4819 - CA2

Sun uneven heating

Wind is a renewable energy source driven by the _______________ of the Earth.

Wind Generation

Sun heats land faster than water → Warm air rises over land → Cool air replaces it → Moving air = Wind.

large increase in power

Small increase in wind speed leads to...

Temperature, Pressure and Altitude

Air Density is Affected by...

Swept Area (A)

Increases with blade diameter².

Betz's Law

Maximum efficiency of a wind turbine rotor is 59.3%

45-50%

Real turbines achieve_____ (≈ 80% of Betz limit).

Pitch, Torque, Yaw

Control strategies to approach Betz limit

Tip Speed Ratio (TSR)

Determines rotor efficiency.

Too low

___ TSR → wind passes without energy capture.

Too high

___ TSR → turbulence, energy loss.

HAWT

Needs Yaw control, Horizontal Axis Wind Turbine

VAWT

Vertical Axis Wind Turbine

Onshore

Cheaper, more mature wind turbine type

Offshore

Higher wind speeds, more expensive to install wind turbine type.

Nacelle

component that Encloses power train.

Tower

component that Raises rotor for higher wind.

height

Wind speed increases with ____.

friction

Smoother terrain → less _____.

Twisting blade

maintains optimal angle along its length

Disadvantages of Wind Energy

Intermittent wind, Noise, Wildlife impact (birds), Visual/aesthetic objections, High initial cost, Grid connection challenges

Angle of Attack (AoA)

angle between blade chord and wind direction.

Angle of Attack (AoA)

Controls lift-to-drag ratio

Stall and loss of lift

Too high AoA leads to

Pitch control

adjusts AoA to regulate rotor speed and power

Region A

Below cut-in speed - No power

Region B

Between cut-in & rated speed - Max power tracking

Region C

Rated to cut-out speed - Constant rated power

Region D

Above cut-out speed - Turbine stops

Region B

Maximum Wind Power Tracking Region (MWPTR)

Region C

Rated Power Output Region (RPOR)

TSR drops

If wind increases beyond rated speed, __________, Coefficient of performance (Cp) drops

Pitch angle increased

If wind increases beyond rated speed, ____________ → ↓ AoA → ↓ lift → maintain power at rated level

Turbine speed control goals

Optimize TSR for max efficiency, Protect turbine during storms

Generator speed control goals

Maintain constant voltage/frequency (50/60 Hz)

Pitch Control

Wind Power Shedding Method that Actively reduce AoA

Active Stall Control

Wind Power Shedding Method that Induces stall to shed power

Passive Yaw Control

Wind Power Shedding Method that Misaligns small turbines from wind

Weibull Distribution

Shape (k), scale (c) parameters -> k=2 → balanced; k=1 → too many low-speed events

Rayleigh Distribution

Simplified Weibull (k = 2), Average Power = 1.91 × Power at avg. wind speed

Clustered turbines

cheaper installation & maintenance.

Downwind turbines

__________ face: Turbulence (wake effect) and Reduced energy capture

Synchronous

Generator Type with Constant speed, needs DC field, gearboxes needed

Asynchronous (Induction)

Generator Type with Rotor spins faster than field, self-exciting

DFIG (Doubly-fed Induction Generator)

Generator Type with Power electronics for wide speed range

PMG (Permanent Magnet Generator)

Generator Type with No external excitation, efficient

Drivetrain & Gearbox

Converts low-speed rotor motion (~15 RPM) to high-speed (~1500 RPM)

Flexible shaft couplings

reduce stress & vibration.

Aerodynamic, Mechanical and Electrical

Type of Braking systems

No power is generated

If wind speed doesn't change across a rotor, then ...

Sunlight

Wind energy does not depend on _____1

16/27

Betz's limit is ____ close to 60%

Bet'z limit

occurs when downstream to upstream velocity ratio is at 1/3

3

Modern wind turbines have ______ blades, not many

Cube

Wind power is proportional to the ____ of wind velocity

Single

Some HAWT designs exist with _______ blade, possibly practical

Increases

Wind velocity ________ with altitude due to lower friction

Lift and Drag

two main forces acting on turbine blades

Sun

is the main cause of wind formation

4x

Doubling rotor radius → ___ power.

VAWT

wind turbine type with generator/control at bottom.

1

Probability of wind < ∞ is __

Geothermal Energy

Heat from Earth's interior due to radioactive decay, Reaches the surface via conduction or convective currents.

Geothermal Energy location

Found at plate boundaries (volcanoes, hot springs, geysers) and in permeable rock regions (~10 km² area, ~5 km depth).

Ring of Fire

Regions with steep geothermal gradients (~100°C/km). Steam and superheated water released under pressure.

Main Uses of Geothermal Energy

Electricity generation (>150°C steam/water), Direct heat use (hot water at 50-70°C), Heat pumps (ambient heat from shallow ground or water).

Dry Steam

Geothermal powerplant that uses Steam directly drives turbine.

Flash Steam

Geothermal powerplant that uses High-pressure water flashed into steam.

Binary Cycle

Geothermal powerplant that uses Water (107-182°C) heats low-boiling working fluid and Uses Organic Rankine Cycle (ORC).

Geothermal powerplant types

Dry steam, Flash steam, Binary Cycle

Direct Heat Applications

Bathhouses, greenhouse heating, fish farming, Crop drying, industrial process heat, district heating.

Geothermal energy advantages

Low emissions: Only 5% of coal plants' CO₂ & SO₂.

✅ Low land use.

✅ Reliable base load energy (24/7, year-round).

✅ Stable electricity prices (no fuel cost).

✅ Minimal maintenance costs.

✅ Energy security (uses local resources).

✅ Low noise.

Geothermal energy disadvantages

❌ Freshwater use & chemical pollution (arsenic, mercury, CO₂, methane).

❌ Geographical limitations (only feasible in tectonically active regions).

❌ Seismic risks (induced earthquakes).

❌ High initial costs (exploration, drilling, system design).

Tidal Power

Caused by gravitational pull of the Moon (main) and Sun.

2

Each location experiences __ high & __ low tides daily.

Spring Tides

Moon, Sun, and Earth aligned → stronger tides.

Neap Tides

Moon and Sun at right angles → weaker tides.

Tidal Barrage

A dam across estuaries, lagoons, or bays -> Gates + turbines convert water level difference into power.

Tidal Barrage features

Works both ways (incoming and outgoing tides). Similar to hydropower dam but with tides.

Tidal Stream Generators

Like underwater wind turbines.

Use moving tidal currents to rotate turbines.

Horizontal & Vertical axis options.

Tidal Stream Generators Risks

Marine animal blade strikes

Marine fouling (biofilm)

Tidal Power Advantages

Unlimited & predictable

80% efficiency possible

Operates day and night

Very long lifespan

Less affected by climate variations

Tidal Power Disadvantages

High construction & maintenance cost

Very site-specific

Requires large tidal ranges (few global locations)

Environmental disruption (fish migration, estuary ecosystems)

Transmission issues (far from demand)

Wave energy

Caused by wind blowing over the ocean, Devices use heave, pitch, or pressure differences.

Shoreline and Offshore Devices

Wave energy devices location types

Shoreline Devices

Close to grid

Easier to maintain

Less vulnerable to extreme waves

Limited by geography & aesthetics

Offshore Devices

Harvest more energy

More powerful waves

Costly to build & maintain

Require strong design

Attenuator

Floats parallel to waves, bends with them. (Wave energy device type)

Point Absorber

Small device moves vertically with waves.

Direction of wave not important.

(Wave energy device type)

Wave Energy Advantages

Highest energy density among renewables.

Up to 90% operational time.

Clean, predictable, abundant.

Small physical footprint.

Wave Energy Disadvantages

Irregular wave patterns need: Energy storage or Device arrays for smoothing output.

Design must tolerate rare 2000 kW/m waves.

Highly corrosive environment = material challenges.

Conversion of random motion (0.1 Hz) is hard.

Minimal environmental disruption in use.

Motion used to generate electricity via internal magnets

Ocean Thermal Energy Conversion (OTEC)

Uses temperature difference between: Surface water (~25°C) and Deep water (~5°C)

Works best in equatorial regions where ΔT ≥ 20°C.

Closed-Cycle

Uses working fluid (e.g., ammonia) with low boiling point.

Heated by surface water, vaporized → spins turbine → cooled by deep water.

Fluid is reused.

(OTEC Type)

Open-Cycle

Uses seawater directly.

Evaporates under low pressure → drives turbine → re-condensed.

Also produces freshwater (desalination).

(OTEC Type)

OTEC Advantages

Continuous power generation (unlike solar/wind).

Desalination in open-cycle systems.

Cold water reuse: for cooling, refrigeration.

No hazardous fluids (open cycle).

Useful in remote tropical islands.

OTEC Disadvantages

High cost (infrastructure, pumping energy).

Limited to tropical oceans (ΔT ≥ 20°C).

Offshore transmission challenges.

Environmental concerns (pipe disruption, leakage).

Open-cycle Disadvantage

chamber must be perfectly sealed (OTEC Type Disadvantage)

Extraterrestrial Radiation (Gon)

Solar radiation outside Earth's atmosphere.

Direct Radiation (GD)

Solar beam reaching surface without scattering.

Diffuse Radiation (Gd)

Scattered by air molecules/clouds.