Motion and performance analysis of an experimental model floating wind turbine

Abstract

Laboratory measurements of the motion and rotor performance of a model floating wind turbine

were undertaken under simple wind and wave conditions. The model consisted of a 40cm diameter

rotor mounted on a fully submerged cylinder supported vertically by four arms to form a tension leg

floater. The laboratory facility involved a low-wind speed straight-through wind tunnel assembled

on the water wave generator. The turbine was connected to a DC generator and an electrical

variable-resistance load to vary the rotor speed. Tests were undertaken for one fixed wind speed,

varying the rotor tip speed ratio and wave conditions. Four different one-dimensional wave

conditions were considered, each with a different wavelength and frequency. Sensors were

installed to measure the unsteady wave height and surge of the turbine platform. Other sensors

were installed to measure the rotor speed and the generator output power.

The measurements show the effects of rotor tip speed ratio and wave condition on the surge

motion of the floater. The presence of aerodynamic damping due to the turbine was evident for

certain wave conditions. The rotor experienced considerable deviations in the power coefficient

characteristics when subjected to waves. The deviations were larger at and above the optimal tip

speed ratio.

Good agreement was obtained when the measurements for the power coefficient and surge

displacement were compared with those from a simplified mathematical model. The model was

based on the Blade-Element Momentum theory for rotor aerodynamics and on the Morison

equation for the hydrodynamic forces on the floater.