IEEE Electrification Magazine - March 2015 - 75

0.42

0.37

0
200

7.25

6.75

6.5

6.25

4.25

0.35
200

5.75

0.36
5.5

400

0.38

5.25

600

0.4
0.39

800

0.41

47.5

Power (W)

1,000

4.5

Aerodynamic
Power Coefficient (Cp)

1,200

Tip Speed Ratio (λ)
220 240

260

280

300

320

340 360

11 m/s
10.5 m/s

Rotational Speed (r/min)

10 m/s
9.5 m/s

9 m/s
8.5 m/s

8 m/s

Figure 14. The power curve of the generator when connected to a

Figure 15. The aerodynamic power coefficient of the rotor blades

48-V battery bank. (Source: rurerg.net.)

with respect to the tip speed ratio. (Source: rurerg.net.)

horizontal-axis small wind turbines. The design tip speed
ratio of the 1.2-m rotor diameter blades has a value of 5
according to the design manual, while its optimal value
was found to be close to 5.75 during the experiments.

small wind turbine and will protect the axial flux generator
from overheating at very high wind speeds. At wind speeds
higher than 10 m/s, the furling angle increases more and
achieves a significant reduction in power production.
The efficiency of the whole wind energy system,
from wind power to electrical power fed into the batteries, is described by the power coefficient (Cp) (Figure 18).
The system efficiency peaks at 0.31 at 5 m/s, while it has
values of above 0.3 for wind speeds ranging from 5 to
6.5 m/s, which is the most frequently occurring wind
speed range in rural applications. At higher wind speeds,
for example, while the wind turbine is operating at a
rated power of 10 m/s, the system's efficiency drops to
0.18 due to power regulation by the furling system. However, during windy conditions, the battery bank manages
to achieve a full charge much more quickly, resulting in
the rejection of most of the energy produced to the heatproducing resistances of the dump load, which makes a
high system efficiency in high winds less significant.
The annual energy production (AEP) predictions for the
wind turbine in question and for sites with mean wind
speeds ranging from 4 to 10 m/s, using a Rayleigh wind
speed distribution, are shown in Table 2. The uncertainties
in the AEP estimations are presented in both kilowatt
hours and as a percentage. For mean wind speeds greater
than 8 m/s, the increase in power production is small due

Outdoor Experiments
The performance of a typical battery-charging wind energy
system using a locally manufactured small wind turbine has
been monitored in the coastal small wind test site of NTUA
in Rafina (Figure 16). The measurements have been conducted according to the standard of the International Electrotechnical Commission 61400-12-1: Power Performance Measurements of Electricity Producing Wind Turbines, and specifically
Annex H, which refers to small wind turbine testing.
The 2.4-m HP small wind turbine has been installed in
a 12-m guyed tower and connected to a 48-Vdc battery
bank with a 75-m-long power cable of a 4-mm2 crosssectional area. A meteorological mast is positioned a few
meters away, equipped with an anemometer at hub
height and a wind direction vane, along with several other
sensors for determining the density of air that measure
the temperature, humidity, and pressure. All meteorological and electrical data, such as currents and voltages, are
measured and logged to provide 1-min averages, which
can then be grouped according to different wind speeds
using the method of bins.
The power curve of the 2.4-m HP locally manufactured
small wind turbine for the aforementioned setup can be
seen in Figure 17. The cut-in wind speed is 3 m/s, a typical
value for most horizontal-axis small wind turbines, while
the maximum or rated power is reached at 10.5 m/s with a
value of 525 W and an uncertainty of ! 11.2 W. The furling
system commences operation at 6.5 m/s by introducing a
small angle between the plane of rotation of the rotor and
the horizontal wind direction, which increases to 20° at
wind speeds of 9-10 m/s and, thus, provides adequate rotor
speed and electrical power regulation for the wind turbine.
A capable furling system mechanism is essential for every

Figure 16. The installation of a small wind turbine with a gin pole
and a rope hoist at the test site of NTUA. (Source: rurerg.net.)

IEEE Electrific ation Magazine / March 2 0 1 5

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