ABSTRACT

In this research work an introduction to the concept of an actively controlled horizontal axis wind turbine through varying blade tip sweep, is presented. The concept refers to variable tip swept rotor blades, that have the ability to pivot collectively aft, about an axis located at the blade tips. Quantities to be controlled are power production and blade loads. The investigation is carried out with a modified Blade Element Momentum (BEM) model that takes into account variable tip swept rotor blades and the modifications are based on results from a lifting line theory based model. The simulations refer to the 5MW NREL reference wind turbine that incorporates a suitable controller and preliminary results show beneficial behaviour in all of the investigated areas.

INTRODUCTION

Over the past few years there is a continuous effort for increasing energy production and reducing dynamic loads of wind turbines. In [1] there is an extensive review of the current status in smart rotor control that goes beyond the borders of conventional control methods like pitch or stall regulation. In particular, smart rotor control refers to an integrated system equipped with sensors, actuators and one or more microprocessors that operate in a feedback loop and control the blade aerodynamic loads. The latter is achieved either by enhancing the flow around the blade with the deployment of microtabs and use of boundary layer control methods (like vortex generators and active synthetic jets) or by altering the shape of the airfoil utilizing camber control, active twist or flaps. However, all of these features have to be carefully designed in order to compensate for their complexity and the fault cases that they may impose. In this research work an introduction to an innovative control method is presented through tip swept rotor blades that have the ability to pivot simultaneously aft (in-plane movement) about an axis located at the blade tips. The swept tip can be either part of the main blade with an internal mechanism or an added surface (add-on) to the blades, as it is shown in Figure 1. The purpose of this control feature is to actively adjust power at specific operating areas and reduce fatigue blade loads or extreme loads during a wind gust through small sweep angle variations in the tip area. Similar research efforts like [2], [3] and [4] have already concluded that aft sweeping of blades plays a beneficial role on reducing the loads while fore sweeping increases them. However, the aforementioned methods refer to passive load control methods, in which changes in wind speed are counteracted through the passively adapting aeroelastic response of the rotor blades (for example tension – torsion, bend – twist, sweep – twist coupling), whereas this research intends to fill in the gap of active load/power control through varying blade tip sweep for an otherwise nondeformable rotor.