Combining the benefits of solar – PV and green roofs

Here in the UK while both green roofs and solar renewables technology are both becoming ubiquitous there is a lag in the specification of the two occupying the same roof area. There are a number of very good reasons why combining rooftop vegetation and renewable energy should be the case: there is somewhat counterintuitively a reciprocal relationship between solar and lvigins roofs.

On the one hand, the vegetation provides a service to the solar panels: primarily, and of most interest to every owner-operator of solar cells, German research has shown that solar panel output is increased are as much as 7% with panels positioned on vegetated green roof substrate compared with those on hard-standing. And with the reduction of the feed-in tariff limiting financial returns and lengthening payback periods, squeezing every last penny out of costly installations makes great economic sense.

How do the green roof components achieve this? Silicate PV panels work optimally at around 23C and American research, including on City Hall in Chicago, has shown that green roofs maintain the ambient air temperature at around 25C. Hard standing on traditional roofs, including concrete, asphalt and metal roofs, all radiate heat and substantially increase the ambient temperature – by as much as 30C in the case of the Chicago.

And the quo for this quid is that the solar panels, by providing a shaded area underneath each unit, can – with certain important but inexpensive design features – create an alternate type of habitat within the green roof expanse that effectively doubles its biodiversity potential. Think about it this way: if a buildings design team design specify a biodiverse wildflower meadow roof, the BREEAM points available are calculated on the number of species that are restituted at roof level. Even with a planting scheme tailored to the local biodiversity action plan, as we encourage our clients to do, there are only a certain number of flora species which will prosper where the roof space has a single aspect, orientation and so on. By providing shade, a differentiated micro-climate emerges in which certain species of plant will prosper but also significantly different kinds of invertebrates’ habitat will form.

It is important to design this symbiotic space appropriately, however. The panels do not only create shade, they also prevent rainwater from reaching the substrate underneath. If no plants are able to survive because the substrate is too dry, both the potential biodiversity benefits and some of the thermal cooling properties are reduced (because those plants would have been evapotranspirating (sweating) directly onto the panel’s underside). To ensure good plant development it is advisable to both increase the substrate depth in shaded areas and to install a wicking layer that carries water from the front of the panel where the water hitting the face falls to the shaded area.

A further design consideration is how the solar panel frame intersects with the structure underneath. We can install mechanically fixed frames but increasingly popular are ballasted system which are weighted down against wind and shear by the green roof itself. This avoids penetrating the membrane. Solar panel frames can now be bought which are pre-integrated with green roof drainage.

With a synergistic approach to building design, letting nature do the work can provide multiple quick wins for all involved. There’s a culture shift required to get the solar/electrical trade, architects, green roof companies, clients and suppliers all talking but the quick wins are out the for those with enough foresight and grasp of the bigger picture to make the effort amply worthwhile.