Flow Batteries: Energy Storage Option for a Variety of Uses

Energy storage is becoming increasingly important to the power industry. Lithium-ion battery technology has been implemented in many locations, but flow batteries offer significant benefits in long-duration usage applications and situations that require regular cycling throughout the day.

The chemistry behind flow batteries has long been proven in the power industry and most analysts agree they are ideal for long-duration energy output with very low degradation of components within larger, utility-scale deployments.

With life spans reaching up to 30 years, depending on the electrolyte chemistry, flow batteries may provide unrivaled cost certainty versus other emerging storage technologies on the market. Though flow batteries currently represent a higher upfront capital investment than a similar-sized lithium-ion configuration, they become more competitive when evaluated on a total cost of ownership over a 20- to 30-year lifecycle. Moreover, costs are dropping for flow batteries as technology advances and manufacturing efficiencies are implemented.

In the utility space, flow batteries are best suited for longer discharge durations (six hours or more) in megawatt-scale power increments. Certain use cases favor flow batteries over other storage types. For 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. In fact, depending on tank configurations, flow batteries can discharge and recharge simultaneously, providing power capacity or voltage support almost indefinitely. Attributes of flow batteries include:

    ■ Demonstrated 10,000-plus battery cycles with little or no loss of storage capacity.
    ■ Ramp rates ranging from milliseconds for discharge if pumps are running, to a few seconds if pumps are not.
    ■ Recharge rates for flow batteries also are reasonably fast.
    ■ Wide temperature ranges for operation and standby modes compared to lithium-ion options.
    ■ Little or no fire hazard.
    ■ Chemistries that pose limited human health risk due to exposures.
    ■ Easy scale-up of capacity by adding electrolyte volume (although that may involve more tanks and piping).

How Flow Systems Work

Though there are dozens of different types of flow batteries, only about 10 to 12 specific chemistries appear ready for commercial applications. All operate 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.

In a redox flow battery, catholyte and anolyte are stored in separate tanks, and pumps are used to circulate the fluids into a 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 customizable 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, containerized 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.

Use Cases

Though specific power markets and load factors will vary widely throughout the world, flow batteries can perform several use cases. More importantly, a flow battery may have greater use case flexibility than lithium-ion systems designed for a specific application.

Lithium-ion systems designed for deep discharge will exhibit greater performance degradation (with potential warranty implications) if they are cycled multiple times per day or used for different applications such as frequency response. Because flow battery performance doesn’t degrade, there are fewer limitations on use cases once the system is installed. More importantly, a flow battery can potentially perform multiple use cases, depending on market signals and energy management system capabilities. With the uncertainty surrounding ISO/RTO storage market development and the dynamically evolving resource mix on the grid, an owner may be more confident that a flow battery asset can stay relevant in the market for a 20- to 30-year project life. Example use cases include:

Arbitrage. Depending on your power market, arbitrage might be the best use case for flow battery capacity. Particularly during high-peak seasons, wholesale power markets have demonstrated rapid pricing swings, and as power costs begin to spike, capacity available from large-scale flow battery configurations can become economical. By syncing flow battery capacity to be available quickly and with durations of six to eight hours or more, while prices per kilowatt are at a peak, significant revenue can be realized.

Behind the Meter. Commercial-scale units are being deployed behind the meter in manufacturing facilities, hospitals, campuses, and even residential locations as a means to shave power demand when premium rates are charged. Depending on rate tariffs and demand response energy curtailment programs available from local utilities, flow batteries can be an economical option to reduce energy costs simply by discharging batteries during periods of voluntary curtailment or when local rates are high.

Volt/VAR Support. Inverter-based generation, such as flow and lithium-ion batteries, can perform similarly to synchronous condensers on feeder circuits due to their ability to provide quick-acting voltage support. With the ability to generate or absorb reactive power as needed to adjust grid voltage, flow batteries connected to smart inverters can provide a distribution grid support function, particularly as systems are designed for emerging peak recharging demand from electric vehicles.

Renewables Pairing. Both flow batteries and lithium-ion batteries are emerging as attractive options to extend the availability of renewable solar and wind resources across more hours of the day to counter well-known intermittency issues. With flow batteries capable of providing power over longer duration, and with the ability to ramp up or down with no degradation, developers and utilities are actively evaluating these technologies.

Black-Start Capacity. Normally, when a power plant is shut down, it will draw power from the grid in order to provide the initial power needed to restart the large generators and return the plant to full service. However, during a widespread outage when backfeed power is not available, generator startup may require an on-site black-start unit. Conventional black-start generators are fueled by diesel or natural gas, but black-start power also can be provided by batteries scaled up to provide the necessary power to return the plant to service.

Resource Diversity. Many utilities have long opted for resource planning strategies that make use of a variety of energy sources, ranging from conventional fossil and nuclear generation to renewables and energy storage. By blending the favorable characteristics of both lithium-ion and flow batteries, utilities can reduce risk across their entire portfolio of energy storage options.

Barriers Flow Battery Technology Must Overcome

Lithium-ion technologies are dominating today’s storage market and there are good reasons for that. But 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.

1. The lifecycle cost comparison depicted here shows the net present value (NPV) including capital, charging, and operations and maintenance (O&M) costs of lithium-ion and flow batteries with similar capacities at the point of interconnect (POI). Courtesy: Burns & McDonnell

Figure 1 shows the results of a lifecycle cost analysis comparing 20-MW, 8-hour (160-MWh) lithium-ion and flow battery systems. The model includes capital, O&M, and charging costs for a 20-year project life. The net present value (NPV) totals are calculated and compared.

The analysis is based on the following key assumptions:

    ■ Single-contract, full-wrap engineer-procure-construct (EPC) methodology.
    ■ Owner’s costs, decommissioning costs, insurance, taxes, and revenues are excluded.
    ■ Costs are based on Burns & McDonnell experience and vendor information for technology available in 2021 and are not representative of any particular technology or original equipment manufacturer (OEM).
    ■ Routine maintenance for 20 years is included. Augmentation is included for lithium-ion technology based on minimal initial overbuild.
    ■ Year 1 roundtrip efficiency values of 70% for the flow system and 84% for the lithium-ion system are included.
    ■ Discount rate is 8.5%. Escalation rate for O&M and energy costs is 2.5%.

With today’s technology, flow battery capital costs are nearly double the cost of a similarly sized lithium-ion system. With longer storage durations and longer lifespans, the economics improve, but it is not expected to achieve parity with lithium-ion at today’s pricing. Decommissioning and recycling costs may favor flow batteries because the electrolyte is more easily recyclable or disposable (depending on technology type), but these costs are still developing. As flow OEMs improve manufacturing scale and supply chain efficiencies, and as EPC contractors gain field experience, costs will continue to plummet. Still, flow batteries are chasing a moving target with falling lithium-ion costs.

The primary barrier to full market penetration of flow battery technologies today is simply the lack of commercialization compared to the heavy installation base of competing lithium-ion technology. In the near term, it is likely that more relatively small flow battery facilities will be installed until a widespread commercial track record is established and use cases, costs, and returns on investment are proven. ■

Tisha Scroggin-Wicker, PE is the flow battery business manager, and Kieran McInerney, PE is a senior engineer at Burns & McDonnell.

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