Investigation of the Operating Principle of Diffuser Augmented Hydrokinetic Turbines

Alternative sustainable technologies for energy production are currently receiving increased attention, one of them being hydrokinetic turbines. For some applications an augmentation of the turbine rotor through a diffuser is a reasonable option offering some advantages. In order to get a better understanding of diffuser augmented turbines an extensive literature study is performed also including relevant publications on ducted wind turbines. The published geometry and performance data are collected and reevaluated applying a new procedure, which references the turbine power on the diffuser outlet area instead of the rotor area. Additionally, geometries are generated, manually optimized and compared to the data gathered in literature. Dominant diffuser attributes are identified independent from the exact diffuser concept such as the shape of the diffuser close to the exit plane and the diffuser area ratio. By using the new evaluation approach a strong correlation between the area ratio and the overall turbine power coefficient is observed. Observations suggest that an increase of the exit area is only reasonable up to a certain limit. To a some extent, this constraint can be overcome through boundary layer control, e.g. by applying multi-stage diffuser concepts. The presented work gives a guide line for effective design of diffusers for hydrokinetic turbines and generally improves the understanding of the operating principle of diffuser augmented turbines.