First, we validated and calibrated the models using real roof temperature and solar energy yield data from roofs in Zürich, Switzerland. Then, we applied them to 13 cities with different climates, from the consistently warm and humid conditions of Singapore to the seasonal extremes of Montreal. For each city, we ran simulations for current conditions and a future climate scenario (year 2090), based on a high-emissions trajectory, assuming greenhouse gas emissions continue at present levels. This allowed us to see how climate change might affect solar panel performance on different roof types. By comparing the results across roof types, cities, and time periods, we could determine how each roofing solution performs in different climates, both now and in the future.
Key Findings
1. Annual Energy Gains Are Modest
Our analysis revealed that annual PV efficiency gains from sustainable roofing, relative to the baseline gravel roofing, remain modest, consistently below 2% in all considered locations. These results challenge widely cited studies reporting gains as high as 8.3%, which were often based on short-term experiments conducted under ideal conditions, such as sunny summer days or constantly saturated green roofs. This difference highlights the importance of evaluating year-round performance to better understand these technologies’ real impact.
2. Green Roofs’ Benefits Depend on Rainfall
The cooling benefits of green roofs are highly dependent on sufficient moisture levels, which are influenced by natural precipitation or irrigation. In rainy climates, such as tropical or temperate zones, green roofs provided consistent cooling benefits due to the evapotranspiration process from the vegetation. However, the lack of moisture significantly reduced their effectiveness in water-scarce or arid climates. Without irrigation, green roofs often underperformed, especially during dry or seasonal drought conditions.
3. White Roofs Outperform Green Roofs
In most scenarios and climates, white roofs provided larger reductions in rooftop temperatures and higher PV efficiency improvements than green roofs. Their performance was particularly strong in hot, arid climates where the reflective properties of white roofs effectively mitigated excessive heat. Unlike green roofs, the performance of white roofs was not dependent on moisture availability, making them a more reliable option across a broader range of climates.
4. Limited Improvements Under Future Climates
Future climate scenarios paint a mixed picture for sustainable roofing. While some cities stand to benefit from increased solar irradiation, which could enhance overall PV yields, rising temperatures are expected to offset these gains by reducing the conversion efficiency of photovoltaic systems, which declines as panel temperatures increase. For example:
- Temperature Rise: Higher global temperatures will reduce the efficiency of converting solar energy into electricity, with declines seen across all examined cities.
- Solar Irradiation: Some cities, particularly in regions with reduced cloud cover, could see a slight increase in absolute PV yields, but this alone does not compensate for the efficiency loss.
- Rainfall Changes: Reduced precipitation in certain regions could further limit the performance of green roofs, especially in future arid climates, while wetter regions may continue to support evapotranspiration.
Combined sustainable roofs help mitigate some of the negative impacts of rising temperatures by reducing rooftop heat and PV panel temperatures. However, their benefits will remain modest and cannot justify replacing existing roofing systems before the end of their functional life. Instead, replacements should be considered as part of natural building and roofing upgrades.
