Flow batteries with longer lives than lithium-ion batteries could be the solution for the energy storage.
Flow batteries, also called redox flow batteries, are a type of hybrid between a battery and a fuel cell. In a redox flow battery, catholyte and anolyte are stored in separate tanks, and pumps are used to circulate the fluids into a cell stack with electrodes separated by a thin membrane. This membrane permits ion exchange between the anolyte and catholyte to produce electricity.
The power produced is dependent on the surface area of the electrodes, while the storage duration is a function of the electrolyte volume.
For some technologies, the power and energy can be scaled independently, allowing for an easily customisable battery. In a hybrid flow battery, electroactive material is deposited on the surface of the electrode during the charge cycle and then dissolved back into the electrolyte solution during discharge. For hybrid technologies, the storage duration is a function of both the electrolyte volume and the electrode surface area. While most hybrid technologies can achieve durations of six to 12 hours, power and energy are not fully decoupled.
Flow batteries can be configured as both a single tank, usually for smaller applications, or as a dual tank, usually on a larger footprint. The single-tank systems typically feature zinc or other metal batteries, while dual-tank systems require electrolyte comprised of saltwater, iron, vanadium, or other minerals.
Flow battery system designs change depending on the application and project size. Behind-the-meter commercial systems are commonly kilowatt-scale packaged units that can fit into a typical utility room. For distribution applications in the 1-MW to 5-MW range, containerised and/or modular solutions exist with varying levels of scalability depending on the storage duration requirements. Utility-scale designs in development may have millions of gallons of electrolyte storage, so the industry is trending toward large quantities of stack modules headered together and piped to large, field-erected tanks.
Power stacks and balance-of-system components, such as piping, pumps, seals, cooling systems, and control instrumentation, require more routine maintenance than lithium-Ion configurations. However, if routine maintenance guidelines are followed, flow battery performance should not degrade within the project lifetime. When the operations and maintenance (O&M) costs are compared to lithium-ion capacity augmentation costs required to offset performance degradation, flow battery annual costs are less expensive.
With life spans reaching as much as 30 years, depending on the electrolyte chemistry, flow batteries may provide unrivalled cost versus other emerging storage technologies on the market.
Dependent on tank configurations, flow batteries can discharge and recharge simultaneously, providing power capacity or voltage support almost indefinitely. Applications where multiple charge/discharge cycles are required each day, flow batteries are available within milliseconds as loads dictate and they can quickly recharge from a variety of available power sources.
A recent report confirmed there is over 70 MW/250 MWh redox flow battery storage capacity deployed to date, in medium to large-scale projects. This is expected to rapidly increase due to the batteries' fast response time, scalability, and not least, their much easier recyclability than lithium-ion batteries.
The flow systems works though dozens of different types of flow batteries, only about 10 to 12 specific chemistries appear ready for commercial applications. All operating on the same basic principle of incorporating liquid electrolyte to function as a source of direct current (DC) electricity that runs through an inverter for conversion to alternating current (AC) power.
Barriers to Overcome
Lithium-ion technologies are dominating the storage market however as renewable energy market penetration increases, more owners are looking at longer-duration storage assets.
Costs are declining for both lithium-ion and flow battery technologies, and it is difficult to predict where and when the prices will settle.
With today’s technology, lithium-ion unit costs ($/kWh) generally flatten out beyond 4-hour storage durations because of the essential addition of higher quantities of the same batteries. However, flow battery unit costs continue to decline as the storage duration increases to eight to 12 hours. The power modules for a 4-hour system are the same for a 12-hour system, so the incremental cost of adding duration/energy to a flow battery is tied to the addition of electrolyte to the system.
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