Rainwater collected from rooftops could help cool buildings and cut air conditioning use during heatwaves, according to a new study from the University of Manchester in the United Kingdom.
The research modeled a system that stores rainwater gathered from roofs and automatically sprays it on buildings during periods of extreme heat. The study was published in the journal Earth's Future.
The researchers found that cooler roofs transfer less heat into buildings, which lowers the need for air conditioning and the energy that cooling normally requires. Less air conditioning also means less residual heat released into the city, which the researchers said could help reduce urban temperatures, cut the number of heat days and ease the intensity of extreme heat episodes.
Timing matters more than tank size
Using Tokyo as an example city, the study found that the timing of when sprinklers switched on, such as when a roof reached a certain temperature, mattered more than the size of the water tank or the amount of water used. Very large tanks brought only a marginal additional reduction in energy use and extreme heat, and using more water did not always improve cooling, since some of it stayed on the roof instead of evaporating.
Dr. Zhonghua Zheng of the University of Manchester, the study's lead author, said cities around the world face growing challenges from extreme heat. He said air conditioning helps keep people safe and comfortable but consumes large amounts of energy and adds heat to the urban environment, and that collecting rainwater from roofs and using it strategically for cooling could be a practical way to reduce both energy consumption and city temperatures.
Zheng added that the benefits of collecting rooftop rainwater grow even larger in hotter years, suggesting the approach could become more valuable as the climate continues to warm. The study could help local authorities and urban planners assess how rainwater-based cooling systems might work in their regions, taking into account practical factors such as cost and water availability.
Junjie Yu, a doctoral student at the University of Manchester, said a rainwater tank also brings the added benefit of reducing extreme urban runoff, describing the approach as a natural solution to natural challenges in which rainwater serves as a resource for easing both heat stress and hydrological extremes.
Why air conditioning falls short
The International Energy Agency estimates that space cooling, mainly air conditioners and fans, consumed around 2,100 terawatt-hours of energy in 2022, about seven percent of the world's electricity generation that year. Experts warn the number of air conditioning units could triple over the next three decades to reach 5.5 billion as extreme heat becomes the new normal, raising concerns about added strain on Europe's already stretched power grid and rising emissions.
Air conditioners also rely on refrigerants such as hydrofluorocarbons and hydrochlorofluorocarbons, potent greenhouse gases that trap thousands of times more heat than CO2. The EU and UK are phasing out fluorinated gases in favor of natural refrigerants such as hydrocarbons and CO2, though propane's flammability complicates installation. The process of pulling heat from inside homes and venting it outside also contributes to the urban heat island effect, which is why big cities tend to face greater heat stress than surrounding rural areas.
The case for 'cool roofs'
A 2024 study by UCL and the University of Exeter examined the cooling effect that white or reflective "cool roofs" would have had on air temperatures in London between June and August 2018, one of the city's hottest summers on record, when the three-month average temperature reached 19.2 degrees Celsius, about 1.6 degrees above the usual seasonal average.
The study found that widespread adoption of cool roofs could have cooled London by about 0.8 degrees Celsius on average. Dr. Charles Simpson of the UCL Bartlett School Environment, Energy & Resources, the lead author, said that if widely implemented, cool roofs could significantly lower ground-level air temperatures across a city, with a cooling effect felt citywide that would save lives and improve quality of life for residents.
UNIAN previously reported that New York had found a simple way to lower roof temperatures by nearly 25 degrees Celsius.
