New view point on the effect of thermal conductivity on phase change materials based on novel concepts of relative depth of activation and time rate of activation: The case study on a top floor room

Abstract

For the reason to explore the latent heat utilization of phase change materials (PCMs) under certain application conditions, the concept of relative depth of activation and time rate of activation was proposed for the first time. The relative depth of activation reflects the degree to which the PCM undergoes phase transition on the spatial scale, while the time rate of activation reflects the degree on the time scale. In addition, the two indices were calculated in a very short time, effectively avoiding the problem of underestimating the heat flux through the PCM. The thermal conductivity of PCM was selected as the optimization parameter, and the two indices were taken as the objective optimization function to evaluate the degree of latent heat utilization when PCM is applied to buildings. Through the software “BuildingEnergy” developed by the authors, it was found that the thermal conductivity which makes the two indices reach the maximum values varies in different application backgrounds. In order to maximize the matching degree between PCM and the applied environment, the thermal conductivity is not certainly the bigger the better or the smaller the better. Increasing thermal conductivity does not necessarily improve the degree of latent heat utilization, and may bring negative effects on it, and vice versa. By calculating relative depth of activation and time rate of activation, parameters of PCM can be directionally optimized to match their application background to achieve the goal of both building energy efficiency and better latent heat utilization.