# Design Options to Improve the Dynamic Behavior and the Control of Small H-Darrieus VAWTs
###### Abstract  
H-VAWTs or straight blades VAWTs are the most common turbine architecture employed for small VAWTs. The manufacture of straight, constant chord blades, coupled with the transport advantages, make this choice technologically simpler, compared to curved (eggbeater) type or curved-bent (Gorlov) type, allowing a large selection of materials, and design solutions. Recently, the strategies to accomplish the task of zero-emission buildings identified wind energy exploitation in the urban environment as one of the most promising. Micro and mini wind turbines installed on buildings (BAWT--building-augmented wind turbines) are considered the candidate technology after that of photovoltaic panels; under certain conditions, these technologies can be combined to obtain the maximum natural resources exploitation in the urban environment. VAWT, compared to HAWT, would ideally perform better in the fast-changing, turbulent winds, typical of the built environment. Additionally, its 3D shape favors a better architectonic integration with the volumes of the building. Nevertheless, despite these claimed advantages, this architecture did still not come to the expected fruition and experience, which revealed that the stochastic nature of the wind resource in the built environment determines a quite challenging context, reflecting not only the structural endurance, but also the operations and the annual energy production. These site characteristics stress the detrimental effect of the high polar inertia of this architecture hampering, be it a reduction in the acceleration and deceleration capability of the rotor, the required adaptation of the rotational speed to the varying wind conditions, or compromising any form of robust control. This leads to poor aerodynamic performance and potential structural damages. This paper contributes to mitigating the issue of the high rotor polar inertia of the H-VAWT without affecting other essential design requirements (strength, performances, needs of smooth control). The work identifies the design parameters governing the rotor acceleration and deceleration and develops a rational design procedure aimed at improving the H-VAWT control and performance. 2 2021 26 September 2021 26 September 2021 3 October 2021 2021 26 September 2021 26 September 2021 26 October 2021 26 September 2021 26 October 2021 26 September 2021 26 September 2  
projects [16]. In the urban field, fluid-dynamic limitation due to the wake interaction between the turbine and the building [7] restricts the size to the order of about 10 kW, limiting rotors to a maximum diameter of about 5-6 m [8]. Concerning distributed wind applications in suburban or rural areas, the adoption of rotors on the top of towers to intercept higher winds poses, again, a limitation on the maximum size of the H-Darrieus turbine caused by the complex structural issues of the rotor-arms-tower connection, and thus, the consequent development remained in the area of very small size turbines. An H-VAWT 200 kW wooden tower prototype has been running discontinuously since 2010 at the University of Uppsala [17], hampered by rotor-tower resonance. This area of application inherited a lack of suitable theoretical and numerical design tools for the design, and a considerable number of projects have a very long commercial latency period, proving the complex design path, and many problems of unreliability and underperformance have been reported for commercial models. All this is to say that H-VAWTs, despite some attractive features, still suffer from some shortcomings, which partly can be handled with a better knowledge of fluid dynamics and structural dynamics. Small straight blades VAWTs are characterized by intrinsically high polar inertia, due to the peripheral arrangements of the blades concerning the rotating shaft. This feature determines a beneficial flywheel effect on the drive train, smoothing down torque oscillations induced by the complex aerodynamics of the rotor.