Enhancing charging/discharging performance of solar-assisted water heater integrated with stearic acid/expanded graphite as form-stable phase change material

Background

Stearic acid (SA) among organic phase change materials (PCMs) is in high demand due to its high latent heat, suitable melting temperature range, chemical stability, non-toxicity, and cost-effectiveness. However, the practical application of SA, similar to organic PCMs, is hindered due to low thermal conductivity and leakage during phase transitions. These shortcomings reduce heat transfer efficiency, increase system complexity, and cost.

Methods

The present research aims to develop a form-stable stearic acid (FOS-SA) composite as a heat storage material. The composite was characterized for thermal conductivity, FTIR, TGA, and DSC before and after 500 thermal cycling treatments to reveal its stable chemical/thermal properties and their long-term utilization. Furthermore, for its practical utilization, the developed composite was then integrated into a packed bed solar water heater (PBD-SWH), and its thermal performance was compared against pure SA at flow rates of 12, 16, and 20 L/h.

Significant Findings

The composite was characterized for thermal conductivity, demonstrating a 345 % enhancement. FTIR confirmed physical interaction between SA and expanded graphite (EG) without chemical alteration, and TGA validated strong thermal stability, whereas DSC revealed its stable thermal characteristics. The developed composite also exhibits favourable results when it has undergone 500 thermal cycling treatments, confirming its reliability for long-term utilization. The results revealed that FOS-SA reduced charging and discharging times by 11–17 % and 17–26 % respectively, and indicated that 20 L/h is the most effective in enhancing the thermal performance of PB-SWH. This research demonstrates, for the first time, the effective integration of FOS-SA into a PB-SWH. The findings establish FOS-SA as a viable and high-performance candidate, not only addressing leakage and low thermal conductivity issues but also substantially improving the system's thermal performance.