Harnessing Tidal Energy: Experimental Analysis of Optimal Contraction Ratios for Velocity Augmentation through a Vertical Axis Tidal Turbine in a Stratford Constriction

Abstract

Tidal energy can be a reliable source of base load power generation due to the periodicity of ocean currents. The average speed of these currents (approximately 1 m/s), however, is too low for commercial energy production and necessitates the exploration of artificial velocity augmentation techniques. One way to increase flow velocity through a turbine cross-section is to design artificial channels around it, while minimizing losses due to the channel-water interaction. This project is an experimental analysis of two configurations of a symmetric converging/diverging, open channel constriction that was designed using the Stratford criterion for turbulent flow separation. Building upon the Master’s thesis of Amanda DeGiorgi (Princeton University, 2013), the project has as its primary goal the identification of optimal inlet to throat contraction ratios rc of the constriction for velocity augmentation at the throat. The losses associated with vertical axis tidal turbines of known efficiencies were modeled by placing stainless steel screens at the constriction throat. Two different constriction configurations were investigated: two constrictions of equal size in parallel as well a smaller constriction nested inside another one twice the size of the former. For both configurations, flow visualization experiments were conducted to identify the largest contraction ratios that showed (a) completely attached flow as well as (b) marginally attached flow along the constriction walls. The freestream velocity gain at the throat was then quantified for the two identified ratios using Particle Image Velocimetry. For the parallel configuration, the flow velocity at the throat was 1.82 times the freestream velocity for a rc of both 2:1 (complete attachment) and 2.47:1 (marginal attachment), yielding a power density increase factor of 6.1 for both cases. Within the two cases, augmenters with a rc of 2.47:1 were found to generate 25% more power than the other case when placed in a parallel array of equal width. For the nested configuration, the velocity gain factor was 1.94 for a rc of 1.71:1 (com- plete attachment) while it was 2.01 for a rc of 2:1 (marginal attachment). Despite a fractionally lower velocity gain, augmenters with an inner rc of 1.71:1 were found to generate 25% more power than ones with a rc of 2:1 when placed in a parallel array of the same width. Furthermore, the nested configuration had a power density increase factor of just over 30% compared to the parallel configuration. This work demonstrates the feasibility of improving the power output for a vertical axis tidal turbine by using optimized augmenter configurations.