A Method for Thermal Dimensioning and for Energy Behavior Evaluation of a Building Envelope PCM Layer by Using the Characteristic Days

Net zero energy buildings (nZEB) require the development of innovative technologies such as the use of phase change materials (PCMs) in walls for the energy requalification of low inertia buildings. The presence of a PCM layer in the external building wall, due to the effect of storage and release of latent energy phenomena, modifies the energy behavior, both during the summer and winter periods. This paper addresses the problem of the definition of the energetic behavior of a layer subject to phase change with periodic non-sinusoidal boundary conditions, characterizing the external walls of air-conditioned buildings. In such conditions, the layer is the site of the formation of one or more bi-phase interfaces, which originate on the boundary surfaces, or are always present and fluctuate within the layer. It is also possible that the layer does not undergo any phase change. The study has been developed by a finite difference numeric calculation model which explicitly determines the number and the position of the bi-phase interfaces that originate in the layer and the temperature and the heat flux fields. The surface heat fluxes are used to evaluate the PCM layer energetic behavior in terms of energy transferred through the boundary surfaces and of stored energy in sensible and latent form. The proposed method employs the characteristic day that it is periodically repeated for all the days of the considered month. The use of the characteristic days drastically reduces the computational burden of the numerical calculation and it allows to obtain guidance on the behaviour of the PCM throughout the year, in accordance with the variability of external climatic conditions, in order to select the PCM with the most suitable thermophysical properties. The methodology developed is applied to PCM layers with different melting temperatures and subject to climatic conditions of two locations, one with a continental climate and the second one with a Mediterranean climate. The results obtained allowed us to identify which PCM is more suitable in improving the energetic performances of building walls in the heating or cooling period during the year. In particular, the energy analysis highlighted that, in both localities, during the winter period: the lowest energy exiting from the indoor environment is ensured by a PCM with a melting temperature of 15 °C; the highest contribution of energy entering the indoor environment, mainly due to solar radiation, is recorded for a PCM with a melting temperature of 26 °C. During the summer period: the lowest value of energy entering the indoor environment is obtained by a PCM with melting temperature of 26 °C; the highest value of energy exiting from the indoor environment is ensured by a melting temperature equal to 20 °C. In both locations, a PCM with a melting temperature intermediate between those of the winter and summer set points of the indoor environment is the best compromise between winter and summer energy needs for an air-conditioned environment, as it allows obtainment of the highest values of the yearly total stored energy.