Why these people are still seeking funding is beyond me. This is by far one of the best medium term energy storage options and there are shot loads of mine shafts in the north of england and east midlands where they could install them and the grid connection infrastructure is also there as its where most of the coal fired power stations were.
Batteries are ok for short term but not environmentally friendly and green Hydrogen for much longer term but isnt very energy efficient, but this to me hits the sweet spot.
www.ft.com