U.S. patent application number 13/643471 was filed with the patent office on 2013-08-01 for rotor for a wind turbine.
This patent application is currently assigned to SE BLADES TECHNOLOGY B.V.. The applicant listed for this patent is Thomas Bjertrup Nielsen, Alejandro Gomez Gonzalez, Rasmus Jensen, Mads Reck, Erik Sloth. Invention is credited to Thomas Bjertrup Nielsen, Alejandro Gomez Gonzalez, Rasmus Jensen, Mads Reck, Erik Sloth.
Application Number | 20130195670 13/643471 |
Document ID | / |
Family ID | 44259798 |
Filed Date | 2013-08-01 |
United States Patent
Application |
20130195670 |
Kind Code |
A1 |
Bjertrup Nielsen; Thomas ;
et al. |
August 1, 2013 |
ROTOR FOR A WIND TURBINE
Abstract
A rotor for a wind turbine, having a diameter of 50 metres or
more, has a blade with at least two blade sections in the
lengthwise direction of the blade, where at least one blade section
has a curvature along the blade and relative to a second blade
section, where said curvature results in the tip end of said blade
being offset (x) relative to the axis of the blade root. A ratio
(x/L) of the offset (x) relative to a distance (L) from the tip end
of the blade to the blade root is between 0 and 0.1.
Inventors: |
Bjertrup Nielsen; Thomas;
(Randers SV, DK) ; Sloth; Erik; (Ronde, DK)
; Gonzalez; Alejandro Gomez; (Aarhus, DK) ;
Jensen; Rasmus; (Ronde, DK) ; Reck; Mads;
(Kobenhavn, DK) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Bjertrup Nielsen; Thomas
Sloth; Erik
Gonzalez; Alejandro Gomez
Jensen; Rasmus
Reck; Mads |
Randers SV
Ronde
Aarhus
Ronde
Kobenhavn |
|
DK
DK
DK
DK
DK |
|
|
Assignee: |
SE BLADES TECHNOLOGY B.V.
ST Hengelo
NL
|
Family ID: |
44259798 |
Appl. No.: |
13/643471 |
Filed: |
April 15, 2011 |
PCT Filed: |
April 15, 2011 |
PCT NO: |
PCT/EP2011/056065 |
371 Date: |
April 15, 2013 |
Current U.S.
Class: |
416/223R |
Current CPC
Class: |
F03D 1/0608 20130101;
Y02E 10/72 20130101; Y02E 10/721 20130101; F05B 2250/71 20130101;
F05B 2250/70 20130101 |
Class at
Publication: |
416/223.R |
International
Class: |
F01D 5/02 20060101
F01D005/02 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 26, 2010 |
DK |
PA201070170 |
Claims
1. The rotor for a wind turbine, where the wind turbine comprises
at least a tower, a nacelle on top of said tower and a rotor, said
rotor having a diameter of 50 metres or more, and being connectable
to a substantially horizontal main shaft, said main shaft being
rotatably mounted in said nacelle, said wind turbine being of the
type where the rotor is facing upwind, where said rotor comprises
at least one blade, three blades and a hub, said blades having an
elongated shape with a root end and a tip end, where said blades at
the root end are connected to said hub and where the interface
between said hub and said blades comprises a pitch mechanism,
further said rotor has a rotor plane, described by the area swept
by the blades, said rotor plane being with a cone shape and having
an angle towards the upwind direction, where the cone shape, at
least partly, originates from the shape of the hub, and where one
blade of said rotor has a combined radius specific solidity
Sol.sub.r at: blade radius r=30% of the rotor radius R, where
Sol.sub.r.ltoreq.0.036; blade radius r=40% of the rotor radius R,
where Sol.sub.r.ltoreq.0.03; blade radius r=50% of the rotor radius
R, where Sol.sub.r.ltoreq.0.0245; blade radius r=60% of the rotor
radius R, where Sol.sub.r.ltoreq.0.021; blade radius r=70% of the
rotor radius R, where Sol.sub.r.ltoreq.0.0175; blade radius r=80%
of the rotor radius R, where Sol.sub.r.ltoreq.0.014; blade radius
r=90% of the rotor radius R, where Sol.sub.r.ltoreq.0.0115; and
blade radius r=96% of the rotor radius R, where
Sol.sub.r.ltoreq.0.0095, and where Sol.sub.r has a level below a
linear interpolation between any of the given blade radii, wherein
a blade of said rotor comprises at least two blade sections in the
lengthwise direction of said blade, where at least one blade
section has a curvature along the blade and relative to a second
blade section, where said curvature results in the tip end of said
blade being offset (x) relative to the axis of the blade root,
where the ratio (x/L) of the offset (x) relative to a distance (L)
from the tip end of the blade to the blade root, said distance (L)
being measured perpendicular to the plane of the root end of the
blade and said offset (x) being measured as the perpendicular
offset from the axis of the blade root is between 0 and 0.1, and
that said rotor is with a cone angle (.beta.) between 0 and 10
degrees, said cone angle (.beta.) being measured between the axis
of the blade root and perpendicular to the axis of the main
shaft.
2. The rotor for a wind turbine according to claim 1, wherein a
blade of said rotor has a combined radius specific solidity
Sol.sub.r at: blade radius r=30% of the rotor radius R, where
Sol.sub.r.ltoreq.0.0345; blade radius r=40% of the rotor radius R,
where Sol.sub.r.ltoreq.0.028; blade radius r=50% of the rotor
radius R, where Sol.sub.r.ltoreq.0.023; blade radius r=60% of the
rotor radius R, where Sol.sub.r.ltoreq.0.019; blade radius r=70% of
the rotor radius R, where Sol.sub.r.ltoreq.0.016; blade radius
r=80% of the rotor radius R, where Sol.sub.r.ltoreq.0.0125; blade
radius r=90% of the rotor radius R, where Sol.sub.r.ltoreq.0.01;
and blade radius r=96% of the rotor radius R, where
Sol.sub.r.ltoreq.0.008, and where Sol.sub.rhas a level below a
linear interpolation between any of the given blade radii.
3. The rotor for a wind turbine according to claim 1, wherein a
blade of said rotor has a combined radius specific solidity
Sol.sub.r at: blade radius r=30% of the rotor radius R, where
Sol.sub.r.ltoreq.0.033; blade radius r=40% of the rotor radius R,
where Sol.sub.r.ltoreq.0.0255; blade radius r=50% of the rotor
radius R, where Sol.sub.r.ltoreq.0.021; blade radius r=60% of the
rotor radius R, where Sol.sub.r.ltoreq.0.017; blade radius r=70% of
the rotor radius R, where Sol.sub.r.ltoreq.0.0135; blade radius
r=80% of the rotor radius R, where Sol.sub.r.ltoreq.0.011; blade
radius r=90% of the rotor radius R, where Sol.sub.r.ltoreq.0.008;
and blade radius r=96% of the rotor radius R, where
Sol.sub.r.ltoreq.0.0065, and where Sol.sub.r has a level below a
linear interpolation between any of the given blade radii.
4. The rotor for a wind turbine according to claim 1, wherein the
ratio (x/L) of the offset (x) relative to a distance (L) is between
0.022 and 0.087.
5. The rotor for a wind turbine according to claim 1, wherein the
ratio (x/L) of the offset (x) relative to a distance (L) is between
0.033 and 0.054.
6. The rotor for a wind turbine according to claim 1, wherein said
rotor is with a cone angle (.beta.) between 1 and 6 degrees, said
cone angle (.beta.) being measured between the axis of the blade
root and perpendicular to the axis of the main shaft.
7. The rotor for a wind turbine according claim 1, wherein said
rotor is with a cone angle (.beta.) between 2 and 5 degrees, said
cone angle (.beta.) being measured between the axis of the blade
root and perpendicular to the axis of the main shaft.
Description
[0001] The present invention relates to a rotor for a wind turbine,
where the wind turbine comprises at least a tower, a nacelle on top
of said tower and a rotor, said rotor having a diameter of 50
metres or more, and being connected to a substantially horizontal
main shaft, said main shaft being rotatably mounted in said
nacelle, said wind turbine being of the type where the rotor is
facing up-wind, where said rotor comprises at least one blade, but
preferably three blades and a hub, said blades having an elongated
shape with a root end and a tip end, where said blades at the root
end are connected to said hub and where the interface between said
hub and said blades comprises a pitch mechanism, further said rotor
has a rotor plane, described by the area swept by the blades, said
rotor plane being with a cone shape, i.e. the rotor plane is with
an angle towards the up-wind direction, where the cone shape--at
least partly--originates from the shape of the hub, and where one
blade of said rotor has a combined radius specific solidity
Sol.sub.r at: [0002] blade radius r=30% of the rotor radius R,
where Sol.sub.r.ltoreq.0.036 [0003] blade radius r=40% of the rotor
radius R, where Sol.sub.r.ltoreq.0.03 [0004] blade radius r=50% of
the rotor radius R, where Sol.sub.r.ltoreq.0.0245 [0005] blade
radius r=60% of the rotor radius R, where Sol.sub.r.ltoreq.0.021
[0006] blade radius r=70% of the rotor radius R, where
Sol.sub.r.ltoreq.0.0175 [0007] blade radius r=80% of the rotor
radius R, where Sol.sub.r.ltoreq.0.014 [0008] blade radius r=90% of
the rotor radius R, where Sol.sub.r.ltoreq.0.0115 [0009] blade
radius r=96% of the rotor radius R, where Sol.sub.r.ltoreq.0.0095,
and where Sol.sub.r has a level below a linear interpolation
between any of the given blade radii.
[0010] Definition of the combined specific solidity Sol.sub.r:
Sol r = n chord 2 .pi. R ##EQU00001##
Where n is the number of blades on the rotor, chord is the length
of the chord of the blade at the specific radius and R is the
maximum diameter of the rotor.
[0011] It is well known in the wind energy industry to have a wind
turbine with a foundation, a tower with a nacelle on top and with a
rotor fixed to a main shaft. During time several different designs
have been developed, but it seems that the type known as the
"Danish Type" has become the most used and the most effective. A
wind turbine of the "Danish Type" is also known as a front runner,
with a substantial horizontal main shaft, and with a rotor having
three blades and facing upwind. As the size of wind turbines is
becoming larger and larger, with rotor diameters of 80 metres and
for the very largest even up to approximately 125 metres, a number
of engineering challenges occurs. One of these challenges is to
secure a sufficient clearance between the tip of the rotating
blades and the tower, under the different working conditions for
the wind turbine.
[0012] Over time several different ways of creating the needed
clearance have been used. The most common ways to obtain sufficient
clearance are to manufacture the blades with certain stiffness and
to tilt the main shaft a few degrees, typically 4 to 6 degrees,
upwards in the end where the rotor are mounted. It is also commonly
known to use a rotor where the blades are mounted to a hub is such
a way that the plane described by the rotor describes a cone, with
an angle of a few degrees relative to a flat rotor plane. This
angle is commonly known as the cone angle .beta..
[0013] Even other attempts has been tried, where the blades on a
rotor are made with a pre-bend, as described in EP 1 019 631 B1 or
with a cone combined with a pre-bend as described in EP 1 596 063
A2.
[0014] It is the object of the invention to provide a rotor for a
wind turbine, where the above mentioned drawbacks with tip to tower
clearance are eliminated and where the necessary tip to tower
clearance are obtained. Further, it is an object of the invention
to provide a rotor structure allowing the blades to have a low
combined radius specific solidity, Sol.sub.r.
[0015] This invention concerns a rotor for a wind turbine according
to the introductory part of this description and according to the
preamble of claim 1, where a blade of said rotor comprises at least
two blade sections in the lengthwise direction of said blade, where
at least one blade section has a curvature along the blade and
relative to a second blade section, where said curvature results in
the tip end of said blade being offset (x) relative to the axis of
the blade root in the flap wise direction of the blade and in the
up-wind direction, where the ratio (x/L) of the offset (x) relative
to a distance (L) from the tip end of the blade to the blade root,
said distance (L) being measured perpendicular to the plane of the
root end of the blade and said offset (x) being measured as the
perpendicular offset from the axis of the blade root, is between 0
and 0.1, and that said rotor is with a cone angle .beta. between 0
and 10 degrees, said cone angle .beta. being measured between the
axis of the blade root and perpendicular to the axis of the main
shaft.
[0016] Having a slender blade profile, and at the same time having
a relatively small ratio (x/L) also called "pre-bending" of the
blade together with a cone shaped rotor plane, has a number of
advantages. First of all the slender blade profiles have the
advantage that the load from the wind is minimized as the blade
area exposed to the wind is smaller and therefore less stresses and
loads are transferred from the blades to the rest of the wind
turbine structure.
[0017] Secondly, it is advantageous that the blades are pre-bend
and thus giving the possibility to be deflected even more by the
wind and still maintaining a sufficient clearance between the tip
of the blades and the tower. Pre-bending does, however, give some
trouble during fabrication and transportation if the ratio (x/L) is
too big.
[0018] Most blades for modern wind turbines are manufactured from
fibres of various types in various types of mats which is
impregnated with a suitably resin using a method well known as
VARTM (vacuum assisted resin transfer moulding). This is a very
attractive process as the impregnating and curing takes place in a
closed mould and thus no fumes or only a very small amount of fumes
will enter the production facilities. If the pre-bend ratio (x/L)
is too big, it becomes difficult to draw the resin by vacuum into
the mould, as a rather large height difference is present. This can
result in difficulties obtaining the preferred and needed quality
in the moulded blades. Another problem with pre-bending with a
large ratio (x/L) is, as mentioned above, transportation. If the
blades have to be transported over land by vehicle the width of the
roads and the height of bridges and tunnels become a relevant
factor. If a blade for example has a length of 50 metres and a
pre-bend ratio of 0.1, this means that the width of the blade will
be 0.1*50=5 meter and then the radius of the root end has to be
added, as we are measuring from the centre of the root end, which
easily could end with a total width of the blade of more than 6
metres. Transportation of such a blade is already a challenge
because of the length, and handling the width makes the challenge
even bigger.
[0019] To be able to have a sufficient tip to tower clearance and
at the same time to have a relatively low pre-bend ratio (x/L), it
is also preferable to have the hub of the rotor manufactured with
the interface for the blades with an angle. By having this angle
the rotor-plane will have the shape of a cone. This cone together
with the pre-bended and slender blades will create a sufficient tip
to tower clearance and at the same time the blades can be
transported by vehicle without extraordinary trouble. Hence there
is a synergy occurring between the single measures slenderness,
pre-bending and coned arrangement of the blades to the hub. This
synergy is getting even more important when it comes to the
definition of the very quantity of each measure.
[0020] In a preferred embodiment of a rotor for a wind turbine
according to the invention, a blade of said rotor can have a
combined radius specific solidity Sol.sub.r at: [0021] Blade radius
r=30% of the rotor radius R, where Sol.sub.r.ltoreq.0.0345 [0022]
blade radius r=40% of the rotor radius R, where
Sol.sub.r.ltoreq.0.028 [0023] blade radius r=50% of the rotor
radius R, where Sol.sub.r.ltoreq.0.023 [0024] blade radius r=60% of
the rotor radius R, where Sol.sub.r.ltoreq.0.19 [0025] blade radius
r=70% of the rotor radius R, where Sol.sub.r.ltoreq.0.016 [0026]
blade radius r=80% of the rotor radius R, where
Sol.sub.r.ltoreq.0.0125 [0027] blade radius r=90% of the rotor
radius R, where Sol.sub.r.ltoreq.0.01 [0028] blade radius r=96% of
the rotor radius R, where Sol.sub.r.ltoreq.0.008, and where
Sol.sub.r has a level below a linear interpolation between any of
the given blade radii.
[0029] In yet another preferred embodiment of a rotor for a wind
turbine according to the invention, a blade of said rotor can have
a combined radius specific solidity Sol.sub.r at: [0030] Blade
radius r=30% of the rotor radius R, where Sol.sub.r.ltoreq.0.033
[0031] blade radius r=40% of the rotor radius R, where
Sol.sub.r.ltoreq.0.0255 [0032] blade radius r=50% of the rotor
radius R, where Sol.sub.r.ltoreq.0.021 [0033] blade radius r=60% of
the rotor radius R, where Sol.sub.r.ltoreq.0.017 [0034] blade
radius r=70% of the rotor radius R, where Sol.sub.r.ltoreq.0.0135
[0035] blade radius r=80% of the rotor radius R, where
Sol.sub.r.ltoreq.0.011 [0036] blade radius r=90% of the rotor
radius R, where Sol.sub.r.ltoreq.0.008 [0037] blade radius r=96% of
the rotor radius R, where Sol.sub.r.ltoreq.0.0065, and where
Sol.sub.r has a level below a linear interpolation between any of
the given blade radii.
[0038] By having the blades with a combined radius specific
solidity Sol.sub.r as described above a very attractive solution is
obtained as the wind turbine structure only is affected with a
minimum of loads. Further, the blades themselves are not exposed to
the same heavy loads and resulting deflection as blades with a
higher combined radius specific solidity. Thus it is a possibility
to manufacture the blades with less rigidity as there is no need
for it, which will affect the choice of the different materials
used for producing the blades. For instance using expensive carbon
or aramid fibres can be minimized or even avoided. Blades made with
some--small or large--content of carbon and/or aramid fibres will
be more expensive and there are a whole lot of production measures
that have to be considered when combining two or more different
types of fibres, e.g. considerations regarding different stiffness,
thermal expansion and electrical conductivity. By not having the
need for the large stiffness the blades can be made of a uniform
type of fibres, e.g. glass fibres and with a suitably resin such as
a polyester or epoxy resin. A further advantage of the slender
blade is that it becomes possible to have rotor blades with less
thickness and thus it becomes possible to use high performance
airfoils which are more efficient.
[0039] In another preferred embodiment of a rotor for a wind
turbine according to the invention the ratio (x/L) of the offset
(x) relative to a distance (L) is between 0.022 and 0.087, or even
more preferred between 0.033 and 0.054, as the pre-bend ratio (x/L)
then will have a size making transportation of long blades of up to
50 metres or more possible by vehicle and at the same time securing
the necessary tip to tower clearance.
[0040] Yet another preferred embodiment of a rotor for a wind
turbine according to the invention can have a rotor with a cone
angle .beta. between 1 and 6 degrees or even more preferred between
2 and 5 degrees, said cone angle .beta. being measured between the
axis of the blade root and perpendicular to the axis of the main
shaft. A cone angle .beta. as described will together with the
previous mentioned construction details fulfil the wish of a wind
turbine with slender and pre-bended blades installed on a hub where
the interface is with an angle. By having this combination the
synergetic effect is archived to make use of blades with less
stiffness and weight and thus the blades will become cheaper to
manufacture and to transport and thus the overall environmental
load will be decreased. The reduced weight together with the
slender profile will also have a positive effect on the complete
structure, as the aerodynamic will decrease and thereby allowing
for further optimization of structural strength of the different
parts of the construction.
[0041] In yet another preferred embodiment of a rotor for a wind
turbine according to the invention the main shaft is tilted with an
angle between -2 and +4 degrees, or between -2 and +2 degrees, or
even between 0 and +2 degrees, said angle being measured at the hub
and between horizontal and the centre line of said main shaft. By
having only a slight positive angle or even a negative tilt angle
on the main shaft, a significant reduction of emitted noise is
obtained. When a traditional wind turbine is running with a main
shaft with an inclination of approximately +4 to +6 degrees noise
is emitted. The noise is especially emitted from the blade moving
downwards. This is caused by the interaction between the blade and
the wind and is increased due to the fact that the downward moving
blade is forced against the wind as a direct consequence of the
angle of the main shaft, also called the tilt angle .alpha.. Thus a
significant tilt angle .alpha. of +4 to +6 degrees leads to the
blades experiencing a higher wind velocity on the downward movement
than on the upward movement and thus more noise is emitted. During
last years problems with noise have been dealt with by erecting the
wind turbines in distant areas and offshore, where no people are
bothered by the noise. By having the tilt angle a in the intervals
as described above this noise is reduced and especially by large
rotor diameters of up to 80 metres or more, this is very
attractive, as it makes it possible to erect large wind turbines
onshore with only a minimum of environmental impact.
[0042] Noise emission from wind turbines has a modulating character
i.e. the emitted noise level is varied over time. At close
distances a major part of this variation originates from the
difference in distance from the observer position to the blade. At
longer distances, this variation in observer distance vanishes, but
modulation of the noise level is still seen from longer distances.
The cause of this is primarily that the blades emit varying noise
as a function of the position in the rotation. When the main shaft
has a tilt angle a and the wind shear is vertical or close to
vertical, the blades will experience variances in the inflow speed
of the wind. This will cause variations in the aerodynamic angle of
attack. These variations will result in the described variation in
noise emission and will be minimized by having only a small tilt
angle .alpha. e.g. between 0 and +2 degrees to horizontal.
[0043] The invention further comprises an embodiment where a blade
for a rotor according to the invention is assembled from two or
more blade sections, and where the two or more sections comprises
co acting means for assembling said blade sections into one
functionally complete blade for a rotor. By having a dividable
blade several advantages regarding manufacturing and transportation
are achieved. As the blades tend to be made longer and longer the
handling of the blades becomes a larger challenge. The
manufacturing and storage facilities have to be very large and
especially during transportation from the place of manufacturing to
the site where the blades is to be installed on a wind turbine,
seems to be quite a challenge. By having the blades build in
sections and assembled on site, a very attractive solution is
reached seen from the mentioned point of view.
[0044] Further there are a number of advantages of the slender
blades, i.e. the low radius specific solidity. When comparing
blades according to the invention with more traditional blades with
a wider chord distribution and thus with a higher Sol.sub.r the
advantages of the slenderness is that the annual yield per load
ratio is higher resulting in lower wind turbine costs per kWh
produced over 20 years. The annual yield increases as the rotor
diameter can be increased with the slender blades for the same load
level compared to a traditional blade with wider chord
distribution. And regarding fatigue loads the same coefficient of
lift--Cl as a function of the angle of attack, will reduce the load
variations over time due to turbulence with a reduced chord
according to the invention. This is with respect to flap wise loads
which will give lower loads on the blades, hub and main frame.
Further with respect to rotor thrust it will give lower loads on
the tower and foundation. Another situation where a rotor for a
wind turbine according to the invention is advantageous is when
designing for extreme loads during idling or stand still of the
rotor at high wind speeds (for instance the 50 year gust). As the
blade area is lower also the extreme loads are lower.
[0045] By having a rotor with only a small tilt angle a noise will
be reduced as mentioned above and a small cone angle .beta. will
result in a rotor with a centre of gravity closer to the main
bearing thus leading to less loads in said main bearing, but also
in the main shaft and in the main frame of the nacelle. Further the
relatively small ratio of the pre-bend will reduce problems during
manufacturing, transportation and handling in general. At the same
time there is the large advantage that the centre of gravity of the
rotor is closer to the main bearing as already mentioned with
regards to the cone angle .beta..
[0046] A further advantage of having a small tilt is the reduction
in unsteady loads. Unsteady loads may be of a purely mechanical
character (for example the own weight of the blades), as well as of
an aerodynamic nature (interaction with the wind). A low rotor tilt
has an influence on the aerodynamic loads, causing a reduction in
the unsteadiness of these. For a rotor with high tilt angles, the
angle between the incident wind and the blades oscillates in time,
causing fatigue loads which are detrimental for the structure. Not
only the fatigue loads are lower, but also the aerodynamic
performance of the whole rotor is higher due to a more optimal
incidence on the blade elements.
[0047] In the following the invention will be described with
reference to the drawing, where
[0048] FIG. 1 shows a part of a wind turbine with normal
blades.
[0049] FIG. 2 shows a part of a wind turbine with slender
blades.
[0050] FIG. 3 shows a rotor with a hub with a cone angle .beta. and
with a pre-bended blade.
[0051] FIG. 4 shows a rotor with a hub with a cone angle .beta. and
with a pre-bended blade.
[0052] FIG. 5 shows a rotor with a hub with a cone angle .beta. and
with a pre-bended blade.
[0053] In FIG. 1 a part of the top of a wind turbine 1 is seen. The
wind turbine 1 consists of a tower 2, a nacelle 3, and a rotor 4,
said rotor 4 consisting of a hub 5 and three blades 6. During
rotation of the blades 6 an area 7 is swept by the blades 6. The
ratio between this area 7 and the area of the three blades 6, which
are facing towards the wind, is an expression for the rotors
solidity. To be able to use a solidity ratio as a function of the
radius a new ratio Sol.sub.r is used. This ratio Sol.sub.r is
called the combined radius specific solidity and is the ratio
between the added length of the chord 8 on all the blades and the
circumference of the circle 15 described by the maximum radius on
the rotor 4.
[0054] Definition of the combined specific solidity Sol.sub.r:
Sol r = n chord 2 .pi. R ##EQU00002##
Where n is the number of blades on the rotor, chord is the length
of the chord of the blade at the specific radius and R is the
maximum diameter of the rotor.
[0055] In FIG. 2 another rotor is seen where the combined radius
specific solidity clearly is less than in FIG. 1. On top of one of
the blades 6 a blade as shown in FIG. 1 is depicted with a dashed
line, just to illustrate the difference in the ratio Sol.sub.r.
[0056] In FIG. 3 the top of a wind turbine 1 is shown as seen from
the side, where the tower 2 has a centre line 10, the nacelle 3 has
a line 11 indicating the direction of the main shaft. Further, a
rotor 4 with a hub 5 and with a blade 6 is shown. The hub 5 is with
an angled interface for the blades 6. The direction of the
interface is indicated with a line 12 parallel to the interface and
the root end of the blade 6. Another line 13 indicates the centre
line 13 of a blade 6 and yet another line 14 is indicating where
the distance x is measured. Further, the distance L is the distance
between the interface line 12 and where the line 14 is intersecting
the line 13. The ratio x/L is used as geometrical expression for
the pre-bend of the blades 6, and the start of the pre-bend is
illustrated with a line 16, and in this embodiment approximately
1/2 of the blade is with a straight pre-bend.
[0057] The angle between the line 11 and horizontal is describing
the tilt angle .alpha.. In this figure the tilt angle .alpha. is
zero. Further the angle between the line 11 and the line 12 is
describing the cone angle .beta..
[0058] In FIGS. 4 and 5 other ways of shaping the pre-bended blades
6 is shown. In FIG. 4 the blade 6 has a straight portion from the
hub 5 and approximately 1/3 of the length of the blade 6, where the
outer 2/3 of the blade has a forward curved shape, illustrated with
the line 16, resulting in the same ratio x/L as seen in FIG. 3. In
FIG. 5 the blade has a straight portion of approximately 2/3 of the
length of the blade 6, and the outer 1/3 is with a curved shape,
once again illustrated by the line 16 and still with the same ratio
x/L as in FIGS. 3 and 4.
REFERENCE LIST
[0059] 1 wind turbine [0060] 2 tower [0061] 3 nacelle [0062] 4
rotor [0063] 5 hub [0064] 6 blade [0065] 7 area [0066] 8 chord
[0067] 9 circumference [0068] 10 centre line [0069] 11 line [0070]
12 line [0071] 13 centre line [0072] 14 line [0073] 15 maximum
radius [0074] 16 line [0075] L distance [0076] x/L ratio [0077]
.alpha. tilt angle [0078] .beta. cone angle
* * * * *