Abstract
Hydrokinetic turbines extract energy from currents in oceans, rivers, and streams. Ducts can be used to accelerate the flow across the turbine to improve performance. The objective of this work is to couple an analytical model with a Reynolds averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) solver to evaluate designs. An analytical model is derived for ducted turbines. A steady-state moving reference frame solver is used to analyze both the freestream and ducted turbine. A sliding mesh solver is examined for the freestream turbine. An efficient duct is introduced to accelerate the flow at the turbine. Since the turbine is optimized for operation in the freestream and not within the duct, there is a decrease in efficiency due to duct-turbine interaction. Despite the decrease in efficiency, the power extracted by the turbine is increased. The analytical model under-predicts the flow rejection from the duct that is predicted by CFD since the CFD predicts separation but the analytical model does not. Once the mass flow rate is corrected, the model can be used as a design tool to evaluate how the turbine-duct pair reduces mass flow efficiency. To better understand this phenomenon, the turbine is also analyzed within a tube with the analytical model and CFD. The analytical model shows that the duct’s mass flow efficiency reduces as a function of loading, showing that the system will be more efficient when lightly loaded. Using the conclusions of the analytical model, a more efficient ducted turbine system is designed. The turbine is pitched more heavily and the twist profile is adapted to the radial throat velocity profile.
Highlights
Hydrokinetic turbines are a means to extract energy from the world’s oceans, rivers, and tidal streams
Understanding the fluid dynamics effects of energy extraction within the duct will allow for the development of more efficient hydrokinetic turbines, which can in turn reduce the overall generation cost of marine renewable energy
The objective of this study is to evaluate the differences between a Reynolds averaged Navier–Stokes (RANS) computational fluid dynamics (CFD) prediction and an analytical model prediction for ducted turbine performance
Summary
Hydrokinetic turbines are a means to extract energy from the world’s oceans, rivers, and tidal streams. Energy extraction from renewable sources can reduce human dependence on fossil fuels and mitigate global warming. Ducts have been used to augment hydrokinetic turbines, wind turbines, and propellers for decades. Understanding the fluid dynamics effects of energy extraction within the duct will allow for the development of more efficient hydrokinetic turbines, which can in turn reduce the overall generation cost of marine renewable energy. The duct is comprised of two sections: an intake (from inlet to throat) and a diffuser (from throat to exit)
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