Solid–Liquid Phase Change Composite Materials for Direct Solar–Thermal Energy Harvesting and Storage

ConspectusSolar–thermal energy storage (STES) is an effective and attractive avenue to overcome the intermittency of solar radiation and boost the power density for a variety of thermal related applications. Benefiting from high fusion enthalpy, narrow storage temperature ranges, and relatively low expansion coefficients, solid–liquid phase change materials (PCMs) have been viewed as one of the promising candidates for large-capacity STES. Organic and inorganic PCMs, however, generally suffer from poor solar absorption, slow thermal diffusion, and leakage problems, which seriously limit their direct STES applications. Preparing composites through compounding functional fillers with PCMs has been investigated as the mainstream route to enhance their thermophysical properties, but optimizing the charging, storage, and discharging performances of PCM systems remains a grand challenge. Most often, improved STES performances were achieved at the sacrifice of reduced energy storage capacity. Simultaneous enhancement of STES performances has been viable in recent years through fabricating PCM composites with lightly loaded functional fillers and tailoring their heat diffusion and solid–liquid phase change kinetics.In this Account, we discuss recent progress in developing large-capacity solid–liquid STES PCM composites that can achieve rapid direct charging, long-term stable storage, and controlled heat release. Such lightly loaded composites take advantage of rapid transportation of solar photons within PCMs to achieve fast direct absorption-based harvesting and storage of solar–thermal energy. Dynamic manipulation of the solar absorbers by external fields can further accelerate the charging process. Moreover, tuning the interaction between the surfaces of solar absorbers and the PCM molecules not only enables seasonable storage of harvested solar–thermal energy under supercooled states but also allows for controllable heat release through triggering the cold crystallization process. First, we describe design and engineering principles of such direct absorption-based solar–thermal PCM composites. Emphases are placed on introducing the desired features of the solar absorbers to comprehensively enhance thermophysical properties of the lightly loaded PCM composites including solar absorptance, thermal conductivity, form stability, and reduced supercooling through tailoring the size, morphology, and surface chemistry of fillers. We further elaborate the movable charging strategies within which minute solar absorbers are driven by external fields to directionally advance the solid–liquid phase-changing interface, thereby accelerating the charging rates while fully retaining the large latent heat storage capacity of PCMs. The latest development of PCM composites that are capable of stably storing solar–thermal energy as latent heat at room temperature for months or even years is also introduced. In the end, emerging applications of direct absorption-based PCM composites in providing domestic heating, personal thermal management and thermoelectric conversion, and future research needs in this field are discussed.