I think what ARES is doing has the biggest potential: running heavy trains on a closed loop uphill to store energy and downhill to generate. They claim 80% char

I think what ARES is doing has the biggest potential: running heavy trains on a closed loop uphill to store energy and downhill to generate. They claim 80% charge/discharge efficiency which is quite good. The technology is very well understood and mature and the environmental effects are lower than building a reservoir.