Practical and commercial issues in the design and manufacture of vanadium flow batteries

Abstract The vanadium flow battery has emerged as one of the most favourable types of flow batteries for a number of reasons, including the lack of cross-contamination that troubled many earlier systems such as the Fe/Cr flow battery. Because the vanadium flow battery employs the same metal ion in both electrolytes, albeit in different oxidation states, there is no cumulative loss in performance, just an effective reversible self-discharge current. The self discharge that occurs in the vanadium flow batteries is limited to the electrolyte volume in the cells. However it can become substantial under low load conditions. The pumps also use power from the battery and may be considered as another source of self discharge. Taking these and maintenance considerations into account the layout of a 10 kW, 100 kWh, 48 V vanadium flow battery was designed as a “Multi-Stage-Operation” system to provide maximum performance at all levels of load, ease of use and optimum maintenance conditions. Experimental A complete energy storage system with 10 kW in power and 100 kWh in energy was designed. It consists of a vanadium flow battery with smart controller and configurable power electronics housed in a weatherproof housing. The battery can be charged and discharged at up to 10 kW and provides up to 100 kWh of energy. The smart controller ensures that the battery operates at maximum efficiency at all times and allows remote observation of various battery parameters, including a reliable state of charge (SOC) measurement. The option of different arrangements of power electronics gives almost complete freedom in specification of electrical output (dc, single or three-phase ac). The battery can also be connected to photovoltaic, wind turbine, diesel/petrol/gas/biogas generators, fuel cells and water turbines to form discrete autonomous power supplies or to be part of a micro-, mini- or smart-grid. The FB10/100 battery for “Multi-Stage-Operation” is comprised of 5 strings of 36–40 cells each in 3 separate fluid circuits. The first fluid circuit, containing a single string, is always actively pumped with electrolyte and electrically connected to the charger and load. The second and third fluid circuits contain 2 strings each and are only actively pumped and electrically connected when the voltage reaches preset limits. When the circuits are in “standby”, i.e. not actively pumped and electrically connected, the self discharge is limited to the small volume of electrolyte in the cells. There is also a significant saving of pumping energy, because 3 pairs of small pumps are used in place of 1 pair of more powerful pumps. Results In “Multi-Stage-Operation” mode, the overall battery performance is improved significantly. This is very important in off-grid installations, where loads are typically small compared to the power levels necessary for charging; i.e. a solar powered telemetric station may use 500 W continuous power but requires fast charging due to the narrow time window when solar energy is available. In example, at a 1 kW load the battery provides 25% more energy when operated in “Multi-Stage-Operation” mode compared to all stacks in operation. Since 2008, several power station have been equipped with FB10/100 storage units and put into operation. Within the presentation a report on the latest results including technical performance and cost issues will be given.