Wind Energy

Flashcards based on the MAC316 Wind Energy lectures covering source of power, basic turbine aspects, systems, drivetrain design, dynamics, and design loads.

What is the primary source of wind on Earth?

Uneven solar heating of the Earth’s surface creates pressure differences, causing air movement (wind). The Coriolis effect deflects winds right in the Northern Hemisphere and left in the Southern Hemisphere.

What are the key advantages of wind power?

Renewable and sustainable, no greenhouse gas emissions during operation, no fuel transportation/storage needed, wind is free and widely available, and it is useful for remote areas.

What are the major disadvantages of wind power?

Intermittent and unpredictable output, energy storage may be required, high installation and infrastructure costs, and environmental concerns such as noise, visual impact, and wildlife.

What are the main types of wind turbines?

1. Horizontal Axis Wind Turbines (HAWTs) 2. Vertical Axis Wind Turbines (VAWTs)

What are the characteristics of modern Horizontal Axis Wind Turbines (HAWTs)?

Low solidity, high rotational speed, aircraft-propeller-like blades, high power output, and the most common commercial design.

Give examples of Vertical Axis Wind Turbines (VAWTs).

Darrieus turbine and Savonius turbine.

What were the approximate sizes of wind turbines in the 1980s?

Power output was below $100\,kW$ and rotor diameters were around $10\text{–}15\,m$.

What are the top capital costs in offshore wind farms?

Wind turbines, foundations, and electrical export systems.

Why is offshore wind attractive?

Higher and steadier wind speeds, larger turbine installation possible, and greater energy production.

What is Hornsea 2?

The world’s largest offshore wind farm (as of 2026), with $1.32\,GW$ capacity and $165$ turbines.

What is cut-in wind speed?

The minimum wind speed at which a turbine begins generating usable power.

What is cut-out wind speed?

The maximum wind speed at which a turbine safely operates before shutdown.

What is rated wind speed?

The wind speed at which the turbine first reaches its rated power.

What is rated power?

The maximum continuous electrical power output of a wind turbine.

State the Atmospheric Boundary Layer (ABL) power law.

$$\frac{V_z}{V_{ref}} = \left(\frac{Z}{Z_{ref}}\right)^{\alpha}$$

What is the typical value of the wind shear exponent $\alpha$ for open land?

Approximately $0.15$.

What distribution is commonly used to model wind speed variations?

Weibull distribution.

What is V50?

The wind speed exceeded for $50\%$ of the time during a year.

State the kinetic power available in wind.

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

Why is wind speed so important in wind energy?

Because wind power is proportional to the cube of wind speed.

What is the power coefficient, Cp?

The ratio of extracted turbine power to total available wind power.

What is the Betz limit?

The theoretical maximum turbine efficiency: $C_{p,max} = \frac{16}{27} = 59.3\%$

Why are aerofoils used in wind turbine blades?

They provide high lift-to-drag ratios, maximizing energy extraction.

Define capacity factor.

$$\text{Capacity Factor} = \frac{E_{year}}{E_{rated}} \times 100\%$$

What does IEC 61400-1 define?

International wind turbine design requirements and safety standards.

What are IEC wind turbine classes based on?

Reference wind speed and turbulence intensity.

What happens to torque when turbine size increases?

Torque increases significantly with rotor radius.

What are the four major parts of a wind turbine?

Foundation, tower, nacelle, and rotor.

What components form the rotor?

Blades and hub.

What is inside the nacelle?

Main shaft, gearbox, generator, transformer, and control systems.

What does the pitch drive do?

Adjusts blade pitch angle for power and load control.

What is the purpose of the yaw drive?

Rotates the nacelle to face the wind direction.

Name common fixed offshore substructures.

Monopile, Jacket, and Twisted jacket.

Name common floating offshore substructures.

Semisubmersible platform, Tension-leg platform, and Spar buoy.

What are the key design considerations for commercial wind turbines?

Power rating, rotor speed, blade control, tip speed ratio, generator type, and gearbox requirement.

Define tip speed ratio.

$$\mu = \frac{\omega R}{V}$$

Why are variable-speed wind turbines advantageous?

They operate at optimum tip speed ratio for maximum efficiency and reduced drivetrain loads.

How is rotor speed controlled below rated wind speed in variable-speed turbines?

Rotor speed changes proportionally with wind speed to maintain optimum tip speed ratio.

What happens above rated wind speed?

Blade pitch control limits power and torque to rated values.

Why do higher tip speed ratios reduce drivetrain weight?

Higher rotational speed reduces torque for the same power level.

What is a wind turbine drivetrain?

All rotating components excluding the rotor hub, including shafts, gearbox, brakes, couplings, and generator.

What is an indirect drivetrain?

A drivetrain using a gearbox between rotor and generator.

What is a direct drivetrain?

A drivetrain without a gearbox, using a low-speed multi-pole generator.

What are the advantages of direct-drive systems?

Fewer moving parts, lower maintenance, and no gearbox failures.

What is the main disadvantage of direct-drive systems?

Large and heavy generators.

What was the gearbox ratio of the NREL 750 kW turbine?

Approximately $1:81.5$.

What gearbox stages were used in the NREL 750 kW turbine?

One planetary stage and two parallel shaft stages.

What is synchronous speed?

$$n_{sync} = \frac{120f}{P}$$

What are the most commonly used generators for variable-speed turbines?

Synchronous generators (SG) and Induction generators (IG).

What does PMSG stand for?

Permanent Magnet Synchronous Generator.

What does SQIG stand for?

Squirrel Cage Induction Generator.

What does WRIG stand for?

Wound Rotor Induction Generator.

What is a key disadvantage of SQIG systems?

They require full-scale power electronic converters and reactive power support.

What is a major advantage of WRIG systems?

Smaller converters (approximately $1/3$ rated power), reducing cost and weight.

What does DFIG stand for?

Doubly Fed Induction Generator.

What is the key feature of a DFIG system?

Power can flow into or out of the rotor through converters.

Why are variable-speed turbines more efficient than fixed-speed turbines?

They maintain operation near optimum Cp over varying wind speeds.

What are the main components of a conventional wind turbine drivetrain?

Main shaft, bearings, gearbox, brake, generator, and couplings.

What is the main purpose of a gearbox?

Increase low rotor speed to high generator speed.

Why are planetary gear stages commonly used?

They provide high power density and compact design.

What are the three main gears in a planetary stage?

Sun gear, planet gears, and ring (annulus) gear.

What are key considerations in gearbox design?

Torque capacity, reliability, weight, efficiency, fatigue life, and load distribution.

Why are wind turbine gearboxes highly stressed?

Due to fluctuating wind loads and transient operating conditions.

What is the function of the high-speed shaft?

Connect the gearbox output to the generator.

Why is drivetrain dynamics important?

To avoid resonance, vibration, and fatigue failures.

What are the two main dynamic properties of drivetrain systems?

Inertia and stiffness.

What does shaft stiffness influence?

Torsional vibration and natural frequency.

What happens if excitation frequency matches natural frequency?

Resonance occurs, causing large vibrations.

Why are equivalent inertia models used?

To simplify complex drivetrain dynamic analysis.

What causes torsional oscillations in wind turbine drivetrains?

Variable aerodynamic torque and gearbox interactions.

What is a free vibration natural frequency?

The frequency at which a system naturally oscillates without external forcing.

Why are design loads important in wind turbine engineering?

They ensure structural safety, reliability, and long service life.

What types of loads act on wind turbine components?

Aerodynamic loads, gravitational loads, inertial loads, operational loads, and extreme loads.

What are fatigue loads?

Repeated fluctuating loads that can cause crack growth and failure over time.

What are extreme loads?

Rare high loads caused by storms, gusts, faults, or emergency shutdowns.

Which components are highly affected by fatigue loading?

Gearbox, bearings, shafts, and blades.

Why must wind turbine gearboxes be designed for transient loads?

Wind conditions constantly fluctuate, creating dynamic torque variations.

What international standard is commonly used for wind turbine design loading?

IEC 61400 series.

What is the purpose of load case analysis?

To evaluate turbine response under different operational and extreme conditions.

What is the ultimate goal of wind turbine mechanical design?

Safe, efficient, reliable, and cost-effective energy generation over the turbine lifetime.