Abstract

In this paper, a new array-type wave energy conversion (WEC) with a liquid metal magnetohydrodynamic (LMMHD) generator is presented. The working principle is described in detail and a time-domain mathematical model of the WEC coupling with the LMMHD type power take-off (PTO) system is established. The system's output performance characteristics under different sea conditions are calculated and analyzed. The results reveal that through the coupling and superposition of multiple oscillating floats, the output power can be effectively increased, and the system output has good stability under different sea conditions.

Introduction

The liquid metal magnetohydrodynamic (LMMHD) wave energy conversion (here after denoted as LMMHD-WEC) uses the reciprocating force of waves to drive the liquid metal with the high conductivity through the MHD channel directly, cutting the magnetic field lines to generate electricity. It has the advantage of matching the speed-torque characteristic of the generator with the mechanical impedance of waves. So it is a very competitive direct wave generation method (Majid, Apandi et al. 2016).

Research works on the LMMHD generator and the LMMHD-WEC has been carried out in Institute of Electrical Engineering, Chinese Academy of Sciences (IEECAS) since 2007 (Lin et al., 2007; Yan et al., 2010; Liu et al., 2015). Zhao et al. (2009a) investigated the influence of the end effect on the electromotive force, induced magnetic field and flow loss of the reciprocating LMMHD generator. The study shows that the end gradient of the external magnetic field and end leakage current are main causes of the end effect in a LMMHD generator. Zhao et al. (2009b) analyzed the effect of different liquid metals on the performance of the LMMHD-WEC. The results show that the system has the largest output power and the highest efficiency when the working medium is Na-K alloy under the same sea conditions. Xiao et al. (2011) calculated the electromagnetic and flow fields of the LMMHD generator by threedimensional numerical simulations, the result shows that a maximum output power of 209.4 Wt is got with a load parameter of 0.44 and inlet velocity of 3.6m/s. Liu et al. (2011) established the hydrodynamic model of a point absorption LMMHD-WEC using the simplification of the linear damping coefficient of the PTO system, and studied the law of the radiation damping, added mass, and floating body capture width ratio with the wave cycle and float diameter. In 2011, IEECAS successfully developed a LMMHD laboratory test prototype, which used the U-47 alloy as the working medium, and obtained 1.1kW power generation (Xu et al., 2012). A 10kW prototype device of a heaving float LMMHDWEC was constructed and operated successfully in sea trial in 2015 (Liu et al., 2018a). Liu et al. (2018b) developed a performance analysis model of a heaving float LMMHD-WEC. Performance analysis was carried out and a 5kW LMMHD-WEC prototype device with the system efficiency of 27% was designed. The results reveal that the best performance can be derived by adjusting loading parameters. It can be seen these research works are all for single-point absorption type LMMHD-WEC. Due to the limitation of the volume and weight of the single oscillating float, the total power to collect wave energy is limited, and the power generation stability is poor, so it cannot be used in the large-scale power generation.

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