Use of a wave energy converter a s a motion suppression device for floa ting wind turbines 25th J a nua ry, 2013 Micha el Borg, C ra nfield University

Outline

• Introduction • System Description • Methodology • WEC Parameters • Numerical Model • Results • Conclusions

Introduction

• Floating platforms subject to large-amplitude motion →Increased fatigue loads →Reduced aerodynamic performance

Mooring lines

Waves & Currents

Turbulent winds Turbine

control

Structure flexibility

Increased cost of electricity

Introduction

• Use of passive damping devices to reduce motion

• Propose that this energy is captured using a WEC →Increased system energy yield →Shared infrastructure and reduced costs

Wind Energy

Wave Energy

FOWT

SYSTEM

Electricity from wind

Wave-induced energy dissipated

Wind Energy

Wave Energy

FOWT

SYSTEM

Electricity from wind AND wave

Methodology

• Hypothetical WEC is considered. • No characteristic constraints • No geometry considered → No hydrodynamic forces

• Assumed to move only in heave • Connected to FOWT with spring-damper system • Identify spring-damper characteristics for two cases:

1. Maximum Motion Reduction 2. Maximum Energy Extraction

System Description

• 5MW Vertical Axis Wind Turbine mounted on Trifloater

• Dogger Bank site, North Sea →JONSWAP spectrum →Hs=4.9 m ; Tz=10s

• Hypothetical WEC: additional degree of freedom in heave

→Connected through PTO spring-damper coupling

W EC Parameters

• Mass → 3 cases: 2.5%, 5% and 10% of FOWT mass → based on Refs. [1], [2]

• Damping → Damping ratio (ζ) varied from 0.17 to 7.7 → 5 cases

• Stiffness → 3 cases: WEC nat. freq.(ωn)= FOWT ωn 1 cases: Varied 25% to 200% FOWT ωn → constant damping

excnn Fxxxm =++ 22 ωζω

Numerical Model

• Based on Marine Systems Simulator Toolbox [3] in the MATLAB/Simulink environment

• Cummins Eqn. used with radiation-force approximation

• Aerodynamics modelled with Double Multiple Streamtube model with modifications [4]

• Gyroscopic forces also included [5]

• Found to occur with largest mass and lowest PTO damping • Shifting WEC ωn reduces power absorbed

Absorbed Power vs. Supply Reliability

Results Maximum Energy Extraction

• Increase in PTO damping led to smaller motion reduction • Damping ratio > 1 → RAO deteriorates

Results Effect of W EC Damping

Results Ma ximum Motion Reduction

• Occurs when WEC ωn is lower than FOWT ωn

• 15% reduction in heave mean amplitude

• 29% reduction in RAO peak response

Conclusions

• Proposed concept of using a WEC to reducing FOWT motion and increase cost-effectiveness.

• Maximum energy extraction from the WEC is achieved by matching the WEC ωn to the FOWT ωn and using low damping ratios.

• Maximum motion reduction of the FOWT is achieved by shifting the WEC to a lower frequency than the FOWT ωn.

• Importance of maximising energy yield per unit area of ocean utilised.

Thank you for your attention

References [1] Peiffer, A., Roddier, D. and Aubault, A. (2011), "Design of a point absorber inside the WindFloat structure", Proceedings of the International Conference on Offshore Mechanics and Arctic Engineering - OMAE, Vol. 5, pp. 247. [2] Muliawan, M. J., Karimirad, M. and Moan, T. (2013), "Dynamic response and power performance of a combined Spar-type floating wind turbine and coaxial floating wave energy converter", Renewable Energy, vol. 50, pp. 47-57. [3] Fossen, T. I. and Perez, T. , MSS. Marine Systems Simulator (2010), available at: http://www.marinecontrol.org (accessed16.04.2012). [4] Shires, A. (2013), "Design optimisation of an offshore vertical axis wind turbine", Proc. Inst of Civil Engineers, Energy, vol.166, no. EN0, pp. 1-12. [5] Blusseau, P. and Patel, M. H. (2012), "Gyroscopic effects on a large vertical axis wind turbine mounted on a floating structure", Renewable Energy.