Horizontal Axis Wind Turbines
The concept of the horizontal axis wind turbine (HAWT) has been around for many years. You're probably more familiar with the term "wind mill." Unlike horizontal wind turbines, the wind mills were primarily used for grinding grain, pumping water, and filling railroad tanks. Modern horizontal wind turbines are generally used for generating electricity. Modern wind turbines are far more efficient that previous models since modern wind turbines stay perpendicular to the wind at all times. Modern wind turbines also use a gearbox that turns slower rotation into quicker rotation, which is more suitable for driving the generator.
Modern wind turbines often come under heavy fire from locals who are opposed to their construction. The most common reason is the effect they have on the appearance of the local landscape.
Tall Structures - Good and Bad
Horizontal wind turbines are usually very tall, reaching heights of 150 feet or more. These tall towers give the blades access to stronger winds with more wind shear. Some some areas, the increased wind shear can increase power output by as much as 34% per 10m of elevation.
The height of the towers has a downfall as well. Being hundreds of feet tall, these towers are difficult to transport. Once they've been delivered to their sites, they still have to be constructed. Construction involves renting very tall, expensive cranes as well as skilled operators. A hole had to be dug where the base will go and filled with enough concrete to support the massive structure, including the heavy blades, generator, turbine, and gearbox. Once the tower is assembled, the generator, blades, turbine, gearbox, and all the little parts have to go up to the top to be installed. This all adds up costs very quickly.
The blades on horizontal wind turbines are actually quite amazing. One would think they simply attach the blades and let them spin, but it's nowhere near that simple.
The blades must always be facing the wind to maximize efficiency so wind vanes are used on smaller horizontal wind turbines and larger turbines use wind sensors with servo motors. They need to be stiff enough that strong winds won't bend them. Bending blades would easily strike a tower and critically damage the unit. The blades can have their pitch adjusted to optimum angles, allowing them to collect the maximum amount of energy.The face of the blades needs to be struck at a constant angle. This ensures a contant wind load and reduces noise and vibration from the wind turbine.
Placing The Wind Turbines
Placement of the horizontal wind turbines is very critical. Turbulence is created behind the blades created cyclic stress on wind turbines downwind. Cyclic stress fatigues the blades, bearings, and axle of the wind turbines and for many years was a major cause of wind turbine failure. Horizontal wind turbines are now placed at least 5 blade lengths apart to reduce damage to surrounding turbines while still conserving space..