Longer passage of airflow in multiple packed-bed thin tanks versus in a short big tank for improved thermal storage performance

A very challenging issue about solar thermal power generation is the use of a high temperature heat transfer fluid (water, oils, or molten salts) for heat transfer and thermal storage material, which may freeze at night or cold weather. When choosing air as the heat transfer fluid, the problem of freezing is eliminated. In order to increase the performance of thermal storage system which uses air as the heat transfer fluid passing through a packed bed (by ceramic spheres of Al2O3), multiple small-diameter tanks are considered to replace a single large-diameter tank with the same packed-bed volume and airflow rate in this paper. Analysis about the thermal storage performance in a short big tank and in cascade thin tanks has been made for comparison. A long passage of airflow and faster flow speed of air in the cascade thin tanks has been found significantly beneficial to thermal storage. Results about the increased thermal storage performance and increased pressure loss will be presented. Longer passage of airflow made it possible to have a longer time of high temperature of outflow air during discharging period. And faster speed of the fluid enhanced the heat transfer between air and thermal storage material. The total effective energy and thermal storage efficiency of cascade thin-tank thermal energy storage (TES) are higher. The thermal storage efficiency in the two types of thermal storage arrangement was compared for optimal design. The obtained results are of great significance to the development of using air as heat transfer fluid and rocks or ceramic spheres as the thermal storage material for thermal storage system in concentrated solar thermal power plants.