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Windmills

Updated on July 6, 2016

Introduction

A windmill is any machine that converts wind energy into rotational energy. It contains rotor blades connected to a hub that enables continuous rotation without hindrances. The usage of windmills spans from a long time in history. They were employed in various applications such as automation of grain grinding, drawing water from wells (water pumping) and so forth. The initial designs featured the vertical axis mechanism, where the rotating blades were placed in a horizontal plane. This feature was after that improved to one in which the rotating blades were placed in a vertical plane, forming a horizontal axis mechanism. The reasons given for the change, which occurred in the western world in 1300, was given as a greater drag type efficiency existing in the horizontal- axis mechanism than the vertical axis. Moreover, the horizontal-axis mechanism idea was taken from the European water wheels model in existence at that time.

Windmills
Windmills | Source

Windmill operation

Windmills in current times are mainly known as wind turbines since they are mostly used to generate electricity. Due to this fact, the technology used has evolved over the years creating more efficient and reliable wind dependent machines. The current crop of windmills mainly uses a three-rotor system connected to a hub that through a shaft is attached to a gear mechanism that controls the rotational output and is finally connected to a generator. The rotor blades are twisted in shape to enable them to either catch the wind or shed it. The wind is shed mainly when there is a large gale of the wind that may damage the internal components of the turbine/ windmill.

Besides the pitching, the efficiency of the mill is affected by the number of blades present. Due to eddy currents, one blade disturbs the air for the following module. Therefore, the number of blades should be kept to a minimum to prevent the disturbance (the minimum should be two for a balance to be reached). The more the blades are, the more the torque (a force that creates rotation) but this also causes a reduction in the speed of rotation. Moreover, the length of the blade also affects the efficiency. The longer the blade, the more the wind energy harvested. However, there is a limit that should be considered on matters of length- the length of the blades should not be too long. The tips of blades that are the too long approach the speed of sound during high-speed rotation and this minimizes efficiency.

The windmill built by the winning team had six blades equally distributed over the circular disc area. The blades were strongly anchored to a central support through the use of stressed strings and two basal-angular supports. Moreover, the blades were short enough to ensure they were within the range of the circular wind flow generated by the fan. The blades were also twisted, connected to the shaft that was intern attached to the vertical anchor support. Also, the vertical support was long enough to ensure that the rotation of the blades was not hindered. The weight that was to be lifted was connected using a string in such a way that it did not hinder the rotation of the blades and the shaft by being placed at a distance from the shaft/blade connection.

The design and construction of the fan were suitable in numerous ways. First, the blades were solidly attached to the shaft using various support mechanisms. This prevented the horizontal movement of the blades caused by the wind energy. The power would have caused vibrations if the blades were not strongly attached and this would have resulted in the rotational motion being hindered. The blades were twisted or fixed at an angle to maximize the amount of wind caught. This caused a maximum number of rotations in the least time possible and the rotational energy that would enable the lifting of the mass. The blades numbered six. As discussed earlier, the higher the number of blades, the more the torque but this, unfortunately, limits the rotational speed.

However, for the winning design, the effect of the number of blades is countered by the twisting of the blades. Therefore, the blades rotate at a constant and appropriate speed that would enable the system to lift the load. Also, the equally distributed blades ensure an equal distribution of the weights of the blades and the system consequently rotated without hindrance. The system also had a burden behind the vertical anchor support that balanced the weight of the system.

For this design, almost the same concept was incorporated. However, the design varied with that of the winning team mainly in the length of the blades. The design employed the use of a slightly longer blade which when compared to the design of the winning team had the same span from the shaft but with shorter basal supports. This meant that the span of the actual blades would be longer. This was done to increases the speed of rotation thereby reducing the time taken to lift the load. The number of blades was maintained at six since the weight of the loads would be equally distributed over the circumference. More blades would lead to a disturbance in the flow of the wind thereby reducing the efficiency.

The span of the blades (the disc size) of the system was determined by the span of a conventional fan that has a diameter of 30 to 40 centimeters. As a result, the design of the windmill had a diameter span of 45 centimeters that would enable it to be within the range of the air flow conveyed by the fan. The blades were tilted at an angle of about 40 degrees in order. Moreover, the width of the blade was determined to be about 10 centimeters. The increased length and width would ensure an increase in the surface area to be acted upon by the wind. The blades are connected to a central axial rod using circular basal supports 1 centimeter in diameter. The design once built is expected to lift the load in about 8.5 seconds due to the factors mentioned above.

Conclusion

The design, build, and test activity is particularly useful for students since it enables them to apply the theoretically acquired knowledge practically. It imparts in the students firsthand knowledge of how a windmill should function efficiently and also allows them to apply the factors by constructing the system. The testing part of the activity ensures that the students design working systems, and this prevents cases of students making the plans and building the windmill for the sake of it. It also provides a competitive environment between groups through the measurement of the time taken to lift the said load. Similar activities should, therefore, be incorporated in future for the students at that time to experience firsthand the features mentioned above. For example, they could design a system that uses the wind to draw water from a water source located below the windmill. They could also design, build and test a system that uses a continuous flow of water to open and close a door.

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