Abstract Heat storage performance of an open thermochemical heat storage (TCHS) system using composite salt hydrates of Wakkanai siliceous shale (WSS) - 9.6 wt.% LiCl was investigated numerically. A two-dimensional model considering the combined heat and mass transfer was developed. The calculation results were validated by the experimental results obtained in our previous study. The inlet air temperature, inlet air relative humidity, humid air flow rate, the ratio of the thickness of air channel and the thickness of composite solid wall, and length of the TCHS unit were evaluated to evaluate the heat storage performance. When the ratio of the thickness of air channel and the thickness of composite solid wall is 5 and the length of the heat storage unit is 0.1 m, the volumetric heat storage density of the open TCHS system can reach 510 MJ/m3.