Energy "storage" via existing hydro
Actually, there's an even better way of doing "storage" that I hear they're using in Norway (and via a power sharing arrangement with Denmark), which is to use the existing hydro capacity as the load balancer. When there is plenty of solar/wind etc. you "store" energy by NOT drawing from your hydro reservoirs and when there's a shortage, then you open the sluices. The only work required to optimise this would perhaps be to increase the generating capacity at existing hydro stations, so that they can generate the same total amount of energy, but in a more flexible manner and with a higher peak power.