image: (a) Conventional bulk PCMs undergo slow, diffusion-limited melting. (b) In the interface-anchored charging system, directly irradiated CPCPs circulate continuously, keeping the phase-change interface anchored at the irradiated surface and enabling rapid solar thermal charging. (c) Matched energy and mass fluxes during solar charging: cold CPCPs are fed into the illuminated irradiation zone, absorb solar energy during surface exposure, and exit as hot particles after completing the charging process. (d) CPCPs temperature evolving with time for diffusion-limited and interface-anchored solar charging modes. (e) Energy flow diagram of the solar thermal energy storage system.
Credit: ©Science China Press
Solar thermal energy storage is central to making solar energy a dispatchable power source after sunset. Phase-change materials are among the most promising candidates for this purpose, as they absorb substantial quantities of latent heat upon melting and release it upon solidification. However, a long-standing bottleneck has constrained their practical efficiency: as the material melts inward from the irradiated surface, the growing liquid layer forms a progressively thickening thermal barrier, impeding heat transfer to the unmelted core. Consequently, the charging rate decays sharply over time, and the overall storage efficiency plateaus below 2%.
Previous efforts have focused on dispersing high-thermal-conductivity nanofillers, such as graphene or carbon nanotubes, into the storage material. Although these additives enhance heat conduction, they fail to address the underlying mechanism: the phase-change front progressively retreats from the irradiated surface, lengthening the heat-transfer path and increasing thermal resistance over time.
Now, a research team led by Prof. Xianglei Liu at Nanjing University of Aeronautics and Astronautics has introduced a fundamentally new storage paradigm. Rather than accelerating heat conduction through the liquid layer, their strategy, termed the "interface-anchoring" storage mode, prevents its accumulation at the source. Published in Science Bulletin, the approach exploits the gravity-driven circulation of engineered photothermal phase-change particles to continuously renew the irradiated surface with unmelted material, thereby keeping the phase-change front anchored within the concentrated solar irradiation zone throughout the charging process.
The team designed composite particles approximately 2 mm in diameter with a core-shell architecture. The outer shell consists of manganese ferrite (MnFe₂O₄), a broadband solar absorber that captures over 91% of the incident solar radiation and converts it into thermal energy. The core comprises an NaCl-KCl eutectic salt embedded in a percolated magnesium oxide (MgO) framework with lamellar hexagonal boron nitride (h-BN), yielding a thermal conductivity 2.6 times higher than that of the pristine salt. Each particle exhibits a total thermal energy storage capacity of 845 kJ/kg, two to four times that of conventional sensible-heat particles.
In operation, unmelted particles are continuously fed into a funnel-shaped solar receiver under concentrated solar irradiation. Each particle absorbs incident radiation through its MnFe₂O₄ shell and stores the absorbed energy as latent heat via core melting. Fully charged molten particles then exit the bottom of the receiver, while fresh solid particles simultaneously replenish the irradiated surface from the top. Because unmelted particles persistently occupy the irradiation zone, the thermally resistive liquid layer is prevented from accumulating at the phase-change front.
The system was evaluated under a solar concentration of approximately 285 suns with an incident solar power of 1.08 kW. Particles completed melting within approximately five minutes and exited the receiver at a stable outlet temperature of 686 °C. The continuous-flow system delivered a charging power of 0.54 kW and a solar thermal storage efficiency of 49.7%, compared with only 0.02 kW and 1.9% for a stationary bed of identical particles under identical irradiation conditions. Durability testing further confirmed that the particles retained 97% of their initial mass and stable thermal performance after 1,000 thermal cycles.
"The key insight is a conceptual shift," said Prof. Liu. "Rather than enhancing heat transport through a stagnant liquid layer, we eliminate its accumulation altogether by keeping the storage material in motion. We define this as the transition from a 'material-static, interface-retreating' mode to a 'material-moving, interface-anchoring' mode."
The experimental operating conditions, approximately 285 suns of solar concentration and outlet temperatures above 680 °C, fall within the typical operating range of commercial concentrating solar power (CSP) plants, indicating that the proposed interface-anchoring strategy is well-suited for practical deployment.
In future work, the team plans to scale up the continuous-flow system and explore its integration with existing CSP infrastructure for high-temperature industrial process heat and power generation.
See the article:
Qiao Xu, Jingwen Zhu, Yan Wang, Xianglei Liu*, Tao Wang, Jianguo Wang, Shushan Lv, Yongliang Li. Anchoring phase change interface enhances solar thermal energy storage. Science Bulletin (2026). DOI: 10.1016/j.scib.2026.04.024