Inside Clean Energy: Flow Batteries Could Be a Big Part of Our Energy Storage Future. So What’s a Flow Battery?

A battery project uses a technology that could be vital for meeting the need for long-duration energy storage.

This shipping container holds a flow battery storage system developed by ESS Tech Inc. of Oregon. The company is aiming to meet the need for long-duration energy storage with batteries that can discharge electricity for up to 12 hours. Credit: ESS Tech
This shipping container holds a flow battery storage system developed by ESS Tech Inc. of Oregon. The company is aiming to meet the need for long-duration energy storage with batteries that can discharge electricity for up to 12 hours. Credit: ESS Tech

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A clean energy development this week in the San Diego area isn’t much to look at. Workers will deliver four white shipping containers that house battery storage systems. Soon after, workers will hook up the containers so they can store electricity from a nearby solar array.

The part that I care about is the “flow battery” technology inside those shipping containers, developed by ESS Tech Inc., an Oregon startup. Flow batteries have the potential to be an important part of the energy transition because they can provide electricity storage that runs for much longer than the typical four hours of the dominant technology, lithium-ion batteries.

So what is a flow battery? A key design element is the use of two external tanks that contain electrolyte fluids that get pumped through the battery as it charges and discharges.


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The duration of the battery, which is how long it can run before recharging, increases based on the size of the tanks. Think of this as the battery equivalent of one of those novelty baseball helmets that hold two cans of soda. If you switch out cans of soda for two-liter bottles, you can drink a lot more.

“For the whole machine, what you need to do is add more liquid rather than adding many, many more batteries,” said Jun Liu, a University of Washington professor and a fellow at the Pacific Northwest National Laboratory. He also is director of the federal government’s Battery500 Consortium, which develops next-generation batteries for electric vehicles.

In contrast to flow batteries, lithium-ion batteries and most other batteries are self-contained, with less flexibility in their design, he said.

Lithium-ion batteries also are highly flammable. Leading flow battery types, like “vanadium redox” flow batteries, have a much lower fire risk. (Vanadium is a metal that doesn’t easily catch on fire, especially when it’s dissolved in a fluid, as it is in a flow battery.)

And one of ESS’ selling points to investors and customers is that it doesn’t rely on rare metals like lithium or vanadium at all. The main ingredients of its fluid are iron, salt and water.

The San Diego area project, developed in partnership with the utility San Diego Gas & Electric, is the largest demonstration of ESS’ technology to date.

Look Inside a Flow Battery

The system will be able to discharge 3 megawatt-hours before being recharged, which is enough electricity to meet the needs of about 100 houses for one day. It includes six shipping containers that house the batteries, the last four of which are scheduled to arrive this week. Each container has stacks of batteries, with tanks of electrolyte fluid for each battery.

ESS also is working on a demonstration project with the utility Portland General Electric in Oregon, which should go online later this year. This will be the debut of a larger and less portable version of the ESS battery called the Energy Center.

Flow batteries can come in many sizes, from as small as a compact refrigerator to as large as stacks of shipping containers. In addition to ESS, the players in this space include Sumitomo Electric of Japan, VRB Energy of Canada and others, a mix of startups and established businesses.

The companies are betting that their technologies can meet the need for energy storage that lasts for eight to 12 hours per charge. The underlying idea is that the grid will need a mix of various storage technologies, with various durations, to fill in the gaps left when wind, solar and other resources are not enough to meet customers’ needs.

That’s a pretty good bet, according to Adarsh Nagarajan, a group manager for power system research at the National Renewable Energy Laboratory in Colorado.

“Flow batteries are here to stay,” he said.

But he is careful to specify what flow batteries are not: They are not a replacement for lithium-ion batteries. Instead, flow batteries will serve a part of the market that barely exists today for energy storage that can last for eight hours or more, while lithium-ion batteries will continue to be the leaders in shorter-duration storage, electric vehicles and consumer electronics. While lithium-ion batteries can be used for durations of eight hours or more, they are better suited for shorter runs of one to four hours.

One of the biggest obstacles to rapid deployment of flow batteries is cost. A 2020 report from Pacific Northwest lab estimated an installed cost of $551 per kilowatt-hour for a 4 megawatt-hour system. A developer could build a lithium-ion storage system at a lower cost, although there are a number of factors that make this comparison difficult, including a longer life expectancy for a flow battery.

ESS has not disclosed the cost of the San Diego area project. The company has said that it is aiming to get the cost down to $200 per kilowatt-hour by 2025, as Bloomberg reported last year.

Current costs are reasonable enough that utilities and other buyers are stepping forward to finance larger flow batteries than before. Among them, Central Coast Community Energy in California announced in November that it was building three flow battery projects, scheduled to go online in 2026, with no costs disclosed. The projects, with a total of 226 megawatt-hours, would be done in partnership with Concentric Power Inc. of Salinas, California, a designer and builder of microgrids. The design would be a vanadium redox flow battery.

ESS, or Energy Storage Systems, was co-founded in 2011 by Craig Evans and Julia Song, who are married. The two had previously worked together at a manufacturer of fuel cells where Evans was a product designer and Song was a chemist.

ESS worked for years on its technology, with financial help from investors and the government. In 2021, the company held an initial stock offering and hired a CEO, Eric Dresselhuys. It has about 220 employees.

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Now, the company is close to the point at which its ideas and designs need to begin translating into large orders from customers.

Last week, ESS reported a first-quarter loss of $5.7 million, with a ledger that includes lots of expenses and almost no income.

In a conference call with analysts, Dresselhuys said the company’s order pipeline was strong, with plans to ship 40 to 50 of its systems this year. But ESS also is facing delays in getting parts, which means some projects that were going to happen in 2022 may get pushed into 2023.

Liu, of the University of Washington, said he is confident that flow batteries will be a success in the market, but he sees potential for wide variability in when that might happen. So 2022 may not be the breakthrough year for flow batteries, but that time is likely coming.

“We need to be patient,” he said.

Other stories about the energy transition to take note of this week:

Department of Energy Launches Program to Support Long-Duration Energy Storage: The Department of Energy has begun a $505 million, four-year program to encourage deployment of energy storage systems that can run for at least 10 hours on a charge. The program will use the expertise of agency and national labs to try to drive down the costs of long-duration storage and help figure out which storage technologies are best suited for wider adoption, as Elizabeth McCarthy reports for Utility Dive. The program is called Long Duration Energy Storage for Everyone, Everywhere, which this acronym-loving agency is shortening to LD ESEE.

Inside the Race for a Car Battery that Charges Fast—and Won’t Catch Fire: Toyota, Ford and Volkswagen are among the automakers working to produce a solid-state battery for electric vehicles, a system that could help to catapult the electric vehicle market. Solid-state batteries use a solid electrolyte as opposed to the liquid in most batteries used today. In an electric vehicle, a solid state battery would be able to charge more quickly and have a longer range. Another benefit is that solid-state batteries would be less flammable. Pranshu Verma of The Washington Post looks at the state of the solid-state race and why it’s so important. “It’s the technology of the future,” said Eric D. Wachsman, director of the Maryland Energy Innovation Institute. “The question is: ‘How soon is that future going to be here?’”

Michael Bloomberg Plans a $242 Million Clean Energy Investment in Developing Countries: Michael Bloomberg is announcing a new initiative to promote clean energy in 10 developing countries. Bloomberg Philanthropies will spend $242 million to work with nonprofits and governments in Bangladesh, Brazil, Colombia, Kenya, Mozambique, Nigeria, Pakistan, South Africa, Turkey and Vietnam, as Maggie Astor reports for The New York Times. The idea will be to help the countries move away from fossil fuels and use technologies that are cleaner and less expensive. “Which strategies are appropriate for each country will really be guided by the in-country partners who know them best,” said Helen Mountford, the president and chief executive of ClimateWorks, a nonprofit working with Bloomberg on the initiative.

Google’s New Campus Has a Different Kind of Solar Roof: The roofs of buildings in Google’s newly opened campus in Mountain View, California don’t look like solar panels, but that’s what they are. The roofs are giant canopies that look like futuristic circus tents, covered in silver-colored photovoltaic panels, as Adele Peters reports for Fast Company. The solar roofs, along with a geothermal system for heating and cooling, will help the campus work toward Google’s company-wide goal of running completely on renewable energy around the clock by the end of the decade. The Google complex was designed by Thomas Heatherwick of the United Kingdom in partnership with the Bjarke Ingels Group of Denmark.

Ford Continues to Reduce Stake in EV Startup Rivian: Ford Motor Co. has cut its stake in Rivian to less than 10 percent following two large sales of shares. Ford is reducing its investment following a severe drop in Rivian’s share price as the startup maker of electric trucks deals with challenges in obtaining parts and filling orders, as Jordan Grzelewski reports for The Detroit News. Despite the challenges, Rivian’s debut truck has gotten strong reviews in the automotive press and has high interest from buyers.

Inside Clean Energy is ICN’s weekly bulletin of news and analysis about the energy transition. Send news tips and questions to