These companies join at least a dozen others around the world vying for their breakout moment. Each is depending on one or more of about 12 different chemistries that appear ready for commercial applications. However, only a handful of these chemistries — vanadium, zinc-bromine and iron — are currently being used in utility-scale applications.

VANADIUM REDOX FLOW BATTERIES

Perhaps most common is the vanadium redox flow battery, which relies on vanadium’s ability to exist in four different oxidation states, each of which has a different electrical charge. When the battery is discharged, ion exchange between the electrolyte and stack generates electricity. Vanadium redox flow batteries do not suffer from degradation like lithium alternatives, meaning that they do not require augmentation to maintain the same power output. Vanadium can also be easily recycled at the end of the project, making this battery a sustainable option.

At least five companies are applying vanadium redox flow battery technology in their long-duration storage solutions. These companies include Invinity, which reported that its battery costs dropped by up to 30% following the RedT and Avalon merger. The small modular design of the company’s revamped product is said to benefit from RedT’s processing and manufacturing experience and the system-level technology and modularization that Avalon contributed.

Austria-based CellCube, which reports having completed more than 130 energy storage installations worldwide, also relies on vanadium redox flow battery technology. Its projects are expected to ramp up production to 240 megawatt-hours (MWh) in 2022. Meanwhile, VRB Energy announced plans in 2021 to build a 100-megawatt (MW) solar photovoltaic and 100-MW/500-MWh vanadium redox flow battery integrated power station in China.

Other players in the vanadium redox flow battery space include Sumitomo Electric and WattJoule.

ZINC-BASED FLOW BATTERIES

Multiple other long-duration solutions rely on zinc-bromine batteries, a hybrid redox flow battery that stores energy by plating zinc metal onto the anode plates in the electrochemical stack during charge. Originally developed by Exxon in the 1970s, zinc-bromine batteries have among the highest energy densities found in flow batteries. Consider, for example, Primus Power, which designs and develops grid-scale electrical energy storage and battery technology solutions using zinc-bromine flow batteries.

Founded in 2009, it has been shipping its small modular designs to both U.S. and international microgrid and utility customers.

On the other side of the world, in Australia, Redflow has developed one of the world’s smallest zinc-bromine flow batteries. The company is now focused on simplifying its manufacturing and reconfiguring for mass production, changes it says should reduce battery costs by a third. It will soon be deploying its first flow battery in the U.S. at a 2-MWh waste-to-energy facility in California.

Two other companies — Zinc8 Energy Solutions and NantEnergy — have developed technologies that employ rechargeable zinc-air batteries that are powered by oxidizing zinc. While not a flow battery, Eos Energy Enterprises also has a rechargeable battery that utilizes zinc chemistry. These high energy density batteries are relatively inexpensive to produce.

Eos reports a backlog of $30 million and booked energy storage orders with Hecate Energy, Azure Power and ZGlobal Inc. Zinc8 raised $15.5 million in early 2021 to fund continued product development after signing an agreement with New York Power Authority to deploy a 100-kilowatt (kW)/1-MWh battery energy storage system to support peak shaving at the University of Buffalo.

IRON FLOW BATTERIES

A third type of long-duration storage, iron flow batteries, uses electrolytes composed of ionized iron salts to store electrical energy as chemical energy. Because iron is abundant and nontoxic, this solution has the potential to reduce the cost of energy storage.

It is also where startups like Form Energy are placing their bets for long-duration storage. Backed by Bill Gates’ Breakthrough Energy, Form Energy introduced a new iron-air battery in August 2021 that can deliver power for 100 hours at one-tenth the cost of lithium-ion. Because of the low-cost materials, Form Energy reports that its washing machine-sized batteries cost just $20 per kilowatt-hour (kWh), compared to the $50 to $80 per kWh cost of lithium-ion batteries.

Form Energy, launched in early 2021, also relies on iron flow battery technology. The energy storage system starts at 3 MW power capacity but can be scaled up for larger projects. In more recent months, ESS has announced a contract to integrate 300 kW/2 MWh of its technology into a solar microgrid in rural Chile. ESS is expected to use proceeds from its merger with a special purpose acquisition company to create 16 GWh of manufacturing capacity on three continents.

Additional flow battery companies and chemistries continue to enter the market. For example, researchers at Warwick University in the U.K. say they have found a way to make a redox flow battery that costs less than $25/kWh. According to ScienceDaily, the redox flow battery technology combines carbon-based electrodes with low-cost manganese or sulfur electrolytes using an electrophoretic deposition of nitrogen-doped graphene additives. The combination is said to significantly enhance electrode durability and performance in highly acidic or alkaline environments.

Meanwhile, Cyclotron Road, a division of the Lawrence Berkeley National Laboratory, has been awarded $3 million in seed funding for a carbon-oxygen flow battery technology it developed. The lightweight, low-cost technology has demonstrated lab-scale proof of concept, and researchers are now working to scale it up to a stack-level demonstration.

Elsewhere in the U.S., flow battery installations have been limited primarily to pilot projects and grant-funded deployments at the utility or state level. For example, the California Energy Commission is helping to fund a handful of microgrids and other Invinity Energy Systems projects designed to provide communities with backup power and lower energy costs.

Adiabatic compressed air energy storage, pressurized liquid air energy storage and other mechanical energy storage systems, along with thermal and gravitational energy storage systems, may also be promising alternatives for bulk electricity storage. Each deserves its own discussion. Given manufacturer efforts to lower their materials and production costs, at least some of these solutions may become price-competitive and commercially viable. Funding from the U.S. infrastructure plan, if passed, may help kick-start the effort.

For the electric grid and those who depend on it, one thing is certain: Now is the time for utilities to begin exploring how long-duration storage might fit within their integrated resources plans.