Robotics (ME-604 (A)) - Unit 3 Short Notes

UNIT 3: RENEWABLE ENERGY TECHNOLOGIES - SHORT NOTES

I. SOLAR ENERGY

A. Solar Radiation & Geometry

• Measurement: Done using Pyranometer (global radiation), Pyrheliometer (direct radiation), and Pyrometer (diffuse radiation). Data is recorded as irradiance (W/m²) or irradiation (Wh/m²/day).

• Key Angles:

  • Latitude (φ): Angular position north/south of equator.

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

  • Hour Angle (ω): Angular displacement of sun from local meridian. ω = 15° × (hours from solar noon).

  • Solar Altitude (α): Angle between sun's rays & horizontal.

  • Solar Zenith (θz): Angle between sun's rays & vertical. θz = 90° - α.

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

• Solar Radiation on Tilted Surface:

$$I_T = I_b \cos\theta + I_d \left(\frac{1+\cos\beta}{2}\right) + I \rho \left(\frac{1-\cos\beta}{2}\right)$$

Where:

*   $$\displaystyle I_b $$ = Beam radiation on horizontal surface

*   $$\displaystyle I_d $$ = Diffuse radiation on horizontal surface

*   $I$ = Global radiation on horizontal surface ($$\displaystyle I = I_b + I_d $$)

*   $\beta$ = Tilt angle of surface

*   $\rho$ = Ground reflectance (albedo)

*   $\theta$ = Incidence angle on tilted surface

[!TIP] Exam Focus: Calculating incidence angle (θ) for a given date, time, location, and collector orientation (azimuth, tilt) is a very common 7-mark question. Use:

$$\cos\theta = \sin\phi \sin\delta \cos\beta - \sin\phi \sin\delta \sin\beta \cos\gamma + \cos\phi \cos\delta \cos\omega \cos\beta + \cos\phi \cos\delta \sin\beta \cos\gamma \cos\omega + \cos\phi \cos\delta \sin\beta \sin\gamma \sin\omega$$

For south-facing collector (γ=0), it simplifies.

B. Solar Thermal Systems

• Solar Collector Classification:

  • Non-concentrating (Flat Plate): Absorber area ≈ aperture area. Operates at low temps (30-100°C). Used for water heating, space heating.

  • Concentrating: Uses reflectors/lenses to increase intensity. Aperture area > absorber area. Achieve high temps (>100°C). Types: Parabolic Trough, Central Receiver (Power Tower), Parabolic Dish.

• Flat Plate Collector Construction:

  DiagramCANVAS: Show glass cover, absorber plate with selective coating, riser tubes, insulation at back, casing.
  • Working: Sunlight passes through glass (transmits shortwave, traps longwave). Absorber plate (black, selective coating) heats up. Heat transfer fluid (water/air/antifreeze) in tubes carries heat away.

• Solar Water Heating Systems:

  • Thermosyphon (Passive): Natural circulation due to density difference. No pump. Storage tank above collector.

  • Forced Circulation (Active): Pump circulates fluid. Storage can be below collector. More control, larger systems.

• Applications: Domestic hot water, industrial process heat, solar drying, solar cooling (via absorption chiller).

C. Solar Photovoltaic (PV) Systems

• Principle of Photovoltaic (P-V) Conversion: Direct conversion of sunlight (photons) into electricity (DC) via the photovoltaic effect in a semiconductor p-n junction. No moving parts, no thermal cycle.

• Working of Solar Cell:

  1. Photon absorption in p-n junction creates electron-hole pairs.

  2. Built-in electric field separates carriers.

  3. Electrons flow to n-side, holes to p-side, creating voltage.

  4. External circuit allows current flow.

• Types of Solar Cells/Materials:

  • First Generation (Crystalline Si): Monocrystalline (high efficiency, expensive), Polycrystalline (lower cost, slightly lower efficiency).

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

  • Third Generation (Emerging): Multi-junction cells, Perovskite cells, Organic PV (OPV). Aim for higher efficiency & lower cost.

• Fabrication: Silicon purification → Ingot/wafer making → Doping (p-n junction formation) → Anti-reflective coating & metallization → Cell lamination → Module assembly (series/parallel connection, encapsulation).

• PV System Applications: Standalone (with battery), Grid-tied (without/with battery), Hybrid (with diesel/wind). Power calculators, street lights, remote telemetry, large solar farms.

• Limitations: Intermittency (no sun at night/cloudy), low conversion efficiency (~15-22% commercial), high initial cost, requires large area for MW-scale, energy storage needed for 24/7 supply.

• Maximum Power Point Tracking (MPPT): Technique to operate PV array at its Maximum Power Point (MPP) which varies with irradiance & temperature.

  • Perturb and Observe (P&O) Algorithm:

    1. Measure current power (P(k)).

    2. Perturb (change) duty cycle (D) slightly.

    3. Measure new power (P(k+1)).

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

    5. If ΔP < 0 → reverse perturbation direction.

    6. Repeat periodically. Simple but can oscillate around MPP under rapid irradiance change.

[!TIP] Common Pitfall: In P&O, if you perturb voltage up and power increases, you keep going up. If power decreases, you reverse direction. Remember it's a hill-climbing method.

II. WIND ENERGY

A. Fundamentals & Aerodynamics

• Power in Wind (Betz's Limit):

  • Kinetic energy flux through area A: $$\displaystyle P_{wind} = \frac{1}{2} \rho A V^3 $$

  • Betz's Limit: Maximum power extractable by an ideal rotor is 59.3% of $$\displaystyle P_{wind} $$. No turbine can exceed this.

  • Power Coefficient (Cp): Actual power extracted: $$\displaystyle P = C_p \cdot \frac{1}{2} \rho A V^3 $$

  • $$\displaystyle \boxed{C_p \leq 0.593} $$ (Betz limit). Typical modern turbines: $$\displaystyle C_p \approx 0.4-0.5 $$.

• Aerofoil (Airfoil): Cross-sectional shape of a wind turbine blade. Generates lift force (perpendicular to airflow) which is the primary force causing rotation.

  • Types: Symmetric (zero lift at 0° AoA), Cambered (positive lift at 0° AoA). Wind blades use highly cambered, twisted aerofoils.

  • Role: Lift-to-drag ratio (L/D) is critical. High L/D means efficient energy capture.

• Power Curve: Graph of turbine power output (kW) vs. wind speed (m/s). Key points:

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

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

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

B. Wind Energy Conversion Systems (WECS)

• Classification:

  • Horizontal Axis Wind Turbine (HAWT): Rotating axis parallel to ground & wind. Most common. Requires yaw mechanism. Higher efficiency.

  • Vertical Axis Wind Turbine (VAWT): Rotating axis perpendicular to ground. Omni-directional (no yaw). Lower efficiency, higher torque ripple. Types: Darrieus (lift-based), Savonius (drag-based).

• Constructional Features (HAWT):

  DiagramCANVAS: Show tower, nacelle, rotor (3 blades), hub, gearbox (if any), generator, yaw system, anemometer, wind vane.
  • Working: Wind blows on aerofoil blades → lift force → rotor rotation → shaft rotation → gearbox (steps up speed) → generator (produces AC electricity).

• Wind-Diesel Hybrid System: Combines wind turbines with diesel generators & often batteries. Wind provides base load/reduces diesel consumption. Diesel provides backup/peak power. Increases reliability in remote/isolated grids.

C. Challenges & Limitations

• Factors Prohibiting Large-Scale Utilization:

  1. Intermittency & Variability: Wind not constant → grid stability issues.

  2. Grid Integration: Requires flexible grid, forecasting, backup power (spinning reserve).

  3. Site Specificity: Requires high, consistent wind resources (often remote).

  4. Visual & Noise Impact: Public acceptance issues.

  5. Impact on Wildlife: Bird/bat mortality.

  6. High Capital Cost relative to fossil fuels (though LCOE now competitive).

  7. Need for Energy Storage to firm output.

[!TIP] Exam Focus: "What prohibits large-scale utilization?" is a very frequent 7-mark question. Structure answer: Intermittency → Grid challenges → Site specificity → Environmental/social concerns → Economics.

III. BIOMASS ENERGY

A. Biomass Resources & Conversion

• Definition & Usefulness: Biomass is organic matter from plants/animals. Useful as renewable, carbon-neutral energy source (CO₂ released ≈ CO₂ absorbed during growth). Can be solid, liquid (biofuels), gaseous (biogas).

• Biochemical Conversion (Anaerobic Digestion): Biological breakdown of organic matter by bacteria in absence of oxygen.

  • Stages: Hydrolysis → Acidogenesis → Acetogenesis → Methanogenesis.

  • Output: Biogas (~60% CH₄, 40% CO₂) + digestate (fertilizer).

• Types of Biomass Gasification: Thermal conversion with limited oxygen.

  • Fixed Bed (Updraft/Downdraft): Fuel bed stationary. Simple, suitable for small scale.

  • Fluidized Bed: Fuel particles suspended in air stream. Better mixing, higher efficiency, handles diverse fuels.

  • Entrained Flow: Finely ground fuel blown with oxygen. High temp, high throughput, for large industrial use.

• Power Generation from Gasification: Producer gas (CO, H₂, CH₄) → Cleaning (removes tars, particulates) → Engine/Gas Turbine (mechanical work) → Generator (electricity). Can also be used in boilers for steam.

B. Biogas Plants & C3/C4 Plants

• Types of Digesters:

  • Batch vs. Continuous: Batch (fill, digest, empty - simple but inconsistent); Continuous (constant feed/output - steady production).

  • Fixed Dome (KVIC): Masonry dome, gas stored under pressure in dome. No moving parts. Common in India.

  • Floating Drum (Deenbandhu): Gas holder (drum) floats on slurry. Gas pressure constant. Moving parts require maintenance.

• Design Considerations: Retention time (15-50 days), temperature (mesophilic ~35°C, thermophilic ~55°C), C/N ratio (20-30:1), pH (6.6-7.6), loading rate.

• C3 & C4 Plant Production (Photosynthesis Pathways):

  • C3 Plants: First stable product is 3-carbon molecule (3-Phosphoglycerate). Calvin cycle only. Examples: Rice, Wheat, Potatoes, Soybeans. Photorespiration loss at high temp/low CO₂.

  • C4 Plants: First stable product is 4-carbon molecule (Oxaloacetate). Spatial separation (mesophyll & bundle sheath cells). No photorespiration. Higher water-use efficiency. Examples: Maize, Sugarcane, Sorghum. Adapted to hot, dry climates.

[!TIP] Common Pitfall: Don't confuse gasification (partial oxidation, high temp, producer gas) with anaerobic digestion (biological, low temp, biogas). Gasification is thermochemical; Digestion is biochemical.

IV. HYDRO ENERGY

A. System Classification & Components

• Differentiation (Based on Installed Capacity):

  • Micro Hydro: ≤ 100 kW. Very small, often run-of-river, for single community/village.

  • Mini Hydro: 101 kW - 1 MW. Small scale, can be grid-connected or isolated.

  • Small Hydro: 1 MW - 25 MW (or up to 50 MW by some definitions). Often run-of-river, minimal storage.

  • (Large Hydro > 25 MW, involves major dams/reservoirs).

• Main Components of Small Hydro System:

  1. Intake/Diversion: Screens, gate.

  2. Penstock/Pressure Conduit: Carries water under pressure.

  3. Turbine: Converts water energy to mechanical.

  4. Generator: Converts mechanical to electrical.

  5. Tailrace: Returns water to river.

  6. Control System: Gates, valves, governor.

B. Turbines & Plant Operation

• Types of Turbines:

  | Type | Principle | Head (m) | Flow (m³/s) | Examples | Application | | :--- | :--- | :--- | :--- | :--- | :--- | | Impulse | Jet of water hits buckets, kinetic energy transfer. | High (>200) | Low | Pelton | High-head, low-flow | | Reaction | Pressure & velocity change in runner, water fills runner. | Low/Medium (<200) | High | Francis (Medium), Kaplan (Low) | Medium/low-head, high-flow |

• Selection of Turbine: Primarily based on Net Head (H) and Design Flow (Q). Use Specific Speed (Ns) concept:

$$N_s = N \sqrt{P} / H^{5/4}$$

(N = rpm, P = kW, H = m). High Ns → Kaplan/Propeller; Low Ns → Pelton.

• Speed & Voltage Regulation:

  • Speed Regulation: Maintain constant generator speed (synchronous) or variable speed (with power electronics). Done by governor controlling water flow through turbine (wicket gates/nozzles).

  • Voltage Regulation: Maintain terminal voltage. Done by excitation system of generator (varying field current). For grid-connected, voltage/frequency set by grid.

[!TIP] Exam Focus: "Selection of turbine" is a recurring 3-7 mark question. Always state: Head & Flow are primary factors. Then mention Specific Speed and give examples (Pelton for high head, Kaplan for low head).

V. GEOTHERMAL ENERGY

A. Resources & Principles

• Types of Geothermal Deposits/Resources:

  1. Convective (Hydrothermal): Hot water/steam in permeable rock. Vapor-dominated (dry steam, e.g., Larderello), Liquid-dominated (wet steam, e.g., Wairakei).

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

  3. Hot Dry Rock (HDR)/Enhanced Geothermal Systems (EGS): Hot, impermeable rock. Requires hydraulic fracturing to create reservoir.

  4. Magma: Molten rock (highest temp, most challenging).

• Thermodynamic Principle: Exploits heat energy from Earth's interior. Basic cycle: Heat source (geothermal fluid) → Heat Exchanger/Flash Separator → Working fluid (often water/steam) expands in turbine → Condenser → Pump → repeat. Governed by Rankine cycle (steam) or Binary cycle (organic fluid for low temp).

• Electricity Generation Process (for hydrothermal):

  1. Production well brings hot fluid (150°C - 350°C) to surface.

  2. For Flash Steam Plant: Fluid at high pressure flashes to steam in separator. Steam drives turbine. Brine may be flashed again (double flash).

  3. For Binary Plant: Hot fluid heats secondary organic fluid (low boiling point) in heat exchanger. Vapor drives turbine. Geothermal fluid is reinjected.

  4. Turbine drives generator → electricity.

  5. Steam/vapor condensed, fluid reinjected.

B. Plant Development

• Site Selection Criteria:

  • High subsurface temperature gradient (heat flow).

  • Permeable reservoir rock & caprock.

  • Sufficient fluid (water) availability (natural or injectable).

  • Proximity to power grid/load center.

  • Environmental constraints (protected areas, emissions).

  • Economic viability (drilling cost, reservoir size).

• Environmental Benefits:

  • Very low greenhouse gas emissions (mostly steam, some non-condensables like CO₂, H₂S - much lower than fossil fuels).

  • Small land footprint per MW.

  • Minimal visual impact.

  • Can be used for direct use (heating, greenhouses) with even lower emissions.

[!TIP] Key Distinction: Flash Steam plants use geothermal fluid itself as working fluid (requires >180°C). Binary plants use a secondary fluid (allows use of lower temp resources, reinjects geothermal fluid completely).

VI. OCEAN ENERGY

A. Tidal Energy

• Principle: Harness kinetic (tidal streams) or potential (tidal range) energy from tides caused by gravitational pull of moon/sun.

• Types of Tidal Power Plants:

  1. Single Basin (One-way/Two-way): Single basin with one set of turbines/gates.

     • Filling (Flood): Basin fills through turbines during incoming tide.

     • Emptying (Ebb): Basin empties through turbines during outgoing tide.

     • Two-way: Bi-directional turbines.

  2. Double Basin: Two basins at different levels. Turbines between them. Can generate power during both flood and ebb as water moves from high to low basin. More complex.

• Energy Calculation (Single Basin, Filling/Emptying):

  Potential energy available:

$$E_{potential} = \rho g A \int_0^H z \, dz = \rho g A \frac{H^2}{2}$$

Where:

*   $\rho$ = density of seawater (~1025 kg/m³)

*   $g$ = acceleration due to gravity (9.81 m/s²)

*   $A$ = basin area (m²)

*   $H$ = **effective head** (tidal range minus minimum operating head, $$\displaystyle H_{eff} = H_{tide} - H_{min} $$)

*   **Energy Generated (kWh):**

$$\boxed{E_{gen} = \frac{\rho g A H_{eff}^2}{2} \times \eta_{turb-gen} \times \frac{1}{3.6 \times 10^6}}$$

Where $$\displaystyle \eta_{turb-gen} $$ is combined turbine-generator efficiency.

> **Example from May 2024 paper:** $$\displaystyle A=30\times10^6 m^2 $$, $$\displaystyle H_{tide}=12 m $$, $$\displaystyle H_{min}=3 m $$, $$\displaystyle \eta=0.73 $$. $$\displaystyle H_{eff}=9 m $$. Plug into formula.

• Significant Wave Height (Hs): In irregular sea waves, Significant Wave Height is the average height of the highest one-third (1/3) of waves in a given sea state. It's the standard statistical measure for wave energy resource assessment.

B. Ocean Thermal Energy Conversion (OTEC)

• Principle: Exploits temperature difference between warm surface seawater (~25-30°C) and cold deep seawater (~5-10°C). Based on Rankine cycle using a low-boiling-point working fluid (e.g., ammonia, R-134a).

• Types:

  • Open Cycle: Warm seawater flash-evaporated in vacuum chamber → low-pressure vapor drives turbine → condensed by cold seawater. Produces desalinated water.

  • Closed Cycle: Warm seawater heats volatile fluid in evaporator → vapor drives turbine → cold seawater condenses vapor in condenser. Most common.

  • Hybrid Cycle: Combines aspects of both.

• Challenges: Very low thermodynamic efficiency (3-4%) due to small ΔT (~20°C). Requires massive seawater flow rates & large heat exchangers. High capital cost.

[!TIP] Exam Focus: Tidal energy calculation is a very high-frequency 14-mark question. Crucial: Use effective head ($$\displaystyle H_{eff} = H_{range} - H_{min} $$), not full tidal range. Always convert Joules to kWh (divide by 3.6e6). State formula clearly.

VII. FUEL CELLS

A. Fundamentals & Classification

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

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

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

  • Electrolyte: Ion-conducting membrane, blocks electrons.

  • Overall: 2H₂ + O₂ → 2H₂O + electricity + heat.

• Classification (by Electrolyte):

  | Type | Electrolyte | Operating Temp (°C) | Fuel | Application | | :--- | :--- | :--- | :--- | :--- | | AFC (Alkaline) | KOH solution | 60-90 | Pure H₂, O₂ | Space (Apollo), specialty | | PEMFC (Polymer Electrolyte) | Solid polymer (Nafion) | 60-80 | H₂ (from reformate) | Vehicles, portable, CHP | | PAFC (Phosphoric Acid) | Liquid H₃PO₄ | 180-210 | H₂ (reformed) | CHP, utility | | MCFC (Molten Carbonate) | Molten Li/K carbonate | 600-700 | H₂, CO, CH₄ (reformed) | Utility-scale, stationary | | SOFC (Solid Oxide) | Solid ceramic (ZrO₂) | 800-1000 | H₂, CO, CH₄ | Utility, auxiliary power | | DMFC (Direct Methanol) | Polymer | 60-130 | Liquid methanol | Portable electronics |

B. Performance & Applications

• Advantages:

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

  • Low/zero emissions (only H₂O for H₂ fuel).

  • Quiet, reliable, modular.

  • Fuel flexible (depending on type).

• System Components: Stack (cells in series), Fuel Processor/Reformer (for hydrocarbon fuels), Power Conditioner (DC-AC inverter), Heat Recovery System, Fuel/Oxidant Supply.

• Applications: Transportation (FCEVs), Stationary power (backup, CHP), Portable power (laptops, military).

[!TIP] Key Comparison: Remember temperature ranges and fuel flexibility. SOFC/MCFC are high-temp, can internally reform hydrocarbons. PEMFC is low-temp, needs pure H₂ (or carefully cleaned reformate). AFC is very pure fuel sensitive.

VIII. ENERGY SYSTEMS, ECONOMICS & ENVIRONMENT

A. Integration & Management

• Energy Storage Management: Critical for intermittent renewables (solar, wind). Technologies:

  • Chemical: Batteries (Li-ion, flow), Hydrogen (via electrolysis).

  • Mechanical: Pumped Hydro, Compressed Air (CAES).

  • Thermal: Molten Salt, Ice storage.

  • Management: Controls charging/discharging to balance supply-demand, reduce peak demand, provide grid services (frequency regulation).

• Hybrid Systems: Combine two or more generation/storage technologies. Examples:

  • Wind-Diesel: Reduces diesel fuel consumption in remote grids.

  • Solar-Wind-Battery: Smoother output, better reliability.

  • PV-Diesel: Diesel acts as backup/peak.

  • Advantages: Increased reliability, reduced emissions/fuel cost, better utilization of resources.

B. Economics & Policy

• Types of Tariffs in Electricity:

  • Flat Rate Tariff: Fixed charge per kWh, independent of time/load.

  • Time-of-Day (TOD) Tariff: Varies by time block (peak, off-peak, normal). Encourages load shifting.

  • Two-Part Tariff: Fixed charge (demand/capacity) + variable charge (energy).

  • Inclining/Declining Block Tariff: Unit price changes with consumption slab.

  • Power Factor Tariff: Incentive/penalty based on power factor (for industrial).

C. Environmental Context

• Greenhouse Effect: Natural process where greenhouse gases (CO₂, CH₄, H₂O, etc.) trap outgoing infrared radiation, warming Earth. Enhanced by human emissions → Global Warming.

• Effect of Fossil Fuels: Primary source of anthropogenic CO₂ emissions → enhances greenhouse effect → global temperature rise → climate change (sea-level rise, extreme weather, ecosystem disruption).

• Sensible vs. Latent Heat:

  • Sensible Heat: Heat exchanged that causes a temperature change (ΔQ = m c ΔT). Measurable with thermometer.

  • Latent Heat: Heat exchanged during phase change (melting, vaporization) at constant temperature (ΔQ = m L). Not sensed by temperature change.

  • Example in Renewables: Latent heat of vaporization is key in solar stills (desalination) and OTEC (evaporation/condensation cycles).

[!TIP] Final Note: For "Effect of fossil fuels on climate change," link: Combustion → CO₂ ↑ → Enhanced Greenhouse Effect → Global Warming → Climate Change Impacts. Be specific about impacts (IPCC reports mention: temperature, sea level, extreme events, ocean acidification).