Metal pad roll instability in liquid metal batteries

N. Weber¹ - P. Beckstein¹ - V. Galindo¹ - W. Herreman² - C. Nore² - F. Stefani¹ - T. Weier¹

¹ Helmholtz-Zentrum Dresden--Rossendorf, Bautzner Landstr. 400, 01328 Dresden, Germany
² Laboratoire d'Informatique pour la Mécanique et les Sciences de l'Ingénieur, CNRS UPR 3251, bat. 508, 91405 Orsay CEDEX and Université Paris-Sud 11, France

Abstract
The increasing deployment of renewable energies requires three fundamental changes to the electric grid: more transmission lines, a flexibilization of the demand and grid scale energy storage. Liquid metal batteries (LMBs) are considered these days as a promising means of stationary energy storage. Built as a stable density stratification of two liquid metals separated by a liquid salt, LMBs have three main advantages: a low price, a long life-time and extremely high current densities. In order to be cheap, LMBs have to be built large. However, battery currents of the order of kilo-amperes may lead to magnetohydrodynamic (MHD) instabilities, which -- in the worst case -- may short-circuit the thin electrolyte layer. The metal pad roll instability, as known from aluminium reduction cells, is considered as one of the most dangerous phenomena for LMBs. We develop a numerical model, combining fluid- and electrodynamics with the volume-of-fluid method, to simulate this instability in cylindrical LMBs. We explain the instability mechanism similar to that in aluminium reduction cells and give some first results, including growth rates and oscillation periods of the instability Figs 8, Refs 42.