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INTERNATIONAL JOURNAL OF ENERGY RESEARCH
Int. J. Energy Res. 2012; 36:829
844
Published online 22 March 2012 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.2912
–
REVIEW PAPER
Blade sections for wind turbine and tidal current turbine
applications
current status and future challenges
—
M. Rafiuddin Ahmed
*
,†
Division of Mechanical Engineering, The University of the South Paci
c, Laucala Campus, Suva, Fiji
SUMMARY
The designers of horizontal axis wind turbines and tidal current turbines are increasingly focusing their attention on the design
of blade sections appropriate for speci
c applications. In modern large wind turbines, the blade tip is designed using a thin
airfoil for high lift : drag ratio, and the root region is designed using a thick version of the same airfoil for structural support.
A high lift to drag ratio is a generally accepted requirement; however, although a reduction in the drag coefcient directly
contributes to a higher aerodynamic ef
cant contribution to
the torque, as it is only a small component of lift that increases the tangential force while the larger component increases the
thrust, necessitating an optimization. An airfoil with a curvature close to the leading edge that contributes more to the rotation
will be a good choice; however, it is still a challenge to design such an airfoil. The design of special purpose airfoils started with
LS and SERI airfoils, which are followed by many series of airfoils, including the newCAS airfoils. After nearly two decades of
extensive research, a number of airfoils are available; however, majority of them are thick airfoils as the strength is still a major
concern. Many of these still show deterioration in performance with leading edge contamination. Similarly, a change in the
freestream turbulence level affects the performance of the blade. A number of active and passive
ciency, an increase in the lift coef
cient does not have a signi
ow control devices have been
proposed and tested to improve the performance of blades/turbines. The structural requirements for tidal current turbines tend to
lead to thicker sections, particularly near the root, which will cause a higher drag coefcient. A bigger challenge in the design of
blades for these turbines is to avoid cavitation (which also leads to thicker sections) and still obtain an acceptably high lift
coef
ow conditions with
a simple control system. The performance of a rotating blade may be signicantly different from a non-rotating blade, which
requires that the design process should continue till the blade is tested under different operating conditions. Copyright ©
2012 John Wiley & Sons, Ltd.
cient. Another challenge for the designers is to design blades that give consistent output at varying
KEY WORDS
wind turbine; tidal current turbine; blade section; lift; drag
Correspondence
*M. Ra
uddin Ahmed, Division of Mechanical Engineering, The University of the South Paci
c, Laucala Campus, Suva, Fiji.
†
E-mail: ahmed_r@usp.ac.fj
Received 8 August 2011; Revised 20 December 2011; Accepted 7 February 2012
1. INTRODUCTION
at varying Reynolds numbers (Re) and poor performance due
to the roughness effect resulting from leading edge (LE) con-
tamination for these proles. NACA airfoils are suitable mainly
for high Re and relatively small angles of attack (
a
) [2,3]. The
magnitude, direction and turbulence levels of the atmospheric
wind are known to vary signicantly with time, which
adversely affects the performance of WTs if NACA airfoils
are employed for the blades. Figure 1 shows the variation of
the section lift coefcient (C
l
)w th
a
at different Re for
NACA23012 airfoil [2]. It can be seen that the C
l
drops sharply
and signicantly as
a
increases beyond the stall angle for differ-
entRe.SuchanabruptdropinC
l
will signicantly reduce the
output of a WT or a TCT. The trend for special-purpose blade
sections started in the early 1980s with the design of LS and
Solar Energy Research Institute (SERI) airfoils after the experi-
ence gained from employing aviation class NACA airfoils
Research efforts directed at maximizing the power output of
horizontal axis wind turbines (WTs) and tidal current turbines
(TCTs) have increased signicantly during the recent years
providing impetus to extensive research on blades and blade
sections appropriate for specic applications. Advances in
the development of WTs and TCTs will have immense bene-
ts in providing solutions to the global energy requirements.
The rotor blade is one of the most important components of
the WTs and TCTs, which is the primary energy conversion
device. For the turbine blade design, the selection of airfoils
for different sections and the distribution of chords and twists
are pivotal [1]. Most of the NACA airfoils are not appropriate
for WTs and TCTs because of the poor stall characteristics, low
structural efciency near the root, inconsistent performance
829
Copyright © 2012 John Wiley & Sons, Ltd.
M. R. Ahmed
Blade sections for wind and tidal current turbines—status and future
Figure 2. Different
a
versus C
l
behaviours.
consuming. Researchers are, at the moment, testing a number
of active and passive ow control devices to improve the
performance of the turbines and to control the load on the
rotor. The present paper discusses the different blade sections
that were used in wind turbines starting from the earliest
special-purpose airfoils that were designed by National
Renewable Energy Laboratory (NREL). The main perfor-
mance characteristics of the popular blade proles used in
WTs and TCTs are discussed; many of these are also tested
for their performance under different operating conditions.
Promising ow control techniques and devices for impro-
vement of turbine performance are discussed in brief. The
performance characteristics of rotating turbine blades are com-
pared with those of non-rotating turbine blades, and nally, the
future challenges for blade designers are briey discussed.
Figure 1. The section lift coef
cient of NACA23012 airfoil at differ-
ent angles of attack.
10
6
;
10
6
;
○ –
Re = 3
□ –
Re=6
◊ –
8.8
10
6
;
– Re=6 10
6
(standard roughness) [2].
highlighted the shortcomings of these airfoils for horizontal
axis wind turbines (HAWTs) [4]. Stall-controlled HAWTs
produced excessive power in stronger winds, which caused
generator damage. The need to gain a better understanding
of the airfoil performance near stall was felt, as some stall-
controlled turbines were operating with some part of the
blade in deep stall a lot of time; the predicted loads were less
than the measured loads, and the LE roughness was affecting
the turbine performance.
Figure 2 shows the
a
versus C
l
behavior of traditional
airfoils and some of the presently used airfoils that have a
gradual upstream movement of the point of separation with
increasing angle of attack. A number of special-purpose air-
foils for WT applications have a gradual upstreammovement
of the location of separation from the trailing edge so that C
l
does not drop sharply and the coefcient of drag (C
d
)does
not rise sharply with an increase in the angle of attack. The
third type of behavior, in which the C
l
value essentially
remains constant over a wide range of angles of attack, is also
shown. With a growing demand for a reduction in energy
costs, the designers are now forced to think of simple passive
control techniques. An airfoil of this type of behavior will
de
2. LIFT AND DRAG
The main focus in the design of blade sections has been to
maximize lift : drag ratio (L/D) mainly by increasing C
l
.It
is still a generally accepted requirement. However, there are
different preferences among blade aerodynamicists regarding
the maximum C
l
depending on the type of control
—
the stall-
controlled turbines restrict C
l,max
to serve two purposes: (i) to
reduce the peak power generated; and (ii) to keep the thrust
on the system small. One of the earlier airfoils that were
designed (in 1986) with a restricted C
l,max
was S809 [6]. This
airfoil was designed with another related objective of
reduced effect of LE roughness so that the value of C
l,max
does not reduce due to LE contamination. On the other
hand, the pitch-controlled turbines require a high C
l,max
.
The pitch-controlled system adjusts
a
such that maximum
L/D is obtained for all the wind speeds up to the maximum
power. A high value of maximum lift coefcient gives higher
aerodynamic efciency as long as the blade structure is satis-
factory. One of the recent designs [7] focused on increasing
the tangential force coefcient (C
t
)ratherthanC
l
,asan
nitely contribute to a reduction in the cost, as its perfor-
mance will not deteriorate with a change in the
ow direction,
and it will help achieve the challenge of
[5],
which is especially important for offshore WTs and TCTs
for which maintenance and repairs are expensive and time
‘
lean design
’
830
Int. J. Energy Res. 2012; 36:829
–
844 © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Blade sections for wind and tidal current turbines—status and future
M. R. Ahmed
increase in C
l
contributes more to the thrust than to the rota-
tion. Having C
t
as the design objective can allow for more
attention on insensitivity to LE roughness rather than on high
lift : drag ratio caused by low C
d
. In most cases, it is desirable
that the design-
a
region is close to C
l,max
because this enables
low rotor solidity and/or low rotational speed [7].
3. LOCATION OF TRANSITION
POINT
It is known that the location of transition of the boundary
layer (BL), especially on the upper surface, is a very impor-
tant factor that signicantly inuences the performance of
the airfoil. The distance from the LE to the point where
transition occurs, x
tr
, reduces with (i) increasing Re; and
(ii) increasing turbulence intensity (Tu). Figure 3 shows
the location of transition on the upper surface of SG6043
airfoil at a lower Re and 5% Tu. When Re is increased, a re-
duction in x
tr
can clearly be seen from Figure 4. A reduction
in x
tr
also can be seen from Figure 5 when Tu is increased to
10%. As x
tr
reduces, the region of laminar boundary layer
reduces, and that of turbulent BL increases; this results in
an increase in the skin friction drag. For higher
a
, the shift
in the location of transition is normally less as the transition
anyway occurs close to the LE. However, for higher
a
,a
reduction in x
tr
may shift the location of separation
towards the TE, resulting in a reduction in the wake thick-
ness and a lower momentum loss.
The freestream turbulence levels of the atmospheric wind
at heights at which wind turbines are normally installed are
generally higher compared with the levels achieved in stan-
dard wind tunnels. Studies performed in the past have
explored the effect of Tu on the airfoil characteristics
[8
–
10] below one million Re. Hoffmann [8] studied the
Figure 4. Location of transition on the upper surface of SG6043
airfoil at high Re and 5% Tu.
effect of varying the freestream turbulence intensity from
0.25% to 9% for NACA0015 airfoil and reported an increase
in the peak lift coefcient because of delayed ow separation
at higher
a
. Devinant et al. [9] varied the turbulence level
from 0.5% to 16% and studied its effect on NACA
65
4
-421; they found that the ow was separating at higher
angles of attack when the turbulence level was increased.
Most of the works on such studies are performed on NACA
airfoils. Recently, Maeda et al. [10] studied the effects of
turbulence intensity on the static and dynamic characteristics
of DU93-W-210 airfoil at Re = 350 000 and two turbulence
levels of 0.15% and 11%. They found that the ow separa-
tion is delayed at higher turbulence levels, and the stall angle
increased.
Also, when the blade LE gathers dust, dirt, and so on, it
causes early transition of the BL. This roughness effect is
known to reduce the C
l,max
[2]. Figure 6 shows the effect of
roughness on C
l,max
at different Re. It can clearly be seen that
increasing roughness reduces C
l,max
. Airfoils with a relative
thickness of 25% or more may suffer a signicant deteriora-
tion in performance because of roughness effects. LE rough-
ness adds thickness to the BL and shifts the location of
transition very close to the LE. The resulting thicker BL leads
to increased drag, a reduction in the effective camber and an
earlier stall because of the weakening of the BL. The associ-
ated reduction in C
l,max
depends on the airfoil geometry and
the degree of contamination of the airfoil LE. A study on the
effects of airfoil thickness and C
l,max
on roughness sensitivity
concluded that the roughness sensitivity is directly propor-
tional to the airfoil thickness [11]. Some turbine blade
designers choose a prole and a design
a
, which has the tran-
sition point close to the LE so that for any reason, if the BL
transitions earlier, the performance of the blade is not much
different from the case with free transition. The SERI series
of airfoils (discussed in the next section) were designed to
Figure 3. Location of transition on the upper surface of SG6043
airfoil at low Re and 5% Tu.
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Int. J. Energy Res. 2012; 36:829
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844 © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
M. R. Ahmed
Blade sections for wind and tidal current turbines—status and future
families were designed for different rotor sizes [13] using
the Eppler code [14,15]. By minimizing the energy losses
because of roughness effects, by optimizing airfoils
’
perfor-
mance characteristics for appropriate Re and thickness and
by limiting C
l,max
, an annual energy improvement of up to
35% was estimated with SERI series of airfoils. For small
to medium blade lengths, 11
–
15% thick airfoils were
designed, whereas for large lengths, 16
–
21% thick airfoils
were designed. Greater thicknesses were intended to provide
greater blade ap stiffness for tower clearance, lower blade
weight important for large machines, and to accommodate
aerodynamic overspeed control devices for stall-regulated
machines. Two of the airfoil families were thin and were suit-
able for downwind rotors of up to 10m blade lengths and 20
to 100 kW power. Figure 7 shows the thin airfoil family
designed for pitch-controlled blades that did not have a
restrained C
l,max
. The tip-region airfoil (S803) is 11.5%
thick, whereas the root-region airfoil (S804) is 18% thick.
The maximum C
l
was found to be 1.5 at the respective Re.
For the stall-controlled blades of this size range, four thin air-
foils, S806A, S805A, S807 and S808, were designed. The
C
l,max
value for the tip-region airfoil, S806A, was only 1.1.
Another thick airfoils family, S820, S819 and S821, was
designed, which had similar characteristics; the tip-region
blade maximum thickness was 16%, which was to accom-
modate speed-control mechanism for stall-regulated turbines
and has a greater stiffness at the cost of slightly higher drag.
For blades of 10
–
15m length and with ratings of
100
–
400 kW, a thick-airfoil family consisting of S826,
S825, S814 and S815 was designed. The geometries and
the design specications are shown in Figure 8. The tip-
region airfoil was 14% thick and was found to have a
C
l,max
of 1.6. The root region (40% radius) airfoil, S814, is
24% thick and was designed with two primary objectives:
(i) to achieve a C
l,max
of at least 1.30 for Re = 1.5
10
6
,
which should not reduce with transition xed near the LE
on both the surface; and (ii) to obtain low prole drag
Figure 5. Location of transition on the upper surface of SG6043
airfoil at low Re and 10% Tu.
have the transition point close to the LE just prior to reaching
C
l,max
. It is interesting to note that below a Re of about 10
5
,
the roughened or turbulated airfoils perform better because
of the transfer of energy from outside the BL to the low
energy region inside the BL. However, above this Re, the
BL tends to become weaker for roughened or turbulated air-
foils and L/D drops [12].
4. SERI AIRFOILS
The development of special-purpose airfoils for HAWTs
began in 1984 as a joint effort between the NREL, formerly
the SERI, and Airfoils Incorporated. In all, nine airfoil
Figure 6. Effect of surface roughness on the maximum lift coef
cient for NACA 63(420)-422 at different Re [2].
832
Int. J. Energy Res. 2012; 36:829
–
844 © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
Blade sections for wind and tidal current turbines—status and future
M. R. Ahmed
Figure 7. Thin NREL airfoil family for medium blades (high tip C
l,max
) [13].
coefcients over the range of C
l
from 0.6 to 1.2 for the same
Re. With these objectives, the drag polar was plotted; the C
d
for the C
l
range of 0.5 to 1.2 increased only slightly because
of the elimination of signicant laminar separation bubbles
on the upper surface that are found on many laminar-ow
airfoils. The drag increases very rapidly outside the low-drag
range because the BL transition point moves quickly toward
the LE with increasing (or decreasing) C
l
. This feature results
in a LE that produces a suction peak at higher C
l
values,
which ensures that transition on the upper surface occurs
very close to the LE, and hence, the value of C
l,max
is insen-
sitive to roughness at the LE [16]. Based on these aspects, the
pressure distributions along the polar were deduced. The
desired pressure distribution for the higher C
l
angle is shown
Figure 8. Thick NREL airfoil family for large blades (high tip C
l,max
) [13].
833
Int. J. Energy Res. 2012; 36:829
–
844 © 2012 John Wiley & Sons, Ltd.
DOI: 10.1002/er
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