Dynamic Response of a Packed Bed for Thermal Energy Storage.

Abstract

This study presents an analysis, with experimental results, for the transient response of a packed bed thermal storage unit. The axisymmetric model considers arbitrary temporal and radial variations in inlet fluid temperature. Both charging and recovery modes are examined analytically and experimentally. The effects of spatial variations in void fraction, velocity, and transport coefficients, the thermal capacitance and longitudinal conduction effects for the container wall, and energy losses at the wall are included in the analysis. Axial and radial thermal dispersion are analyzed and the influence on the governing equations and experimental results is determined. An unconditionally stable numerical model is developed which predicts the transient response of both solid and fluid phases, and is valid for various Pr and tl number fluids, and for a wide range of Reynolds numbers. Improved boundary conditions associated with thermal dispersion are identified. Detailed experimental measurements of temperature distributions in two beds of r and omly packed uniform spheres, with air as the working fluid, compare favorably with the numerical model, over a broad range of Reynolds number. Spatial variations in void fraction are found to have significant influence on the dynamic response. Experimental results indicate that the local heat transfer coefficient in packed beds is underestimated by typical literature values which are generally volume averaged values. In addition, results are presented for commercial size rock beds which show the effects of void distribution in such systems to be more pronounced than in beds of uniform particle size.