Reducing the cost of energy storage to make competitive energy mixes 100% renewable without CO2 emissions.
Investment cost: 20 times cheaper than batteries, 10 times cheaper than pumped storage hydroelectricity. Capital cost = $25/kWh (4000 meters depth) and $50/kWh (2000 meters depth)
Energy efficiency: > 80% total roundtrip.
No technical barrier: just buy existing components and put together.
No environemental impact: no land use, no chemical contamination.
For locations far from the sea (> 500 km) Giant pumped-storage hydroelectricity is also an option with an acceptable cost and land use. Unlike conventional pumped-storage, it can be developed almost everywhere, even in flat areas. Open pit mine like réservoirs and economy of scale make the cost of energy storage can be lower than $220 / kWh for 50 GWh units.