Design of Wind Turbines - Wind Energy: Renewable Energy and the Environment - page 98

6 Design of Wind Turbines
6.1 INTRODUCTION
The design of wind turbines has developed from a background of work on propellers, airplanes,
and helicopters. Computer codes developed for analyzing aerodynamics, forces, and vibration have
been modified for wind turbines. Theory and experimental procedures are well developed, and no
scientific breakthroughs are needed for wind turbines. However, there are problems of predicting
loads from unsteady aerodynamics. These loads lead to fatigue and less life than predicted by the
design codes. Part of the time, wind turbine blades operate in regions of large attack angles, which
is quite different than for airplane wings.
Someone made the comment that you could use brooms for blades and the rotor would turn.
Of course, the efficiency would be low, control would be a problem, and the strength would not be
adequate. A large number of airfoils were developed for wings on planes and sailplanes, which were
later used for wind turbine blades.
In the beginning the aerospace industry thought that the design of wind turbines and their con-
struction would simply be the transfer of technical knowledge from airplanes and helicopters.
However, this was an erroneous conclusion. One big difference is that airplanes and helicopters
move in response to large loads from wind gusts, whereas a wind turbine is tied to the ground.
Because power in the wind increases as the cube of the wind speed, the blades must have the
strength and flexibility to withstand the high variable loads, and then there must be a control mecha-
nism for shedding power in high winds.
There has been a lot of research and development, primarily by national labs and universities, and
later by the manufacturers of wind turbines, which has resulted in today's wind industry. The design
of wind turbines requires a broad cross section of knowledge: aerodynamics, mechanical engineer-
ing, electrical engineering, electronics, materials and industrial engineering, civil engineering, and
meteorology. The design process is iterative from first concept to the final design. Remember, it is
easier to fix problems at the design stage than to have the cost of retrofits in the field.
6.2 AERODYNAMICS
The analysis of aerodynamic performance begins with a disk or area in a stream flow of air.
Conservation of energy and momentum are used to determine the limit on the amount of extract-
able energy.
Forces of lift and drag on airfoils are measured experimentally in wind tunnels. As previous
measurements were for use with airplanes, a lot of airfoil data [1] are available from national labs.
Almost any shape can serve as an airfoil, even a flat plate, and the design of airfoils is almost an
art. As wind turbine blades operate in different wind speeds than airplane wings, airfoil data with
low Reynolds numbers [2] became available. Most of the lift and drag data were limited to attack
angles up to stall and a few degrees pass stall, because after the stall point, the airplane loses lift,
stalls out, and falls. Lift and drag data for attack angles up to 180° were only available for a few
airfoils. Airfoils, which had a large ratio of lift to drag, were developed for sailplanes. Which air-
foils are used for wind turbines depends on a number of factors, not just the ratio of lift to drag. As
the requirements are different for wind turbines, starting in the late 1980s airfoils were designed
specifically for wind turbines. A major change was to design airfoils that were less sensitive to
surface roughness.
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