To add flexibility to heating and power generation energy systems, long-term, high-capacity energy storage will be essential to balance supply and demand. As variable renewable energy makes an increasing contribution to power generation, the requirement for hydrogen as an energy storage medium will increase dramatically to balance the power transmission grid.
The optimum method of power and energy storage depends on the storage duration, energy release rate and storage capacity. Batteries and flywheels are suitable for kilowatt-hour (kwh) of power to be released within seconds, minutes, or hours. Pumped-hydro schemes can release gigawatt-hour (gwh) of power over a time span of minutes or hours. Underground molecular energy storage of hydrogen or natural gas is effective when gwh or terawatthour (twh) of energy storage is required over days, weeks, or months.