By pv magazine Germany

The German PV company Polarstern and the Wilhelm Büchner University have published a study stating that the output power of photovoltaic systems can be increased by up to 8% when installed on a green roof. The cooling effect of the plants through transpiration and evaporation, their ability to absorb dust and the diffuse reflection of sunlight by the foliage are the main factors determining this performance improvement.

Due to various effects such as transpiration and evaporation by the foliage and the casting of shadows, as well as the absorption of light for photosynthesis, green roofs have a lower ambient temperature than conventional roofs and as a result the operating temperature of a PV system is also lower. According to the study, the output power of solar panels can improve by an average of 4.35%. Depending on the technology used and the region, this value can even rise to approximately 8%.

The plants also act as an air filter by absorbing the dust washed off the modules. According to company estimates, a green roof binds about two kilograms of particulate matter per square meter per year. The amount also depends on the size of the leaf surfaces and the dust resistance of the plants. Dust-induced energy yield losses in photovoltaic systems in urban regions are estimated to be up to 30%.

According to the company, not only avoiding dust, but also the reflection of the light from the leaves increases the performance of the solar panels, as they can use more radiation from the solar spectrum. This diffuse reflection causes a higher low light radiation, which can lead to higher yields, especially with thin film modules. Citing unspecified field studies, Polarstern said a green roof can increase irradiance by up to 32% compared to a gravel roof.

In addition to the increased efficiency of the photovoltaic systems, the green roofs also ensure more biodiversity due to the water-storing capacity of the base plate.

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