Energy storage devices and methods of using same

A system for energy storage, comprising:

• an energy storage device comprising a first mass;
• a motor/generator having a rotatable shaft configured to rotate in a first direction when electricity is received by said motor/generator and to rotate in a second direction opposite the first direction to cause said motor/generator to produce electricity;
• a spool coupled to said rotatable shaft, whereby said spool and said rotatable shaft rotate in a same direction;
• and a tether attached at a first end to said first mass and wound about said spool between said first end and a second end;
• and a second mass, attached to said tether at a second end opposite said first end, whereby rotation of said spool by said rotatable shaft in the first direction causes said tether to wind onto said spool thereby pulling said first end of said tether and said attached first mass toward said spool, and allowing said second end of said tether to un-wind from said spool, pulled by the movement of said second mass away from said spool, and whereby a movement of said first mass away from said spool causes said first end of said tether to un-wind from said spool thereby rotating said rotatable shaft in the second direction, and also causing said second end of said tether to wind onto said spool thereby pulling said second mass towards said spool;
• and
• an electric cable electrically connected at a first end to an electrical load and electrically connected to said motor/generator at a second end opposite said first end;
• wherein said energy storage device is disposed in a body of water.