Efficient thermal energy conversion and storage enabled by hybrid graphite nanoparticles/silica-encapsulated phase-change microcapsules

The ability to achieve efficient solar energy utilization via photo-thermal conversion underscores the need for efficient working fluids in solar thermal collectors. However, traditional working fluids suffer from a set of disadvantages, including low heat storage density, low efficiency, and poor heat transfer efficiency, thereby restricting effective use of solar energy. Herein, we report the crafting of robust microencapsulated phase change composites as a heat transfer fluid that manifests high heat storage capacity and outstanding photo-thermal conversion performance. Specifically, microscopic composites, composed of paraffin/nano-graphite microspheres that are capped by a silica (SiO2) shell, are constructed via in situ polycondensation of tetraethoxysilane (yielding SiO2) in the presence of nano-graphite-encapsulated paraffin microspheres, where paraffin functions as a phase change material. The resulting composites are referred to as paraffin/nano-graphite (NG)@SiO2 microcapsules and exploited as a heat transfer fluid. The melting temperature and latent heat of paraffin/NG@SiO2 microcapsules are 49.4 °C and 82.5 J g−1, respectively. Particularly, the thermal conductivity of microcapsules with a hybrid NG/SiO2 shell (1.562 W m−1 K−1) is found to be enhanced by ∼500% compared to that of paraffin (0.256 W m−1 K−1). More importantly, the visible light absorption rate of microcapsules with a hybrid NG/SiO2 shell has increased up to 80% compared with that of single SiO2 shell microcapsules (almost zero absorption). The addition of the as-crafted microcapsules in water not only enhances the thermal conductivity and heat capacity of the base fluid, but also boosts its optical–thermal performance as a heat transfer medium in direct-absorption solar collectors (DASCs). Notably, the receivers based on this heat transfer fluid (i.e., paraffin/NG@SiO2 microcapsules) are found to retain more than 80% of solar-to-thermal efficiency over a temperature range from room temperature to 60 °C. Importantly, paraffin/NG@SiO2 microcapsules maintain their phase transition after 50 melting–freezing cycles with no leakage of paraffin found. Such a high heat storage capacity and photo-thermal conversion capability highlight the potential of paraffin/NG@SiO2 microcapsules to store solar energy for practical applications.