Exergy Analysis of the Discharge of Sensible Heat Thermal Energy Storage Systems: Granular Material vs. Solid Blocks

Thermal energy storage (TES) systems are essential components of concentrating solar power (CSP) plants that enable uniform generation of green electricity and process heat. The performance of sensible heat TES systems based on granular material and on solid blocks was experimentally analyzed during the discharge process. A granular material TES system was studied in its operation as a regular fluidized bed and as a confined bed that restricted the motion of the granular material via mechanical confinement. The pressure drop required to circulate the heat transfer fluid (HTF) through the TES system made of alumina blocks was measured to be two orders of magnitude lower than that used in the granular material TES system, being uniform and close to 0.2 bar for operation of the fluidized bed and decreasing from 0.65 to 0.3 bar for the confined bed. The evolution of the exergy efficiency of the TES systems along the discharge process was determined by obtaining an inversed exponential reduction for the fluidized bed TES system, a parabolic reduction for the alumina block TES system, and a roughly uniform value for the confined bed TES system. The exergy efficiency of the confined bed TES system, considering the HTF pressure drop as a parasitic consumption of the plant, remained higher than 90% during the complete discharge process.