An experimental study of melting behavior of the phase change material in cylindrical capsules for thermal energy storage

Intermittent solar energy needs energy storage to store during its peak availability. Phase change materials (PCM) store heat energy under a near isothermal condition. The PCM has a high energy storage density but its low thermal conductivity reduces its melting. The present study uses three heat transfer fluid (HTF) flow configurations, outer, inner, and combined flow (inner and outer), with gradually decreasing PCM capsule sizes, to expedite melting in a horizontal cylindrical thermal energy storage container. Three copper PCM capsules are attached to the inner heat transfer tube. The HTF at 65, 70, and 75 degrees C are used to analyze the PCM melting. The HTF at 75 degrees C expedites PCM melting due to more temperature gradients. The energy efficiency during PCM melting is about 17 % for the outer, 15 % for the inner, and 24 % for combined flow. The PCM melts at 16, 20, and 10 min for outer, inner, and combined flow. The solidification characteristics were studied by passing HTF at 30 degrees C after melting. The PCM solidifies at 46, 62, and 24 min for outer, inner, and combined flow. The combined flow is suitable for the expedited PCM fusion due to enhanced heat transfer and hence suggested for flat plate solar collectors and heat recovery systems.