Influence of flow distribution on the thermal performance of dual-media thermocline energy storage systems

Dual-media molten-salt thermocline thermal energy storage (TES) systems can be used to maintain constant power production at Concentrated Solar Power (CSP) plants independent of weather changes at costs less than that of traditional two-tank molten-salt storage systems. The flow distribution is a critical parameter affecting the thermal performance but has rarely been considered for dual-media TES systems in previous studies. This study analyzes the influence of the flow distributions at the inlet and outlet of a salt-rock dual-media thermocline TES tank on the thermal performance. The flow distribution is characterized by radial component, and a two-temperature model is used to investigate the thermal performance of the thermocline tank. The model is first validated against experiment data available in the literature and then used to study the discharge process of the thermocline thermal storage tank for various flow distributions. The results show that even with a large (80% of the area) flow blockage at the inlet, the flow distribution has only a limited influence on the useable energy output (<3% change) of the dual-media storage tank. In fact, the flow non-uniformities reduce the thickness of the thermocline layer and slightly increase the useable energy output, whereas non-uniformities at the top outlet slightly decrease the output. An entropy generation analysis, including the effects from diffusion and interstitial heat transfer, is performed to further explain these phenomena. The interstitial heat transfer is found to be the main cause for the entropy generation in the discharge. Flow non-uniformities are also found to reduce the entropy generation.