Heat transfer enhancement induced by the geometry of a LHTES device

In the present contribution the authors investigate by means of numerical simulations the thermal performance of two geometries of Latent Heat Thermal Energy Storage (LHTES). The numerical model takes into account the mass, momentum and energy equations for the PCM together with an Entalphy-Porosity model for the phase change and a Bussinesq approximation for the buoyancy force. Indeed in such a system the convective motion within the melted PCM plays an important role, as confirmed by several studies. The scope of the research has been the enhancement of heat transfer performance of LHTES increasing the convection by means of modifications on the shape of the PCM enclosure. In particular, in this study two geometries have been considered to investigate the influence of the convective motion on the overall melting time. The first case consists of two concentric cylinders with the axis aligned with the vertical direction, in order to realize an internal heating of the Phase Change Material (PCM) that is confined between the internal and the external cylinder. It is a well-known "shell-and-tube" configuration, generally adopted in LHTES systems, even in CSP applications. In the second case, the authors propose a simple cylindrical geometry filled with the PCM, with the cylinder axis aligned with the vertical position. Thus, the heated surface corresponds to the external lateral surface of the cylinder. The PCM volume (V) and the net heat transfer surface (A) have been kept constant in the two cases, in order to be able to compare them under the same operative conditions. The tests reveal a sensible difference between the thermal performance of the two proposed solutions. The external heating configuration shows an important enhancement of the convective motion within the PCM that induces a more efficient heat transfer with respect to the standard "shell-and-tube" configuration. The complete melting time of the PCM with external heating is reduced by about 50%.