Abstract: A heat-storage apparatus providing for especially efficient charging and discharging of a storage medium with heat and greatly facilitating simultaneous charging and discharging of different regions of the medium. The apparatus comprises a vessel having insulating walls and containing a heat transfer fluid bath and a plurality of mobile heat storage elements, each of which contains a phase change material for the storage of heat, and which are disposed within the fluid bath in a sufficiently loose configuration to allow heat-storage elements to migrate about the vessel. A charging conduit and a discharge conduit are disposed within the vessel in the fluid bath so as to induce a migratory circulation of heat-storage elements between regions where elements are being charged and regions where elements are discharged.