Cool materials performances analysis in building and urban scale

Abstract

Climate changes have in global warming one of the most obvious manifestations. The increase in temperature is a generalised phenomenon, associated with a number of environmental energy risks. The urban heat island is a typical effect, in which global phenomena are in addition to specific local conditions that lead to significant temperature rises in densely urbanised areas with respect to rural ones. Another trend is the increase in consumption for air conditioning in summer and, consequently, of electricity in the civil sector, as demonstrated by data collected in recent years, at national and European level, regarding consumption by sector. Cool materials represent a possibility to mitigate both phenomena, since they allow to reduce the overheating of construction materials due to the high absorption of solar radiation. Cool materials, characterised by high solar reflectance and thermal emissivity, have many possible applications and are the subject of numerous studies to optimise their thermal-physical properties and check their effects on urban area and on buildings. This study aims at evaluating the efficiency of the cool material both on building scale and on urban scale. Among cool materials there are products used as building roof coatings, the so-called cool roofs. These products are available in the European market for nearly a decade and nearly two decades in American ones. Although they are a consolidated technology, their use remains limited to a few buildings mostly experimental. This also makes impossible to massively exploit their properties. Within this report, the application of a cool roof has been tested on a single floor building with a low-sloped roof. The experimental and numerical analyses have shown the potential of this passive technique on increasing in thermo-hygrometric comfort influencing the decrease in internal temperatures of about 2.5 °C as well as the energy savings associated with the energy demand for cooling. A little investigated aspect of this technology is the study of the decay of the reflection power related to natural phenomena of ageing and soiling. A measurement campaign still in progress aims at assessing the reflectance of materials used as roof coatings with regular experimental analyses. After 18 months there was a decrease of about 24% of the reflection power for samples that had an initial solar reflectance greater than 0.8. In general, this decay is a function of roughness and reflectance spectral response shape. Nonetheless in many cases spectral data measured after 12 and 18 months demonstrate a convergence trend with broadband values however higher than that of conventional materials. Despite the benefits reported in this and other studies on real applications, there are no procedures for the energy classification of these materials. National energy policies are aimed at reducing consumption related to space heating, with restrictions on thermal transmittance values of the building envelope, neglecting those related to space cooling. For this reason it has been proposed a cool roof classification system as a function of their thermal properties and their influence on the energy performance of buildings normalised as a function of the geometry of some reference buildings and climatic zones. The cool materials used for urban applications as asphalt and concrete tints, the cool paving, are a technology still in its infancy, with a negligible market penetration. The optical and solar characterisation of these products confirmed their high reflectance especially compared to conventional materials with surface temperatures even lower than 20 °C. A lower surface temperature reduces the heat transferred to the air for convection. The potentialities of these materials applied on the asphalt of a neighbourhood of Rome were analysed by numerical analysis with a type SVAT tool (Soil, Vegetation, Atmosphere, and Transfer). Results showed a decrease in average air temperature of 5.5 °C compared to those obtained in presence of conventional asphalts. The temperature decrease is also reflected at an altitude over the average height of buildings. This shows how cool materials, when used on an urban scale, can be exploited as a contributory factor to the reduction of the heat island. The use of different energy analysis tools employed for numerical purposes has highlighted a limit concerning the modelling of the building surfaces of the buildings. These models consider the reflectance as a constant and independent on the incidence angle of radiation that hits the surfaces (Lambertian reflection). Instead, the building materials, especially those used as roof coatings, present mixed reflection modalities with not negligible regular components, which are a function of angle of incidence as demonstrated in the experimental campaign shown in this report. Thus, it was obtained a function that linked the solar reflectance to incidence angle starting from the measured data. It has been used in order to modify the calculation model of a dynamic tool in order to optimise the solar gains calculation. The results demonstrate the limits of current thermo-physical models generally used to conduct energy analyses.