A Strategic Assessment of Flow Batteries

As renewable energy resources continue to gain market share, Battery Energy Storage Systems (BESS) are increasingly being used to support use-cases such as renewables firming, peak management, ancillary services, energy and carbon arbitrage, storage as a transmission asset, and capacity for resource adequacy. Among monetary based use cases, the economics of new solar and  wind installations can generally be improved when paired with energy storage.

Flow Batteries (FB) are now being evaluated in utility resource planning scenarios, and depending on the use case may be a viable alternative to lithium-ion batteries (LIB). FBs are proving to be a viable technology that will begin to displace LIB deployments due to their potential to provide low-cost Long Duration Energy Storage (LDES) (e.g. 10+ hour duration) and maintain performance over lengthy contract terms (20+ years).

Use-Cases and Energy Storage Solutions

For a FB asset to remain relevant in the market for its 20-to-30 year expected life span, FB developers / owners are looking for clear use cases that can result in a reasonable return on investment. Matching the duration of an energy storage technology to the use-case demands (FIGURE 1) is critical to consider when selecting a storage technology. FBs capability spans over most use-cases including:

• Asset Deferral is the process of using energy storage to extend the life of existing electric system infrastructure, including natural gas power plants and transmission lines, by deferring or eliminating future investment in these capital-intensive assets
• Capacity Markets in many RTO regions are experiencing policy and regulatory changes and have had higher capacity auction prices. The capacity market is a potentially lucrative use-case for storage.
• Energy Arbitrage is the ability to store energy during low energy price periods and discharge during high energy price periods. Wholesale power markets have demonstrated rapid pricing swings, particularly during volatile pricing seasons. In addition to energy arbitrage, there is an emerging concept of carbon arbitrage as a potential area to demonstrate the value of LDES systems and FBs.
• Renewables Integration with BESS is emerging as an attractive option to enhance the economics by extending the availability of renewable resources across more hours of the day.
• Demand Response, particularly within the Commercial and Industrial (C&I) space, energy storage is being deployed as a “behind-the-meter” resource in manufacturing facilities, hospitals, campuses and even residential locations as a means to shave power demand when premium rates are charged.
• Ancillary Services include spinning and non-spinning reserves, and VAR support are potential value streams for energy storage resources depending on the region.
• Black Start Capacity is valuable during a widespread outage when backfeed power is not available for generation resources to provide power thus preventing generator startup.
• Resiliency has become a high-profile use-case in recent years related to weather related events which has inhibited normal operation of gas assets, transmission and distribution system, and other power generation/delivery resources.

Principle of Operation

A FB is a rechargeable fuel cell in which an electrolyte containing dissolved electroactive elements flows through an electrochemical cell stack that reversibly converts chemical energy directly to electricity (FIGURE 2). In short, FBs work by pumping negative and positive electrolytes through electrodes stacks, allowing energy to be stored and released as needed through electrochemical energy storage. Flow batteries allow power and energy to be scaled separately to certain degrees depending on technology. For example, the energy (MWh) component is driven by the volume of electrolyte fluid in the tanks, while the power is driven by the surface area of the electrode stacks, depending on the design. Due to manufacturing and design constraints, this flexibility is limited, but ow batteries are typically well suited for LDES applications.

Flow Battery Supply Chain

Flow batteries are a maturing technology, with many original equipment manufacturers (OEMs) developing products / platforms suitable for utility scale projects. Though there are a multitude of FB types, only about 3-4 specific chemistries appear ready for utility applications. Vanadium Redox flow batteries (VRFBs), Iron ow batteries (IRFBs), and Zinc-Bromine flow batteries (ZBFBs) all operate on the same basic principle and are discussed in greater detail below.

Vanadium Redox Flow Batteries. A VRFB cell consists of two electrodes or “stacks” (made from carbon felt, plastics, and metal alloys) and two circulating electrolyte solutions (a positive/cathode-side electrolyte or catholyte, and a negative/anode-side electrolyte or anolyte) that are separated by an ion exchange membrane. The conversion from electrical energy to chemical potential energy (charge) and vice versa (discharge) occurs instantly within the electrodes as the liquid electrolytes flow through the cell. Vanadium flow batteries use only a single element in both half-cells which eliminates the problem of cross-contamination across the membrane.

Hybrid Flow 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 6-12 hours, power and energy are not fully decoupled.

Iron Redox Flow Batteries. The IRFB stores and releases energy through the electrochemical reaction of iron salt. The configuration of IRFBs is similar to other redox-ow battery types. IRFBs consists of two tanks, which in the uncharged state store electrolytes of dissolved iron ions. The electrolyte is pumped into the battery cell which consists of two separated half-cells. The electrochemical reaction takes place at the carbon-based porous electrodes within each half-cell. IRFB OEM landscape is not well developed but ESS Inc. is currently constructing several systems throughout the U.S.

Zinc-Bromine Flow Battery. A zinc-bromine battery is a rechargeable battery system that uses the reaction between zinc metal and bromine to produce electric current, with an electrolyte composed of an aqueous solution of zinc bromide. The ZBFB is a hybrid ow battery which utilizes a solution of zinc bromide (electrolytes) stored in two tanks. When the battery is charged or discharged, the electrolytes are pumped through a reactor stack from one tank to the other. The ZBFB OEM landscape is not well developed but Redflow has >50 MWH deployed and is currently constructing dozens of systems throughout the U.S. and world.

Performance Summary of Commercially Available Technologies

The elimination of repeated ion insertion and de-insertion in RFB electrodes as occurs in other batteries, preserves the structural and mechanical integrity of the cells/stacks, enabling a long-cycle life of the battery. In a true VRFB that utilizes liquid electrolytes on both positive and negative sides, its cycle life is independent of the battery’s State of Charge (SOC) and depth of discharge (DOD). This is not the case with traditional batteries that store energy in their solid electrodes that causes energy capacity degradation over the life of the product. Unlike LIBs, FB systems have less limitations on cycling characteristics that cause performance degradation or impacts to warranties. However, they have lower energy densities and lower efficiencies than LIB, which lead to larger footprints for similarly sized power systems.

Technical Viability

Cost models for utility-scale BESS are based on a bottom-up cost model using the data and methodology for utility-scale BESS. There are two common references used for BESS costs analysis: (1) National Renewable Energy Laboratory (NREL) Annual Technology Baseline Report¹ and (2) the Pacific Northwest National Laboratory (PNNL) Cost and Performance Database². The bottom-up BESS model accounts for major components, including the battery pack, inverter, and the balance of system (BOS) needed for the installation, Fixed Operation and Maintenance. Using PNNL’s detailed cost models for BESS installation allows for an illustrative capital cost comparison for a 10 MW / 100 MWh BESS facility (FIGURE 3).

Conclusion

FIGURE 4 shows the results of a life cycle cost analysis comparing 10 MW LIB and VRB systems at various durations. The model includes capital, O&M and performance losses for a 20-year project life based on cost public references.

For more information or to comment on this article, please contact:
MATT SMITH, SENIOR PROJECT MANAGER
GDS Associates, Inc. 
Marietta, GA
770-799-2478 or
matt.smith@gdsassociates.com

*References / Credits
¹ https://www.pnnl.gov/lithium-ion-battery-lfp-and-nmc
² https://atb.nrel.gov/electricity/2023/data
Page 4 image, 3 MWH Iron Flow Battery (OEM: ESS, Inc.) Deployment for Sacramento Municipal Utility District