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.

Direct drive wave energy conversion (DD-WEC) is a typical low-speed long-stroke application. Harvesting wave energy presents enormous challenges, because wave energy naturally exists in the form of extremely low speed, high force, or torque motion. However, the frequency of flux linkage in field-modulated machine can be increased by the magnetic gear effect, and the output voltage and power density can also be effectively enhanced. In this article, based on the field-modulated principle, a field-modulated transverse flux linear generator (FM-TFLG) is proposed. Different combinations of permanent magnet (PM) number, slot number, and translator number are selected and their performances are compared and analyzed. Results show that the electric frequency is enhanced as the modulation ratio increases. Compared with 18/12/17 (PM number/slot number/translator number) topology, 12/12/11 FM-TFLG has advantages of lower thrust ripple and total harmonic distortion (THD) of back electromotive force (EMF), higher thrust/power density, and efficiency. The proposed 12/12/11 FM-TFLG is a good candidate for DD-WEC.

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.

Even under open-circuit, Halbach permanent magnet (PM) machines may partially suffer from an irreversible demagnetization issue. In this paper, the relationship between PM segment number per pole and electromagnetic performance reduction caused by demagnetization under both open-circuit and on-load is investigated in air-cored Halbach PM machines. The PM segment number has influence on effective PM thickness, which is related to on-load demagnetization withstand capability. The higher segment number, the larger effective PM thickness, and the better on-load demagnetization withstand capability. However, when segment number >3, the further improvement in effective PM thickness becomes less significant, and hence, the further improvement in on-load demagnetization withstand capability becomes less significant. Meanwhile, the influence of geometric demagnetization factors, including airgap length, PM thickness, and PM angular width, is investigated. It is found that Halbach PM machines with larger airgap length, and thicker PM have higher demagnetization withstand capability. Compared with the Halbach machines with 100% PM width ratio, a gap between adjacent PMs can mitigate the influence of demagnetization. An air-cored Halbach machine with PM 2-segments per pole has been fabricated and tested to verify the finite element results, such as cogging torque, back EMF, static torque, and average torque.