- An ionic membrane divides each cell. The electrolytes are acid vanadium sulphate.
- The oxidation states of the vanadium are V2+ to V3+ on the negative side, and V5+ to V4+ on the positive side.
- The open circuit potential across each cell is 1.35V in the 50% charged state.
- The power (kW) depends on the membrane area. The capacity (Ah) depends on the volume of electrolyte.
- A conventional horizontal stack of vertical cells
- The cells are connected electrically in series
- The electrolytes are fed to the cells in parallel
Solutions to Parellel Flow
- Electrical shunt currents in the channels feeding the electrolytes to the cells bypass the intended electrical path through the cells
- These shunt currents cause corrosion, loss of energy efficiency, and non-uniform distribution of the current to the cells
- To ensure uniform electrolyte distribution to the cells and prevent blockages, flow rates are higher than needed for the electrochemical reactions
Bipolar Electrochemical Cell Stacks
- Left: Undivided cells
- Right: Cells divided by membranes.
- Top: Parallel flow of electrolyte.
- Bottom: Series flow of electrolyte.
Series Flow with Bipolar Electrodes
- In the charging mode shown the reactions are V3+ + e- = V2+ on the negative side of the membrane and V4+ = V5+ + e- on the positive side. These reactions are reversed when the battery delivers electrical power.
- The open circuit potential across each cell varies from 1.1 volts in the uncharged state to 1.6 volts in the fully charged state.
Benefits of the Novel Flow Architecture
- The cells are horizontal in a vertical pile with electrolytes fed upwards through the cells in series
- This almost eliminates bypass currents
- It is safe because a blockage would be detected at once and the electrolysis of water can be quickly prevented
- It is also efficient because electrolyte flow rates (and pumping power) needed are lower than with parallel feed
Lead-Acid Battery Comparison
- Critical problems at present encountered in lead-acid batteries
- Lead-acid batteries have a lifetime of only 3-4 years
- Lead-acid batteries cannot tolerate fluctuating charging and discharging rates
- Lead-acid batteries cannot operate at high temperatures 50 degress C
- Storage and inverting systems have overall efficiencies no more than 55%
- Solutions offered by vanadium flow batteries
- They are expected to last at least 20 years
- They are not affected by rapid charging and discharging variations
- They operate normally at 80 degrees C
- The life-cycle costs of conventional vanadium batteries have been estimated at 60% of lead-acid systems. The new systems will make life-cycle costs even less
Preparation of the Electrolyte
- Commercially available V2O5 is only slightly soluble in sulfuric acid
- Existing chemical methods of electrolyte preparation are costly
- This new method reduces the vanadium at the negative electrodes, but oxidation at the positive electrode is prevented by the small area
- Measured amounts of V2O5 powder and H2SO4 are added in a continuous process
The Novel Inductionless Power Converters
- Left: An "auto-transformer" for stepping up the voltage of a continuous direct current.
- Right: The principle of the inductionless inverter for delivering alternating current from the battery.
MPPT and Inductionless Power Converter
- Electrical taps connected to the electrodes control the voltage of the input and output currents with the help of electronic power switching circuits
- The alternating current drawn directly from the battery is practically harmonic-free
Cells with Monopolar Electrodes
- The cells are divided into elements, each having a separate membrane
- The separators between cells are electrical insulators impervious to the electrolytes
- Switches enable the number of operating electrodes and membranes to be varied
- The use of monopolar electrodes enables one to control the voltage as the current load varies by varying the membrane area
- Individual cells in a stack may have different numbers of built-in elements; this is important when a sinusoidal output is created using the switching method
- There is no hydraulic pressure stress across the electrodes
- Different materials can be used for the positive and negative electrodes