Numerical investigation on simultaneous charging and discharging process of molten-salt packed-bed thermocline storage tank employing in CSP plants

Based on the local thermal non-equilibrium theory, a modified transient two-dimensional numerical model is developed to investigate the simultaneous charging and discharging performance of a molten-salt packed-bed thermocline tank employed in concentrated solar power plants. Two different types of operation including that of stable and periodic charging with steady discharging are analyzed. Three different solid fillers are utilized, including quartzite rock, slag pebbles, and alumina ceramics. The effects of charging/discharging flow rate ratio and non-charging periods on thermocline thickness and energy storage power have been investigated. The results show significant influences of the charging/discharging flow rate ratio. When charging is steady, the discharge performance of a packed-bed tank is better than that of a pure molten-salt tank at lower flow rate ratios. The thermocline thickness of the packed-bed tank is higher than that of a pure molten-salt tank for different solid materials. Compared with quartzite rock and slag pebbles, alumina ceramics achieves a higher thermal gradient. In periodic charging mode, as the cycle's number of charging periods increases, thermocline thickness increases linearly. Operating at higher charging rates and shorter non-charging periods, the discharge outlet temperature of molten-salt becomes more stable for the steam generation process and hence, the discharge performance is improved.