Design optimization of a diffuser augmented, dual-rotor hydrokinetic turbine utilizing 2D actuator disk theory

Abstract

An estimated 58,400 ± 109 TWh/year of untapped energy exists in rivers across the globe.Installing renewable technology at this capacity would mean significant reductions in greenhouse gas emissions and a slowdown of global warming. Hydrokinetic turbines have gained attraction in recent years to fulfill this need as they offer competitive power production with minimized environmental impact when compared to conventional fully dammed hydropower. Even more recently, researchers and manufactures have investigated how diffuser-augmentation of a hydrokinetic turbine can positively increase the power coefficient of these devices. Several design optimizations of these diffusers have been completed and some generalizations of their suggested shape have been made. Often neglected; however, are the adverse outcomes of diffuser augmentation, such as increased structural loading, increased manufacturing costs, and a reduction in compactness for array installation. The present study has developed a robust methodology for quantifying and implementing these adverse effects into the design optimization process. A total of 40, two-dimensional CFD numerical simulations are conducted, testing a wide range of diffuser shapes from flange style to airfoil style. Additionally, the actuator disk method is utilized across all design cycles to replicate the presence of a turbine rotor, creating a more realistic flow field; a step often neglected in existing two-dimensional design optimizations. The output parameter of maximizing interest is efficiency; a metric calculated by how effectively a diffuser shape can accelerate fluid through at the rotor plane. The output parameters of minimizing interest are diffuser pressure, diffuser material volume, and wake deficit; all metrics significantly influencing manufacturing costs and deployment constraints. A maximum of 29.4% and near minimum 24.9% efficiency values are recorded by the sharp flange and airfoil diffusers, respectively. However, where the flange diffuser excelled in maximizing efficiency, it under-performed in reducing the adverse effects. The contrary can be stated for the airfoil diffuser. An ‘optimal’ design is identified that shares characteristics to both diffuser shapes. Discussion includes insight on why this optimal design (DC 16) is most effective at this and how a future economic analysis could reveal a new optimal design dependent on the turbine deployer’s needs and manufacturing costs.