Scientists in the Netherlands have assessed how the so-called blue-green roofs can help reduce the operating temperature of rooftop PV panels and have found they provide a significant cooling effect. The irrigation system used for the research project relies on an additional water supply sourced from grey water from showers that is transformed into irrigation water.

A research group led by the Netherlands’ Water Research Institute has investigated how a blue-green roof (BGR) may act as a cooling agent for rooftop PV systems and has found that this kind of roof may lower the roof surface temperature by up to 4.64 C compared to a conventional bitumen roof (BiR).

Blue-green roofs are roofs that use “green” technologies, such as lateral drainage and irrigation for plant and crop growth, as well as “blue” technologies like rainwater storage and dosage.

The scientists conducted a series of tests on a PV system installed on a so-called constructed wetroof in an apartment building in Amsterdam. Constructed wet roofs are roofs that use natural processes involving wetland vegetation, soils, and their associated microbial assemblages to improve water quality. The irrigation system used for the research project relies on an additional water supply sourced from grey water from showers that is transformed into irrigation water.

The roof was also equipped with a Permavoid 85s rainwater retention system, a capillary irrigation system, and a substrate layer of 6 cm. “Using the treated grey water, the water level in the water storage layer below the vegetation is kept at a minimum of 50 mm, ensuring a sufficient water supply for the vegetation,” the Dutch group explained, adding that 26 native plant species native were sown on the roof’s blankets.

The group’s findings were presented in the paper “Increasing solar panel output with blue-green roofs in water-circular and nature inclusive urban development,” which was recently published in Building and Environment.