Renewable Energy:

Wind Turbines

Wind turbines convert the wind’s kinetic energy into electric power. While windmills of various types were used for mechanical purposes in agricultural operation from as early as 200 BC, windmills for electricity generation was first devised in 1887 by a Scottish inventor.

Immediately after this the first electricity-generating windmill was constructed in the USA in 1889. By the 1930s windmills for power generation were commonplace in rural areas in the USA. The modern design of the wind turbine is based on the earliest design developed in the USSR. In 1954, the first wind turbine connected to an electricity grid became operational in 1954.

Modern wind turbines essentially have the rotar component that include the blades, a generator component, an electronic control component, gear box component for converting low speed winds to electricity and support structures like tower and rotar pointing components.

These components may be arranged horizontally or vertically in relation to the ground, each for specific situations and each have specific advantages or disadvantages. Besides, these designs special designs for cater to unique needs are also seen.

Horizontal- Axes Wind Turbine (HAWT)

In the HWAT design, the axes of the turbine is horizontally mounted atop a steel towers ranging in height between 200-300 feet. The blades are connected to a computerized motor and placed facing the direction of the wind. Generally the rotation speed is 10- 22 revolutions per minute, however, some designs have provisions to step-up this speed to generate collect/transmit more power when possible. All designs have features to protect equipment from damage through over speeding. The power produced by a turbine is dependent on the area spanned by the fans (rotar), the density of air and the cube of the speed of air.

HWAT turbine allows turbines the optimal angle to gather wind power and can be controlled remotely and adjusted for changing conditions. The tall tower base also helps in access to strong winds.

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The predominant reasons for failure in HAWT are cyclic stress on blades, axle and bearing. The reasons for this are that HAWT; the horizontally mounted equipment undergoes cyclic twists with dip in force and torque during high-wind conditions. HWAT installations close to the ground are both are difficult to operate and prone to damage, while the tall towers prove onerous to transport. Installation costs are high, equipment more investment intensive and personnel needs more specialized and also need extra yaw machines for control. HWAT are also accused of spoiling the aesthetics of the area.

Vertical - Axes Wind Turbine (VAWT)

Wind turbines with a vertical axis are used wherever the winds are highly variable and the turbine needs to gather momentum from winds in all directions. Here, the rotar shaft is arranged vertically and this arrangement does not require for the blades to face the direction of the wind. The gearboxes of vertical turbines are present in the ground for easy maintenance. The VAWT can have variable shaped blades and arrangement to suit local variations. The Derrieus Wind turbine is shaped like an eggbeater that does not require guy wires to hold, consequently do not require superstructures to support them. Nevertheless there is a lot of fatigue produced by this design and the blade also needs an external source of power to begin rotation. The Giromill vertical turbine is similar to the Derrieus design, except that straight instead of curved blades are used. This difference renders the design self-starting with the variable turbine having a pulsating torque that offers better performance and better efficiency with lower blade speeds, thereby reducing wear and tear. Another type of vertical turbine design is the Savonius wind turbine design, which may have low-efficiency but high reliability.

The advantages of VAWT are that they do not require long towers or yaw mechanisms and can be located close to the ground. This makes VAWTs adaptable tore areas. The area of blade sweep is higher and they typically start easily without supply of initial energy, creating electricity at speeds as low as 10km/hr. VAWT is less prone to damage under high wind conditions compared to HWAT. VAWT do not need to face the wind. They are also considered more eco-friendly, as birds avoid them easily.

This design is however, difficult to install and produces an undesirable pulsating torque resulting in a drag during rotatory movements. The drag in the equipment allows VAWT to produce only 50% energy compared to HAWT.
As they are often ground mounted with other heavier parts placed above them, changing parts during repair gets near impossible. To hold the structure in place, “guy wires,” are used. Guy wires cause a down ward thrust in wind needing a superstructure to hold a top bearing to correct the problem. The other disadvantage is that airflow near the ground creates air turbulence increasing vibrations and noise pollution and also decreasing the service life of the equipment.
It has been recommended that the height of rooftop mounted turbines be at least 50% of the height of building for maximum energy and minimum turbulence.

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