performance of the proposed system under any number of working generators, from
1 to 4. In order to maintain system safety, the windmill is designed to stop spinning
when revolutions exceed the upper limit (155 rpm).
Figure 12.7a shows the relationship between generator output and windmill
revolution speed when only one generator is working. Output tends to increase
gradually as revolution speed increases. The relationship between output and
speed with all generators working is depicted in Fig. 12.7b. Jumps in output occur
at 80, 120, and 140 rpm, due to changes in the number of operational generators. The
solid red line shows output at high revolution speeds if only one generator is
working, for comparison.
If only one generator is used, output at the maximum speed of 155 rpm is
approximately 220 W. However, output at this speed can be increased to approximately 900 W when all generators are operational. Total output of four operating
generators is always higher than the output of one generator. This raises the question
of whether a large-scale generator with a high capacity can generate significant
electric power more efficiently than one set of the proposed multi-type generators.
As indicated by the dashed red line in Fig. 12.7b, this type of large-scale generator
requires higher wind velocity to initiate windmill revolution due to higher total
resistance of the generating system. Wind velocity has to exceed a critical level; in
the case of Fig. 12.7b wind velocity must be high enough to cause a revolution speed
of 120 rpm. Although the behavior of the large-scale generator is not understood in
detail, it is clear that the lower bound of wind velocity to initiate revolution of the
windmill is much higher than that required for one small generator in the proposed
system. Thus, the electric power generation system presented here can efficiently
produce electric power under wind conditions in which a large-scale generator
cannot work (0–120 rpm).
12.3.4 Effect of Blade Configuration on Power Generation
Performance
It is assumed that the cross-sectional shape of the blade has an important effect on
power generation performance. Initially, the cross section of NACA4412, popular as
an airplane wing, was accepted for the cross-sectional shape of the blade. In the usual
flight of an airplane, wind blows from ahead of the wing. Thus, the wing is put in the
optimum conditions to obtain the maximum lifting force for the airplane. However,
in the proposed windmill system, the air stream against the blade is always changing
direction due to revolution of the windmill axle. Thus, when the circumferential
velocity of the blade is less than the wind velocity, the wind direction varies from
0
to 360
during one revolution of the windmill axle. In such a situation, the cross
section of NACA4412 would not give the optimum configuration.
The effect of the cross-sectional configuration of the blade on revolution power
was investigated. For ease of comparison, continuum fluid dynamics (CFD) analyses
12 Building a Global Low-Carbon Society Based on Hybrid Use of Natural Clean Energy 225
1 to 4. In order to maintain system safety, the windmill is designed to stop spinning
when revolutions exceed the upper limit (155 rpm).
Figure 12.7a shows the relationship between generator output and windmill
revolution speed when only one generator is working. Output tends to increase
gradually as revolution speed increases. The relationship between output and
speed with all generators working is depicted in Fig. 12.7b. Jumps in output occur
at 80, 120, and 140 rpm, due to changes in the number of operational generators. The
solid red line shows output at high revolution speeds if only one generator is
working, for comparison.
If only one generator is used, output at the maximum speed of 155 rpm is
approximately 220 W. However, output at this speed can be increased to approximately 900 W when all generators are operational. Total output of four operating
generators is always higher than the output of one generator. This raises the question
of whether a large-scale generator with a high capacity can generate significant
electric power more efficiently than one set of the proposed multi-type generators.
As indicated by the dashed red line in Fig. 12.7b, this type of large-scale generator
requires higher wind velocity to initiate windmill revolution due to higher total
resistance of the generating system. Wind velocity has to exceed a critical level; in
the case of Fig. 12.7b wind velocity must be high enough to cause a revolution speed
of 120 rpm. Although the behavior of the large-scale generator is not understood in
detail, it is clear that the lower bound of wind velocity to initiate revolution of the
windmill is much higher than that required for one small generator in the proposed
system. Thus, the electric power generation system presented here can efficiently
produce electric power under wind conditions in which a large-scale generator
cannot work (0–120 rpm).
12.3.4 Effect of Blade Configuration on Power Generation
Performance
It is assumed that the cross-sectional shape of the blade has an important effect on
power generation performance. Initially, the cross section of NACA4412, popular as
an airplane wing, was accepted for the cross-sectional shape of the blade. In the usual
flight of an airplane, wind blows from ahead of the wing. Thus, the wing is put in the
optimum conditions to obtain the maximum lifting force for the airplane. However,
in the proposed windmill system, the air stream against the blade is always changing
direction due to revolution of the windmill axle. Thus, when the circumferential
velocity of the blade is less than the wind velocity, the wind direction varies from
0
to 360
during one revolution of the windmill axle. In such a situation, the cross
section of NACA4412 would not give the optimum configuration.
The effect of the cross-sectional configuration of the blade on revolution power
was investigated. For ease of comparison, continuum fluid dynamics (CFD) analyses
12 Building a Global Low-Carbon Society Based on Hybrid Use of Natural Clean Energy 225