Critical review of Design and Material Modifications in Thermal Energy Storage Systems

TES systems are increasingly recognized as essential components for enhancing the flexibility, reliability, and efficiency of renewable-energy-dominated power networks. However, their widespread adoption continues to be constrained by intrinsic material and design limitations: among other problems, low thermal conductivity, substantial cycling degradation, phase separation, leakage in latent-heat media, and slow heat-transfer dynamics limit charge-discharge efficiency. In response, this paper provides an extensive critical review of state-of-the-art design and material modifications developed to advance the performance of sensible, latent, and thermochemical TES systems. On the design side, great interest has been focused on the enhanced heat-transfer structures of extended-surface fin geometries, spiral and helical heat-exchanger coils, metal-foam matrices, and internal conductive networks, while cascaded multi-temperature PCM configurations collectively enhance temperature uniformity, charging rates, and overall exergy utilization. These recent developments have yielded impressive experimental and numerical performance gains; however, they often rely on increased mechanical complexity, higher fabrication expense, and an uncertain reliability upon long-term cycling. In addition, encapsulation technologies, from macro-capsules to microencapsulation and microfluidic shell-formation techniques, have arisen as effective solutions for mitigating PCM leakage and thermal instability but create challenges for shell integrity, thermal contact resistance, and scalability in manufacturing. Material-level modifications have also come to the fore. Nano-Enhanced PCMs with metallic, oxide, carbon-based and hybrid nanoparticles have shown notable improvement in effective thermal conductivity and heat-storage density. Similarly, composite PCMs with graphite foams, expanded graphite matrices, carbon nanotubes, or embedded metal structures have shown improved charge-discharge performances. On the other hand, thermochemical materials modified by using appropriate stabilizers or dopants or composite supports also exhibit improved reversibility and reduced cyclic degradation. Despite their promise, these materials pose a number of challenges in the form of nanoparticle agglomeration, increase in viscosity and sedimentation effects, long-term thermal instability, and environmental concerns during material synthesis and disposal.