Low-cost hygroscopic packed beds for energy storage in solar dryers: Sizing rules from a dual-scale heat and mass transfer model

To overcome the inherent limitations of solar dryers, a simple, low cost and efficient solution is proposed to store solar energy in hygroscopic packed beds. Energy is stored as latent heat in the bed, which drastically reduces thermal losses. The packed bed acts as direct heat/mass exchanger and regeneration during the day occurs at a moderate temperature which allows a low cost container to be used. A macroscopic formulation able to consider non-local equilibrium at the particle level was derived and used to formulate the coupled heat and mass transfers in the packed bed. The particle kernel function needed in this approach was characterized using dynamic sorption tests for four different hygroscopic particles: pine chips, spruce chips, hemp shives and zeolite beds. Results are given as thermo-activated mass diffusivity and GAB parameters of the sorption isotherm. The dual-scale fields embedded in the kernel function, together with the efficient computational solution, allow a packed-bed to be simulated in a few seconds for several night/day cycles (adsorption-regeneration). Profiles of the state variables as well as the inlet and outlet air characteristics over time are depicted for a selected set of configurations. The yields of a packed bed (mass of water and energy stored during one reference night/day cycle) are depicted as a function of the bed height for the four different particles, including the effect of particle size. Yields up to 45 MJ per cubic meter of bed were reported, which represents nearly 100 MJ per cubic meter of particles. To promote the deployment of such low-cost storage systems, two time constants, at the particle and bed levels, are introduced to derive sizing rules. • Packed beds of simple materials are proposed for moisture buffer and energy storage. • A comprehensive dual-scale formulation of coupled transfer is provided. • Detailed simulation and analysis of the packed bed during a night/day cycle. • Two characteristic times introduced to analyze the dual-scale mechanisms. • A full set of yield results and good practice rules provided to design the bed.