Performance Evaluation and Optimum Design of Ventilation Roofs with Different Positions of Shape-Stabilized PCM

Abstract

Environmental pollution and energy shortages have become increasingly prominent. Building energy conservation is an important part of a low-carbon strategy. Integrating phase change material (PCM) into a building’s roof is effective in altering the space cooling load, however less effective in reducing it. To reduce the cooling load, a novel ventilation roof with shape-stabilized PCM (VRSP) is introduced. The mechanical ventilation is used at night by embedding ducts in the roof to remove the solidification heat of the PCM. To identify the best position of PCM and an optimum design, the thermal performances of three kinds of VRSPs were compared and investigated through CFD simulation: ventilation roofs with outer-layer shape-stabilized PCM (VRSPO), middle-layer shape-stabilized PCM (VRSPM) and inner-layer shape-stabilized PCM (VRSPI). The effects of PCM and ventilation parameters on the thermal performance of three roofs were analyzed on a typical design day in summer in Wuhan. The results show that for VRSPO, VRSPM and VRSPI, the proper thicknesses of PCM are 35 mm, 25 mm and 15 mm; melting temperatures are 35~37 °C, 33~35 °C and 29~31 °C, respectively; the proper ventilation speeds are 2.5~2.6 m/s; and the optimum cavity radii are all 40 mm. The best performance can be obtained by placing PCM on the outer layer. The PCM of VRSPO has the highest number of days in which the phase change process occurs (specifically, 75 days in the summer). The application of VRSP can effectively reduce the internal surface temperature of the roof, by an average of 1.77 °C. The maximum and average inner surface temperatures of VRSPO in different weather conditions can be calculated using the daily average outdoor sol-air temperature or average dry bulb temperature by fitting equations. The structure can be used as a passive and active envelope in areas with hot and long summers.