title: Assessing the performance of horizontal axis marine current turbines : the impact of evaluation methods and inflow parameters
Creators: name: Rahimian, M
Publisher: name: University of Tasmania
Year: 2018

description: The oceans offer a considerable sustainable energy resource in the form of wave, marine currents and thermal energy and provide a potential alternative to fossil fuels. Marine current, or tidal, energy can be harnessed using horizontal axis marine current turbines (HAMCTs). In this work, the performance of a 2-bladed HAMCT was extensively investigated using experimental, theoretical and numerical models under different flow conditions. The motivation behind the study was to assess the influence of various parameters, associated with an evaluation method and those involved in the environment of a deployment site, on turbine performance. The employed evaluation methods were compared to find the best practice for performance assessment in various operational conditions. For this purpose, two physical scale models with diameters of 500 mm and 800 mm were tested in the Australian Maritime College Towing Tank and Circulating Water Channel (CWC). Towing tank results from the United States Naval Academy were also employed for facility comparison as well as CFD model verification. The experimental results provide the hydrodynamic characteristics of the turbine under the different inflow conditions in each facility. The difference between the performances of the two scale models was found to be mainly due to the effect of Reynolds number and possibly attributed to the blockage effect. To predict the performance of a full-scale turbine, experiments on a scale model in conditions with Reynolds number independency, in this study Re &gt; 2×10\(^5\), in a facility with little blockage is suggested. Unlike the towing tanks, the CWC did not have a uniform inflow. Using the equivalent flow velocity, obtained from kinetic energy flux over the rotor swept area, in dimensional analysis of the CWC data resulted in a better correlation with the towing tank results. It shows that although the shear flow profile practically has little effect on the mean power output per se, selecting the flow velocity by which the performance is analysed is essential in scale model tests. The impact of facility bias on the performance assessment appeared to be induced mainly from blockage and partially from the flow velocity profile. An experimentally validated Blade Element Momentum (BEM) model was modified to consider shear flow profile and Reynolds number in the performance calculations. In addition, QBlade software was employed as a tool to investigate the effect of using local Reynolds number over the blade span. No significant changes were seen in the BEM model results by incorporating the shear flow in the code. Comparing the QBlade and BEM model results showed that using local sectional Reynolds numbers in the prediction may be not worth the effort to achieve results that were slightly more accurate than the model with a single reference Reynolds number. BEM theory provides reasonable performance predictions for turbines in steady flow conditions. To establish detailed hydrodynamic characteristics of the turbine in ideal flow conditions, an experimentally validated numerical model using Computational Fluid Dynamics (CFD) was developed. Different CFD approaches were applied to the model to find the best numerical practice for the performance evaluation of HAMCTs. The CFD model was modified to account for sheared inflow and surface waves using volume of fluid (VOF) and single-phase methods. A steady moving reference frame (MRF) simulation of the whole turbine model using the k-ω SST turbulence model with the wall-function model was found to be the best approach for the performance prediction of HAMCTs under steady inflow conditions in a balance between simulation time and result accuracy. Conversely, a transient solution with the sliding mesh method provided a better fit to the experimental results for turbine under waves and sheared inflow velocity profiles. The CFD simulations showed that a sheared inflow velocity profile had a cyclic effect on the blade loadings and almost no significant effect on the mean power production of the turbine. The effect of turbine depth and waves on the mean power yield was also negligible when the tip immersion depth of the turbine was more than half the turbine radius. Since both wave and sheared flow velocity affect the quality of power output even in deeper positions, they should be considered during the turbine design stage. Regardless the angular position of the blades, the maximum values of C\(_P\) and C\(_T\) occurred at the passing wave peaks and the minimum values at the troughs. This study comprehensively evaluated the methods available to predict the performance of HAMCTs and provided a detailed discussion of the different parameters that affect both turbine and model performance. Overall, the BEM model provided accurate performance results in the steady flow condition, though it was unable to capture the effect of shear velocity on the turbine hydrodynamics. The QBlade model yielded similar results to the BEM model with the possibility of investigating Reynolds number effect; a user-friendly tool for quick performance prediction of a full-scale turbine. The CFD approach provided detailed information about the turbine hydrodynamics in both steady and unsteady conditions; however, an extensive verification and validation of the model is essential to achieve trustworthy results. The scale model tests were found to be a reliable way for performance assessment of HAMCTs. However, it is important to know how to account for blockage, inflow velocity profile and model scale effects when extrapolating the results to the full-scale turbine.