Vanadium Redox Flow Battery (VRFB) Technology

The Vanadium Redox Flow Battery (VRFB) is a new concept – a means of storing electrical energy in liquid form thereby avoiding many of the short life, high maintenance issues associated with conventional batteries. Sometimes referred to as a regenerative fuel cell, the VRFB consists of a bipolar stack fed by a positive and a negative electrolyte which are separated by an ion selective membrane .


Fuel Cell Diagram


During discharge the positive ions migrate through the membrane and power is delivered to the load from the end electrodes; during recharging the polarity is reversed and the electrolyte gradually returns to the recharged condition. The unique multivalent Vanadium chemistry has a high round trip efficiency with repeatable deep cycle capability over long periods. The greatest engineering challenge in developing an end product for utility markets has proved to be in the containment of the electrolyte both hydraulically and electrically within the stack assembly. REDT have now proven a 5kW rated stack and 6 hour storage system which is in pilot production for BTS telecom markets.

Simple electrochemistry: The flow battery concept was first developed in the 1960s by NASA for the US lunar space programme, but later the all Vanadium system designed at the University of New South Wales in the 1980’s for solar energy storage has proven to be the best electrochemical couple for flow battery purposes. The simple fact that similar Vanadium compounds exist on both sides of the cell allows for any cross mixing through the membrane and ensures reliable very long life operation – most other flow battery systems (ZnBr, FeCr, BrPolysulphide) suffer from irreversible cross mixing which results in capacity run-down over time and requires complex chemical recycling or even complete reprocessing of electrolyte for recovery. The Vanadium based electrolyte, manufactured by electrochemical dissolution of Vanadium Pentoxide (V2O5) in mild concentrations of sulphuric acid, has a virtually unlimited life and a high residual value for re-use.

In summary; the VRFB has many unique advantages over conventional batteries:

· Power and energy are independent variables - allows great flexibility in matching power and duration for specific applications, this reduces capital cost and allows for maximum return on investment for the customer.

· Fundamentally safe system using non-flammable electrolytes operating at ambient temperature and pressure.

· Longevity results from the fundamental use of stable and balanced liquid reagents in the high efficiency active ion exchange energy storage process. The electrolyte has virtually unlimited life and retains a high residual value even after 20 years.

· Bi-polar electrodes do not participate in reactions and do not degrade significantly over time. The ion exchange membrane does degrade slowly but allows for a stack design life of 8 to10 years or 10,000 deep cycles.

· Unique stack design for high efficiency. REDT has developed a patented design stack which minimises internal ‘shunt’ current losses and therefore gives a better roundtrip efficiency over other similar stack designs. The system operates at relatively low pressure compared with other designs.

· Cross-membrane contamination eliminated. A fundamental and unique benefit of the all Vanadium system is that it tolerates the unavoidable migration of molecular compounds across the membrane. There are no irreversible side reactions and this eliminates the imbalance problems suffered by other flow systems, and can be quickly rebalanced by opening a mixing valve between the two electrolyte tanks, allowing complete mixing. The valve is then closed and the system recharged in the balanced condition. In normal operation this process may be carried out automatically twice per year with minimum downtime.

· Very low maintenance. The foregoing features result in a very low maintenance device which is ideal for utility and remote operation. This feeds through to the lowest total cost of operation (TCO) of any electrochemical storage system.

·  Excellent thermal management Flow-through electrodes have inbuilt cooling/heating capability with large heat sink effect of electrolyte store. Electrolyte operates between 5°C and 40°C. Cells are at uniform temperature and state of charge - fed from common manifold – eliminates cell imbalance, a major cause of failure in conventional batteries.

· Charge retention is total - with the pumps switched off - except for a small amount of electrolyte remaining in the stack.

· High Charge/Discharge ratio at 1:1 unmatched in most conventional batteries which require lower charge rates to avoid localised heating and polarisation.

· Fast response to changing loads (typically 1ms).  Large overload capacity (400% for 10 seconds, 200% for 5 minutes); response times  are usually limited by electrical equipment.

· Low whole life cost: When combined with low maintenance costs, the VRFB energy storage systems has the lowest projected cost over life of any comparable electrochemical energy storage system.

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