Co-locating solar panels with “green roofs” in tropical climates can boost solar energy output, improve greenery growth and reduce indoor building temperatures, according to a joint study by researchers at CDE, the Building and Construction Authority (BCA), and the National Parks Board (NParks).
Increasingly adopted worldwide as sustainable urban solutions, solar photovoltaics (PV) help cut reliance on fossil fuels while green roofs reduce energy use for cooling and mitigate heat build-up in cities.
The study, led by Associate Professor Stephen Tay (Department of the Built Environment), shows that combining the two systems delivers multiple additional benefits compared to deploying them separately. These include boosting solar panel efficiency, increased support for greenery growth, and a lowering of both roof and indoor ceiling temperatures.
“These findings highlight the potential of integrating renewable energy generation and greenery through solar photovoltaic with rooftop greenery to advance sustainable urban development in Singapore and beyond,” Assoc Prof Tay said.
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 renewable energy generation, plant growth, and roof temperature.
The four rooftop set-ups comprised:
- a bare concrete roof,
- a bare green roof,
- solar panels on bare concrete, and
- solar panels on a green roof.
The study found that the solar panels with green roof set-up delivered the highest overall performance, achieving both the highest PV 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 solar panel-green roof set-up improved significantly compared to the bare green roof set-up, with a 19.8 per cent higher horizontal coverage.
Other benefits were also highlighted in the study. Firstly, in the solar panel with green roof 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 solar panel–concrete set-up.
Secondly, the co-location of solar panels and green roofs allow buildings to stay cooler. Roof surface temperatures were reduced by up to 4.7 degrees Celsius compared to the bare concrete roof, while indoor ceiling surface temperatures decreased by as much as 3 deg C, as the roof is shielded from direct sunlight. Additionally, the solar panel–green roof set-up exhibited the lowest variability in indoor ceiling temperatures, with average ceiling temperatures remaining below 30.5 deg C throughout the day and night, in contrast to the solar panel–concrete roof set-up, where indoor ceiling temperature can exceed 33 deg C in the day.
The research also tested five shade-tolerant plant species for green roof applications, four of which achieved an average of 20 per cent higher horizontal coverage for the solar panel–green roof set-up. Notably, a herbaceous plant called Pilea depressa, commonly known as ‘Baby Tears’, showed the most significant improvement when compared between the solar panel–green roof and bare green roof set-ups, indicating its suitability for co-located solar panel–green roof installations.
For building owners, the outcomes of the study demonstrate the technical feasibility and added benefits of solar panel–green roof set-ups. Cumulatively, the impact from these benefits may translate into potential cost savings.
Given Singapore’s limited rooftop space, the co-location of solar panels and green roofs represents a smarter use of space, contributing to green building initiatives and advancing its vision of becoming both a “Low-Carbon City” and a “City in Nature”.
