Return of investment analyses in residential and utility-scale multi-service storage systems

In all-vanadium flow batteries, electrolyte oxidation from atmospheric oxygen and/or hydrogen evolution due to operations at extreme states of charge may occur. After events like these, an electrolyte imbalance affecting the battery capacity appears that cannot be recovered by a simple mixing operation and a more complex rebalancing process must be undertaken. Among the different rebalancing processes which have been proposed in the literature, this work consider the electrochemical method that uses a specific electrolysis reactor to reduce V(V) in the positive electrolyte to the level of V(II) in the negative electrolyte, so as to equate the states of charge in the two tanks. In order to adjust the balancing mechanism to the system, the rated power of the reactor performing this process should be optimized to the VFB rated energy, imbalance level (namely, the process periodicity) and the rebalancing process duration. In turn, the rated power defines the size and thus the cost of the reactor, namely area and number of cells, given their current density and internal area specific resistance. This work presents a model that assembles these parameters and their interplay, allowing to investigate how the process and the reactor can be optimized. Such model also provides an economical evaluation of the investment and operational expenditures of the rebalancing process, thus providing design criteria for the minimization of these cost figures. For the sake of example, numerical calculations are performed referring to a 500-kWh VFB, that is a quite common rating for VFBs, and they indicate that a cost reduction of almost 70% is achieved if 10-hour processes are run four times per year instead of once a year. At this rebalancing frequency, the 500 kWh VFB needs a 9-kW reactor costing around €15.000.