On the energy impact of cool roofs in Australia

Abstract

Urban overheating is a serious environmental issue with a significant impact on cooling loads, peak electricity demand, indoor air temperature, health, pollution, sustainability, and the economy. Cool roofs – which are characterised by high solar reflectance (ρ) or albedo and high thermal emissivity (ε) – are one of the most effective technologies to mitigate the urban overheating effect and its undesirable impacts. The existing literature on the energy impacts of cool roofs in various building types and climate zones is vast but fragmented and predominantly assessed the building-scale and urban-scale benefits separately. Here, we computed the energy conservation potential of cool roofs in 17 building archetypes with different occupation hours, internal heat gains, thermal insulation levels, and building heights under various Australian climates. This study presents a holistic approach to assessing the magnitude and spatial variation of the cooling load savings, the peak electricity demand reductions, and indoor air temperature reductions generated by the building-scale and combined building-scale and urban-scale implementation of cool roofs in all representative building types under various climate conditions in Australia. In Sydney, a city with a warm/mild temperate climate, cool roofs can reduce the cooling loads of a typical low-rise office building without roof insulation by 10.2–13.8 kWh/m2 (37.6–42.0 %) during the summer months of January and February. When cool roofs are implemented in both individual buildings and the whole urban area, the cooling load conservation potential for the same building increases to 14.9–17.4 kWh/m2 (50.3–63.7 %). The corresponding peak electricity demand and indoor air temperature reductions by the combined building-scale and urban-scale application of cool roofs are 53–70 % and 11.2–12.0 °C, respectively. The energy conservation potential of cool roofs can be noticeable even for high-rise buildings with a high level of thermal insulation. We estimated that the cooling conservation potential by combined building-scale and urban-scale application of cool roofs can range between 4.0 and 7.1 kWh/m2 (29.3–48.3 %) in a new high-rise residential building with high levels of thermal insulation to 18.5–24.5 kWh/m2 (23–29.6 %) in an existing low-rise shopping mall centre with low levels of thermal insulation in Sydney. Cool roofs lead to additional savings as the reduced temperature increases the energy efficiency of air conditioning systems, with cooling load savings between 0.6 and 3.7 kWh/m2 (3–11 %) for a typical low-rise office building without roof insulation in Sydney. Also, the heating load penalties induced by cool roofs are negligible compared to the cooling load savings in all building types and Australian cities/climate zones except for non-commercial buildings in the cold climate of Hobart. Policymakers can use the results of this study to obtain a deep knowledge of the expected performance of cool roofs when applied city-wide to mitigate urban overheating and counterbalance its adverse impacts on energy uses and indoor thermal comfort.