This work presents the impact of reactor design on the thermal performance and energy storage during the dehydration of salt hydrate of thermochemical material; magnesium chloride hexahydrate (MgCl2.6H2O). The dehydration process is performed by flowing hot air through the material. A transient 2D axisymmetric mathematical model for an open thermochemical long-term heat storage reactor by using COMSOL Multiphysics software is presented. Two configurations of the reactor design are considered; cylinder and truncated cone having the same height and volume of seven designs; cylindrical (base design), convergent truncated cones of inlet to outlet area ratio (AR) 1.4, 4 and 5.8 and divergent truncated cones of AR 0.71, 0.25 and 0.17. Results show that the reactor of lower AR has the lower charging time and higher pressure drop and temperature difference. However, the reactor design hasn’t great impact on the maximum value of water content concentration inside the thermochemical material. Maximum variation of the energy storage of the thermochemical material is about 25.5% and the dehydration time more than three times due to design reactor changing. Maximum stored energy is achieved for the reactor truncated cone of AR 1.4 while the minimum desorption time is obtained for the cone of AR 0.17%.