Effect of inclination angle on the performance of two-and-three axial cascaded latent heat thermal energy storage systems

Coupling two passive enhancement techniques, such as the cascaded arrangement and storage orientation, substantially improves the melting performance of the cascaded latent heat thermal energy storage system (CLHTESS). This paper studies the effect of seven inclination angles (0, 15, 30, 45, 60, 75, and 90°) on the melting performance of two- and three-axial CLHTESS. The melting performance is assessed in terms of melting time, dynamic effectiveness, cycle charging capacity, and charging rate. The four performance indicators are compared under full and partial charging of 50 %, 70 %, 90 %, 98 %, 99 %, and 100 %. Three types of phase change materials (PCMs), specified with their nominal melting temperatures as PCM40, PCM50, and PCM60, are used for the CLHTESS. PCM40/PCM60, with their best volume fractions of 80/20 %, is tested for the two-stage-axial cascaded systems, and PCM40/PCM50/PCM60, with optimum volume fractions of 49/40/11 %, is used for the three-stage CLHTESS. The analysis is conducted using a three-dimensional transient model. The results indicated that the maximum cycle charging capacity occurs at the horizontal storage orientation for all investigated CLHTESS. Accepting a deviation of 2 % from the optimum performance allows the generalization of the best orientation of the two-and-three-stage axial CLHTESS. The two-and-three-stage axial CLHTESS shows the highest average effectiveness at inclination angles of 30° and 45° for charging levels of 90 % and 98–100 %, respectively. Also, the shortest charging time and the highest charging rate for the two-and-three-stage axial CLHTESS appear at 45, 45–60, and 75–90° for charging levels of 90 %, 98–99 %, and 100 %, respectively. Generally, the charging performance for the three-stage is higher than the two-stage axial CLHTESS. Also, the charging rate and charging time are more sensitive to the storage orientation than the effectiveness and the cycle charging capacity of the CLHTESS.