An energy storage device (2) is provided. The energy storage device (2) comprises a storage chamber (8), wherein the storage chamber (8) comprises a port (4, 6) configured to pass a heat transfer medium via piping (16) connectable to the port, a heat storage material (10) disposed in the storage chamber (8) and configured to exchange thermal energy with the heat transfer medium when the heat transfer medium flows through the energy storage device (2) via the port (4, 6), and a flow control arrangement (20) arranged in a transition area (18) between the port (4, 6) and the heat storage material (10). The flow control arrangement (20) is controllable to adjust a flow of the heat transfer medium through a boundary region (25) between the transition area (18, 19) and the heat storage material (10).