Extract

INTRODUCTION:
The combination of urban overheating due to urbanization and global climate change are leading to an increase in building cooling loads and outdoor air temperatures. This topic has been widely investigated, but a comprehensive appraisal of the direct and indirect benefits of heat mitigation is still needed. Here, we present the first results of a project on the assessment of the costs and benefits of cool roofs in Australia.

METHODS:
Meso-scale climate modelling was performed for the cities of Adelaide, Brisbane, Melbourne, Perth, and Sydney for January and February 2017, assigning roof albedo equal to 0.15 (conventional roof) and 0.85 (cool roof) to all roofs in the city. Then, weather data for the period from May 2016 to April 2017 have been collected from several stations of the Bureau of Meteorology: 11 in Sydney, 7 in Melbourne and Brisbane, and 5 in Adelaide and in Perth. With the first dataset, by means of EnergyPlus, we performed building energy simulations for the hottest summer months (January and February) in three scenarios: with conventional roofs on all buildings, with a cool roof on the simulated building in the unmitigated climate (conventional roofs on all other buildings), and with a cool roof on the simulated building in a mitigated climate. Then, we performed whole-year simulations using the data from the Bureau of Meteorology, with and without a cool roof in an unmitigated climate. We modelled 17 commercial and residential buildings, with poor insulation levels (pre-code) and current insulation level (according to the National Construction Code). In all cases, we computed the heating and cooling energy needs (i.e., heating and cooling loads) and the indoor air temperature in free-floating conditions (i.e., without heating or cooling).

RESULTS:
Cool roofs reduce the ambient temperature in all cities, with peak ambient temperature reductions in Sydney equal to 2.4 °C, while in Melbourne and Brisbane the peak reductions are of 2.1 °C and 2.5 °C, respectively. By reducing the ambient temperature in metropolitan areas due to the limited absorption of solar energy and release of turbulent sensible heat, cool roofs also reduce the height of the planet boundary layer over the city, which causes a reduction in the mixing with advective flows from inland. Also, lower temperatures favour the penetration of the sea breeze front, further cooling the city.

Cool roofs lead to substantial reduction in the cooling energy needs of low-rise uninsulated offices, for instance. Savings account to 22-26% in Sydney when a reflective roof is applied only onto a single building, with heating penalties of less than 1%. Instead, when all roofs have high albedo, the cooling load savings range between 38 and 54%, depending on the location. Cooling load savings in the range between 18% and 39% can be achieved even for high-rise insulated offices, because of the lowered outdoor air temperature. Then, the indoor air temperatures in houses are reduced, even by 4 °C in new houses with high insulation with a reduction of the number of hours of even 100 hours per summer month, when the indoor air temperature exceeds 26 °C with respect to a conventional roof.

CONCLUSIONS:
Reflective roofing can both mitigate the outdoor air temperature and reduce the cooling loads of buildings, in all Australian cities. The use of cool roofs is associated with heating penalties – i.e., higher heating energy needs – which are largely outweighed by the cooling energy savings, already in the present climate context. Cooling energy savings are not limited to poorly insulated low-rise buildings. Also, high-rise insulated building benefit from a city-wide application of reflective roofing, because of the reduced outdoor air temperature.

PDF

Download