Robotics (ME-604 (A)) - Unit 5 Short Notes

I. Introduction to Renewable Energy

Definition: Energy derived from naturally replenishing sources (solar, wind, biomass, etc.) on a human timescale, as opposed to finite fossil fuels.

Need for Renewable Energy:

• Depleting fossil fuel reserves.

• Environmental pollution and climate change.

• Energy security and independence.

• Sustainable development.

Classification of Renewable Energy Sources:

Energy Storage Management: Techniques to store excess energy (e.g., batteries, pumped hydro, thermal storage) for use during low generation periods, ensuring grid stability.

Climate Change & Global Warming:

Fossil fuel combustion releases CO₂, CH₄, N₂O (greenhouse gases) → traps heat → global temperature rise → extreme weather, sea-level rise.

Greenhouse Effect:

Natural process where atmospheric gases (H₂O, CO₂, CH₄) absorb and re-emit infrared radiation, warming Earth. Enhanced by anthropogenic GHG emissions.

Sensible vs. Latent Heat:

• Sensible Heat: Heat exchanged causing temperature change (e.g., heating water).

• Latent Heat: Heat exchanged during phase change without temperature change (e.g., evaporation/condensation).

[!TIP]

Exam Focus: Distinguish between greenhouse effect (natural) and enhanced greenhouse effect (human-induced). Know the formula for latent heat of vaporization of water (~2260 kJ/kg).

II. Solar Energy

A. Solar Radiation Fundamentals

Measurement:

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

• Pyrheliometer: Measures direct normal irradiance (DNI).

• Sunshine recorder: Measures sunshine duration.

Sun-Earth Relationship:

• 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 altitude (α) & azimuth (γₛ): Determine Sun's position.

Solar Geometry:

• Altitude angle: \( \alpha = \sin^{-1}(\cos\phi \cos\delta \cos\omega + \sin\phi \sin\delta) \)

• Incidence angle (θ) on tilted surface:

  \( \cos\theta = \cos\phi \cos\delta \cos(\omega - \gamma) + \sin\phi \sin\delta \)

  where \(\phi\) = latitude, \(\gamma\) = surface azimuth (0° for south-facing).

• Solar radiation on tilted surface:

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

  \(I_b\) = beam, \(I_d\) = diffuse, \(I_r\) = ground-reflected, \(\beta\) = tilt angle.

Wavelength-Energy Conversion:

Photon energy \(E = \frac{hc}{\lambda}\)

Given \(\lambda = 1\ \mu m = 10^{-6}\ m\), \(h = 6.626 \times 10^{-34}\ J\cdot s\), \(c = 3 \times 10^8\ m/s\),

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

Convert to eV: \(1\ eV = 1.602 \times 10^{-19}\ J\) → \(E \approx 1.24\ eV\).

\boxed{E(eV) = \frac{1240}{\lambda(nm)}}

[!TIP]

Common Pitfall: In incidence angle formula, \(\omega\) is solar hour angle, not clock time. Use solar time: \(\omega = 15^\circ \times (t_{solar} - 12)\).

B. Solar Thermal Systems

Solar Collectors Classification:

Flat Plate Collector (FPC):

• Construction: Insulated box, transparent cover (glass), black absorber plate with serpentine tubes.

• Working: Solar radiation passes through glass, absorbed by plate → heats → transfers to circulating fluid (water/air) → useful thermal energy.

Solar Water Heating Systems:

• Thermosyphon System:

  Natural circulation due to density difference (hot water rises, cold sinks). No pump. Requires elevated tank.

• Forced Circulation System:

  Pump circulates fluid. Controlled, flexible installation, requires power.

Applications: Water heating, space heating, industrial process heat, solar drying, distillation.

C. Photovoltaic (PV) Systems

Principle of Photovoltaic Conversion:
Photoelectric effect: Photons with energy > bandgap excite electrons from valence to conduction band → electron-hole pairs → internal electric field (p-n junction) separates charges → DC current.

Solar Cells - Semiconductor Types:

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

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

• Emerging: Perovskite, organic PV.

Fabrication of PV Cells & Modules:

1. Ingot growth (Czochralski for mono-Si, casting for poly-Si).

2. Wafering (slicing ingot into thin wafers).

3. Cell processing: Texturing, doping (p-n junction), anti-reflection coating, metallization.

4. Module assembly: Cells interconnected, encapsulated (EVA), framed, junction box.

SPV Systems Applications:

• Standalone: Remote homes, street lights, water pumping.

• Grid-connected: Rooftop, solar farms.

• Hybrid: With diesel/generator, battery storage.

Limitations of SPV Systems:

• Intermittency (day/night, weather).

• High initial cost.

• Low efficiency (15-22% typical).

• Requires large area for utility-scale.

• Energy storage needed for 24/7 supply.

Maximum Power Point Tracking (MPPT):

• Perturb and Observe (P&O) Algorithm:

  Step-by-step:

  1. Measure initial \(V_{pv}\), \(I_{pv}\) → calculate \(P_{pv}\).

  2. Perturb voltage (increase or decrease by ΔV).

  3. Measure new \(P_{pv}\).

  4. If \(P_{new} > P_{old}\) → continue same perturbation direction.

  5. If \(P_{new} < P_{old}\) → reverse perturbation direction.

  6. Repeat periodically.

• Advantage: Simple, no prior knowledge of PV characteristics.

• Disadvantage: Oscillates around MPP under steady conditions; slow response to rapid changes.

[!TIP]

Exam Focus: Be ready to draw I-V and P-V curves showing MPP. In P&O, emphasize it’s a hill-climbing method. For solar cell efficiency: \(\eta = \frac{P_{mp}}{P_{in}} = \frac{V_{mp} I_{mp}}{G A_{cell}}\).

III. Wind Energy

A. Wind Fundamentals

Wind Regimes & Energy Estimation:

• Characterized by Weibull distribution: \(f(v) = \frac{k}{c} \left( \frac{v}{c} \right)^{k-1} e^{-(v/c)^k}\)

  \(k\) = shape parameter (2-3 typical), \(c\) = scale parameter (mean wind speed).

• Mean power density: \(P_{avg} = \frac{1}{2} \rho \int_0^\infty v^3 f(v) dv = \frac{1}{2} \rho c^3 \Gamma\left(1+\frac{3}{k}\right)\)

Power Developed from Wind (Derivation):

1. Mass flow rate through rotor: \(\dot{m} = \rho A v\) (A = swept area).

2. Kinetic energy per unit time (available power): \(P_{available} = \frac{1}{2} \dot{m} v^2 = \frac{1}{2} \rho A v^3\).

3. Betz limit: Maximum power extracted = \(\frac{16}{27} \times \frac{1}{2} \rho A v^3 = \frac{8}{27} \rho A v^3\).

4. Actual power: \(P = C_p \cdot \frac{1}{2} \rho A v^3\), where \(C_p \leq 0.593\).

\boxed{P = \frac{1}{2} C_p \rho A v^3}

Aerofoil (Airfoil):

Cross-sectional shape of blade generating lift.

• Types: Symmetrical (zero lift at 0° AoA), Cambered (positive lift at 0° AoA).

• Characteristics: Lift coefficient \(C_L\), drag coefficient \(C_D\), lift-to-drag ratio \(C_L/C_D\) (higher → more efficient).

B. Wind Energy Conversion Systems (WECS)

Components:

• Rotor blades: Capture wind energy.

• Nacelle: Houses gearbox, generator, control systems.

• Gearbox: Increases rotational speed (low-speed shaft to high-speed shaft).

• Generator: Converts mechanical to electrical energy (induction/synchronous).

• Tower: Supports rotor at sufficient height.

• Yaw system: Orients rotor into wind.

• Brakes: Safety shutdown.

Types of WECS:

Wind-Diesel Hybrid System:

Combines wind turbines with diesel generators and often battery storage. Wind reduces diesel consumption; diesel provides backup. Controls manage power flow.

C. Wind Turbine Performance

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

  • Cut-in speed: ~3-4 m/s (start generation).

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

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

  • Survival speed: ~60 m/s (structural limit).

D. Challenges and Limitations

• Intermittency & variability → grid integration issues.

• Noise pollution (aerodynamic, mechanical).

• Visual impact and land use.

• Threat to wildlife (birds, bats).

• Initial capital cost high.

• Transmission needs (remote windy sites).

[!TIP]

Exam Focus: Derive Betz limit using momentum theory (actuator disk). Know typical values: air density \(\rho \approx 1.225\ kg/m^3\), \(C_p\) max = 0.593. In power curve, remember power ∝ \(v^3\) below rated speed.

IV. Biomass Energy

A. Biomass Resources and Conversion

Biomass Definition: Organic material from plants/animals (wood, crops, residues, manure) used as fuel.

Usefulness: Renewable, carbon-neutral (CO₂ recycled), waste management, rural employment.

Conversion Types:

Biomass Gasification Types:

• Updraft: Air enters from bottom → high tar, low temp.

• Downdraft: Air enters middle → lower tar, medium temp.

• Crossdraft: Air enters side → high temp, low tar.

• Fluidized bed: Good mixing, uniform temp.

B. Anaerobic Digestion

Process: Microbial breakdown of biomass in absence of oxygen → biogas (CH₄ ~60%, CO₂ ~40%) + digestate (fertilizer).

Types of Digesters:

• Fixed dome (Chinese type): Concrete dome, gas holder above digestate. Low cost, but gas leakage issues.

• Floating drum (Indian type): Movable gas holder (steel drum) floats on digestate. Good gas sealing.

• Plug flow: Long narrow tank, continuous feed.

• Anaerobic lagoon: Shallow pond, low-rate.

Biogas Plant Design Components:

• Inlet (feedstock entry).

• Digester tank (anaerobic zone).

• Gas holder/compensation chamber.

• Outlet (digestate removal).

• Gas outlet pipe.

C. Photosynthesis

C3 Plants:

• First stable product: 3-carbon compound (3-phosphoglycerate).

• Calvin cycle only.

• Examples: Rice, wheat, soybeans.

• Limitation: Photorespiration at high temp/low CO₂ → lower efficiency.

C4 Plants:

• Spatial separation: CO₂ fixed in mesophyll cells (4-carbon compound) → transported to bundle sheath → Calvin cycle.

• Examples: Maize, sugarcane, sorghum.

• Advantage: Reduced photorespiration, higher water-use efficiency, better in hot/dry climates.

[!TIP]

Exam Focus: Compare C3 vs C4 in table form (pathway, photorespiration, temperature adaptation, efficiency). In gasification, know downdraft is common for engines due to low tar.

V. Hydropower

A. System Classification

Main Components of Small Hydropower:

• Intake: Screens, gate.

• Penstock: Pressure pipe.

• Turbine: Converts head & flow to rotation.

• Generator: Converts rotation to electricity.

• Tailrace: Discharges water.

• Control gate, surge tank, draft tube (for reaction turbines).

B. Turbines

Types:

• Impulse Turbines: (Pelton) High head, low flow. No pressure change in runner; jets strike buckets.

• Reaction Turbines: (Francis, Kaplan) Medium/low head, high flow. Pressure drop occurs in runner.

Explain Francis Turbine (Reaction):

• Suitable for: Medium head (10-300 m), medium flow.

• Working: Water under pressure enters spiral casing → stays and guide vanes direct flow onto runner blades at optimal angle → pressure and kinetic energy drop across runner → shaft rotation → draft tube converts velocity head to pressure head, reduces exit loss.

Turbine Selection: Based on head (H) and flow (Q):

• High head (H > 300 m): Pelton.

• Medium head (30-300 m): Francis.

• Low head (H < 30 m): Kaplan (propeller), Bulb.

C. Plant Operation

Speed Regulation:

• Governor: Senses speed deviation → adjusts wicket gate opening (flow) to maintain constant speed (e.g., 50/60 Hz synchronization).

• Hydraulic/Pneumatic/Electronic governors.

Voltage Regulation:

• Automatic Voltage Regulator (AVR): Controls generator excitation current → maintains terminal voltage constant despite load changes.

• Methods: Thyristor-based excitation systems.

[!TIP]

Exam Focus: Know specific heads for turbine types. Francis is most common for small hydro. Draft tube in reaction turbines reduces exit kinetic energy loss, increases net head.

VI. Geothermal Energy

A. Resources and Site Selection

Types of Geothermal Deposits:

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

• Liquid-dominated (hot water): High-temperature water/brine (flash steam plants).

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

• Hot dry rock (HDR): Impermeable hot rock → requires hydraulic fracturing (EGS).

Site Selection Criteria:

• Geological: Tectonic activity, volcanism, faults, fractures.

• Hydrological: Aquifers, permeability.

• Reservoir: Temperature > 150°C for electricity, depth, size, sustainability.

• Environmental: Remote areas, land use, emissions.

• Economic: Proximity to grid, drilling costs.

B. Power Generation

Generation Process:

1. Exploration: Geological, geophysical surveys, test wells.

2. Production wells: Tap reservoir.

3. Power plant:

   • Dry steam: Steam directly drives turbine.

   • Flash steam: High-pressure hot water → flash tank → steam + brine → steam to turbine.

   • Binary cycle: Hot water heats secondary fluid (low boiling point, e.g., isobutane) → vapor drives turbine (closed loop).

4. Condensation & reinjection: Condensate + brine reinjected to sustain reservoir.

Thermodynamics Principle:

Based on Rankine cycle (vapor power cycle).

• Heat source: geothermal fluid.

• Working fluid: water (dry/flash) or organic fluid (binary).

• Key improvement: Binary cycle allows lower temperature resources (85-150°C).

C. Environmental Aspects

Benefits:

• Very low GHG emissions (mostly steam, minor non-condensables).

• Small land footprint per MW.

• Minimal solid waste.

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

• Sustainable if properly managed (reinjection).

[!TIP]

Exam Focus: Differentiate dry steam, flash, binary. Binary cycle is most versatile for low temps. Reinjection prevents reservoir depletion and subsidence.

VII. Ocean Energy

A. Tidal Energy

Principle:

Harness kinetic or potential energy of tides. Tidal range (difference between high/low tide) → store water in basin → release through turbines.

Single Basin Tidal Power Plant Calculation:

For a basin of area \(A\), tidal range \(R\), minimum operating head \(h_{min}\), turbine-generator efficiency \(\eta\).

• Available potential energy per filling/emptying cycle:

  \(E_{available} = \rho g \int_{h_{min}}^{R} A h \, dh = \rho g A \frac{(R^2 - h_{min}^2)}{2}\)

• Electrical energy generated:

  \(E_{elec} = \eta \times E_{available}\)

\boxed{E_{elec} = \eta \rho g A \frac{(R^2 - h_{min}^2)}{2}}

Diagram:

Energy in Filling vs. Emptying:

• Filling: Basin empty at low tide → sea water flows in through turbines → generation during filling until head = h_min.

• Emptying: Basin full at high tide → water flows out to sea through turbines → generation during emptying until head = h_min.

• Typically, both directions generate if two-way turbines.

B. Ocean Thermal Energy Conversion (OTEC)

Principle:

Utilize ocean temperature gradient: Warm surface water (25-30°C) vs. cold deep water (5-10°C) → heat engine.

• Closed-cycle: Warm seawater evaporates low-boiling fluid (e.g., ammonia) → vapor drives turbine → cold seawater condenses vapor.

• Open-cycle: Warm seawater flash-evaporated in vacuum → steam drives turbine → condenses to desalinated water.

• Hybrid: Combination.

Challenges: Low efficiency (3-4%), large pipe systems for cold water, biofouling.

C. Wave Energy

Significant Wave Height (\(H_s\)):

Statistical measure: average height of the highest one-third of waves in a wave spectrum.

\(H_s = 4 \sqrt{m_0}\), where \(m_0\) is the zero-order moment of wave spectrum.

[!TIP]

Exam Focus: Tidal energy formula derivation: integrate \(dE = \rho g A h \, dh\) from \(h_{min}\) to \(R\). For OTEC, remember Carnot efficiency limits: \(\eta_{Carnot} = (T_{hot} - T_{cold})/T_{hot}\).

VIII. Fuel Cells

A. Classification

B. Working Principle

Electrochemical Reaction (H₂-O₂ Fuel Cell):

• Anode: \(H_2 \rightarrow 2H^+ + 2e^-\) (oxidation)

• Cathode: \(\frac{1}{2}O_2 + 2H^+ + 2e^- \rightarrow H_2O\) (reduction)

• Overall: \(H_2 + \frac{1}{2}O_2 \rightarrow H_2O + \text{heat} + \text{electricity}\)

Diagram:

Key Features: No combustion, high efficiency (40-60%, up to 85% with CHP), low emissions (water vapor).

C. Performance

Advantages:

• High efficiency (electrochemical, not Carnot-limited).

• Low emissions: Only water, negligible NOₓ/SOₓ (if H₂ from clean source).

• Modular & scalable.

• Quiet operation.

• Fast refueling (for H₂ tanks).

Disadvantages: High cost (catalysts, materials), hydrogen storage/transport infrastructure, durability issues.

[!TIP]

Exam Focus: Compare fuel cell types by electrolyte, temp, fuel flexibility. SOFC/MCFC tolerate CO (internal reforming). PEMFC requires pure H₂ (sensitive to CO).

IX. Additional Topics

Various Types of Tariffs in Electricity:

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

• Block Rate Tariff: Slab system (higher consumption → higher rate per unit).

• Two-Part Tariff: Fixed charge + variable energy charge.

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

• Seasonal Tariff: Varies with season (e.g., higher in summer for cooling).

Applications of Solar Collectors (Driers):

• Solar Driers: Use solar thermal energy to remove moisture from agricultural products (grains, fruits, fish).

• Types:

  • Direct (through-pass): Product exposed to solar radiation.

  • Indirect (collector-heated air): Air heated in collector → passed through product bed.

  • Mixed-mode: Combination.

• Advantages: Low cost, preserves quality, reduces post-harvest losses.

[!TIP]

Exam Focus: For tariffs, know TOD promotes load shifting. Solar driers: indirect type gives better control and avoids contamination.