Harvesting rainwater from rooftops and spraying it onto buildings during heatwaves could cut air conditioning use and lower urban temperatures, research suggests.
The study, from the University of Manchester, found that a system storing rainwater collected from rooftops and automatically spraying it onto buildings during hot weather could reduce the number of heatwave days a city experiences and lessen the intensity of extreme heat events.
Cities around the world are facing rising temperatures, putting pressure on public health, infrastructure and energy systems. As more people rely on air conditioning to stay cool, energy demand increases – and waste heat released from buildings can make urban areas even hotter.
Urban watering technologies are becoming an important tool for reducing extreme heat in cities, but their use is often limited by the availability of water.
Researchers used process-based numerical simulations and artificial intelligence (AI) to test the rooftop system, using Tokyo as a case study. They found that cooling rooftops reduced the energy needed for air conditioning: cooler roofs transferred less heat into buildings, while lower air conditioning use meant less waste heat was released into the city. Together, these effects helped cut urban temperatures.
The study found that when the sprinklers switched on – for example, once a roof reached a certain temperature – had a greater impact on performance than either the size of the storage tank or the volume of water applied. Very large tanks delivered only modest additional reductions in energy use and extreme heat, while applying more water did not always lead to more cooling, as some of it remained on the roof rather than evaporating.
Growing challenges
The researchers said the approach could help local authorities and urban planners evaluate how rainwater-based cooling systems might work in their own regions, while balancing considerations such as cost, water availability and local regulations.
Dr Zhonghua Zheng, co-lead for environmental data science and AI at the Manchester Environmental Research Institute (MERI) and associate professor in data science and environmental analytics at the University of Manchester, 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.”
Junjie Yu, PhD researcher at the University of Manchester, added: “The rainwater tank also provides an additional co-benefit on reducing the extreme urban runoff. This approach exemplifies a ‘natural solution to natural challenges’, in which rainwater serves as a natural resource to mitigate both thermal stress and hydrological extremes.”
Image credit: Anna Nikonorova/Shutterstock
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