High albedo materials to counteract heat waves in cities: An assessment of meteorology, buildings energy needs and pedestrian thermal comfort

Climatological data show that the frequency of Heat Waves (HWs) has increased since the 1950s and that it will continue to increase. These phenomena exasperate the urban heat island (UHI) phenomenon. In this study, we investigated the impact of a HW on the UHI in Milan (Italy); we also analyzed the effects of the application of high albedo materials on the vertical and horizontal urban built surfaces. We performed numerical experiments with the Weather Research and Forecasting model, whose output were used as input for the EnergyPlus software and the computation of the Mediterranean Outdoor Comfort Index (MOCI). Our results showed that the HW induces a maximum increase of about 4 °C in the temperature at 2 m height and that the use of high albedo materials covering all urban surfaces only partially counteracts this increase. The wind speed decreases due to the HW and the introduction of high albedo materials leads to its further decrease. The cooling energy consumption for a building located in the city doubles in the presence of HW and high albedo materials have a negligible positive effect when applied to the surrounding urban environment. The HW brings an increase up to about 0.7 of the MOCI and the use of high albedo materials further worsens thermal sensation. Therefore, this mitigation strategy leads to considerable benefits in terms of temperature and energy, while it determines a penalization of the well-being of the pedestrian. Its application requires a careful evaluation of benefits and side effects.