Page 300 - Alternative Energy Systems in Building Design

274 WIND ENERGY TECHNOLOGIES

The convection cycle causes portions of the atmosphere to warm differently. Hot air
rises, reducing the atmospheric pressure at the earth’s surface, and cooler air is drawn
in to replace it. The result is wind. Air has mass, and when it is in motion, it contains
the energy of that motion, or kinetic energy. Some portion of that energy can be
converted into other forms, including mechanical force or electricity, that we can use
to perform work.

Structural Considerations

Wind turbines typically are mounted on open, four-legged steel towers similar to
supports for electric transmission lines or single circular steel columns similar to flag
poles. Both support systems are mounted on concrete foundations at the ground level.
The steel support column is connected to the concrete foundation with steel anchor bolts
through a baseplate, which is welded to the column.
The vertical loads that the structural engineer must consider in the design include
the weight of the turbine, the weight of the support column, and the weight of the
foundation. There are also horizontal loads that are combined with the vertical load.
The horizontal loads are induced either by earthquake or wind. In the case of wind
towers, wind-induced horizontal loads typically govern the design. These wind loads
include the horizontal thrust produced by the rotation of the turbine blades plus the
wind load on the tower itself.
The horizontal forces resulting from the wind loads, which are applied at levels
above the ground, produce a bending torque throughout the height of the structure.
The stress induced by this bending effect is most critical at the welded interface
between the column and the steel baseplate. There are two other critical elements that
require structural engineering design expertise: the anchor bolts holding the column
on the foundation and the concrete foundation that counteracts the wind loads and
keeps the entire structure in place. Of course, the column itself also will be designed
to resist both the vertical and horizontal loads.
For proper design of the foundation, the structural engineer will require that a geot-
echnical engineer investigate the soil’s properties at the location where the wind tower
will be constructed and make recommendations with regard to the soil’s capacity to
resist vertical and horizontal loads.
The structural engineer will design the foundation such that the entire structure will
not slide, will not experience differential settlement along its base, and most impor-
tant, will not overturn or be pulled out of the ground. It is important to point out that
wind turbines are designed and installed with an internal feature that limits the speed
of rotation of the blades. The structural engineer designs all aspects of the structure for
maximum horizontal thrust, resulting from the maximum design speed of blade rotation.
To limit the speed of rotation of the turbine blades, the turbine normally is designed
in such a way that when the blades reach the maximum design rotation speed, the
turbine yaws and changes its angle with respect to the direction of the blowing wind,
thus controlling rotation speed. This yawing feature in the turbine is very critical.
Should it fail, the entire structure could fall over.