Renewable Energy Sources (CE-504 (C)) - Unit 1 Short Notes

UNIT 1: Renewable Energy Technologies and Systems

I. Introduction and Overview

Prospects of Non-Conventional Energy Sources in India

• Abundant Resources: India has high solar insolation (~4-7 kWh/m²/day), extensive coastline (7500 km) for wind/wave, large agricultural residue base for biomass, and Himalayan/riverine potential for small hydro.

• Energy Security: Reduces dependence on imported fossil fuels (crude oil, gas).

• Environmental Benefits: Mitigates greenhouse gas emissions and local pollution.

• Rural Electrification & Employment: Decentralized systems (solar, biomass) can power remote areas; manufacturing/installation creates jobs.

• Government Initiatives: National Solar Mission (target 100 GW solar by 2030), wind power policies, biomass cogeneration incentives, and international commitments (Paris Agreement).

Exam Tip: Link prospects directly to India's specific geographical and socio-economic context. Mention key government targets.

Role and Potential of Renewable Energy (Global and Indian Context)

• Global Role: Rapidly growing share in electricity mix (~30% of global power capacity additions). Drives technological cost reduction (solar PV, wind). Key for climate change mitigation.

• Global Potential: Theoretical potential far exceeds current/future demand (e.g., solar radiation incident on Earth in 1 hour > annual global energy consumption).

• Indian Potential & Role:

  • Solar: ~5000 trillion kWh/year potential. Leader in low-cost solar.

  • Wind: ~300 GW onshore, ~100 GW offshore potential.

  • Biomass: ~25 GW from agricultural residues.

  • Small Hydro: ~20 GW potential.

  • Role: Critical for achieving 500 GW non-fossil capacity by 2030 (Panchamrit commitment). Supports "Make in India" via manufacturing.

Strategies for Efficient Energy Management

1. Energy Conservation: Reducing energy consumption through behavioral changes and efficient technologies (e.g., LED bulbs, efficient motors).

2. Energy Efficiency: Using less energy for the same service output (e.g., 5-star rated appliances, high-efficiency turbines).

3. Load Management: Shifting demand from peak to off-peak hours (demand-side management).

4. Energy Audit: Systematic examination to identify energy-saving opportunities (see Section X).

5. Integration & Grid Management: Smart grids, forecasting, and integration of variable renewables (solar/wind).

6. Cogeneration/Trigeneration: Simultaneous generation of electricity and useful heat/cooling (common in biomass/solar thermal).

7. Waste Heat Recovery: Capturing heat from industrial processes.

II. Solar Energy Technologies

Solar Radiation Fundamentals

• Definition: Electromagnetic energy emitted by the sun.

• Extraterrestrial Radiation ($$\displaystyle I_{sc} $$): Solar constant (~1367 W/m²) at Earth's mean distance.

• Radiation at Earth's Surface: Affected by:

  1. Earth-Sun Distance: Varies ±1.7% annually.

  2. Solar Declination (δ): Due to Earth's axial tilt (23.45°), varies ±23.45° annually.

  3. Hour Angle (ω): 15° per hour from solar noon.

  4. Atmospheric Effects:

     • Absorption: By ozone (UV), water vapor, CO₂ (IR).

     • Scattering: By air molecules (Rayleigh), aerosols, clouds (Mie).

     • Reflection: From ground and clouds.

• Key Parameter: Air Mass (AM): Ratio of path length through atmosphere to vertical path. AM1.5 is standard for terrestrial PV testing.

Common Pitfall: Confusing reasons for variation with types of solar radiation (direct, diffuse, reflected). Focus on atmospheric and geometric factors.

Solar Thermal Systems: Principle of Conversion

• Principle: Concentrate solar radiation to increase intensity, convert to heat in a receiver, transfer via heat transfer fluid (HTF) to a thermal storage or directly to a heat engine/utility.

• Concentration Technologies:

  • Low-Temp (<100°C): Flat plate collectors (water heating).

  • Medium-Temp (100-250°C): Evacuated tube collectors, linear concentrators (process heat).

  • High-Temp (>250°C): Parabolic troughs, power towers, parabolic dishes (for power generation via steam Rankine/Brayton cycles).

• Basic Equation: $$\displaystyle Q_{useful} = A_c \cdot I_T \cdot F_R \cdot (\tau \alpha) - A_c \cdot F_R \cdot U_L \cdot (T_i - T_a) $$

  • $$\displaystyle A_c $$: Collector area, $$\displaystyle I_T $$: Transmitted radiation, $$\displaystyle F_R $$: Heat removal factor, $(\tau\alpha)$: Transmittance-absorptance product, $$\displaystyle U_L $$: Loss coefficient, $$\displaystyle T_i $$: Inlet temp, $$\displaystyle T_a $$: Ambient temp.

Photovoltaic (PV) Systems

Solar Cell Materials and Technologies

Maximum Power Point Tracking (MPPT) for PV Systems

• Need: PV $I$-$V$ curve has a single Maximum Power Point (MPP) where $$\displaystyle P = V \times I $$ is max. MPP varies with irradiance ($G$) and temperature ($T$).

• Goal: Keep PV array operating at MPP despite changing conditions to maximize energy harvest.

• Common Algorithms:

  1. Perturb and Observe (P&O): Perturb voltage, observe power change direction. Simple but oscillates around MPP, can falter during rapid irradiance changes.

  2. Incremental Conductance (IncCond): Uses condition $$\displaystyle dI/dV = -I/V $$ at MPP. More accurate, faster response than P&O, but slightly complex.

  3. Constant Voltage (CV): Maintains $$\displaystyle V_{pv} = k \cdot V_{oc} $$ (k≈0.76-0.8). Simple, but not true MPPT under all conditions.

III. Wind Energy

Wind Power Generation Systems: Specifications of Windmills

• Rotor Diameter (D): Determines swept area ($$\displaystyle A = \pi D^2/4 $$) and thus power capture.

• Hub Height: Higher = access to higher, less turbulent wind speeds ($$\displaystyle V \propto H^{\alpha} $$, α≈0.14-0.4).

• Rated Power (P_rated): Maximum continuous electrical output at rated wind speed ($$\displaystyle V_r $$).

• Cut-in Wind Speed ($$\displaystyle V_{ci} $$): ~3-4 m/s. Minimum speed to start generation.

• Rated Wind Speed ($$\displaystyle V_r $$): ~12-15 m/s. Speed at which P_rated is reached.

• Cut-out Wind Speed ($$\displaystyle V_{co} $$): ~25 m/s. Speed at which turbine shuts down for safety.

• Power Coefficient ($$\displaystyle C_p $$): Ratio of mechanical power extracted to wind power through rotor area. Betz limit: $$\displaystyle C_{p,max} = 16/27 \approx 0.593 $$.

• Specific Power (W/m²): $$\displaystyle P_{rated} / A $$. Lower values often indicate larger rotor for given power (better low-wind performance).

• Capacity Factor: Actual energy output / (P_rated × 8760 h). Typically 20-40% onshore.

IV. Biomass Energy

Biomass Conversion Processes (Detailed)

V. Ocean Energy

Wave Energy Conversion

• Principle: Capture kinetic & potential energy of surface waves (primarily wind-generated).

• Common Converters:

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

  • Point Absorber: Buoy moves with waves, drives linear generator/pump.

  • Attenuator: Long multi-segment floating structure (like Pelamis), flexes at joints to drive hydraulics.

  • Overtopping Device: Waves overtop into reservoir, water runs down through turbine.

• Advantages: High energy density (~5-50 kW/m of crest vs. ~0.5 kW/m² for wind/solar), predictable (based on weather), vast resource.

• Limitations: Harsh marine environment (corrosion, storms), maintenance costs, intermittent, grid connection challenges, potential environmental/maritime navigation impacts.

Ocean Thermal Energy Conversion (OTEC)

• Principle: Exploit temperature difference ($\Delta T$) between warm surface water (~25-30°C) and cold deep water (~5-10°C) to run a heat engine.

• Closed Cycle OTEC System:

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

  2. Evaporator: Warm surface water vaporizes working fluid.

  3. Turbine: Vapor expands through turbine, generating power.

  4. Condenser: Cold deep water condenses vapor back to liquid.

  5. Pump: Liquid working fluid pumped back to evaporator.

• Open Cycle OTEC: Uses seawater itself as working fluid. Warm seawater flash-evaporated in vacuum chamber, steam drives turbine, then condenses to produce desalinated water.

• Advantages of Closed Cycle over Open Cycle:

  • Higher Efficiency: Working fluid has optimized thermophysical properties (latent heat).

  • Simpler Turbine: Vapor pressure/temperature more controlled; no corrosion from seawater vapor.

  • Dual Product Flexibility: Primarily power; can add desalination via second-stage flash.

  • No Scaling/Fouling: Working fluid is closed loop; seawater only in heat exchangers (manageable).

  • Disadvantage: Requires working fluid charge, potential leakage.

VI. Hydro Energy

Small Head Hydro Power Development

• Concept: Hydro plants with head (water fall) < 30 m (often < 15 m). Also called Low Head/Low Head Hydro.

• Types:

  • Run-of-River (ROR): No or small reservoir; diverts part of river flow through channel/penstock to turbine, returns to river. Minimal storage, environmentally less disruptive.

  • Pondage: Small storage (pond) to meet short-term (hourly/daily) peak demands.

• Implementation:

  • Site Selection: Requires consistent flow (Q) and sufficient head (H). $$\displaystyle P_{avail} = \rho g Q H \eta $$ (η = overall efficiency ~0.5-0.8).

  • Civil Works: Weir/diversion, intake, headrace channel/tunnel, surge tank, penstock, tailrace.

  • Turbines: Kaplan (propeller with adjustable blades) or Francis (reaction) for low heads; Bulb turbines for very low heads (<10m) in tidal/estuarine.

• Significance:

  • Decentralized Power: Ideal for remote hilly/riverine areas, grid extension difficult.

  • Renewable & Clean: No fuel cost, low emissions.

  • Complementary to Large Hydro: Can be developed faster, with less social/environmental impact.

  • Pumped Storage Potential: Small heads can use existing water bodies for small-scale pumped storage.

VII. Geothermal Energy

Classification of Geothermal Sources

• Based on Temperature/Enthalpy:

  | Type | Temp Range | Typical Resource | Primary Use | | :--- | :--- | :--- | :--- | | Low-Temp | < 100°C | Warm aquifers, coproduced fluids | Direct use (heating, spas, greenhouse) | | Medium-Temp | 100°C - 150°C | Geothermal wells | Binary cycle power (ORC) | | High-Temp | > 150°C | Vapor-dominated (dry steam), liquid-dominated (wet steam) | Flash steam power | | Super-High | > 300°C | Magma, very high-enthalpy wells | Enhanced Geothermal Systems (EGS) research |

• Based on Resource Characteristics:

  • Convective Systems: Natural circulation of water/steam (most common: liquid-dominated, vapor-dominated).

  • Conductive Systems: Heat transfer through solid rock (Hot Dry Rock - HDR/EGS). Requires hydraulic stimulation.

VIII. Advanced and Emerging Technologies

Magneto-Hydrodynamic (MHD) Generation

• Principle: Direct energy conversion. Ionized hot combustion gas (plasma) passed through a magnetic field. According to Faraday's law, moving charged particles (ions/electrons) in a magnetic field ($B$) induce an electromotive force (EMF) perpendicular to both velocity ($v$) and $B$: $$\displaystyle E = v \times B $$.

• Basic Configuration:

  1. Combustor/Seedbed: Air + fuel (e.g., natural gas, coal) + seed material (alkali metal vapor like Cs or K to increase electrical conductivity).

  2. Nozzle: Accelerates hot, conducting plasma.

  3. Channel: Rectangular duct with electrodes on opposite walls (positive/negative) and magnetic field perpendicular to flow and electrodes.

  4. Diffuser: Decelerates plasma, recovers pressure.

  5. Seed Recovery: Condenses and recycles seed material from exhaust.

• Advantages: High theoretical efficiency (50-60%), no moving parts in generator, fast start-up.

• Challenges: High temperature materials (1500-2000°C), seed recovery/corrosion, high magnetic field requirements, integration with conventional steam bottoming cycle.

IX. Power Electronics and Control in Renewable Systems

Electronic Load Controllers (ELC)

• Function: Regulate power output from variable speed renewable sources (especially wind and micro-hydro) to maintain constant frequency/voltage for grid or fixed-speed AC loads.

• Application in Wind/Micro-Hydro:

  • Variable speed turbine generates variable frequency AC.

  • ELC (typically using power electronic converters like AC/DC/AC or DC/AC) converts this to fixed frequency/voltage AC.

  • Dump Load Control: When generator output exceeds load demand, ELC diverts excess power to a resistive dump load (heater) to maintain safe turbine speed and generator frequency. Prevents overspeed.

• Key Components: Power semiconductor switches (IGBTs/MOSFETs), controller (microprocessor), sensors (voltage, current, frequency), dump load resistor.

Thyristors (SCR - Silicon Controlled Rectifier)

• Role in Power Conversion & Control:

  • Phase Angle Control: Varying firing angle ($\alpha$) of thyristor in AC circuit controls RMS output voltage/current. Used in soft starters for induction motors (reducing inrush current) and fan/pump speed control.

  • AC/DC Conversion (Rectifiers): For controlled DC output from AC (e.g., in battery charging, HVDC, DC motor drives).

  • Inverters & Cycloconverters: As switching elements to convert DC to AC or directly change AC frequency.

  • Renewable Specific: Used in grid-tie inverters (as switching devices), MPPT charge controllers, and HVDC links for remote wind/solar farms.

• Key Feature: Latching device (once on, stays on until current falls below holding value). Requires gate pulse to turn on. Cannot be turned off by gate signal.

X. Energy Efficiency in Industrial and Motor Systems

Energy-Efficient Motors

• Factors Affecting Performance & Efficiency:

  1. Stator Losses (I²R): ~35-40% of total loss. Reduced by using thin, high-grade silicon steel laminations and increasing conductor cross-section (larger stator).

  2. Rotor Losses (I²R): ~20-25%. Reduced by optimizing cage bar design/material (copper vs. aluminum).

  3. Core (Iron) Losses: ~15-20% (hysteresis + eddy current). Reduced by thinner, higher-grade electrical steel (low core loss grade), improved stamping/insulation.

  4. Friction & Windage Losses: ~10-15%. Reduced by high-quality bearings, optimized fan design, precision balancing.

  5. Stray Load Losses: ~10-15%. Reduced by optimized magnetic design, manufacturing precision.

• Efficiency Standards: IE (International Efficiency) classes (IE1, IE2, IE3, IE4, IE5). IE3 is "Premium Efficiency" (mandatory in many applications). Efficiency = (Output Power / Input Power) × 100%.

• Other Factors: Proper sizing (avoid under-loading), power factor correction, voltage balance, regular maintenance.

Energy Audit

• Concept: A systematic procedure to obtain information on existing energy consumption patterns, identify areas of energy wastage, and recommend energy conservation measures (ECMs) with cost-benefit analysis. It's the first step in energy management.

• Types of Energy Audit:

  | Type | Depth/Scope | Typical Use | Output | | :--- | :--- | :--- | :--- | | Preliminary/ Walk-through Audit | Quick, visual inspection, utility bill analysis. | Initial screening, low-cost/no-cost ECMs. | List of obvious inefficiencies & potential savings. | | Detailed/Standard Audit | In-depth, measurements, data logging, detailed analysis of all energy systems. | Comprehensive ECM identification. | Detailed report with technical & economic feasibility of all ECMs. | | Targeted Audit | Focuses on specific system/equipment (e.g., compressed air, HVAC). | Deep dive into known problem area. | Detailed recommendations for that system. | | Investment-Grade Audit | Very detailed, long-term data (1+ year), sophisticated modeling. | For large capital projects (>1M USD). | Guaranteed savings performance contract basis. |