NOVELTY - The method involves temporarily flowing working fluid, which is provided for heat supply to and heat removal from a heat storage (33), through a heat storage unit (22). The storage unit is provided with a ballast (24) including ballast elements (25). The ballast is provided with continuous pores between the elements. The storage unit in a heat storage space (23) i.e. container inner space (5), is temporarily mechanically loosened by selectively displacing a portion of the elements into motion relative to each other and by moving the portion into the ballast by a mechanical force effect.    USE - Method for operating a heat storage (claimed) i.e. pressure container, which is used as an energy storage for intermediate storage of heat for an energy production and/or energy storage system e.g. solar thermal and/or solar dynamic power plant, and adiabatic compressed air storage power plant. Can also be used for an energy storage for intermediate storing of mechanical energy, potential energy, and chemical energy.    ADVANTAGE - The heat storage unit in the heat storage space is temporarily mechanically loosened by selectively displacing the portion of the ballast elements into motion relative to each other and by moving the portion into the ballast by the mechanical force effect, thus building a smaller horizontal pressure, avoiding risk of increase of a critical horizontal pressure, and preventing damage or destruction of walls of the heat storage space and/or a heat insulation. The method provides the heat storage with reduced manufacturing expenditure, improved installation space usage, and improved long-term stability.    DETAILED DESCRIPTION - The portion of the ballast is made of a ceramic material. An INDEPENDENT CLAIM is also included for a heat storage.    DESCRIPTION OF DRAWING(S) - The drawing shows a sectional view of a heat storage accommodating a heat storage unit.    Container inner space (5)    Heat storage unit (22)    Heat storage space (23)    Ballast (24)    Ballast elements (25)    Heat storage (33)