# Design Options to Improve the Dynamic Behavior and the Control of Small H-Darrieus VAWTs
###### Abstract
On the other side, high inertia rotors suffer from huge response times, and the acceleration/deceleration rate is far from the required one under the presence of wind unsteadiness, wind gusts, and wind drops. This behavior results in a large idling time, which is felt to be one of the leading causes of the poor performance claimed for these wind energy conversion systems in gusty environments, as urban sites are. It was proved [18] that the power reduction caused by the inertia of wind turbines changed proportionally with their natural time constants. Additionally, the frequency of the wind speed variation and the average of the absolute value of the rate of wind speed change demonstrated to play an important role about the output power: in fact, when the wind speed varies rapidly, the above two factors of the wind speed increase as well, determining a greater reduction in wind turbines power output. Many works have been devoted to understanding the factors that influence such behavior and the mitigation effects, although the papers analyzed the partial design characteristics, such as blade chord, number of blades, rotor solidity, and rotor aspect ratio, as well as the parametric effect of these single variables on turbine inertia [19; 20; 21; 22; 23; 24; 25; 26]. More comprehensive work was undertaken by [PERSON] et al. [27], who investigated the response of a straight-bladed VAWT to pulsating winds. The phase delay between the rotational speed and the wind variation was experimentally and numerically analyzed. The results show a constant value by changing the moment of inertia, the wind cycle, and the wind amplitude. The phase delay resulted to be approximately \(\pi/2\) when the wind amplitude was large. The experimental evidence of the rate of change in the rotational speed suggested a relation between the rotational speed width and the pulsating wind amplitude. The efficiency of the H-VAWT in a pulsating wind with unvarying amplitude stayed almost constant under both changes in the moment of inertia and the wind cycle, but a decrease was recorded when the wind amplitude was large. [PERSON] and [PERSON] [28] combining measured wind data and blade element momentum (BEM) simulations, investigating an H-VAWT response capability in turbulent winds. Even though the energy production adopting an ideal tip speed ratio controller resulted in being much higher than that obtained with a fixed angular speed, a marked efficiency drop was registered when the rotor was no longer capable of closely tracking the gusts. The same authors [29] also investigated different VAWT architectures (acting on height-to-diameter aspect ratio, blade airfoil shape, rotor solidity, and moment of inertia) to determine the best configuration to maximize the energy harvest from urban gusty winds, showing that the optimal power coefficient versus tip speed ratio curve is not necessarily the one exhibiting the highest peak, but rather the broadest shape. Such works indicate that not all of the parameters are of identical importance for a successful final product. Some of them (such as the choice of airfoils, supporting strut configuration, shape, solidity, and the material) are more sensitive and critical than others. As is evident from the preceding review, most of the works pointed to the effect of a single or a few design parameters, without linking them in a unique and rational design path. The long in-field experience collected by the author over about 15 years of tests in open areas and wind tunnels [30] recognized the high polar inertia of the rotor as the most detrimental characteristic of small VAWTs, leading to a severe drop in output energy expectation and structural failure. This issue can only partially be alleviated by the control strategy. Based on these studies, this paper proposes a design procedure aimed to improve the control of small H-VAWTs without affecting essential design requirements (strength, performances, needs of smooth control). The procedure can be easily extended to other VAWT architectures.