There is growing interest in using direct contact heat transfer in thermal storage with phase change materials (PCM). Previous research has predominantly focused on the heat transfer improvement mostly using liquid as the heat transfer fluid, with limited consideration for volume change and pumping losses both of which reduce the useful energy storage density of the system. An experimental investigation was undertaken using air as the heat transfer fluid and water as the PCM subject to freezing only. Unity heat exchange effectiveness was identified over the entire phase change process demonstrating the excellent heat transfer characteristics of this concept. A volume increase of 30% was measured with potential for significant reduction. Pumping losses were found to be significantly higher than expected, and should represent the primary focus of future research. If pumping losses can be reduced, gas based direct contact PCM storage can potentially achieve a higher useful storage density than conventional PCM systems which rely on a large heat exchange area. (C) 2014 Elsevier Ltd. All rights reserved.