IEEE Electrification Magazine - March 2015 - 76

600
Power Coefficient (Cp)

0.35

Power (W)

500
400
300
200
100
0

9 10 11 12 13 14 15

Wind Speed (m/s)
Figure 17. The power curve of the 2.4-m HP wind turbine measured
at the coastal test site of the NTUA. (Source: rurerg.net.)

to the effect of the furling system, which would classify
this small wind turbine as a design for low-wind-speed
areas. For a typical rural installation with a 5-m/s mean
wind speed, the 2.4-m HP wind turbine would be expected
to produce 1,271 kWh/year with an uncertainty of
! 111 kWh, which would amount to an average of
106 kWh/month, although this prediction will depend on
the constancy of the mean wind speed during the different seasons of the year.

Operation and Maintenance
During the field tests, many secondary aspects of the
small wind turbine operation have been studied, such as
the response time of the furling system and the starting
wind speed of the rotor, while the maintenance procedure
required in a highly corrosive coastal environment has
been recorded. In addition, and because of the high mean
wind speed of the test site, the small wind turbine has
been operated under extreme weather conditions to
observe the robustness of the design.
The response time of the furling system during strong
wind gusts and the maximum power produced in this
case has been examined. The maximum response time

Table 2. The estimation of the aeP of the
2.4-m hP wind turbine according to the mean
wind speed. (Source: rurerg.net.)

Mean wind
speed (m/s)

AEP (kWh)

Uncertainty (%)

751.61

9.9

1,270.85

7.94

1,747.85

6.76

2,124.21

6.05

2,392.87

5.62

2,569.72

5.34

2,673.97

5.13

I E E E E l e c t r i f i c ati o n M agaz ine / March 2015

0.3
0.25
0.2
0.15
0.1
0.05
0

2 3

5 6 7 8 9 10 11 12 13 14 15
Wind Speed (m/s)

Figure 18. The power coefficient of the complete wind energy system for the 2.4-m HP wind turbine measured at the coastal test site
of the NTUA. (Source: rurerg.net.)

was measured to be 3 s and the maximum instantaneous
power produced 1,093 W, which is an increase of 100% of
the turbine's rated power, a situation that can be taken
into account when sizing the diversion load controller's
resistive loads. In addition, the starting wind speed of the
rotor, defined as the wind gust needed to move the rotor
from standstill, which is different from the cut-in wind
speed, was measured to be 4.45 m/s (Figure 19).
The highly corrosive environment of the coast, which is
one of the harshest environments a small wind turbine will
have to face, has proven to increase the degradation of
materials but not significantly. A yearly maintenance check
of a few hours has been conducted for the past four years,
during which the wind turbine is lowered from the tower.
The wooden rotor blades are painted, the yaw and furling
mechanisms are greased, and the metal frame is painted, if
required, while it is possible to add grease to the rotor hub
bearing. The back iron disks of the rotor of the axial flux
generator have been hot-dip galvanized to increase resistance in corrosion and, thus, improve protection of the neodymium magnets, which are highly corrosive. The rotor
blades have been constructed with European pine, which is
softer than other varieties such as Oregon pine, resulting in
the degradation of the leading edges of the blades due to
sand particles in the wind. This requires the use of resin
and fiberglass putty to act as a filler in the degraded parts
before the blades are painted. Overall, the maintenance
requirements of the small wind turbine have been low in
terms of cost, time, tools, and skills required, especially
when compared to the cost, time, tools, and skills required
to construct the turbine itself.
The 2.4-m HP small wind turbine has proved its robustness in extreme weather conditions when operated continuously for two days in average wind speeds of 90 km/h,
while the power curve of Figure 20 was recorded for up to
25 m/s. The highest wind gust recorded by the meteorological mast during this period was 31 m/s. For safety reasons, during storm conditions such as the ones described,
it is recommended to stop the operation of the small wind
turbine using its electrical brake.

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Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2015