Search

A prerequisite in undertaking large-scale developments and utilisation of tidal current energy is to comprehend the hydrodynamic characteristics of the wake field of tidal current turbines. The wake of an upstream turbine can strongly affect the power output of downstream turbines and their arrangement. The structure of turbine wakes was investigated. One experiment was conducted on a two-bladed horizontal turbine in a laboratory flume. Actuator disc momentum theory, combined with Reynolds-averaged Navier–Stokes equations, was used to parameterise the tidal turbine. The numerical model was validated using experimental data. The degree of wake mixing improved when the axial distance was eight times the turbine diameter (8D) or larger. The influence of the arrangement and spacing of turbines was investigated at two tidal farms, one with three aligned turbines and one with seven staggered turbines. The incoming flow velocity of the downstream turbine was reduced by 31% when turbines were linearly aligned. The extent of influence of the third column was larger than that of the second column, indicating an expanding wake. Different lateral spaces of staggered turbines were analysed. Different distances between rows strongly influenced both the incoming velocity of downstream turbines and the expansion of the whole wake. The incoming flow of the third column reduced by 31, 17 and 16% with increasing lateral space. The flow velocity increased as water passed over the turbine tip. For staggered turbines, a centreline spacing of up to 1.5D for adjacent rows is recommended.

The exploitation of wave energy will benefit remote islands considering environmental, social, and economic aspects. Located more than 250 kilometres off the Hainan Island, Xisha Islands is an archipelago in the South China Sea (SCS) that is yet to be grid-connected. Previous studies suggested the SCS has vast and untapped wave energy. Here, high-resolution wave hindcast generated by a nesting SWAN model is used for the assessment of wave energy resource in Xisha Islands. The result of the spatial-temporal analysis highlights the seasonal variability in wave energy, as the mean wave power density ranging from 2 to 12 kW/m between summer and winter. Spatial variation of wave energy majorly caused by depth-induced wave breaking, and blockage effect of islands and reefs. Available wave conditions for the operation of WECs are also discussed, showing that wave energy can provide stable and sustainable energy to islands in Xisha.

Two numerical wake model were established, one based on full geometry turbine and one based on actuator disc theory. The Actuator disc momentum theory assumes turbine as a constant sink of momentum in numerical domain. Results of numerical models were compared with experimental results, the experimental data is in good agreement with the simulation data. The difference between two models were analyzed in detail. The the difference between simulation results of actuator disc and test results are slightly larger in near wake region, the difference gradually decreased as the axial distance increased. The lack of the simulation of rotation of the model may lead this difference. The effect to wake recovery of blockage ratio was studied using numerical model based on actuator disc theory, 11 cases were studied, the results showed that the blockage ratio can heavily effect the recovery speed of turbine wake. Along the centerline, the degree of wake recovery decreased as the blockage ratio increased before X/D=10.8, and the degree of wake recovery increased as the blockage ratio increased after X/D=10.8.

The effect of stanchion and reverse flow on turbine power performance is studied based on Computational Fluid Dynamics (CFD). The turbine in this paper is developed by National Ocean Technology Center (NOTC), the experiment study has been carried out in year 2014. The simulation model is validated based on experimental results. The CFD model can simulate the marine current conditions up to an acceptable accuracy and is cost-effective compared with experimental work. The mesh dependent including prism layer cell mesh has been studied. There are two methods in simulating the turbine power performance which aer steady-state and transient model. We compared the results of these two methods, the difference of Cp was within 2% based on the same mesh size. The results of two methods showed little difference. We chose the steady-state method in the rest of the calculation to maintain the better economically computational model. The effect of different size of the rotating zone is calculated in the model. The results showed that the biggest gap is 1.06%, the size of rotation zone has little impact to the turbine performance. We simulated cylindrical stanchions with different distance against to turbine. Simulation model is established based on the validated model. The power coefficient curves were given based on simulation results. The results showed that the largest drop reached 25.3% between different distances. The effect of bi-directional flow to turbine power performance is studied through simulation model. The power decrease could reach 21.7% when the turbine is backward to the tidal flow is calculated and graphed. The numerical model established in this paper can simulate the performance of turbine well, The result can provide technical support for engineering application of tidal turbine.