Performance - Wind Energy: Renewable Energy and the Environment - page 162

8.7.1 F ARM W INDMILL
The farm windmill is an old technology, with no design changes since the 1930s. It is well designed
to pump small volumes of water for livestock and residences. It is comparable to a drag device,
because of the large solidity (close to 1), and the wind rotor has a peak efficiency of 15-18% at a
tip speed ratio around 1. The wind rotor efficiency is higher than the overall efficiency, because the
pump efficiency limits the system performance. Since it is connected to a positive displacement
pump, the rotor needs a lot of blades to obtain a high starting torque. For the mechanical farm
windmill with a positive displacement pump, the water flow rate is directly related to the number
of strokes per minute. Overall, efficiency or average annual efficiency (wind to water pumped) is
around 5-6% [17]. The curve for water flow is similar to the efficiency curve. In general for the farm
windmill, tables are provided to estimate performance for different wind regimes (Table 8.5). The
same information is shown in Figure 8.13 ; however, the strong wind data from Table 8.5 were not
plotted since they were close to the fair wind data.
Performance tests of eight farm windmills [18] show little difference between the four units,
which had reciprocating pumps, two of which had the conventional reduction gear and two that did
not. The windmill equipped with a Moyno pump performed well, but the three airlift units had poor
performance. So the advantage of no moving parts in the well was offset by the lower efficiency of
the pump and air compressor.

8.7.2 E LECTRIC TO E LECTRIC
A very promising development is a wind, electric-to-electric, water pumping system [19]. The wind
turbine alternator is connected directly to a motor, which is connected to a centrifugal or turbine
water pump. This system is a better match between the characteristics of the wind turbine rotor
and the load. The overall annual efficiency is 12-15%, which is double the performance of the
farm windmill. The water flow is higher at the higher wind speeds for the wind-electric system
( Figure 8.14 ) , as there is more wind power in this region. This is also the region where the farm
windmill is furled, limiting the power out.
The farm windmill and a 1.5 kW wind-electric system [20] using a submersible pump are essen-
tially the same size, and the costs are almost the same. The wind-electric system pumped twice the

TABLE 8.5
Estimated Water Pumped by Farm Windmill

Light Wind
3-4.5 m/s
cubic m/h

Fair Wind
5-7.5 m/s
cubic m/h

Strong Wind
8 m/s
cubic m/h

Depth
m

Pump Diameter
cm

9 3.6 8.1 12.5 13.7

17 2.7 4.6 7.1 7.8

24 2.2 3.2 4.9 5.4

38 1.8 2.0 3.1 3.4

49 1.6 1.6 2.4 2.6

67 1.3 1.1 1.8 2.0

79 1.2 0.9 1.5 1.6

91 1.1 0.8 1.2 1.4

110 1.0 0.6 1.0 1.1

140 0.89 0.5 0.7 0.8

171 0.84 0.5 0.7 0.8

183 0.78 0.4 0.6 0.6