Evaluation of stearic acid/coconut shell charcoal composite phase change thermal energy storage materials for tankless solar water heater

•Cost-effective form-stabilized composites based on coconut shell charcoal (CSC) were prepared.•H2O2 modification on CSC support can increase the loading ratio of PCM.•Enthalpy and temperature in melting are 76.69 J g−1 and 52.52 °C, respectively.•Thermal performances of composite in tankless solar water heater were evaluated. This work presents a cost-effective and environment-friendly form-stabilized phase change material (PCM) and corresponding solar thermal application in the tankless solar water heater (TSWH). Coconut shell charcoal (CSC) as supporting material was modified by moderate oxidant of H2O2 with different concentrations, and then stabilized stearic acid (SA) to prepare composite PCMs through vacuum impregnation. It found that CSC support causes a 15.70% improvement of SA loadage after treated by 15% H2O2 due to coefficient enhancement by physical interaction and surface modification. The modified CSC15 support appears more super macropores which contribute to the impregnation of SA than non-modified CSC0 support verifying from SEM and BET results. And the content of oxygen functional groups was increased after oxidation modification, also motivating SA stabilization by hydrogen bond interaction in XPS analysis. FTIR results proved there is no chemical reaction happened between SA and CSC. Moreover, the latent heat and phase transition temperature of the as-prepared SA/CSC15 composite are 76.69 J g−1 and 52.52 °C, respectively. All composites exhibit excellent thermal stability under a working temperature of 180 °C and form stability during phase change. Thermal energy storage-release test within 70 °C presents the composite has fast heat transfer efficiency than pure SA. The composite filled in TSWH system has 0.75 W m−1 K−1 thermal conductivity which is 2.88 times higher than that of pure SA (0.26 W m−1 K−1). Besides, the TSWH system with a flow rate of 0.004 kg s−1 could heat water effectively after sunset and the energy obtained from the thermal storage system within 1830 s testing times is about 0.15 kW h. In all, SA/CSC composite with good physical-thermo properties has potential in thermal energy storage application, especially in solar energy storage.