The invention relates to a heat storage system using sand as a solid heat storage medium, characterized in that it comprises the following components: a storage tank (1) for cold sand; a storage tank (2) for hot sand; a fluidized bed heat exchanger (3) arranged therebetween, which is separated from the storage tanks (1, 2) by weirs (4, 5) and divided into a plurality of chambers (7) by weirs (6), wherein the weirs (4, 5, 6) are arranged as a combination of overflow and underflow weirs; wherein means (8) for transferring heat from a heat source to the sand fluidized therein as well as means (9) for transferring heat from the sand fluidized therein to a heat transport medium are provided in the chambers (7) of the heat exchanger (3); and at least one blower (14) positioned underneath the heat exchanger (3) for fluidizing the sand.