Green Roofs Could Help Improve Solar Panel Efficiency

There has been a movement in architecture in recent decades to help connect major urban developments with plants and greenery. We’ve seen a few buildings, and hundreds more renders, of tall skyscrapers and large buildings covered in vegetation.

The aesthetic is often beautiful, but the idea is done just as well for the tangible benefits as it is for the sheer visual glory. Of course, there’s the obvious boost of plants converting carbon dioxide into delicious, life-giving oxygen. However, according to a recent study, greenery on the roofs of buildings can also help improve the yield of solar installations from Sydney, Australia.

The solar installation on top of Daramu House, Sydney, complete with greenery on the roof. Source: UTS report

The study was led by Dr. Peter Irga of the University of Technology Sydney, with a report published for the city of Sydney. The opportunity for the research suddenly came about thanks to two similar office buildings side by side in central Sydney. Each building had a photovoltaic solar system installed on the roof to generate electricity. On one building, plenty of plants were placed on the roof and around the solar panels, while the other building was left bare.

Over an eight-month period, the green-loaded roof was 3.6% more efficient than the bare roof over the course of the experiment. The difference between the two was as much as 20% at peak times. This led to the green roof with 69 MWh of electricity versus 59.5 MWh for the bare roof. The additional 9.5 MWh generated during the experiment is worth $2595 at local market rates.

The key to the difference in performance came down to the temperature. Solar panels don’t work as well at higher temperatures, with Irga noting that “temperatures above 25 degrees make photovoltaic panels less efficient.” This can be problematic in a place like Australia, where sunlight is abundant in the summer months but daily temperatures are routinely 30 to 45 degrees Celsius.

A graph showing the difference in performance between the solar installations on the two buildings. Arrows A, B and C point to areas where city shadow caused a discrepancy.
However, pay attention to the big boost that the green roof gets at peak times. The cooling effect is most important in full sun when temperatures are highest.
Source: UTS report

Green roofs cool buildings through a process called evaporation, or rather, the twin processes of evaporation and transpiration. Water from the soil and other roof surfaces is evaporated, reducing the heat in the air. In addition, small holes in the plants of the green roof, called stomata, are essentially the pores through which the plant exchanges gases with its environment. The plants lose water to the atmosphere through these stomata, which further aids the cooling process. Ideally, the vast majority of this water comes from rainfall, avoiding irrigation costs that can compromise the efficiency and environmental benefits of the roof as a whole.

Reportedly, temperatures on the green roof were in the order of 20°C lower than in the otherwise identical bare-roofed office building. This is a remarkable figure, and one that speaks to the quality of the green roof design in the experiment. This comes down to a careful selection of the right plant species, which can survive and thrive on the roof while providing good cooling performance.

This significant drop in temperature thus allowed the solar panels to remain in a much more efficient operating range, leading to that 3.6% efficiency gain. This figure was determined under simulated lighting conditions, to prevent differences in the urban environment around the two buildings from spoiling the result. It may not sound like much, but huge amounts of money are spent every year researching percentage gains in solar panels. In comparison, offering a cheap natural cooling solution can have a remarkably outrageous effect.

Garage with
Both rooftop solar and green roofs must be carefully designed to capture shade from surrounding buildings. This can affect the amount of power that can be generated, as well as the use of suitable plant species for the amount of sun available.

The green roof also offers other advantages. The study found that the roof absorbed nearly 9 tons of greenhouse gases during the experiment and also significantly reduced stormwater runoff. The plants were also highly valued by the local wildlife. The team noted that insects and birds flocked to the greenery quickly. Even predator species were spotted atop the building, which was surprising to see on a tower in Sydney’s central business district.

Overall, it is a project that shows many net benefits. Moreover, it does not have to be limited to green roofs only. Other solar installations could benefit from co-located greenery that naturally cools the environment and leads to better solar panel performance. Expect more research in this area, especially in a localized way. Green roofs and similar technologies are highly dependent on local climatic conditions and often must also be designed to work with the local flora and fauna. For those who dive in, it looks like there are significant gains to be had!

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