Harnessing ocean wave energy is an old challenge that has gained momentum in recent years. In this paper, we present the flow and electrical characterization of a prototype of an alternate liquid metal magnetohydrodynamic (MHD) generator at a laboratory scale which has the potential to make use of the energy of marine waves for its conversion into electrical energy. The eutectic alloy Galinstan, used as a working fluid, was driven in oscillatory motion in a duct of a rectangular cross-section exposed to a transverse magnetic field generated by permanent neodymium magnets. The electric current induced by the motion of the liquid metal in the magnetic field was collected through copper electrodes and delivered to the load. The oscillatory axial velocity component along the duct was measured using ultrasonic Doppler velocimetry for different oscillation frequencies. In turn, the output currents and voltages were measured for different operation conditions and the electric power and efficiency were estimated from experimental measurements. The coupling of this generator to a wave energy converter (WEC) is discussed.
The zero-mean oscillatory flow of a liquid metal in an alternate magnetohydrodynamic electric generator is explored analytically. The flow, confined in a two-dimensional insulating wall duct under a transverse magnetic field, is driven by an externally imposed oscillatory pressure gradient. The flow behaviour is analyzed in two different regions. First, asymptotic solutions for low and high oscillating frequencies in the uniform magnetic field region far from the magnet edges are used to explore the phase lag produced by the Lorentz force between the velocity and the axial pressure gradient. In addition, the entrance flow region where the oscillatory fluid motion interacts with the non-uniform magnetic field is studied. A perturbation analysis of the boundary layer flow in this region reveals that non-linear effects lead to the appearance of steady streaming vortices superimposed on the harmonic flow. The influence of these vortices on the performance of the generator is analyzed.
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