Windpumping
Practical Action
rotary force produced by the rotor) than low-solidity machines but are inherently less efficient
than low-solidity machines. The windpump is generally of this type. Low-solidity machines
tend to be used for electricity generation. High solidity machines will have a low tip-speed
ratio and vice versa.
The choice of rotor is dictated largely by the
characteristic of the load and hence of the end use.
Some common rotor types and their characteristics
are shown in Table 1 below.
Figure 1: Low solidity rotor
Photo: Practical Action
Figure 2: High solidity rotor
Photo: Practical Action
Type
Horizontal axis
Multi blade
Three-bladed
aerofoil
Vertical axis
Panemone
Darrieus
Speed
Low
High
Torque
High
Low
Cp
0.25 - 0.4
up to 0.45
Solidity (%) Use
50 – 80
Mechanical power
Less than 5 Electricity production
Low Medium less than 0.1 50
Moderate Very low 0.25 - 0.35 10 - 20
Mechanical power
Electricity production
Table 1: Comparison of rotor types
Water pumping
Matching rotor and pump
When installing a windpump it is important to match the characteristics of the pump and the
wind machine. A good interaction between pump and rotor is essential. The most common
type of pump used for water pumping (especially for borehole water pumping) in conjunction
with a windmill is the reciprocating or piston pump. The piston pump tends to have a high
torque requirement on starting because, when starting, the rotor has to provide enough torque
to overcome the weight of the pump rods and water in the rising main - once the rotor is
turning, the torque requirement decreases because of the momentum of the revolving rotor.
The windspeed can then drop to about 2/3 of the start-up windspeed before the windpump will
stop.
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