Computational Fluid Dynamics Analysis on the Improvement of Hybrid Savonius-Darrieus NREL S809 at Various Fluid Flows

Among all types of vertical axis wind turbines, Savonius and Darrieus are the most popular to study. Savonius has a very large torque and requires a low wind velocity to start the rotation, but the efficiency is below 20%. Meanwhile, Darrieus is not capable of self-starting if the wind velocity is below 4 m/s, but the efficiency is the best among all types of vertical axis wind turbines, which is around 40–45%. So far, many researchers have applied the hybrid Savonius-Darrieus to combine their performances. However, the conventional hybrid Savonius-Darrieus cannot produce an optimal Power Coefficient at a high Tip Speed Ratio. In this study, an improvement in the performance of the hybrid Savonius-Darrieus has been analyzed using the Computational Fluid Dynamics method at various fluid flows. This improvement was carried out with the aim of obtaining a high Moment Coefficient and Power Coefficient in all turbine Tip Speed Ratios compared to the conventional hybrid Savonius-Darrieus. The hybrid model used is the 2-bladed Savonius as the internal rotor and the 3-bladed Darrieus NREL S809 as the external rotor with a rotor radius ratio of 0.5. Improvements were made by changing the turbine rotation mode from hybrid at a low Tip Speed Ratio to single Darrieus at a high Tip Speed Ratio. This method was tested in the wind velocity range of 3–7 m/s using the Realizable k-e turbulence model, enhanced wall treatment. As a result, the best performance occurs at a wind velocity of 7 m/s, which is a Moment Coefficient value of 110% better than single Darrieus at low TSR and a maximum Power Coefficient value of 50% better than conventional hybrid.