Micro/nano-encapsulated phase-change materials (ePCMs) for solar photothermal absorption and storage: Fundamentals, recent advances, and future directions

Building on their dual functionality for solar photothermal absorption and storage, slurries/dispersions of micro/nano-encapsulated phase-change materials (ePCMs) are capable of revolutionizing the solar-thermal industry. Yet, to facilitate their transition from the research and development stage into market adoption and penetration, there is a dire need for a methodical understanding of the design criteria, fabrication techniques, application areas, and technical challenges of these novel solar concepts in light of state-of-the-art advances. This work thoroughly addresses these needs with a focus on slurries/dispersions with solid-liquid PCM cores for latent heat storage and surface-engineered shells for solar radiation extinction. By dividing this study into four parts, we start with an overview of the material types, desired attributes, and key challenges of PCMs; the different types of PCM systems; and their potential applications in the solar energy industry. We then focus in the second part on ePCMs in indirect (surface-based) and direct (volume-based) solar-absorption systems in terms of their functional requirements, encapsulation methods, stability metrics and assessment, compositional and structural characterization techniques, measurement of thermophysical properties, and key design parameters. The third part of this work is dedicated to the theoretical foundation necessary to model and simulate solar ePCM systems, including continuum, discrete, and multi-scale modeling approaches for flow and heat transfer in ePCM slurries/dispersions; thermophysical property correlations; melting theory in PCM capsules; radiation transfer and optical properties evaluation; and energy performance analysis. In the final part, recent breakthroughs in multi-functional shell engineering, molten-salt encapsulation, multi-scale modeling, contrasting ePCMs and nanofluids, and ePCM-based optical filtration are highlighted. By striking a balance between fundamentals and applications, this work aims to serve as a comprehensive foundation for newcomers into this promising field of research as well as an updated critique for experts looking to identify knowledge gaps, technical bottlenecks, latest advances, and future directions.