Experimental study of the discharge process of a thermal energy storage system based on granular material operated as a fluidized or confined bed

• Published in: Journal of energy storage 2023-12, Vol.73, p.109173, Article 109173
• Main Authors: Soria-Verdugo, A., Guil-Pedrosa, J.F., Hernández-Jiménez, F., García-Gutiérrez, L.M., Cano-Pleite, E., García-Hernando, N.
• Format: Article
• Language: English
• Subjects: Confined bed; Discharge process; Fluidized bed; Granular material; TES system
• ISSN: 2352-152X; 2352-1538
• DOI: 10.1016/j.est.2023.109173

The stable generation of green electricity by Concentrated Solar Power (CSP) plants is subjected to the proper performance of Thermal Energy Storage (TES) systems, which constitute a critical subsystem of the plant. TES systems based on granular material have already been tested for industrial scale, allowing the operation at high temperature. Depending on the particle size of the granular material and the velocity of the working fluid, the bed conforming the TES system may be operated in a bubbling fluidized or a fixed bed regime. Further, beyond minimum fluidization conditions the bed can operate as fixed if the particles are mechanically confined. The performance of the discharge process of a lab-scale TES system (20.5 cm of diameter, 1 m bed height and 55 kg of silica sand as bed material) of granular material operated as a fluidized bed and as a confined bed was experimentally analyzed for various air volumetric flow rates. For the fluidized bed configuration, the bed temperature was found to be uniform along the bed height due to the high axial mixing rate caused by gas bubbles. During the discharge process of the fluidized bed TES system, the bed temperature decreased exponentially from the initial bed temperature to the temperature of the cooling fluid, with a faster temperature reduction for higher flow rates of fluid. The confined bed operation resulted in a stable temperature of the fluid at the outlet, close to the maximum temperature of the system, for around 45 min when discharging the bed with 700 Nlpm of cold air, whereas after 45 min of the discharge process, the outlet temperature of gas for the fluidized bed was roughly half the temperature obtained from the confined bed. However, the average air pressure drop during the discharge of the TES system of granular material operated as a fluidized bed is stable at around 0.16 bar, while the pressure drop of the confined bed is significantly higher, decreasing from 0.65 to 0.30 bar as the discharge process of the bed progressed. •A lab-scale TES system based on granular material was experimentally tested.•The operation of the system as a fluidized bed and as a confined bed was compared.•The confined bed is optimal in terms of fluid outlet temperature during discharge.•The fluid pressure drop is lower for the fluidized bed compared to the confined bed.•The fluidized bed operation is relevant when a homogeneous temperature is desired.