Sustainably cooling one of the world's hottest cities

New research from UNSW Sydney has demonstrated how to significantly cool a major city in a hot desert climate while also reducing energy costs. The findings have been published in Nature Cities.

Riyadh, the capital of Saudi Arabia, is one of the hottest cities in the world, with temperatures that can exceed 50°C during summer. Climate change and rapid urbanisation are further increasing the city’s magnitude of overheating.

Conducted in collaboration with the Royal Commission of Riyadh, the UNSW study is reportedly the first to investigate the large-scale energy benefits of modern heat mitigation technologies when implemented in a city.

UNSW Scientia Professor Mattheos (Mat) Santamouris, senior author of the study, specialises in developing heat mitigation technologies and strategies to decrease urban temperatures.

“Limited greenery and large artificial surfaces made of conventional building materials like asphalt and concrete trap heat, meaning the city continues to heat up,” Santamouris said. “Additional heat from car pollution and industrial activities also increases the city’s temperature.”

For the study, the team ran climatic and energy simulations of the Al Masiaf precinct of Riyadh, including the energy performance of 3323 urban buildings, under eight different heat mitigation scenarios to evaluate optimal strategies for lowering the temperature of the city and reducing cooling needs.

The modelling, which considered different combinations of super-cool materials, vegetation types and energy retrofitting levels, found it is possible to decrease the outdoor temperature in the city by nearly 4.5°C during summer. The strategy would also improve cooling-energy conservation for the city by up to 16%.

This type of modelling is important, as a blanket implementation of urban cooling techniques not based on detailed and advanced scientific optimisation, like the use of non-irrigated greenery, may result in a substantial increase in the city’s temperature.

The recommended heat mitigation (or cooling) scenario for Riyadh includes using super-cool materials implemented in the roofs of buildings as well as more than doubling the number of irrigated trees to improve transpiration cooling.

“By implementing the right combination of advanced heat mitigation technologies and techniques, it is possible to decrease the ambient temperature at the precinct scale,” Santamouris said. “For a sweltering city the size of Riyadh, significantly reducing cooling needs is also tremendous for sustainability.”

The research also simulated the energy impact of retrofitting measures for all 3323 buildings alongside heat mitigation technologies implemented at the urban scale. Combining the optimal cooling technologies with energy retrofitting options — namely, improving the building envelope through better windows, insulation, solar and cool roofs — could decrease the cooling demand by up to 35%.

“This represents a substantial reduction to the energy needs for Riyadh that would help further reduce costs associated with cooling for the city while improving the quality of life for the local population,” Santamouris said.

The researchers now hope to work with the Royal Commission of Riyadh to begin implementing the tailored heat mitigation plan in the city, which they said would be the largest of its kind in the world.

The study team also included researchers from the University of Sydney, Lawrence Berkeley National Laboratory, the Royal Commission of Riyadh City, the University of Calcutta and the University of Athens.

Image credit: iStock.com/JohnnyGreig