The Diffuser Augmented Wind Turbines (DAWTs) have been widely studied, since the diffusers improve the power coefficient of the wind turbine, particularly of small systems. The diffuser is a device which has the function of causing an increase on the flow velocity through the wind rotor plane due to pressure drop downstream, therefore resulting in an increase of the rotor power coefficient. This technology aids the turbine to exceed the Betz limit, which states that the maximum kinetic energy extracted from the flow is 59.26%. Thus, the present study proposes a mathematical model describing the behavior of the internal velocity for three conical diffusers, taking into account the characteristics of flow around them. The proposed model is based on the Biot-Savart's Law, in which the vortex filament induces a velocity field at an arbitrary point on the axis of symmetry of the diffusers. The results are compared with experimental data obtained for the three diffusers, and present good agreement.

The use of diffusers around of the horizontal-axis wind turbines have been widely studied, since the diffuser provides an improvement in the turbine power coefficient. These diffusers are often called Diffuser Augmented Wind Turbines (DAWT’s). The DAWT’s have the feature to make efficiency exceeding the Betz limit (maximum energy flow extracted = 59.26%), due to the increasing of the internal mass flow by influence of the diffuser presence. Thus, the present work proposed a mathematical model describing the behavior of the velocity profile internally to a diffuser according to the characteristics of flow and geometry of a conical diffuser. The model results were compared with experimental data and showed good agreement.

In recent years, great attention has been given to the study of hydrokinetic turbines for power generation. Such importance is due to the use of clean energy technology by using renewable sources. Therefore, this work aims to present a relevant methodology for the efficient design of horizontal-axis hydrokinetic turbines with variable rotational speed. This methodology includes the Blade Element Method (BEM) for determining the turbine power coefficient, since BEM is widely used in the hydrokinetic turbine design due to its good agreement with experimental data. In addition, the dynamic equation of the driveline is used, taking into account the BEM to provide the rotor hydrodynamic torque coupled with the drive train model, including the multiplier and the electric generator. In this case, the modeling of the whole system comprises the hydrodynamic information of the rotor, the mass-moment of inertia, frictional losses and electromagnetic torque imposed by the generator. The theoretical results were obtained for the transient rotational speed and compared with field data measured from small hydrokinetic turbine installed at the Arapiranga-Açu creek, which is located in the city of Acará, Pará, Brazil.