Two kinds of new bio-chars were produced from carbonization of waste hazelnut wood as low-cost and eco-friendly supporting matrices to simultaneously solve the seepage and low thermal conductivity problem of capric acid (CA) used as a phase change materials (PCM) for thermal management applications. In the prepared seepage-free composites, the CA was impregnated by 52 and 64 wt% into porous structure of carbonized hazelnut wood (CHW) and activated carbonized hazelnut wood (ACHW), respectively. The SEM analysis exhibited that the CA was well confined by CHW and ACHW. FTIR and XRD investigations confirmed the existence of good chemical compatibility between CA and CHW/or ACHW. DSC results indicated that the seepage-free CHW/CA and ACHW/CA composites have melting points of 28.5 degrees C and 28.9 degrees C, and melting enthalpies of 111.3 and 110.3 J/g, respectively. TG analyses revealed that the functioning temperatures of the composites were considerably lower than their thermal degradation temperatures. The LHTES properties and chemical structure of the composites was not altered throughout 1000-cycling thermal test. The thermal conductivity of CHW/CA and ACHW/CA were 2.70 to 3.05 times higher than that of pure CA. The decrease in melting/solidification time of the composites proved the improvement in their thermal conductivities compared with pure CA. Consequently, the produced bio-chars can be evaluated as supporter and thermal conductivity enhancer materials (TCEMs) for PCMs; besides, the fabricated composite PCMs can be considered as hopeful admixtures to fabricate of cost-effective, eco-friendly and energy-efficient new types of mortar, concrete and plaster which can be used for thermal management implementations in buildings.