I looked at that in the past: storing energy in a massive ball going up and down a well. But you need tons of mass traveling a long distance. The energy density

I looked at that in the past: storing energy in a massive ball going up and down a well. But you need tons of mass traveling a long distance. The energy density is just too low. To give you an idea: E(energy in Joules) = mgh(mass in kg x gravity in m/s2 x height in m). So one ton (1kkg) traveling 10m would store about 100J which is about 27Wh (1J = 1Ws), which is about nothing.