Robotics (ME-604 (A)) - Unit 2 Short Notes

UNIT 2: RENEWABLE ENERGY TECHNOLOGIES

1. SOLAR ENERGY

Solar Radiation Fundamentals

• Measurement Instruments:

  • Pyranometer: Measures global solar radiation (beam + diffuse) on a horizontal surface.

  • Pyrheliometer: Measures direct beam radiation only (tracks the sun).

  • Sunshine Recorder (Campbell-Stokes): Measures sunshine duration (hours).

• Sun-Earth Geometry:

  • 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.

  • Solar Time: Based on sun's position; differs from clock time.

• Solar Angles:

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

  • Azimuth Angle (γₛ): Projection of sun's rays on horizontal plane, measured from south (N. Hemisphere).

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

• Radiation on Tilted Surface:

  • $$\displaystyle I_T = I_b \cos\theta + I_d \frac{1+\cos\beta}{2} + I \rho_g \frac{1-\cos\beta}{2} $$

  • Isotropic Model: Assumes uniform diffuse sky.

  • Anisotropic Models (e.g., HDKR): Account for circumsolar diffuse component.

• Energy Estimation: Uses solar radiation maps, clearness index (Kₜ), and monthly/ annual averages for site assessment.

[!TIP] Exam Focus: Derivation of incidence angle formula and application of isotropic model are frequent.

Solar Thermal Systems

• Classification:

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

  • By Flow: Active (pumped), Passive (thermosyphon).

• Flat Plate Collector (FPC):

  • Construction: Absorber plate (selective coating) → Tubes → Glazing (transparent, low iron) → Insulation (side/back) → Casing.

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

• Solar Water Heating Systems:

  • Thermosyphon (Natural Circulation): Density difference drives flow. No pump, reliable, simple.

  • Forced Circulation: Pump controls flow. Allows storage tank placement above/below collector, better control.

• Applications: Water heating (domestic, industrial), crop drying (bin, tunnel, rotary), distillation (multi-effect), cooking (box, parabolic).

Photovoltaic (PV) Systems

• Principle: Photoelectric Effect in p-n Junction.

  • Photons with energy > bandgap excite electrons from valence to conduction band.

  • p-n junction creates internal electric field → separation of electron-hole pairs → DC current.

• Solar Cell Technologies:

  • Crystalline Si (c-Si): Mono (high efficiency, costly), Poly (lower cost, lower efficiency).

  • Thin Film: Amorphous Si (a-Si), CdTe, CIGS. Lower efficiency, cheaper, flexible.

• PV Module: Series/parallel connection of cells for desired voltage/current. Encapsulated (EVA), framed, with junction box.

• System Components:

  • Modules: Generate DC.

  • Charge Controller: Prevents overcharge/discharge of battery.

  • Batteries (Storage): Lead-acid, Li-ion.

  • Inverter: Converts DC to AC (for grid/appliances).

• Maximum Power Point Tracking (MPPT):

  • Perturb & Observe (P&O):

    1. Measure $P(V,I)$.

    2. Perturb voltage (ΔV).

    3. If ΔP > 0 → continue perturbation in same direction.

    4. If ΔP < 0 → reverse direction.

    5. Iterates to MPP. Simple but oscillates at MPP.

• Limitations of SPV:

  • Low efficiency (15-22% commercial).

  • Intermittency (no sun = no power).

  • High initial cost.

  • Requires storage/battery for 24/7 supply.

  • Degradation over time (~0.5%/year).

• Applications: Standalone (home, street light), Grid-tied (rooftop, solar farm), Pumping (DC pump), Lighting.

[!TIP] Exam Focus: Draw p-n junction band diagram, explain P&O algorithm steps clearly, list limitations with brief explanation.

2. WIND ENERGY

Wind Energy Fundamentals

• Kinetic Energy & Power:

  • Mass flow rate: $$\displaystyle \dot{m} = \rho A V $$

  • Kinetic energy/sec (Power): $$\displaystyle P = \frac{1}{2} \dot{m} V^2 = \frac{1}{2} \rho A V^3 $$

  • Cube Law: Power ∝ $$\displaystyle V^3 $$. Small change in V → large change in P.

• Wind Regime Assessment:

  • Measurement: Anemometer (speed), Wind vane (direction), at hub height (10-50m).

  • Estimation: Weibull Distribution (k = shape, c = scale). Probability density: $$\displaystyle f(V) = \frac{k}{c} \left(\frac{V}{c}\right)^{k-1} e^{-(V/c)^k} $$

  • Energy Assessment: $$\displaystyle E = \int_0^\infty P(V) f(V) dV \times 8760 $$ hrs/year.

• Factors Affecting Power: Air density (ρ), Rotor swept area (A), Wind speed (V).

Wind Turbine Aerodynamics

• Aerofoil: Cross-section of blade. Generates lift (L) and drag (D).

  • Symmetric: Zero lift at 0° AoA.

  • Cambered: Positive lift at 0° AoA (used in wind blades).

• Betz Limit: Maximum possible $$\displaystyle C_p $$ (power coefficient) = $$\displaystyle \frac{16}{27} \approx 0.593 $$ (59.3%).

  • Reason: Air must retain some kinetic energy to pass through rotor.

• Power Curve:

  • Cut-in Speed (Vᵢ): ~3-4 m/s. Below this, turbine doesn't generate.

  • Rated Speed (Vᵣ): Generator reaches rated power.

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

Wind Energy Conversion Systems (WECS)

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

• Classification:

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

  • Speed: Fixed speed (single generator), Variable speed (with power electronics).

  • Location: Onshore, Offshore.

• Working (HAWT): Wind → blades (lift force) → rotor rotation → gearbox → generator → electricity. Yaw system aligns nacelle to wind.

Wind Energy Challenges & Hybrid Systems

• Limitations/Barriers:

  • Intermittency & variability.

  • Site-specific (good wind resources needed).

  • Noise (aerodynamic, mechanical).

  • Visual impact.

  • Threat to birds/bats.

  • Grid integration issues (voltage/frequency fluctuations).

• Prohibitions to Large-Scale Use: Technical (grid stability), Economic (high capital, LCOE), Infrastructural (transmission to remote sites).

• Wind-Diesel Hybrid:

  • Configuration: Wind turbine(s) + Diesel generator + Battery bank + Controller.

  • Operation: Wind supplies load when available; diesel supplements/charges batteries; batteries buffer fluctuations, reduce diesel runtime → fuel saving, lower emissions.

[!TIP] Exam Focus: Derive Betz limit (using momentum theory), draw power curve, explain wind-diesel hybrid operation with block diagram.

3. BIOMASS ENERGY

Biomass Resources & Conversion

• Definition: Organic matter from plants/animals (carbon-based).

• Sources: Agricultural residues (straw), Forest waste (sawdust), Energy crops (miscanthus), Municipal Solid Waste (MSW), Animal waste.

• Usefulness: Renewable, carbon-neutral (in cycle), waste-to-energy.

• Conversion Pathways:

  • Thermal: Combustion (direct), Gasification (partial oxidation), Pyrolysis (thermal decomposition without oxygen).

  • Biochemical: Anaerobic digestion (biogas), Fermentation (ethanol).

  • Chemical: Transesterification (biodiesel).

Biomass Gasification

• Principle: Partial oxidation of biomass at 700-900°C with limited air/oxygen → producer gas (CO, H₂, CH₄, CO₂, N₂).

• Gasifier Types:

  • Fixed Bed: Fuel bed stationary.

    • Updraft: Air from bottom, gas from top. High tar, high efficiency.

    • Downdraft: Air from top, gas from bottom. Low tar, common.

    • Cross-draft: Air/gas from opposite sides.

  • Fluidized Bed: Fuel particles suspended in air stream.

    • Bubbling: Lower velocity.

    • Circulating: Higher velocity, particles carried out & recycled.

• Products & Applications: Producer gas → Engine (electricity), Boiler (heat), Synthetic fuels (via Fischer-Tropsch).

Anaerobic Digestion & Biogas

• Biochemical Stages:

  1. Hydrolysis: Complex organics → sugars, amino acids.

  2. Acidogenesis: Sugars → VFAs, alcohols, CO₂, H₂.

  3. Acetogenesis: VFAs → acetic acid, H₂, CO₂.

  4. Methanogenesis: Acetic acid/H₂+CO₂ → CH₄ + CO₂ (methanogens).

• Biogas Composition: CH₄ (50-70%), CO₂ (30-50%), traces H₂S, H₂O.

• Uses: Cooking (burner), Electricity (engine-generator), Vehicle fuel (after purification).

• Digester Types:

  • Floating Drum (KVIC): Gas collects under floating steel drum → constant pressure.

  • Fixed Dome (Deenbandhu): Gas under dome, pressure varies.

  • Bag Type: Flexible PVC bag, low cost.

  • Plug Flow: Long, narrow, continuous feed.

• Biogas Plant Design: Sizing based on feedstock & retention time (20-50 days). Feedstock preparation (chopping, mixing).

Biochemical Conversion & Biofuels

• Fermentation (Ethanol): Sugars (from starch/sugar crops) → yeast → ethanol + CO₂. Distillation purifies.

• Photosynthesis Pathways:

  • C3 Plants (e.g., rice, wheat): First product is 3-carbon compound (3-PGA). Photorespiration loss at high T/low water → lower efficiency.

  • C4 Plants (e.g., maize, sugarcane): Spatial separation (mesophyll → bundle sheath). Minimizes photorespiration → higher yield, better in hot/dry climates.

[!TIP] Exam Focus: Draw 4-stage AD process, compare C3/C4 in table, explain downdraft gasifier working.

4. HYDRO ENERGY

Hydropower System Classification

• By Capacity:

  • Micro: < 100 kW (village).

  • Mini: 100 kW - 1 MW (small community).

  • Small: 1 MW - 10 MW (isolated grid).

  • Large: > 10 MW (grid-connected).

• Small Hydro Components: Intake (screen, gate) → Penstock (pressure pipe) → Turbine → Generator → Tailrace.

Hydraulic Turbines

• Classification by Head & Flow:

  • Impulse (High Head, Low Flow): Pelton wheel. No pressure change in runner, jets strike buckets.

  • Reaction (Low/Medium Head, High Flow): Francis (medium), Kaplan/Bulb (low). Pressure change in runner.

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

  • Pelton: Low Nₛ (10-50)

  • Francis: Medium Nₛ (50-300)

  • Kaplan: High Nₛ (300-1000)

• Pelton Wheel (Example):

  • Construction: Runner with double-cup buckets, nozzle with needle valve, casing.

  • Working: High-velocity jet from nozzle strikes bucket center → impulse → rotation. Deflection ~180°. Efficiency ~90%.

Operation & Control

• Speed Regulation: Governor (mechanical/hydraulic) senses speed deviation → adjusts wicket gate opening (flow) → maintains constant speed (frequency).

• Voltage Regulation: Excitation System controls generator field current → maintains terminal voltage.

Hydro Power Calculations

• Available Hydraulic Power: $$\displaystyle P_{avail} = \rho g Q H $$

  • ρ = 1000 kg/m³, g = 9.81 m/s², Q = flow (m³/s), H = net head (m).

• Actual Power Output: $$\displaystyle P_{out} = \eta_{turb} \times \eta_{gen} \times P_{avail} $$

• Energy Generation: $$\displaystyle E = P_{out} \times t $$ (kWh). Consider head variation (reservoir), minimum operating head.

[!TIP] Exam Focus: Draw Pelton wheel, explain governor working, solve numerical: given Q, H, efficiencies → find E.

5. GEOTHERMAL ENERGY

Geothermal Resources

• Types of Deposits:

  • Vapor-dominated (Dry Steam): Steam under pressure (e.g., The Geysers, USA).

  • Hot Water: Pressurized hot water (common).

  • Hot Dry Rock (HDR): Impermeable hot rock. Requires hydraulic fracturing (EGS).

  • Geopressured: Hot water under high pressure with dissolved methane.

• Resource Assessment: Temperature gradient, Depth, Permeability, Fluid chemistry (corrosion/scaling).

Geothermal Power Generation

• Site Selection: High subsurface temperature, reservoir permeability/porosity, fluid recharge, proximity to grid, environmental constraints.

• Power Plant Types:

  • Dry Steam: Direct use of geothermal steam → turbine → condenser. Simplest.

  • Flash Steam: Hot water → flash vessel (pressure drop) → steam → turbine. Brine may be reinjected.

    • Single Flash: One flash vessel.

    • Dual/Triple Flash: Multiple flashes for higher efficiency.

  • Binary Cycle: Geothermal fluid heats secondary working fluid (low boiling point, e.g., isobutane) in heat exchanger → vapor → turbine → condenser. No direct contact, no emissions. Most common for low-temp resources.

• Generation Process: Heat extraction (production well) → heat exchange (if binary) → vapor expansion (turbine) → electricity (generator) → condensation → reinjection.

Thermodynamic Principles

• Rankine Cycle Modification: Geothermal replaces boiler. Working fluid may be water (flash) or organic (binary - ORC).

• Efficiency: Low (10-20%) due to low temperature vs. ambient. Increases with higher resource temperature.

• Binary Fluid Cycle (ORC): Organic Rankine Cycle. Better match to low-temperature heat sources.

Environmental Aspects

• Benefits: Low GHG emissions (mostly steam/water), Small land footprint, Baseload capability (24/7), Minimal fuel use.

[!TIP] Exam Focus: Compare dry steam, flash, binary in table. Explain binary cycle working with T-s diagram sketch.

6. OCEAN ENERGY

Tidal Energy

• Principle: Harness potential energy from tidal rise/fall (gravitational pull of moon/sun).

• Configurations:

  • Tidal Barrage: Dam across estuary. Basins fill/empty through turbines.

  • Single Basin: One basin, generation during filling/emptying (4 periods/day).

  • Double Basin: Two basins, one fills while other empties → more continuous.

  • Tidal Stream Turbines: Underwater "wind turbines" in fast tidal currents.

• Energy Calculation (Single Basin, Filling):

  • Potential energy of water: $$\displaystyle E_{pot} = \frac{1}{2} \rho g A H^2 $$ (H = tidal range).

  • Available Energy: $$\displaystyle E_{avail} = \frac{1}{2} \rho g A H^2 \times \eta_{turb-gen} $$

  • Note: Only generates when head > min operating head (H_min). Effective head = H - H_min.

• Turbines: Bulb (in barrage), Kaplan (low head), Propeller.

Ocean Thermal Energy Conversion (OTEC)

• Principle: Utilize temperature gradient (ΔT ~20-25°C) between warm surface water (~25-30°C) and cold deep water (~5-10°C).

• System Types:

  • Closed Cycle: Working fluid (e.g., ammonia, low boiling point) evaporates in evaporator (warm water) → turbine → condenser (cold water) → liquid → pump → repeat.

  • Open Cycle: Warm seawater → flash evaporator → steam → turbine → condenser (cold seawater) → fresh water condensate.

  • Hybrid: Combines aspects of both.

• Challenges: Very low thermal efficiency (3-4%), large infrastructure (pipes), biofouling, high capital cost.

Wave Energy

• Wave Characteristics:

  • Significant Wave Height (Hₛ): Average height of highest 1/3 of waves in a sea state.

  • Wave Period (T): Time between successive crests.

  • Energy Flux (Power/Width): $$\displaystyle P = \frac{\rho g^2}{64\pi} H_s^2 T $$ (kW/m).

• Conversion Devices:

  • Oscillating Water Column (OWC): Wave drives air column → turbine.

  • Point Absorber: Buoy moves with waves → hydraulic/pneumatic system.

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

[!TIP] Exam Focus: Derive tidal energy formula (E = ½ ρgAH²η), explain OTEC closed cycle with diagram, define Hₛ.

7. FUEL CELLS

Fundamentals & Working

• Principle: Electrochemical conversion of fuel (H₂, CH₄, etc.) + oxidant (O₂) → electricity + heat + byproducts (H₂O).

• Main Components:

  • Anode: Fuel oxidation (e⁻ released).

  • Cathode: Oxidant reduction (e⁻ consumed).

  • Electrolyte: Ion conductor (H⁺, O²⁻, CO₃²⁻), blocks electrons.

  • Catalyst: (Usually Pt) speeds reactions.

• Overall (H₂-O₂): $$\displaystyle 2H_2 + O_2 \rightarrow 2H_2O + \text{ Electricity} + \text{Heat} $$

• Electrode Reactions (PEMFC):

  • Anode: $$\displaystyle H_2 \rightarrow 2H^+ + 2e^- $$

  • Cathode: $$\displaystyle \frac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O $$

Classification (by Electrolyte)

Fuel Cell Systems & Performance

• System Components: Stack (series cells), Reformer (for hydrocarbon fuels → H₂), Power conditioner (DC-DC/AC), Heat recovery.

• Advantages: High efficiency (40-60%, up to 85% CHP), Low emissions (only H₂O if H₂ fuel), Modular, Quiet.

• Limitations: High cost (catalysts, materials), Durability (degradation), Fuel infrastructure (H₂ production, storage, distribution challenges).

[!TIP] Exam Focus: Draw and label fuel cell diagram, write reactions for PEMFC, compare types in table, list advantages/disadvantages.

8. SYSTEM INTEGRATION, ECONOMICS & MANAGEMENT

Hybrid Renewable Energy Systems

• Concept: Combine two+ renewable sources (e.g., solar-wind) or with conventional (diesel) → improve reliability, reduce storage size, optimize cost.

• Configuration: Common bus (AC/DC), with/without battery. Energy Management System (EMS) prioritizes sources, controls battery.

• Example (Solar-Wind-Battery): Solar/wind supply load; excess charges battery; battery discharges when generation < demand; diesel backup.

Energy Storage

• Role: Mitigate intermittency, time-shift energy, provide ancillary services (frequency regulation).

• Technologies:

  • Batteries: Lead-acid (cheap, mature), Li-ion (high energy density, costly).

  • Pumped Hydro: Mature, large-scale, geographical constraints.

  • Flywheels: Short-term, high power, high cycle life.

  • CAES: Compressed air in underground caverns.

Grid Integration & Power Quality

• Challenges: Variability → frequency/voltage fluctuations, need for forecasting, grid stability (inertia loss with inverters), fault ride-through.

• Solutions: Smart grids, Grid-forming inverters (provide voltage/frequency reference), Advanced controllers, Synthetic inertia.

Economic & Tariff Aspects

• Electricity Tariffs:

  • Flat Rate: Fixed price/unit.

  • Tiered (Block): Inclining (higher rate for more consumption) or Declining.

  • Time-of-Use (TOU): Different prices for peak/off-peak hours.

• RE Support Mechanisms:

  • Feed-in Tariff (FiT): Guaranteed price for RE fed into grid.

  • Renewable Purchase Obligation (RPO): Mandate for DISCOMs to buy minimum % from RE.

• Cost Metrics:

  • Levelized Cost of Energy (LCOE): $$\displaystyle LCOE = \frac{\text{Total lifetime cost}}{\text{Total lifetime energy output}} $$ ($/kWh). Key for comparison.

  • Capital Cost (CAPEX), O&M Cost (OPEX).

Energy Storage Management

• Strategies: Peak shaving, load leveling, arbitrage (buy low/sell high), firming RE output, emergency backup. Controlled by EMS based on forecasts, tariffs, state of charge (SoC).

[!TIP] Exam Focus: Draw hybrid system block diagram, explain FiT vs RPO, define LCOE formula, list storage applications.

9. ENVIRONMENTAL & SUSTAINABILITY ASPECTS

Environmental Benefits

• GHG & Pollutants: Displacement of fossil fuels → ↓ CO₂, SOₓ, NOₓ, particulates.

• Sustainability: Inexhaustible sources, energy security.

• Water Footprint: Most RE (wind, solar PV) use negligible water vs. thermal/nuclear.

Climate Change & Global Warming

• Fossil Fuels: Release stored carbon → enhanced greenhouse effect → global warming.

• Role of RE: Decarbonization of energy sector → mitigation.

Life Cycle Assessment (LCA)

• Purpose: Cradle-to-grave analysis of environmental impact.

• Key Metrics:

  • Energy Payback Time (EPBT): Time to generate energy equal to that used in manufacturing/installation.

  • Carbon Footprint: gCO₂eq/kWh over lifetime.

• Example: PV modules have EPBT ~1-3 years, lifetime 25-30 years → net positive.

[!TIP] Exam Focus: Contrast water use of RE vs thermal, define EPBT, explain carbon cycle disruption by fossils.