Experiments have been performed on the stable supercooling characteristics of sodium acetate trihydrate (CH3COONa:3H2O) within three shapes of thermal storage units: sharp rectangular, rounded rectangular, and cylindrical. The phase change material (PCM) in the rounded rectangular unit was prone to be more stably supercooled than the PCM in the sharp rectangular one. The cylindrical unit is also desirable for stable supercooling but contains much less PCM. The effects of several factors on stable supercooling were then investigated in the rounded rectangular unit; these included charging time, cooling rate, the inner surface roughness of the PCM container, the carboxymethyl cellulose (CMC) content, and the salt water mass ratio in mixture. The experimental results showed that more stable supercooling could be achieved with longer charging periods for fully melting crystals, a relatively lower cooling rate, an appropriate CMC content, and a lower inner surface roughness. Additionally, stable supercooling is achieved with the salt water mass ratios of 3:2, 1:1, and 2:3 in mixture. The results in this paper will be useful for the unit and system design of solar energy seasonal storage for space heating, by utilizing the supercooled inorganic phase change materials. (C) 2017 Elsevier Ltd. All rights reserved.