A bulk energy storage device for an energy source. The energy storage device includes a circular or conveyor, such as a rail track, which may be formed with slopes. An electrical conductor, such as a third rail or overhead electric cable, connects to an electric grid. Coupled masses, such as rail cars, which travel on the conveyor, have motor/generators that are electrically connected to the conductor. The coupled masses can be raised on a magnetic field and driven by a linear electric motor having a stator embedded in the track and a rotor attached to the undersurface of the masses. Power from the grid is drawn via the third rail to propel and store energy in the ring of masses. When the grid needs to draw power from storage, the linear motor acts as a generator, supplying power to the grid by converting mass momentum into electric power.