Planted green roofs deliver a boost to solar output, study reveals

While researchers around the globe work tirelessly to produce incremental increases in solar cell efficiency, a Sydney-based study has found that putting PV panels on a planted “green roof” can boost their output by an average of 3.6 per cent, compared to a conventional rooftop array.

The study, backed by the City of Sydney and led by the University of Technology Sydney, compared the solar power output of two adjacent roofs of two relatively new buildings in Barangaroo, on the north-western edge of the Sydney CBD.

The green roof on Daramu House, a 6 Star Green Star rated commercial and retail building by property developer Lendlease, was installed by local “living infrastructure” company Junglefy in September 2019, made up of a selection of native grasses and herbaceous plants to cover 78.4% of the total roof space (the solar panels cover 40.6%).

Accounting for differences in size and efficiency of the two rooftop solar systems – outputs for the green roof were corrected for differences in panel efficiency as well as system size and and age – the green roof’s solar system emerged as the superior energy generator.

The team found that the green, or “biosolar” rooftop solar system was, on average, 3.63 per cent more efficient at generating power than the conventional rooftop system on any given day over produced the 8-month period of testing.

This might seem like a a small amount, but in an industry where fractions of a percentage of improvement in efficiencies are celebrated, it is quite an improvement – and really adds up when applied to larger rooftop arrays.

In this experiment, where the rooftop systems were sized at 132kW (green) and 110kW (regular), the green roof recorded an average production increase of 11.3kW a day, or a total of 9.5MWh more than the conventional roof over the 8-month period.

“The environmental impact of this is substantial,” the report concludes. “The difference in energy generated is equivalent to 110 trees planted, with an additional 1.1 t-CO2 potentially mitigated by photosynthetic activity of the plant foliage on the green roof.”

The research concluded that the increased output of the biosolar system appeared to come down to the cooling effect the greenery had on the building’s rooftop and its solar panels.

The study notes that the below-panel temperature observations for the green roof were consistently lower than the conventional roof across all seasons for both the average and maximum temperatures.

The maximum temperatures recorded within the soil on the green roof was 32.5 °C, compared to 63 °C on the conventional roof. The minimum temperatures however were similar between the two roofs above and below the panel, thus highlighting the thermal insulative properties of the green roof, too.

“Green roofs stabilise ambient temperatures and improve solar panel efficiency by creating more suitable temperature conditions for energy production,” the report says.

“There is a correlation between the reliability and performance of PV panels and surrounding ambient temperatures. As PV module surfaces heat up beyond optimal conditions, the panels’ efficiency decreases.”

And there are other benefits to having green rooftops, beyond boosted solar output, including hosting biodiversity, counteracting air pollution, reducing the urban heat island effect, and decreasing city-scape surface runoff from rainwater.

“Various forms of green infrastructure, such as green roofs, green walls and other space efficient green solutions are becoming increasingly valued,” said Peter Irga, the chief investigator of the UTS team.

“While it is understood that residential and roadside vegetation provides important urban functions, research looking at the way in which green roofs provide these same benefits is relatively understudied.”

Lucy Sharman, the sustainability manager at Lendlease said th integration of greenery into urban environments was vital to creating more liveable cities that coud mitigate the impacts of the changing climate.

“This research has made a valuable contribution to our understanding of the multiple, measurable benefits of green roofs,” Sharman said.

“We hope this extremely positive collaboration between industry, researchers and government inspires other organisations to look at the benefits of biosolar systems.”