Phase Change Materials (PCM) for Building Envelope Applications: A Review of Numerical Models

Phase Change Materials (PCM) present a great potential for energy efficiency gains in thermal systems, e.g. by storing solar energy in buildings or heat loads in industrial processes. This is because a great amount of energy can be stored per mass unit within a small temperature range. Significant applications of this peculiar characteristic of PCM regard the effective adoption of macro -encapsulated PCM into building envelopes. Several studies on this topic tend to be limited to a sort of "material selections" on PCM and a lack of systematic analysis has consequently emerged. In order to guarantee an effective use coupled with economic feasibility, a deep understanding of the phase transition phenomenon is needed. The study of PCM using computational fluid dynamics (CFD) is documented in several works, in accordance with the current trend of CFD to become increasingly widespread. Numerical studies on solidification and melting processes use a combination of formulations to describe the physical phenomena related to such processes, mainly the latent heat and the velocity transition between the liquid and the solid phases. The methods used to describe the latent heat are divided in three main groups: (i) source term methods (E-STM), (ii) temperature transforming models (E-TTM) and (iii) enthalpy methods (E-EM). The description of the velocity transition is in turn divided in three main groups: (i) switch-off method (SOM), (ii) source term method (STM) and (iii) variable viscosity method (VVM). In this context, the main objective of the present paper is to review the numerical approaches used to describe solidification and melting processes.