Thermocline stability criterions in single-tanks of molten salt thermal energy storage

Abstract

Thermal storage with molten salt is considered to be an important subsystem for solar thermal power stations due to the fluctuation of sunshine over time. A molten salt thermal storage tank, in which the fluid is stratified in temperature with “hot” on the upper level and “cold” in the lower level due to the density difference of the fluids (Criterion 1), is preferred for the system efficiency. Porous media, such as quartzite rock or silica sand, are used to fill the tank in order to reduce the inventory of the molten salt and manage the mixing between the hot and cold molten salt, so as to form a stable layer of thermocline. However, in the flow of molten salt, either the hot fluid displaces the cold one or vice versa, phenomena of viscous fingering and/or channeling are likely to occur, which may disturb the stability of the thermocline, resulting in an over widened temperature transitional zone. To circumvent the problems, criterions are proposed in this work based on the analysis of Darcy’s flow in porous medium, i.e. Criterion 2, when hot molten salt displaces cold molten salt, the flow velocity must be under the critical value (vc); Criterion 3, when cold molten salt displaces hot molten salt from the top down, though this is unusual, the flow velocity must exceed a critical value (wc); and Criterion 4, in the thermocline region, if the mobility ratio is less than 1, displacement is stable, otherwise it is unstable. Criterion 1 concerns the hydrostatics factor, i.e. density and gravity. Criterion 2–4, however, takes hydrodynamics into consideration as well, which involve velocity, density and viscosity of the fluid, and porosity and permeability of the porous medium.