Spraying buildings with water collected from rooftops could help cities reduce the impact of heatwaves, a study has found.
Using sprinklers in hot weather to distribute rainwater stored in water butts could cut air conditioning use and lower urban temperatures, according to research from the University of Manchester.
Rising temperatures globally are putting pressure on public health, infrastructure and energy systems. In Europe, a summer of heatwaves has led to concerns over power production due to low river levels needed for cooling, while wildfires have come alarmingly close to a nuclear power plant in the UK.
As people rely more on air conditioning to stay cool, energy demand increases and waste heat released from buildings can make towns and cities even hotter. A simulation model using Tokyo as a case study proved that automatically cooling rooftops once they reached a certain temperature could reduce the amount of energy used by A/C units.
In the study, published in Earth’s Future, cooler roofs transferred less heat into buildings, while lower air conditioning use meant less heat was released into the immediate surroundings. The researchers found this approach helped reduce urban temperatures, cutting the number of heatwave days overall and lessening the intensity of extreme heat events.
Lead author Dr Zhonghua Zheng of Manchester Environmental Research Institute (Meri), said: "Cities around the world are facing growing challenges from extreme heat. Air conditioning can help keep people safe and comfortable, but it also consumes large amounts of energy and releases additional heat into the urban environment.
"Our study shows that harvesting rainwater from roofs and using it strategically for cooling could provide a practical way to reduce both energy demand and urban temperatures. What is particularly encouraging is that the benefits become even greater during hotter years, suggesting this approach could become increasingly important as the climate continues to warm.”
The researchers believe introducing rooftop cooling systems could have a significant impact. In the Tokyo model, the system saved around 100 MWh of AC electricity per year, with even bigger savings in warmer years. Over the 10-year simulation period, the extreme heat hours on roof surfaces were cut by 592 to 1,221 hours, and street extreme temperature hours dropped by 86 to 297 hours.
However, Dr Zheng told The National there was no one-size-fits-all solution as any cooling system must be evaluated against considerations such as cost, water availability and local regulations. Choosing the correct moment to switch on the sprinklers had a greater impact than either the size of the tank or the amount of water applied, the study showed.
The researchers also found that bigger is not always better. Very large tanks delivered only modest additional reductions in energy use and extreme heat, while applying extra water did not always lead to more cooling because some of it remained on the roof instead of evaporating.
“The trigger is roof surface temperature,” he said. “For the Tokyo case, the model found the sweet spot between 35°C and 40°C. But that's site-specific as local climate and building characteristics vary across different sites, so no single temperature works everywhere. However, our AI-enabled framework can be used to figure that out for any other location.”
“The model showed that a 4.4mm tank per roof area got you most of the benefit, scaling up to 20mm didn't add much, in our case study. Water only gets released when the temperature threshold is hit.”
Junjie Yu, PhD researcher at the University of Manchester, said the rainwater tank shows a “natural solution to natural challenges, in which rainwater serves as a natural resource to mitigate both thermal stress and hydrological extremes”.
