Frost Protection - UTL Repository

ACTIVE PROTECTION METHODS
FIGURE 7.9
Temperature field response to wind machine on 26 March 2000 in northern Portugal
H E I G H T ( m )
24
16
8
A
before wind machine
2 min. after
4 min. after
6 min. after
D I S T A N C E ( m )
220
165
110
55
0.6
0.4
0
0
-1 1 3 0 45 90 135 180 225 270
TEMPERATURE (° C)
0.2
Wind drift: 0.1 m s -1 ; Direction: 5°
0.8
1.0
0.8
0.6
0.4
0.0
0.4 0.2
0.8
D I S T A N C E ( m )
(A) Temperature profiles (30 m from wind machine) before and after wind machine; and (B) 1.5 m
temperature response pattern produced by wind machine after 2 complete rotations around the
tower (after Ribeiro et al., 2002).
B
0.2
0.4
0.0
Generally, a 75-kW wind machine is necessary for each 4 to 5 ha (i.e. a radius
of about 120 m to 125 m). If one wind machine is used, about 18.8 kW of engine
shaft power per hectare is typically needed. About 15 kW of engine shaft power
is suggested per machine per hectare when several machines are used. Protection
decreases with distance from the tower, so some overlap of protection areas will
enhance protection. Usually, the protection area is an oval rather than a circle
shape because of wind drift. For example, Figure 7.9B shows the 1.5 m height
temperature response pattern to wind machine operation in an apple orchard
(Ribeiro et al., 2002).
Starting and stopping
Wind machines are typically started when the air temperature reaches about 0 °C.
Under stable inversion conditions, air tends to stratify near the ground and
mixing is believed to become less. However, trials in California (USA) and
Portugal have shown that starting fans after inversions have formed has little
influence on fan effectiveness. In less than half-an-hour after starting, the 2 m
temperature typically rises, sometimes approaching the 10 m temperature of an
unprotected orchard (Figure 7.8). However, because the temperature of a
radiating surface during a frost night is usually lower than the air temperature, it
is wise to have the wind machines operating when the air temperature reaches the
critical damage temperature (T c ). If the fruit is wet during the day or evening of
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