Orientation impact on structural integrity of a shell and tube latent heat thermal energy storage system

Latent heat thermal energy storage systems can regulate the intermittency of electricity generation in a grid powered by renewable energy sources. Such systems could be used in conjunction with Brayton power generation cycles operating at high temperatures (520 degrees C-750 degrees C) delivering higher efficiency. However, a trade-off of higher thermal efficiency can be higher thermal stresses. The isothermal latent heat thermal energy storage and release can reduce the thermal stresses for a specific design. This study investigates the impact of the orientation on the melting process and the consequent thermal stresses in a lab-scale latent heat shell and tube system using a combination of transient computational fluid dynamics and finite element analyses. With a high temperature phase change material melting at 705.8 degrees C, two initial temperatures of 700 degrees C and 650 degrees C were considered in transient analyses, leading to a 50 K and 100 K initial temperature difference across the tubes, respectively. For the structural analysis, an elastic method was employed for the condition of 50 K temperature difference, and an elastic-plastic analysis for the condition of 100 K to account for plastic deformation. The results of the elastic analysis presented the maximum thermal stresses of 63.5 MPa and 106.7 MPa in tubes in the horizontal and vertical orientations, respectively. The stress in the tube sheet was slightly higher (94.5 MPa) for the horizontal system compared with 90.4 MPa in the vertical position. For the condition of 100 K, the results of the elasticplastic analysis identified the highest stresses of 172.5 MPa and 147.4 MPa in the tube sheets of the horizontal and vertical systems, respectively. For both initial temperature conditions, the stress level in tubes is lower in the horizontal orientation, which is where most cost occurs in a shell and tube system. Particularly important for the 100 K condition, the results show a lower stress of 100 MPa in tubes for a horizontal system compared with 140 MPa in a vertical orientation. In general, the lowest thermal stress occurs where a melt layer forms around tubes during the early stages of a melting process. A higher stress occurred in parts in contact with argon. The highest level of stress occurred in parts in contact with solid PCM, which has direct implications for larger vessel designs. This study provides insights into hydrothermal and mechanical design of a PCM system to prevent and/ or manage localised maximum thermal stresses for a cost-effective thermal energy storage system.