School of Civil and Environmental Engineering

Ph.D. Thesis Defense Announcement

Assessing Hydrokinetic Tidal Energy Extraction for Rose Dhu Island, Georgia:

A Case Study for Tidal Rivers with Marsh Environs

by:

Brittany L. Bruder

Advisor: 

Dr. Kevin Haas

Committee Members: 

Dr. Herman Fritz, Dr. Thorsten Stoesser, Dr. Donald Webster, & Dr. Annalisa Bracco - EAS

Date & Time: May 5, 2015 1PM

Location:  Mason Room 3132

ABSTRACT:

Hydrokinetic tidal power is a novel and emergent technology undergoing continuous advancement with much of the progress focused on large utility scale projects. This resource is potentially underutilized because much of the coastal United States, despite having substantial tidal currents, do not have the deep and wide environments required by the current developing turbine technology. Wetland estuaries with shallow and narrow tidal rivers have unique hydrodynamics that foster moderately strong currents and are a vast untapped resource for the United States. This dissertation evaluates the feasibility for tidal energy projects in estuaries with extensive tidal marshes through a hydrokinetic energy resource assessment for Rose Dhu Island, Georgia.

To obtain hydrodynamic data for the assessment, numerous boat-based field campaigns were executed in the immediate vicinity of Rose Dhu Island from 2010-2014. Measurements of water surface elevations, current velocities, and channel volume fluxes were obtained using a coupled Global Positioning System (GPS) and Acoustic Doppler Current Profiler (ADCP) assembly. The measurements are used to identify areas of persistent hydrokinetic power, calibrate an existing numerical model for yearly energy predictions, and characterize estuarine and local hydrodynamics.

To resolve the estuarine hydrodynamics, data from various numerical simulations of the encompassing Ogeechee Estuary, modeled by Sandeep Bomminayuni, are evaluated against field measurements. Wetland dominated estuaries commonly have a high degree of non-linear distortion which govern the relative durations and strengths of the tidal stages and thus the overall hydrodynamics and incoming hydrokinetic energy. In this dissertation, distortion is uniquely parameterized by the statistical parameters skewness and asymmetry. The distortion of the Ogeechee estuary is primarily governed by intertidal storage; the estuary is ebb dominant with peak ebb and flood volume fluxes near high tide.

Measurements are analyzed to isolate areas of persistently high current velocities suitable for tidal energy extraction. A location near the southwest coast of the island is identified as a hydrokinetic energy hotspot. A kinematic and dynamic analysis is performed to identify key physical processes behind its formation. In accordance with subcritical flow, a localized bump in the seabed accelerates currents at that point. During flood tide the bump acts as a boundary for co-rotating circulation cells induced by centrifugal acceleration, concentrating axial momentum throughout the water column at their interface. During ebb tide axial momentum distribution is governed by the two counter-rotating circulation cells from the confluence of two upstream channels, concentrating axial momentum against the bump.

The effects of tidal distortion on the projected hydrokinetic energy resource is explored with synthetic data. Distortion has little effect on the theoretical resource but the technical resource can vary greatly. Thus for wetland estuaries with high distortion, accurate resource assessments require high quality information about distortion. The most optimum turbine therefore may not necessarily have the highest rated power. For the theoretical resource assessment for the island, tidal constituents are calculated from the model and used to predict yearly velocity and power timeseries in the local domain. Using the method of Garret and Cummins, it is estimated that the maximum average channel power is 8.80 MW. For the hotspot it is estimated that there is theoretically 30.3 MWh available to capture yearly with an average power of 3.46 kW for a turbine with an area of 10$m^2$. For the technical resource assessment, synthetic turbine efficiencies are applied to analyzed points. A rotor prototype was tested at Rose Dhu; however data was unusable. Because of the spatially varying velocity magnitudes, projected technical energy calculations varied greatly across the domain. For a generalized turbine with a cut-in and rated speed of 0.8 m/s and 1.5 m/s respectively, a cross sectional area of 10m$^2$ and a maximum mechanical to electrical efficiency of 41\%, the hotspot could provide a yearly energy of 10.9 MWh with an average power of 1.25kW.

This dissertation highlights the large spatial and temporal variability of hydrokinetic energy for a wetland environment. These considerations are more pertinent when performing a hydrokinetic energy resource assessment in a marsh estuary than for large scale bay-ocean exchange environments, the present industry focus.