A flow battery is a rechargeable battery that stores energy in liquid electrolytes kept in external tanks. The liquids are pumped through an electrochemical cell stack, where chemical energy is converted to electrical energy or stored again during charging. This design is useful for renewable energy because solar and wind power often need storage for hours or days.
Instead of making the battery cell much larger, engineers can increase storage by using bigger tanks of liquid.
Key Facts
- Energy capacity mainly depends on electrolyte tank volume and concentration.
- Power output mainly depends on the size and number of cells in the electrochemical stack.
- During discharge, chemical energy is converted to electrical energy through redox reactions.
- Cell voltage is related to chemical potential difference: Ecell = Ecathode - Eanode.
- Electrical energy delivered is approximately E = VIt, where V is voltage, I is current, and t is time.
- Power is the rate of energy transfer: P = VI.
Vocabulary
- Flow battery
- A rechargeable battery that stores energy in liquid electrolytes pumped through an electrochemical cell stack.
- Electrolyte
- A liquid or solution containing ions that can carry charge and participate in battery reactions.
- Redox reaction
- A chemical reaction in which electrons are transferred through oxidation and reduction.
- Cell stack
- A group of electrochemical cells connected together to produce the desired voltage and power.
- Membrane
- A thin separator that allows selected ions to pass while keeping the two electrolytes mostly apart.
Common Mistakes to Avoid
- Thinking the tanks produce electricity by themselves, which is wrong because the electrochemical reactions occur in the cell stack.
- Confusing energy and power, which is wrong because tank size mostly controls stored energy while stack size mostly controls power output.
- Assuming the two electrolytes should freely mix, which is wrong because uncontrolled mixing can waste stored chemical energy and reduce efficiency.
- Ignoring pump energy losses, which is wrong because moving liquid through pipes and cells uses some of the stored energy.
Practice Questions
- 1 A flow battery delivers 50 kW for 6 hours. How much electrical energy does it deliver in kWh?
- 2 A cell stack operates at 400 V and supplies 75 A. What is its electrical power output in kW?
- 3 Explain why increasing the electrolyte tank size can increase storage duration without necessarily increasing the maximum power output.