by As concerns regarding climate change mount and renewable energy sources like solar and wind proliferate, the need for improved large-scale energy storage solutions has never been greater. Lithium-ion batteries have dominated the consumer electronics market for years but face challenges scaling up to the megawatt-hour levels needed by the electric grid. A newer battery technology known as flow batteries may provide the breakthrough needed. Possessing attributes like long lifetime, flexible size, and intrinsic safety advantages over lithium-ion, flow batteries are positioned to transform how utility-scale renewable energy is stored.
What are Flow Batteries?
Flow batteries store energy in chemical solutions contained within external tanks rather than within the battery itself. During charging, electrolyte solutions containing redox-active species of two different chemical compositions are pumped from their storage tanks through a electrochemical cell where they facilitate oxidation and reduction reactions. The electroactive materials dissolved in the liquid electrolytes serve as the charge carriers rather than solid electrodes. During discharging, the flow of electrolytes is reversed and the energy stored in the oxidized and reduced species is released as electricity. This architecture allows flow batteries to decouple power capacity from energy capacity through the size of electrolyte storage tanks rather than internal battery components.
Types of Flow Batteries
The two main types of commercially available Flow Batteries are vanadium redox batteries (VRBs) and zinc-bromine (Zn-Br) batteries. VRBs use aqueous solutions of vanadium in different oxidation states as the positive and negative electrolytes. They feature long lifetime, high efficiency, and ability to sustain thousands of charge-discharge cycles. Zn-Br batteries employ zinc and bromine dissolved in an organic electrolyte like methanol to carry out redox reactions. While initially faster to recharge than VRBs, Zn-Br batteries have faced lifetime issues with electrolyte degradation over extended use. Promising new variants under development employ polymers, aqueous organic molecules, and multivalent metal ions to enable low cost, high efficiency systems tuned for specific applications.
Advantages over Lithium-ion Batteries
When it comes to energy storage at the multi-megawatt-hour scale, flow batteries hold several advantages over lithium-ion technology. Firstly, they independently scale in power and energy limited only by the size of electrolyte tanks rather than the internal components of a battery pack. This flexibility enables flow batteries to be tailored to nearly any capacity requirement for renewable energy smoothing, backup power, or grid-level storage. In addition, as their electroactive materials are stored externally rather than inside each cell, flow batteries feature intrinsic safety benefits like mechanical simplicity and thermal stability in the event of overcharging. Finally, the liquid electrolyte chemistry employed allows theoretical cycle lifetimes in the thousands to tens of thousands of charge-discharge cycles, far exceeding current lithium-ion technology and reducing long term replacement costs.
Issues Facing Wider Deployment
While flow battery characteristics make them promising candidates for renewable integration at utility scales, several challenges have slowed widespread adoption to date. Firstly, the cost per kilowatt-hour of energy capacity has remained higher than alternative battery technologies like lithium-ion and lead-acid. Economies of scale and materials innovation are helping to drive costs down but significant reductions are still needed. Secondly, round trip efficiency between charging and discharging current sits around 75-85% for VRB and Zn-Br systems compared to >90% for high-quality lithium-ion batteries. This translates to more energy lost per cycle over the lifetime. Finally, due to relatively modest deployment to date, storage power markets have remained fragmented with few global suppliers providing full turnkey solutions and support. Consolidation will be important to drive further cost reductions and build confidence in these systems.
Despite current obstacles, progress is being made to address flow battery shortcomings and ramp up commercial deployments. Materials breakthroughs are yielding electrolyte formula with better energy density, lifetime, and round trip efficiencies. Improved cell fabrication and reactor design concepts promise lower costs. Global leaders in each chemistry are driving manufacturing scale upwards. Policy and regulatory frameworks around emissions and renewable integration are underpinning meaningful deployments worldwide. Applications are diversifying from simple peak shaving to include microgrid backup, frequency regulation services, and hybrid renewable generation-plus-storage plants. With the long lifetimes and scalability aligned to the needs of grid operators, flow batteries are positioned to scale up as the backbone technology enabling the renewable grid of tomorrow if technology and market development continue on current trajectories. Concerted RD&D efforts coupled with sustained demand growth could see flow batteries displacing lithium-ion for energy storage duties beyond the 100MWh level within the next decade.
As a battery technology optimized for large-scale, utility-grade energy storage, flow batteries represent a potential game changer in enabling renewable energy to power the grid. With attributes like flexibility to independently scale in both power and energy capacity, intrinsic safety advantages, and potential for extremely long lifetimes far exceeding alternative technologies, flow batteries fill a critical need. Though challenges around current costs, efficiency, and supply chain maturity remain, the dual value propositions of optimizing renewable integration and reducing emissions mean flow batteries merit continued support and development. With steady innovation and deployment experience, flow batteries may realize their potential to bank renewable energy and stabilize grids worldwide.
