Fluid-Structure Dynamics for Extreme-Scale Offshore Wind and Riverine Energy Harvesting

As the need for energy increases globally, renewable energy has become the fastest growing segment of the energy sector, providing cleaner sources of energy that will have reduced impact on escalating environmental concerns. To achieve a carbon-free electric grid, no single form renewable energy can be relied upon due to the intermittency of renewables. To that end, two forms of renewable energy are explored to address energy generation capabilities at large and small scales.
The abundance of wind resources offshore and the decreasing costs associated with larger wind turbines supports the development of offshore turbines at unprecedented scales. Such large rotors pose structural challenges due the extreme size and weight the blades. A novel methodology for designing extreme-scale rotors was developed to optimize aerodynamics for maximum power production and minimum rotor weight and cost. The approach consists of three stages used to computationally design blade and rotor geometry while considering aeroelastic effects and steady state performance. To achieve this, a design space was identified based on novel empirical models of 2-D airfoil characteristics at extreme Reynolds numbers to help determine ideal lift coefficient and chord distributions for blades at these very large scales. The method was then applied to produce a 25 MW rotor, one of the largest ever to be designed.
Variable pitch capabilities are another crucial design consideration to maximize extreme-scale turbine performance. These pitch systems must be able to sufficiently actuate very long and heavy blade to handle the high wind speeds and turbulence levels expected offshore. However, a generalized approach for sizing the peak and average power of blade pitch systems is not publicly available. Thus, a method for estimating peak pitch requirements was developed herein and applied to reference turbines ranging from 5 MW to 50 MW in terms of rated power. In doing so, scaling laws were determined to estimate maximum blade root pitching moments, required actuator torque, and pitch power consumed based on the product of blade mass and average blade chord length. The results indicate that wind speeds slightly above rated conditions are the primary drivers for pitch system power requirements.
Small scale hydrokinetic devices can be installed in riverine environments to generate relatively consistent power from the incoming flow. However, conventional rotary turbines post threats to marine wildlife and are not ideal for operating in shallow riverine depths and handling seasonal changes in flow conditions. Oscillating hydrofoil systems can address these issues but require further development to improve the understanding of flow physics that lead to high power generation efficiencies. Experiments were conducted to investigate the impact of freestream turbulence on hydrofoil performance for a single hydrofoil at laboratory scale. The results indicate that elevated turbulence levels actually improve hydrofoil kinematics, forces, and thus power. A more in-depth analysis of the flow physics associated with this efficiency benefit is recommended for future work.
The relatively shallow depths of rivers also present an opportunity to further increase oscillating hydrofoil efficiency through flow confinement effects. A 2-D numerical approach was validated and utilized to assess the impact of horizontal confinement using a vertically stacked dual hydrofoil configuration. Both laboratory scale and field scale Reynolds numbers were considered and the results indicated that the confinement due to interactions between the hydrofoils and the free surface and riverbed provide significant enhancement of hydrodynamic efficiency. Confinement between two hydrofoils provide a smaller benefit, and, surprisingly, sometimes an efficiency reduction. Additional work is recommended to account for three-dimensional effects that are expected to negatively impact hydrofoil performance.