Numerical study of circular-elliptical double-pipe thermal energy storage systems

Solar energy is one of the unlimited sustainable energy resources that can be stored for different applications by using latent heat thermal energy storage systems. These systems utilize the phase change process (melting/solidification) of a phase change material (PCM) for the store and release energy. In the present study, an annulus with an inner ellipse as a thermal energy storage system was numerically investigated which the outer wall is considered as a isolation wall, and the inner wall is employed at a constant temperature. This study investigated the effects of various aspect ratios of the inner ellipse diameters (1, ¾, ½ and 1/3) as well as the angular position of the inner ellipse (90°, 60°, 45°, 30° and 0°) during both melting and solidification processes. The results show that the lowest aspect ratio (W = 1/3H) has the best performance during charging (minimum melting time); however, the aspect ratio has no considerable effect on the solidification process. On the study of the ellipse angle, the results reveal that lower melting time is achieved for a higher angle while the solidification time is not varied significantly. The melting time reduces by almost 61% and the efficiency enhances by 26% for the case of W = 1/3H with the angle of 90º compared with the case of aspect ratio=1 (circular case).