Two kinds of bio-carbons were developed by carbonization/and chemical activation of walnut shell (WS) as lightweight, porous, low-cost and environmental friendly supporting matrix. The produced walnut shell carbon (WSC) and activated WSC (AWSC) were evaluated as novel shape stabilizer and thermal conductivity enhancer for methyl palmitate (MP) preferred as phase change materials (PCM) for thermal controlling applications in buildings. The surface area and pore volume of WS was increased by 1.47 and 1.44 times, respectively after its chemical activation process. WSC and activated AWSC significantly prevented the leakage of liquid MP through phase change due to their suitable adsorption capacities reaching 43 and 55%, respectively. The SEM investigation demonstrated that the MP was well confined into pores of CHW and ACHW carriers. The FTIR and XRD examinations proved the presence of well physicochemical compatibility between the components of the leak free composites. The DSC measurements indicated that the melting temperatures of WSC/MP and AWSC/MP were 26.27 and 26.65 degrees C, and corresponding fusion enthalpies were 108.3 and 138.1 J/g, respectively. The leak-free composite PCMs were showed remarkable chemical structural stability and thermal reliability after 1000 heating/cooling cycles. Both composites had considerably high thermal degradation stability. The thermal conductivity of leak-free WSC/MP and AWSC/MP was measured as about 1.9 and 1.58 times higher than that of MP. All findings revealed that the leak-free composite PCMs can be used as admixture in the manufacture of lightweight, cost-effective, eco-friendly and energy-saving construction elements utilized for solar thermal controlling of buildings.