Nano-Engineered Organic Solid–Liquid and Solid–Solid Phase Change Materials for Efficient Thermal Energy Storage in Renewable Energy Systems: A Comparative Review

Efficient and sustainable thermal energy storage is essential for advancing renewable energy systems and reducing carbon emissions. Organic phase change materials are attractive for this purpose because of their high latent heat, chemical stability, and design flexibility. However, a clear comparative understanding of nano-engineered solid–liquid and solid–solid systems remain limited. This review analyzes developments from 2015 to 2025 to evaluate how nano-engineering strategies including nanofiller incorporation, encapsulation, polymer crosslinking, and hybrid composite design affect thermal performance, durability, and sustainability. The findings show that optimized nano-enhancement can increase thermal conductivity by up to an order of magnitude while maintaining more than ninety percent of the latent heat capacity and improving long-term cycling stability. Solid–liquid materials provide higher energy density but require containment to prevent leakage, whereas solid–solid systems offer superior mechanical integrity and service life. Hybrid and bio-derived composites achieve balanced performance while lowering environmental impact. This review establishes a unified techno-economic-environmental framework for designing advanced nano-engineered phase change materials, emphasizing recyclable matrices, sustainable nanofillers, and circular design strategies as key enablers of scalable, low-carbon thermal energy storage.