Effect of internal channels on energy storage in macro-encapsulated phase change material

A feasible design modification of introducing internal flow channels in spherical encapsulated phase change material (PCM) systems is proposed. The internal flow channels enhance the melting and energy storage rate by faster heat transfer to the center of the capsules. The proposed design is easier to implement as compared to other methods for heat transfer enhancement, such as introduction of fins within the capsules. As the PCM volume decreases with increasing channel radius (r), a trade-off exists between a higher energy storage rate and overall energy storage capacity. The present study addresses the effects of varying channel-to-capsule radius ratios (r/R) for 4 mm and 8 mm capsule radii. Four different r/R values are considered: 0.250, 0.333, 0.416, and 0.500, each with two values of R (4 mm and 8 mm). Results show that although the final storage capacity decreases with increasing values of r/R, the energy storage rates during most of the charging period are highest for intermediate values of r/R. For the 4 mm capsule packed bed system, it is seen that depending upon the total energy requirement, the energy storage rates are highest for either r/R = 0.333 or r/R = 0.416, while for the 8 mm system, r/R = 0.333 shows the highest energy storage rate. The energy storage density of PCM for 4 mm and 8 mm capsule systems increases with increasing r/R ratio but tends to saturate beyond r/R = 0.416.