Renewable Energy Sources (CE-504 (C)) - Unit 5 Short Notes

UNIT 5: RENEWABLE ENERGY SYSTEMS AND MANAGEMENT

1.0 Introduction & National Context

1.1 Prospects of Non-Conventional Energy Sources in India

• Resource Availability: India has abundant solar (5-7 kWh/m²/day), wind (onshore potential ~300 GW at 80m hub height), biomass (agricultural residues ~500 MT/year), and small hydro (potential ~20 GW) resources.

• Government Policies & Targets:

  • National Solar Mission: Target of 100 GW solar by 2022 (achieved ~70 GW), now aiming for 280 GW by 2030.

  • National Wind-Solar Hybrid Policy: Promotes hybrid parks for better grid stability.

  • National Bioenergy Mission: Focus on biomass pellets, biogas, and 2G ethanol.

  • International Commitments: 50% cumulative electric power installed capacity from non-fossial sources by 2030 (Panchamrit).

• Key Drivers: Energy security (reduce import dependence), climate commitments (NDC targets), declining technology costs, and rural electrification.

1.2 Role and Potential of Renewable Energy

• Global Scenario: RE (mainly solar & wind) is the fastest-growing energy source. Share in global electricity generation ~30% (2023).

• Indian Scenario: RE installed capacity ~190 GW (as of early 2024), ~43% of total installed capacity. Solar and wind dominate.

• Benefits:

  • Energy Security: Reduces dependence on imported fossil fuels.

  • Environmental: Low/zero GHG emissions during operation, reduces air pollution.

  • Economic: Creates jobs in manufacturing, installation, and O&M; stabilizes power costs long-term.

  • Social: Enables decentralized generation, improving access in remote areas.

[!TIP] Exam Focus: Always link India's prospects to specific policies (NSM, NDC) and quantitative targets/resource potentials.

2.0 Solar Energy Systems

2.1 Solar Radiation and Its Variability

Solar radiation reaching Earth's surface (Global Horizontal Irradiance, GHI) is less than the extraterrestrial radiation due to:

1. Atmospheric Scattering: By air molecules (Rayleigh) and aerosols (Mie), redirecting sunlight (diffuse radiation).

2. Atmospheric Absorption: By ozone (UV), water vapor, CO₂ (infrared).

3. Cloud Cover: Major attenuator; reflects and absorbs radiation; causes high variability.

4. Earth's Rotation & Orbit: Causes diurnal (day/night) and seasonal variations; orbital eccentricity causes ~6.8% annual variation.

Key Formula for Clear-Sky Radiation:

$$ I = I_{sc} \left[ 1 + 0.033 \cos\left(\frac{360n}{365}\right) \right] \cos \theta_z $$

Where:

• $$\displaystyle I_{sc} $$ = Solar constant (~1367 W/m²)

• $n$ = Day number

• $$\displaystyle \theta_z $$ = Solar zenith angle

[!TIP] Common Pitfall: Do not confuse extraterrestrial (top of atmosphere) with surface radiation. Variability is primarily due to atmospheric effects (clouds, aerosols).

2.2 Solar Thermal Energy Conversion

Principle: Concentrate sunlight to heat a working fluid (water, oil, molten salt) to generate steam for a turbine (Rankine cycle) or for direct thermal applications (heating, drying).

2.3 Photovoltaic (PV) Systems & MPPT

MPPT (Maximum Power Point Tracking): Algorithm to continuously operate PV array at its Maximum Power Point (MPP), which varies with solar irradiance (G) and cell temperature (T).

Key I-V & P-V Characteristics:

• Short Circuit Current ($$\displaystyle I_{sc} $$): Increases with irradiance.

• Open Circuit Voltage ($$\displaystyle V_{oc} $$): Decreases with temperature.

• MPP: Point on P-V curve where $$\displaystyle dP/dV = 0 $$ (or $$\displaystyle dI/dV = -I/V $$).

Common MPPT Algorithms:

1. Perturb & Observe (P&O): Simple, most common. Perturbs voltage, observes power change. Can oscillate at MPP and fail during rapid irradiance changes.

2. Incremental Conductance (IncCond): More accurate, faster. Uses condition $$\displaystyle dI/dV = -I/V $$ at MPP. More complex implementation.

3. Constant Voltage (CV): Simple, less efficient. Maintains $$\displaystyle V_{pv} \approx k * V_{oc} $$ (k~0.76-0.8). Works only if temperature stable.

[!TIP] Exam Focus: Be prepared to sketch a PV I-V and P-V curve, marking $$\displaystyle I_{sc} $$, $$\displaystyle V_{oc} $$, and MPP. Explain why MPPT is needed (G & T variation).

2.4 Solar Cell Materials and Technologies

Efficiency ($\eta$) defined as:

$$ \eta = \frac{P_{mp}}{G \cdot A_{cell}} \times 100\% $$

where $$\displaystyle P_{mp} $$ is max power, $G$ is irradiance, $$\displaystyle A_{cell} $$ is cell area.

[!TIP] Crucial: Know the dominant commercial technology (c-Si) and one key advantage/disadvantage of at least one thin-film type (e.g., CdTe's low cost vs. toxicity).

3.0 Wind Energy Systems

3.1 Wind Power Generation

Power in Wind:

$$ P_{wind} = \frac{1}{2} \rho A V^3 $$

Where $\rho$=air density, $$\displaystyle A=\pi R^2 $$ (swept area), $V$=wind speed. Power ∝ V³ – critical for site assessment.

Turbine Specifications & Key Terms:

• Rotor Diameter (D): Determines swept area ($A$).

• Hub Height: Higher = better wind resource (less surface friction).

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

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

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

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

• Survival Speed: Extreme wind turbine can withstand (~50-60 m/s).

• Capacity Factor (CF):

$$ CF = \frac{Actual\ Annual\ Energy\ Output}{P_{rated} \times 8760\ hours} \times 100\% $$

Typical onshore CF: 25-40%; offshore: 40-60%.

Power Curve: Graph of electrical power output vs. wind speed at hub height. Shows regions: below $$\displaystyle V_{ci} $$ (0 power), between $$\displaystyle V_{ci} $$-$$\displaystyle V_r $$ (increasing), $$\displaystyle V_r $$-$$\displaystyle V_{co} $$ (constant at $$\displaystyle P_{rated} $$), above $$\displaystyle V_{co} $$ (0).

[!TIP] Exam Focus: Be able to sketch a wind turbine power curve and label $$\displaystyle V_{ci} $$, $$\displaystyle V_r $$, $$\displaystyle V_{co} $$, and $$\displaystyle P_{rated} $$. Remember the $$\displaystyle V^3 $$ dependence.

4.0 Biomass Energy

4.1 Biomass Conversion Processes

A. Thermochemical Conversion (High Temperature, Low Oxygen)

B. Biochemical Conversion (Enzymes/Microorganisms)

[!TIP] Key Distinction: Thermochemical = heat-driven (gas, liquid, char). Biochemical = biological (gas from AD, liquid from fermentation). Biogas (AD) vs. Producer Gas (Gasification) – composition and calorific value differ significantly.

5.0 Ocean Energy

5.1 Wave Energy Conversion

Principle: Convert kinetic & potential energy of ocean surface waves into electricity. Common Devices:

• Oscillating Water Column (OWC): Wave compresses air in chamber, driving a Wells turbine.

• Point Absorber: Buoy moves with waves, drives a linear generator or hydraulic pump.

• Attenuator: Long, multi-segment floating structure (like Pelamis) flexes with wave, driving hydraulic pumps.

• Overtopping Device: Captures wave water in a reservoir, releases through low-head turbine.

Advantages:

• High energy density (energy flux ~30-40 kW/m vs. wind ~1-2 kW/m²).

• Predictable (weather-driven, hours-ahead).

• Abundant resource, especially western coasts.

Limitations:

• Harsh Environment: Corrosion, storms, biofouling → high maintenance, short lifespan.

• Variability: Irregular, multi-frequency waves.

• Technology Maturity: Mostly TRL 4-6 (prototype/demonstration). Few grid-connected plants.

• Environmental: Potential impact on marine ecosystems, navigation, fisheries.

• Grid Connection: Often remote from demand centers.

5.2 Ocean Thermal Energy Conversion (OTEC)

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

Carnot Efficiency Limit:

$$ \eta_{Carnot} = \frac{T_{warm} - T_{cold}}{T_{warm}} $$

(T in Kelvin). With $\Delta T$ ~20K, $$\displaystyle \eta_{Carnot} $$ ~6-7%. Actual cycle efficiency ~40-50% of Carnot.

Advantages of Closed-Cycle:

• Higher thermodynamic efficiency due to optimized working fluid.

• Smaller, more efficient turbine (vapor density higher than steam).

• No direct contact of working fluid with seawater in cycle → less scaling/corrosion in heat exchangers.

• Can be hybridized with other RE sources.

[!TIP] Critical: OTEC requires $\Delta T$ ≥ 20°C (tropical/subtropical zones). Closed-cycle is more efficient and practical for power generation. Open-cycle's main product is freshwater.

6.0 Geothermal Energy

6.1 Geothermal Resource Classification

Based on hydrothermal state and temperature:

Temperature-Depth Relationship: Average geothermal gradient ~25-30°C/km. Economic resource typically requires T > 100°C at drillable depths (<3-4 km).

[!TIP] Mnemonic: Dry Steam (direct vapor), Flash Steam (liquid flashes), Binary (separate fluid loop). Binary is key for low-temperature resources.

7.0 Hydro Energy (Small-Scale)

7.1 Small Head Hydro Power Development

Definition: "Small Hydro" (SHP) typically < 25 MW (India: < 10 MW for 'mini', < 100 kW for 'micro'). "Small Head" refers to low hydraulic head (< 15-20m).

Site Selection Criteria:

1. Hydrology: Reliable year-round flow (Q), good Flow Duration Curve (FDC). High Design Flow (Q_d) for >90% of time.

2. Head (H): Available gross head (difference in water levels). Net head = Gross head - losses.

3. Topography & Geology: Suitable for penstock/turbine house; stable foundations.

4. Accessibility: For construction & O&M.

5. Environmental & Social Impact: Minimal displacement, fish passage, flow requirements for ecology.

6. Grid Proximity: Cost of evacuation.

Turbine Types for Low Head (Kaplan & Cross-Flow):

Environmental & Social Considerations:

• Positive: Low GHG, renewable, can provide irrigation/drinking water.

• Negative: Alters river flow, affects sediment transport, fish migration, local ecology. Requires environmental flow (e-flow) release. Social impact from land submergence (though small for low-head).

[!TIP] Distinguish: Kaplan = very low head, high flow, adjustable blades. Cross-Flow = simple, robust, good for variable flow (common in micro-hydro).

8.0 Power Electronics for Renewable Energy

8.1 Electronic Load Controllers (ELC)

Function: Protect self-excited induction generators (SEIG) used in wind/biomass (micro-hydro) systems from over-speed and over-voltage when load is disconnected or reduced.

• Principle: Maintains constant power output by diverting excess power to a dump load (resistive heater) when consumer load is low.

• Operation: Senses voltage/frequency. If they rise above setpoint (indicating less load), ELC switches in dump load (via thyristors/relays) to keep generator loading stable.

• Need for SEIG: SEIGs are simple, rugged, cheap. But their excitation (capacitors) is load-dependent. Sudden load drop → voltage/frequency spikes → damage. ELC solves this.

Block Diagram:

SEIG → Voltage/Frequency Sensor → Controller → Switching Circuit → Dump Load (Heater)

                                                              ↑

                                                          Consumer Load

8.2 Thyristor-Based Power Control

Thyristor (SCR - Silicon Controlled Rectifier): 4-layer (PNPN), 3-terminal device. Acts as a switch that turns ON when gate pulse is applied and current > holding current. Turns OFF when current falls to zero (natural commutation in AC).

Key Applications in RE:

1. Phase Angle Control (AC Power Control): Vary firing angle ($\alpha$) of thyristor in AC circuit to control RMS voltage/current to load (e.g., heater in ELC, fan speed).

   • Output voltage: $$\displaystyle V_{rms} = \frac{V_m}{\sqrt{2}} \sqrt{1 - \frac{2\alpha}{\pi} + \frac{\sin 2\alpha}{\pi}} $$ (for resistive load).

2. AC/DC Conversion (Rectification): Single-phase/three-phase rectifiers. Controlled rectifiers allow variable DC output voltage.

3. Inverters (DC/AC): Thyristors (or IGBTs/MOSFETs now) form the switching legs of voltage source inverters (VSI) to generate AC from DC (PV, battery).

4. AC Voltage Controllers: For soft-starting induction motors (wind turbine pumps).

[!TIP] Crucial: Thyristor is a latching switch – needs gate pulse to turn ON, current must drop to zero to turn OFF. This makes it ideal for AC circuits (natural zero-crossing commutation).

9.0 Energy Management and Efficiency

9.1 Strategies for Efficient Energy Management

1. Demand-Side Management (DSM): Modify consumer demand pattern.

   • Peak Clipping: Reduce load during peak times (e.g., time-of-use tariffs, direct load control).

   • Valley Filling: Increase off-peak load (e.g., storage charging, industrial processes).

   • Load Shifting: Move load from peak to off-peak.

2. Integration of RE Sources:

   • Hybrid Systems: Combine complementary sources (solar+wind, solar+diesel) for better reliability.

   • Grid Integration: Use inverters with grid-support functions (voltage/frequency ride-through, power factor correction).

   • Energy Storage: Batteries, pumped hydro to manage variability.

3. Grid Stability: RE integration requires:

   • Inertia & Frequency Support: From synchronous generators or grid-forming inverters.

   • Voltage Control: Reactive power support from inverters (Q control).

   • Forecasting: Improve scheduling with solar/wind forecasts.

9.2 Energy-Efficient Motors

Efficiency ($\eta$):

$$ \eta = \frac{Output\ Power}{Input\ Power} = \frac{P_{out}}{P_{in}} $$

Losses (in % of input):

1. Stator Copper Loss ($$\displaystyle I^2R $$): ~30-40%. Reduced by larger conductor cross-section.

2. Rotor Copper Loss (for wound rotor): ~20-25%. Reduced by deeper bars (squirrel cage).

3. Core (Iron) Loss: ~15-20%. Hysteresis + Eddy current. Reduced by high-grade silicon steel (thin laminations).

4. Friction & Windage: ~10-15%. Bearings, air drag.

5. Stray Load Loss: ~10-15%. Harmonic effects, etc.

Premium Efficiency Standards (IE Codes - IEC 60034-30):

• IE1: Standard Efficiency

• IE2: High Efficiency

• IE3: Premium Efficiency (mandatory in many applications)

• IE4: Super Premium Efficiency

• IE5: Ultra Premium (emerging)

Factors Affecting Efficiency:

• Design (magnetic/electrical loading, slot/pole combination)

• Materials (steel grade, copper quality)

• Manufacturing (laminations insulation, air gap uniformity)

• Operating Conditions (Voltage/frequency deviation, derating)

9.3 Energy Audit

Concept: Systematic examination of energy use & flows to identify opportunities for energy conservation and cost savings.

Types of Energy Audit (by depth):

Methodology (AS/NZS 3598 or ISO 50002):

1. Planning & Preparation: Define scope, team, data requirements.

2. Data Collection: Energy bills, process data, equipment inventory, operating schedules.

3. Analysis: Identify energy uses, benchmark (against norms/peers), calculate savings potential for each ECO.

4. Reporting: Present findings, recommendations, action plan, financial analysis.

[!TIP] Hierarchy: Preliminary → Detailed → Investment-Grade. Investment-Grade is required for major projects seeking external finance.

10.0 Advanced/Alternative Technologies

10.1 Magneto-Hydrodynamic (MHD) Generation

Principle: Direct conversion of thermal energy of hot, ionized gas (plasma) into electricity, bypassing mechanical rotation (no turbine). Working:

1. Ionization: Combust fossil fuel (coal, natural gas) with seed material (e.g., Potassium carbonate, Cs) to produce high-temperature (~2000-3000K), electrically conductive plasma.

2. Acceleration: Plasma flows at high velocity through a magnetic field (from superconducting magnets).

3. Electrode Interaction: According to Faraday's Law of Induction, moving conductor (plasma) in magnetic field induces an EMF perpendicular to both flow and field directions.

4. Power Extraction: Electrodes placed on channel walls collect the DC current directly.

Key Components:

• Combustor/Ionizer: With seed injection & recovery system.

• MHD Channel: Non-conducting walls (ceramic), electrode pairs.

• Superconducting Magnets: High B-field (3-5 T) for high voltage output.

• Seed Recovery System: Critical for cost (seeds are expensive) and environmental (remove alkali metals from exhaust).

Advantages:

• High theoretical efficiency (50-60% for combined cycle with bottoming steam cycle).

• No moving parts in generator → potentially higher reliability, faster start-up.

• Compact size.

Current Status:

• Experimental/Prototype Stage. Major programs in USA (1970s-80s), Soviet Union, India (BARC, 1980s-90s).

• Challenges: Material science (channel walls at high T, corrosion), seed recovery cost, magnet system complexity, integration with bottoming cycle.

• Not commercially deployed. Research continues for niche applications (space power, military).

[!TIP] Remember: MHD is a topping cycle for a fossil fuel plant. It generates DC directly. Seed material (alkali metal) is essential for ionization but must be recovered.