Dynamic Modelling of the Drive Train of Small Vertical Axis Wind Turbines

The design of the electromechanical train, brake and controller logics for small VAWT is only possible when the unsteady behaviour of the wind turbine is accurately predicted. Steady state approach is largely insufficient for a proper design of the wind turbine, firstly because the aerodynamics is inherently unsteady, even with spatially uniform and stationary wind profile, secondly because the wind is actually turbulent and causes random aerodynamic loads, and finally because, from the mechanical point of view, inertia, mass unbalance and flexibility of the members (blades, arms, shaft and tower) can add further periodic loads. All such items can be dealt with an aeroelastic approach, although the level of design complexity and engineering skill is actually not straightforward. From the aerodynamic point of view, BEM algorithms and vortex methods are not accurate enough to predict high-solidity rotors aerodynamics (typical for small VAWT), while complex CFD computations are still hardly linked into unsteady routines. It is of experimental evidence that the dynamics of the system under highly transient winds is necessary for a correct design of the electromechanical components, including the rheostatic brakes that carry out the main safety function for typical fix-geometry turbines. No commercial aeroelastic codes are available yet for small VAWT, and the impact of real turbulent environment and members flexibility on turbine behaviour is often addressed only empirically during the testing phase of the prototypes. As a consequence the design of many components and sub-systems of a small VAWT still lies nowadays on a try and error approach. The lack of commercial aeroelastic codes for small-VAWT leaves open also the issue of the load assessment for certification within IEC 61400-2 standard. The present contribution discusses a possible approach for solving the aforementioned issues. A combined method conjugating a measurement campaign with a structural analysis based on a modal approach offers the capability to capture main WT behaviour for a safe design and a benign certification path.