Design and performance evaluation of a variable-diameter vertical axis wind turbine

Abstract

Vertical Axis Wind Turbines (VAWTs) offer simpler designs than horizontal-axis counterparts. However, adoption was limited by lower power generation and poorer self-starting. This research, therefore, proposed and investigated a variable-diameter VAWT. The goal was to increase power production and widen the operational tip-speed ratio (TSR) range. A review of active systems showed mechanisms aimed to optimise aerodynamic flow and delay stall. These systems included slotted blades, morphing blades, and variable pitch or flaps. Whilst several approaches existed, variable pitch and variable solidity demonstrated the most promise for improving power yield and self-starting. However, variable pitch systems had been investigated more extensively. This was because dynamically varying a VAWT's solidity was considered mechanically complex. Previous studies focused on altering the blade chord or number. This approach left the most feasible parameter, rotor diameter, largely unexplored. This long-standing design compromise fundamentally limited the performance of Darrieus-type turbines. This research introduced and validated a novel adaptive solution called the Variable-Diameter Vertical Axis Wind Turbine (VD-VAWT). An engineered mechanism actively altered the rotor's diameter to modulate solidity. This innovation decoupled the conflicting aerodynamic requirements. The design enabled a compact, high-torque configuration for start-up. It subsequently transitioned to an expanded, high-efficiency state for power generation. The concept's viability was rigorously evaluated through a multi-stage methodology. This process involved conceptual design, Computational Fluid Dynamics (CFD) simulations, and comprehensive wind tunnel testing. A laboratory-scale prototype was used to validate the numerical models. Experimental results confirmed the VD-VAWT's superior performance. Initial tests of fixed-diameter configurations identified an optimal diameter of 600 mm, which increased the maximum power coefficient (CP,max) by 68% compared to an 800 mm diameter. The actively controlled system boosted the overall power yield by 25%, outperforming the fixed 600 mm and 800 mm turbines by 34% and 68%, respectively. Peak power reached 19.92 W at 224 RPM, with a net output of 13.61 W after accounting for motor consumption (32%); the power – RPM trend aligned with predictions. The study also validated a hybrid morphing strategy where the turbine employed a high-solidity state for robust self-starting before shifting to a low-solidity state for optimal power generation. This expanded operational range improved wind capture and enabled a storm protection mode by contracting during extreme winds. This study concluded that a variable-diameter design was a transformative strategy for overcoming the inherent limitations of VAWTs. CFD analysis provided critical insight into the aerodynamic mechanisms responsible for these performance gains. The analysis revealed that the adaptive low-solidity state successfully mitigated adverse aerodynamic effects that impair high-solidity turbine performance. The significance of the VD-VAWT lies in resolving the long-standing trade-off between starting torque and running efficiency. This research validated a step-change from static to adaptive turbine design by actively managing its aerodynamic regime. It presents a viable pathway toward the enhanced performance and commercial feasibility of VAWT technology.

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