Robotics (ME-604 (A)) - Unit 4 Short Notes

UNIT 4: RENEWABLE ENERGY TECHNOLOGIES & SYSTEMS

This unit covers the fundamental principles, technologies, and system-level considerations of major renewable energy sources, with emphasis on solar, wind, biomass, hydro, geothermal, marine, and fuel cells, as frequently examined in past papers.

I. SOLAR ENERGY TECHNOLOGIES

A. Solar Radiation & Geometry

• Measurement of Solar Radiation:

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

  • Data: Expressed in kWh/m²/day or W/m². Key parameters: Global Horizontal Irradiance (GHI), Direct Normal Irradiance (DNI).

• Sun-Earth Relationship:

  • Earth's orbit is elliptical; distance variation causes ~6.7% solar irradiance change.

  • Declination (δ): Angle between sun's rays and equatorial plane. Varies ±23.45° annually.

  • Solar Constant (G_sc): ~1367 W/m² (extraterrestrial radiation on a surface perpendicular to sun's rays at 1 AU).

• Solar Geometry Key Angles:

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

  • Solar Altitude Angle (α): Angle between sun's rays and horizontal plane. $$\displaystyle \sin \alpha = \sin \phi \sin \delta + \cos \phi \cos \delta \cos \omega $$

  • Solar Zenith Angle (θ_z): Angle between sun's rays and vertical. $$\displaystyle \theta_z = 90° - \alpha $$.

  • Solar Azimuth Angle (γ_s): Angle of sun's projection on horizontal plane from south (N. Hemisphere).

  • Surface Azimuth Angle (γ): Angle of surface normal's projection from south.

  • Angle of Incidence (θ): Angle between sun's rays and normal to surface. For tilted surface: $$\displaystyle \cos \theta = \sin \phi \sin \delta \cos \beta - \sin \phi \cos \delta \sin \beta \cos \gamma + \cos \phi \cos \delta \cos \beta \cos \omega + \cos \phi \sin \delta \sin \beta \cos \gamma \cos \omega + \cos \delta \sin \beta \sin \gamma \sin \omega $$ where β = tilt angle, ω = hour angle.

  [!TIP] For a south-facing surface (γ=0) at latitude tilt (β=φ), the formula simplifies significantly.

• Solar Radiation on Tilted Surface:

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

  • $$\displaystyle I_b $$: Beam radiation, $$\displaystyle I_d $$: Diffuse radiation, $$\displaystyle I_g $$: Ground-reflected radiation.

  • Implication: Optimal tilt (β) maximizes annual/sessional energy capture.

B. Solar Thermal Systems

• Solar Collectors Classification:

  • By Concentration: Non-concentrating (Flat Plate), Concentrating (Parabolic Trough, Dish, Tower).

  • By Heat Transfer Fluid: Liquid (water, oil, glycol mix), Air.

  • By Movement: Fixed, Single-axis tracking, Dual-axis tracking.

• Flat Plate Collector (FPC) - Construction & Working:

  • Components: Transparent cover (glass), absorber plate (selective coating), header/riser tubes, insulation, casing.

  • Working: Solar radiation passes through cover, absorbed by plate, converted to heat. Heat transferred to fluid in tubes via conduction/convection. Insulation minimizes losses.

  • Efficiency: 30-50%. Used for low-temperature applications (<100°C).

  [!DIAGRAM] Search: "solar flat plate collector cross-section diagram"

• Solar Water Heating Systems:

  • Thermosyphon System: Passive. Cold water enters tank at bottom, hot water exits from top. Natural convection drives flow. No pump, no control. Reliable, simple.

  • Forced Circulation System: Active. Pump circulates fluid from tank through collector. Requires controller (thermostat) to prevent nighttime losses. More flexible in design.

• Applications of Solar Thermal: Water heating (domestic, industrial), space heating, industrial process heat, solar drying, solar refrigeration (absorption chillers).

• Solar Driers:

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

  • Main Applications: Agricultural products (grains, fruits, vegetables, spices, fish), industrial raw materials.

C. Photovoltaic (PV) Systems

• Principle of Photovoltaic Conversion:

  • Photoelectric Effect: Absorption of photon (energy $$\displaystyle E = h\nu = \frac{hc}{\lambda} $$) by semiconductor material ejects electron from valence to conduction band, creating electron-hole pair.

  • p-n Junction: Built-in electric field at junction separates charge carriers, generating DC voltage/current.

  • Key Equation: $$\displaystyle P_{max} = V_{mp} \times I_{mp} $$ (Maximum Power Point).

• Solar Cells: Types & Fabrication:

  • Silicon-Based (95% market):

    • Monocrystalline (mono-Si): High efficiency (15-22%), uniform dark blue color, expensive.

    • Polycrystalline (poly-Si): Moderate efficiency (13-16%), blue speckled, cheaper.

    • Amorphous Silicon (a-Si): Thin-film, low efficiency (6-8%), flexible, performs better in low light.

  • Thin-Film: CdTe, CIGS, a-Si. Lower cost, flexible, lower efficiency.

  • Fabrication Steps (Silicon): Purification (Si from quartz) → Ingot casting (Czochralski for mono) → Wafer slicing → Doping (p-n junction formation) → Anti-reflective coating → Metallization (contacts) → Encapsulation (EVA, glass, backsheet).

• PV System Components:

  • PV Modules (series/parallel) → Charge Controller (protects battery) → Battery Bank (storage) → Inverter (DC to AC) → AC Load/Grid.

  • Balance of System (BOS): Mounting structures, wiring, protection devices.

• Applications in PV Generation Systems:

  • Stand-alone (remote homes, telemetry, water pumping).

  • Grid-connected (rooftop, solar farms).

  • Hybrid (PV-diesel, PV-wind).

• Limitations of SPV Systems:

  • Intermittency (no sun at night, cloudy weather).

  • Low conversion efficiency (~15-22% for commercial Si).

  • High initial capital cost.

  • Requires energy storage (batteries) for 24/7 supply, adding cost & maintenance.

  • Large area requirement for utility-scale.

• Maximum Power Point Tracking (MPPT) - Perturb & Observe (P&O):

  • Procedure:

    1. Measure initial PV voltage ($V$) and current ($I$), calculate power $$\displaystyle P = V \times I $$.

    2. Perturb (increase or decrease) the duty cycle of DC-DC converter (changes load impedance).

    3. Re-measure $V$, $I$, $$\displaystyle P_{new} $$.

    4. If $$\displaystyle P_{new} > P_{old} $$: Continue perturbation in same direction.

    5. If $$\displaystyle P_{new} < P_{old} $$: Reverse perturbation direction.

    6. Repeat continuously to track MPP under changing irradiance/temperature.

  • Disadvantage: Oscillates around MPP, can lose track during rapid irradiance changes.

• Numerical: Photon Energy

  • Energy of a photon: $$\displaystyle E = \frac{hc}{\lambda} $$

  • $$\displaystyle h = 6.626 \times 10^{-34} \ \mathrm{J \cdot s} $$, $$\displaystyle c = 3 \times 10^8 \ \mathrm{m/s} $$, $$\displaystyle \lambda = 1 \ \mu\mathrm{m} = 10^{-6} \ \mathrm{m} $$

  • $$\displaystyle E = \frac{(6.626 \times 10^{-34}) \times (3 \times 10^8)}{10^{-6}} = 1.988 \times 10^{-19} \ \mathrm{J} $$

  • Convert to eV: $$\displaystyle 1 \ \mathrm{eV} = 1.602 \times 10^{-19} \ \mathrm{J} $$ → $$\displaystyle E = \frac{1.988 \times 10^{-19}}{1.602 \times 10^{-19}} \approx 1.24 \ \mathrm{eV} $$.

  \boxed{E \ (\mathrm{in \ eV}) \approx \frac{1.24}{\lambda \ (\mathrm{in \ \mu m})}}

II. WIND ENERGY TECHNOLOGIES

A. Fundamentals & Aerodynamics

• Wind Regimes & Energy Estimation:

  • Wind speed varies randomly. Characterized by Weibull Distribution: $$\displaystyle f(v) = \frac{k}{c} \left( \frac{v}{c} \right)^{k-1} e^{-(v/c)^k} $$ where $k$ = shape parameter, $c$ = scale parameter (m/s).

  • Wind Power Density (WPD): $$\displaystyle P_{density} = \frac{1}{2} \rho v^3 $$ (W/m²). Average WPD over time is critical for site assessment.

  • Energy Estimation: $$\displaystyle E = P_{density} \times A \times T $$, where $A$ = swept area, $T$ = time.

• Aerofoil (Airfoil):

  • Definition: Cross-sectional shape of a blade designed to generate lift (pressure difference) when air flows over it.

  • Principle: Bernoulli's principle + Coanda effect. Cambered shape causes faster flow over top surface → lower pressure → lift force perpendicular to relative wind.

  • Types:

    • Symmetrical: Zero camber, used in vertical axis turbines (e.g., Darrieus).

    • Cambered: Positive camber, higher lift-to-drag ratio, standard for horizontal axis turbines.

    [!DIAGRAM] Search: "wind turbine blade airfoil cross-section diagram"

• Derivation of Power from Wind (Betz's Law):

  • Consider wind of density $\rho$, velocity $$\displaystyle v_1 $$ upstream, area $A$.

  • Mass flow rate: $$\displaystyle \dot{m} = \rho A v_1 $$

  • Kinetic energy/sec (power) in wind: $$\displaystyle P_{wind} = \frac{1}{2} \dot{m} v_1^2 = \frac{1}{2} \rho A v_1^3 $$

  • Turbine extracts power. Downstream velocity = $$\displaystyle v_2 $$. Power extracted: $$\displaystyle P_{turbine} = \frac{1}{2} \dot{m} (v_1^2 - v_2^2) = \frac{1}{2} \rho A v_1 (v_1^2 - v_2^2) $$

  • Thrust: $$\displaystyle \dot{m}(v_1 - v_2) = \rho A v_1 (v_1 - v_2) $$

  • Maximize $$\displaystyle P_{turbine} $$ w.r.t $$\displaystyle v_2 $$ (or axial induction factor $$\displaystyle a = (v_1 - v_2)/v_1 $$). Optimal $$\displaystyle a = 1/3 $$ → $$\displaystyle v_2 = 2v_1/3 $$.

  • Maximum Power (Betz Limit): $$\displaystyle P_{max} = \frac{16}{27} \times \frac{1}{2} \rho A v_1^3 = \frac{8}{17} \rho A v_1^3 $$

  • Power Coefficient: $$\displaystyle C_p = \frac{P_{turbine}}{P_{wind}} \leq \frac{16}{27} \approx 0.593 $$ (Betz limit).

  \boxed{P_{turbine} = \frac{1}{2} \rho A v_1^3 C_p \quad \text{where} \ C_p \leq 0.593}

B. Wind Energy Conversion Systems (WECS)

• Components:

  1. Rotor (blades, hub)

  2. Nacelle (gearbox, generator, controller)

  3. Tower

  4. Foundation

  5. Yaw system (for HAWT)

  6. Braking system

• Types of WECS:

  • By Axis:

    • Horizontal Axis Wind Turbine (HAWT): Main rotor shaft horizontal, yaw mechanism needed. Most common (3-bladed).

    • Vertical Axis Wind Turbine (VAWT): Main shaft vertical. No yaw needed, can accept wind from any direction. Lower efficiency, higher torque ripple (e.g., Darrieus, Savonius).

  • By Location: Onshore (land), Offshore (sea, higher wind, lower turbulence, expensive).

• Wind Turbine Power Curve:

  • Graph of power output vs. wind speed.

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

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

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

  • Area under curve: Total energy produced at a site.

  [!DIAGRAM] Search: "wind turbine power curve graph"

• Limitations & Barriers to Large-Scale Utilization:

  • Intermittency & Variability: Non-dispatchable, requires backup/storage/grid flexibility.

  • Grid Integration: Voltage/frequency fluctuations, need for power electronics (converters), grid codes.

  • Environmental: Noise, visual impact, bird/bat mortality, shadow flicker.

  • Economic: High capital cost, transmission infrastructure needed for remote windy sites.

  • Social: Public acceptance (NIMBYism).

C. Hybrid & Advanced Systems

• Wind-Diesel Hybrid System:

  • Concept: Combine wind turbines with diesel generators to supply power to isolated grids/communities.

  • Working: Wind power used as primary source. Diesel genset provides backup when wind is low and charges battery bank (if present). Control system manages load sharing, stabilizes frequency/voltage.

  • Benefits: Reduces diesel fuel consumption, emissions, operating cost; increases reliability.

III. BIOMASS & BIOENERGY CONVERSION

A. Biomass Resources & Production

• Definition: Organic matter derived from plants or animals, available on a renewable basis.

• Sources: Woody biomass (forest residues, energy crops), Herbaceous biomass (agricultural residues: straw, bagasse), Animal waste, Municipal solid waste (organic fraction), Aquatic biomass (algae).

• Utility: Direct combustion for heat, conversion to biofuels (bioethanol, biodiesel, biogas), biochemicals.

• Photosynthesis - C3 vs C4 Pathways:

  • C3 Plants: First stable product is 3-carbon compound (3-phosphoglycerate). Calvin cycle only. Most plants (wheat, rice, potatoes). More efficient under cool, moist conditions. Photorespiration loss at high T/low CO₂.

  • C4 Plants: First stable product is 4-carbon compound (oxaloacetate). Spatial separation (mesophyll & bundle sheath cells). Minimizes photorespiration. Higher efficiency under high light, temp, drought. Examples: maize, sugarcane, sorghum. Higher biomass yield potential.

B. Conversion Technologies

• Biochemical Conversion - Anaerobic Digestion (Biogas):

  • Process: Microbial breakdown of organic matter in absence of oxygen.

  • Stages: Hydrolysis → Acidogenesis → Acetogenesis → Methanogenesis.

  • Output: Biogas (CH₄ ~55-65%, CO₂ ~35-45%, traces H₂S, H₂O).

• Types of Biomass Gasification:

  • Fixed Bed: Biomass fed from top.

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

    • Downdraft: Air blown from top, gas exits bottom. Low tar, common for engines.

    • Crossdraft: Air/gas flow perpendicular.

  • Fluidized Bed: Sand bed fluidized by air. Good mixing, uniform T, high throughput. Bubbling Fluidized Bed (BFB), Circulating Fluidized Bed (CFB).

  • Output: Producer gas (CO, H₂, CH₄, CO₂, N₂). Used in engines, boilers, or for synfuel.

• Types of Digesters (Biogas Plants):

  • Batch: Fill, digest, empty. Simple, uneven gas production.

  • Continuous: Feed continuously, digestate removed continuously. Steady gas output.

  • Fixed-Dome: Masonry dome, gas stored under pressure. No moving parts, but gas leakage issues.

  • Floating-Drum: Movable steel drum on slurry, acts as gas holder. Good pressure regulation, moving parts need maintenance.

  [!DIAGRAM] Search: "floating drum biogas plant diagram" or "fixed dome biogas plant diagram"

• Design of Biogas Plant (Key Considerations):

  • Feedstock type & availability: C/N ratio (20-30:1 ideal), moisture content (85-90% for wet digestion).

  • Retention Time: 15-60 days depending on temp, feedstock.

  • Temperature: Psychrophilic (<20°C), Mesophilic (30-38°C), Thermophilic (50-55°C). Mesophilic most common.

  • Digester Volume: Based on daily feed, retention time.

  • Mixing: Ensures contact, prevents scum.

  • Gas Holder Capacity: ~50-60% of daily gas production.

• Thermochemical Conversion - Biomass Gasification for Power:

  • Process: Partial oxidation at 700-1000°C with air/oxygen/steam.

  • Gas Cleaning: Remove particulates, tars, alkali metals, sulfur compounds to protect engines/turbines.

  • Power Generation: Clean gas fuels internal combustion engine (coupled to generator) or gas turbine. Efficiency ~20-25% (gasifier + engine).

C. Other Biochemical Processes

• Fermentation (Biofuels):

  • Bioethanol: Saccharification (cellulose → sugars) + Fermentation (yeast, sugars → ethanol + CO₂) + Distillation.

  • Biodiesel: Transesterification (vegetable oil/animal fat + alcohol + catalyst → biodiesel + glycerol).

IV. HYDROENERGY TECHNOLOGIES

A. Hydro System Classification

• Based on Installed Capacity:

  • Micro Hydro: < 100 kW. Very small, often run-of-river, for village/individual.

  • Mini Hydro: 100 kW - 1 MW. Small community/industrial use.

  • Small Hydro: 1 MW - 25 MW (or up to 50 MW, definitions vary). Can be run-of-river or with small storage.

  • Large Hydro: > 25/50 MW. Major dams, significant storage.

• Main Components of Small Hydropower System:

  • Diversion Structure/Weir: Diverts river flow into channel.

  • Headrace Channel/Tunnel: Conveys water to forebay/surge tank.

  • Penstock: High-pressure pipe delivering water to turbine.

  • Turbine & Generator: Converts hydraulic to mechanical to electrical energy.

  • Tailrace: Returns water to river.

  • Control Gates, Trash Rack: Regulate flow, prevent debris.

B. Turbines & Plant Operation

• Types of Turbines:

  • Impulse Turbines: Energy transferred by high-velocity jet(s) striking buckets. Head: High (>300m). Pelton Wheel: Most common impulse. Buckets shaped to deflect jet, minimize thrust. Efficiency ~90%.

  • Reaction Turbines: Pressure drop occurs in both stationary (stator) and rotating (rotor) blades. Head: Low to medium (2-300m).

    • Francis Turbine: Most common reaction. Spiral casing, adjustable guide vanes, runner. Used for medium head/flow.

    • Kaplan Turbine: Axial flow, adjustable blades. Used for low head, high flow.

    • Turgo Turbine: Impulse-reaction hybrid, for medium head.

  [!DIAGRAM] Search: "pelton turbine diagram" or "francis turbine diagram"

• Selection of Turbine (based on Net Head & Design Flow):

  • Use Specific Speed ($$\displaystyle N_s $$) concept: $$\displaystyle N_s = N \sqrt{P} / H^{5/4} $$ (N in rpm, P in kW, H in m). Higher $$\displaystyle N_s $$ → lower head, higher flow preference.

  • High Head (>300m): Pelton.

  • Medium Head (30-300m): Francis.

  • Low Head (<30m): Kaplan, Bulb, Propeller.

• Speed and Voltage Regulation in Hydropower Plant:

  • Speed Regulation: Maintain constant generator speed (synchronous speed) despite load changes. Done by Governor system (mechanical/hydraulic/electro-hydraulic) adjusting turbine guide vane/nozzle opening.

  • Voltage Regulation: Maintain terminal voltage. Done by AVR (Automatic Voltage Regulator) adjusting generator excitation field current. In isolated grids, voltage/frequency are coupled; in grid-connected, grid dictates frequency.

C. Energy Estimation & Numerical

• Energy Estimation of Hydropower:

  • Theoretical power: $$\displaystyle P_{th} = \rho g Q H $$ (W)

  • Practical Power: $$\displaystyle P = \eta_{turbine} \times \eta_{gen} \times \rho g Q H $$

  • $\rho$ = density (1000 kg/m³), $g$ = 9.81 m/s², $Q$ = design discharge (m³/s), $H$ = net head (m), $\eta$ = overall efficiency (~0.7-0.9).

  • Energy (E): $$\displaystyle E = P \times t $$ (Joules or kWh; 1 kWh = 3.6e6 J).

• Numerical: Tidal Power (Single Basin):

  • Principle: Basin separated from sea by barrage with turbines. Potential energy from head difference.

  • Energy per Cycle (filling/emptying): $$\displaystyle E = \frac{1}{2} \rho g A H^2 \eta_{turbine-gen} $$

    • $A$ = basin area (m²)

    • $H$ = mean tidal range (m) BUT turbine stops at minimum head $$\displaystyle H_{min} $$. Effective head for generation is $$\displaystyle H_{eff} = H - H_{min} $$.

    • Actual energy derived from integration, but approximate: $$\displaystyle E \approx \frac{1}{2} \rho g A (H^2 - H_{min}^2) \eta $$

  • Example from Paper: $$\displaystyle A = 30 \times 10^6 \ \mathrm{m}^2 $$, $$\displaystyle H = 12 \ \mathrm{m} $$, $$\displaystyle H_{min} = 3 \ \mathrm{m} $$, $$\displaystyle \eta = 0.73 $$.

    • $$\displaystyle E = \frac{1}{2} \times 1000 \times 9.81 \times 30 \times 10^6 \times (12^2 - 3^2) \times 0.73 \ \mathrm{J} $$

    • $$\displaystyle E = 0.5 \times 1000 \times 9.81 \times 30e6 \times (144 - 9) \times 0.73 $$

    • $$\displaystyle E = 0.5 \times 1000 \times 9.81 \times 30e6 \times 135 \times 0.73 \ \mathrm{J} $$

    • Convert to kWh: divide by $$\displaystyle 3.6 \times 10^6 $$.

    • $$\displaystyle E_{kWh} = \frac{0.5 \times 1000 \times 9.81 \times 30 \times 10^6 \times 135 \times 0.73}{3.6 \times 10^6} $$

    • $$\displaystyle E_{kWh} = \frac{0.5 \times 9.81 \times 30 \times 135 \times 0.73}{3.6} \times 10^3 $$ (since $$\displaystyle 10^6/10^6=1 $$, but 1000/3.6e6 gives $$\displaystyle 10^3 $$ factor)

    • Calculate stepwise: $$\displaystyle 0.5 \times 9.81 = 4.905 $$; $$\displaystyle 4.905 \times 30 = 147.15 $$; $$\displaystyle 147.15 \times 135 = 19865.25 $$; $$\displaystyle 19865.25 \times 0.73 = 14501.6325 $$; $14501.6325 / 3.6 \approx 4028.23$; $$\displaystyle \times 10^3 = 4,028,230 \ \mathrm{kWh} $$.

    • Result: $$\displaystyle \boxed{4.03 \times 10^6 \ \mathrm{kWh} \ \text{or} \ 4028 \ \mathrm{MWh}} $$ (per filling/emptying cycle).

V. GEOTHERMAL ENERGY

A. Resources & Principles

• Types of Geothermal Deposits:

  • Hydrothermal: Hot water/steam in porous/fractured rock. Most common (vapor-dominated, liquid-dominated).

  • Geo-pressured: Hot brine under high pressure, contains dissolved methane.

  • Hot Dry Rock (HDR): Hot, impermeable rock. Requires artificial fracturing (EGS - Enhanced Geothermal Systems).

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

• Principle of Thermodynamics:

  • Heat from Earth's interior (radioactive decay, residual heat) transferred to fluid (water/steam) in reservoir.

  • Rankine Cycle: Standard for power generation. Working fluid (water/ORC fluid) is pumped to high pressure, heated in boiler (heat exchanger with geothermal fluid) to vapor, expanded in turbine, condensed, and pumped again.

  • Key: Temperature gradient (geothermal gradient ~25-30°C/km).

B. Power Generation & Site Selection

• Generation Processes:

  • Dry Steam: Direct use of geothermal steam (>235°C) from reservoir to drive turbine. Simplest, oldest (The Geysers, USA).

  • Flash Steam: High-pressure hot water (>180°C) from reservoir is throttled (flashed) into steam in separator. Steam drives turbine, leftover brine may be flashed again or reinjected.

  • Binary Cycle: Geothermal hot water heats secondary fluid (isobutane, pentane) with lower boiling point in heat exchanger. Secondary fluid vapor drives turbine. Allows use of lower T resources (85-175°C). Closed loop, minimal emissions.

• Site Selection Criteria:

  • High subsurface temperature (>150°C for power).

  • Permeable reservoir with sufficient fluid.

  • Near surface to reduce drilling cost.

  • Accessibility for infrastructure.

  • Environmental constraints (protected areas, emissions).

  • Proximity to load center to minimize transmission loss.

C. Environmental Aspects

• Environmental Benefits:

  • Very low greenhouse gas emissions (mostly non-condensable gases like CO₂, H₂S, but much lower than fossil fuels).

  • Minimal air pollution (particulates, NOx, SOx).

  • Small land footprint per MW.

  • Baseload capability (high capacity factor >90%).

  • Sustainable if managed properly (reinjection maintains reservoir pressure).

VI. MARINE & OTHER RENEWABLE SOURCES

A. Tidal Energy

• Principle of Tidal Energy Conversion:

  • Harnesses kinetic energy (tidal currents) or potential energy (tidal range/head).

  • Tidal Range (Barrage): Dam across estuary/bay. Basins fill at high tide, empty at low tide through turbines. Single basin: Generation during either flood or ebb (or both with two-way turbines). Double basin: Two basins at different phases, allows more continuous generation.

  • Tidal Current (Stream): Underwater turbines (like wind turbines) in fast-flowing tidal channels. No dam, less environmental impact.

  [!DIAGRAM] Search: "single basin tidal barrage diagram"

• Numerical: As derived in Section IV.C.

B. Ocean Thermal Energy Conversion (OTEC)

• Principle:

  • Exploits temperature difference ($\Delta T$) between warm surface water (25-30°C) and cold deep water (5-10°C).

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

  • Closed Cycle: Warm surface water evaporates working fluid in evaporator → vapor drives turbine → cold deep water condenses vapor in condenser → liquid pumped back.

  • Open Cycle: Warm seawater itself flashed to steam in vacuum chamber (low pressure) → steam drives turbine → condensed to fresh water.

  • Hybrid: Combination.

  • Challenge: Very low $\Delta T$ (~20°C) → low thermodynamic efficiency (Carnot ~3-4%), requiring massive flow rates.

C. Wave Energy

• Concept: Capture energy from ocean surface waves (caused by wind).

  • Devices: Oscillating Water Columns (OWC), Point Absorbers, Attenuators (Pelamis), Overtopping devices.

• Significant Wave Height ($$\displaystyle H_s $$):

  • Definition: Average height of the highest one-third of waves in a wave spectrum.

  • Statistically equivalent to the root-mean-square (RMS) wave height for a narrow spectrum. Used for energy resource assessment.

  • Wave power density: $$\displaystyle P \approx 0.5 \ \mathrm{kW/m} \times (H_s)^2 \times T_e $$ (where $$\displaystyle T_e $$ = energy period).

VII. FUEL CELL TECHNOLOGY

A. Fundamentals & Classification

• Working Principle:

  • Electrochemical device converting chemical energy of fuel (H₂, hydrocarbons) and oxidant (O₂) directly into electricity.

  • Components: Anode (oxidation), Cathode (reduction), Electrolyte (ion conductor), Catalyst (Pt, Ni, etc.).

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

  • Half-reactions:

    • Anode: $$\displaystyle H_2 \rightarrow 2H^+ + 2e^- $$ (or $$\displaystyle H_2 + OH^- \rightarrow H_2O + e^- $$ in alkaline)

    • Cathode: $$\displaystyle \frac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O $$ (or $$\displaystyle O_2 + 2H_2O + 4e^- \rightarrow 4OH^- $$)

  • Key: Electrolyte determines ion type (H⁺, OH⁻, O²⁻) and operating T.

• Classification (by Electrolyte):

  • AFC (Alkaline Fuel Cell): KOH electrolyte. High efficiency, used in space. Sensitive to CO₂.

  • PEMFC (Polymer Electrolyte Membrane / Proton Exchange Membrane): Solid polymer (Nafion). Low T (60-80°C), quick start, high power density. Used in vehicles, backup power.

  • MCFC (Molten Carbonate): Molten carbonate (Li/K). High T (650°C), can use CO, CH₄ directly (internal reforming). Efficiency ~60%.

  • SOFC (Solid Oxide): Ceramic (ZrO₂). Very high T (800-1000°C), all fuels, high efficiency (~60%), slow start.

  [!DIAGRAM] Search: "fuel cell diagram pemfc"

B. System & Applications

• Fuel Cell System (Beyond Cell):

  • Fuel Processor/Reformer: Converts hydrocarbon fuel (methane, methanol) to H₂-rich gas (if not using pure H₂).

  • Fuel Cell Stack: Multiple cells in series for desired voltage.

  • Power Conditioner: DC-DC converter, inverter (for AC output).

  • Heat Recovery System: Utilize waste heat (cogeneration/CHP).

  • Sensors & Controls: Monitor T, pressure, flow, humidity.

• Advantages:

  • High efficiency (40-60% electrical, >80% with CHP).

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

  • Modular, scalable.

  • Quiet operation.

  • Fast refueling (vs. batteries).

• Limitations/Challenges:

  • High Cost: Catalyst (Pt), materials, manufacturing.

  • Hydrogen Infrastructure: Lack of production, storage, distribution network.

  • Durability: Catalyst poisoning (CO, sulfur), membrane degradation, thermal cycling.

  • Storage: Low volumetric energy density of H₂ (compressed, liquid, chemical carriers).

VIII. CROSS-CUTTING SYSTEM & ECONOMIC TOPICS

A. Energy Systems & Storage

• Energy Storage Management (Role & Technologies):

  • Role: Balance supply-demand mismatch (especially for variable RE), provide grid stability (frequency, voltage), time-shift energy, backup power.

  • Technologies:

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

    • Electrochemical: Batteries (Li-ion, lead-acid, flow batteries).

    • Chemical: Hydrogen (via electrolysis), Synthetic fuels.

    • Thermal: Molten salt (CSP), chilled water, phase change materials.

    • Electrical: Capacitors, Superconducting Magnetic Energy Storage (SMES).

B. Economics & Policy

• Various Types of Tariffs in Electricity:

  • Feed-in Tariff (FiT): Fixed, above-market rate paid to renewable generators for electricity fed into grid. Promotes deployment.

  • Time-of-Use (TOU) Tariff: Price varies by time of day (peak/off-peak) to reflect generation cost and encourage load shifting.

  • Net Metering: Consumer-generator (e.g., rooftop solar) exports excess to grid, offsetting imports. Billing based on net usage.

  • Fixed Charge + Variable Charge: Common residential/commercial. Fixed monthly charge + per kWh charge.

  • Dynamic Pricing: Real-time pricing (RTP) based on spot market prices.

C. Environmental Context

• Need for Renewable Energy (vs. Fossil Fuels):

  • Energy Security: Reduce dependence on imports.

  • Climate Change Mitigation: Lower GHG emissions.

  • Air Quality: Reduce pollutants (SOx, NOx, PM).

  • Sustainability: Inexhaustible sources.

  • Economic: Job creation, technology export.

• Effect of Fossil Fuel Use on Climate Change & Global Warming:

  • Combustion releases CO₂, CH₄, N₂O (greenhouse gases).

  • GHG trap outgoing longwave infrared radiation → Enhanced Greenhouse Effect → Global temperature rise.

  • Consequences: Sea-level rise, extreme weather, glacial melt, ocean acidification, ecosystem disruption.

• Greenhouse Effect (Natural):

  • Solar radiation (shortwave) passes through atmosphere, warms Earth's surface.

  • Surface emits longwave infrared radiation.

  • Greenhouse gases (CO₂, H₂O, CH₄, O₃) absorb and re-radiate this IR, trapping heat and keeping Earth ~33°C warmer than it would be. Essential for life. Enhanced by human activities.

> [!TIP] EXAM STRATEGY:

• Solar & Wind: Master derivations (Betz, solar geometry), diagrams (FPC, PV cell, WECS, aerofoil), and numericals (tidal, photon energy, solar angles).

• Biomass: Clearly differentiate gasifier types, digester types, C3/C4.

• Hydro: Know turbine selection logic, energy formula, and draw Pelton/Francis.

• Geothermal & Marine: Focus on principles, types (binary, tidal basin), and site selection.

• Fuel Cells: Classify by electrolyte, draw PEMFC, list pros/cons.

• Cross-cutting: Be precise on tariffs (FiT vs. Net Metering) and environmental terms (GHG, greenhouse effect).