Wind for electricity generation
Practical Action
Currently, the use of wind power for electricity production in developing countries is small, the
main area of growth being for very small battery charging wind turbines (50 – 500 Watts).
Other applications for small wind machines include water pumping, telecommunication power
supplies and irrigation.
Wind into watts
Power in the wind is explained in the Practical Action technical brief Energy from the Wind.
Although the power equation in that Brief 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, or the
sophistication of blade design. In reality, this figure is usually around 45% (maximum) for a
large electricity producing turbine and around 30% to 40% for a wind pump, (see the section
on coefficient of performance below). So, modifying the formula for ‘Power in the wind’ we can
say that the power which is produced by the wind machine can be given by this equation:
PM = ½.Cp.ρ.A.V3
where, PM is power (in watts) available from the machine
Cp is the coefficient of performance of the wind machine
There are a variety of important wind speeds to consider:
Start-up wind speed : the wind speed that will turn an unloaded rotor
Cut-in wind speed : the wind speed at which the rotor can be loaded
Rated wind speed : the wind speed at which the machine is designed to run
(this is at optimum tip-speed ratio
Furling wind speed : the wind speed at which the machine will be turned out
of the wind to prevent damage
Maximum design wind speed : the wind speed above which damage could
occur to the machine
The choice of rotor is largely dictated by the characteristic of the load, and hence of the end
use. Slow machines - such as multiple-blade designs – are used for pumping water, while fast
machines are used for electricity generation, such as the horizontal axis design with two or
three blades.
Grid connected or battery charging
Depending on the circumstances, the distribution of electricity from a wind machine can be
carried out in one of various ways.
Larger machines are connected to a grid distribution network. This can be the main national
network, in which case electricity can be sold to the electricity utility when an excess is
produced, then purchased back when the wind is low, this providing an agreement can be
made between the producer and the grid. Using the national grid helps provide flexibility to
the system and does away with the need for a back-up system when wind speeds are low. See
Grid Connection Technical Brief.
Micro-grids distribute electricity to smaller areas, typically a village or town. When wind is used
for supplying electricity to such a grid, a hybrid system consisting of a diesel generator set is
often used as a backup for when wind speeds are low. Alternatively, electricity storage can be
used but this is an expensive and impractical option.
Hybrid systems use a combination of two or more energy sources to provide electricity in all
weather conditions. The capital cost for such a system is high but subsequent running costs
will be low compared with a pure diesel system.
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