Development Of Sustainable Thermal Energy Storage Materials For Solar Energy Applications: A Low-Cost Composite Phase Change Material For Pakistan's Solar Thermal Sector

Abstract

Pakistan's rapidly expanding solar energy sector faces a persistent mismatch between the intermittent availability of solar radiation and continuous energy demand, a gap that thermal energy storage (TES) can help close for both domestic solar water heating and utility-scale concentrated solar power (CSP) applications. Commercially available phase change materials (PCM), while effective at storing latent heat, are frequently imported, costly, and prone to two well-known limitations: low intrinsic thermal conductivity and gradual degradation of latent heat capacity over repeated melt–freeze cycling. This study reports the development and laboratory characterization of a low-cost, locally formulated composite PCM comprising commercial-grade paraffin wax (PW) as the base storage medium, expanded graphite (EG) as a thermal-conductivity enhancer, and locally sourced bentonite clay as a form-stabilizing and leakage-preventing support matrix. Composite samples with EG loadings of 0–20 wt.% were prepared and characterized using differential scanning calorimetry (DSC), thermal conductivity measurement, and accelerated thermal cycling (500 melt–freeze cycles). The optimized composite (15 wt.% EG, bentonite-stabilized) exhibited a melting temperature of approximately 43–44°C, a latent heat of fusion of 178 J/g, and a nearly four-fold improvement in thermal conductivity (0.79 W/m·K versus 0.21 W/m·K for pure paraffin), while retaining over 95% of its initial latent heat capacity after 500 thermal cycles, compared with 63% retention for pure paraffin wax under identical conditions. A preliminary cost comparison indicates the locally formulated composite can be produced at roughly 40–50% of the cost of imported organic PCM products. These results demonstrate that regionally available materials can be used to develop durable, thermally efficient, and economically viable TES media suited to Pakistan's solar water heating, building thermal management, and CSP storage needs.