Flow batteries get their name because they supply electricity by pumping two electrolyte solutions through an electrochemical cell. When exposed to each other, these two solutions exchange ions through a specially designed membrane, producing voltage. Although they are less common than lead-acid and lithium-ion batteries, flow batteries offer potential as a complement for commercial solar arrays and other renewable generation systems.
The basic construction of a flow battery consists of an electrochemical cell, two tanks for the electrolyte solutions, and two pumps. To be rechargeable, a flow battery must also be capable of restoring the solutions after use, though the process of electrolysis.
In flow batteries, the electrolyte solution tanks are the energy storage medium, while the electrochemical cell produces power. Since they are separate components, there is greater design flexibility, allowing you to combine any storage capacity with any power output capacity. Lithium-ion batteries do not have this flexibility, since manufacturers normally offer them in modular designs with fixed increments of storage capacity (kWh) and power output (kW).
The use of separate components for energy storage and power generation also means that flow batteries are not subject to a charge and discharge cycle. The electrochemical cell can deliver power indefinitely as long as the electrolyte solutions are available, and the tanks can be replenished at any time without interrupting power output. This flexible operation makes flow batteries an excellent complement for electricity sources with a variable output, such as commercial solar arrays.
Flow batteries are also characterized by a long service life, which can range from 20 to 30 years. For comparison, the best lithium-ion batteries are normally rated for 5,000 cycles, which is equivalent to slightly under 14 years if charged and discharged daily. Since commercial solar power systems typically last for more than 20 years, a flow battery can be used with no need for a replacement halfway through the photovoltaic array service life.
Since a flow battery requires electrolyte tanks and pumps, it takes up much more space than a lithium-ion battery system of the same storage capacity. For this reason, flow batteries can be deployed in fixed applications, but are impractical for use in vehicles.
Flow batteries also have a lower efficiency than their lithium-ion counterparts. While flow batteries tend to have a round-trip efficiency in the range of 75% to 80%, there are now lithium-ion cells that offer an efficiency above 95%.
The long service life of flow batteries comes with a higher upfront cost, compared with other types of batteries. This can be a barrier for their adoption if the user is unwilling or unable to commit the required capital.
Flow batteries are a very flexible energy storage technology, allowing any combination of storage capacity and instantaneous power as required by the application. They are less efficient and more bulky than lithium-ion batteries, but compensate this with a much longer service life. Commercial solar power systems can be enhanced with flow batteries to provide electricity on demand, regardless of how much power the photovoltaic array is currently producing.