Phase change material pack

A phase change material (PCM) pack (1) for an air conditioning system comprises phase change material sealed between a first thermally conductive layer (2) forming a first outer surface of the PCM pack and a second thermally conductive layer (2) forming a second outer surface of the PCM pack. At least the first or second outer surface of the PCM pack takes the form of a substantially planar surface having defined therein a plurality of depressions (4) deviating from the planar surface towards the interior of the PCM pack in a direction perpendicular to the planar surface. The depressions improve heat transfer between the pack and an airflow passing over the surface of the PCM pack.