Collaborative Research: A multi-scale approach for optimizing tidal kinetic energy extraction for sustainable power generation
Tidal in-stream energy conversion (TISEC) facilities provide a highly predictable and dependable source of energy. Given the economic and social incentives to migrate towards renewable energy sources there has been tremendous interest in the technology. However, at present it is not yet apparent what the most efficient design would be for real ocean conditions. A second open issue pertains to environmental impact. Such devices are able to extract a significant fraction of energy from the water column and can produce local modifications to the bed stress resulting in a morphodynamic adjustment in locations where the substrate is mobile. When sited in an inlet or constriction, a turbine array can influence the exchanges of water between an embayment and the adjacent ocean. An accurate quantification of environmental impact is critical for site planning and permitting.
Presently, researchers are examining scientific issues related to tidal kinetic energy extraction using a variety of computational tools that range from commercial CFD codes for evaluating device performance to ocean models for evaluating impact and site selection. Taken independently, such approaches are forced to rely on simplifying assumptions that decrease the accuracy and utility of the computations. The device scale CFD models typically employ a uniform inflow velocity, ignoring realistic site-specific ocean conditions that influence device performance such as vertical shear, surface waves, free-stream turbulence, and eccentricity of the tidal ellipse. Large-scale hydrodynamic models typically employ subgrid-scale approaches to model energy extraction using simple parameterizations to represent the influence of the device on the flowfield.
This project will apply a multi-model approach to link the scales of interest. Device scale simulations will be performed using a RANS CFD code with an optimal shape design capability. The shape design procedure uses a highly efficient approach based on control theory for construction of the gradient. For the larger scales necessary for impact assessment and site evaluation, an unstructured grid ocean model will be employed. The models will be coupled using a two-way approach. The device scale simulations will provide momentum loss and changes in turbulent kinetic energy needed for accurate subgrid-scale parameterization of energy extrac- tion in the ocean model. The ocean model will provide realistic flow conditions for the device scale simulations. The coupled approach will be applied to both idealized and realistic domains and will be capable of improving turbine performance in realistic conditions while simultaneously considering the potential impacts to the environment.
This project will develop and test a process for optimizing and evaluating tidal kinetic energy extraction across the entire range of critical scales. In particular, the multi-model approach will enable (1) automatic device shape design for increased efficiency in realistic flow conditions, (2) assessments of impact on circulation and sediment transport using accurate parameterizations of turbine effects. An important outcome of this research will be the development and demonstration of a suite of tools to support the planning and permitting processes for tidal energy installations.
