Cool Roofs and Solar Panels Combat London's Heat

By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Oct 10 2024

A recent article published in Nature Cities proposed using advanced urban climate modeling to assess the impact of cool roofs (high-albedo roofs) and rooftop photovoltaics (RPVs) on air temperature and heat-related mortality in London during the record-breaking summer heatwave of 2018.

Study Overview: Modeled Temperature, Mortality Impact, and External Benefits of Cool Roofs and RPVs

Heat-related deaths are becoming a growing concern in the United Kingdom as heatwaves increase in frequency and intensity due to climate change. The summer of 2018 was the hottest on record in England, with unprecedented average maximum temperatures, bringing the issue of extreme heat to the forefront.

Urban areas are particularly affected by the urban heat island effect, where dense infrastructure absorbs and traps heat, raising temperatures compared to rural surroundings. One proposed solution is the installation of cool roofs made from high-albedo materials that reflect sunlight, helping to reduce heat absorption by buildings. This strategy not only cools indoor spaces but also lowers the demand for air conditioning. Another promising approach is the use of RPVs, which generate renewable energy while occupying no extra land, although their cooling effect is less pronounced than that of cool roofs.

Urban climate models, which combine urban energy dynamics with atmospheric data, provide a valuable tool for estimating the city-wide effects of these technologies on air temperature. This study utilized such models to evaluate the potential impact of cool roofs and RPVs on air temperatures and heat-related mortality in London during the heatwave of 2018.

Methods

The study focused on the period between June and August 2018, chosen for its record-breaking temperatures. The research team used the Weather Research and Forecasting (WRF) model (version 4.3), enhanced by the Building Effect Parameterization (BEP-BEM) module, to simulate urban temperatures. This model incorporates urban structures and heat emissions to reflect the local climate dynamics more accurately.

Data on urban morphology and land-use classes were sourced from the European Local Climate Zone (LCZ) map through the World Urban Database and Access Portal Tool (WUDAPT), while albedo and other thermal properties were set based on LCZ classifications. RPVs were modeled to account for both convective and radiative heat fluxes, adjusting for the panels' conversion efficiency based on temperature changes. To improve accuracy, the model was calibrated with real-time data from personal weather stations across the area.

The research team calculated the relationship between temperature exposure and heat-related mortality using exposure-response functions (ERFs), and population data from the 2021 census was used to weight exposure by population. The analysis was confined to Greater London. The economic impact of heat-related mortality was calculated using the UK Government’s statistical life valuation, and solar power generation estimates were embedded within the climate model's BEP-BEM framework.

Results and Discussion

The analysis showed varying degrees of temperature reduction under different scenarios. Population-weighted mean urban canopy temperatures were reduced by −0.3 °C, −0.8 °C, and −1.9 °C in the RPV, cool roof, and nonurban scenarios, respectively. The model also exhibited a root mean squared error of 1.0 °C when comparing predicted urban temperatures to actual observations.

Temperature reductions were not uniform across London’s population. Both the cool roof and RPV scenarios showed notable decreases in mean maximum temperatures in Greater London, while daily minimum temperatures remained relatively stable. In contrast, the nonurban scenario led to a significant drop in daily minimum temperatures with minimal impact on maximum temperatures.

The spatial patterns of temperature reduction differed significantly between the cool roof and nonurban scenarios, emphasizing the conceptual difference between reducing the urban heat island effect (minimizing urban-rural nighttime temperature differences) and addressing overall high temperatures. The model further estimated that under full RPV coverage, electricity output in Greater London would reach 20 TWh from June to August 2018, nearly half of London’s total electricity usage that year (37.8 TWh).

A mixed scenario, where 65 % of the roof area is dedicated to RPVs and 35 % to cool roofs, offered substantial benefits. This combination could have prevented an estimated 150 deaths and saved between 6.2 and 13 TWh of electricity, resulting in economic savings of £357 million to £4.7 billion compared to the baseline scenario.

Conclusion

In summary, the researchers successfully employed urban climate modeling to assess the potential impacts of cool roofs and RPVs on temperature exposure and electricity demand during the 2018 London heatwave. The study also examined the corresponding mortality rates and economic implications.

Their analysis concluded that the widespread adoption of cool roofs would significantly reduce outdoor air temperatures compared to existing low-albedo roofs, offering greater cooling benefits. While RPVs also contributed to temperature reductions, their effect was less pronounced. This comprehensive assessment of urban heat mitigation strategies provides critical insights for shaping policies that incentivize the adoption of cool roofs and RPVs, promoting both environmental sustainability and economic savings in urban areas.

Journal Reference

Simpson, C. H. et al. (2024). Modeled temperature, mortality impact and external benefits of cool roofs and rooftop photovoltaics in London. Nature Cities. DOI: 10.1038/s44284-024-00138-1, https://www.nature.com/articles/s44284-024-00138-1

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