Solar photovoltaics (PV) and green roofs are increasingly being adopted worldwide as sustainable solutions for urban environments. While PV systems help to reduce reliance on fossil fuels and lower greenhouse gas emissions, green roofs lower building energy use for air conditioning, mitigate urban heat island effects, and enhance the aesthetics of rooftops.
When combined, these systems allow for more efficient use of rooftop space by harnessing the cooling benefits of rooftop greenery and the generation of renewable electricity from solar panels.
A joint research study by the National University of Singapore (NUS), the Building and Construction Authority (BCA), and the National Parks Board (NParks) demonstrates the benefits of co-locating solar panels and green roofs in tropical climates – an area that is less well-studied compared to temperate regions.
Leading the research effort from NUS is Associate Professor Stephen Tay from the Department of the Built Environment at the College of Design and Engineering in NUS. The research findings were published online in the scientific journal Applied Energy on 4 June 2025.
“Our study shows that co-locating solar photovoltaics with green roofs in a tropical climate is technically feasible with multiple benefits – from improving solar panel performance and supporting greenery growth, to lowering roof surface temperature. These findings highlight the potential of integrating solar energy generation with rooftop greenery to advance sustainable urban developments in Singapore and beyond,” said Assoc Prof Tay.
Comparative analysis of four different set-ups
To evaluate the performance of co-located solar panels and green roofs under tropical conditions, an experimental study was conducted on the seventh-floor rooftop of Alexandra Primary School. Four different set-ups were monitored over a 12-month period, from November 2021 to October 2022, and a comparative analysis was conducted to assess the impact on solar energy generation, plant growth, and roof temperature.
The four rooftop set-ups comprised a bare concrete roof (BR-CT), a bare green roof (BR-GR), solar panels on bare concrete (PV-CT), and solar panels with green roof (PV-GR).
The study found that the PV-GR set-up delivered the highest overall performance, achieving both the highest photovoltaic performance, along with the lowest and most stable roof surface temperature. This enhances solar energy generation while providing effective cooling through greenery and shading. In addition, greenery growth in the PV-GR set-up improved significantly compared to the BR-GR set-up, with a 19.8 per cent higher horizontal coverage.
Other benefits were also highlighted in the study. Firstly, in the PV-GR set-up, evapotranspiration from plants was found to have a cooling effect on solar panels, which increased the performance ratio of the solar panels by an average of 1.3 per cent under clear sky conditions as compared to the PV-CT set-up.
Secondly, the co-location of solar panels and green roofs allows buildings to stay cooler. Roof surface temperatures were reduced by up to 4.7 deg C when compared to the BR-CT set-up, while indoor ceiling surface temperatures decreased by as much as 3 deg C when compared to the PV-CT set-up, as the roof is shielded from direct sunlight. Additionally, the PV-GR set-up exhibited the lowest variability in indoor ceiling temperatures, with average ceiling temperatures remaining below 30.5 deg C when compared with the PV-CT set-up, where indoor ceiling temperature can exceed 33 deg C in the day.
Thirdly, the research also tested five shade-tolerant plant species for green roof applications. Among them, Pilea depressa, Pilea nummulariifolia, Heterotis rotundifolia, and Sphagneticola trilobata achieved an average of 20 per cent higher horizontal coverage for the PV-GR set-up. Notably, Pilea depressa showed the most significant improvement when compared to the BR-GR set-up, indicating its suitability for co-located solar panel–green roof installations.
