Horizontal-Axis Wind Turbine: All You Wanted to Know
Self-reliance is highly important where resources are concerned. The ability to provide your own electricity without relying on the grid is commendable. One way to achieve this is installing a wind turbine.
Wind energy has a very low carbon footprint, which makes it one of the most attractive alternatives to traditional energy sources. Furthermore, unlike solar, you get it all year-round, even during winter.
There are two types of wind turbines, and we are going to focus on one: the horizontal-axis wind turbine. Keep reading to find out the facts about horizontal-axis wind turbines.
The Two Types of Wind Turbines
1. Horizontal-axis wind turbines - They are the most commonly used type of wind turbine. They have all their components (shaft, blades, generator) atop a tall tower. The blades face directly into the wind, and the shaft is positioned horizontal to the ground. The blades are connected to the shaft, and rotation happens when the wind hits the blades. Consequently, a gear on the shaft’s end turns the generator.
2. Vertical-axis wind turbines – Unlike with horizontal turbines, the shaft that the blades are connected to is positioned vertical to the ground. In this type, all the main components are located close to the ground – in fact, the wind turbine itself is near the ground. This type is easy to maintain, does not incur high construction and transportation costs, and does not have to face the wind.
Design and Performance of Horizontal-Axis Wind Turbines
Horizontal-axis wind turbines are more commonly used than vertical-axis wind turbines. This is in spite of requiring a mechanism for orienting the blades to ensure they are facing the wind’s direction.
Horizontal-axis wind turbines (also known by the acronym HAWT) have a higher aerodynamic yield than vertical-axis turbines.
They feature blades aerodynamically shaped to resemble airplane wings. While in the case of airplane wings, lift is used to maintain the aircraft in flight, for a HAWT, lift is used to generate a driving torque, and that causes rotation.
The number of blades typically used for generating electric power varies: it may be one, two, or three blades. When three blades are used, there is a compromise between cost, power coefficient, and rotation speed of the wind sensor.
Horizontal-access turbines are less expensive than vertical-access turbines. Furthermore, they have less exposure to mechanical stresses. Having the receiver several tens of meters above ground makes them very efficient.
The main rotor shaft and electrical generator of a HAWT are positioned at the top of a tower. They must be pointed into the direction the wind is coming from – for small turbines, a wind vane suffices to make this happen. Large turbines use a wind sensor and a servo motor.
The turbine’s gearbox makes the blades rotate quicker so as to drive the electrical generator.
Since a tower tends to create turbulence behind itself, for best performance, the turbine is typically positioned upwind of the tower supporting it.
Turbine blades are stiff, which ensures they aren’t slammed/pushed into the tower by the huge force of high winds. Furthermore, turbine blades are positioned quite a distance ahead of the tower – sometimes they may even be tilted forward just a little into the wind.
Note, however, that not all horizontal-axis wind turbines are upwind turbines. Downwind wind turbines exist too.
Downwind Horizontal-Axis Wind Turbines
A downwind wind turbine is a HAWT whose rotor is positioned downwind (on the lee side) of the tower,
The theoretical advantage of downwind turbines is that they do not necessarily need a yaw mechanism, if the rotor and nacelle feature a suitable design which enables the nacelle to follow the wind passively.
This advantage is doubtful when considering large wind turbines. Cables are required to lead the current away from the generator – so how do you untwist the cables, considering the machine has been doing passive yawing in one direction for an extended period, if there is no yawing mechanism?
The more concrete advantage of downwind turbines is that it’s possible to make the rotor more flexible. This advantage is relevant both in terms of weight and the machine’s structural dynamics. It means that the turbine blades will bend at high speeds, and thus reduce the tower’s load.
The crucial advantage of downwind wind turbines is that they can be built lighter than upwind turbines.
On the other hand, the main disadvantage is wind power fluctuation caused by the rotor passing through the tower’s wind shade. As a result, the turbine may have to bear more fatigue loads than with an upwind design.
Parts of a Horizontal Wind Turbine
Since horizontal-axis wind turbines feature tall towers, they have access to stronger winds, particularly on sites which experience wind shear.
They can also be installed in a forest above tree-line. The tall towers also enable HAWTs to be installed on uneven ground or in offshore locations.
Furthermore most horizontal-axis wind turbines are self-starting.