f high demand.
Storing renewable energy isn’t the only way we will reach net zero. As well as reducing our energy consumption there is also a push towards interconnecting electricity grids across Europe and ultimately even across the world, so renewable energy can be moved to where it’s needed. 2021 saw the North Sea Link begin operating, a 720km 1400MW sub-sea interconnector linking the UK and Norway electricity systems, and adding to existing links to Belgium, France and the Netherlands. ‘It is definitely part of the solution,’ says Woodward, but it can’t guarantee a supply – storage capacity will still be crucial.
Batteries and physics
Lithium-ion batteries are the one storage technology that has made progress in the last decade (see Building better batteries). ’There’s been massive investment going into this technology simply because [of] electric vehicles,‘ says Schmidt. But while the lithium-ion battery is king for short-term storage – up to four hours – the technology isn’t ideal for the medium- to long-term storage that the grid needs. The batteries suffer from power fading over multiple cycles, limiting their lifetimes, and there are also safety and reliability issues – a number of fires caused by thermal runaway have been reported, such as the major data centre fire in Pangyo, South Korea, in 2022 which caused disruption to internet service for days.
With lithium prices rising (see The lithium rush), the costs are likely to be too high for long-term storage, which Schmidt defines as ‘any technology that is economic when discharging for more than eight hours’. One alternative idea is to use gravity in ways that are less geographically limited than pumped hydrothermal systems. It can be as simple as using a crane to lift concrete blocks. Schmidt says it’s a little more sophisticated than it sounds but has the advantage of using ‘super cheap materials’. One such project comes from Edinburgh-based Graviticity, who plan to cite their storage in disused mineshafts. In 2021 the company constructed a 250kW grid-connected demonstration project at the Port of Leith, using a 15m high rig with two 25 tonne weights suspended by steel cables.
Another concept is storing energy in compressed gases, which can be released to drive a turbine. This is also not new – the first compressed air plant was built in 1978 in Huntorf in Germany, where the air is housed at a pressure of 100 bar in 310,000m3 salt caverns, 600m deep. It can power a 320MW turbine to produce two hours’ worth of electricity. In the UK, the technology is being developed, including a project proposing the air is stored in deep ocean bags, attached to the seabed, with the water above acting to pressurise the system.
A large part of the solution is likely to be electrochemical, says Pasidu Pallawela, founder and chief technology officer of Edinburgh-based battery developer StorTera, because this can provide more flexible systems that can be installed in any location. The current landscape is ‘very noisy,’ says Pallawela, with many labs claiming they have the right technology to crack the problem. But from his experience, most will reach ‘a valley of death’. ‘Even if you have a great working battery, it’s so difficult to take it out there and install in a custom location. There’s so much to do to get to that point.’
‘Eventually, every technology will come down to how much it costs you to store one kilowatt hour,’ says Stefano Mezzavilla, chemistry lead at battery developer RFC Power. And that is where some electrochemical systems that use expensive and scarce metals fall down. Batteries making use of cheaper metals are receiving a lot of attention, says Schmidt, including an iron-air battery from US-based Form Energy which has been designed to store electricity for 100 hours using only iron, water and air. The company describe the electrochemical process as ‘reversible rusting’, supplying oxygen to discharge the battery, converting iron metal to rust and using an electrical current to charge the battery and reverse the reaction. Toronto-based e-Zinc have developed a 