A simplified dynamic model for mast design of H-Darrieus vertical axis wind turbines (VAWTs)

Abstract The use of small scale vertical axis turbines (VAWTs) in the urban environment is becoming increasingly popular. VAWTs are usually supported on steel masts to increase their efficiency. The resonance between the supporting masts and the VAWTs can disturb the power generation and trigger potential fatigue issues in the mast structure. As the turbine itself influences greatly the free vibration modes of the mast, VAWTs should not be modelled as a single lumped mass during a dynamic analysis. A simplified analytical model of an H-Darrieus VAWT is developed to allow the dynamic analysis of the supporting masts taking into account the coupling of the mast and turbine. The VAWT has a height of 5 m, a diameter of 3 m and a power output of 6 kW whereas the height of the masts ranges between 3 and 15 m. The coupled structure is analysed both analytically, through a multi-degree of freedom system, as well as numerically through the finite element (FE) method. The correlation of the estimated natural frequencies of the system through the FE method with site measurements was found to be within 10% and can allow designing adequately the mast. Harmonic forces representative of the turbine out of balance and cyclic aerodynamic forces are proposed and used to predict peak acceleration at resonance.

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