Should I Stay or Should I Flow…

I saw a LinkedIn post recently from Root Power – they’d submitted a tender for a UK Long Duration Energy Storage (LDES) contract – and I was excited. For years I’ve been banging on about advanced batteries, even back when people (over a decade ago) didn’t see the significance.

Now the industry has caught on. The pros and cons of lithium-ion battery energy storage systems (BESS) are well known: they typically deliver full power for only 1–4 hours, making them relatively short-duration storage. They’re still expensive (though costs have plummeted, roughly halving every few years), but incredibly fast-acting – able to respond in milliseconds.

This fast response provides much-needed grid stability in renewables-heavy grids that lack the natural inertia of big spinning turbines. see my inertia blog article and my YouTube video.

Lithium BESS are superb for smoothing out second-to-second fluctuations and short peaks, but they’re not built for multi-hour or multi-day energy shifting.

Long-duration energy storage (LDES), addresses an entirely different problem and, is not a new concept. The oldest form of LDES is hydroelectric storage. This has been used for decades to balance grids. In fact, pumped hydro “water batteries” still account for over 90% of the world’s energy storage capacity hydropower.org.

However, pumped hydro is geographically constrained (you need mountains, and a lot of water) and each plant’s power capacity is essentially fixed once built. You can’t install a new pumped hydro anywhere or easily expand one beyond its design. This is why the search has been on for new LDES technologies that are more flexible in location and scale.

So what about flow batteries – and if they’ve been around in labs for years, why are we only now seeing them move into real projects?

The technology of a flow battery is fundamentally different from lithium-ion, NiCad, or old lead-acid batteries…

All batteries store energy via chemical reactions, but the where and how differs. In a lithium-ion cell, charging the battery forces lithium ions (and electrons) into atomic “slots” within the electrode materials. There are lots of empty sites when the battery is discharged, so it charges quickly at first, but as those sites fill up, the process slows – which is why your EV or phone charges rapidly when near empty and then tapers off as it approaches full charge.

Traditional lead-acid or other chemically-rechargeable batteries store charge by altering the composition of solid plates: for example, in a lead-acid battery, forcing electrons in during charging converts lead sulphate back into lead dioxide at the positive plate (storing energy in chemical form).

The battery then discharges when the reaction is allowed to reverse and electrons flow back, converting the material to its original state.

Flow batteries, by contrast, store energy in a liquid electrolyte that flows through electrochemical cells. Instead of the energy being stored in static solid electrodes, it’s stored in electrolyte solutions which are pumped across electrode plates (through a membrane) – hence the name “flow” battery.

In a typical design, two tanks hold liquid electrolytes (often one positively charged, one negatively charged). When you charge the battery, a chemical redox reaction (redox is short for oxidation-reduction) occurs in the cell stack, and the energy ends up stored in the electrolyte itself (as oxidation increases in the solution, electrons are lost. as oxidation decreases electrons are gained).

To get the energy back out, you run the flow in reverse, and the chemical energy in the fluids is converted back to electricity. Essentially, the electrolyte is the fuel. This architecture has a massive advantage: the energy capacity is decoupled from power.

The power (MW) is set by the size of the cell stack (how many cells, how big the electrodes are), while the energy (MWh) is determined by how much electrolyte you have in tanks. If you want more duration or capacity, you just use bigger tanks or more liquid – in theory, virtually unlimited storage is possible by scaling up the fluid volume.

Because of this decoupling, flow batteries excel at long duration. Need 8 hours, 24 hours, or even multi-day storage? Just add more electrolyte. One can even imagine charging up one set of electrolyte tanks, then swapping them out for fresh (discharged) electrolyte to keep going – the capacity is limited only by how much charged fluid you can store.

This scalability makes flow batteries highly suitable for grid storage requiring many hours of discharge. Moreover, the technology has other perks: the components can be long-lived and handle thousands of deep charge/discharge cycles with minimal degradation. Flow systems also use abundant materials with low flammability and no risk of thermal runaway, improving safety.

The key, of course, lies in the chemistry of the electrolyte. Researchers have experimented with all sorts of redox pairs over the years. The most common flow battery chemistry uses vanadium ions in different charge states (vanadium redox flow batteries), which was pioneered back in the 1980s.

Vanadium is popular because you can use the same element on both sides (vanadium can exist in several ionic states), avoiding cross-contamination. Other chemistries have been tried too: iron–chromium systems were early examples, as were zinc–bromine, and newer variants use organic molecules or metal pairs like iron–salt solutions.

One flow battery product (from ESS Inc) uses iron in a saltwater electrolyte showing that simple, non-toxic salt solutions can store energy. A particularly interesting one is a “saltwater” flow battery being piloted by a Dutch start-up, AquaBattery. Their system uses table salt (sodium chloride) in water along with acid and base solutions to store energy via reversible water chemistry. This saline solution approach promises a safe, abundant, and low-cost electrolyte.

If flow batteries have such great potential and have been talked about for decades, why are they only now emerging from the lab into real projects? The truth is, they’ve been around for a while, but only in limited pilot projects.

As with any new technology, early units were expensive and investors (and utilities) wanted to see the tech prove itself at scale. It’s a classic chicken-and-egg: you need volume to cut costs, but need lower costs to get volume. Over the last few years, though, we’ve started to see flow batteries move out of trial phase.

For instance, Japan embraced the idea early – Sumitomo Electric ran what was for a long time the world’s largest flow battery project, a 60 MWh vanadium flow system in Hokkaido that went into operation in 2015. That record was only recently overtaken by a larger 700 MWh flow battery in China.

In Europe, adoption was slower, but it’s picking up now. In the UK, a 5 MWh vanadium flow battery – the country’s largest – was installed as part of the Energy Superhub Oxford project in 2022 integrating with a large lithium battery to combine strengths.

And as noted earlier, companies are now actively bidding flow battery projects into national schemes. Root-Power, just announced it has submitted four flow battery projects (total 2.4 GWh across 8-hour duration systems) into Ofgem’s new LDES cap-and-floor support tender.

In the Netherlands, a saltwater-based flow battery pilot launched in late 2024 at the Deltares campus in Delft a partnership with Statkraft to test 10-hour energy storage with simple salt solutions. These are big steps out of the lab and into the ground.

Early adopters and demonstration projects have been crucial to prove that flow batteries can work reliably at scale. Costs are gradually coming down as manufacturing ramps up and companies learn from pilots.

We’re also seeing supportive policies (for example, government incentives for long-duration storage and grid resiliency needs) that make these projects more viable. Europe, in particular, is waking up.

As more projects get built and run, it creates a positive feedback: investors gain confidence, production scales, and costs fall, which in turn spurs more projects. It’s the same trajectory lithium batteries followed a decade ago.

In summary

Flow batteries promise to revolutionize energy storage by providing long-duration capacity. They store charge in liquid tanks rather than solid materials, allowing virtually unlimited scalability in capacity.

While the technology had a slow start, real-world deployments are now happening – from Japan to China, the UK and Europe – proving their worth outside the lab.

I don’t often make predictions but here goes… In 2015 I said the global Lithium Ion battery market (BESS) would be huge as we move towards sources with less inherent inertia – it’s worth around $30 billion in 2025 and predicted to be well over $100 billion by 2032.

Today (unless we see the rapid rollout of some game changing generation technology like Nuclear fusion) flow batteries will be the next $100 billion dollar energy sub-industry. There, I’ve said it.