Plans for a 100%-renewable powered Los Angeles took a big step forward with what’s claimed to be a world-first order for gas turbines designed to run on green hydrogen produced using wind and solar, and stored in vast underground caverns the size of the Empire State Building.

The two combined-cycle gas turbines will in 2025 replace coal-fired generation at the Intermountain Power Plant (IPP) site in Delta, Utah, run by the Los Angeles Department of Water and Power (LADWP) and which already sends power to the city and other parts of California.

The gas turbines will enter operation fuelled by a 30% blend of green hydrogen, with the remainder natural gas. But the 840MW of turbine capacity is due to complete a phased switch to 100% green hydrogen in 10 years or sooner, creating what LADWP claims will be a dream match of the zero emissions of renewables and the dispatchable, reliable supply of a conventional generator.

Turbine provider Mitsubishi Hitachi Power Systems said the turbines bought by LADWP for the $1.9bn project mark the industry’s first “specifically designed and purchased as part of a comprehensive plan to sequentially transition from coal, to natural gas and finally to renewable hydrogen fuel”.

The hydrogen used in the turbines will be produced on an adjacent site at a linked project called Advanced Clean Energy Storage (ACES). That will use wind and solar electricity imported from Utah, Wyoming and elsewhere in California to power electrolysis – the process of splitting water molecules into hydrogen and oxygen using an electric current.

The green hydrogen produced will be stored in more than 100 underground caverns, each the size of the Empire State Building, taking advantage of what LADWP called a “one of a kind geological feature” of salt formations that are ideally suited to contain gas.

The green gas could be stored in the caverns for months, way beyond the duration of large battery systems, and used by the IPP’s hydrogen-burning turbines to meet demand peaks or periods when wind and solar output dips.

Each salt cavern will hold 5,500 tonnes of H2, enough to fill up a million fuel cell vehicles, representing storage 84 times greater than a 1.2GWh battery, according to LADWP.

The project has faced questions over the ongoing, albeit diminishing, role for gas, and the cost of the power produced compared to other renewables.

But LADWP insists the combination of ACES and IPP can produce power competitively, and play an important part in helping Los Angeles meet its ambitions for a 100%-renewable electricity mix by 2045 while ensuring supply reliability to a city whose power demands are expected to double by then.

We have prospered as Los Angeles because we have controlled our energy future.

“We have prospered as Los Angeles because we have controlled our energy future, we control our own assets and we have been able to keep the lights on regardless of what’s going around us,” LADWP general manager Martin Adams told a Board of Commissioners meeting in December. “We need to stay in that arena.”

IPP and ACER are among an ever-growing number of hugely ambitious projects globally to put zero-carbon hydrogen centre-stage in the energy transition. They include Shell-led plans for the world’s largest offshore wind deployment to power renewables off the Netherlands, and research to tether hydrogen production to nuclear generation in the UK.