Evaluation of the energy flow characteristics and efficiency of photothermal functional phase change materials for efficient solar thermal harvesting

Photothermal functional phase change materials (PCMs) have attracted considerable attention due to their large energy density, which can solve the inherent imbalance defects of solar energy. However, the efficiency determination of the PCM-based photothermal utilization process (including photon absorption, photothermal conversion, thermal storage, and thermal release) is still unclear, especially with different or even contradictory quantitative indexes for the same process, resulting in inaccurate photothermal utilization performance and unfair comparison in various efforts. Herein, we clarified the photothermal utilization sub-processes of phase change composites via optical characterizations and photothermal conversion experiments and highlighted the energy dissipation mechanism of photothermal conversion process by fluorescence and femtosecond transient absorption examination. Besides, the energy flow features of these sub-processes were explored by determining the energy flow and optical/thermal losses. More importantly, we standardized the efficiency of the four subprocesses by proposing evaluation indexes and established the relationship among the four sub-processes to derive the total photothermal utilization efficiency. This work provides a paradigm for a comprehensive investigation of PCM-based photothermal utilization systems, especially establishing consistent criteria for subsequent efficiency determination, laying a solid foundation for developing and quantifying solar thermal utilization systems.