08-06-2013, 12:17 PM
WIND ELECTRICITY GENERATION
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Introduction
Windpower technology dates back many centuries. There are historical claims that wind
machines which harness the power of the wind date back beyond the time of the ancient
Egyptians. Hero of Alexandria used a simple windmill to power an organ whilst the
Babylonian emperor, Hammurabi, used windmills for an ambitious irrigation project as early
as the 17th century BC. The Persians built windmills in the 7th century AD for milling and
irrigation and rustic mills similar to these early vertical axis designs can still be found in the
region today. In Europe the first windmills were seen much later, probably having been
introduced by the English on their return from the crusades in the middle east or possibly
transferred to Southern Europe by the Muslims after their conquest of the Iberian Peninsula.
It was in Europe that much of the subsequent technical development took place. By the late
part of the 13th century the typical ‘European windmill’ had been developed and this became
the norm until further developments were introduced during the 18th century. At the end of
the 19th century there were more than 30,000 windmills in Europe, used primarily for the
milling of grain and water pumping.
Modern wind generators
The first wind powered electricity was
produced by a machine built by Charles
F. Brush in Cleveland, Ohio in 1888. It
had a rated power of 12 kW (direct
current - dc). Direct current electricity
production continued in the form of smallscale,
stand-alone (not connected to a
grid) systems until the 1930's when the
first large scale AC turbine was
constructed in the USA. There was then a
general lull in interest until the 1970's
when the fuel crises sparked a revival in
research and development work in North
America (USA and Canada) and Europe
(Denmark, Germany, The Netherlands,
Spain, Sweden and the UK). Modern
wind turbine generators are highly
sophisticated machines, taking full
advantage of state-of-the-art technology,
led by improvements in aerodynamic and
structural design, materials technology
and mechanical, electrical and control
engineering and capable of producing
several megawatts of electricity.
Wind generation for developing countries
Unlike the trend toward large-scale grid connected wind turbines seen in the West, the more
immediate demand for rural energy supply in developing countries is for smaller machines in
the 5 - 100 kW range. These can be connected to small, localised micro-grid systems and
used in conjunction with diesel generating sets and/or solar photovoltaic systems (see hybrid
systems section later in this fact sheet). Currently, the use of wind power for electricity
production in developing countries is limited, the main area of growth being for very small
battery charging wind turbines (50 - 150 Watts). In Inner Mongolia there are over 30,000 such
machines used by herders for providing power for lighting, televisions, radios, etc. (Spera
1994). Other applications for small wind machines include water pumping,
telecommunications power supply and irrigation.
Technical
The power in the wind
The wind systems that exist over the earth’s surface are a result of variations in air pressure.
These are in turn due to the variations in solar heating. Warm air rises and cooler air rushes
in to take its place. Wind is merely the movement of air from one place to another. There are
global wind patterns related to large scale solar heating of different regions of the earth’s
surface and seasonal variations in solar incidence. There are also localised wind patterns
due the effects of temperature differences between land and seas, or mountains and valleys.
Wind speed generally increases with height above ground. This is because the roughness of
ground features such as vegetation and houses cause the wind to be slowed.
Windspeed data can be obtained from wind maps or from the meteorology office.
Unfortunately the general availability and reliability of windspeed data is extremely poor in
many regions of the world. However, significant areas of the world have mean annual
windspeeds of above 4-5 m/s (metres per second) which makes small-scale wind powered
electricity generation an attractive option. It is important to obtain accurate windspeed data
for the site in mind before any decision can be made as to its suitability. Methods for
assessing the mean windspeed are found in the relevant texts (see the ‘References and
resources’ section at the end of this fact sheet).
Wind into watts
Although the power equation above gives us the power in the wind, the actual power that we
can extract from the wind is significantly less than this figure suggests. The actual power will
depend on several factors, such as the type of machine and rotor used, the sophistication of
blade design, friction losses, and the losses in the pump or other equipment connected to the
wind machine. There are also physical limits to the amount of power that can be extracted
realistically from the wind. It can been shown theoretically that any windmill can only possibly
extract a maximum of 59.3% of the power from the wind (this is known as the Betz limit).
Anatomy and characteristics of the wind generator
A typical small wind generator has rotor that is directly coupled to the generator which
produces electricity either at 120/240 volt alternating current for direct domestic use or at
12/24 volt direct current for battery charging. Larger machines generate 3 phase electricity.
There is often a tail vane which keeps the rotor orientated into the wind. Some windmachines
have a tail vane which is designed for automatic furling (turning the machine out of
the wind) at high wind speeds to prevent damage. Larger machines have pitch controlled
blades (the angle at which the blades meet the wind is controlled) which achieve the same
function. The tower is of low solidity to prevent wind interference and are often guyed to give
support to the tower.