Heat transfer and thermal performance investigation on a visualized latent heat storage system in pilot-scale: Scalable medium-temperature thermal energy storage system

Abstract

Large-capacity medium-temperature (100–300 °C) and high-temperature (>300 °C) latent heat storage (LHS) systems can enhance the reliability and performance of solar thermal utilization systems due to their characteristics of balancing the gap between energy supply and demand. In this study, a pilot-scale LHS system was developed and investigated, incorporating a mass of 270 kg of phase change material (PCM), specifically a commercial nitrate mixture. Moreover, the thermal energy storage unit featured a rectangular aperture at its center, covered by a quartz visualization window to facilitate the observation of the melting and solidification behavior of the PCM. The charging process lasted 111 min, with a mean average power, accumulated energy and efficiency of 11.90 kW, 79.33 MJ, and 77.67%, while those of the discharging process were 56.5 min, 12.99 kW, 44.11 MJ, and 71.41%, respectively. Furthermore, a comparative analysis was conducted concerning heat transfer performance and mechanisms during the charging and discharging processes. The heat transfer coefficients and average thermal resistances of the LHS system during charging and discharging were 613.08 W/(m2·K), 295.28 W/(m2·K), and 0.001984 (m2·K)/W, 0.005285 (m2·K)/W, respectively. The average thermal resistance observed during the charging process was notably lower than that observed during the discharging process, attributable to the variance in the predominant heat transfer mechanism. For a substantial portion of the charging process duration, heat transfer was dominantly governed by natural convection, whereas throughout a significant duration of the discharging process, heat transfer was primarily driven by thermal conduction.