Self-Starting Enhancement of Small-Scale H-Darrieus VAWTs for Urban Low-Wind Conditions: A Taxonomy of Strategies, Biomimetic Flow Control, and Design Guidelines

Poor self-starting under low and turbulent urban winds remains a key limitation for small-scale H-Darrieus vertical-axis wind turbines (VAWTs). This review presents a comprehensive and design-oriented synthesis of strategies to enhance self-starting performance in micro-scale H-VAWTs. The study systematically classifies existing approaches into five major categories: rotor configuration modifications, aerodynamic blade design strategies, passive flow control techniques, biomimetic innovations, and active/semi-active mechanisms. Fundamental startup physics, including static and dynamic torque behavior, low tip-speed ratio dead-band, and Reynolds number effects, are critically examined in relation to urban wind characteristics. A detailed comparative evaluation of over 150 studies is conducted to identify the effectiveness, limitations, and trade-offs of each strategy. Particular emphasis is placed on emerging biomimetic approaches such as leading-edge tubercles and J-shaped blades, which demonstrate significant potential in mitigating stall and enhancing low-speed torque without substantial efficiency penalties. Manufacturing considerations, including additive manufacturing feasibility and structural implications, are also discussed. Based on this synthesis, practical design guidelines are developed to support the development of reliable, low cut-in speed VAWTs for urban applications. The review highlights critical research gaps, including the need for standardized testing protocols and long-term field validation. Overall, this work provides a structured roadmap for overcoming the self-starting bottleneck and advancing the viability of decentralized urban wind energy systems.