Redox Flow Batteries - The Alternative Battery

Author: Petra Dotzauer, ZAE Bayern (As of December 2016) International research and development work on redox flow systems has produced pilot plants with outputs in the kW and MW range. According to this research, redox flow batteries are very well suited as stationary storage systems for intermediate storage of fluctuating renewable power sources [1].  Main difference of a redox flow battery compared to other battery systems is that in redox flow batteries the storage medium is stored in external tanks (Fig. 1). In the best-known system, the vanadium flow battery, the storage medium is vanadium ions dissolved in different oxidation states in an aqueous sulfuric acid (electrolyte). During battery operation, the electrolyte is pumped through the cells of a stack to charge or discharge the medium. The cells are each composed of two porous carbon electrodes with an ion exchange membrane, e.g. of Nafion, between them. The following reactions take place at the surfaces of the electrodes through which flow occurs during charging [2]:

On discharge, the reactions proceed in the reverse direction. At 25°C and state of charge 50%, a voltage of 1.4 V is obtained. The electrons delivered to the negative electrode can then be used in an external circuit by a consumer. To grant charge balance in the cell, protons (H+) simultaneously diffuse across the exchange membrane [2]. H+ transport occurs by means of H3O+ ions, thus "piggybacking", as it were, on H2O molecules:

Since 2012, a 1 MW/5 MWh vanadium flow battery in combination with a 100 kW PV system has been installed at the factory site redox flow battery manufacturer Sumitomo Electric in Yokohama, Japan, which is successfully used for factory energy management [4]. In a currently ongoing project at Fraunhofer ICT, a 2 MW/20 MWh pilot plant is being set up as part of a publicly funded project by the state of Baden-Württemberg and the German Federal Ministry of Education and Research, which will be used to investigate the coupling with a 2 MW wind turbine. Such an interconnection could be an attractive solution for island systems of, for example, industrial sites or energy villages [5]. This year, Rongke Power further announced the construction of the world's largest 200 MW, 800 MWh battery in northern China. The battery is expected to be used for grid services from 2020 [6]. 

As part of the ZAE-ST project (funded by the Bavarian Ministry of Economic Affairs), redox flow battery systems are being technically and  economically analyzed at ZAE Bayern in their application as grid storage. The aim is to develop operating conditions and an optimized cell design for efficient operation. For this purpose, the loss mechanisms in the battery are first identified and analyzed experimentally. In a further step, the experimental data of the battery will be linked to a cost model to enable a comprehensive techno-economic analysis of the redox flow battery for different use cases.

For the technical investigations, a redox flow battery test system was developed at ZAE Bayern, which can be used to measure and characterize industrial converter units as well as own prototypes and test cells in a power range of up to 15 kW (Fig. 2). Electrochemical impedance spectroscopy (Fig. 3) on the cells and voltage-current characteristics allow voltage losses to be determined. Furthermore, thermal and hydraulic losses are quantified via sensor technology in the test system.

Current research steps include a detailed breakdown of battery costs at the component level and taking into account manufacturing processes, as well as technical testing of new materials for membranes and electrodes on a laboratory scale to enable further increases in cell performance.