Experimental and Numerical Assessment of a Vertical Latent Heat Storage Unit: Effects of PCM Container Geometry and Heat Transfer Fluid Velocity

This study investigates the charging of a vertical latent heat thermal energy storage unit containing a single coaxial paraffin container. Measurements for smooth and internally finned containers are compared with transient conjugate simulations that account for forced and buoyancy-driven flow in water and molten paraffin. Latent heat is represented by the apparent-heat-capacity method, and motion in solidifying material is suppressed by a mushy-zone resistance term. Modeled temperatures agree with four experimental sensor records, with root mean square errors of 0.46–0.96 °C and coefficients of determination of at least 0.996. At fixed container height, increasing the PCM diameter from 40 to 80 mm increases the PCM mass fourfold and the calculated time to 99% liquid fraction from 166 to 419 min. For a 40 mm container, increasing inlet velocity from 0.005 to 0.05 m/s reduces that time from 244 to 95 min. At approximately equal PCM volume, the model predicts that annular and eight-ray star containers shorten charging by 41% and 39%, respectively, compared with a smooth cylinder. The two designs are practically equally fast (98 and 101 min), but the star container requires 1.9 times as much steel as the annulus. These results identify geometries that shorten charging within the investigated operating range; charging time is evaluated separately from full-system energy and exergy efficiency.