Numerical study on stratification performance of cascaded three-layered packed-bed in the thermal storage process

Packed-bed thermal storage tank (PBTST) has broad application prospects in solar water heating systems, thermoelectric decoupling in power plants, and industrial waste heat recovery. In the present work, a three-dimensional PBTST model with three-stage phase change material (PCM) and three-layered diameter capsules was proposed, its thermal energy storage behavior and thermal stratification performance were studied by numerical simulation. The effects of PCM bed height ratio, fluid flow rate, and inlet temperature on the stratification performance of the new tank were investigated. The results reported that due to the increased heat transfer area and expanded heat-transfer temperature difference: (1) Compared with the conventional non-cascaded single-layered diameter capsule PBTST, the charging efficiency of the cascaded three-layered diameter capsule PBTST increased from 24% to 61% at the end of charging; (2) the cascaded three-layered diameter capsule PBTST shows significantly improved stratification properties compared to the conventional ones, with the Richardson number increased from 20.78 to 49.47; (3) increasing the height of the bottom PCM bed, raising the inlet temperature, and reducing the flow rate all can improve the stratification performance.