Wind: Causes and Wind Rose Diagrams

• Wind is a natural movement of air at any velocity, especially as a current from a specific direction.
• Causes of wind:
  • Unbalanced radiation insolence
  • Earth's rotation

Unbalanced Radiation Insolence

• Earth's surfaces absorb and reflect solar radiation differently causing air temperature to vary.
• Air pressure is directly proportional to temperature.
• Mechanical instability results from air temperature variation on surfaces.
• To maintain mechanical equilibrium, high-pressure air moves from regions of low pressure and conversely.
• The resulting movement is wind.

Earth's Rotation

• Earth's rotation affects the direction of wind.
• The Coriolis Effect is the tendency for any moving body on or above the earth's surface to drift sideways from its course because of the earth's rotation.
• Northern Hemisphere deflection is right.
• Southern Hemisphere deflection is left.
• The Coriolis Effect stops air at roughly 30° latitude.

Wind Rose Diagrams

• A wind rose diagram is a tool which graphically displays wind speed and wind direction at a particular location over a period of time.

Windmills vs Wind Turbines

• Windmills convert kinetic energy of air into mechanical energy.
• Wind turbines convert kinetic energy of air into electricity.

Types of Wind Turbines

• Horizontal Axis Wind Turbine (HAWT)
• Vertical Axis Wind Turbine (VAWT)

Comparison of Horizontal Axis Wind Turbines (HAWT) and Vertical Axis Wind Turbines (VAWT)

Advantages of HAWTs:

• Higher power generation captures higher wind speed
• Greater efficiency

Disadvantages of HAWTs:

• More complicated blade design
• The turbine's axis must always be aligned with wind direction.
• Difficult access to generator repair

Advantages of VAWT:

• The generator is placed on the ground making it easy to maintain.
• No yaw mechanism is needed for wind direction
• Easier to design blades

Disadvantages of VAWT:

• Lower wind speeds at ground level
• Less efficient
• Requires an initial force during start-up.

Operating Principle of Wind Turbines

• Wind power is the fastest-growing energy source.
• Turbines are mounted on towers 100 or more feet above the ground for faster and less turbulent winds.
• When the blades start moving, they spin a shaft that leads to a generator.
• The generator consists of a conductor, such as a coiled wire, surrounded by magnets.
• The rotating shaft turns the magnets around the conductor, generating an electrical current.
• Sensors cause to turbine to rotate to face into the wind, and the blades change the angle.
• The blades are flexible and stop spinning if the wind is too strong.
• Wind turns the turbine blades, which then spin the shaft
• Shaft spinning Generates electricity
• An inverter converts electricity from DC to AC
• Electricity is either connected directly to the mains power, connected to a battery bank, or connected to the electricity grid

Typical Wind Turbine Installation

• Main parts: Nacelle, Blades, Hub, Tower, Foundation, and Connecting Cables
• Offshore installation: Turbine, Transformer

Parts of a Wind Turbine (HAWT)

• Rotor
• Nacelle (Turbine housing)
• Tower
• Foundation
• Low-speed shaft
• High-speed shaft
• Controller
• Gear box
• Brake
• Generator
• Blade
• Yaw drive
• Pitch control
• Wind and weather measuring instruments
• Blades: Lift and rotate when hit by wind, causing the rotor to spin.
• Rotor: Combination of the blades and hub.
• Pitch System: Turns blades out of the wind to control rotor speed and stops the rotor from spinning in conditions where wind is blowing too slow or too fast.
• Generator: Produces 60-cycle AC electricity within the turbine.
• Controller: Starts and stops the turbine from working, depending on conditions.
• *Yaw Drive:*Controls upwind turbines to orient them should wind direction change.
• Tower: The base of the turbine built to support the rest of the structure.

Main Parts of a Wind Turbine

• Rotor: collect energy from the wind, consists of the hub, three blades and a pitch regulation system. The blades are airfoils, which depend on aerodynamic lift to move the blades and cause rotation, Design speed of the rotor is 16 rpm.
• Transmission System: Transmit mechanical power to Generator, regulates rotation of the shaft with a gearbox, braking system plus the auxiliary lubricating and cooling system. Gearbox increases rotor's speed from 16 rpm to the 1800-rpm speed of the generator. Braking system is designed to lock the rotor when shut down
• Generator converts the mechanical energy to electrical energy. They produce either alternating current (AC) or direct current (DC), and equipped with features to produce the correct voltage (120 or 240 V) and constant frequency (60 cycles) of electricity, even when wind speed is fluctuating.
• Yaw and control systems: A fully automatic microprocessor-based control and monitoring system. The yaw system turns the nacelle into the actual wind direction using a rotary actuator and a gear mechanism at the top of the tower.

Design Criteria: Wind Speed

• 0-10 mph: Too low for generating power. The turbine is not operational and the rotor is locked.
• 10-25 mph: 10 mph is the minimum operational speed or "Cut-in speed." In this range, generated power increases with wind speed.
• 25 – 50 mph: Wind Turbines reach rated power (maximum operating power) at wind speed of 25 mph (called Rated wind speed). Increasing wind speed won't increase generated power substantially by design
• More than 50 mph: The turbine shuts down when wind speed is higher than 50 mph, also called "Cut-out" speed, to prevent structure failure.

Design Criteria: Elevation

• Rotor Diameter (m) and Wind Turbine (Kw) increase with the passing of time

HAWT vs VAWT diagram

VAWT Turbine types

• Savonius

  • S-shaped if viewed from above
  • Drag-type VAWT turns relatively slowly, but yields a high torque
  • Most useful for grinding grain, pumping water, and many other tasks, but its slow rotational speeds make it unsuitable for generating electricity on a large-scale

• Darrieus

  • The Darrieus turbine is the most famous vertical axis wind turbone, and it is normally built with two or three blades
  • It is characterised by its C-shaped rotor blades which give it its eggbeater appearance.
  • Not self starting

• Giromill

  • Powered by two or three vertical aerofoils aerofoils attached to the central mast by horizontal supports
  • Giromill turbines work well in turbulent wind conditions and are an affordable option where a standard horizontal axis windmill type turbine is unsuitable

Design Criteria: Type of Blade

• Drag Blade
  • Wind literally pushes the blades out of the way
  • Slower rotational speeds and high torque capabilities.
• Lift Blade
  • The blade is an airfoil or wing.
  • A wind speed and pressure differential is created between the upper and lower blade surfaces when air flows past the blade.
  • Higher pressure at the lower surface acts to "lift" the blade.
  • Lift-powered wind turbines have much higher rotational speeds than drag types and are, therefore, well suited for electricity generation.

Lift vs Drag

• Lift turbines can theoretically capture 59% of the wind (Betz Limit).
• The theoretical maximum efficiency for a wind turbine is 59.3%, conjectured in 1919 by German physicist Albert Betz.
• This means that at most only 59.3% of the kinetic energy from the wind can spin the turbine and generate electricity.
• Drag turbines can theoretically capture 15% of the wind.
• Drag turbines require more material

Aerodynamic Lift Explained by Newton's Laws of Motion

• Lift occurs when a moving flow of air is turned by a solid object. The flow is turned in one direction, and the lift is generated in the opposite direction, according to Newton's Third Law of action and reaction.

Angle of Attack (Blade Angle)

• The angle between the chord line of the airfoil and the flight direction
• A large effect on the lift generated by an airfoil
• Typical values range from 1.0 to 15.0 degrees.
• Angle of Attack
• Lift
• Drag
• Chord Line
• Relative Wind
• Lower and Upper Camber
• Trailing and Leading Edge
• Turbine Blade
• Aircraft wing Aerofoil Motion
• Turbine Blade
• Angle of Attack
• AOA too low: (5°) Low-Lift, Low Drag, Laminar Flow and Separation Point
• AOA maximum (15°): Separation Point and the Start of Turbulence
• AOA too high: (25°) Lift, and Drag
• AOA insufficient to sustain aircraft flight, Turbulent Air Flow and Separation Point
• Low Lift to sustain generate turbine, High Drag

Design Criteria: Number of Blades

• The determination of the number of blades involves design considerations of aerodynamic efficiency, component costs, system reliability, and aesthetics.
• Aerodynamic efficiency increases with the number of blades but with diminishing return. Increasing the number of blades from one to two yields a 6% increase in efficiency. Further increasing the blade count from two to three yields gives only an additional 3% in efficiency.
• A rotor with an even number of blades causes stability problems because at the very moment when the uppermost blade bends backwards, because it gets the maximum power from the wind, the lower most blade passes into the wind shade in front of the tower. This produces uneven forces on the rotor shaft and rotor blade.

Number of Turbine Blades vs Performance

• The decision to design three-blade turbines was a compromise. Due its reduced drag, a one-blade design is the optimal number for maximum efficiency. However, a single blade causes imbalance and, hence, is not practical.
• A number of blades greater than three produces greater wind resistance, lower power generation and less efficient than three-blade turbines. For example, two-blade wind turbines face an unbalanced torsional force acting.
• For these reasons, turbines manufactured with three blades represent an ideal compromise between high energy output, high stability, light weight, and turbine durability
• The mechanical behavior of turbines with three or more blades is the same for every possible blade orientations. The maximum number of blades that will produce maximum power on a particular size turbine is four.
• Mechanical problems with two-blade turbines can be solved in a four-blade design, but ignoring the extra weight of the blade, economically four blades are more expensive than three blades.

Tip Speed Ratio

The ratio of the rotational speed of the blade to the wind speed. The larger this ratio, the faster the rotation of the wind turbine rotor at a given wind speed.

• Electricity generation requires high rotational speeds.

• Lift-type wind turbines have maximum tip-speed ratios of around 10

• Drag-type wind turbines approximately 1. Given the high rotational speed requirements of electrical generators, it is clear that lift-type wind turbines is the most practical for this application.

• How do you calculate tip speed ratio, ∅ ? The tip speed ratio is given by dividing the speed of the tips of the turbine blades by the speed of the wind, if a 20 mph wind is blowing on a wind turbine and the tips of its blades are rotating at 80 mph, then the tip speed ratio is 80/20 = 4.

• for electricity generation, Lift-type wind turbines: Omax = 10 Drag-type wind turbines: Omax = 1

• The number of blades and the total area they cover affect wind turbine performance. For a lift-type rotor to function effectively, the wind must flow smoothly over the blades. To avoid turbulence, spacing between blades should be great enough so that one blade will not encounter the disturbed, weaker air flow caused by the blade which passed before it.

Power Generation calculation formula

• Power = 1/2 (p)(A)(V)³ (η)
• where we assumed the turbine efficiency is 40%.

Turbine Efficiency

• Maximum theoretical efficiency of a wind turbine: 59.3%
• Maximum theoritcal under Betz Law an ideal wind turbine would slow down the wind by 2/3 of its original speed.
• Typical efficiency is 35% to 45%