Tidal and Ocean Current Energy

Renewable energy resources (for example, wind, solar or ocean currents, among others) possess an enormous potential, owing to the fact that they are a pollution-free and inexhaustible resource. However, although the technological developments for harnessing energy from the wind and the sun have been studied long-term, the technology for harnessing energy from tidal and ocean currents is still in its infancy.

In this Special Issue, we present contributions that focus on research in support of the development of tidal and ocean current energy in order to outline the current state of the art and highlight the emerging trends and challenges in this field, ranging from mathematical modeling to methodological aspects. We expect that these contributions will help in future designs of technologies for harnessing energy from tidal and ocean currents. The five papers in this Special Issue address the knowledge gap towards the development of tidal and ocean current energy technologies.

Yan et al. proposed in [1] the design of swept-back symmetrical airfoil blades based on three kinds of swept design methods, and discussed the influence of different designs on the hydrodynamic performance of the adaptive variable pitch blade. The software of SolidWorks was used to build the 3D model of the hydraulic turbine. Based on the pre-processing software ICEM CFD, the solver Fluent, and the post-processing software CFD-Post, axial thrust coefficient, energy acquisition characteristics, and starting torque of the turbine models were analyzed and discussed. In addition, physical model experiments of the selected model were carried out to verify the performance of the numerical simulation.

Cai et al. developed in [2] a novel output power internal control strategy based on a pseudo-tip-speed ratio and an adaptive genetic algorithm (PTSR-AGA) to improve the anti-interference ability and reliability. The proposed control scheme consists of two parts. The first part proposes the PTSR method for the maximum power point tracking (MPPT) to predict the tidal stream turbine’s operating point, which contributes reducing the logical errors assigned to swell disturbances. The second part designs an AGA for the optimization of the pitch controller to conduct its angle delay. A reduced pitch control strategy was applied to the preprocessing of the pitch controller to reduce the mechanical wear over the rated power. The comparative simulation results validated that the tidal stream turbine system could obtain a higher power efficiency of energy capture and a smoother power output with the proposed control strategies at full range of tidal current speeds.

Schmitt et al. presented in [3] several field tests of a tidal turbine, performed using a self-propelled barge in real tidal flow and still water conditions, that were compared to a towing tank test. Factors influencing the performance characteristics, such as the choice of velocity sensor, vessel handling, and data processing techniques were investigated in this paper. Direct comparison with test results of the exact same turbine obtained in an experimental test facility further confirmed that field testing with robust data analysis capabilities was a viable, time- and cost-efficient alternative to characterize tidal turbines.

Zhou et al. numerically studied in [4] the key structural parameters of the spring–mass system that govern the dynamics of the double-elastic-constrained flapping taking the rigid NACA0012 airfoil as the object. A two-dimensional numerical model, based on the CFD software FINE/Marine, was established to investigate the influence of the spring stiffness coefficient, frequency ratio, and damping coefficient on the motion and performance of the flapping hydrofoil. This study demonstrated that when the structural parameters were adequately adjusted, the power factor exceeding 1.0 had been achieved, and the corresponding efficiency was up to 37.8%. Moreover, this system could start and work within a wide range of damping coefficients. However, the hydraulic efficiency and power coefficient are sensitive to the change in damping coefficient, so it was very necessary to design an appropriate power output. Lastly, the most obvious parameter affecting the energy acquisition performance was the spring stiffness coefficients. Frequency ratios in the two directions had little influence on the peak value of the power coefficient, but they will cause the change of damping coefficients of the peak point. The key structural parameters studied in this paper provided a useful guideline for an optimized design of this interesting system through searching for the best performance.

Liu et al. investigated in [5] the influence of swept blades on the performance and hydrodynamics of the bidirectional horizontal-axis tidal turbines (BHATTs). A three-dimensional (3D) numerical model based on OpenFOAM was adopted to simulate a full-scale BHATT. The numerical framework was validated using two well-known experiments, and the mesh convergence was taken into consideration. The results indicated that the forward- and backward-swept blades had a limited impact on the performance and hydrodynamics of the BHATT.