A heat energy storage system may have a shape-stabilized composite prepared using an easy impregnation method involving a porous Ca2+-doped MgCO3 matrix and PEG as the functional phase. The heat storage capability, microstructures, and interactions with the PEG/CaMgCO3 composite can be characterized by DSC, SEM imaging, FT-IR spectroscopy, and TGA. Likely because of the synergistic phase change effect of CaMgCO3 and PEG, the PEG/CaMgCO3 composites can have high thermal enthalpies, and their enthalpy efficiencies are substantially higher than those of traditional shape stabilized PCMs. The functional material PEG can permeate porous CaMgCO3 matrices under capillary action. Liquid PEG can be stabilized within the porous matrix, and/or the CaMgCO3 matrix can improve the thermal stability of the PEG. The high heat energy storage properties and good thermal stability of such organic-inorganic composites offers utility in a range of applications, including thermal energy storage.