**4. Conclusions**

To extract as much potential as possible, there is a need to increase the wind turbines efficiency. One way of increasing the efficiency of wind turbines is by reducing the sources of drag and for wind turbines one such source are the blade tip vortices.

The study aimed to find out if the application of split winglets at the tip of HAWT blades would improve its aerodynamic performance, and the results showed that adding a split winglet to the NREL phase VI sequence H design can increase its power generation. Accounting for the difference in length due to adding a split winglet, power generation on an average increased by 2.53%. In comparison, a winglet that bends toward the suction side improves power generation on an average by 1.23%. Results also show that while winglets reduce performance at low tip speed ratios, the performance benefits at higher tip speed ratios where the majority of the power is generated more than make up for it.

The study also shows that the increase is achieved through two factors. First is that the winglet and split winglet drastically reduced the low-pressure area at the trailing edge of the blade tip that pulls back the blade, creating drag. This reduction in vortex core region was a result of the large drop in the tangential wind velocity going from the pressure to the suction side of the blade tip due to the winglets. Second is that the winglets themselves generate lift as can be seen on the pressure contour plot at the blade tip. The difference in pressure at the blade tip between the pressure and suction side responsible for creating lift is still present on the winglet and in case of the split winglet improved.

The results, however, are not all positive. One negative is that the winglets increased the thrust experienced by the rotor blade. Accounting for the difference in blade span of the base blade and ones with winglet, thrust increased by 0.83% and 2.05% for the blade with winglet and split winglet, respectively. This means that additional structural support is required to help support the additional load brought by the addition of winglet to the blade.

In the case of this study, attaching a split winglet to a wind turbine rotor blade increased power generation by a small but not insignificant amount. The study also showed that the split winglet is better than a winglet in increasing power generation. The opposite is true, on the other hand, in terms of the additional thrust experienced by the blade.

**Author Contributions:** Investigation, M.S.S.; methodology, M.S.S. and B.E.A.; supervision, L.A.M.D.; writing—original draft, M.S.S.; writing—review & editing, B.E.A. and L.A.M.D. All authors have read and agreed to the published version of the manuscript.

**Funding:** There is no funding that was availed in the conduct of this study.

**Acknowledgments:** The authors would like to thank Lee Fingersh of NREL for the experimental data provided in this study.

**Conflicts of Interest:** The authors declare no conflict of interest.
