A vanadium redox flow battery is a rechargeable battery designed to store large amounts of renewable electricity for many hours. Instead of storing most of its energy inside solid electrodes, it stores energy in liquid electrolytes held in external tanks. Pumps send the liquids through an electrochemical cell stack where electrons are released or absorbed.
This makes flow batteries useful for smoothing out solar and wind power when production changes during the day.
Key Facts
- Energy capacity mainly depends on electrolyte tank volume and vanadium concentration.
- Power output mainly depends on the size and number of cells in the electrochemical stack.
- Electrical power is P = VI, where P is power, V is voltage, and I is current.
- Stored electrical energy is E = Pt for constant power over time.
- A vanadium flow battery uses different oxidation states of vanadium, such as V2+, V3+, VO2+, and VO2+.
- Round-trip efficiency is efficiency = energy out / energy in × 100%.
Vocabulary
- Redox reaction
- A chemical reaction in which electrons are transferred between substances through oxidation and reduction.
- Electrolyte
- A liquid or solution containing ions that can carry electric charge.
- Cell stack
- A group of electrochemical cells connected together to produce useful voltage and power.
- Membrane
- A thin separator that allows certain ions to pass while keeping the two electrolytes mostly separate.
- Round-trip efficiency
- The fraction of stored energy that can be recovered after charging and discharging a battery.
Common Mistakes to Avoid
- Thinking the tanks produce electricity by themselves is wrong because the tanks mainly store chemical energy in the electrolytes, while the cell stack converts that energy to electrical energy.
- Confusing power with energy is wrong because power is the rate of energy transfer, while energy is the total amount delivered over time.
- Assuming a larger tank automatically gives higher power is wrong because larger tanks increase storage duration, but the stack size controls how fast energy can be delivered.
- Treating vanadium flow batteries like ordinary lithium-ion batteries is wrong because flow batteries use pumped liquid electrolytes and can scale energy capacity separately from power.
Practice Questions
- 1 A vanadium flow battery delivers 250 kW for 8 hours. How much electrical energy does it deliver in kWh?
- 2 A flow battery stores 6,000 kWh during charging and later returns 4,800 kWh to the grid. What is its round-trip efficiency?
- 3 A wind farm often produces extra electricity at night and less during evening demand. Explain why a vanadium flow battery can help match supply to demand better than using the wind farm alone.