Fluid Mechanics and Thermodynamics of Turbomachinery, 5e

caused by this increased roughness are 20 to 30%. The newer NREL turbine blades
described below are much less susceptible to the effects of fouling.
Developments in blade manufacture
Wind Turbines 363
Snel (1998) remarked, “in general, since blade design details are of a competitive
nature, not much information is present in the open literature with regard to these
items”. Fortunately, for progress, efficiency, the future expansion of wind energy power
plants (and this book), the progressive and enlightened policies of the US Department
of Energy, NASA and the National Renewable Energy Laboratory allowed the release
of much valuable knowledge to the world concerning wind turbines. Some important
aspects gleaned from this absorbing literature are given below.
Tangler and Somers (1995) outlined the development of special-purpose aerofoils
for HAWTs which began as a collaborative venture between the National Renewable
Energy Laboratory (NREL) and Airfoils Incorporated. Seven families of blades comprising
23 aerofoils were planned for rotors of various sizes. These aerofoils were
designed to have a maximum CL that was largely insensitive to roughness effects. This
was achieved by ensuring that the boundary layer transition from laminar to turbulent
flow on the suction surface of the aerofoil occurred very close to the leading edge, just
before reaching the maximum value of CL. These new aerofoils also have low values
of CD in the clean condition because of the extensive laminar flow over them. The
tip–region aerofoils typically have close to 50% laminar flow on the suction surface
and over 60% laminar flow on the pressure surface.
The preferred choice of blade from the NREL collection of results rather depends on
whether the turbine is to be regulated by stall, by variable blade pitch or by variable
rotor speed. The different demands made of the aerofoil from the hub to the tip preclude
the use of a single design type. The changing aerodynamic requirements along
the span are answered by specifying different values of lift and drag coefficients (and,
as a consequence, different aerofoil sections along the length). For stall-regulated turbines,
a limited maximum value of CL in the blade tip region is of benefit to passively
control peak rotor power. Figures 10.22 to 10.25 show families of aerofoils for rotors
originally designated as “small-, medium-, large- and very large-sized” HAWTs*,
designed specifically for turbines having low values of maximum blade tip CL. A noticeable
feature of these aerofoils is the substantial thickness–chord ratio of the blades,
especially at the root section, needed to address the structural requirements of “flap
stiffness” and the high root bending stresses.
According to Tangler (2000) the evolutionary process of HAWTs is not likely to
deviate much from the now firmly established three-bladed, upwind rotors, which
are rapidly maturing in design. Further refinements, however, can be expected of
the various configurations and the convergence towards the best of the three options
of stall-regulated, variable-pitch and variable-speed blades. Blades on large,
*With the top end size of HAWTs growing ever larger with time, the size catagories of “large” or
“very large” used in the 1990s are rather misleading and, perhaps, better described by stating either
the relevant diameter or power range.