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Delegates are invited to meet and discuss with the poster presenters in this topic directly after the session 'Aerodynamics and rotor design' taking place on Wednesday, 12 March 2014 at 09:00-10:30. The meet-the-authors will take place in the poster area.

Davide Medici GL Garrad Hassan, The Netherlands
Co-authors:
Lindert Blonk (1) F P Patrick Rainey (1)
(1) GL Garrad Hassan, Heerenveen, The Netherlands

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Abstract

Evaluation of a slender, high tip-speed-ratio, blade for a very large offshore wind turbine

Introduction

The advantages of slender, flexible blades in the context of large offshore wind turbines have been known for a
long time. The authors have completed an in-depth design comparison and Lifetime Cost of Energy (LCoE) analysis
and found that high-tip-speed ratios can make notable reductions to support structure cost and LCoE. This R&D is
focusing on ‘near term’ technology that is applicable to UK Round 3 offshore projects, to reduce LCoE through a
fully integrated approach to design of the wind turbine, blades, support structure and controller.

Approach

The analytical case for preferring slender, high-speed rotors was established by considering a linear model in
closed loop with the pitch-speed controller. The analysis is straight-forward yet by including the controller in a
closed loop represents an innovation in methodology.

A baseline turbine design was selected to be representative of the designs that are currently proposed for UK
offshore round 3. The blade was redesigned using an in-house conceptual structural blade design tool to obtain
the spanwise mass, flapwise stiffness and edgewise stiffness distributions. The tool is based on section analysis
according to beam theory. Strength reserve factors on fatigue and extreme strength were assessed. The design
was first assessed and then optimized through iteration by carrying out a reduced set of load calculations. The
GH Bladed wind turbine design software was used for the simulations.

The baseline and the final design load calculations were completed according to IEC61400-3 using IEC 1A wind
and site specific, directional wave loading at Dogger Bank, North Sea (30-40m depth), using a fully defined
jacket support structure. SACS software was used in the support structure analysis and optimisation. The
turbine control algorithm included nacelle acceleration feedback and individual pitch control and was optimised
in parallel to the blade design.

An integral part of the study was the assessment of LCoE for the various design iterations. Inputs to this
financial calculation were the Capital Expenditure, Operation Expenditure (OpEx), and Annual Energy Production
(AEP). The assessment of CapEx for the Rotor Nacelle Assembly (RNA) was performed using in-house turbine
cost modelling tools. The tower, jacket and pile CapEx was obtained from cost assessments of the optimized
designs that were created for these for the different iterations. OpEx cost was acquired from analysis using the
in-house O2M operation and maintenance cost assessment tool. The AEP figures for the different iterations
were acquired from the dynamic load calculations.


Main body of abstract

Slender blade profiles are naturally more flexible as the spars that carry the flapwise loads are closer together.
In addition they naturally favour higher tip speed operation as the optimal tip speed ratio is increased. In
combination these changes can lead to a reduction in turbulence induced fatigue loads. The slender profile
reduces the partial derivative of lift with respect to wind speed variation. The increased flexibility reduces the
amplitude of high frequency loading that is passed to the hub through inertial relief. Higher rotor speeds reduce
drive train torque and cost, but also increase the partial derivative of lift with respect to wind speed variation.
The objectives of this research were to verify that more slender blades are structurally feasible, to quantify what
load reductions are possible, and to calculate the benefit on LCoE.

To complement the IEC load calculations a novel analytical method was developed for used during the early
design phase. The aim was to quantify the relative effects of reduced blade chord and increased rotor speed. A
simple linear MIMO turbine model was constructed and analysed in closed loop with the pitch-speed controller.
Scaling laws were derived that demonstrated that reducing blade chord in combination with a corresponding
rotor speed increase does reduce the load variation at high frequencies.

By iterating blade designs and running reduced simulation sets of IEC load calculation, it was found that for
upwind blades without carbon reinforcements, the highest feasible tip speed ratio was 10.0, with a maximum
tip speed of 100m/s. Any further reduction of blade cord is penalised by requiring an increase in the thickness
to chord ratio of the aerofoil sections in order to design in sufficient flapwise strength. The optimum design had
a small increase in thickness to chord ratio resulting in a 0.5% loss in annual energy yield. Pre-bend and cone
angles only needed moderate increases to ensure that tower clearance constraints were met.

The downscaling of the chord length and twist distribution of the baseline blade was done using optimal rotor
theory based on blade element momentum theory, with inclusion of Prandtl tip correction. The chords lengths
were reduced by up to 20%. Thickness increases of up to 10% in the midspan and outboard region were
introduced to mitigate the reduced structural efficiency associated with the reduced chords. In addition,
decreases in fatigue bending loads due to the reduced chords helped to mitigate this effect further. The mass
of the high TSR speed blade is slightly higher than that of the baseline blade. The effect of this slight mass
increase on the edgewise fatigue loading was taken forward in the mass assessment of the edgewise stiffener.

The tower base fatigue was reduced by 8%. The cost savings are concentrated in the jacket support structure
as it makes up 25% of the farm CAPEX. The influence of the load changes on the cost of the blade, pitch
bearing, pitch drives, hub, shaft, gearbox, mainframe and yaw bearing were assessed using component cost
models. 2.2% LCoE reduction is associated with the support structure while 0.4% is associated with the
reduction in rated torque, more than offsetting the slight loss of annual energy yield.


Conclusion

UK offshore round 3 projects are still in the planning stages and represent a very large potential market for wind
turbine manufacturers who are prepared to take on the design challenge. Foundation costs, relatively deep water
environments, and large distances to shore all motivate turbines designs of a larger scale than currently in
production. To design these very large systems to work structurally yet to deliver a lifetime cost of energy that is
competitive in the UK energy market is the main challenge facing the industry. The design and manufacture of
wind turbines is now a maturing industry, so step changes in performance or costs are not realistic. However for
this new generation of larger turbines there is a necessity to re-examine all the design assumptions and optimise
the designs for the new environment.

The findings from this thorough design study reveal that large offshore turbines could benefit from more flexible
rotor designs with a higher tip speed ratio and a higher maximum tip speed. There is an optimised blade
slenderness that is governed by the trade-off between cost of material structure and annual energy yield.
Further reduction in blade chord are penalised by a loss in aerodynamic efficiency. When a slender blade
design is fully integrated with the design of the controller and support structure the LCoE can be reduced
significantly. The cost reduction is not transformative, but the baseline model used in this study represents the
relatively mature nature of current designs. There are many design parameters that should be re-evaluated
when considering the new scale of turbines; to find that these cost reductions are possible as a direct
consequence of blade plan-form optimisation represents a very positive result for the industry.


Learning objectives
The evaluation of the effect of increasing rotor speed on LCoE for a large offshore turbine. An analytical method
for estimating the associated fatigue reductions and how this compares against full IEC61400-3 load calculations.
That LCoE can be reduced through the use of a fully integrated design approach incorporating advanced rotor
technology, wind turbine, controller design and support structure design. Work on any one area in isolation is
unlikely to yield notable savings to LCoE.