-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 similar system using zinc, where charging the battery deposits the metal onto electrodes from a basic solution of zincate ions, periodically wiping off the metal which falls through to another chamber where it can be re-oxidised by air to discharge the battery.
Enter the flow
There is also intense activity around redox flow batteries – which use reduction and oxidation reactions but store the redox-active species in two liquid electrolyte solutions. These are pumped through a central stack containing inert electrodes separated by a membrane. The oxidised and reduced products are kept in separate external reservoirs, where they can sit until the battery needs to be charged or discharged. ‘In principle, they are infinitely scalable, because you decouple the storage of the fuels from the electricity generation pod,’ explains Clare Gray, a materials and battery chemist from the University of Cambridge, UK.
Redox flow batteries could be the ideal technology for energy storage. ‘They’re very flexible compared to these things that need salt caverns and big geographical spaces,’ says Kathryn Toghill, an electrochemist at Lancaster University in the UK. Plus they can deliver back electricity very quickly to respond to demand. They have a longer life than lithium-ion batteries because there are no structural changes at the electrodes to interfere with performance. ‘If you have a fading capacity, you can top up the tank, which is clearly less disruptive than replacing a stack,’ says Mezzavilla.
The next step is finding the right chemistry to optimise the energy density of flow batteries. One important element is choosing substances that have high solubility to increase the concentration of active species. In aqueous solution the cell needs to run at a potential where water is stable or it will itself electrolyse. According to Gray, this means that ‘you’ve only got about 1.2V to play with’.
The most commercially advanced redox flow battery technology is the vanadium redox flow cell. Several companies are already supplying th