Transient Thermo-mechanical analysis of a shell and tube latent heat thermal energy storage for CSP plants

The structural integrity of a lab-scale shell and tube latent heat thermal energy storage under transient conditions was investigated. The system was designed to use sodium at 750 degrees C as a heat transfer fluid with a high temperature phase change material, melting at 705.8 degrees C, as the heat storage medium. Stainless steel 347H (SS347H) was used as the tube and shell material. One quarter of the lab-scale system was used for the CFD model (using ANSYS Fluent) while a temperature profile was generated from the CFD modelling which was then linked to the validated ANSYS FEA model. The operating pressure of the system was below 1 bar, therefore, the thermally induced stress-strain due to high temperature gradient (up to 100 K) was the determining factor. The difference between the thermal expansion coefficient of the PCM and metal tubes was found to be of lesser influence as the ullage area provided sufficient space for expansion of the PCM. In the case of 50 K temperature gradients, a thermoelastic analysis showed that the maximum equivalent stress is approximately 106 MPa, which is below the allowable design stress for SS347H. However, the maximum equivalent stress under the 100 K temperature difference was 250 MPa, well above the allowable stress. Therefore, the structural integrity of the system is sound under transient operating conditions within a 50 K temperature gradient for 1000 h. Furthermore, an elastic-plastic (bilinear) analysis showed similar results for operation within a temperature difference of 50 K, however, a lower equivalent stress of 140 MPa for 100 K. Therefore, operation under a higher temperature gradient of up to 100 K is possible considering plastic strain and deformation while the system is reasonably protected against creep failure during the first 1000 h of operation. The current study provides new knowledge and insight into the structural integrity of a shell and tube system, in particular a system using an austenitic steel material, under high operating temperature which can assist in the design of a cost-effective latent heat thermal energy storage system.