Fast response of kW-class Vanadium Redox Flow Batteries

An experimental and numerical time-domain analysis of the early electric response of two kw-class Vanadium Redox Flow Batteries (VRFBs) under different state of charge, electrolyte flow and load is presented. The numerical analysis resorted to an equivalent circuit whose parameters were identified from electrochemical impedance spectroscopy measurements. Two discharge modes were investigated: natural discharge on resistors, to detect the spontaneous battery evolution, and discharge driven by an electronic load, forcing a galvanostatic evolution. Two different timescales were investigated in each discharge modes, namely 20ms and 120s. The millisecond mode revealed stack current and voltage swings in the is very short timescale (7 ms). This behavior suggests an initial turbulent electrochemical phase involving successive activations of vanadium species associated with coordination complexes at the positive electrode. In the latter mode, experimental and numerical results revealed that both stacks reached steady conditions much faster, in few hundreds of milliseconds. These results demonstrate that vanadium redox flow batteries are eligible for fast services in 5060 Hz grids, provided the discharge current is driven in a current-source mode by proper interface power electronics. To the best of our knowledge, this is the first time that the fast time-domain response of large VRFBs is reported.