Horizontal Axis Wind Turbines

From Wind wiki

Horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and may be pointed into or out of the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Most have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.

Since a tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount.

Downwind machines have been built, despite the problem of turbulence, because they don't need an additional mechanism for keeping them in line with the wind, and because in high winds, the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since turbulence leads to fatigue failures, and reliability is so important, most HAWTs are upwind machines.

HAWT advantages

• Variable blade pitch, which gives the turbine blades the optimum angle of attack. Allowing the angle of attack to be remotely adjusted gives greater control, so the turbine collects the maximum amount of wind energy for the time of day and season.

• The tall tower base allows access to stronger wind in sites with wind shear. In some wind shear sites, every ten meters up, the wind speed can increase by 20% and the power output by 34%.

• High efficiency, since the blades always move perpendicularly to the wind, receiving power through the whole rotation. In contrast, all vertical axis wind turbines, and most proposed airborne wind turbine designs, involve various types of reciprocating actions, requiring airfoil surfaces to backtrack against the wind for part of the cycle. Backtracking against the wind leads to inherently lower efficiency.

HAWT disadvantages

• The tall towers and blades up to 90 meters long are difficult to transport. Transportation can now cost 20% of equipment costs.

• Tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators.

• Massive tower construction is required to support the heavy blades, gearbox, and generator.

• Reflections from tall HAWTs may affect side lobes of radar installations creating signal clutter, although filtering can suppress it.

• Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition.

• Downwind 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.

Cyclic stresses and vibration

Cyclic stresses fatigue the blade, axle and bearing; material failures were a major cause of turbine failure for many years. Because wind velocity often increases at higher altitudes, the backward force and torque on a horizontal-axis wind turbine (HAWT) blade peaks as it turns through the highest point in its circle. The tower hinders the airflow at the lowest point in the circle, which produces a local dip in force and torque. These effects produce a cyclic twist on the main bearings of a HAWT. The combined twist is worse in machines with an even number of blades, where one is straight up when another is straight down. To improve reliability, teetering hubs have been used which allow the main shaft to rock through a few degrees, so that the main bearings do not have to resist the torque peaks.

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, pre-cessive 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.