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 study the impact of cool roofs (high-albedo roofs) and rooftop photovoltaics (RPV) on air temperature and heat-related mortality in London during the record-breaking hot summer of 2018.

Background

With thousands of deaths attributed to heat in recent years and the increasing frequency and intensity of heat waves due to climate change, heat is becoming a major concern in the United Kingdom (UK). Notably, the summer (June to August) of 2018 was the hottest on record in England (maximum average temperature).

The urban areas experience the heat island effect due to their built environment, altering the energy balance of the land surface. As a solution, applying high-albedo materials to roofs can reduce heat flux into buildings, passively reducing indoor temperature and cooling demand. Alternatively, RPV, with a lower albedo than a typical roof, can provide renewable energy without taking up additional land.

Urban climate models, combining the energy balance of the urban environment with an atmospheric model, help estimate the thermal effects of roofing technologies at the city scale. Thus, this study employed urban climate modeling to estimate the effect that cool roofs and RPV could have had on urban canopy air temperature in London during the 2018 summer. 

Methods

June to August 2018 was chosen as the study period due to its record-breaking average temperature. The Weather Research and Forecast (WRF, version 4.3) regional climate model was used to simulate urban temperatures in London and Southeast England during this period. The embedded three-dimensional (3D) building effect parameterization model combined with the building energy model (BEP-BEM) was used to simulate the impacts of cities and urban anthropogenic heat emissions on the local climate.

Urban morphological parameters and land-use classes were derived from the European local climate zone (LCZ) map of the World Urban Database and Access Portal Tool (WUDAPT) following the WUDAPT-TO-WRF strategy and using the Python code. Albedo and other thermal parameters were set according to LCZ.

The RPV were modeled in detail to include convective and radiative heat fluxes from the bottom and top of the panel and conversion efficiency response to temperature. The baseline was adjusted for bias using data from personal weather stations (PWS).

The relation between exposure (temperature) and response (risk of mortality) was derived from exposure-response functions (ERF). The population from the 2021 census was used to population weight the temperature. Notably, only the areas in the Greater London boundary were included.

The economic burden of attributable mortality was determined by multiplying it with the value of statistical life sourced from the UK Government appraisal guidance. Finally, total power generation was determined from the solar power model within the BEP-BEM model.

Results and Discussion

Different scenarios were analyzed using the proposed climate model. Notably, the mean differences from the baseline scenario in population-weighted mean urban-canopy temperature were −0.3, −0.8, and −1.9 °C in the RPV, cool roof, and nonurban scenarios, respectively. Additionally, the root mean squared error between the modeled urban temperature and observations was 1.0 °C.

Different temperatures were observed for different scenarios distributed across the population of London. The Greater London region experienced a lower mean temperature in the cool roof and RPV scenarios. Additionally, the mean daily maximum temperature was lower in these scenarios, but the difference in mean daily minimum temperature was little. Alternatively, the nonurban scenario exhibited greatly reduced daily minimum temperature but little difference in daily maximum temperature.

The spatial pattern was significantly different between the cool roof and nonurban scenarios, emphasizing the conceptual difference between minimizing the urban heat island intensity (urban-rural nighttime differences) and mitigating heat (high temperatures). Additionally, the urban climate model estimated electricity output at 20 TWh in June-August 2018 for a full RPV coverage scenario in Greater London. The order is comparable to London’s total electricity usage in 2018 (37.8 TWh).

Considering a mixed scenario of different rooftop technologies, 65% of the roof area was estimated to be suitable for RPV, and the remaining 35% for cool roofs. This would have avoided 150 deaths and saved 6.2 and 13 TWh of electrical energy worth £357 million and £4.7 billion compared to the baseline.

Conclusion

Overall, the researchers successfully employed urban climate modeling to determine the possible influence of cool roofs and RPV on the temperature exposure of people and electricity demand in London during a hot summer like 2018. Moreover, the related mortality rates and economic influences were analyzed.

It was concluded that wide adoption of cool roofs would more effectively decrease average outdoor air temperature compared with existing low-albedo roofs. RPV would also reduce temperatures but not as effectively. Such a comprehensive evaluation of different urban temperature mitigating strategies can help design strategic and economic policies incentivizing or enabling the adoption of cool roofs and RPV 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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