A study, led by researchers from the University of Technology in Sydney, suggests that a ‘biosolar’ rooftop system – one that combines a solar system installed on a roof that is partially or completely covered with vegetation – provides significant improvements in panel efficiency and energy production.

“As solar panels heat up beyond 25℃, their efficiency decreases markedly,” the researchers said. “Green roofs moderate rooftop temperatures so we wanted to find out could green roofs help with the problem of heat reducing the output of solar panels.”

The research team, which also included academics from the University of Canberra and Charles Sturt University, compared a conventional rooftop PV system with a combined solar and integrated green roof system over an eight-month period that spanned summer and winter.

The two buildings used in the study are the same height, size and shape and located next to each other in Sydney’s central business district. Both rooftops encompass about 1,860 square metres, with about one third covered by solar panels. Vegetation covered about 78% of the green roof and the solar panels covered 40% of this planted area.

Both the green and conventional rooftop systems employed four SolarEdge three phase inverters rated to operate at a 98 % efficiency. The green roof comprised 335 SunPower Maxeon 395 W panels, with a total system size of 132.33 kWp. The conventional roof used 346 LG 320 W panels with a peak nominal power of 110.72 kWp.

“We measured the impacts on biodiversity and solar output, as well as how the plants coped with having panels installed above them,” the researchers said, noting that the green roof had reduced surface temperatures by up to 9.63℃ for the solar panels and 6.93℃ for the roof surfaces.

The research team said the reduction in temperature had delivered significant improvements in solar panel efficiency which led to increased solar generation and overall energy production.

“This lowering of temperatures increased the maximum output of the solar panels by 21-107%, depending on the month,” they said. “Performance modelling indicates an extensive green roof in central Sydney can, on average, produce 4.5% more electricity at any given light level.”

The researchers said the benefits of combing solar panels with a living roof extended beyond increased energy production with the vegetation on the biosolar roof flourishing in the areas directly below and surrounding the solar panels.

“This was surprising,” they said. “It was not expected the plants would prefer the shaded areas under the panels to the open areas. This shows that shading by solar panels will not prevent the growth of full and healthy roof gardens.”

The green roof had also supported more species of birds and insects than the conventional roof, decreased stormwater runoff, and insulated the building from extremes of temperature.