Wind Power production and Turbines

1. Wind Power
Wind – Atmospheric air
in motion
Solar radiation differentially
absorbed by earth surface
converted through convective
processes due to temperature
differences air motion

2. Brief History – Early Systems
Harvesting wind power isn’t exactly a new
idea – sailing ships, wind-mills, wind-pumps
1st Wind Energy Systems
– Ancient Civilization in the Near East / Persia
– Vertical-Axis Wind-Mill: sails connected to a vertical
shaft connected to a grinding stone for milling
Wind in the Middle Ages
– Post Mill Introduced in Northern Europe
– Horizontal-Axis Wind-Mill: sails connected to a
horizontal shaft on a tower encasing gears and axles
for translating horizontal into rotational motion
Wind in 19th century US
Wind-rose horizontal-axis water-pumping wind-mills
found throughout rural America

4. Rise of wind power electricity

5. Origin of Wind

8. Installed Wind Capacity

10. Power from Wind
 Power extracted from wind
Cp= Power coefficient
A= coss sectional area intercepted by turbin

11. Turbine Aerodynamics

12. Lift & Drag

13. Lift & Drag

14. Understanding Wind

15. Wind Turbine Characteristics
 The drag force or air resistance is the force that is working against the blades causing them to
slow.
 The lift force FL is the component perpendicular to FD. The use of the word ‘lift’ does not mean
FL is necessarily upwards, and derives from the equivalent force on an airplane wing.
 Rotational movement of the air occurs as the airstream flows off the blade. This may be apparent
as distinct vortices and eddies (whirlpools of air) created near the surface. Vortex shedding occurs
as these rotating masses of air break free from the surface and move away, still rotating, with this
airstream.
 The air is disturbed by the blade movement and by wind gusts, and the flow becomes erratic and
perturbed. This turbulence occurs before and after the rotating blades, so each individual blade
may often be moving in the turbulence created by other blades.
 The wind turbine presents a certain solidity to the airstream. This is the ratio of the total area of
the blades at any one moment in the direction of the airstream to the swept area across the
airstream. Thus, with identical blades, a four-bladed turbine presents twice the solidity of a two-
bladed turbine.
 The aerodynamic characteristics of the blades are crucial; roughness and protrusions should be
avoided. Note that the predominantly 2-dimensional air flow over an airplane wing becomes 3
dimensional, and therefore more complex, for a rotating wind turbine blade.

16. Wind Turbine Characteristics
 Is the axis of rotation parallel or perpendicular to the airstream? The former is a
horizontal axis machine, the latter usually a vertical axis machine in a cross-wind
configuration.
 What is the solidity?
The total blade area as a fraction of the total swept disc area is known as Solidity of
Turbine.
For many turbines this is described by giving the number of blades. Large solidity machines
start easily with large initial torque, but soon reach maximum power at small rotational
frequency.
Small solidity devices may require starting, but reach maximum power at faster
rotational frequency. Thus large solidity machines are used for water pumping even in
light winds. Small solidity turbines are used for electricity generation, since fast shaft
rotational frequency is needed.
 What is the purpose of the turbine? Historic grain windmills and water pumping wind
turbines produce mechanical power. The vast majority of modern wind turbines are for
electricity generation; generally large for grid power and small for autonomous, stand-
alone power.

17. Turbine Characteristics
 Is the frequency of rotation controlled to be constant, or does it vary with wind
speed? A wind turbine whose generator is connected directly to a strong AC
electrical grid will rotate at nearly constant frequency. However a turbine of
variable frequency can be matched more efficiently to the varying wind speed
than a constant frequency machine, but this requires an indirect connection
through a power-electronic interface.
 Is the mechanical shaft directly coupled to its generator, or is connection
through an intermediate step that acts as a smoothing device? A decoupling
of this kind filters out rapid torque fluctuations, and allows better matching of
rotor to wind, and generator to load, than direct coupling. Partial decoupling of
the turbine from the generator is a soft coupling. Since wind velocities fluctuate
rapidly, the inertia of the wind turbine and the ‘softness’ of the rotor/generator
coupling are used to prevent these fluctuations appearing in the electricity
output. Similar effects occur if the blades are independently hinged against a
spring, which smoothes forces and decreases mechanical stress.

18. Wind Turbine Classification
Horizontal-Axiz Machines
 The dominant driving force is lift. Blades on the rotor may be in
front (upwind) or behind (downwind) of the tower.
 Upwind turbines need a tail or some other yawing mechanism, such
as electric motor drives to maintain orientation. Downwind turbines
are, in principle, self-orienting, but are more affected by the tower,
which produces wind shadow and extra turbulence in the blade
path. Perturbations of this kind cause cyclic stresses on the
structure, additional noise and output fluctuations. Upwind and
downwind machines of rotor diameter more than about 10m use
electric motors to control yaw.
 Two- and three-bladed rotors are common for electricity generation.
The three-bladed rotors operate more ’smoothly’ and, generally,
more quietly than two-bladed. Gearing and generators are usually at
the top of the tower in a nacelle.
 Multi-blade rotors, having large starting torque in light winds, are
used for water pumping and other low frequency mechanical power.

19. Horizontal Axis Turbines

20. Vertical Axis Turbines
By turning about a vertical axis, a machine can accept wind from any direction
without adjustment, whereas a horizontal-axis machine must yaw (i.e.turn in the
horizontal plane to face the wind). An expectation for vertical axis wind turbine
generators is to have gear boxes and generators at ground level
Types:
1. Cup anemometer.
This device rotates by drag force. The shape of the cups produces a nearly linear
relationship between rotational frequency and wind speed, so measurement of
the number of rotations per time period correlates to average wind speed over that
period. The device is a standard anemometer for meteorological data.
2. Darrieus rotor.
This rotor has two or three thin curved blades with an airfoil section. The rotor
shape is a catenary, with the aim of the rotating blades being only stressed along
their length.

23. Turbine Solidity
Low solidity (0.10) = high speed, low torque
High solidity (>0.80) = low speed, high torque
Solidity = 3a/A