Progress in Thermal Energy Storage Modeling

Economic analyses for concentrated solar power (CSP) systems indicate that the cost of electricity can be reduced through the use a molten-salt thermal energy storage system (TES). Use of a thermocline in the tanks to keep cold and hot fluids separate in the tank has been proposed to further reduce the cost for TES to about $30/kWh thermal. This paper studies the details for molten-salt storage where the tank is filled with nominally 1-inch-size quartzite rocks and 1/4-inch sand particles. These filler materials are envisioned to occupy 75% of the tank’s volume. Experiments at Sandia National Laboratories indicate that these filler materials are compatible with the molten salt, and a well-established thermocline can be maintained in these tanks. The National Renewable Energy Laboratory attempts to look into the next level for the tank design, modeling heat transfer between solid filler and molten salt. Loose filler materials exert added hoop stresses in the tank that are likely to be unmanageable in the long run because of thermal cycling. Structured brick-like stacked shapes are needed that can maintain their structure without requiring sidewall support for the fill material. Large tanks with a volumetric capacity of 100,000 kL maintain a reasonable thermocline during charge and discharge. We report on progress in computational fluid dynamics (CFD) modeling of such tanks. However, for operational convenience for the plant, we suggest the use of multiple tanks.