A metastable miscibility gap in the C-Al binary phase diagram has been exploited to produce macroscopically solid phase-change enhanced thermal energy storage materials. With 50% by volume of Al or Al-12.7%Si dispersed in a graphite matrix, the materials have thermal conductivity of similar to 150 W/m K, energy densities of 0.9 and 1.1 MJ/L for Delta T = 100 degrees C and energy storage/delivery temperatures centred around 660 degrees C and 577 degrees C respectively. These characteristics are matched to both direct-steam and fluid-mediated concentrated solar thermal power systems using conventional Rankine cycle steam turbine-generator technology. Powder metallurgy processing combined with a low-temperature binder ensures a non-percolating inverse microstructure in which the phase change Al or Al-Si particles are securely encapsulated, thereby overcoming the usual containment problems associated with metallic phase change materials. The graphite and binder system used have been shown to be stable upon short-term thermal cycling or holding at the maximum expected operating temperature.