A gravity-block storage tower banks electricity as the gravitational potential energy of one heavy block riding a cable — the 2D cross-section of the same crane-tower principle as pumped hydro, but with solid mass:
E_stored = m · g · h
Charging (rise): P_mech_up = eta_motor · P_grid
dh/dt = P_mech_up / (m·g)
Discharging (fall): P_mech_down = P_grid / eta_generator
dh/dt = -P_mech_down / (m·g)
P_grid_out = eta_generator · P_mech_down = P_grid (rated)
Round-trip (theory): eta_motor · eta_generator
Round-trip (measured): E_out(full cycle) / E_in(full cycle)
- Charge — the motor draws rated grid power and converts it to mechanical winch power at motor-up efficiency; the leftover heats the motor and is permanently lost. The block climbs at the speed that mechanical power implies for its weight.
- Discharge — the falling block drives the winch as a generator; only generator-down efficiency of the mechanical power extracted from its falling weight reaches the grid, the rest is lost to friction and electrical resistance.
- Round trip — because both stages lose energy, the fraction of grid electricity you get back after a full charge/discharge cycle is the product of the two efficiencies, never either one alone. The simulator tallies actual joules in and out across a full cycle and compares that measured number to the theoretical product live.
Real-world relevance: this is the operating principle behind commercial gravity-storage systems such as Energy Vault's crane towers — a long-duration, non-chemical alternative to lithium-ion batteries for grid balancing.