Module 12: Renewables & Ocean | Power Plant Engineering

Module 12A: Wind Turbine Components & Types

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Remaining Components

6. Hub: Central solid part connecting blades to low speed shaft.

7. Propeller: Revolving shaft with blade. Two types: Up wind & Down wind.

8. Needle: Assembly consisting of gears, bearing, and generator mounted in housing.

9. Yaw Control: Orients the axis of the wind turbine to keep rotor facing the wind.

Horizontal vs Vertical Axis (HAWT vs VAWT)

HAWT (Horizontal Axis): Parallel to wind direction. High efficiency (35-45%), needs yaw control, very tall, used for large utilities.
VAWT (Vertical Axis): Perpendicular to wind direction. Lower efficiency (15-35%), omnidirectional, lower height, used for small scale.
i. Savonius Rotor (15% η, S-shape section, self-starting)
ii. Darrieus Rotor (35% η, Troposkein shape, not self-starting)

Site Selection & Wind Shear

Four suitable sites: Plane land, Hill top, Sea shores, Off shore.

$$ P \propto kV, \quad V \propto H^{\alpha} $$

$$ V_H = V_{ref}\left(\frac{H}{H_{ref}}\right)^\alpha $$

Hellmann exponent $\alpha \approx 1/7$

HAWT (45% η)

Darrieus (35% η)

Savonius (15% η)

≈≈≈ Wind Direction ≈≈≈

Module 12B: Biomass Energy

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Biomass Definition & Sources

Biological material based in carbon, hydrogen, and oxygen. Six direct sources: Garbage, Wood, Plants, Waste, Land fill gases, Alcohol fuels.

Three Conversion Processes

1. Thermal Conversion:
Combustion ($\eta \approx 20\text{-}25\%$), Pyrolysis (without $O_2 \rightarrow$ Bio-oil), Gasification ($\rightarrow$ Syngas: CO + $H_2$).

2. Chemical Conversion:
Transesterification (Vegetable oil $\rightarrow$ Biodiesel $C_{17}H_{35}COOCH_3$).

3. Biochemical/Anaerobic Digestion:

$$ C_6H_{12}O_6 \xrightarrow{\text{bacteria}} 3CH_4 + 3CO_2 $$

Biogas ($\eta \approx 20\text{-}30\%$): $60\%\,CH_4 + 40\%\,CO_2$. CV is $20\text{-}25$ $\text{MJ/m}^3$.

Module 12C: Ocean Thermal Energy (OTEC)

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Temperature Gradients

Uses the thermal gradient between warm surface waters ($26^\circ\text{C}$) and cold deep waters ($4^\circ\text{C}$ at $800\text{-}1000$m).

$$ \Delta T = T_{warm} - T_{cold} = 26 - 4 = 22^\circ\text{C} $$

Efficiency Limit

$$ \eta_{Carnot} = 1 - \frac{T_{cold}}{T_{warm}} $$

$$ = 1 - \frac{277\,\text{K}}{299\,\text{K}} = 7.36\% $$

Actual OTEC efficiency is even lower, around 3-5%. Requires $\Delta T > 20^\circ\text{C}$ to be viable.

Cycles & Applications

1. Closed Cycle: Uses NH$_3$ working fluid. Warm sea $\rightarrow$ Evaporator $\rightarrow$ Turbine $\rightarrow$ Condenser (Cold sea).

2. Open Cycle: Seawater is flash evaporated. Steam drives turbine, then condensed as fresh water.

Surface Temperature $T_{warm}$ (°C): 26

Deep Sea Temp $T_{cold}$ (°C): 4

$\Delta T = 22^\circ\text{C}$

Carnot Max $\eta$

7.36%

Est. Actual $\eta$

3.68%

Viable Gradient

Module 12D: Tidal & Wave Energy

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Tidal Generation & Formulas

705 tidal cycles annually. Minimum head range $h > 5$m needed.

$$ E_{tidal} = \frac{1}{2}\rho g A h^2 \quad (\text{per cycle}) $$

$A$: Area of basin (m$^2$)
$h$: Head range between high & low tide

Tidal Stream Generators (TSG)

Like underwater wind turbines. Water density $\rho \approx 1025$ kg/m$^3$, which is 854× greater than air, meaning same size rotors yield massively more scale.

$$ P_{TSG} = \frac{1}{2}\rho_{water} A v^3 C_p $$

Wave Energy

$$ P_{wave} \approx 0.5 \times H_s^2 \times T_e \quad \text{kW/m} $$

Where $H_s$ = Sig. wave height (m), $T_e$ = Energy period (s).

Head difference drives turbines beneath the barrage during incoming/outgoing tides.

Module 12E: Tidal Barrage: Complete Analysis

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2. Tidal Barrage

$$ \text{Makes use of POTENTIAL energy} $$

$$ \text{= difference in height (head)} $$

Usually built across an estuary or bay. Consists of turbines, sluice gates, embankments, and ship locks.

Tidal Barrage Energy Formula

$$ \boxed{E = \frac{1}{2}\rho g A h^2} $$

$\rho$ = Sea water density ($1025\,\text{kg/m}^3$)
$g$ = 9.81 m/s²
$A$ = Area of tidal basin (m²)
$h$ = Tidal range / head height (m)

$$ P = \frac{E}{T_{cycle}} = \frac{\rho g A h^2}{2T} $$

$T_{cycle} \approx 12.4\,\text{hrs}$, $E_{annual} = E_{per\,cycle} \times 705 \times 2$

Two Types of Systems

Single Basin: One basin. Intermittent power (ebb or flood).

Double Basin: Two basins (high & low). Continuous power. Costlier.

Barrage vs Tidal Stream (TSG)

Type	Barrage	TSG
Energy	Potential (½ρgAh²)	Kinetic (½ρAv³Cp)
Structure	Dam + gates	Underwater rotors
Cost/Impact	High / High	Moderate / Low

h = 8m

E = 80kWh (scales h²)

Module 12F: Dynamic Tidal Power (DTP) & Challenges

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Dynamic Tidal Power (DTP)

Untried but promising technology. A 30-50 km dam built perpendicular to the coast blocks coast-parallel tidal currents, creating a pressure head.

$$ E_{DTP} = E_{potential} + E_{kinetic} $$

$$ = \frac{1}{2}\rho g A h^2 + \frac{1}{2}\rho A v^2 L $$

Advantage: No estuary enclosed (lower eco impact).
Potential: 100-300 GW (Yellow Sea/Korea).

7 Challenges of Wave/Tidal Energy

1. High Cost 2. Variable Supply 3. Limited Locations 4. Harsh Marine Env 5. Eco Impact (Barrage) 6. Maturing Tech 7. Storm Damages

Marine Corrosion Defense

Seawater ($35\text{g/kg}$ salinity) is highly corrosive. Equipment relies on Titanium alloys, fiber composites, and Cathodic Protection (sacrificial anodes like Zn or Mg).

DTP blocks natural flow, forcing water through base turbines.

Module 12G: Turbines for Tidal Power

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Specific Speed & Selection

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

Higher $N_s$ → suitable for low head (ideal for tidal).

Turbine	Head (H)	η	Blades
Kaplan	2-40m	90%+	Adjustable
Propeller	5-30m	85-92%	Fixed
Bulb	2-10m	88-93%	Fixed/Adj

Kaplan: Variable pitch. η remains optimal across varying tides. $N_s = 300\text{-}900$.
Propeller: Drops efficiency fast at off-design conditions. Low cost.
Bulb: Compact axial flow. Generator inside a waterproof bulb. Used at La Rance.

Head Fluctuation:

Module 12H: Complete Tidal Energy Reference

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Global Reference

$\rho_{seawater}$ 1025 kg/m³

Tidal cycles/year 705

Min viable head $h > 5\text{m}$

India Tidal Potential

Gulf of Khambhat: $h=11\text{m}$, $\approx 7000\,\text{MW}$
Gulf of Kutch: $h=8\text{m}$, $\approx 900\,\text{MW}$
Sunderbans: stream currents, small scale
Total India Potential: $\approx 8000\,\text{MW}$

World's Operating Plants

Sihwa Lake (Korea): 254 MW (Largest)
La Rance (France): 240 MW (First)
Annapolis (Canada): 20 MW
MeyGen (Scotland): 6 MW (TSG Stream)

Module 12 Master Refresher

What type of energy does a Tidal Barrage utilize?