## 4 Summary and Conclusions  
A comparative fatigue study was conducted for three small wind turbine power/rotor speed control schemes, one based on stall regulation and two others on pitch regulation. The rotor of the stall-regulated turbine was designed previously to co-optimize the objectives of high power coefficient at partial load, a flat power curve under nominal conditions with small excursions of the rotor speed, and favorable starting performance. The first pitch-controlled turbine design was based on a published large-scale blade, which was downscaled to the blade length of the stall design and afterwards fine-tuned. A second pitch-regulated wind turbine was designed by emulating the rotor speed function of the stall-regulated turbine; the objective was to separate any rpm-related effects in the fatigue damage from the intrinsic behavior of the stall and pitch regulation, respectively.  
A methodology for fatigue damage assessment based on a combination of an aeroelastic simulation and the simplified model proposed in the IEC-61400-2 standard was designed and implemented. Fatigue life curves for two levels of turbulence intensity (adjusted to their respective average wind speeds) were calculated for all three regulations schemes as a function of the wind speed class. An exponential decay of the fatigue life was observed over a large range of average wind speeds, from 7 m/s to about 16 m/s, for all schemes. While the fixed-rpm pitch-regulated schemes continued to decay with the same law for even higher wind speeds, the fatigue life curves for the stall-regulated turbine were found to level off afterwards; the reduced rotor thrust coefficient, resulting from a reduced rotor speed at high wind speeds, was identified as the main underlying cause. Consistently, a similar behavior was observed for the pitch design with variable rotor speed.  
Average fatigue life curves as a function of the Weibull scale factor were calculated for all cases and for different Weibull shape factors. Under typical wind regime conditions and small to moderate average wind speeds, the stall turbine exhibited a larger average fatigue damage, as expected. This difference was less pronounced for wind regimes with smaller shape factors, given the broader wind speed distributions. In all cases, both pitch- and stall-regulated wind turbines showed an average fatigue life in excess of several decades for wind speeds typically encountered in small wind turbine locations.  
Figure 12: Average fatigue life as a function of the Weibull scale factor \(v_{s}\) for all three power regulation schemes, different Weibull shape factors, and low turbulence intensity. Upper row: average fatigue life. Lower row: corresponding Weibull distributions for selected \(v_{s}\) values.  
**Author Contributions:** Conceptualization, O.P.; data curation, B.L. and J.P.-C.; formal analysis, O.P.; funding acquisition, O.P.; investigation, B.L., J.P.-C., and L.I.M.; methodology, D.C. and O.P.; project administration, O.P.; resources, D.C. and L.I.M.; software, B.L.; supervision, O.P.; visualization, B.L. and O.P.; writing, original draft, L.I.M. and O.P.; writing, review and editing, O.P.  
**Funding:** Three of the authors ([PERSON], [PERSON], and [PERSON]) acknowledge funding from FSE/CONACYT through the project P19 (\"Control strategies for reliability in small wind turbines\") of the Mexican Center for Innovation in Wind Energy (CEMIE Eolico). [PERSON] and [PERSON] acknowledge a tuition scholarship for their M.Sc. studies at Tecnologico de Monterrey and a stipend from CONACYT/PNPC under Program Number 5602.  
**Acknowledgments:** [PERSON], [PERSON], and [PERSON] appreciate the opportunity to participate in project P09 (\"Design of wind turbine blades\") of the CEMIE Eolico. [PERSON] and [PERSON] would also like to thank the Energy and Climate Change group at Tecnologico de Monterrey for support. [PERSON] appreciates support from Tecnologico de Monterrey and Universidad de Cuenca.  
**Conflicts of Interest:** The authors declare no conflict of interest.