The concept of segmented wind turbine blades: a review

The concept of segmented wind turbine blades: a review Mathijs Peeters 1,†,‡ ID *, Gilberto Santo 2, ID , Joris Degroote2, and Wim Van Paepegem 1, 1 Department 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 1 of energy (COE). This causes manufacturing and transportation issues which have given rise to the 2 concept of segmented wind turbine blades. In this concept multiple segments can be transported 3 separately. While this idea is not new, it has recently gained renewed interest. In this review paper the 4 concept of wind turbine blade segmentation and related literature is discussed. The motivation for 5 dividing blades into segments is explained and the cost of energy is considered to obtain requirements 6 for such blades. An overview of possible implementations is provided, considering the split location 7 and orientation as well as the type of joint to be used. Many implementations draw from experience 8 with similar joints such as the joint at the blade root, hub and root extenders and joints used in rotor 9 tips and glider wings. Adhesive bonds are expected to provide structural and economic efficiency, but 10 in-field assembly poses a big issue. Prototype segmented blades using T-bolt joints, studs and spar 11 bridge concepts have proven successful, as well as aerodynamically shaped root and hub extenders. 12


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Over the past decades wind turbines have been developing rapidly. Most notably, the size of the 15 rotor diameter and the corresponding power output has been increasing steadily to rotor diameters 16 of up to 180 m, with rated powers as high as 9.5 MW [1-3]. This up-scaling trend is still ongoing, 17 especially offshore and is motivated by an expected reduced cost of energy (COE) for larger rotors as a 18 result of increased economies of scale [4][5][6][7]. However, this up-scaling leads to issues which can cause 19 a steep increase in costs related to the production and handling of blades, to the extent that further and pre-curving. An overview of the maximum allowed dimensions and weights is given in Table 1. allowing the blade tip to be in front of the truck while using a lighter vehicle. To allow larger blades to 90 be transported by rail, Landrum [39] proposed using two coupled rail cars and using a sliding support.

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Another approach is to deform the blade to alter its dimensions. Modern wind turbine blades are often 92 pre-curved and swept. For larger blades however, the amount of pre-curving is less than desirable, due 93 to the difficulty of transport [40]. This issue could be reduced by applying a load to "straighten" out the 94 blades while they are transported [40]. In addition, to improve blade transportation by rail, Schibsbye

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[31] advocated using bumpers to bend the more flexible outboard portion of the blades during turns 96 so that there would be no overhang. An overview of these methods can be seen in Figure 3. Further, 97 the transportation of blades over water is less restricted. Grabau [29] proposed to take advantage of   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(AEP net ), the land lease cost (LLC), operations and maintenance (O&M) cost and the levelized replacement cost (LRC).

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The ICC depends on manufacturing transportation and installation cost of the turbine.

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Manufacturing costs increase because of the additional material, labour and production steps required  Further, the annual energy production (AEP) has a very strong influence on the COE since it 128 has to offset all the costs including those not related to the rotor. The performance of the rotor will 129 decrease by alterations to its outside shape. Therefore, joints should use holes that can be covered or   sections. An overview is provided in Figure 4.   Blade segments can be joined using structural adhesive bonds. An overview is given in Figure 5.

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The efficiency of the joint depends on the chosen geometry. Finger joints were used in the wood-epoxy   they are easy to inspect but require some maintenance.    The stud or insert root joint relies on longitudinal bolts attached to studs or inserts. Typically, the 297 inserts are female threaded and made of steel, causing a thermal and flexural mismatch [104]. This is 298 countered by tapering the studs on the out or inside and by using a thicker laminate [     is extended, placing the pitching mechanism further out-board forming a hub extender or partial pitch shape with a large chord length is required. This can be made feasible by using a root extender with an 356 aerodynamic shape as suggested by Curtin [127]. An overview of these methods is shown in Figure 9. Some blade segmentation concepts cannot directly be traced back to a particular other application.

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An overview of these methods can be seen in Figure 10. spar of adjacent segments. These methods can be seen in Figure 11.