Numerical investigation of a vertical triplex-tube latent heat storage/exchanger to achieve flexible operation of nuclear power plants

The current numerical investigation is carried out to assess the possibility of integrating a latent heat thermal energy storage (TES) in a nuclear power plant (NPP) system. This would allow improving the capability of such plants to operate in a load-following mode by eliminating the gap between energy supply and energy demand by the grid. In contrary to previously published TES designs, the phase change material (PCM) triplex-tube containers are vertically oriented in the present study to take advantage of melting/solidification enhancements induced by the buoyancy forces. In addition, two different fluids simultaneously co-circulating within the storage, hence, allowing for this component to operate as a storage system (under extreme charging/discharging conditions) or as simple heat exchanger (under normal steady-state conditions). The computational domain, in this study, is reduced from the full TES geometry to a single triplex-tube container owing to the homogeneous triangular positioning of the annular tubes within the storage vessel. The results show that the time required to fully melt the solid PCM is around 27 hours during charging mode, thus, allowing for a relatively long-time margin even under an extreme postulated accident scenario consisting in the inability of the secondary loop to remove heat from the system. In the opposite discharging mode, the time needed to fully solidify the 100% liquid PCM is 17 hours. During the simultaneous charging and discharging (SCD) mode, with constant load, the fraction of liquid PCM is nearly fixed with time. Hence, TES in this mode is said to take the role of a standard heat exchanger. Finally, simulations conducted for the SCD mode, with variable load, demonstrate that the coupled system can follow the daily variations in the grid demand without having any significant impact on the constant operation of the reactor. In this operating mode, the PCM is changing phases from solid to liquid and vice versa to adapt to the energy demand variations as anticipated.