When planning an environmentally friendly roof, many people look at two options: either solar panels or a vegetated green roof. Often, however, there is a third option that is overlooked: solar panels and a green roof.
The addition of vegetation beneath solar panels actually increases the efficiency of the panels and means more electricity is produced. Similarly, PV cells positively influence the performance of a green roof and create different habitats that will benefit the local environment.
Vegetated areas are naturally cooler than built up areas. This is evident from the Urban Heat Island Effect, where built up areas absorb and reemit heat, increasing the temperature in the city. Plants perform evapo-transpiration, where water vapour is released into the atmosphere, taking with it latent heat, thus reducing the temperature of that surface. The reduction of heat creates ideal conditions for PV panels which perform better in cooler temperatures. On a hot day, a standard roof can easily reach temperatures of 45o; PV panels in this temperature will produce around 10% less electricity compared with the panels installed on a green roof under the same conditions.
PV panels also increase the diversity of green roofs by providing different niches for organisms to live. Many plants and animals, especially insects, prefer shaded areas and therefore cannot normally survive on living roofs. By adding these areas of shade, the number of species that will habituate the roof increases. This increases the species richness, and therefore the bio-diversity of the roof system.
Diversity can also be increased by contouring the soil. Areas of different depth soil suit different fauna and flora and increase the number of species able to survive on the roof. Not only will this help the wildlife, but also the solar panels above. The deeper substrate will store more water; increasing the evapo-transpiration on the roof and therefore keeping the temperature nearer to the solar panels’ optimum.
One of the main challenges with solar panels is installing them on a roof. The panels must be secure enough so that they will not be moved by strong winds, which often require holes to be drilled into the roof to provide anchor points. This threatens the integrity of the waterproofing membrane. Systems of PV panels have been developed that incorporate the green roof into their design. Using the weight of the system to hold the panels in place protects the membrane, which now does not need to be penetrated for anchor points and reduces its exposure to UV radiation. As UV exposure is one of the main causes of membrane degradation, a green roof is perfect to keep your roof healthy, as well as the environment.