U.S. patent application number 14/074786 was filed with the patent office on 2015-05-14 for rotor blade of a wind turbine.
This patent application is currently assigned to SIEMENS AKTIENGESELLSCHAFT. The applicant listed for this patent is Kevin J. Standish, Jason Stege, Arni T. Steingrimsson, Henrik Stiesdal, Alonso O. Zamora Rodriguez. Invention is credited to Kevin J. Standish, Jason Stege, Arni T. Steingrimsson, Henrik Stiesdal, Alonso O. Zamora Rodriguez.
Application Number | 20150132141 14/074786 |
Document ID | / |
Family ID | 51485575 |
Filed Date | 2015-05-14 |
United States Patent
Application |
20150132141 |
Kind Code |
A1 |
Stege; Jason ; et
al. |
May 14, 2015 |
ROTOR BLADE OF A WIND TURBINE
Abstract
A rotor blade of a wind turbine with a raked tip portion is
provided. The tip portion is raked, i.e. curved, in a rotor blade
plane comprising the tip base chord and a line which is parallel to
the pitch axis of the rotor blade. Additionally, the orientation of
the chords with reference to the chord at the tip base changes
between the tip base and the tip. In other words, the trailing edge
of the tip portion is twisted. Optionally, the tip portion is
additionally swept out of the rotor blade plane, which is
characterized by a cant angle.
Inventors: |
Stege; Jason; (Brande,
DK) ; Stiesdal; Henrik; (Odense C, DK) ;
Zamora Rodriguez; Alonso O.; (Boulder, CO) ;
Steingrimsson; Arni T.; (Erie, CO) ; Standish; Kevin
J.; (Erie, CO) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Stege; Jason
Stiesdal; Henrik
Zamora Rodriguez; Alonso O.
Steingrimsson; Arni T.
Standish; Kevin J. |
Brande
Odense C
Boulder
Erie
Erie |
CO
CO
CO |
DK
DK
US
US
US |
|
|
Assignee: |
SIEMENS AKTIENGESELLSCHAFT
Munich
DE
|
Family ID: |
51485575 |
Appl. No.: |
14/074786 |
Filed: |
November 8, 2013 |
Current U.S.
Class: |
416/228 |
Current CPC
Class: |
F03D 1/0675 20130101;
Y02E 10/72 20130101; F05B 2250/712 20130101; F05B 2240/302
20130101; Y02E 10/721 20130101; F05B 2250/71 20130101; F03D 1/0633
20130101; F05B 2250/711 20130101 |
Class at
Publication: |
416/228 |
International
Class: |
F01D 5/14 20060101
F01D005/14 |
Claims
1. A rotor blade of a wind turbine, comprising a blade body
extending between a root portion of the rotor blade and a tip
portion of the rotor blade, a trailing edge and a leading edge,
wherein the rotor blade is arranged and prepared for being mounted
to a hub of the wind turbine and for being pitched about a pitch
axis, and wherein a rotor blade plane is defined by the plane
comprising the chord at the tip base and a line which is parallel
to the pitch axis, wherein the tip base is the part of the rotor
blade at which the tip portion joins the blade body, wherein the
trailing edge of the tip portion has a curved shape as projected on
the rotor blade plane such that a trailing edge sweep angle
increases from the tip base to the tip of the rotor blade, and
wherein orientation of the chords with reference to the chord at
the tip base changes between the tip base and the tip such that a
chord tilt angle changes between the tip base and the tip.
2. The rotor blade according to claim 1, wherein the trailing edge
sweep angle increases by at least 2 degrees from the tip base to
the tip.
3. The rotor blade according to claim 1, wherein the trailing edge
sweep angle increases by at most 40 degrees from the tip base to
the tip.
4. The rotor blade according to claim 1, wherein the chord tilt
angle varies by at least 2 degrees between the tip base and the
tip.
5. The rotor blade according to claim 1, wherein the chord tilt
angle varies by at most 30 degrees between the tip base and the
tip.
6. The rotor blade according to claim 1, wherein in a first section
of the tip portion the trailing edge is twisted towards the suction
side of the blade body, thus leading to negative chord tilt angles
and/or in a second section of the tip portion the trailing edge is
twisted towards the pressure side of the blade body, thus leading
to positive chord tilt angles.
7. The rotor blade according to claim 6, wherein the first section
is adjacent to the tip base and the second section is adjacent to
the tip.
8. The rotor blade according to claim 6, wherein the rate of
decrease of the chord length of the tip portion is greater in the
second section than in the first section.
9. The rotor blade according to claim 1, wherein the leading edge
of the tip portion has a curved shape as projected on the rotor
blade plane such that a leading edge sweep angle increases from the
tip base to the tip.
10. The rotor blade according to claim 9, wherein the leading edge
sweep angle increases by at least 10 degrees from the tip base to
the tip.
11. The rotor blade according to claim 9, wherein the leading edge
sweep angle increases by at most 80 degrees, in particular by at
most 60 degrees, from the tip base to the tip.
12. The rotor blade according to claim 1, wherein the tip portion
is curved towards the trailing edge of the blade body.
13. The rotor blade according to claim 1, wherein, as viewed from
the suction side of the rotor blade, the trailing edge of the tip
portion is concavely shaped and the leading edge of the tip portion
is convexly shaped.
14. The rotor blade according to claim 1, wherein the trailing edge
of the tip portion as projected on a reference plane, wherein the
reference plane is defined as the plane comprising the pitch axis
and being perpendicular to the chord at the tip base, has a curved
shape such that a cant angle increases from the tip base to the
tip.
15. The rotor blade according to claim 14, wherein the cant angle
increases by at least 20 degrees from the tip base to the tip.
16. The rotor blade according to claim 14, wherein the trailing
edge sweep angle and/or the cant angle increases monotonically from
the tip base to the tip.
17. The rotor blade according to claim 1, wherein at least one
segment of the trailing edge of the tip portion is shaped as a
straight line and/or at least one segment of the leading edge of
the tip portion is shaped as a straight line.
18. The rotor blade according to claim 1, wherein the length of the
tip portion, as projected on the rotor blade plane and measured
along the pitch axis, is between 0.5% and 10%, of the length of the
entire rotor blade as projected on the rotor blade plane and
measured along the pitch axis.
19. The rotor blade according to claim 1, wherein the rotor blade
is shaped such that the tip is separated from a tangential plane
which is defined to be perpendicular to the pitch axis and
tangential to the tip portion.
20. The rotor blade according to claim 14, wherein the rotor blade
deforms under wind loads such that the trailing edge sweep angle
and/or the chord tilt angle and/or the cant angle changes at least
about 5% compared to an unloaded state of the rotor blade.
21. The rotor blade according to claim 1, wherein the tip portion
is added to the blade body as a retrofit.
22. The rotor blade according to claim 1, wherein the rotor blade
is manufactured as a single piece.
23. The rotor blade according to claim 1, wherein the trailing edge
and the streamline of an airflow which flows from the leading edge
across the rotor blade comprise an angle which is smaller than 80
degrees at the tip portion.
Description
FIELD OF INVENTION
[0001] The present invention relates to a rotor blade of a wind
turbine. In particular, the invention relates to means for
improving efficiency of a wind turbine with such a rotor blade.
BACKGROUND OF THE INVENTION
[0002] Wind turbines are a very attractive option for generating
electricity. A wind turbine typically comprises one or several
rotor blades, which are rotatable mounted to a hub of the wind
turbine. A rotor blade in general comprises a root portion, by
which the rotor blade is connected to the hub. Furthermore, a rotor
blade comprises a blade body, which is also referred to as the
airfoil-shaped portion of the rotor blade, and a tip portion, which
is opposite to the root portion.
[0003] The tip portion of a rotor blade encounters conflicting
design requirements which shall be satisfied. On the one side, the
tip portion shall unload the aerodynamic loading on the rotor blade
and shall provide a minimum tip vortex to reduce downwash. This
requires a design that reduces the aerodynamic loading
progressively along the span line of the tip portion and thereby
lowers local axial induction. On the other hand, the tip portion
shall produce as much power as possible and shall thus contribute
to the overall purpose of a rotor blade, which is the improvement
of efficiency of a wind turbine comprising this rotor blade. For
the tip portion, this requires maintaining an axial induction as
close as possible to the aerodynamic optimum.
[0004] Advantageously, these two design requirements are met
without inducing flow separation of the airflow which would
generate undesirably high levels of aerodynamic noise.
[0005] Current tip portions of rotor blades are typically designed
to accomplish the first design requirement: Aerodynamic unloading
with the provision of small tip vortices. This requirement may, for
instance, be met by so-called winglets, which comprise a curvature
of the rotor blade in the section of the tip portion out of a plane
of the rotor blade. However, no satisfying solution for a design of
a tip portion of a rotor blade, wherein the tip portion
satisfactorily meets both design requirements mentioned above, has
been presented yet.
[0006] Thus, there exists an urgent need to provide a rotor blade
of a wind turbine, wherein the rotor blade comprises a tip portion
and the tip portion is designed such that efficiency of a wind
turbine comprising such a rotor blade is improved.
SUMMARY OF THE INVENTION
[0007] This objective is achieved by the independent claims. The
dependent claims describe advantageous developments and
modifications of the invention.
[0008] In accordance with the invention there is provided a rotor
blade of a wind turbine wherein the rotor blade comprises a blade
body extending between a root portion of the rotor blade and a tip
portion of the rotor blade. The rotor blade comprises a trailing
edge and a leading edge. The rotor blade is arranged and prepared
for being mounted to a hub of the wind turbine and for being
pitched about a pitch axis. A rotor blade plane is defined by the
plane comprising the chord at the tip base and a line which is
parallel to the pitch axis, wherein the tip base is the part of the
rotor blade at which the tip portion joins the blade body.
Furthermore, the trailing edge of the tip portion has a curved
shape as projected on the rotor blade plane in a way that a
trailing edge sweep angle increases from the tip base to the tip of
the rotor blade. Additionally, orientation of the chords with
reference to the chord at the tip base changes between the tip base
and the tip in a way that a chord tilt angle changes between the
tip base and the tip.
[0009] Figuratively speaking, the trailing edge of the tip portion
of the rotor blade comprises a curved or swept shape as projected
on the rotor blade plane and, at the same time, the chords of the
tip portion of the rotor blade change their orientation with regard
to the tip base chord. Thus, superimposing the curvature of the
trailing edge in the rotor blade plane and the changing orientation
of the chords results in a curved and twisted trailing edge between
the tip base and the tip.
[0010] Rotor blades with a curved tip portion including a curved
trailing edge with the curvature realized in the rotor blade plane
are known to the person skilled in the art. This invention,
however, discloses a tip portion, where the chords and thus the
trailing edge as well are additionally twisted. By the combination
of sweep and twist at the tip portion of the rotor blade the two
conflicting design requirements which have been described above are
successfully met. More specifically, the curved and twisted
trailing edge of the tip portion involves both a lowering of the
local axial induction in one segment of the tip portion, thus
reducing the aerodynamic loading, and at the same time maintenance
of the axial induction as close as possible to the aerodynamic
optimum on a different segment of the tip portion, thus improving
the power generating potential of the rotor blade.
[0011] Note that in the context of this application, a chord, which
always is a straight line and which, for instance, is not curved of
twisted itself, may be tilted, in other words inclined, with
reference to a reference line, which may be the chord at the tip
base. The result of a plurality of tilted chords comprising
different chord tilt angles is a trailing edge, which is twisted,
in other words bent, and which is thus referred to as a twisted
trailing edge.
[0012] Additionally, note that as, for instance, the rotor blade
may be curved to the suction side or to the pressure side such that
the pitch axis and the chord at the tip base do not intersect and
thus cannot define a plane, the rotor blade plane is defined by any
line which is parallel to the pitch axis and which can validly
define a plane. This is in particular the case for a line being
parallel to the pitch axis which intersects the chord at the tip
base.
[0013] An advantage of the disclosed rotor blade is an increased
power production capability of the wind turbine where the rotor
blade is attached to due to higher pressure recoveries for the
rotor blade near the tip portion. Additionally, acoustic noise may
be reduced due to an advantageous angle between the trailing edge
and the streamline of the airflow which flows from the leading edge
across the rotor blade.
[0014] Another advantage of the inventive tip portion is a
reduction, or even a prevention, of flow separation, thus leading
to a promotion of tip vortices which are small compared to tip
vortices of a similar, conventional rotor blade.
[0015] In other words, the tip portion may be described as being
raked in an edgewise direction of the rotor blade.
[0016] A span line of the rotor blade is referred to as a
longitudinal axis of the rotor blade extending from the root to the
tip. Compared to the pitch axis, which always is a straight line,
the span line follows the actual shape of the rotor blade. This
means that if e.g. the blade body comprises a slight aft-swept, the
span line is slightly aft-swept, too. The span line follows the
shape of rotor blade in its tip portion likewise. Thus, a curved
tip portion results in a curved span line.
[0017] At each radial position along the span line, in particular
at each radial position of the blade body and of the tip portion, a
chord can be assigned to the rotor blade. In the context of this
application, the chord is defined as being the line connecting the
trailing edge and the leading edge in a plane perpendicular to the
pitch axis. This plane is also referred to as the specific
cross-section profile or airfoil of the rotor blade at a radial
position.
[0018] The tip base is a plane which is perpendicular to the pitch
axis. The chord which is determined at the tip base is also
referred to as the tip base chord.
[0019] In particular, the tip base may be understood as the plane
being perpendicular to the pitch axis at the specific radial
position of the pitch axis where the trailing edge starts to
describe a concave curvature and/or the leading edge starts to
describe a convex curvature, as viewed from the suction side of the
rotor blade.
[0020] Furthermore, a reference plane can be assigned to the rotor
blade. The reference plane is defined as the plane comprising the
pitch axis and being perpendicular to the tip base chord.
[0021] The trailing edge sweep angle is defined as the angle
between the pitch axis and the trailing edge and is determined at
the tip base. As known from geometry, an angle in general is
defined by a first line, a second line and an intersection of both
lines, which is also referred to as the vertex. Thus, precisely
speaking, the first side of the trailing edge sweep angle is the
tangent of the trailing edge and the second side of the trailing
edge sweep angle is a line which is parallel to the pitch axis and
which intersects the intersection of the tangent of the trailing
edge and the tip base. This intersection may be referred to as the
vertex.
[0022] For any point along the trailing edge of the tip portion, a
trailing edge sweep angle may be defined. It is one aspect of the
invention that the trailing edge sweep angle increases from the tip
base to the tip. In other words, the trailing edge sweep angle
measured at the tip is greater than the trailing edge sweep angle
measured at the tip base.
[0023] It has to be noted that the expression of the curved shape
of the protection of the trailing edge of the tip portion on the
rotor blade plane comprises a continuously curved shape of the
trailing edge, but also a shape of the trailing edge resembling a
polygon. In other words, the trailing edge of the tip portion may
partially or fully be stepped or segmented.
[0024] The chord tilt angle is defined as the angle between the tip
base chord and a specific chord at a specific radial position
between the tip base and the tip. Mathematically speaking, the
first side of the chord tilt angle is represented by the specific
chord and the second line is represented by a line which is
parallel to the tip base chord and intersects the specific
chord.
[0025] In other words, one aspect of the invention is characterized
by the fact that the orientation of the chords of the tip portion
changes between the tip base and the tip, compared to the chord at
the tip base.
[0026] The blade body may be substantially straight, meaning that
the span line of the blade body substantially coincides with the
pitch axis. However, the curved and twisted tip portion may also be
combined with a blade body which itself comprises a swept or curved
shape. Thus, the raked tip portion may be combined with any shape
of the blade body of the rotor blade.
[0027] A wind turbine, which is also referred to as a wind power
plant, is a device that converts kinetic energy from the wind into
electrical energy.
[0028] The tip base is a virtual or imaginary surface or area
within the rotor blade for defining the limit of the tip portion.
If, as an example, the tip portion is manufactured separately with
regard to the blade body, the tip base may be the real and actual
area where the tip portion is connected or attached to the blade
body. However, this is not necessarily the case, as the rotor blade
comprising the tip portion may also be manufactured unitarily, i.e.
as a single piece.
[0029] It has to be noted that the trailing edge sweep angle does
not necessarily have to increase along the whole way from the tip
base to the tip. However, there have to be parts or sections of the
trailing edge of the tip portion, wherein the trailing edge sweep
angle increases.
[0030] In an advantageous embodiment, the trailing edge sweep angle
increases by at least 2 degrees, in particular by at least 5
degrees, from the tip base to the tip.
[0031] In another advantageous embodiment, the trailing edge sweep
angle increases by at most 40 degrees, in particular by at most 30
degrees, from the tip base to the tip.
[0032] Note that the given values for the trailing edge sweep angle
represent advantageous minimum and maximum values, respectively.
These minimum and maximum trailing edge sweep angles refer to a
comparison of the trailing edge sweep angle determined at the tip
base with the trailing edge sweep angle determined at the tip. If,
for instance, the trailing edge is curved evenly from the tip base
to the tip and the trailing edge sweep angle increases by 2 degrees
between the tip base and the tip, this would imply that the
trailing edge sweep angle determined halfway between the tip base
and the tip only increases by 1 degree compared to the tip
base.
[0033] In the exemplarily case of a straight blade body, the
trailing edge may be parallel to the pitch axis at the tip base. In
this case, the trailing edge sweep angle is 0 degree at the tip
base. Due to the curved trailing edge of the tip portion, the
trailing edge sweep angle may for example be 10 degrees at the tip.
Then, the trailing edge sweep angle is described as increasing by
10 degrees between the tip base and the tip. If, however, in
another example the trailing edge comprises an angle of 7 degrees
with the pitch axis at the tip base, which may well be the case of
a swept blade body, and the trailing edge sweep angle is 10 degrees
at the tip, then the trailing edge sweep angle increases by only 3
degrees between the tip base and the tip.
[0034] In other words, the advantageous minimum and maximum values
of 2 degrees and 40 degrees relate to a relative increase of the
trailing edge sweep angle from the tip base to the tip.
[0035] In another advantageous embodiment, the chord tilt angle
varies by at least 2 degrees, in particular by at least 5 degrees,
between the tip base and the tip.
[0036] In another advantageous embodiment, the chord tilt angle
varies by at most 30 degrees, in particular by at most 20 degrees,
between the tip base and the tip.
[0037] Similarly as for the trailing edge sweep angle, the given
advantageous values of minimum and maximum chord tilt angles,
respectively, refer to a relative variation of the chord tilt angle
between the tip base and the tip.
[0038] It may be advantageous that the chord tilt angle features
both sections with increase and sections with decrease along the
pitch axis between the tip base and the tip. In particular, the tip
portion may comprise a first section, where the trailing edge is
twisted towards a suction of the blade body, and/or a second
section, where the trailing edge is twisted towards a pressure side
of the blade body. Suction and pressure side are commonly used
expressions in the art of rotor blade aerodynamics.
[0039] In other words, it may be advantageous that the chords are
inclined or orientated to one side in the first section, thus
exhibiting negative chord tilt angles in the first section, and
that they are inclined or orientated to the other side in the
second section, thus exhibiting positive chord tilt angles in the
second section. An inclination towards the suction side is referred
to negative chord angles and an inclination towards the pressure
side is referred to positive chord angles
[0040] The radial position where the chord tilt angles change from
a decreasing slope to an increasing slope is also denoted as a
toe.
[0041] It may be advantageous that the first section is adjacent to
the tip base, while the second section is adjacent to the tip.
[0042] Figuratively speaking, coming from the tip base and heading
towards the tip, in one embodiment of the invention the chords are
firstly inclined towards the suction side of the blade body, reach
a maximum inclination to the suction side, which is referred to as
the toe, and subsequently are increasingly inclined to the pressure
side of the blade body until reaching the tip with a considerable
inclination towards the pressure side.
[0043] It may be advantageous that the increase of the chord tilt
angle in the second section is steep compared to its decrease in
the first section.
[0044] In another advantageous embodiment, the chord length
decreases at a higher rate in the second section compared to the
first section.
[0045] It has to be found that the combination of a relatively
strong decrease of the chord length in a section close to the tip,
combined with a relatively strong increase of the chord tilt angle
may provide an exceptionally large improvement of the efficiency of
the wind turbine and an exceptionally large reduction of the
aerodynamic noise generated by the rotor blade.
[0046] In another advantageous embodiment, the projection of the
leading edge of the tip portion on the rotor blade plane has a
curved shape such that a leading edge sweep angle increases from
the tip base to the tip.
[0047] The leading edge sweep angle is defined as the angle between
the pitch axis and the leading edge and is determined at the tip
base. Precisely speaking, the first side of the leading edge sweep
angle is the tangent of the leading edge, and the second side of
the leading edge sweep angle is a line which is parallel to the
pitch axis and which intersects the intersection of the tangent of
the leading edge and the tip base. This intersection may be
referred to as the vertex of the leading edge angle.
[0048] Advantageously, the leading edge sweep angle increases by at
least 10 degrees, in particular by at least 20 degrees, from the
tip base to the tip.
[0049] In another advantageous embodiment, the leading edge sweep
angle increases by at most 80 degrees, in particular by at most 60
degrees, from the tip base to the tip.
[0050] As in general the tip portion does not only comprise a
trailing edge but also a leading edge, the tip portion may also be
characterized by the leading edge sweep angle. The leading edge
sweep angle is defined similarly to the trailing edge sweep angle.
If the tip portion is curved towards the trailing edge of the blade
body, the leading edge sweep angle is generally larger than the
trailing edge sweep angle. If the tip portion is curved towards the
leading edge of the blade body, the leading edge sweep angle is
generally smaller than the trailing edge sweep angle.
[0051] Advantageously, the tip portion is curved towards the
trailing edge of the blade body, which is also referred to as an
aft-sweep.
[0052] A curvature towards the trailing edge of the blade body has
the advantage of an advantageous angle between the streamline of an
airflow which flows from the leading edge across the rotor blade
and the trailing edge, which may lead to an overall reduction of
aerodynamic noise, i.e. a reduction of the sound pressure level
generated by the rotor blade. This is due to directivity
effects.
[0053] Note that the advantageous minimum and maximum values for
the leading edge sweep angles have to be understood similarly as
the minimum and maximum values for the trailing edge sweep
angle.
[0054] In another advantageous embodiment, the trailing edge of the
tip portion, as viewed from the suction side of the rotor blade, is
at least partially concavely shaped and the leading edge of the tip
portion, as viewed from the suction side of the rotor blade, is at
least partially convexly shaped.
[0055] In particular, the whole trailing edge of the tip portion
may be concavely shaped and/or the whole leading edge of the tip
portion may be convexly shaped.
[0056] In another advantageous embodiment, the projection of the
trailing edge of the tip portion on the reference plane has a
curved shape such that a cant angle increases from the tip base to
the tip.
[0057] The cant angle is defined as the angle between the trailing
edge and the line which is parallel to the pitch axis and
intersects the intersection of the tangent of the trailing edge
with the tip base. This intersection is referred to as the vertex
of the cant angle.
[0058] Figuratively speaking, the cant angle refers to a sweep of
the tip portion out of the rotor blade plane, while the trailing
edge sweep angle refers to a sweep of the trailing edge within the
rotor blade plane. The twist of the trailing edge, characterized by
the chord tilt angle, differs from the cant of the tip portion,
characterized by the cant angle, in that the twist basically
relates to a displacement or off-set of solely the trailing edge,
while the tip portion as a whole substantially remains in the rotor
blade plane, compared to the cant, which basically relates to a
displacement of off-set of the tip portion as a whole out of the
rotor blade plane.
[0059] It may thus be advantageous to provide a tip portion wherein
the trailing edge comprises a sweep within the rotor blade plane,
characterized by the trailing edge sweep angle, a sweep out of the
rotor blade plane, characterized by the cant angle, and a twist of
the trailing edge, characterized by the chord tilt angle. By
combining all three features, an optimal design of the tip portion
can be realized.
[0060] The sweep of the tip portion which is characterized by the
cant angle is also referred to as a flapwise rake of the rotor
blade. Thus, in other words, an edgewise rake and a flapwise rake
may be superimposed to achieve an optimum design of the tip
portion.
[0061] Note that the tip portion may be bent towards the tower of
the wind turbine or away from the tower, in case that the rotor
blade is mounted to a wind turbine which comprises a tower.
[0062] It may be advantageous that the cant angle increases by at
least 20 degrees, in particular by at least 40 degrees, from the
tip base to the tip.
[0063] An advantageous maximum for the cant angle may be at 80
degrees. However, note that in principle also cant angles exceeding
80 degrees or even exceeding 90 degrees are possible.
[0064] In another advantageous embodiment, the trailing edge sweep
angle and/or the cant angle increases monotonically from the tip
base to the tip of the rotor blade.
[0065] In another advantageous embodiment, the leading edge sweep
angle increases monotonically from the tip base to the tip of the
rotor blade.
[0066] Thus, note that the sweep angles and/or the cant angle may
monotonically increase in the region from the tip base to the tip.
However, they do not necessarily have to exhibit this monotonic
behavior. According to the specific design of the rotor blade as a
whole and its designated application, a non-monotonic design of the
sweep angles and/or the cant angle may be advantageous, too.
[0067] In another advantageous embodiment, at least one segment of
the trailing edge of the tip portion is shaped as a straight line
and/or at least one segment of the leading edge of the tip portion
is shaped as a straight line.
[0068] It may for example be beneficial from a manufacturing point
of view to include one or more substantially straight segments of
the trailing edge or the leading edge in order to facilitate
manufacturing of the tip portion. Such a segment is also referred
to as a sub-segment of the trailing edge or of the leading edge,
respectively.
[0069] In another advantageous embodiment, the length of the tip
portion, as projected on the rotor blade plane and measured along
the pitch axis, is between 0.5% and 10%, in particular between 1%
and 5%, of the length of the entire rotor blade from the root to
the tip as projected on the rotor blade plane and measured along
the pitch axis.
[0070] In other words, the tip portion comprises the part of the
rotor blade which is the outermost part in a radial direction and
comprises 0.5% to 10% of the total radial length of the rotor
blade. Exemplarily, the tip portion is defined as the outermost 0.5
meters to 3 meters of the rotor blade.
[0071] Note that the given relative and absolute values refer to a
distance along the pitch axis which is not necessarily equal to a
distance along the span line of the rotor blade.
[0072] In another advantageous embodiment, the rotor blade is
shaped such that the tip is separated from a tangential plane,
wherein the tangential plane is defined to be perpendicular to the
pitch axis and tangential to the tip portion.
[0073] Figuratively speaking, the tangential plane is tangential to
the outermost point of the rotor blade, wherein outermost is
defined as furthest away from the root and measured along the pitch
axis.
[0074] Particularly in a case where the tip portion is heavily
curved and/or in a case where the blade body comprises a
significant sweep, there may be a separation or off-set between the
tip and the tangential plane. However, advantageously, this
separation between the tip and the tangential plane is below 1% of
the total length of the rotor blade, defined by the distance from
the tip to the root along the pitch axis. In absolute values, the
separation may be in a range between 1 centimeter and 100
centimeters.
[0075] It shall be noted that in the context of the whole
application, the tip of the rotor blade may also be characterized
as the point where the leading edge meets the trailing edge.
[0076] In another advantageous embodiment, the rotor blade deforms
under wind loads such that the trailing edge sweep angle and/or the
chord tilt angle and/or the cant angle changes at least about 5%,
in particular about 15%, compared to an unloaded state of the rotor
blade.
[0077] It shall be stressed that in the context of this
application, features such as angles, dimensions or positioning
relative to each other relate to a rotor blade in an unloaded
state, i.e. in a state without wind loads acting on the rotor
blade. These features may change if wind loads act on the rotor
blade. In particular, the tip portion is susceptible to a
considerable structural deformation under wind loads. Thus, the
given values for the mentioned feature may change if wind loads are
acting on the rotor blade.
[0078] In another advantageous embodiment, the tip portion is added
to the blade body as a retrofit.
[0079] On the one hand, a retrofit refers to a part, in this case
the tip portion, which is added on an existing rotor blade. For
instance, a wind turbine may already be in operation and at a given
moment the tip portion is added to the existing rotor blade. This
may be done without disassembling the wind turbine, in particular
without dismounting the rotor blade from the hub of the wind
turbine. However, it may also be possible to dismount the rotor
blade, attach the retrofit tip portion and subsequently remount the
rotor blade to the hub of the wind turbine.
[0080] On the other hand, retrofitting shall relate to the fact
that the rotor blade as such is not manufactured as a single piece
but is manufactured with a conventional tip portion, or no tip
portion, and subsequently the inventive tip portion is added to the
blade body during the manufacturing process.
[0081] In another advantageous embodiment, the rotor blade is
manufactured as a single piece, i.e. unitarily.
[0082] An advantage of manufacturing the rotor blade unitarily is a
good mechanical stability at the tip base. However, it has to be
kept in mind that transportation of the manufactured rotor blade
has to be considered, too, which might be a challenge, in
particular if the tip portion is heavily curved and/or the tip
portion has a considerable extension.
[0083] Finally, in another advantageous embodiment, the trailing
edge and the streamline of an airflow which flows from the leading
edge across the rotor blade comprise an angle which is smaller than
80 degrees, in particular smaller than 50 degrees at the tip
portion of the rotor blade.
[0084] It has been found that an angle which is smaller than 80
degrees between the trailing edge and the streamline may reduce
aerodynamic noise which is generated by the rotor blade under
operation. This is due to directivity effects on the sound pressure
level. A separation of the airflow may also be the cause of
undesired aerodynamic noise. One effect of the curved and twisted
trailing edge of the tip portion is that the streamline of the
airflow is guided such that the angle between the streamline and
the trailing edge is reduced compared to a conventional tip
portion.
BRIEF DESCRIPTION OF THE DRAWINGS
[0085] Embodiments of the invention are now described, by way of
example only, with reference to the accompanying drawings, of
which:
[0086] FIG. 1 shows a wind turbine;
[0087] FIG. 2 shows a rotor blade in a top view onto the rotor
blade plane;
[0088] FIG. 3 shows a tip portion of a rotor blade in a top view
onto the rotor blade plane;
[0089] FIG. 4 shows a rotor blade with a swept blade body in a top
view onto the rotor blade plane;
[0090] FIG. 5 shows a perspective view of a rotor blade comprising
a cant angle;
[0091] FIG. 6 shows a conventional tip portion and a raked tip
portion of a rotor blade;
[0092] FIG. 7 shows a raked tip portion added as a retrofit to a
conventional tip portion of a rotor blade;
[0093] FIG. 8 shows a tip portion of a rotor blade in a top view
onto the rotor blade plane;
[0094] FIG. 9 shows the chord lengths of the tip portion shown in
FIG. 8;
[0095] FIG. 10 shows the chord tilt angles of the tip portion shown
in FIG. 8;
[0096] FIGS. 11a-f shows the cross-sections of the tip portion
shown in FIG. 8 at specific radial positions;
[0097] FIG. 12 shows the deflection of the streamline of an airflow
which flows from the leading edge across the rotor blade;
[0098] FIG. 13 shows a tip portion of a rotor blade in a top view
onto the reference plane; and
[0099] FIG. 14 shows another embodiment of a rotor blade.
[0100] The illustration in the drawings is schematically. It should
furthermore be noted that numerals which refer to similar features
or elements are referred to with the same numeral throughout the
drawings.
DETAILED DESCRIPTION
[0101] In FIG. 1, a wind turbine 10, erected on a ground 14, is
shown. Note that, in general, wind turbines may be installed
onshore or offshore. The wind turbine 10 in FIG. 1 comprises a
tower 11 and a nacelle 12, wherein the nacelle 12 is rotatable
mounted to the tower 11 by means of a yaw bearing. The wind turbine
10 furthermore comprises a hub 13. The hub 13 is rotatable mounted
to the nacelle 12 by means of a bearing. The hub 13 is connected to
a rotor which, inside the nacelle 12, is connected to a rotor part
of a generator. The generator is arranged and prepared to convert
rotational movement from the hub 13 into electrical energy. Three
rotor blades 20 (two of them are shown in FIG. 1) are connected to
the hub 13. The rotor blades 20 are arranged and prepared for being
pitched about a respective pitch axis 23. The pitch axis 23
coincides with a longitudinal axis of the rotor blade 20. A control
mechanism adjusts a pitch angle such that an optimum power is
generated by the wind turbine 10 depending, amongst other
parameters, on the actual wind speed and wind direction. In
general, the wind turbine may be a direct drive wind turbine,
meaning that the rotor is directly connected to the generator.
Alternatively, the wind turbine may also be a geared wind turbine.
In that case a gearbox may be present between the hub 13 and the
generator.
[0102] FIG. 2 shows a rotor blade 20 in a top view onto a rotor
blade plane and onto a suction side 22 of the rotor blade 20. The
rotor blade 20 is divided into three areas. A first area is
referred to as the root portion 21 comprising a root 41.
Advantageously, the root 41 comprises a shape of a circular
cylinder. The root 41 is arranged and prepared for being mounted to
the hub 13 of the wind turbine 10. A pitch axis 23 extends through
the center of the circular root 41. The extension of the root
portion 47 may exemplarily comprise 5% compared to a total
extension 46 of the rotor blade 20 from the root 41 to a tip 33 of
the rotor blade 20.
[0103] Adjacent to the root portion 21 is the blade body 22 of the
rotor blade 20. The blade body 22 is also referred to as an airfoil
section of the rotor blade 20. The blade body 22 is configured such
that it is mainly based on aerodynamical considerations. Compared
to the blade body 22, the design of the root portion 21 is mainly
driven by structural considerations. The blade body 22 comprises a
leading edge 24 and a trailing edge 25. The leading edge 24 may,
for instance, comprise a substantially cylindrical shape, while the
trailing edge 25 may, for instance, have a substantially sharp
edge. A chord is defined at each radial position of the rotor blade
20 as the shortest straight line between the leading edge 24 and
the trailing edge 25. A span line or centerline may be defined as
halfway between the leading edge 24 and the trailing edge 25.
[0104] The area of the blade body 22, where the chord is maximum,
is referred to as the shoulder 42 of the rotor blade 20.
Furthermore, a suction side 26 and a pressure side 27 are assigned
to the rotor blade 20.
[0105] The third area of the rotor blade 20 is referred to as the
tip portion 30 of the rotor blade 20. The tip portion 30 joins the
blade body 22 at the tip base 31. Note that in general the tip base
31 is an imaginary or virtual area of the rotor blade 20. In
particular, the tip base 31 may also be understood as the plane
being perpendicular to the pitch axis 23 at the specific radial
position of the pitch axis 23 where the trailing edge 25 starts to
describe a concave curvature and/or the leading edge starts to
describe a convex curvature, as viewed from the suction side 24 of
the rotor blade 20. The chord at the tip base 31 is referred to as
the tip base chord 32.
[0106] To give an idea of the respective dimensions of a rotor
blade, an exemplary rotor blade 20 may have a total length 46
characterized by the distance between the root 41 and the tip 33 of
70 m (meters), while the root portion 21 only measures 2 m and the
tip portion 30 measures 3 m, the latter characterized by the
distance 45 between the tip 33 and the tip base 31.
[0107] FIG. 3 shows a tip portion 30 of a rotor blade 20 in a top
view onto the rotor blade plane and onto a suction side 22 of the
rotor blade 20. The rotor blade 20 comprises a straight blade body
22 and the pitch axis 23 intersects the tip base chord 32 halfway.
Note that the span line 43 of the rotor blade 20 comprises a curved
shape in the region of the tip portion 30, following the shape of
the tip portion 30. The tip base 31 comprises the cross-section of
the rotor blade 20 at the specific radial position where the tip
portion 30 joins the blade body 22.
[0108] The tip portion 30 comprises a leading edge 24 and a
trailing edge 25. Furthermore, it comprises a tip 33 which is
slightly separated from a tangential plane 29 which is defined to
be the plane which is perpendicular to the pitch axis 23 and
tangential to the tip portion 30. Exemplarily, the distance between
the tip base 31 and the tangential plane 29 is 2.5 m, while the
distance between the tangential plane 29 and the tip 33 is 1.5 cm.
A trailing edge sweep angle 34 determined at the tip base 31 can be
given for every point on the trailing edge 25 of the tip portion
30. The trailing edge sweep angle 34 comprises 0 degree at the tip
base 31, as it is just measured at the tip base 31. It increases
monotonically until the tip 33. The trailing edge sweep angle 34 at
the tip 33 comprises exemplarily 20 degrees.
[0109] The leading edge sweep angle 35 is determined likewise.
Again, the leading edge sweep angle 35 at the tip base is 0 degree
while the leading edge sweep angle 35 at the tip 33 is 36 degrees.
Note that the leading edge sweep angle 35 increases monotonically
from the tip base 31 to the tangential plane 29 and slightly
decreases between the tangential plane 29 and the tip 33.
[0110] In FIG. 4, a rotor blade 20 with a swept blade body 22 in a
top view onto the rotor blade plane and onto a suction side 22 of
the rotor blade 20 is shown. Similar to a rotor blade with a
straight blade body, the rotor blade 20 with a swept blade body 22
can be divided into a root portion 21, the blade body 22 and a tip
portion 30. In FIG. 4, both a conventional tip portion 301 and a
tip portion 30 according to one embodiment of the invention are
illustrated. The trailing edge sweep angle 341 at the tip base 31
comprises 35 degrees and the trailing edge sweep angle 34 at the
tip 33 comprises 60 degrees. Thus, the trailing edge sweep angle 34
increases by 25 degrees between the tip base 31 and the tip 33.
Likewise, the leading edge sweep angle increases as well between
the tip base 31 and the tip 33 which for sake of clarity is not
shown in FIG. 4.
[0111] FIG. 5 shows a rotor blade 20 comprising a cant angle 37 in
a perspective view. The rotor blade 20 can be viewed from the
leading edge 24. The tip base is defined by the plane which
comprises the lines I-I', II-II' and the tip base chord 32. FIG. 5
shows an advantageous embodiment of the invention of a rotor blade
20 with a tip portion 30 which comprises a cant angle 37 of
approximately 50 degrees. In other words, the tip portion 30 is
curved out of the rotor blade plane, in the example of FIG. 5
towards the pressure side 27 of the rotor blade 20.
[0112] It has to be noted that the tip portion 30 additionally
comprises a trailing edge which is curved towards the trailing edge
of the blade body 22 and a trailing edge of the tip portion 30 that
is twisted. However, the curvature of the trailing edge of the tip
portion 30 and the twist of the trailing edge of the tip portion 30
is not visible in the perspective view of FIG. 6. A tip portion
with a cant angle 37 is also referred to as a winglet. An advantage
of a tip portion 30 with such a cant angle 37 is a further
improvement of the efficiency and a further noise reduction of the
rotor blade 20 compared to a rotor blade without a winglet.
[0113] FIGS. 6 and 7 show two comparisons of exemplary raked tip
portions 30 according to the invention and conventional tip
portions 301.
[0114] In FIG. 6, the raked tip portion 30 comprises a similar
length as the conventional tip portion 301, wherein the length is
defined as the distance between a first imaginary line A, which can
be identified with the tip base, and a second imaginary line B,
which is the projection of the tip 33 onto the pitch axis 23.
[0115] In FIG. 7, the raked tip portion 30 represents an extension
or an add-on to the conventional tip portion 301. While the length
of the conventional tip portion 301 extends from the imaginary line
A to the imaginary line B1 and is similar to the length of the
conventional tip portion 301 of FIG. 6, the length of the raked tip
portion 30 in FIG. 8 extends from line A to line B2 and thus is
greater than the length of the conventional tip portion 301. An
embodiment as represented in FIG. 7 may advantageously be used when
retrofitting an existing rotor blade.
[0116] FIG. 8 shows a tip portion 30 of a rotor blade in a top view
onto the rotor blade plane. Chords 44 at the specific radial
positions A-A', B-B', C-C', D-D', E-E' and F-F' are shown. The
chord at the position A-A' is also referred to as the tip base
chord 32. At the tip 33, the chord length approaches zero, thus no
chord can be shown.
[0117] FIGS. 9 and 10 show the radial chord length 52 and the
radial chord tilt angle 36 of the rotor blade shown in FIG. 8.
[0118] Specifically, FIG. 9 shows the chord length 52 with regard
to a radial distance from the root of the rotor blade. It can be
seen that the chord length 52 slightly decreases in the radial
positions between A and E. Between the radial positions E and G,
the chord length 52 decreases significantly more.
[0119] As can be seen in FIG. 10, the significant decrease of the
chord length between the radial positions E and G is accompanied by
a sharp increase of the chord tilt angle 36 in this radial section.
Note that the radial position E, where the slope of the chord tilt
angle 36 equals zero, is denoted also as toe.
[0120] FIGS. 11a to 11f show the airfoil profiles, i.e. the
cross-sections perpendicular to the pitch axis, at the radial
positions A to F. The cross-section of the radial position G is not
shown as the chord length at the radial position G approaches zero.
The dimensions of the cross-sections in FIGS. 11a to 11f are
normalized to the cross-section at the tip base, i.e. at the radial
position A.
[0121] In FIG. 11a, no chord tilt angle is shown, as it equals zero
at the tip base. For radial positions extending away from the tip
base and towards the tip, the chord tilt angle increases, as it is
shown for the section between the radial position A and the radial
position E in FIG. 10. Additionally, the chord tilt angle 36 is
negative in this section, which corresponds to an inclination of
the chords 44 towards a suction side 26 of the blade body, as can
be seen in FIGS. 11b to 11e. At the radial position E, i.e. the
toe, the inclination of the chord 44 towards the suction side 26 is
maximal. Going further towards the tip, the inclination first
decreases and subsequently flips to the pressure side 27 of the
blade body. It can be seen in FIG. 11f that the inclination of the
chord 44 at the radial position F is considerable. In particular,
an angle of attack for a designed rotor speed of zero degree may be
realized at the radial position F.
[0122] FIG. 12 shows a tip portion 30 of a rotor blade 20 with a
raked tip portion 30, wherein the tip portion 30 is curved towards
the trailing edge 25 of the blade body 22. It can be seen how the
streamlines 61 of an airflow which flows from the leading edge 24
across the rotor blade is deflected by the tip portion 30. In a
region close to the tip base, where the leading edge 24 and the
trailing edge 25 are substantially perpendicular to the streamline
61, the streamline 61 is hardly deflected by the rotor blade 20.
However, due to the curvature of the tip portion 30, a deflection
of the streamlines 61 is visible. Thus, there results an angle 62
between the trailing edge 25 and the streamline 61 which is smaller
than 90 degrees. It can also be seen that the angle 62 is different
at different positions along the trailing edge 25 of tip portion
30. A consequence of the reduced angle 62 is a reduction of
aerodynamic noise generated by the rotor blade 20.
[0123] In FIG. 13, another way of illustrating the twist of the
trailing edge 25 is shown. More specifically, a top view of the tip
portion 30 onto the reference plane is shown, wherein the reference
plane is defined as the plane comprising the pitch axis 23 and
being perpendicular to the tip base chord. In other words, FIG. 13
shows a top view onto the trailing edge 25. If the trailing edge 25
were not twisted at all, the trailing edge 25 would just be a
straight line if projected onto the reference plane. In FIG. 13,
however, the trailing edge 25 is twisted which is manifested by the
curved line of the trailing edge 25. It can be seen that in the
example shown in FIG. 13, the trailing edge 25 is first twisted
towards the suction side 26, before being twisted towards the
pressure side 27. Note that no chords are shown in FIG. 13 as
chords extend into, or out of, the plane which is illustrated in
FIG. 13.
[0124] Finally, FIG. 14 illustrates the definition of the rotor
blade plane by showing an exemplary rotor blade 20, where the blade
body 22 is curved towards the suction side 26. As the pitch axis 23
and the tip base chord 32 do not intersect, the rotor blade plane
is defined by the line 231, which is parallel to the pitch axis 23,
and the tip base chord 32. The line 231 intersects the tip base
chord 32. Note that in the example of FIG. 14, the tip 33 of the
rotor blade 20 is swept out of the rotor blade plane, thus
comprising a cant angle.
* * * * *