Someone check my working here:<p>"bigger than an Olympic-size swimming pool" - not helpful but I'll assume not much bigger, or they'd compare it to something bigger - so 2.5 ML (million litres), pumped 200 m against gravity with 2.5 x the density of water, in MWh:<p>(2.5e6 * 2.5 * 9.81 * 200) / (3600e6) = 3.4 MWh. That's:<p>- about 5 minutes feeding the 50 MW turbine,
- most of an hour replacing the output of a large (10 MW) wind turbine at typical capacity factors,
- about 0.03 % the capacity of Dinorwig (I believe the UK's largest hydro plant),
- 1 - 2 tonnes CO2 emissions from a natural gas peaker plant,
- the usage of about 3 - 5 g of typical nuclear fuels.
- don't know or care about coal, sorry.<p>If you really want to avoid using gas or nuclear in the UK, and not freezing pensioners, I'd be looking at storing the order of 1-2 TWh. I'll bet that the best competition for water is just building more wind turbines than you need. Any working fluid that doesn't <i>literally fall from the sky</i> just won't scale, and is at best a distraction. Yes, fluid.<p>I'll note an interesting relation between global nuclear deaths and global hydro storage deaths.