Abstract: An Sr2+-doped BaCO3 matrix incorporating polyethylene glycol (PEG-6000) was developed using a simple impregnation process to build a shape-stabilized functional phase transition composite for application in thermal energy storage systems. A direct hydrothermal process was used to prepare a unique anhydrous rhombohedral BaCO3. The developed rhombohedral material, characterized following a hydrothermal reaction in the presence of 5 mol% Sr2+, was used to prepare extremely tiny porous particles (5 % SrBaCO3). The 5 % SrBaCO3 powder was hydrothermally produced and utilized to encapsulate PEG to create a phase change material (PCM) that prevents leakage of PEG during phase change. The shape-stabilized composite PCM (ss-PCM) that was prepared with nano- and micro-sized porous SrO powder and containing pure BaCO3 exhibited consistent and superior performance throughout a significant number of heat cycles. DSC results indicated that the melting temperature of the PEG/5BaCO3 is 60.17 ◦C, and the latent heat is 148.8 J/g. The R% (Impregnation ratio) and Eeff% (Efficient energy per unit mass of PEG) values of 5 % SrBaCO3 were more than those of 10 % SrBaCO3 and 15 % SrBaCO3, probably due to the presence of micro- and macro-pores and high pore volume. Additionally, it was clear from the XRF, EDX, and XPS data that Ba2+ segregation in 5 % SrBaCO3 was considerably less than that in 10 % SrBaCO3 and 15 % SrBaCO3. Compared to the long-established ss-PCMs, the developed composites exhibited much greater thermal enthalpy, higher thermal conductivity, and lesser super cooling. The presence of OH− 1 on the surface of the PCM contributed to a reduction in the super cooling. Furthermore, the supporting materials improved the thermal conductivity of the PEG, overcoming the organic PCM’s poor thermal conductivity. As pure PEG is infused in 5 % SrBaCO3, its super cooling effect is diminished by 15.5 %. Overall, the PCM with 5 % SrBaCO3/PEG preserves its capacity to store and release energy without a substantial modification its properties after multiple thermal heating and cooling cycles.