Flow Batteries' Special Ingredients Are No Secret- How They Scale Remains a Mystery

In Brief

A battery that stores energy for hundreds of hours might mean solar power's economics outshine fossil fuels' costs all day, every day. 

So startups and established tech companies have sped up work to build, deploy, and sell such a battery. The most compelling model so far involves a system called "flow." 

This story introduces the chemistry and some of the competitors that make flow batteries a potential sparkplug for jobs and investment after the Covid crisis. 

Batteries for the long store?

“Flow batteries,” which last many more years and many more hours than the kind we carry in our phones, could make storage of solar or wind power much more straightforward. Firms that make them have commanded high valuations all spring, sometimes with grand and mysterious promises of energy revolution. To understand the technology’s use in business, though, let’s review some chemistry. New entrants using a new material promise to deliver on flow batteries’ potential. 

These entrants follow a course that the government started. In the 1970s, NASA started research on redox flow batteries technology to find long-duration energy storage for its space missions. In 2020, we are closer than ever to its deployment around the world.

Today, storage serves mainly commercial purposes. The total global market size for energy storage is estimated at $59 billion, projected to grow over 900% by 2035. The continual growth in energy demand and call for diversification of energy sources, though, has run hotter. This has fueled the search for further improved storage technology.

Since 2012, electrochemical storage has comprised the principal growth in cumulative rated power growth in the US, surpassing pumped hydroelectric storage projects in the past two years. Within the sector, lithium-ion battery (LIB) remains dominant, benefiting strongly from the spillover effects of electric vehicle deployment. In the media, LIB has become almost synonymous with frontier developments in battery as small-scale devices like laptops, smartphones, and power banks become more energy efficient with a shorter evolution cycle.

However, for renewable solar arrays and wind turbines, LIB fails to meet some tests of larger-scale and longer-duration applications. Developers and utilities investing in storage want to see factories, school campuses, and even municipalities run on renewable energy 24/7. Such an increase in the intensity of power production requires LIB manufacturers to either install additional generation units or pack them with denser active chemicals, both of which would make costs unfeasible for a battery that needs replacing every ten years. This intensification process would also involve exposure to higher fire hazard due to the nature of LIB chemistry. It would also force investors and executives to reckon with the poor sustainability of cobalt, an essential element in the construction of LIB whose extraction pipeline runs through ethically flawed regimes.

If cities look to power, say, entire office parks on storage, they might need a more sustainable, risk-proof, and flexible technology. A promising solution is flow batteries.

The most widely studied flow battery technology, redox flow battery (RFB), utilizes the same reduction-oxidation as LIB. Yet unlike conventional batteries, flow batteries store electrical charge in reservoirs of liquid-state electrolytes separate from the flow-through carbon electrodes during operation. At the selective membrane surface of the redox reaction, stored electrons flow from the negative half-cell side to the positive, completing the circuit in the system and transferring back the spent electrolyte to the tank.

This completely reversible process implies that the tanks of electrolyte can remain inactive until energy is needed, that they can stay undischarged for long periods of time with no degradative effects, and that scaling of RFB only requires scaling the tanks and not the operator itself. In fact, laboratory-based research has shown that RFB demonstrates up to 30 years of longevity, durability under temperature and other weathering conditions, and large discharge capacity—seemingly perfect for municipal energy projects of the next decade. 

They can work at a third of their power capacity and run for three times as long at a cost as low as 5-6 cents per kWh, possessing the flexibility inherently attractive to grid service providers. Consumer growth trends also support the expansion of flow batteries. They’re 100% recyclable, nonflammable, easily rechargeable commodities, boasting round-trip efficiency comparable to the modern LIB.
A wave of startups hoping to build RFB systems at scale has attracted curiosity in the energy market. Invinity Energy Systems, previously US-based Avalon Batteries merged with European RedT, is one of the biggest players in the Western world in the field of RFB. The company utilizes vanadium, a metal known to have several valence states while being found in more abundant quantities than lithium
Compared to other metals or hybrid models, vanadium RFBs able to reliably discharge for thousands of cycles while requiring simple monitoring or controls, representing the current state-of-the art. From intermittent but clean solar generation, vanadium RFB energy storage can alleviate the timeshift excess outputs and avoid high import costs as a way to “futureproof” against rising costs.

Joe Worthington, Invinity’s Head of External Communications, puts the business case simply: “We believe very strongly in creating products that are economic and return good investments, mostly in stationary energy storage applications.” One of Invinity’s primary target is the commercial & innovation space, including large farmhouses, milking parlors, shopping malls, data centers, or water utilities facilities, where there is “an obvious case for flow batteries.” These places can sit off the typical grid while requiring capacity-level energy day and night. Some other potential areas of development include stand-alone battery systems, microgrids or strictly off-grid mining sites, as well as hybrid models. At the moment, Invinity is working behind the Energy Superhub Oxford towards delivering the UK’s largest flow battery as part of “a world-first project decarbonizing transport”, electricity and heat across the city of Oxford, UK.

Other companies are trying to go bigger. Rongke Power, in Dalian, China, is also concentrating on a similar-scale vanadium flow battery, which should launch in 2020, with the potential of storing 800 MWh, enough to power thousands of homes. CellCube, headquartered in Toronto, Canada and dubbed the top flow battery vendor by Navigant Research in 2019, has managed to deploy 130 systems in Siberia, Vietnam, Australia, and South Africa, and is looking to leverage its vanadium-rich properties in Nevada to build a subsidiary focused on enriching vanadium for future exploration.

...a world-first project decarbonizing transport, electricity and heat across the city...

The potential of RFBs lies beyond behind-the-meter unit usage, especially given the evolving dynamics of resource mix offered on the ISO market. Burns & McDonnell flow battery business manager Tisha Scroggin-Wicker and engineer Kieran McInerney considers the asset values of flow batteries that can stay “relevant in the market for a 20- to 30-year project life.” Investors might be able to arbitrage wholesale power when long-storage power becomes more common. 

Flow batteries can also help address peak demand, insure against power outages, and provide the frequency regulation and volatility that might bring about a diversified energy portfolio and even grid independence. Market estimations are similarly optimistic of the potential of flow batteries. A 2020 Allied Market Research publication reports that the global redox flow battery market garnered $130.4 million in 2018, and predicts a compounded annual growth rate of 15.2% up until 2026. Stratistics MRC’s 2020 Market Research Report agrees, predicting an even higher CAGR at 16.8% to $527.55 million by 2027

Yet commercialization of flow batteries faces roadblocks. One involves the high cost of vanadium, which according to the journal Minerals is normally extracted as a by-product of titanomagnetite or uranium ores. Lithium, on the other hand, is best-known to be extracted from brine from salt flats, which is not only better studied but also shares techniques with many other water-intensive industries.

The immaturity of the vanadium market is also a contributing factor: demand for vanadium has increased along with its cost, and the capital cost of vanadium extraction techniques is still not cheap enough. UniEnergy Technologies, co-founded by Dr. Z. Gary Yang utilizing a research success of the Pacific Northwest National Laboratory, managed to drive down the cost of its vanadium RFB to about $400 per kWh capacity. This may pencil out for systems that require four or more hours of energy, up to 10 megawatts of power.

In practice, the capital expense can still be twice as much as LIB. Long-term savings might be a great appeal, but the slow return on investment perhaps has contractors still waiting for more.
One possible solution is making vanadium extraction more efficient. After redT struggled to stabilize, Avalon has created an arrangement for renting vanadium from mining company Bushveld Minerals, which might have provided the necessary torque for the now-merged Invinity to see initial success. Mr. Worthington explains, “Because the electrolyte is a non-degrading commodity, we can roll up the capital expense and invest in the operating expense of the battery,” making for potentially more flexible financial models for project developers. Vanadium-rich inputs also contain by-products like other metals or oily fly ash, which can be resold to subsidize some of the costs. This is what WattJoule, an RFB company likewise dedicated to revolutionizing energy storage, is looking to do over the next decade.

The other approach is to continue the scientific quest for better models of the RFB. ESS Inc, a rising company based outside of Portland, Oregon, has been exploring iron flow batteries. With the identical chemical basis, ESS Inc claims to provide cleaner, more economically viable flow battery technology that also delivers almost all of the long-term benefits that flow batteries provide. A microgrid project for Stone Edge Farm (SEF) that ESS built, for instance, interfaces an all-iron RFB with many other technologies to export excess clean power 24/7 to neighbors through a communal aggregator. This model might be much more attractive in states like California, as it can tap solar or wind baseload.

In the lab, advances in RFB occur nonstop. The DoE’s Joint Center for Energy Storage Research has been looking into nonaqueous flow batteries, sulfur-based batteries, or even organic polymers as the next turning point in storage technology. A joint effort between the University of Sydney, the University of New South Wales, and Utah State University has modeled perovskite-silicon solar-flow batteries reminiscent of PV cells with RFB advantages. Even with existing vanadium RFBs, different leasing schemes that deprioritize upfront capital costs show potential for lower levelized costs for battery owners. Evolving engineering designs in the directions of lower cost and higher efficiency—the ultimate litmus tests for commercial innovation of today’s technology—are constantly propelling RFB startups every day.

“We have a huge R&D team that works iteratively and constantly on trying to drive our performance up and costs down,” Mr. Worthington adds. Invinity hopes that its leading role in the market will also contribute meaningfully towards energy access and energy justice. “We are active in all major energy storage markets such as North America, Europe, sub-Saharan Africa, Australia, Southeast and East Asia,” working on World Bank-funded electrification projects, agribusinesses, and telecom-based stations for local communities. 

To achieve a net-zero future, the current energy storage markets face three main challenges. According to Ed Porter, the Business Development Director at Invinity, developers, investors, and managers alike need to overcome the bias towards current technologies (in this case LIB), barriers to standardization of financing models and legal structures for large-scale deployment, and global policy variability. As we are seeing diversification in applications where renewable energy is coming into play, we should also anticipate similar diversification in energy storage technology usage, emphasizing flexibility and specialization.

Yet this requires active public investments: lithium-ion batteries took 40 years to achieve the widespread integration it has today, and the call for energy innovation is more pressing than ever. Flow batteries present an appealing case for long-duration energy storage: clean, adaptable, and sustainable. If market conditions allow, RFB can help solve energy management issues in major US cities as much as bring energy independence as an empowering tool for many remote communities. The rate of growth for RFB’s economic viability is already several years faster than expected, despite its checkered history. As we become more aware of RFB and peer technologies’ benefits, financial investors and policy actors need to support technical progress step-by-step to realize this potential to the fullest.

And for scientists, the search for ever-better technology marches on.

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