The UK needs a battery that can run for 100 hours. It won’t be easy

In your phone or laptop, batteries earn their keep by being used near-constantly. But the next generation of grid-scale energy storage has the opposite challenge – it needs to sit largely idle for months at a time, before providing electricity continuously for more than four days.

Innovate UK is offering up to £3m funding for studies into new electrochemical storage technologies capable of supplying power for at least 100 hours at rated output, the maximum level of power they are designed for under standard conditions. The systems also need a working life of at least 25 years, with demonstration projects planned by 2030.

Building a battery that can keep going for 100 hours is not as easy as taking today’s lithium-ion batteries and making them much bigger.

“Lithium-ion is very happy at a four-hour duration,” says David Howey, professor of engineering science at the University of Oxford, whose research includes the modelling and management of energy storage systems. “That’s the sweet spot in terms of the ratio of energy to power.” Stretching the same technology to 100 hours quickly becomes very uneconomical.

Longer-term battery power is needed as the UK’s electricity system decarbonises. Wind and solar are world-changing technologies when they work, but there are long stretches in the British autumn and winter when it can be cold, grey and still for days. German has a word for those energy deserts – dunkelflaute, or ‘dark doldrums’.

To fill those gaps, Innovate UK is seeking a class of emerging battery to operate at total ownership costs below $40/kWh by 2030. It is also asking for more experimental systems, intended to compete with gas generation, that can demonstrate a potential route to costing less than $15/kWh by the early 2030s.

None of that is likely to be possible using existing materials. Lithium-ion batteries are prized for how much of the electricity used to charge them eventually comes back out. The alternatives that could last longer are less efficient – iron-air batteries, for example, exploit the reversible reaction between iron and oxygen by repeatedly rusting and de-rusting iron. They are cheaper and more abundant in terms of materials, but their efficiency is only about 50%.

Ordinarily, losing half the energy put into a battery sounds disastrous, but it might not matter. “You don’t care so much about round-trip efficiency,” says Howey, when the electricity that would otherwise be wasted is effectively free.

Those longer-running batteries encounter another engineering problem, however. “You might have issues with pumps and seals and bearings wearing out,” says Howey. Metal-air systems add complicated interfaces between solids, liquids and gases that result in engineers having to worry about erosion contaminants and air electrodes becoming blocked.

All of which makes it a difficult sell to build a battery that is ultimately designed as an energy supply of last resort. “You might be charging and discharging a single-figure number of times over a winter period,” says Jamie Speirs, reader and deputy director of the Centre for Energy Policy at the University of Strathclyde and co-director of the UK Energy Research Centre (UKERC). “How do you make a first investment decision on that plant, on the basis that you have got really uncertain future revenue?”

Technology development is happening alongside changes to the electricity market. Ofgem’s broader long-duration electricity storage scheme requires systems to discharge for at least eight hours, and uses a cap and floor arrangement that gives developers a minimum revenue level if earnings are poor. (If revenues rise above an agreed ceiling, they are returned to consumers.) 16 projects using pumped hydro, compressed air, lithium-ion and vanadium redox flow batteries have been provisionally selected, with final decisions expected this autumn.

The hope is that ultra-long-duration batteries can eventually occupy a gap that lithium-ion cannot economically fill. Howey thinks the UK and Europe are unlikely to wrest conventional lithium-ion manufacturing away from China and other East Asian countries, but says long-duration storage remains an area where there is still “room for breakthroughs” in the West. The trick will be getting one of those breakthroughs out of the lab and onto the grid quickly enough to matter.

Going from an eight hour discharge to 100 hours will be another leap entirely. “It’s really challenging to imagine that you go from having, really, no electrochemical ultra-long-duration storage technologies operating at commercial scale now, but we will have them operating in the early 2030s,” says Speirs. “But that’s not impossible.”