Operational Experience of 5 kW/5 kWh All-Vanadium Flow Batteries in Photovoltaic Grid Applications

The purpose of this work was to analyse and characterize the behavior of a 5 kW/5 kWh vanadium battery integrated in an experimental facility with all the auxiliary equipment and determine whether it would be possible to ascertain the most appropriate application for storage of electricity in photovoltaic (PV) grid applications. The battery has been in operation for 9–10 months. During this time the battery has achieve a full cycle efficiency of approximately 65%. A slight reduction in efficiency is the result of the constant auxiliary power consumption from pumps amounting to 8–9% of rated power, meanwhile the stack is quite efficient showing a cycle efficiency of 73%.The operation of the pumps has been adjusted to fix the current density applied together with the state of charge in order to reduce unnecessary consumption related to the energy required for pumping. According to the results obtained, the intended improvement in global efficiency for the system has not been achieved by this proposed strategy. However, the flow factor evolution selected at this stage needs further optimization in order to establish a trade-off between the concentration overpotential and a detrimental loss in energy due to pumping. Therefore, one should be able to improve system performance through a better configuration of flow factors in order to reach total system efficiencies of 70–75% required for achieving a commercially viable product.