.

B. Solar Photovoltaic (PV) Systems

• Principle of Photovoltaic Conversion: Direct conversion of solar radiation into DC electricity via the photovoltaic effect in a semiconductor p-n junction. Photon absorption generates electron-hole pairs, separated by the junction's electric field.

  Key Formula: $$\displaystyle P_{out} = V \times I $$ (Power output).

• Solar Cell Construction: Thin wafer of semiconductor (Si). p-type (Boron doped) and n-type (Phosphorus doped) layers form a junction. Metal contacts on front (grid) and rear.

• Types of Solar Cells/Modules:

  | Type | Material | Efficiency | Features | | :--- | :--- | :--- | :--- | | Crystalline Si | Mono-Si, Multi-Si | 15-22% | High efficiency, long life, costly. | | Thin-Film | a-Si, CdTe, CIGS | 7-13% | Low cost, flexible, lower efficiency. |

• Applications: Stand-alone systems (remote homes, street lights), grid-connected systems, space satellites, consumer electronics.

• Limitations of SPV:

  • Intermittent (day/night, weather).

  • Low conversion efficiency (~20% max).

  • High initial cost.

  • Requires energy storage or backup.

  • Large area required for utility-scale.

• Maximum Power Point Tracking (MPPT): Algorithm to operate PV at its Maximum Power Point (MPP) where $$\displaystyle dP/dV = 0 $$.

  • Perturb and Observe (P&O):

    1. Perturb (change) PV voltage $$\displaystyle V_{pv} $$ by small $\Delta V$.

    2. Measure change in power $\Delta P$.

    3. If $$\displaystyle \Delta P > 0 $$, continue perturbation in same direction.

    4. If $$\displaystyle \Delta P < 0 $$, reverse perturbation direction.

    [!TIP] Simple but can oscillate around MPP under rapid irradiance changes.

C. Solar Thermal Systems

• Classification of Collectors:

  • By Temperature: Low (<100°C), Medium (100-250°C), High (>250°C).

  • By Concentration: Non-concentrating (flat plate), Concentrating (parabolic trough, dish, tower).

• Flat Plate Collector (FPC):

  • Construction: Absorber plate (black coated), riser tubes, glazing (glass), insulation (rockwool), casing.

  • Working: Solar radiation passes through glazing, absorbed by plate, heats fluid in tubes. Glazing reduces convective/radiative losses.

  Diagram:

  DiagramSEARCH: flat plate solar collector diagram labelled

• Solar Water Heating Systems:

  • Thermosyphon System: Passive. Cold water enters tank at bottom, hot water rises naturally to tank. No pump, reliant on density difference. Suitable for domestic use.

  • Forced Circulation System: Active. Pump circulates fluid from collector to storage tank. Controlled by differential thermostat. Suitable for large installations.

• Solar Dryers: Use solar heat to remove moisture from agricultural products.

  • Types: Direct (product in sun), Indirect (air heated in collector), Mixed (combination).

  • Applications: Drying grains, fruits, vegetables, fish, leather.

II. WIND ENERGY CONVERSION SYSTEMS (WECS)

A. Fundamentals

• Wind Regime & Energy: Wind energy density $$\displaystyle P/A = \frac{1}{2} \rho v^3 $$. Cubic relationship with velocity makes site assessment critical.

• Aerofoil: Cross-sectional shape of a blade (like an aircraft wing). Generates lift (primary force) and drag.

  • Types: Symmetric, Cambered (asymmetric). Wind turbine blades use cambered aerofoils for high lift-to-drag ratio.

• Power Curve: Graph of power output vs. wind speed. Key points:

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

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

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

B. Wind Turbine Aerodynamics & Power

• Betz's Limit Derivation (Key Exam Derivation):

  1. Consider wind turbine of area $A$ with upstream velocity $$\displaystyle v_1 $$, downstream $$\displaystyle v_2 $$, velocity at turbine $v$.

  2. Mass flow rate $$\displaystyle \dot{m} = \rho A v $$.

  3. Power extracted: $$\displaystyle P = \dot{m} (v_1^2 - v_2^2)/2 = \frac{1}{2} \rho A v (v_1^2 - v_2^2) $$.

  4. From continuity, $$\displaystyle v = (v_1 + v_2)/2 $$.

  5. Substitute and maximize $P$ w.r.t. $$\displaystyle v_2/v_1 $$. Optimal $$\displaystyle v_2/v_1 = 1/3 $$.

  6. Maximum Power: $$\displaystyle P_{max} = \frac{16}{27} \cdot \frac{1}{2} \rho A v_1^3 $$.

  7. Power Coefficient: $$\displaystyle C_p = P / P_{wind} \leq 16/27 \approx 0.593 $$ (Betz limit).

  \boxed{P_{max} = \frac{16}{27} \cdot \frac{1}{2} \rho A v^3}

  [!TIP] Remember: Betz limit is theoretical max. Actual $$\displaystyle C_p $$ ~0.4-0.5.

• Factors Affecting Power Output: Wind speed ($$\displaystyle v^3 $$), air density ($\rho$), swept area ($A$), $$\displaystyle C_p $$ (blade design, losses).

C. WECS Configuration & Components

• Types:

  • Horizontal Axis Wind Turbine (HAWT): Main shaft parallel to ground/wind. Most common. Needs yaw mechanism.

  • Vertical Axis Wind Turbine (VAWT): Main shaft perpendicular to ground. Omnidirectional (no yaw), but lower efficiency. Types: Darrieus (lift-based), Savonius (drag-based).

  Diagram:

  DiagramSEARCH: HAWT vs VAWT comparison diagram

• Main Components:

  1. Rotor (Blades + Hub): Captures wind energy.

  2. Nacelle: Housing containing gearbox, generator, controller.

  3. Gearbox: Increases rotor speed (low) to generator speed (high).

  4. Generator: Converts mechanical to electrical energy (usually induction or synchronous).

  5. Tower: Supports rotor/nacelle. Height increases wind speed.

  6. Yaw System: Rotates nacelle to face wind (HAWT).

  7. Braking System: Safety.

D. Challenges & Applications

• Limitations/Barriers:

  • Intermittent and variable.

  • Noisy, visual impact.

  • Threat to birds/bats.

  • Grid integration challenges (voltage fluctuations).

  • High capital cost, remote locations.

• Wind-Diesel Hybrid System: Combines wind turbines with diesel generators. Wind reduces diesel consumption. Requires control system to manage power balance and stabilize frequency/voltage in isolated grids.

III. BIOMASS ENERGY

A. Biomass Resources & Potential

• Definition: Organic matter (plant/animal origin) that can be converted to energy.

• Usefulness: Renewable, carbon-neutral (closed CO₂ cycle), widely available.

• Types/Production:

  1. Energy Crops: Dedicated (e.g., switchgrass, sugarcane for ethanol).

  2. Agricultural Residues: Crop waste (straw, husks, bagasse).

  3. Forestry Residues: Wood chips, sawdust.

  4. Municipal Solid Waste (MSW): Organic fraction.

  5. Animal Waste: Manure.

B. Biomass Conversion Technologies

• Biochemical Conversion:

  • Anaerobic Digestion: Biological breakdown in absence of O₂ → Biogas (CH₄ + CO₂) + digestate (fertilizer).

  • Fermentation: Conversion of sugars (e.g., from corn, sugarcane) to bioethanol using yeast.

• Thermochemical Conversion - Biomass Gasification:

  • Partial combustion at high T (700-900°C) with limited air/oxygen → Producer Gas (CO + H₂ + CH₄ + N₂).

  • Types:

    | Type | Air Flow | Gas Temp | Efficiency | Tar | | :--- | :--- | :--- | :--- | :--- | | Updraft | Bottom-up | Low (exit) | High | High | | Downdraft | Top-down | High (exit) | Medium | Low | | Cross-draft | Side | High | Medium | Low | | Fluidized Bed | Bottom | Uniform | Very High | Very Low |

• Power Generation from Gasification:

  1. Gasifier → Clean producer gas.

  2. Gas engine (IC) or gas turbine → Mechanical/Electrical power.

  3. Or, gas used in boiler for steam turbine.

C. Anaerobic Digestion & Biogas

• Digester Types:

  • Floating Drum (KVIC): Gas collects under floating steel drum. Moves with gas pressure. Common in India.

  • Fixed Dome (Deenbandhu): Brick/cement dome. Gas pressure pushes slurry in outlet. No moving parts.

  • Bag Digester: Flexible plastic bag. Simple, low cost.

  Diagram:

  DiagramSEARCH: floating drum fixed dome biogas plant diagram

• Biogas Plant Design: Based on retention time (20-50 days), feedstock (C/N ratio ~25:1), temperature (mesophilic ~35°C). Sizing: Volume = (Daily feed × Retention time).

• Photosynthesis - C3 vs C4 Plants:

  | Feature | C3 Plants | C4 Plants | | :--- | :--- | :--- | | First Product | 3-PGA (3-carbon) | OAA (4-carbon) | | Pathway | Calvin Cycle only | Spatial separation (mesophyll, bundle sheath) | | Photorespiration | High (wastes energy) | Very Low | | Efficiency | Lower | Higher (better in high light, temp, drought) | | Examples | Rice, Wheat, Soybean | Sugarcane, Maize, Sorghum |

IV. HYDROPOWER

A. Hydropower System Classification

• Micro Hydro: < 100 kW. Run-of-river, no dam. For village/micro-grid.

• Mini Hydro: 100 kW - 1 MW. Small dam/weir.

• Small Hydro: 1 MW - 25 MW (or up to 50 MW, country-dependent). Can have storage.

  Differentiation: Based on installed capacity and degree of water flow regulation (storage vs. run-of-river).

B. Turbines

• Definition: Machines converting water's kinetic & potential energy to rotational mechanical energy.

• Types & Selection:

  | Type | Head (m) | Flow (m³/s) | Principle | Specific Speed | | :--- | :--- | :--- | :--- | :--- | | Impulse | High (>300) | Low | Jet impacts buckets | Low | | Pelton | 150-2000 | Low | Buckets on wheel | Very Low | | Turgo | 50-300 | Medium | Jet hits between buckets | Low-Medium | | Reaction | Low-Medium | Medium-High | Pressure + kinetic | Medium-High | | Francis | 10-300 | Medium-High | Radial flow | Medium | | Kaplan | < 30 | High | Axial flow, adjustable blades | High |

• Selection Criteria: Primarily Net Head and Design Flow.

  Rule of Thumb: High Head → Impulse (Pelton); Medium Head → Francis; Low Head → Kaplan.

• Explanation - Francis Turbine (Reaction):

  • Construction: Spiral casing (volute), stay vanes, guide vanes, runner (curved blades), draft tube.

  • Working: Water enters volute, pressure drops as it flows through stay/guide vanes onto runner blades. Both pressure and velocity change cause rotation. Draft tube recovers kinetic energy, creates suction.

  Diagram:

  DiagramSEARCH: Francis turbine labelled diagram

C. Plant Operation & Management

• Speed Regulation: Maintain constant generator speed (e.g., 3000/1500 RPM for 50/60 Hz) despite load changes. Uses governor (mechanical/hydraulic) to adjust wicket gate opening (flow control).

• Voltage Regulation: Maintain terminal voltage. Uses AVR (Automatic Voltage Regulator) to control generator excitation current. In isolated grids, voltage also affected by reactive power balance.

V. GEOTHERMAL ENERGY

A. Principles & Resources

• Thermodynamic Principle: Utilizes Earth's internal heat. Flash Steam: High-pressure hot water (>180°C) flashed to steam in low-pressure tank. Dry Steam: Direct use of natural steam. Binary Cycle: Heat exchanger heats secondary fluid (low boiling point) to drive turbine.

• Geothermal Deposit Types:

  1. Hydrothermal: Hot water/steam in porous/fractured rock (most common). Vapor-dominated (steam), Liquid-dominated (hot water).

  2. Geopressured: Hot water under high pressure in deep sedimentary basins. Contains dissolved methane.

  3. Hot Dry Rock (HDR): Impermeable hot rock. Requires Enhanced Geothermal Systems (EGS) - inject water to create fractures.

  4. Magma: Molten rock. Highest T, least accessible.

B. Power Generation & Plant Siting

• Generation Process (Typical Liquid-Dominated):

  1. Production well taps hot water (150-300°C).

  2. Water flashes to steam in separator.

  3. Steam drives turbine-generator.

  4. Condensate + remaining brine injected back via injection well (re-injection).

• Site Selection Criteria:

  • High geothermal gradient/heat flow.

  • Presence of reservoir (permeability, porosity).

  • Suitable chemistry (non-corrosive, non-scaling).

  • Proximity to load centers.

  • Environmental and social acceptability.

C. Environmental Aspects

• Benefits:

  • Very low GHG emissions (mostly steam, some dissolved CO₂/H₂S).

  • Small land footprint.

  • Minimal visual impact.

  • Base-load capability (unlike solar/wind).

  • Note: Potential for induced seismicity (EGS), water use, emissions of H₂S (rotten egg smell) if not abated.

VI. OCEAN ENERGY

A. Tidal Energy

• Principle: Harness kinetic energy of tidal currents or potential energy of tidal height difference. Tidal Range (difference between high/low tide) is key for barrage systems.

  Diagram:

  DiagramSEARCH: single basin tidal barrage diagram

• Types of Tidal Power Plants:

  1. Single Basin (One-way/Two-way): One basin separated by dam with turbines. Fills on flood tide (generates) or empties on ebb tide. Two-way generates on both.

  2. Double Basin: Two basins at different levels. Pump water from lower to upper using excess generation, then release for generation on demand (more control).

• Numerical Problem (Filling/Emptying Process Energy):

  • Given: Basin area $A$, tidal range $R$, min operating head $$\displaystyle H_{min} $$, efficiency $\eta$.

  • Available Potential Energy: $$\displaystyle E_{avail} = \frac{1}{2} \rho g A R^2 $$ (Energy in water column of height R over area A).

  • Usable Energy: Only the volume from mean sea level down to $$\displaystyle H_{min} $$ (or up) generates.

    • Effective head for generation: $$\displaystyle H_{eff} = R - H_{min} $$ (for one-way, single basin).

    • Energy Generated: $$\displaystyle E_{gen} = \eta \times \frac{1}{2} \rho g A (R^2 - H_{min}^2) $$.

    \boxed{E_{gen} = \eta \cdot \frac{1}{2} \rho g A (R^2 - H_{min}^2)}

  [!TIP] For May 2023/May 2024 problem: $$\displaystyle A=30\times10^6 m^2 $$, $$\displaystyle R=12 m $$, $$\displaystyle H_{min}=3 m $$, $$\displaystyle \eta=0.73 $$. Compute $$\displaystyle E_{gen} $$ in kWh (1 kWh = 3.6e6 J).

B. Ocean Thermal Energy Conversion (OTEC)

• Principle: Uses ocean's thermal gradient (warm surface water ~25-30°C, cold deep water ~5-10°C). Requires ΔT > 20°C (tropical regions).

  • Closed-Cycle: Working fluid (e.g., ammonia, low boiling point) evaporates in warm seawater heat exchanger, drives turbine, condenses in cold seawater exchanger.

  • Open-Cycle: Warm seawater itself flashed to steam in vacuum chamber (low pressure), steam drives turbine, condenses to desalinated water.

  Diagram:

  DiagramSEARCH: OTEC closed cycle diagram

C. Wave Energy (Brief)

• Significance of Wave Height: Wave energy flux (power/m) is proportional to square of wave height ($$\displaystyle H^2 $$) and wave period ($T$). $$\displaystyle P \propto H^2 T $$.

• Irregular Amplitudes: Real sea state has spectrum of waves. Significant Wave Height ($$\displaystyle H_s $$): Average height of highest 1/3 of waves in a sea state. Used for resource assessment and device design.

VII. FUEL CELL TECHNOLOGY

A. Fundamentals & Working

• Working Principle: Electrochemical device converting chemical energy of fuel (H₂, CH₄, etc.) and oxidant (O₂/air) directly into electricity, with water/heat as byproducts. No combustion.

  • Anode: Fuel oxidized (e.g., H₂ → 2H⁺ + 2e⁻).

  • Cathode: Oxidant reduced (e.g., ½O₂ + 2H⁺ + 2e⁻ → H₂O).

  • Electrolyte: Conducts ions (H⁺, O²⁻, CO₃²⁻) but blocks electrons.

  • External Circuit: Electrons flow from anode to cathode → DC power.

  Diagram:

  DiagramSEARCH: fuel cell diagram anode cathode electrolyte

• Main Components: Fuel processor (reformer), fuel cell stack (cells in series), power conditioner (DC-AC inverter), heat recovery system.

B. Classification & Types (By Electrolyte)

C. Evaluation (Advantages)

• High efficiency (40-60%, up to 85% with CHP).

• Low/zero emissions (if H₂ from renewable).

• Quiet, reliable, modular.

• Fast refueling (for H₂ vehicles).

• Disadvantages: High cost (catalysts like Pt), fuel infrastructure (H₂), durability challenges (SOFC thermal cycling).

VIII. INTEGRATED & MISCELLANEOUS TOPICS

A. Energy Systems & Economics

• Types of Tariffs:

  • Flat Rate: Fixed charge per unit, independent of consumption.

  • Block Rate: Different rates for different consumption blocks (slab system). Higher consumption → higher rate.

  • Two-Part: Fixed charge + variable charge (per unit).

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

• Energy Storage Management (Brief): Critical for balancing intermittent renewables (solar/wind). Technologies: Batteries (Li-ion), Pumped Hydro, Flywheels, Thermal Storage, Hydrogen. Management involves charging/discharging strategies to minimize cost, maximize life, and support grid stability.

B. Environmental Context

• Effect of Fossil Fuels: Release of CO₂ (primary GHG), SOₓ, NOₓ, particulates. Leads to:

  • Global Warming: Enhanced greenhouse effect → rising temperatures.

  • Climate Change: Extreme weather, sea-level rise, ecosystem disruption.

  • Air Pollution: Smog, acid rain, health issues.

• Greenhouse Effect: Natural process where GHGs (CO₂, CH₄, H₂O) trap outgoing infrared radiation, warming Earth. Enhanced by human emissions.

• Sensible vs Latent Heat:

  • Sensible Heat: Heat that causes a temperature change ($$\displaystyle Q = m c_p \Delta T $$). Example: Heating water from 20°C to 80°C.

  • Latent Heat: Heat that causes a phase change at constant temperature ($$\displaystyle Q = m h_{fg} $$). Example: Evaporation of water at 100°C. Crucial in thermal power cycles (steam) and cooling.

C. Foundational Concepts

• Definition & Need for Renewable Energy: Energy from naturally replenishing sources (solar, wind, biomass, hydro, geothermal, ocean). Need: Depleting fossil fuels, environmental pollution, energy security, sustainable development.

• Classification of Renewable Sources:

  1. Solar (direct/indirect)

  2. Wind

  3. Hydropower (large/small)

  4. Biomass (solid, liquid, gas)

  5. Geothermal

  6. Ocean (tidal, wave, OTEC)

  7. Hydrogen (as energy carrier, if produced renewably).

END OF UNIT 4 NOTES