A model for a variable-volume, density-stratified, liquid desiccant storage tank.

Abstract

A stratified, single-vessel liquid desiccant storage tank is an attractive option for the seasonal storage of solar energy. Maintaining stratified storage preserves the chemical energy stored in concentrated liquid desiccant solutions. Simulations of desiccant-based storage systems have been limited to modeling well-mixed storage. The subject of this paper is a density-stratified liquid desiccant storage tank model. The one-dimensional storage is composed of a series of well-mixed segments of a liquid desiccant (CaCl2 or LiCl) solution in a variable-volume, density-stratified storage tank. The density of each segment depends on both the temperature and the salt concentration of the desiccant solution. The model incorporates mixing due to mass diffusion and buoyancy and accounts for the heat of dilution in the energy balance. Thermal losses to the surroundings are accounted for and auxiliary heat input is permitted. Stored energy is a combination of sensible energy (a function of temperature) and chemical potential energy (a function of salt concentration and temperature). Fluid movement within the tank is simulated with a plug flow approach. Fluid enters the tank through fixed inlets or through an idealized inlet manifold. Fluid exits the tank through fixed outlets. For use with the Transient Systems Simulation (TRNSYS) software, the model is labeled TRNSYS Type 209. The user is able to define the storage characteristics (e.g. capacity, insulation, number and location of inlets and outlets, etc.), the initial conditions, and the boundary conditions of TRNSYS Type 209. Several test cases demonstrate TRNSYS Type 209's capacity for modeling liquid desiccant storage using the plug flow approach. These cases use initial and boundary conditions that have been arbitrarily selected from within the expected operating ranges.