Plurion | Technology | Flow Vs. Conventional Batteries

Plurion Technology Platform | Flow Batteries Vs. Conventional Batteries
Plurion Advantages | Product Engineering

Conventional batteries store electricity in the form of chemical energy. Electrodes form the pathway through which electricity is stored into or released from an "electrochemical couple," which is contained in an "electrolyte" that is often but not necessarily an acid. The "couple" is the specific combination of chemical elements that define a battery's chemistry. Examples of electrochemical couples include lead/lead oxide (as used in typical automotive batteries); nickel/cadmium; and sodium/sulfur. In general:

The type and amount of electrolyte determines the energy storage capacity of a battery, which is defined in SI units as watt hours per liter or kilogram (Wh/l or Wh/kg);
The type of couple determines the voltage per pair of electrodes or "cell" (i.e., 2.0 v/cell for lead/lead oxide, ~2.2v per cell for cerium/zinc, ~1.2v per cell for vanadium/vanadium);
The conductivity of the electrolyte and the distance between the electrodes and the cell voltage combine to effectively determine the power density (W/l or W/kg).

Conventional batteries (lead-acid, lithium-ion, sodium-sulfur, etc.) store their electrolyte between the electrodes. So increasing the amount of electrolyte requires greater spacing between electrodes. For a given couple and electrolyte, increased cell-gap results in increased internal resistance. This feature places a practical limit on the amount of electrolyte that can be stored. Increasing resistance between the electrodes limits the practical power delivery by reducing the output voltage, and converting stored energy into waste heat.

Flow batteries solve the energy/power constraint of conventional batteries by separating energy storage capacity from power delivery. This is achieved by storing the majority of the electrolyte outside of the electrodes. A flow battery has an electrolyzer that contains the electrodes and is optimized for power delivery. The energy storage capacity is therefore determined primarily by the volume of electrolyte stored in the external tanks.

This separation of electrolyte from electrolyzer allows flow batteries to store much larger amounts of energy than is possible with conventional batteries. One important trade-off is that flow batteries are more complex than conventional batteries and require systems to pump electrolyte into and out of the electrolyzers. A flow battery's pumps must be powered by the battery during discharge, which represents a parasitic loss. However, these parasitic losses tend to be fixed, and also tend to become less important as battery size increases.

One practical result of this characteristic is that flow batteries are well suited to very high capacity storage. They are not typically suited to systems of less than 20kW of delivered power or 40kWh of delivered energy.