Tidal inflow and energy resource characterization is needed for planning tidal energy projects and designing tidal energy converters (TEC). Recommended metrics and best field measurement practices for characterization are given in several International Electrotechnical Commission (IEC), Task Committee 114 (TC114), technical specifications, viz., 62600-201 (Tidal Energy Resource Assessment), 62600-200 (Tidal Power Performance), and 62600-2, (Marine Energy System Design). The intent of the IEC standards is to ensure consistent metrics, methods, and analysis for TEC projects, and the design and certification of TECs, without being overly prescriptive. Nevertheless, inconsistencies between methods and analysis are often difficult to avoid and can complicate comparisons between tidal inflow and energy resource characteristics between sites. In this study, a recent literature review reveals a lack of consistency in establishing a slack current threshold when reporting tidal resource site assessment. As used in this study, the term “slack currents” include a loosely defined range of weak flow that encompasses the flow reversal between flood and ebb tides. The inclusion or exclusion of these slack flow measurements and the slack current threshold are found to significantly impact flow statistics and subsequent inflow characterization at a site.
We analyze datasets from tidal flow measurements worldwide and examine how the slack current threshold affects critical inflow characterization metrics, e.g., mean velocities, turbulence intensity values, and power spectra. These metrics influence the TEC device size and material selection, as well as estimates of device and component lifespan and energy harvesting estimates. Sensitivity studies illustrate the effects of varying the slack current threshold on a range of reported tidal inflow metrics. In New York City’s East River, for example, the reported mean velocity increases by 18%, and site-averaged turbulence intensity decreases by over 50% based on the selected slack threshold. The sensitivity study also reveals the frequency of slack tides depends on this threshold, which could be directly correlated to the estimated daily operation time for a deployed TEC.
This work aims to further standardize methods for tidal inflow characterization to promote consistency for comparison and collaboration. This work addresses the effects of the slack threshold on the slack tide frequency and duration, which is needed to plan periods for inspection, maintenance, and repair. In addition, a survey of current TEC devices will be presented that analyzes the cut-in and cut-out speeds, which should be considered when determining a slack threshold in characterizing inflow statistics.