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The thermochemical energy storage properties of calcium hydride (CaH2) destabilised with either silicon (Si) or CaxSiy compounds at various molar ratios, were thoroughly studied by a combination of experimental and computer assisted thermodynamic calculations. Particularly, the destabilisation effect of Si on CaH2 at five different molar ratios (1:1, 1:2, 2:1, 3:4, 5:3 CaH2 to Si) was extensively investigated. Theoretical calculations predicted a multi-step thermal decomposition reaction between CaH2 and Si forming CaxSiy at varying temperatures, which was confirmed by in-situ synchrotron X-ray diffraction, differential scanning calorimetry, thermogravimetric analysis and mass-spectroscopic measurements. The most suitable destabilisation reactions between CaH2 and Si or CaxSiy that meet the criteria of a thermal energy storage system for the next-generation of concentrated solar power (CSP) plants were identified. The CaH2 and CaSi system (in a 2:3 molar ratio of CaH2 to CaSi) showed desirable operating conditions with a decomposition temperature of 747 ± 33 °C at a hydrogen pressure of 1 bar. Pressure composition isothermal measurements were conducted on this system to determine its practical enthalpy of decomposition to form Ca5Si3. The calculated value (107.3 kJ mol−1 H2) was lower compared to the experimentally determined value (154 ± 4 kJ mol−1 H2). This mismatch was mainly due to the formation of CaO and a CaSi solid solution in addition to the desired Ca5Si3 phase.