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Detailed modeling of the spanwise joint and root cuff was performed for the baseline 65 m case, then scaled for larger blades. The connection weight and cost were estimated using the following steps: 1) The root connection (laminate build-up and hardware) were isolated in the NREL 61.5 m cost model 2) A 35% reduction factor was applied to laminate build-up as estimated shift from T-bolt to embedded stud style connection 3) The total resulting blade mass (including adjusted root build-up) was scaled up to 65 m and checked against IEA Task 37 blade and industry commercial trend-line 4) The mid-span joint was estimated based on bending loads and dimensional characteristics of local blade cross-section: a.

Based on evaluation of DNV GL loads database, the peak flapwise bending moment was estimated to be 20% of the flapwise bending moment.

b.

Using the available geometry, a bolt pattern was estimated and evaluated.

c.

Mid-span joint parameters were iterated in an attempt to match peak bolt stress (mid-span bolt stress equal to peak for root connection).

d.

Laminate build-up was estimated to match final joint configuration.

e.

Cost for laminate part estimated as $10.45/kg.

f.

Bolt hardware weight and cost estimated by direct calculation.

5) Cost and weight for the root cuff were calculated based on skin weight from NREL 61.5 m laminate schedule, scaled as R2.2 for larger blade lengths, with estimated factors to account for fastener connections. Fastener hardware weight and cost was estimated directly based on assumed joint configuration. While there is significant uncertainty in these estimates, the root cuff weight and cost represent a small contribution to overall weight and cost for the modular blades. Due to geometric constraints such as available locations for connection laminate and required bolt spacing, the predicted bolt stresses for the final mid-span joint configuration were estimated as 129% of the peak root connection bolt stress. While this suggests a non-conservative result, there is significant uncertainty in the initial root connection build-up estimation. The mid-span joint was also evaluated based on ratios of approximated engineering parameters, rather than a detailed design or optimization process, so the result was considered as a reasonable approximation for the purposes of this study. For the 65 m blade case, the final estimated mid-span root connection consisted of 36 M30 bolts (18 each blade shell), and a laminate build-up with weight approximately 26% of the root connection. Due to reduced structural efficiency at mid-span, this is greater than the bending load ratio (20% of root bending load), but only modestly, indicating that the estimate is at least reasonable. However, whereas the root laminate buildup is one-sided (with a steel pitch bearing/hub assembly on the other side), the mid-span bolted connection would require a laminate build-up on both the inboard and outboard parts. As a result, the total joint laminate build-up was estimated to be about 52% of the root build-up. Table 6-2 provides a summary of estimated weight and cost for both monolithic and modular blades produced in a traditional off-site factory. The weight and cost adders for modular blade components range

DNV GL – Document No.: 10080081-HOU-R-01, Issue: C, Status: FINAL www.dnvgl.com

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Supersized wind turbine blade study  

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