A Review of Solar Thermal Energy Storage Systems Using Phase Change Material

—Solar thermal energy systems are widely used for renewable energy applications such as water heating, space heating, solar drying, and industrial thermal processes. However, the intermittent nature of solar radiation limits their ability to provide a continuous and reliable thermal energy supply. Thermal Energy Storage (TES) systems employing Phase Change Materials (PCMs) have emerged as an effective solution to overcome this limitation due to their high latent heat storage capacity and nearly constant temperature operation during phase transition. This review paper presents a comprehensive analysis of PCM-based solar thermal energy storage systems. Various thermal energy storage technologies and classifications of PCMs, including organic, inorganic, and eutectic materials, are discussed along with their thermal properties, advantages, and limitations. The review examines the application of PCMs in solar water heating systems, solar dryers, photovoltaic/thermal (PV/T) systems, concentrated solar power (CSP) plants, and building thermal management applications. Performance enhancement techniques such as nano-additives, fin-assisted heat transfer, encapsulation, and metal foam integration are critically evaluated. A comparative analysis of different PCM categories and enhancement methods is presented based on thermal performance, energy storage capability, and practical feasibility. Furthermore, current research gaps, technical challenges, and future research opportunities are identified. Experimental and numerical studies reported in the literature indicate that PCM integration significantly improves thermal efficiency, heat retention capability, charging–discharging performance, and energy availability during periods of low solar radiation. The review concludes that PCM-based thermal energy storage systems offer a promising approach for improving the efficiency, reliability, and sustainability of solar thermal energy applications. Continued advancements in advanced materials, hybrid storage technologies, and intelligent thermal management strategies are expected to further accelerate their commercial adoption in future renewable energy systems.