J.M. MacInnes
University of Sheffield,
United Kingdom
Keywords: mass transfer, flow battery, rotating channel
Summary:
We have been developing a general technology for controlled contacting of any pair of fluid phases. Most attention has been on gas-liquid pairs although the method applies equally to immiscible liquid-liquid systems. Recent interest by a consulting company alerted us to the possibility that the same ideas and technology may apply to the important emerging area of flow battery development. Current flow battery power units require membranes to maintain separation between the two electrolyte solutions; but membranes are expensive, introduce a significant barrier to current flow and are susceptible to chemical incompatibility, fouling and mechanical damage. Development of flow batteries without membranes has remained in an exploratory stage with experiments restricted to simple laboratory devices delivering power densities that are an order of magnitude lower than for membrane devices. Use of a rotating spiral channel has been shown by us to allow counter-current flow with simultaneous control of phase flow rate ratio and relative layer thicknesses for immiscible liquids. Further, mass transfer is augmented by secondary motion in each phase generated by a combination of curvature and Coriolis acceleration. The rotation can easily produces centrifugal acceleration that is hundreds of times that of terrestrial gravity and this allows surface forces to be controlled even for sub-millimetre channel size. Since species flux, and hence electric current flux, scales as the inverse square of channel size, power density rises rapidly as channel size is reduced. Work to date has developed a wide-channel approximation capable of predicting flow and mass transfer for any two fluid phases and transferring solute species. The model has been used to design a research apparatus that has now generated a range of verifying experimental data reported in four papers in the open literature. A model for finite channel width where the curvature and Coriolis effects enter has also been used for detailed prediction of flow and mass transfer. The liquid electrolyte solutions commonly used in flow batteries are miscible rather than immiscible, so at first the rotating spiral work may not seem relevant. However, since preventing mixing of the electrolyte solutions is an important objective, the miscible solutions will remain distinct to a good approximation and the theoretical and experimental understanding developed for immiscible fluids will largely carry over for the regimes of interest. Also, recent research has emphasised organic-aqueous electrolyte pairs, which are immiscible. Either way, it appears that a rotating spiral can be an effective way of contacting electrolyte solutions without the need for membranes, and very likely with considerably greater power density. Our recent work has demonstrated two orders of magnitude improvement in species mass flux in comparison with a membrane contactor. The presentation will introduce the rotating spiral contactor, review results and capabilities developed to date and consider the technical challenges associated with application to flow battery power units.