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The Concept of Segmented Wind Turbine Blades: A Review

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There is a trend to increase the length of wind turbine blades in an effort to reduce the cost of energy (COE). This causes manufacturing and transportation issues, which have given rise to the concept of segmented wind turbine blades. In this concept, multiple segments can be transported separately. While this idea is not new, it has recently gained renewed interest. In this review paper, the concept of wind turbine blade segmentation and related literature is discussed. The motivation for dividing blades into segments is explained, and the cost of energy is considered to obtain requirements for such blades. An overview of possible implementations is provided, considering the split location and orientation, as well as the type of joint to be used. Many implementations draw from experience with similar joints such as the joint at the blade root, hub and root extenders and joints used in rotor tips and glider wings. Adhesive bonds are expected to provide structural and economic efficiency, but in-field assembly poses a big issue. Prototype segmented blades using T-bolt joints, studs and spar bridge concepts have proven successful, as well as aerodynamically-shaped root and hub extenders.
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Review
The concept of segmented wind turbine blades: a
review
Mathijs Peeters 1,†,‡ *, Gilberto Santo 2, , Joris Degroote2, and Wim Van Paepegem 1,
1Department of Materials, Textiles and Chemical Engineering, Ghent University, Tech Lane Ghent Science
Park – Campus A, Technologiepark-Zwijnaarde 903, 9052 Zwijnaarde
2
Department of Flow, Heat and Combustion Mechanics, Ghent University, Sint-Pietersnieuwstraat 41 – 9000
Ghent, Belgium
*Correspondence: Mathijs.Peeters@UGent.be
Academic Editor: name
Version July 16, 2017 submitted to Energies
Abstract:
There is a trend to increase the length of wind turbine blades in an effort to reduce the cost
of energy (COE). This causes manufacturing and transportation issues which have given rise to the
concept of segmented wind turbine blades. In this concept multiple segments can be transported
separately. While this idea is not new, it has recently gained renewed interest. In this review paper the
concept of wind turbine blade segmentation and related literature is discussed. The motivation for
dividing blades into segments is explained and the cost of energy is considered to obtain requirements
for such blades. An overview of possible implementations is provided, considering the split location
and orientation as well as the type of joint to be used. Many implementations draw from experience
with similar joints such as the joint at the blade root, hub and root extenders and joints used in rotor
tips and glider wings. Adhesive bonds are expected to provide structural and economic efficiency, but
in-field assembly poses a big issue. Prototype segmented blades using T-bolt joints, studs and spar
bridge concepts have proven successful, as well as aerodynamically shaped root and hub extenders.
Keywords: wind turbine blades; segmented/split blades; modular design; blade joints;
1. Introduction
Over the past decades wind turbines have been developing rapidly. Most notably, the size of the
rotor diameter and the corresponding power output has been increasing steadily to rotor diameters
of up to 180 m, with rated powers as high as 9.5 MW [
1
3
]. This up-scaling trend is still ongoing,
especially offshore and is motivated by an expected reduced cost of energy (COE) for larger rotors as a
result of increased economies of scale [
4
7
]. However, this up-scaling leads to issues which can cause
a steep increase in costs related to the production and handling of blades, to the extent that further
up-scaling may no longer be beneficial. As a consequence, optimal rotor sizes exist for on- and offshore
turbines which can increase as a result of technical improvements [
2
]. Furthermore, methods to reduce
the loads on the rotor have proven successful for reducing the COE. The increase in size of the blades
has led to interest in the concept of so-called "segmented" blades. Instead of the conventional single
piece blades, these are manufactured as a number of segments, which can be transported individually
and assembled at the site of the turbine. While the "segmented", "split" or "modular" blade concept is
not new, it has recently gained increased interest. This paper intends to provide the reader with an
overview of the concept. Design options include span-wise or chord-wise segmentation, the purpose
and location of the division as well as the use of a static joint or a variable mechanism. The available
options are discussed along with their advantages and limitations. Furthermore, the feasibility of
different methods is discussed.
Submitted to Energies, pages 1 – 19 www.mdpi.com/journal/energies
Version July 16, 2017 submitted to Energies 2 of 19
2. Wind turbine blade manufacturing
While initially, aerospace methodologies were used, most modern wind turbine blades are
manufactured from composite materials using methods derived from ship building [
8
,
9
]. Large
clamshell moulds are used to manufacture separate pressure sides (PS) and suction sides (SS) and
a number of shear webs. This is done using processes such as the lamination of pre-impregnated
material, bladder moulding, wet-layup or vacuum assisted resin transfer moulding (VARTM) [
10
12
].
The material is currently placed manually but can be automated [
13
,
14
]. Most frequently the separate
components are joined together using thick adhesive bonds [
15
]. As the blade size increases this leads
to issues. Firstly, tolerances increase resulting in thickness variations of the adhesive bonds which
add weight and cause stress concentrations [
16
]. Secondly, heating and temperature control become
more difficult while very thick laminates give exothermic reactions which can damage the blade [17].
Thirdly, defects become more severe and prevalent in larger volumes resulting in a lower strength than
assumed from coupon data [
18
]. These defects lower the load-carrying capacity of the blade and may
require scrapping the part, which is more expensive for a larger blade [
19
,
20
]. Lastly, modern blades
are often designed with a pre-curved shape, to ensure sufficient tower clearance under extreme load
without using a very stiff design [21].
Modifications have been suggested to counter the issues with manufacturing large blades.
Typically, these allow production in separate components, allowing better quality of the individual
pieces. Frequently, a separately cured spar structure is used [
18
]. Furthermore, Hayden [
22
] suggested
to build the spar cap out of thin pultruded planks glued on top of each other to avoid thick laminates.
Hayden [
17
] suggested producing the blade root in multiple segments for better temperature control.
Kontis [
23
] suggested producing large parts of the blades separately and joining them together using
adhesive bonds before transport. This approach has the advantage of manufacturing segments and
avoids the difficulty of on-site assembly. Additionally, to improve the quality of the adhesive bonds at
the shear webs, Sorensen [
24
] advocated producing the internal spar of a blade in two pieces, of which
the height can be adjusted to order to obtain the desired bond thickness.
3. Transportation of wind turbine blades
In general, wind turbine blades are manufactured at a production facility and subsequently
transported to the installation site [
25
]. Due to local legislation, the total number of transports
and various other factors, transportation costs are highly route dependent. Every haul requires
investigation of the optimal route and transportation method [
26
]. While wind turbine blades are
frequently transported by road, typically, lengths of over 45m need to be transported as oversized
and overweight (OSOW) load requiring specialized trucks with rear steering escorted by service
cars [
27
]. The route has to be analysed to ensure blade transport vehicles can be accommodated
[
27
]. Furthermore, modifications to the road may be required and local regulations may restrict road
transportation to night-time, specific weather conditions and may impose special licenses [
28
,
29
].
Licenses with a limited validity period introduce lead-times and additional costs in the case of a delay
[
30
]. Wind turbine blades can also be transported by rail. While blade lengths are not limited to the
size of a single rail car, trains have to go slower when part of the blade is hanging over board [
31
].
Further, blades are also transported over waterways and seas. However, to prevent twisting of the
ship from damaging the blades, expensive fixtures, custom to every blade type, have to be used [
29
].
As a last resort, blades can also be transported by air lifters. Because helicopters are expensive and
risky, blimp like air lifting devices are under development [
26
,
32
]. Increased difficulty of transporting
larger blades results in a non-linear increase in costs. Beyond certain breakpoints there is a sudden
steep increase [
26
]. On the road, transportation costs rise sharply for blade lengths over 46m and
can be prohibitive for blades longer than 61m [
18
]. Furthermore, there are actual limitations to the
dimensions of components that can be transported for each method [
33
]. These apply to the bounding
box surrounding the blade. As can be seen in Figure 1, the height and width of the box is determined by
Version July 16, 2017 submitted to Energies 3 of 19
Table 1.
Maximum allowed dimensions and weights for the transportation of wind turbine blades,
based on [26].
transportation method max. weight (tonne) max. length (m) max. height (m) max. width (m)
rail 163 27.4 4 3.4
road (over weight) 36 45.7 4.1 2.6
water (barge) >200 76.2 - 16.5
the blade’s maximum chord length and the blade root diameter as well as the amount of pre-bending
and pre-curving. An overview of the maximum allowed dimensions and weights is given in Table 1.
Figure 1.
Top and side view of a modern wind turbine blade, giving an overview of blade transportation
critical dimensions. The solid line shows a blade without pre-curving or sweep, while the dashed line
shows a swept and pre-curved blade. 1) maximum chord length, 2) blade root diameter, 3)blade sweep,
4)blade pre-curving
Various improvements have been made to the conventional transportation methods. One possible
approach is to make the position in which the blade is carried variable. Jensen [
34
] suggested a system
where the blade is suspended at both ends, which can each be lifted. This allows the blade to be lifted
over small obstacles. Similarly, Wobben [
35
] suggested to rotate the blade to pass under obstacles
such as bridges. These systems can be seen in Figure 2. Likewise, Kawada [
36
] proposed connecting
only the blade root to a truck with a system that enables tilting the tip upwards. This allows larger
blades to get past a narrow corner. Furthermore, Nies [
37
] suggested tilting the blade and reducing
the length of the carrying vehicle. Additionally, Pedersen [
38
] improved upon these tilting concepts,
allowing the blade tip to be in front of the truck while using a lighter vehicle. To allow larger blades to
be transported by rail, Landrum [
39
] proposed using two coupled rail cars and using a sliding support.
Another approach is to deform the blade to alter its dimensions. Modern wind turbine blades are often
pre-curved and swept. For larger blades however, the amount of pre-curving is less than desirable, due
to the difficulty of transport [
40
]. This issue could be reduced by applying a load to “straighten” out the
blades while they are transported [
40
]. In addition, to improve blade transportation by rail, Schibsbye
[
31
] advocated using bumpers to bend the more flexible outboard portion of the blades during turns
so that there would be no overhang. An overview of these methods can be seen in Figure 3. Further,
the transportation of blades over water is less restricted. Grabau [
29
] proposed to take advantage of
the similarity between blades and composite boats. When all gaps are sealed, the blades can float in
the water and towed behind a ship. Alternatively, Berry [
41
] investigated producing blades in a small
on-site factory using material kits prepared at the main factory. However, there were difficulties with
handling the blades at the temporary facility.
Version July 16, 2017 submitted to Energies 4 of 19
Figure 2.
Blade road transportation solutions that temporarily change the way the blade is handled
a) Solution where the blade can be rotated to pass under obstacles such as bridges or tunnels. [
35
] b)
system where the blade can be lifted to pass over low obstacles [
34
] c) system where the blade can be
tilted at the root [42]
Figure 3.
An overview of blade transportation solutions that deform the blade to ease transportation. a)
straightening of the pre-curved blade to simplify transportation [
40
] b) temporary deforming the blade
to simplify transportation c) Deforming the outboard portion of the blade during rail transportation to
prevent overhang during turns [31].
4. The cost of energy: requirements for segmented blades
4.1. Cost of energy components
The overall aim of the wind energy industry is to provide energy at the lowest possible cost. This
cost is affected by segmenting. The cost of energy (
COE
) can be modelled as suggested in [
43
], as
can be seen in
(1)
. The
COE
depends on the fixed charge rate (
FCR
), the initial captial cost (
ICC
),
the net annual energy production of the turbine(
AEPnet
), the land lease cost (
LLC
), operations and
maintenance (O&M) cost and the levelized replacement cost (LRC).
COE =FCR ·ICC
AEPnet
+LLC +O&M+LRC
AEPnet (1)
4.2. The initial capital cost
The
ICC
depends on manufacturing transportation and installation cost of the turbine.
Manufacturing costs increase because of the additional material, labour and production steps required
for producing the joint and reinforcing the inboard part of the blade [
44
]. On the other hand, a
Version July 16, 2017 submitted to Energies 5 of 19
cost reduction is possible due to economic benefits. Production facilities can be smaller [
44
46
] and
components can be standardised. For example, a single root segment can be combined with different tip
segments to obtain blades for different wind conditions [
47
,
48
]. Additionally, using different materials
at different locations along the span is economically interesting but requires a difficult transition. This
can be simplified by segmenting [
49
,
50
]. Furthermore, segmentation simplifies quality assurance [
49
].
Blade segmentation can decrease transportation costs [
44
]. Moreover, many sites that are suited for
wind turbines are located in complex terrain with poor infrastructure. Their development may become
cost effective with segmented blades [
51
,
52
]. Installation costs increase because of additional assembly
steps required to make the final blade. In this respect, speed and simplicity of assembly are important.
4.3. Operations and maintenance cost
The cost for operations and maintenance (
O
&
M
) increases because of additional inspections or
maintenance. It may be required to verify the pre-stress of bolts or the protection against water ingress
[
53
]. Minimal additional maintenance and good access and inspectability to the joint are required to
limit this cost increase. Therefore, sensors can be included to monitor the joint [54].
4.4. Levelized replacement cost
The use of a detachable joint could allow replacement of a single segment rather than the complete
blade in the case of damage [
55
,
56
]. This would allow a reduction of the
LRC
, which represents the
cost of replacements over the life of the turbine.
4.5. Net annual energy production
Further, the annual energy production (
AEP
) has a very strong influence on the
COE
since it
has to offset all the costs including those not related to the rotor. The performance of the rotor will
decrease by alterations to its outside shape. Therefore, joints should use holes that can be covered or
blind holes from the inside of the blade [
50
]. Furthermore, a lower rotor inertia makes it easier for
the control system to keep the ratio of the rotational speed of the rotor to the wind speed optimal
under fluctuating wind conditions, thereby resulting in a higher
AEP
[
57
]. The additional inertia
resulting from the joint may therefore reduce the
AEP
. Additionally, the
AEP
will be decreased if a
local stiffened portion is included [58].
4.6. General considerations for segmented blades
In order to minimize the
COE
resulting from a segmented blade the different cost components
have the following considerations based on [44,59].
Initial capital costs
manufacturing costs
tolerance requirements
production complexity and accuracy
ability to use with conventional production methods
quality control
positioning accuracy and speed of assembly
Annual energy production
reliability
aerodynamics
weight of the joint
Annual operating expenses
requiring minimal inspection
easy to repair during service
possibility of disassembly for replacing segments
Version July 16, 2017 submitted to Energies 6 of 19
Table 2. An overview of blade segmentation strategies.
Segmentation strategy Type of division Advantages Drawbacks
Reducing component lengths Span-wise Potential cost reductions Goes against historical trend
Slender blades reduce available space
Optimal split transport/structure differs.
Division of structural spar
Reducing component width/height Chord-wise No division of structural spar. Transfer of edge-wise loads
Obtaining variable rotor loading Span-wise: telescopic blades Increased power output. Division of structural spar
Chord-wise: trailing edge flaps Reduced extreme and fatigue loads.
No need to divide structural spar. Increased complexity
4.7. Cost effectiveness of blade segmentation
Segmenting blades is useful if this results in a reduced COE. For example, Dutton [
44
] reported
an expected increase in blade cost of approximately 19% for a 60m blade, while the transportation
costs decreased only about 5% of the total price of the blade, thus overall resulting in an elevated COE.
However, from Dutton [
44
] it is clear that the relative added cost of segmenting a blade decreases with
the size of the blades. Further, at a turbine level, the optimum scale is determined by the ratio between
capital costs and other costs [
2
]. Because the fixed costs are significantly higher for offshore turbines
than for their onshore counterparts, the optimum size for offshore turbines is larger than onshore [
2
].
Additionally, for land based turbines, transportation costs may be extremely high for certain sites that
do allow for a high
AEP
. Therefore, segmentation is most likely to be cost effective for either very
large, typically offshore turbines or on-shore turbines that are installed on sites that allow a high yield
but are otherwise difficult to access.
5. Blade segmentation strategy
Blade segmentation can be done following different strategies. These are detailed in the following
sections. An overview is provided in Figure 4.
Figure 4.
Different segmentation strategies. a) Blade with a separate TE-segment to reduce the blades
width b) Blade with separate LE and TE panel segments to reduce the blade width. c)Blade divided to
reduce the length of the components. d) Telescopic wind turbine blade.
5.1. Segmenting to obtain a reduced component length
Large blades cannot get past narrow corners. This issue can be alleviated by splitting the blades
into in-board and out-board segments. However, such a division requires the use of highly loaded
structural joints to transfer loads between the segments. Introducing such additional joints goes
against the historical trend in aerospace and wind energy of reducing the number of components [
18
].
Furthermore, fatigue design is better off without joints [
60
]. Additionally, there is a trend to produce
more slender blades with higher tip speed ratios (TSR) and reduced chord lengths resulting in less
space for a segmentation joint [
61
,
62
]. While the split location may be determined as to minimize
transportation costs, it may also be influenced by structural consequences. The blade loads increase
non-linearly towards the root. Meanwhile, modern blade designs use very thick airfoils near the
root, where structural requirements dominate the design and very thin ones toward the tip, where
aerodynamic performance dominates. As a consequence, the ratio of section forces to the available
cross-section is the highest around the center of the blade [
59
]. At this location, a very heavy joint
Version July 16, 2017 submitted to Energies 7 of 19
would be required. The ratio of section forces to the available cross-section is lower towards the
tip portion and towards the root portion, with the tip region expieriencing the lowest section forces.
However, while this was also true for the 61.5m blade considered in [
48
], a mid-span location was still
selected.
5.2. Segmenting to obtain a reduction in width and height of the components
On straight roads, the width and height of the blade’s bounding box are the main limiting factors.
The area of maximum chord length is typically critical since it can easily reach a size of 6m [
50
]. To
counter this problem, [
63
] tried to alleviate the transportation issues by truncating a blade around the
area of the maximum chord length. However, in this particular study, the prototype blade was found
not to perform as expected. More beneficially, the blade can be segmented to obtain a separate trailing
edge segment [
50
,
64
,
65
]. This segmentation strategy can be applied without dividing the blade’s
structural spar. As a consequence the segmentation joints are not highly loaded and typically the
trailing edge segment does not transfer loads coming from the tip region to the root. Alternatively, the
blade can be split in a load-bearing structural spar and a non-structural aerodynamically shaped skin
to reduce the width of the structure. Multiple authors [
66
71
] have suggested to consider the blade as
a structure consisting of a load-bearing part (the spar) and an aerodynamic skin. In this approach it is
possible to maintain a single part for the load bearing component, while making separate segments for
the blade skin. However, conventionally, the skin transfers shear loads between the spar and trailing
edge reinforcements originating from edge-wise loads. The decoupled skin concept should avoid to
break up the structure that handles the edgewise loads [18].
5.3. Segmenting to obtain a variable rotor loading
Control strategies such as varying the blade pitch or the rotor speed are used to produce the
maximum amount of energy while limiting the load to the turbine’s rated power. Additionally, various
strategies are used to reduce the extreme and fatigue loads on the rotor. Reducing the loads on the rotor
can affect the loads on other components such as the bearings, gearbox and generator and could reduce
the
COE
. Such strategies include cyclic pitch, individual pitch control and aeroelastic tailoring [
72
].
Alternative strategies using the relative displacement of different blade segments are possible. One
such approach uses telescopic blades. In that case, one segment is retracted into the other to vary the
swept area of the rotor [
73
76
]. This allows the turbine to produce more power at low wind conditions
while avoiding the extreme loading at high wind speeds. However, this requires a mechanism to
perform the retraction that has to transmit all the loads from the outboard segment to the inboard
segment. Alternatively, various active ’smart’ control strategies are under development [
72
]. These
use distributed sensors and actuators along the blades. The actuators include trailing edge flaps.
Castaignet [
77
] demonstrated this concept on a turbine with 13m long blades. The average flap-wise
blade root moment decreased by 14% along with 20% of the amplitude of the 1P loads. [
78
,
79
] tested
trailing edge flaps on a turbine with 9m blades. An average load reduction of 14% was reported.
6. Adhesive joints in segmented blades
6.1. Cost of energy
Adhesive joints can be structurally efficient, light and cheap. They have low stress concentrations
and good damage tolerance. However, when used in segmented blades they result in high installation
costs due to the the need for specialized equipment and the number of added time consuming steps
during on-site assembly. Various improvements have proposed approaches to alleviate these issues.
One problem is the lack of inherent self-alignment of adhesive joints. This increases the complexity
and time required to assemble the blade from its segments [
80
]. Baker [
81
] presented a system to
align blade segments on different carriers using laser positioning. Alternatively, Zirin [
82
] suggested
using brackets attached to the spar caps to ease alignment, after which the adhesive bond can be
Version July 16, 2017 submitted to Energies 8 of 19
Table 3. My caption
Adhesive joint issue Suggested remedies
Time of assembly: Alignment of the segments -Alignment using laser-positioning
-Brackets attached to spar cap
-Alignment pins
-Overlapping portions
Curing of the bonds -Resistance heated bonds
Bond-quality bond thickness -Bonding grid
-Shims
-Producing the segments in a single mould
air entrapments -Flooding of a cavity
-Infusion
formed. Livingston [
83
] proposed using alignment pins. Additionally, Baehmann [
84
] and Riddell [
85
]
suggested using different types of overlapping portions to ease alignment. Further, Kyriakides [
86
]
proposed using joint portions that are offset in span-wise direction to create an overlap.
A second issue is the difficulty of producing a high quality bond on-site compared to under
controlled conditions [
56
]. Surface preparation, temperature and humidity affect the quality of
adhesive joints [
16
]. Good control over the bond thickness is important to avoid stress concentrations.
In [
87
] the use of a bonding grid is proposed. This grid is incorporated into the joint to obtain a very
accurate bond thickness. Zirin [
82
] suggested using shims to ensure a constant minimum distance
between the parts to be adhered. To ensure a perfect fit between two segments, Riddell [
85
] advocated
producing the segments in a single mould. By folding in a vacuum bag with release agent the two
adjacent segments can be manufactured while in contact with each other. Afterwards, they can be
separated easily and will have a very good fit at the interface. Further, air entrapments can drastically
reduce the strength of adhesive joints. Arelt [
88
] suggested to put the connecting surfaces in place first,
creating a cavity which can subsequently be flooded or infused to create the joint while avoiding air
entrapments. Similarly, Baehmann [
84
] suggests a segmented blade with overlapping spar caps, which
cause the formation of a spar cap cavity, which is subsequently filled with adhesive. Another issue
is the assembly time and requirement of specialized equipment such as ovens, heat tents and heater
blankets to cure the bonds [
89
]. Up to ten hours at elevated temperature may be required to fully cure
the adhesive [
88
]. Driver [
89
] suggested the use of resistance heated bonds to alleviate these issues.
Also, the O&M costs are lower for adhesive joints compared to mechanical connections.
6.2. Implementations
Blade segments can be joined using structural adhesive bonds. An overview is given in Figure 5.
The efficiency of the joint depends on the chosen geometry. Finger joints were used in the wood-epoxy
blades of the MOD-5A turbine [
90
]. However, the use of this type of joint in modern fiberglass blades
may be impeded by the higher modulus of elasticity and strains as well as issues with tooling. Similarly,
diamond shaped splice-inserts can be adhered to the segments to form the joint [
91
]. Likewise, Bech
[
92
] improved upon this approach by using longer connections providing higher stiffness and strength.
Bhat [
93
] used finite element modelling to investigate the option of bonded strap plates. For general
geometries, scarf joints and stepped lap joints have the highest efficiencies [
94
]. Concepts using scarf
joints were suggested by [
82
84
,
87
]. To avoid fragile protrusions, Hayden [
95
] proposed using a double
scarf joint. Segmentation using stepped lap joints was suggested by Baker [
81
]. Further, Frederiksen
[
96
] suggested not infusing the fibres in the joint areas when fabricating the segments, so that they can
be joined by overlapping, infusing and curing the dry fibres.
Version July 16, 2017 submitted to Energies 9 of 19
Figure 5. Blade segmentation concepts using adhesive bonds. a) Finger joint b) Splice insert joint [90]
c) Adhesive cavity joint d) Single lap joint [85] e) Stepped lap joint [81] f) Double scarf joint [95]
Table 4. My caption
Blade root connection Advantages Drawbacks Implementations
Flange type - Inferior fatigue behavior
Hub type Heavy
T-bolt type Cheap and simple Packing limitation of the T-bolts DEBRA, JOULEIII, MEGAWIND
Stud/insert type Allows for the lightest joint UpWind,
7. Mechanical joints in segmented blades
7.1. Cost of energy
Mechanical joints are heavy and expensive, but are fast and easy to assemble [
44
,
46
]. Furthermore,
they are easy to inspect but require some maintenance.
7.2. Experience from blade root connections
Conventionally wind turbine blades are attached to a steel hub using a detachable mechanical
joint. These root joints are highly loaded and experience a very high number of load cycles. Because of
the existing experience in this field and the similarities with the joints for segmented blades these joint
types are candidates for blade segmentation. The most frequent root types are seen in Figure 6.
7.2.1. Flange type
Blades with a flange type root have a flange formed by moulding the material outwards. This
flange is then bolted to the hub. Bundles of fibers can be looped around bushings with the flange to
capture them mechanically. This type of root is known as the Hütter root connection [8,97].
Version July 16, 2017 submitted to Energies 10 of 19
Figure 6.
An overview of blade root joints. a) Flange root connection b) Hub type root connection c)
T-bolt root connection d) Stud root connection.
7.2.2. Hub type
The hub type root connection uses a tapered metal cylinder embedded or adhered to the root
laminate and bolted to the hub. Assuring correct bond thickness is difficult, but critical for the
performance of the joint[
41
]. Strain incompatibilities are present, resulting in large stress concentrations.
Furthermore, in some implementations the hub has a lower diameter than the actual root [
97
]. This
reduces the second moment of area of the section trough which the loads are transferred, reducing
the structural efficiency of the joint. Hosseini-Toudeshky [
98
] investigated the progressive debonding
of a hub type joint using finite element methods. It was demonstrated that an overloading such as a
gust can cause damage to the bonding of the root joint, which grows due to fatigue loading. The used
method was able to predict the life reduction of this joint caused by various loadings.
7.2.3. T-bolt joint
T-bolt joints have cross-bolts positioned perpendicular to the root cylinder surface. Longitudinal
bolts connect the hub to the cross-bolt [
99
]. T-bolts rely on the contact between the cross-bolt and
the laminate to transfer loads. Martinez [
100
] investigated the T-bolt joint both numerically and with
experiments and concluded that the T-bolt joint is reliable and cheap but has a low structural efficiency,
resulting in a high weight compared to other solutions such as inserts. Packing limitations exist
and lead to a significant laminate build-up. Furthermore, the load factors of the bolts are critical to
the integrity of the connection. Multiple improvements to the conventional T-bolt joint have been
suggested. Harismendy [
101
] suggested the use of two longitudinal bolts for each cross bolt outside
the blade laminate. While Quell [
102
] suggested using other shapes of cross bolts than cylindrical.
Additionally, Doorenspleet [
103
] suggested using multi-row T-bolts in order to increase the packing
limit.
7.2.4. Stud/insert type
The stud or insert root joint relies on longitudinal bolts attached to studs or inserts. Typically, the
inserts are female threaded and made of steel, causing a thermal and flexural mismatch [104]. This is
countered by tapering the studs on the out or inside and by using a thicker laminate [
104
]. Hayden
[
104
] suggested to use a threaded insert made from a composite tube to improve compatibility to allow
a reduced root wall thickness. Furthermore, to reduce the stress concentration at the tip of the inserts,
Vronsky [
105
] suggested using inserts of different lengths. Often, the studs are glued into the blade. In
wood composite blades the studs are placed in holes that are drilled, while in glass fibre blades the
holes are preferably formed during fabrication [
41
,
106
]. Positioning of the stud is vital to the quality of
Version July 16, 2017 submitted to Energies 11 of 19
the joint as a non-uniform adhesive thickness causes stress concentrations [
107
]. Typically, fixtures
are used to position and bond the studs simultaneously. Often, the adhesive is injected into the hole
around the insert by using a secondary hole or through the gap between the laminate and the stud.
Alternatively, modified studs can allow the adhesive to flow through the center of the insert to form
the bond through the stud [
41
]. Additionally, the joint quality can deteriorate because of macroscopic
voids [
41
]. To avoid these voids, Raina [
107
] suggests to improve the tru-stud bonding method to
allow vacuum infusion by adding a second channel to the stud. Alternatively, the studs can be directly
embedded during the lamination process. This requires less fabrication process steps, tooling and
allows the root laminate to be much thinner, but increases the complexity of the lamination process
[
41
,
108
]. Sorensen [
109
] suggested using a holder with spaced recesses to hold the bushings. As an
alternative, Bendel [
110
] and Kildegaard [
111
] both suggested inserts that can easily be positioned and
form a smooth outer and inner surface onto which the fibre mats of the root laminate can be applied.
In general, to provide sufficient pull-out strength, inserts have to be long. Various improvements have
been suggested to increase the pull-out strength, allowing shorter, lighter inserts. Grove-Nielsen [
112
]
suggested to include longitudinal grooves on the outside of the inserts to increase the contact area
with the laminate. Further, in similarity with the Hütter root, Mcewen [
108
] proposed to capture the
inserts mechanically by looping fibres around it. Additionally, Feigl [
113
] suggested putting fibres in
between the inserts for a better contact, whereas Schmidt [
114
] suggested stitching together the fibres
surrounding the bushings.
7.2.5. Comparison
The blade root design is mainly driven by cost as it represents between 7 and 20 percent of the
total blade cost [
100
,
115
]. The weight of the joint is less important, since it is added close to the hub and
the center of rotation. As a consequence it does not have a big impact on the blade’s eigenfrequencies,
and edge-wise and dynamic loads. This is different for blade segmentation joints which are placed
further outboard. Due to the superior fatigue performance of T-bolts and studs other blade root designs
have become rare. Jackson [
116
] performed the preliminary design of a 50 m blade. Blade roots were
designed considering a T-bolt joint and a stud joint. The stud connection allowed a larger number of
connections because of packing limitations of T-bolts. This lead to a reduced root laminate build-up
resulting in a lower weight and price, despite cheaper T-bolt hardware.
7.2.6. Implementations in segmented blades
The T-bolt joint has been used in several prototype segmented blades, seen in Figure 7. It was
first used on the DEBRA 25 wind turbine [
117
]. T-bolts joined the blades to the hub and connected
the C-spars of the two 8.5 m blade segments. The turbine was successful and needed only limited
additional maintenance to verify bolt pre-tension. Dutton [
44
] also investigated the use of a T-bolt
joint for a segmented blade by using a single row of T-bolts in a prototype 23.3m blade. The blade
survived both static and fatigue testing. Later, Vionis [
51
] also investigated the use of a T-bolt joint by
using a double row of T-bolts in a 30 m segmented blade. The blade survived static testing but bolts at
the spar caps failed during fatigue testing at one fifth of the 1E6 load cycles. Prototypes using inserts
have also been made, as shown in Figure 8. Within the UpWind project, a 42.5m sectional blade using
inserts was developed [
118
]. Furthermore, Saenz [
48
,
119
] developed a joint for blade segmentation
that increases the number of connections by alternating long and short bolts. The joint was used to
design a 61.5 m segmented blade since this was the optimal location for blade transport.
7.3. Experience from blade root and hub extenders
To use existing blades on turbines at sites of a lower wind class than the blades were designed for,
blade root extenders are placed in-between the hub and the blade roots, increasing the rotor diameter
[
120
]. Blade extenders are generally made out of metal but can also be manufactured from composite
material [
121
]. They can incorporate a pre-coning [
122
] or sweep [
123
]. In a similar approach, the hub
Version July 16, 2017 submitted to Energies 12 of 19
Figure 7.
Prototype segmented blades using a T-bolt joint. a)DEBRA-25 blade [
117
]. b) split blade
tested under the JOULEIII project [44] c) blade tested under the MEGAWIND project [51].
Figure 8.
Prototype segmented blades using inserts. a) blade joint developed during the UpWind
project [54]. b) Blade joint with alternating long and short bolts [48].
is extended, placing the pitching mechanism further out-board forming a hub extender or partial pitch
system [
124
]. This concept was already used in the NASA Mod-2 turbine [
125
]. Lu [
126
] investigated a
segmented blade of which the inboard portions were essentially blade extenders connected by a truss
structure to reduce loads. Furthermore, to provide sufficient solidity at the blade root, an aerodynamic
shape with a large chord length is required. This can be made feasible by using a root extender with an
aerodynamic shape as suggested by Curtin [
127
]. An overview of these methods is shown in Figure 9.
Figure 9.
Blade extension methods. a) Blade root extender [
120
] b) Partial pitch mechanism [
124
] c)
Blade root extender with an aerodynamic shape [
127
] d) Segmented blade with the inboard segment
made from steel.
7.4. Experience from rotor tips and glider wings
To reduce turbulence at the tips, aircraft often employ winglets. Similar tips are used on wind
turbines to limit noise production [
128
]. However, such angled blade tips form delicate components
during transportation and make manufacturing more complex and expensive. Therefore, they are
often made as separate components and connected to the blades at the installation site. The blade tips
can be connected by means of tubular guides and locked by means of a bolt, either transversely to the
Version July 16, 2017 submitted to Energies 13 of 19
joint as suggested in Olthoff [128] or longitudinally as suggested in Hoffmann [129]. Furthermore, in
the past, tip brakes were used to prevent over-speed on rotors with stall control [8,97,130]. These can
rotate 90 degrees to create a drag. They are typically connected by means of a tube. Similarly, Moroz
[
49
] suggested to alleviate loads with a segmented tip. In addition, the fabrication methods, structural
layout and slenderness of gliders and wind turbine blades are similar making joints used in gliders
suitable candidates for blade segmentation [
117
]. Gliders often have detachable wings to allow easier
transport and storage [
131
]. Modern glider carry-trough structure configurations have one or two
tongues next to each other to transfer bending loads [
132
]. Similar spar-bridge strategies with one or
more protrusions have been suggested and tested for segmented blades. For example, Rudling [
133
]
suggested a segmented blade that relies on joining the shear webs of a number of structural spars
with shear pins. Loads are distributed using shear blocks attached to the webs. Segmented blades
using spar-bridge joints were suggested in various studies [
134
137
]. Further, Dutton [
44
] designed
and tested prototype of a segmented version of a 13.4 m blade with a connecting tube. The tube
was attached to the blade using two bulkheads similar to the concept suggested by [
138
]. The blade
underwent three static load tests (flap-wise towards the suction side and both edge-wise directions)
followed by a 5 million cycle fatigue test in the flap-wise direction after which the static tests were
repeated. It was concluded that no damage occurred in the blade, but that the loads were sometimes
transmitted trough the locking device instead of via the fitting which had resulted in fretting corrosion.
7.5. Other concepts
7.5.1. Cables
Some blade segmentation concepts cannot directly be traced back to a particular other application.
An overview of these methods can be seen in Figure 10. There are a number of segmentation
joints that rely on cables to form a connection. [
139
] suggested using pre-tensioned straps to hold
together eccentric transversal bolts, attached to neighbouring segments. However, due to friction
the pre-stress accuracy is limited and difficult to ensure [
56
]. Furthermore, this concept leads to
high stress concentrations [
88
]. Alternatively, pre-tensioned cables can be used to hold the different
segments together by pulling them towards the root. Kootstra [
140
] proposed to incorporate a joining
segment that is pulled towards the root using a pre-tensioned cable. Similarly, in Doellinger [
141
]
using pre-tensioned steel cables running through channels in the skin and shear webs as an alternative
to a structural spar is suggested. The cables are attached at the blade root and fastening points on
every blade segment. Likewise, Cairo [
142
] suggested using pre-tensioned cables running through
conduits in the blade skin. Further, Klein [
143
] suggested using U-shaped cable loops embedded into
the laminate.
Figure 10.
Unique blade connections relying on cables to connect the different segments. a) Blade
using pre-tensioned steel cables to hold together the different segments as an alternative to a spar
structure [
141
]. b) Joint using pre-tensioned straps around eccentric bolts [
139
]. c) U-shaped loops [
143
]
d) Segmented blade joint relying on pre-tensioned cables to pull the outer segment towards the hub
[140].
Version July 16, 2017 submitted to Energies 14 of 19
7.5.2. Joints using transverse fasteners
Joining the segments with fasteners in a transverse direction has also been considered. Torres
[
144
] suggested joining the blades by riveting. Petri [
55
] suggested transversely bolting overlapping
plates to the segments. To increase laminates the bearing strength, Birkemeyer [
59
] suggested using
fibre metal laminate (FML) in the region of the joint. Llorente [
145
] suggested using lugs to connect the
spar of adjacent segments. These methods can be seen in Figure 11.
Figure 11.
Transverse joining concepts. a) Segments joined with lugs [
145
] b) segments joined by
intermediate pieces [55] c) Segments joined with rivets [144]
8. Conclusion
The feasibility of a segmented blade largely depends on the risk of the chosen concept. In this
respect concepts that require only limited changes from existing approaches pose less risk and are more
likely to succeed. For example, concepts that do not require division of the blade’s main structural
components such as the use of separate leading or trailing edge segments are only small modifications
since these require only limited loads to be transmitted across the connections. For this reason, active
trailing edge flaps are more likely to succeed than telescopic blades. Similarly, aerodynamically shaped
root extenders pose only a small modification from existing root extenders, which are well known in
the industry. Furthermore, concepts incorporating a spar-bridge are close to joints used in sail-planes
and tip brakes and have been shown to be feasible. Joints using longitudinal bolts have also been
successful and are well known from the blade root design. The fact that large modern blades typically
prefer the use of inserts to form a lightweight joint indicates that such joints are better suited for
segmentation than flanged, hub type and T-bolt joints. On the other hand, breaking up the blade’s
main structural components poses significant challenges regarding production, maintenance costs
and reliability. The failure of the T-bolt prototype blade in Vionis [
51
] and the unfavourable cost
calculation for the T-bolt prototype blade in Dutton [
44
] indicate the difficulty of making this approach
successful. Adhesive joints are also well known in the industry and are sometimes preferred because of
their structural and economic efficiency [
46
]. However, the step from controlled conditions to in-field
production of such connections is large. Yet, it may be possible to assemble the blade segments using
local, perhaps temporary facilities. Furthermore, to avoid air entrapments, such adhesive joints would
most likely be produced using vacuum infusion. The issues related to the manufacturing of larger
blades are already being countered by manufacturing separate blade components. These are mostly
assembled in the factory using either adhesive or mechanical joints. While blade segmentation poses
serious challenges, the wide variety of possibilities and the potential benefits are bound to lead to
further developments in this field. Furthermore, segmentation appears most likely to be cost effective
for very large, offshore turbines or on-shore turbines promising conditions but accessibility issues.
Acknowledgments:
The work leading to this publication has been supported by VLAIO (Flemish government
agency for Innovation and Entrepeneurship) under the SBO project "OptiWind: Serviceability optimisation of the
next generation offshore wind turbines" (project no. 120029).
Version July 16, 2017 submitted to Energies 15 of 19
Conflicts of Interest:
The authors declare no conflict of interest. The founding sponsors had no role in the design
of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the
decision to publish the results.
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... Secondary spars are usually positioned on the TE side and, alike primary spars, they are composed of spar caps to support the flapwise bending moments and shear webs to withstand the flapwise shear loads. Secondary spar caps are typically manufactured out of CFRP, hybrid CFRP/GFRP, or GFRP materials, whereas secondary shear webs are sandwich structures [13], [45]. ...
... Reinforcement components are located at both LE and TE where the two shells are bonded together, and they are made of either CFRPs or GFRPs. Their principal function is to carry the edgewise bending moments and shear loads [13], [45]. ...
... The purpose of the shells is to provide the aerodynamic aerofoil shape that converts the wind flow pressure into aerodynamic forces responsible for the rotation of the blades [13]- [16]. In addition, the shells also transfer the edgewise shear loads from the primary spar to the secondary spar [45]. Figure 2.4 shows that the shells are sandwich structures composed of a core material encapsulated within two layers of GFRP. ...
Thesis
Full-text available
Modern wind turbine blades are equipped with a lightning protection system to intercept the lightning and conduct its current, preventing the direct attachment to internal conductors. In such conditions, resin thermal degradation develops at the equipotential bonding (EB) connections between down conductors (DCs) and carbon fibre reinforced polymer (CFRP) spars. This problem was investigated in this work by combining experimental studies and finite element method (FEM) simulations. The experimental work focused on the characterisation of the input material properties to be used in the FEM models. An experimental-numerical procedure was established to determine the electrical contact resistivity of EB joints. Besides, the thermal degradation of a commercial epoxy was studied to determine its reaction kinetics. The developed FEM models solve a weakly coupled formulation of the electromagnetic-thermal problem to predict lightning current paths and thermal damage at the bonding interfaces. The validation of the models against conducted current test data showed that they can assist in the design of EB joints. High current densities and temperatures were predicted at the sparking locations found during the test, which allowed a qualitative prediction of potential thermal degradation areas upon the solution of the Arrhenius equation. In addition, such models can be used to assess the potential risk of flashover between the blade conductors due to high electric fields. Finally, typical EB materials were compared using the developed FEM models to provide guidelines and suggestions for the implementation of EB joints. It was seen that materials with high in-plane electrical conductivities, such as ECF and BIAX CFRP, can reduce the electric field below the insulation breakdown strength and prevent flashovers. Besides, hot spots at the bonding interfaces can be controlled by changing the arrangement of the EB layers, or by using a material with low contact resistivity and high thermal diffusivity like ECF.
... To improve the reliability and reduce the cost of sectional blades, several attempts have been made to create new joints, and optimized multi-bolted joint designs and composite layouts as well as to investigate these blades' structural responses and availability. Numerous patents referring to connection configurations for sectional blades have been applied for [10,11]. However, the concepts of these patents have seldom been employed in feasible studies or for commercial sectional blades, and only a few groups and enterprises have carried out practical studies, such as laboratory testing and field testing in limited WTs [9,11]. ...
... Numerous patents referring to connection configurations for sectional blades have been applied for [10,11]. However, the concepts of these patents have seldom been employed in feasible studies or for commercial sectional blades, and only a few groups and enterprises have carried out practical studies, such as laboratory testing and field testing in limited WTs [9,11]. These pioneering efforts have provided invaluable knowledge allowing us to understand the feasibility of such joints and layout concepts for sectional blades. ...
Article
The structural reliability of large sectional blades has always been a challenging issue with regard to the mild cost increment incurred compared with corresponding non-segmented blades. The use of excellent connection configurations and optimized composite layouts are cost-efficient approaches to achieve this goal as well as better understanding of the structural responses of sectional blades under extreme loads. This work presents the design and fabrication of a sectional blade, based on a 38-meter typical commercial wind turbine blade, with unique connections and a composite layout. The prototype blade is experimentally studied under extreme static loads according to international standards. The strains of the spar caps, sandwich structures, and shear webs are collected, and the contact status of adjacent segments and bolt loads are also monitored as well. The results revealed that the sectional blade successfully sustained the extremely static loads with only a moderate manufacturing cost increment compared with corresponding non-segmented blades. The advantages of the proposed strategies are validated for sectional blades. Regarding the structural integrity of the sectional blade, blade buckling, composite failure and residual deflection were not observed. Flap-wise loads are more critical for the bolted joint of segment connections than edge-wise loads, as detrimental bolt load increases tend to occur due to the prying effect at the leading and trailing edges. An improved analysis based on the load factor from multi-bolted joint box-beam models and experiments could predict the bolt loads of segmented connections with desirable accuracy and could be suitable for the multi-bolted joint design of sectional blades.
... A comprehensive AutoNuMAD 27 structural optimization, with contributions from control and aerodynamic teams, has been performed, the new designs have achieved 26% mass reduction and 32% cost reduction comparing to the SUMR50 S5. 23 The feasibility of such a blade requires that it be manufactured in segments for manufacturing and transportation constraints. Although this is a vital design aspect of making such a wind turbine a reality, this study is focused on providing a benchmark for structural properties of a 50-MW blade, and targeted segmentation methods 34 are the focus of future research. Since this paper focuses more on system-level design, only the external geometry of the blade is elaborated and the detailed structural work can be found in Yao et al. 30 It should be noted in this work that we have not focused on the detailed hinge system. ...
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Wind turbine design encompasses many different aspects including aerodynamic, structural, electrical, and control system design. To achieve optimal plant performance, a system design approach is utilized in which the performance of the whole wind turbine is evaluated and quantified during operational scenarios with subsystem interactions. In this paper, the design for a Segmented Ultralight Morphing Rotor (SUMR) 50‐MW wind turbine is presented utilizing levelized cost of energy (LCOE) for design choices, with additional quantification of simulated performance shortcomings at the 50‐MW scale. The multi‐disciplinary design process results in a final ultra‐scale turbine configuration that outperforms other existing offshore wind farms regarding the LCOE.
... Modular WTB can help to overcome the challenges of fabricating and transporting large WTB (Peeters et al., 2017;Garate et al., 2018). Moreover, modular WTB can be fabricated with thermoplastics using AM, and the process can be scaled for large blades. ...
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Is predicted that around 42 million tonnes of composite waste from wind turbine blades will need to be recycled annually worldwide by 2050. This poses a potential environmental crisis that must be timely mitigated. Therefore, this study proposes an integrated multilevel product stewardship to address the environmental impact of wind turbine blade waste. This product stewardship integrates circular economy, cleaner production, eco design, and industry 4.0 technologies. To tailor the proposed product stewardship, a systematic literature review that extracted a total of 267 studies and industry reports was performed. A large variety of technologies were identified under seven different potential pathways that can be taken and combined to address the environmental impact of wind turbine blades. Moreover, a technology roadmap and a project strategy plan composed of 5 milestones, to be achieved by 2050, were presented envisioning the maturation and adoption of the proposed solutions.
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In this research work, smart composite beam with Shape Memory Alloy (SMA) embedded at neutral layer of the Glass Fiber Reinforced Polymer (GFRP) beam, clamped at both ends has been considered for study of the effect of SMA on frequency shift on passive condition. Mathematical model for natural frequency and magnification factor of a smart beam has been developed considering the continuous beams as lumped mass system with constant damping. This research work also deals with development of numerical model to study the effect of SMA on frequency with super elasticity nature. The obtained numerical and analytical results were compared with experimental work to understand the agreement between each other. From the final results it was observed that, due to overall increase in density of composite beam the continuous decrease of frequency was occurred. Also, though the young’s modulus value of SMA is much higher in GFRP, overall increase in density of composite beam is much higher than increase in young’s modulus.
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Rotor blade or rotor blade segment for a wind turbine, having at least one cable for fixing the rotor blade or rotor blade segment to a rotor hub or to a further rotor blade segment, wherein the at least one cable is redirected in a U-shaped manner within the rotor blade or rotor blade segment.
Patent
Full-text available
Wind turbine systems and methods are disclosed herein. A representative system includes a wind turbine blade having an inner region that has an internal load-bearing truss structure, and an outer region that has an internal, non-truss, load-bearing structure. In particular embodiments, the truss structure can include a triangular arrangement of spars, and/or can include truss attachment members that connect components of the truss without the use of holes in the spars. Spars can be produced from a plurality of pultruded composite members laminated together in longitudinally extending portions. The longitudinally extending portions can be connected at joints that interleave projections and recesses of each of the spar portions. The blades can include fan-shaped transitions at a hub attachment portion, formed by laminated layers and/or a combination of laminated layers and transition plates.
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Although wind energy exploitation dates back five thousand years ago, contemporary societies are based almost exclusively on fossil fuels for covering their electrical energy needs. On the other hand, during the last thirty years, security of energy supply and environmental issues have reheated the interest for wind energy applications. In this context, the present work traces the long and difficult steps of wind energy development from the California era to the construction of huge offshore wind parks worldwide, highlighting the prospects and the main challenges of wind energy applications towards the target of 1000 GW of wind power by 2030.
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The paper proposes a novel design concept for the wind turbine rotors. The design is composed of the segmented blades and a hinged-rods support structure (SBHR) as a means of reducing weight through alleviating the moment on the blade. A prototype of the design is manufactured. Focusing on the hinged-rods support structure (HRSS), a method combining the experiments and numerical calculation is developed to analyze its feasibility. The experiments in the wind tunnel platform were conducted to measure the loads at the root of the isolated blade and in the rods. A numerical model was developed to describe the designed wind turbine rotor using the measured loads in experiments. In the model, the mounting locations of the hinged rods significantly affected the moment distribution on the blade. Thus, two dimensionless indexes were determined to analyze its influences. The model perfectly explain the characteristics of the novel structure under different configurations. The results demonstrated that the moment of the blade below the hinged location were alleviated, which reduced the requirements for the material. A 43.1% reduction of the maximum moment can be achieved in the design. In addition, the gross reduced weight of the blade was estimated to be 35.4% based on the blade mass distribution along the span.
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In this paper, the aerodynamic performance of the telescopic blade wind turbine concept has been analysed experimentally and computationally. A model-scale wind turbine with two-stage telescopic blades, having a chord ratio of 0.6, was studied in the wind tunnel for different blade extensions (i.e. different lengths of the second section, ranging 0–40% of the first section). The experimental setup allowed the measurement of rotor speed, shaft torque, and thrust, under varying wind speeds. It has been established that the effect of a step change in blade chord is significant for the range of blade extensions studied. In particular, the power coefficient Cp of the wind turbine was found to decrease with extension, with 25% decrease in maximum Cp obtained for a 20% blade extension. This is attributed to additional losses arising from the step change in chord. Correlations proposed to quantify losses arising from the step change in the chord of a telescopic blade are found to be in good agreement with experimental data. In spite of the detrimental effects of a step change in blade chord, the power output of a telescopic blade wind turbine is found to increase for all blade extensions considered in this study.
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Knight & Carver was contracted by Sandia National Laboratories to develop a Sweep Twist Adaptive Rotor (STAR) blade that reduced operating loads, thereby allowing a larger, more productive rotor. The blade design used outer blade sweep to create twist coupling without angled fiber. Knight & Carver successfully designed, fabricated, tested and evaluated STAR prototype blades. Through laboratory and field tests, Knight & Carver showed the STAR blade met the engineering design criteria and economic goals for the program. A STAR prototype was successfully tested in Tehachapi during 2008 and a large data set was collected to support engineering and commercial development of the technology. This report documents the methodology used to develop the STAR blade design and reviews the approach used for laboratory and field testing. The effort demonstrated that STAR technology can provide significantly greater energy capture without higher operating loads on the turbine.
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The United States Department of Energy (DOE) through the National Renewable Energy Laboratory (NREL) implemented the Wind Partnership for Advanced Component Technologies (WindPACT) program. As part of the WindPACT program, Global Energy Concepts, LLC (GEC), was awarded contract number YAM-0-30203-01 to examine Technical Area 1-Blade Scaling, Technical Area 2-Turbine Rotor and Blade Logistics, and Technical Area 3-Self-Erecting Towers. This report documents the results of GEC's Technical Area 1-Blade Scaling. The primary objectives of the Blade-Scaling Study are to assess the scaling of current materials and manufacturing technologies for blades of 40 to 60 meters in length, and to develop scaling curves of estimated cost and mass for rotor blades in that size range.