Active Pitch Control of a Vertical Axis Wind Turbine: Enhancing performance in terms of power and loads including dynamic stall effects

Due to its inherent dynamic stability the Vertical Axis Wind Turbine (VAWT) is a strong candidate for large (10-20 MW) floating offshore applications. However, there is room for improvement considering the VAWT’s aerodynamic and structural performance, so as to increase its cost-effectiveness. This thesis explores the VAWT’s potential of enhancing its performance, using active pitch control, in terms of the following three objectives: 1.) power maximization, 2.) power minimization, and 3.) thrust relief. For this purpose the Modified-Linear Actuator Cylinder Model (Mod-Lin ACM) is used as optimization tool. Another inherent feature of the VAWT is that its blades experience large angles of attack, and it is believed that dynamic stall effects will play a significant role in the optimization. The quasi-steady aerodynamic force coefficients in the Mod-Lin ACM are therefore replaced by their dynamic equivalents, which are obtained from a Beddoes-Leishman type dynamic stall model. The phenomenon of leading edge separation is modelled separately, as “plug-in” to this dynamic stall model, following the formulation presented in. The analysis in this work is carried out for the symmetrical NACA0018 airfoil. The achievable increase of maximum power is only 1%, which is even less when unsteady effects are considered in the optimization. For power coefficients at tip-speed ratios and solidities other than the initial optimum point, increases up to 15% are documented. This implies that values close to the initial maximum power can be achieved for lower solidities, decreasing the total weight, and therefore the cost, of the VAWT. A close correlation between the potentials of power increasal and thrust alleviation is found, which is caused by the fact that local power losses due to thrust relief must be compensated on other peripheral positions. Therefore, in order to obtain the highest trust reductions the optimizer will maximize the local power production on the energy efficient parts of the rotor, similar to the power maximization target. Depending on tip-speed ratio and solidity, the given pitch allowance is able to decrease the power production between 65% and 100%. Generally, the decrement is only enhanced by any occurring unsteady effects. This reveals the potential of using the VAWT’s blades as air brake.Aerospace EngineeringAerodynamics, Wind Energy & Propulsion / Wind Energy & DTU Wind EnergyEuropean Wind Energy Master (EWEM