Development of correlations for Nusselt number and friction factor of packed bed solar thermal energy storage system having spheres with pores as packing elements

摘要

Solar energy is an emerging alternative of fossil fuels to fulfill the global energy demand as it is available in plenty and eco-friendly in behaviour. However, solar energy systems requires storage unit due to its intermittent nature. The packed bed storage system (PBSS) as storage unit is considered to be a feasible option for solar thermal energy systems having operating temperature range under 100 degrees C. The heat transfer and pressure drop within PBSS are influenced by shape of packing element as convective heat transfer coefficient is a function of it. Under the current study, correlations are developed for Nusselt number as well as friction factor using the results obtained through experimental investigation for the PBSS built up of spherical shaped packing elements having pores on the surface. The developed correlations are the function of Reynolds number, ratio of pore diameter to sphere diameter (d/D), perforation index (PI) and ratio of pore depth to sphere diameter (t/D) within range of 200 to 800, 0.066 to 0.2, 0.06 to 0.22 and 0.05 to 0.2, respectively. Values obtained from the developed correlations are compared with the values obtained through experimentations and it is found that the correlations for Nusselt number and friction factor can predict the values with mean absolute deviation of 6.7% and 7.5%, respectively. These correlations may be helpful for the researchers working in the same research area and manufacturers developing the low temperature solar thermal energy systems. This study may also be useful to investigate the effect of similar shape of packing elements in latent heat based PBSSs.