For a chemical heat pump based on reversible solid-gas reactions, two criteria are very important to reach good performances i.e. the energetic density and power, both linked to the reactive medium implementation. An experimental optimization is done on the solid-gas pair CaCI,/CH3NH 2 adding a filling material (expanded graphite) which improves the heat and mass transfer. The results obtained under fixed conditions of temperature and pressure present a maximum in the curve power vs packing density of the solid beyond 25% (in mass) of binding material. The interpretation of the behaviour that the reactive mixing of the solid-gas reactor undergoes could be explained by the theory of percolation. NOMENCLATURE DE energetic density (MJ m -3) AH enthalpy of the reaction (kJ tool -m) ms mass of anhydrous salt put at the initial time (g) M s molar mass of the salt (kg mol-I ) n number of moles exchanged during the reaction Pc constraint pressure (Pa) Pm average power per unit of mass (W kg-i salt) Pv average power per unit of volume (Wm -3 mixing) SP salt packing (mol m-3) t time (s) Te constraint temperature (ยฐC) V useful volume for the reactive medium (m 3) X conversion rate