Impact of Post-Stall Extrapolation and Rotational-Augmentation Models on the Performance of Stall-Controlled Wind Turbines

Abstract

While most wind energy comes today from utility-scale machines, small wind turbines SWTs can still play a role in off-grid installations or in the context of distributed production and smart energy systems. Due to cost reasons, SWTs (especially up to 100kW) are usually controlled via progressive stall of the blades. Accurate airfoils polars are therefore key in engineering simulation models, not only to ensure predictive blade performance, but are also pivotal for proper controller tuning and an effective evaluation of loads. This study presents an analysis on the impact of post-stall extrapolation methods for airfoil polars, combined with the rotational-augmentation correction, on the aero-servo-elastic simulation of a stall-controlled wind turbine. The selected test case is the UNIFI 50kW RWT, a 50kW reference turbine recently developed by Università degli Studi di Firenze. Aero-servo-elastic simulations are carried out with the open-source code OpenFAST by NREL considering typical turbine Design Situations. Several post-stall extrapolation methods are tested on the same set of polars, also including the 3D correction. Results prove that the performance of turbines like the one studied here is strongly dependent on these methods, especially after the needed correction for rotational-augmentation is applied. In this view, dedicated studies and experimental validations in the next future could help increase the predictivity of low-order numerical models, establish new engineering best-practices, and finally increase the economic feasibility of small wind turbines.

Design Loads in Small Wind Turbines: a Detailed Comparison Between Pitch and Stall Regulation

“…For the purposes of the present study, it is worth noting that the turbine has been originally designed using airfoil polars extrapolated with the Montgomerie method [10] and corrected for rotational-augmentation with the Bak's method [11]. The reader is referred to the GitHub repository for any additional detail about the turbine; a detailed explanation of the variable speed, stall-control strategy can be also found in [12].…”

On the influence of twist and taper of HAWT blades on the rotational augmentation phenomenon: the NREL Phase VI – Phase II comparison

“…However, the author themselves recognized the necessity of further investigation in order to generalize the methodology, particularly for larger rotors. Using the UNIFI experimental turbine, recently developed by Università degli Studi di Firenze, Papi et al [19] evaluated the impact of poststall extrapolation methods with particular attention on the stall delay effects. The results showed that the impact of the methodology was significant therefore the designers should pose more attention to a wise selection of the method used for post-stall extrapolation.…”