How to enhance thermal energy storage effect of PCM in roofs with varying solar reflectance: Experimental and numerical assessment of a new roof system for passive cooling in different climate conditions

In last decades, the effectiveness of thermal energy storage (TES) systems for energy efficiency has been widely studied when integrated to building envelope, often as latent strategy by means of Phase Change Materials (PCM). In this view, the present study investigates the dynamic thermal performance of an innovative cool polyurethane-based membrane with PCMs inclusion for roofing applications. Such membranes were developed in previous works when the new composite material was characterized in terms of thermal-optical and morphological properties, when integrating PCMs with transition point at 25 °C and 55 °C. In this work, such materials is finally implemented into a roof multilayer structure, and additional experimental tests are carried out in a dedicated environmental simulation chamber, where the thermal performance of the novel lightweight roof stratigraphy is investigated. To this aim, two types of membranes for roofing applications, i.e. the innovative proposed cool membrane and a more traditional dark bitumen membrane, are considered. These two roof configurations are tested with and without the integration of PCMs, with varying the melting temperature range, in two different climate boundary conditions, i.e. Mediterranean climate of Rome, Italy, and Hot desert climate of Abu Dhabi, UAE. Moreover, numerical analysis of the roof thermal behavior is carried out through transient heat transfer calculation. Therefore, the two roof configurations are simulated and compared, in the same selected climate conditions and with varying PCMs. Findings of coupled experimental tests and numerical analysis show that the combination of cool membrane and PCMs with suitable melting temperature range allows to significantly reduce the roof surface temperatures and, therefore, the heat flux through the roof. In detail, results of numerical analysis demonstrate how in Rome the integration of PCMs with melting point around 25 °C and 45 °C in the cool and the dark roof membrane, respectively, has a great effect in reducing the external roof surface temperature. On the contrary, in Abu Dhabi, the configuration allowing the higher inward heat flux reduction is represented by the integration of PCMs with melting point around 35 °C and 55 °C in the cool and dark membrane, respectively. On the other hand, experimental tests show how the inclusion of a PCM layer with melting point around 25 °C and 31 °C under the cool and dark coating membrane, respectively, allows the best thermal performance in both climate conditions. Therefore, this work showed the promising effectiveness of PCMs inclusion to improve the thermal performance of waterproof membranes for roofing applications. However, as demonstrated, the selection of the proper PCM melting temperature should be operated by taking into account not only the climate conditions, but especially the roofing medium and the position in the roof stratigraphy where thermal energy storage materials are integrated. In fact, roof coating thermal behavior is largely influenced by its thermal-optical properties, e.g. solar reflectance and thermal emittance.