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

Small wind turbines (SWTs) have known alternate fortune but can now play a key role in distributed production to foster energy transition. Among the typical features of small machines, the use of stall regulation is a distinctive one, since the ubiquitous variable speed pitch regulation used in utility-scale rotors is hampered by space constraints in small nacelles. This reduced the cost and boosts overall reliability; on the other hand, compromises in the aerodynamic design of the blades must be adopted to ensure smooth power regulation above rated windspeed. This has implications not only on the final Annual Energy Production (AEP) of the rotors, but also on loads. The current study aims at investigating the differences in peak loads between a stall and a pitch regulated SWT. The UNIFI 50kW SRWT, a reference turbine designed by Università degli Studi di Firenze, is used as a case study thanks to the fact that is available both with stall and pitch control options. Differently from most of the studies to date, simulations are carried out for a variety of power production and parked load cases, significant for extreme loading of the rotor and tower of the wind turbine, thus strongly impacting on the design. Results demonstrate how a pitch-regulated machine can ensure a significant reduction of the main maximum loads, thus paving the way for more optimized and cost-effective designs of future SWTs.