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Vertical axis wind turbine

This document discusses horizontal axis wind turbines (HAWTs) and vertical axis wind turbines (VAWTs). HAWTs face issues like requiring large towers and complex yaw mechanisms, while experiencing high stresses from cyclic loads. VAWTs have simpler designs that do not require yawing and can harvest multi-directional winds. The two main VAWT types are Savonius and Darrieus turbines. Savonius turbines have drums that produce torque from differential wind forces. Darrieus turbines use curved blades whose lift forces generate torque as they rotate. While more efficient than Savonius designs, Darrieus turbines require external starting torque. Overall, VAWTs have advantages like omni-directional operation and simpler installation

Engineering◦
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Presented by… K.DAVIS LAZARUS (17IS1D0205)
Wind Turbine Principle: Wind turbines convert the kinetic energy in the wind into mechanical work which is converted into electrical power Working: The energy in the wind turns two or three propeller-like blades around a rotor. The rotor is connected to the main shaft, which spins a generator to create electricity. Types:  Horizontal axis wind turbine [HAWT]  Vertical axis wind turbine[VAWT]
Drawbacks of Horizontal Axis Wind Turbine  Massive tower construction is required to support the heavy blades, gearbox, and generator.  Components of a horizontal axis wind turbine (gearbox, rotor shaft and brake assembly) being lifted into position.  Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.  Down wind variants suffer from fatigue and structural failure caused by turbulence when a blade passes through the tower’s wind shadow (for this reason, the majority of HAWTs use an upwind design, with the rotor facing the wind in front of the tower).
 HAWTs require an additional yaw control mechanism to turn the blades toward the wind.  HAWTs generally require a braking or yawing device in high winds to stop the turbine from spinning and destroying or damaging itself.  Cyclic Stresses & Vibration – When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it pivots, gyroscopic precession tries to twist the turbine into a forward or backward somersault. For each blade on a wind generator’s turbine, force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbines.  Noise occurs when blades are rotating.
Vertical Axis Wind Turbine (VAWT):  VAWTs are a type of wind turbine where the main rotor shaft is set transverse to the wind (but not necessarily vertically) while the main components are located at the base of the turbine  Vertical axis turbines are powered by wind coming from all 360 degrees, and even some turbines are powered when the wind blows from top to bottom. Types of VAWT:  The Savonius wind turbine  The Darrieus wind turbine
CONSTRUCTION UNDER ROTOR AXIS
Savonius wind turbine  The Savonius rotor is an extremely simple vertical-axis device that works entirely because of the thrust force of wind.  The basic equipment is a drum cut into two halves vertically.  The two parts are attached to the two opposite sides of a vertical shaft.  As the wind blowing into the structure meets with two dissimilar surfaces-one convex and the other concave-the forces exerted on the two surfaces are different, which gives the rotor a torque.
 By providing a certain amount of overlap between the two drums, the torque can be increased.  This is because the wind blowing into the concave surface turns around and gives a push to the inner surface of the other drum, partly cancelling the wind thrust on the convex side.  It has been found that an overlap of about one-third the drum diameter gives the optimum result.  The Savonius rotor is inexpensive and simple, and the material required for it is generally available in any rural area, enabling onsite construction of such windmills.  However, its utility is limited to pumping water because of its relatively low efficiency.
Darrieus wind turbine  In 1931, a vertical-axis device for wind energy conversion was invented by G.J. Darrieus of the United States, but was forgotten for a long time.  The energy crisis renewed interest in windmill development in the 1970s, which reinvented the use of the Darrieus rotor for wind energy conversion.  The peculiarity of the Darrieus rotor is that its working is not at all evident from its appearance.  Two or more flexible blades are attached to a vertical shaft as shown in Figure.
 The blades bow outward, taking approximately the shape of a parabola, and are of symmetrical aerofoil section.  At first sight it appears that the forces on the blades at the two sides of the shaft should be the same, producing no torque.  In fact, the torque is zero when the rotor is stationary.  It develops a positive torque only when it is already rotating.  This means that such a rotor has no starting torque and has to be started using some external means.
The principle of operation is shown in Figure. One blade of the rotor is shown in three successive positions along the path of rotation. At each position the blade velocity vector u, the wind velocity vector υ, the relative wind w , the lift force FL, and the drag force FD are shown. It can be seen that at each position the lift force has a positive component in the direction of rotation, giving rise to a net positive torque. This torque is not the same in all the positions. It varies from zero when the blade is moving directly upwind or downwind to a maximum about a quarter of a revolution later. Fig: Top view of a three-blade VAWT showing the velocity components relative to the blade. The linear velocity of the blade b W can be obtained by deriving its position with respect to time.
Advantages of VAWTs  They can produce electricity in any wind direction. Strong supporting tower in not needed because generator, gearbox and other components are placed on the ground.  Low production cost as compared to horizontal axis wind turbines.  As there is no need of pointing turbine in wind direction to be efficient so yaw drive and pitch mechanism is not needed.  Easy installation as compared to other wind turbine.  Easy to transport from one place to other.  Low maintenance costs.  They can be installed in urban areas.  Low risk for human and birds because blades moves at relatively low speeds.  They are particularly suitable for areas with extreme weather conditions, like in the mountains where they can supply electricity to mountain huts.
Disadvantages of VAWTs  As only one blade of the wind turbine works at a time, efficiency is very low compared to HAWTS.  They need an initial push to start; this initial push that to make the blades start spinning on their own must be started by a small motor.  When compared to horizontal axis wind turbines they are very less efficient because of the additional drag created when their blades rotate.  They have relative high vibration because the air flow near the ground creates turbulent flow.  Because of vibration, bearing wear increases which results in the increase of maintenance costs.
VAWTs (vs) HAWTs
Thank you…. Signing off….