cience of energy
UNDERSTANDING WIND ENERGY
WINDFACTS
There is evidence that wind energy was used to propel boats along the Nile as early as 5000BC.
The earliest known windmills were in Persia (Iran) and looked like large paddle wheels.
In 200BC people in China and the Middle East used windmills to pump water and grind grain.
The UK is the windiest country in Europe, so much so that we could power the whole country several
times over using wind energy.
The highest gust from a high level site is 150 knots (173 mph) at Cairngorm Summit (1,245 m) 20 /3/86
Highest windspeed recorded in the UK is 123knots (142mph 13/2/89 at Fraserburgh in Aberdeenshire
The world’s windiest place is Antarctica, where winds blow 100 km/hr + for 5 months of the year.
An average wind speed of 10-14mph (5-7 m/sec) is required to convert wind energy into electricity.
The European Union had more than 56 GW of installed wind power capacity by the end of 2007. This is
almost enough to power the entire electrical demands in the UK
The world's largest-capacity wind turbine is the Enercon E-126 at 7.58 MW. It has an overall height of
198 m (650 ft), and a diameter of 126 m (413 ft)
The tallest wind turbine is Fuhrländer Wind Turbine Laasow. Its axis is 160 metres above ground and its
rotor tips reach a height of 205 metres.
TURBINES
Turbines take the kinetic energy available in air and water currents and convert it into
electricity. This is a three stage process.
MECHANICAL ENERGY
KINETIC ENERGY
ELECTRICAL ENERGY
Wind Blows Turning Blades Dynamo
INTERMITTENCY
Much has been made of the fact that wind turbines only generate when the wind is blowing and
winds are slight during winter anticyclones when we have the coldest weather and hence highest
heating demands,. However, wind is fairly predictable over a whole year and the output of wind
farms can be predicted with reasonable accuracy from weather statistics.
TYPES OF TURBINES
There are two types depending on how the turbine is aligned to the wind:
Horizontal Axis Wind Turbines
HAWT
Vertical Axis Wind Turbines
VAWT
Commercial wind turbines as seen in wind farms are mostly horizontal axis turbines.
These are very suitable for large scale electricity generation fed into the NATIONAL GRID
HAWTs have higher efficiency ratings in use and are best suited to operate in clear air.
They need towers to position the turbine blades above turbulent airflow nearer the ground.
They are not as efficient in turbulent air as VAWTs which are very useful for lower output
building fixed locations
Savonius turbines ( a type of VAWT) have the benefit that they continue to generate
electricity in the strongest winds without being damaged
Types of Vertical Axis Wind Turbine
There are two main types of VAWT
Darrieus uses lift forces generated by aerofoils, The Giromill is a variant of the Darrieus type
Savonius uses drag forces (pushing)
SAVONIUS
DARRIEUS/GIROMILL
The GIROMILL is typically powered by two or three vertical aerofoils attached to the central mast
by horizontal supports. It is cheaper and easier to build than a standard Darrieus turbine but is
less efficient and also requires a strong a motor to start,. However, they do work well in turbulent
wind conditions and are an affordable option where a standard horizontal axis windmill type
turbine is unsuitable.
A DARRIEUS wind turbine can spin at many times the speed of the wind.
It generates less torque (turning force) than a Savonius but it rotates much faster.
This makes Darrieus wind turbines much better suited to electricity generation rather than water
pumping and similar activities.
The centrifugal forces generated by a Darrieus turbine are very large and act on the turbine blades
which therefore have to be very strong - however the forces on the bearings and generator are
usually lower than are the case with a Savonius.
SAVONIUS types are drag VAWT which operate in the same way as a cup anemometer.
They typically only have an efficiency of around 15-20% . Savonius type turbines turn more slowly
but generate a high torque. They are not very suitable for large scale electricity generation but
they are suited to applications such as pumping water and grinding grain for which slow rotation
and high torque (turning force) are essential.
ADVANTAGES OF VAWTs FOR SMALL SCALE USE
They are very quiet
They are omni-directional and will catch the wind from wherever it is blowing
They handle turbulence very well
They offer benefits in low wind situations
They can be placed close together
VAWTs can offer up to 30% efficiency
They do not place as much stress on the support structure
They are easy to build and very suitable to the competent DIYer
DISADVANTAGES OF VAWT
They have a tendency to stall under gusty winds.
The blades of a VAWT are prone to fatigue as the blade spins around the central axis.
Darrieus wind turbines are not self-starting and need a small motor to get them up to
rotational speed. (An alternative is to fit two small Savonius rotors are mounted on the shaft of
the Darrieus turbine to start rotation. These slow down the Darrieus turbine when it gets going
however they make the whole device a lot simpler and easier to maintain).
Self Starting Darrieus Wind Turbines
combine aerofoil
and savonius foils
SA VONIUS
DARRIEUS GIROMILL
Horizontal Axis Wind Turbines
ISLE OF GIGHA COMMUNITY WIND TURBINES
The Dancing Ladies
Wind turbines generate electricity to be used
directly (OFFGRID) or fed into the national
power grid (ONGRID) to be used elsewhere
SA VONIUS - Anenometer
SA VONIUS - Helix
DARRIEUS
POWER CURVES
These give the output at any given wind speed and are different for every turbine
HAWT turbines ‘cut in’ – start to generate at about 3m/s and ‘cutout’ at above 12 m/s.
However, many turbines reach their rated speed about 10 m/s and can generate at that full
output right up to their shut down speed when the blades are ‘feathered’ to protect from
storm damage.
An average wind speed of 5 m/s is the ideal threshold for wind turbine installation.
HAWTs usually have their hub height well above ground level and average wind speeds can
double at heights over 12m above ground level depending on landform and obstacles.
VAWTS can have quite different power curves and Savonius perform better at lower wind
speeds.
This turbine has a rated capacity of
The cut in speed is
The rated wind speed is
TP 6/11
WIND NUMBERS
It’s not as hard as it looks !
Wind is made up of moving air molecules which have mass.
Any moving object with mass carries kinetic energy which is given by the equation:
Kinetic Energy = 0.5 x Mass x Velocity2
(where the mass is measured in kg, the velocity in m/s, and the energy is given in joules.)
Air has a density of around 1.23 kg/m3 at sea level , so the mass of air hitting a wind turbine
(sweeping a given area) each second is given by the following equation:
Mass/sec = Velocity x Density x Area (m2)
So the power (i.e .amount of energy per second) in the wind hitting a wind turbine with a certain
swept area is given by inserting the mass per second calculation into the standard kinetic energy
equation above giving:
Power = 0.5 x Swept Area x Air Density x Velocity3
where Power is in Watts (i.e. joules/second), the Swept area is in square metres; Air density is in
kilograms per cubic metre, and the Velocity is in metres per second.
THE BETZ LIMIT
According to the Betz limit, no turbine can convert more than 59.3 % of the kinetic energy in wind
into mechanical turning energy (which then generates electricity) .
As wind turbines do not operate at 100% efficiency the real limit is actually well below the Betz
Limit. Even well designed wind turbines only convert 10-30% of the power of the wind into useable