How VRFB's work
How VRFB's work
The amazing feature of vanadium is in it's ability to exist in four different oxidation states (V2+, V3+, V4+, and V5+) This allows vanadium to be both the anode and cathode of the battery. (Positive and Negative)
All four states of vanadium can hold a different electrical charge. The electrolyte in the negative half-cell contains V3+ and V2+ ions, whilst the electrolyte in the positive half-cell contains VO2+ and VO2+ ions.
The VRFB battery utilises the flow of this vanadium electrolyte separated by an ion exchange membrane. A reversible electrochemical reaction allows electrical energy to be stored and subsequently returned. The system works much like the everyday rechargeable battery but on a much larger scale.
- 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.
Discharging VRFB storage
- A redox reaction occurs changing the composition of the electrolyte and creating a surplus of electrons at the negative terminal, relative to the positive terminal.
- When the battery is in use the electrons flow from the negative terminal to the positive terminal, generating an electrical current.
Charging VRFB Storage
- The reaction is reversed by applying an electrical current to the terminals, returning the battery to its original state.
Modular Energy Storage
In conventional batteries all of the components are in a sealed casing, this limits the energy storage capacity.
VRFBs are not constrained by these limits as the electrolyte is stored outside of the cell, separating power and energy. The setup of the electrolyte and the membrane stack can be compared to that of an engine and fuel tanks. The engine delivers power rated in kilowatts (kW) (the membrane stack) and while the fuel (the vanadium electrolyte) delivers energy rated in kilowatt hours (kWh). This makes VRFB's energy storage solutions easily scalable as more electrolyte or stacks can be added as required.
The VRFB electrical energy storage system is a vanadium-based redox regenerative fuel cell that converts chemical energy into electrical energy. VRFBs offer durability, rapid response time and extremely large capacity that is suited to storage for applications in load levelling, renewable energy systems (e.g. wind and solar), and uninterruptible power supplies.
A VRFB is a type of flow battery which generates power by pumping liquid from external tanks to a central stack where the liquids are mixed. Energy is stored chemically in different valence forms of vanadium in a dilute sulphuric or hydrochloric acid electrolyte.
The electrolyte is pumped from storage tanks into flow cells that use an exchange membrane to separate an oxidised electrolyte from a reduced-state electrolyte. The flow of electrons across the membrane creates a current which flows through the electrodes to the external circuit. The reaction is reversible, allowing the battery to be charged, discharged and recharged.
A VRFB consists of two electrolyte tanks containing active vanadium in different states and operates at ambient temperature and pressure. The cathode electrolyte (catholyte) is a V (IV)/V (V) couple; the anode electrolyte (anolyte) is a V (II)/V (III) couple.
These energy-bearing liquids are circulated through the cell stack by pumps. The stack typically consists of many cells comprising two half-cells separated by a membrane. In the half-cells, the electrochemical reactions take place on inert carbon electrodes from which current may be used to charge or discharge the battery.
20-25 year battery life - Cross-contamination of ions through the membrane separator is not an issue in VRFB because both electrolytes are acidic vanadium sulphate solutions and half-cell solutions can be remixed to return the system to its original condition.
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
The open circuit cell voltage for each vanadium species varies depending on the cell design and configuration. The relatively fast kinetics of the vanadium redox complex allow high coulombic and voltage efficiencies.
Advantages of VRFB battery technology are in its storage and discharge performance, speed of cycle, relative immunity from increase of electrode resistance, scalability through cell stack size and long life.
VRFBs have estimated lifespans in excess of 10,000 cycles. VRFBs can be left completely discharged for long periods with no adverse effects and can be recharged simply by replacing the electrolyte if no power source is available to charge it. Also, if electrolytes are accidently mixed, the battery suffers no permanent damage.
Further, evidence has suggested that VRFBs have amongst the lowest ecological impact of all energy storage devices. The fact that the two electrolytes are the same when completely discharged, gives VRFB an advantage over other types of flow batteries. Shipment and storage is cheaper, more practical and electrolyte management is simplified.
|Table 26: Battery density comparison|
|Batteries||Energy density (Wh/L)||Power Density (W/L)|
|Flow cell systems|
|Nickel metal hydride||100-150||330|
|Source: Bhaskar Garg, University of Stanford|
The main disadvantage of VRFB batteries is simply the cost of electrolyte which is factor of the absence of domestic supply and associated cost reducing process technologies.
VRFBs require a high level of vanadium pentoxide purity, a factor which is of minimal concern to vanadium producers supplying the steel industry. Pilot plant testing of the Lac Dore Vanadium Project mineralization resulted in 99.9% vanadium electrolyte purity. The pilot plant facility was located at SGS Lakefield.
It is understood that an estimated two-thirds of VRFB costs are associated with the electrolyte, and vanadium prices therefore have a significant bearing on VRFB economics. Most difficulties with the commercialisation of VRFB technology have been associated with the electrolyte cost.
The conventional comparative measure for battery costs is US$ per kWh per cycle. Zinc-air batteries are the most expensive whereas pumped hydro batteries are the least, and cost around $0.01 per kWh per cycle. Flow batteries vary in cost, although it is estimated that VRFBs in general have costs of around $0.08 per kWh per cycle with the potential for lower costs because of possible refurbishment A lower electrolyte cost would improve the commercial potential of VRFBs. Learn more about electrolyte here...
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