Study on standby process of an air-based solid packed bed for flexible high-temperature heat storage: Experimental results and modelling

An air-based solid packed bed thermocline tank is a low-cost high-temperature thermal energy storage technology, which is especially suitable for concentrated solar power plants when combined with Brayton cycle. Standby process between charging and discharging is a practical operation concern to improve the flexibility and economy of a concentrated solar power plant, and it is one of the most frequent states of the heat storage tank and is often related to the destratification in the thermocline. Whereas, till date little attention has been paid on to this static process as well as its effects on the heat storage performance, thus this work aims at filling this gap and getting more insights into the standby stage of the air-based solid packed bed thermocline tank by means of both experiments and modelling. In the present study, 1-D two-phase transient models were developed for both the dynamic (charging and discharging) and static (standby) processes, and experiments were conducted on a pilot-scale air-based solid packed-bed thermal energy storage testing system. It was found that the thermal stratification of the testing system decays by 30% after a 6-h standby stage, indicating a largely deteriorated heat storage performance. It was also found that the numerically predicted temperatures agree pretty well with the experimentally measured data under both a charging-standby-discharging cycle and a charging-standby process. In addition, sensitivity analysis was conducted to better understand the heat transfer mechanisms during the standby stage, and contribution of each mechanism to the thermocline destratification was also studied. The results indicate that thermal diffusion and heat loss contribute more to the degradation of the thermocline during the standby stage than natural convection, all of which cannot be simply ignored in the modelling. Moreover, the developed models and the experimental data reported here are reliable and valuable for further extended studies on more systematic and complex problems of related industrial engineering applications.