A heat storage system using sand as a solid heat storage medium has a fluidized bed heat exchanger (3) arranged between and separated from a storage tank (1) for cold sand and a storage tank (2) for hot sand by weirs (4, 5). The heat exchanger (3) is divided into a plurality of chambers (7) by weirs (6). The weirs (4, 5, 6) are arranged as a combination of overflow and underflow weirs. Fluidized sand is produced in the chambers (7) by a blower (14) positioned underneath the heat exchanger (3). Heat is transferred from a heat source to the sand fluidized and from the fluidized sand to a heat transport medium by transferring mechanisms (8, 9) in the chambers (7). The sand is redirected in a horizontal direction by horizontally acting blowers and/or installations (12) projecting into a respective chamber from a side.