Fig. 4: Volumetric energy density versus temperature plots for various types of heat storage materials.

The data other than the present data are taken from the references^{1,2,3,4,5,6,7,8,9}. The heat storage materials represented by blue and red change the initial host structure when accommodating gas molecules, where gas molecules are kept inside heat storage materials in the blue category but the bonding of molecules are broken inside heat storage materials in the red category. In contrast, heat storage materials denoted by green can absorb gas molecules while maintaining the host structure. The figure in parentheses denotes the ratio of “number of atoms in guest” to “number of atoms in host”. The dashed lines are depicted for water (heat capacity 4.18 kJ K⁻¹ kg⁻¹, density 1000 kg m⁻³) and cast iron (0.46 kJ K⁻¹ kg⁻¹, 7280 kg m⁻³) given that the reference temperature is set to 0 °C. The green curve indicates the ideal maximum energy density per dehydrated substance volume (based on the manner presented here) that is estimated for water-intercalation materials.