Performance of a packed bed solar energy storage system having large sized elements with low void fraction

Packed beds are the commonly used storage units for air-based solar system. Stratification improves the efficiency of collector with the bed and a decrease of void fraction lead to an increase in stratification. Therefore an experimental study in a low range of void fraction is carried out in this paper so as to increase the stratification. Five different shapes of large sized storage elements of concrete were used and arranged differently to obtain four values of void fraction including the lowest possible void fractions that could be obtained for spherical elements. Correlations were developed for Nusselt number and friction factor by taking into account the effect of system parameters on them. The Nusselt number was maximum for spherical elements (w = 1.0) and at the minimum value of void fraction (0.275) whereas the friction factor is found to be minimum for cubical elements (w = 0.80) and at the maximum value of void fraction (0.48). The stratification coefficient has been found to be maximum for the minimum value of void fraction (0.275) and for the bed of spheres (w = 1.0). With spherical elements and for lowest void fraction the thermo-hydraulic performance is having the maximum value. 2012 Elsevier Ltd. All rights reserved.