Effect of shroud on the performance of horizontal axis hydrokinetic turbines

To investigate the effect of shroud on the performance of hydrokinetic turbines in inline and off-axis flows, a horizontal axis model turbine and two shrouds were designed and fabricated. Consistent power, torque, and thrust coefficients for the shrouded and unshrouded turbines were obtained for a complete range of performance curve in water tunnel experiments. A maximum power enhancement of 91% over the unshrouded turbine is obtained with the straight wall diffuser design. Power coefficients of the model turbine with the diffuser calculated based on the diffuser exit area, in agreement with the van Bussel momentum theory, is found comparable to the performance coefficients of the unshrouded turbine with the size of the diffuser exit diameter. In off-axis flows the output power of a hydrokinetic turbine is observed to decrease. The reduction is negligible up to 10° yaw angle but increases as the yaw angle increases beyond 10°. The model turbine with the convergent–divergent wall shroud design experiences negligible performance loss in yaw operations compared to the unshrouded turbine. Using the experimental results and in an analogy to the cosine rules of the linear momentum theory, similar cosine relations for the shrouded turbines are proposed for the first time. The optimum power of the model turbine with the shroud is experimentally observed to decrease with cosine of yaw angle. Based on this, the turbine with the shroud in a 45° yaw angle would generate 29% less power than in an inline flow. The unshrouded turbine, based on the cosine cubed rule of the momentum theory, would generate 65% less power in the same flow. This study addresses the fact why the reported augmentation factors for diffuser augmented wind turbines in the literature has a broad range from -36% to 760% which shows disagreements between researchers.