Passive cooling solutions and mitigation strategies for urban heat islands
Urban areas occupy about 0.5% of the Earth's land surface but host more than half of the world's population.
The clustering of people and industry generates a phenomenon known as urban heat islands (UHI) — metropolitan areas that are warmer than the surrounding rural areas. This relative warmth, most perceivable at night, is caused by the absorption of solar radiation or “heat trapping” due to the built environment's configuration, the building materials' heat-absorbing properties, reduced ventilation and greenery, and waste or residual heat generated from domestic, transportation, and industrial activities. Most people compensate for the heat by turning up the air-conditioning, which further strains existing electrical infrastructure and contributes to the heat island effect by pumping hot air out to the street.
Cities worldwide are appointing Chief Heat Officers (CHOs) to protect city residents from the effects of excess heat through sustainable long-term mitigation and adaptation strategies. Scientists, urban labs, activists, and research organisations, all dedicated to solving complex challenges in the built environment, are looking increasingly at passive cooling solutions. Modern approaches include green roofs, urban forests, clever architectural designs, and high-reflectivity coatings, all leveraging natural processes to reduce temperatures in urban areas.
Green roofs consist of vegetation planted on building rooftops, which provide insulation, absorb solar radiation, and release moisture through evapotranspiration. This process not only cools the surrounding air but also reduces stormwater runoff and improves air quality by filtering pollutants. Using green roofs in cities or other built environments with limited vegetation can moderate the heat island effect, particularly during the day.
Urban forests promote growing trees strategically throughout cities to create cooling microclimates, lower ambient temperatures, and enhance overall urban biodiversity. Research shows that urban forests achieve temperatures that are on average 1.6°C lower than unforested urban areas, allowing them to mitigate UHIs by providing shade, absorbing carbon dioxide, and releasing oxygen.
Clever principles in architectural and building design incorporate features like orientation, shading, and ventilation to optimize airflow, minimise heat gain, and reduce reliance on mechanical cooling systems. For example, using thermal mass materials such as concrete and clay helps stabilize indoor temperatures by absorbing excess heat during the day and releasing it at night.
High-reflectivity coatings effectively reflect incident light, which bounces off the surface and is scattered away instead of absorbed and converted into heat. Factors such as the type of material used, the surface roughness, and the angle of incident light affect the performance of such coatings and their ability to keep surfaces cool.
Pirta’s mission is to expand the currently available passive cooling options. As a passive cooling R&D platform, we develop ingenious passive cooling and net zero solutions to address climate change. During this journey, we have developed Pirta paint, an affordable, non-toxic, water-based, and VOC-free cooling paint formulation that showcases a combination of strong solar reflectivity and thermal emittance, the abilities to reflect the sun’s energy and release heat away from a surface, respectively. This novel approach triples the cooling potential of currently available solutions from 40w/m2 to 120w/m2. It produces a cooling effect of 5 to 7°C below ambient temperatures, providing thermal comfort and reducing the need for energy-intensive cooling systems and carbon emissions.
The cities of the future must embrace novel technologies to successfully withstand the future challenges of urbanisation. Pirta Paint can be applied together with other passive cooling solutions to amplify their impact and mitigate the heath and financial costs of UHIs. By leveraging the synergies between available passive cooling approaches, cities can work towards creating more sustainable, resilient, and livable urban environments.
Sources:
1 World Urbanization Prospects. United Nations (2014)
2 Reuters: How 'chief heat officers' keep cities cool as the world warms (November, 2022)
3 Knight, T., S. Price, D. Bowler, et al. 2021. How effective is ‘greening’ of urban areas in reducing human exposure to ground-level ozone concentrations, UV exposure and the ‘urban heat island effect’? An updated systematic review. Environmental Evidence 10, 12.
