The redox flow battery is very interdisciplinary battery technology. This is because the power and the storage are decoupled and it is a flow battery the system. Which means it is a mixture of system engineering, the construction of the stack and electrochemical engineering for the materials. On the top the battery has to be connected to a grid or a sink and source.

To understand the behaviour of the redox flow system with a stack thirst a single cell was constructed and tested. Out of these results a small stack (5 cells 250cm²) was build up to learn more about the stack and system behaviour. This system was tested in a test site at the Fraunhofer. The test side was special manufactured for the stack and several measuring values were measured, like the pressure drop, temperature, state of charge, single cell voltage, stack voltage, stack current and pump consumption. An analysis of the influence of the flow rate to energy efficiency of the stack was carried out. Defined cycles were driven to analyse the interrelationship from the stack voltage to the state of charge and the current density. The energy efficiency was analysed according to the current density.

With the results and experience of the small stack a system design was made for a 1 kW 6 kWh redox flow system, which is the redox flow system for the “solarhaus” in Freiburg (WP6). The system was layout to the required power of the stack and the capacity of 6 kWh of the system.

This means the electrolyte volume was adjusted. Also the geometric design of the stack was analysed considering that it would connect to the AC Grid with a standard inverter. The pumps were selected in terms of the maximal flow rate and the ability to control the flow rate of the electrolyte. The SoC range was adjusted through the higher energy efficiency and the flow rate of the pumps.

Image 1: 1 kW – 6 kWh redox flow battery

Image 2: Computational Fluid Dynamics simulation of a single cell.