1 Load Monitoring for Active Control of Wind Turbines Aubryn Cooperman*, Marcias Martinez Faculty of Aerospace Engineering, Delft University of Technology, Delft, Building 62, Kluyverweg 1, 2629HS Delft, P.O. Box 5058, 2600 GB Delft, The Netherlands *Corresponding author. a.m.cooperman@tudelft.nl +31(0)15 278 8233 Abstract This review article examines the range of sensors that have been proposed for monitoring wind turbine blade loads for the purpose of active load control over the past decade. Wind turbine active load control requires sensors that are able to detect loads as they occur, in order to enable a prompt actuation of control devices. Loads may be detected based on structural effects or inferred from aerodynamic measurements. This paper is organized into the following sections: wind turbine control, requirements for load monitoring sensors, sensing technologies and field tests of load control. The types of sensors examined in this article include fiber optic sensors, inertial sensors, pressure measurements and remote optical sensing. Keywords: load monitoring, active load control, fiber Bragg grating, Rayleigh backscattering, MEMS, lidar 1. Introduction Wind turbines are an important source of renewable electricity generation. Worldwide, electricity-generating capacity from wind energy has exceeded 300 GW [1] and it continues to grow, contributing 22% of the new capacity installed in 2011 [2]. As a clean, renewable energy source, wind energy capacity is expected to continue growing. Wind turbines are not only becoming more numerous but also larger. Increasing the size of individual rotors reduces the total number of turbines needed to produce a given amount of energy. This is particularly important for offshore wind farms where the costs of installing and connecting each foundation are much higher than onshore. Taller turbines also capture more energy from faster wind speeds found higher in the atmosphere. Increased energy capture produces higher aerodynamic loading on the structure, while there is also a need to minimize the structural weight of the blades in order to reduce gravitational loads. Monitoring and controlling these loads will become more important as turbines become larger and are located farther from shore. Load sources on wind turbines can be described as aerodynamic, operational (caused by control actions), gravitational or inertial (gyroscopic and centrifugal) [3]. The rotor is the source of the most significant loads on the turbine. Reducing loads on the blades also reduces loads throughout the drivetrain and tower [4, 5]. Turbines are becoming large enough that rotors span significant changes in wind speed including gusts, low-level jets, wind shear and turbine wakes [6-10].