Undersea direct-drive wave energy converters (UDDWEC) have the advantages of high energy absorption and good resistance to wind and waves. This paper proposes a robust faster joint control for the hybrid energy storage system (HESS) in the application of islanded microgrid with UDDWEC. Wave power systems capture maximum power under different conditions, considering regular and irregular waves. The interconnection damping assessment-passivity based controller (IDA-PBC) used as an outer voltage controller regulates the DC link voltage to the desired value by generating reference values for the supercapacitor current and the battery current. The model predictive control (MPC) used as an inner controller tracks the reference state variables generated by the outer voltage controller. HESS is employed to mitigate the discrepancy in average and instantaneous power between the wave generation system and the DC load. The output of the wave power system is investigated under frequency and magnitude variations in wave elevation. It was verified that the system can maintain stability quickly when the load varies. The proposed control structure is verified through simulation and experimental results.

Wave energy has great prospect among many forms of marine renewable energy for its high density and storage. This paper proposes an underwater direct drive wave energy converter (UDDWEC), which is composed of a submerged point absorbing buoy and a linear-rotating axial flux permanent magnetic generator (LR-AFPMG). In addition, a maximum energy capture control strategy, resonance control, is derived for UDDWEC, based on small amplitude oscillation and hydrodynamic analysis. The proposed control strategy assumes the availability of sea condition such as wave height and period. This control strategy has three main characteristics. Firstly, this control strategy is derived based on hydrodynamic analysis of the submerged point absorber. Added mass, radiation damping and other hydrodynamic parameters are obtained to participate in UDDWEC dynamic model. Secondly, a LR-AFPMG is applied as power take-off device to realize energy conversion, which can improve the power density. Thirdly, small amplitude oscillation can be changed into long stroke rotary motion through the LR-AFPMG. The reliability and effectiveness of the proposed control strategy are assessed at various operation conditions for a heaving system and the validity for the UDDWEC is verified.

Over the past two decades, linear generators have been widely used in direct drive wave energy converters (DD-WECs), thus simplifying the system structure and improving conversion efficiency. While these linear generators are desirable for wave energy converter because of their high reliability, they must be very large to generate significant electrical power from such low-speed motion. These linear generators have demerits of large volume, low power density and low conversion efficiency when they are adopted in DD-WEC. Magnetically geared machine and field-modulated permanent magnet linear generator can significantly decreases the volume and cost of the required generator, also the speed of travelling magnetic field is enhanced and high power density capability is ensured by the ‘magnetic gearing effect’. A field-modulated PM transverse flux linear generator (FM-PMTFLG) is proposed and analyzed in this paper. The field modulation mechanism in FM-PMTFLG with tilted translator is introduced in detail. The electromagnetic characteristics of different combinations of stator slots and translator poles and different tilt angle of magnetic tilted bars and nonmagnetic tilted bars are comparatively investigated. The FM-PMTFLG prototype is manufactured to validate the results of the selected design parameters and the relationship between magnetic gearing effect and tilted translator. A fundamental experiment was conducted. A good agreement between 3D-FEA and experiment results proved the validity of the generator design.

Direct drive wave energy converter (DDWEC) needs fewer moving parts since the translator in the linear generator is directly coupled with the wave energy converter. Many linear generators are used to achieve high power density in DDWEC and transverse flux linear generator is a decent choice to achieve high power density, which is suitable to be used in typical low-speed high thrust application, like DDWEC. In this paper, a field modulated transverse flux linear generator (FMTFLG) is proposed. The magnetic gear effect is ensured by the interval arrangement of silicon steel sheets and nonmagnetic sheets and the machine operation principle and field modulation mechanism are discussed in detail. The results show that the air gap flux density distributions and harmonics can be modulated and the field modulated transverse flux linear generator is proved to achieve higher thrust density than its conventional counterpart.

The output power of a direct-drive wave energy converter (DDWEC) is inherently influenced by the real sea state, resulting in significant intermittency. This variability poses challenges for maintaining a stable power supply to the load, thereby complicating control strategies. This paper presents the interconnection damping assessment-passivity based controller (IDA-PBC) for island microgrid applications with DDWEC and hybrid energy storage system (HESS). Unlike other control methods, IDA-PBC does not separate the control strategy into outer and inner loops. This allows for a more efficient control strategy to ensure system stability. In conjunction with passivity-based control, a nonlinear disturbance observer (NDO) is proposed to enhance the robustness of the system against parameter mismatches and variations in load conditions. This controller maintains the DC link voltage at the desired level by generating reference values for the current of the supercapacitor and the battery. HESS is utilized to address the difference in average and instantaneous power between the wave generation system and the DC load. To validate its rapid transient response and anti-disturbance performance, both simulation and experimental results are presented.