Experimental and numerical study on performance of shrouded hydrokinetic turbines

Hydrokinetic turbines, also known as marine current turbines, have the potential to be a major component of the world's renewable energy portfolio. Improving the efficiency of turbines is critical to making this technology more widespread and cost-effective. This work focused on raising power output for a given turbine blade design and flow speed through the addition of a straight-diffusing shroud. A shroud works by lowering pressure at the turbine outlet and thereby accelerating the flow through the turbine. The performance (measured by the power coefficient, C_P) of a shrouded horizontal axis hydrokinetic turbine was studied experimentally in a tow tank at the United States Naval Academy. Two different shroud designs were built and tested, and performance was compared to the bare turbine. Both shrouds featured the same diffuser angle (20°) but different area ratios, cross-sectional shapes, and tip gaps. The second shroud design was optimized using numerical flow simulation data, and resulted in a 21% increase in power output compared to the bare turbine. Both shroud designs lowered the stall speed of the turbine and allowed operation at lower tip speed ratios.