Advanced controls for floating wind turbines

Abstract

Floating Offshore Wind Turbines (FOWT) is a technology that stands to spearhead the rapid growth of the offshore wind energy sector and allow the exploration of vast high quality wind resources over coastal and offshore areas with intermediate and large water depths. This generates the need for a new generation of Wind Turbine control systems that take into account the added complexity of the dynamics and wave-induced motions of the specific floater. The present work presents a simulation study of advanced controls for Tension Leg Platform (TLP) FOWT that attempts to enhance the power output of the Wind Turbine by conversion of the surge kinetic energy of the TLP into wind energy. The public access data of the NREL 5MW offshore wind turbine have been used to perform the study. After establishing a theoretical upper bound for the possible wave energy extraction using frequency-domain methods, a time-domain state-space dynamic model of the FOWT with coupled dynamics of platform surge motion and turbine rotation has been developed that includes both a simplified model of the turbine aerodynamics and the floater surge hydrodynamics. This simulation model has then been used to implement advanced controls that maximize energy extraction by the Wind Turbine in the below rated power region. The proposed controllers are variations of a Linear-Quadratic Regulator (LQR), considering both a steady-state case and a non-stationary, finite horizon LQR case. The latter requires wave-elevation forecasting to be implemented and therefore two different forecasting algorithms have also been developed according to existing literature. While the wave-induced annual energy yield enhancement of the studied FOWT in the two considered locations is small (around 0.02% the baseline annual energy yield of the studied turbine in the two locations) the study is not exhaustive and other FOWT topologies might achieve better results. The present results clearly indicate, however, that the existing correlation between strong wind and waves makes FOWTs a sub-optimal choice as energy extraction mechanism for ocean wave energy harvesting.