A heat storage device (100, 300, 600) for an air conditioning apparatus comprises: a phase change material, PCM (102, 302, 602); a plurality of thermally conductive elements (104, 304, 604); and a heat spreader (106, 306, 606) in thermal contact with the thermally conductive elements, the heat spreader for transferring heat from a heat exchanger of the air conditioning apparatus to the thermally conductive elements, wherein the thermally conductive elements are embedded within the PCM.