Thermal energy harvesting technologies based on composite phase change materials (PCMs) are capable of harvesting tremendous amounts of thermal energy via isothermal phase transitions, thus showing enormous potential in the design of state-of-the-art renewable energy infrastructure. Great progress has been recently made in terms of enhancing the thermal energy storage capability, transfer rate, conversion efficiency and utilization of composite PCMs. Although there are some recent reviews on composite PCMs, they are mainly concentrated on the thermal transfer enhancement and conventional utilization of PCMs. There are few systematic reviews concerning optimization strategies of PCM for thermal energy conversion. In particular, advanced multifunctional utilization of PCMs is still in its infancy. Herein, we systematically summarize the optimization strategies and mechanisms of recently reported composite PCMs for thermal energy storage, thermal transfer, energy conversion (solar-to-thermal, electro-to-thermal and magnetic-to-thermal conversion) and advanced utilization (fluorescence emission, infrared stealth technologies, drug release systems, thermotherapy and thermal protection), including some novel supporting materials (BN nanosheets and metal organic frameworks (MOFs)). Simultaneously, we provide in-depth and constructive insights into the correlations between the structural optimization strategies and thermal performances of composite PCMs. Finally, future research trends, alternative strategies and prospects are also highlighted according to up-to-date optimization strategies.
