# Design Options to Improve the Dynamic Behavior and the Control of Small H-Darrieus VAWTs
## 5 Conclusions  
Experimental and analytical evidence show that the inertia of small H-VAWTs is orders of magnitude higher than HAWTs counterparts of comparable size. The consequence is that the acceleration/deceleration capability of an H-Darrieus VAWTs away from cut-in and cut-out wind speeds is partially hampered since the characteristics time of the rotor is much higher than the characteristic time of the gusts. This issue inhibits and makes partially ineffective any turbine control strategy, leading to an insufficient energy harvest and potential structural issues. It was demonstrated that the wind tracking index (WTI) is a highly informative parameter to understand the dynamic behavior of a wind turbine rotor. Its maximization improves the gust tracking capability of the turbine. To be more specific with concern to H-VAWT, a rise of a factor of about 2 to 5 times for the low gusting site and of about 10 to 20 times for a moderate-high gusting site is necessary to provide an acceptable acceleration/deceleration capability. This goal is accomplished by adopting a multi-front design tool. A rational approach is given by the well-known Ashby method, which was demonstrated and adopted in the paper. Its application limits designers' subjective judgments and the adoption of irrelevant alternatives. This approach consists of expressing the design objective, in the present case, the WTI, by independent performance indexes, namely the \(E\) (environment), the \(G\) (geometry), the \(F\) (functional) and the \(M\) (material) indexes, with respect to some constraints (dimensions, stiffness, strength, etc.). The WTI maximization can be pursued by the maximization of one or more of such indexes. The analysis allowed us to conclude that, generally speaking, the H-VAWT accelerating capability results in being penalized under the condition of rotors of relatively large diameters installed in moderate to high gusty urban environments. Mitigation of this setback can be achieved by a specific design approach providing a well-designed blade spar, leading to a WTI improvement of about 5 times. The accurate characterization of blade materials and fabrication can further add a gain of about 1.6 times. Besides high-performing aerodynamics, which is always beneficial, halving the tip speed ratio leads to a gain of about 2 times. This latter choice should be analyzed under the light of considerations on turbine control and other aerodynamic issues. Finally, moving from simple materials, such as wood or aluminum, to composite materials (GFRP or CFRP) will increase the wind tracking capability from 2 times to 3 times, depending on the original material adopted. The combination of such design choices is strongly suggested to improve the performance of small H-VAWTs. The adoption of variable geometry (blade pitch, and variable diameter) could, in principle, be beneficial to alleviate the long idling times of such rotors, but more investigation is needed in this area in view also of the mechanical complexity and endurance of the blade pitching mechanism as well as the extensible/collapsible arms or blades. As a general statement, it is clear that, besides the beneficial effect during the starting phase (not addressed in the paper), control systems adopting the variable rotational speed become partly (or totally) ineffective concerning the goal of tracking the optimum power if the WTI parameter remains below one. This means that the actual rotational speed history experienced from a VAWT could differ substantially from the one computed in the design phase. As the fatigue damage depends on the duty cycles accumulated, this could be eventually considered an explanation of the premature and unexpected failure of several VAWTs. This research received no external funding. Not applicable. I thank the technicians of the Turbomachinery Laboratory of the University of Trento, Dipl. Eng.