Photovoltaic Panels, Frieburg, Germany - this city probably has the highest density of green roofs and PV Panels

The issue of competition at roof space for various 'green' approaches often raises it's head especially when it comes to a percieved conflict between Solar Power and green roofs. This could not be further from the truth - there is no conflict and in fact there are many advantages of combining green roof technology and Solar Energy technology.

Rear of Photovoltaic Panel, Frieburg, Germany

Combing Photovoltaic Panels and Green Roofs

The combination of the two is actually a good example of an ecosystem services approach to the built environment. Whilst the green roof provides a range of benefits to the environment - reductio in the Urban Heat Island Effect, thermal cooling and insulation , reduction in airbourne particles, reduction in flash floods and stormwater management and increase in biodiversity, the positioning of Photovoltaic panels on roofs provide a renewable energy source for the building. There are a number of studies in germany that actually demonstrate that the evapotranspiration form the plants on a green roof can have a very positive effect on the energy efficiency of PV panels. In general PVs are relatively inefficient and this inefficicency can be compounded on a roof in the height of summer. Black and grey roofs can get very hot when direct sunlight is bounding the surface. This is especially true in the height of summer when sun at it's peak in terms of the production of energy. However the surface temperature of roofs can effect the ambient temperature of the air around the panels thus reducing their efficiency. Research suggests that PVs work best with an ambient air temperature of about 24 degrees C. A PV panel can lose 0.5% efficiency per degree C above 25 degrees C.

Photovoltaic Panels, Zinco Headquarters, Neurtingen, Germany

A green roof, however, maintains a lower surface temperature compared to a black or grey roof and because the plants on a roof transpire, this can help maintain the ambient air temperature around a PV panel between 20 - 28 degrees C. Thus the green roof helps maintain the efficiency of the PV panels.

A further advantage of using green roofs and PVs together is installation. It is quite common in Germany and Switzerland for the legs of PV panels to be pre-installed into drainage layers. Once on the roof the substrates/growing medium to be used acts as ballast, holding the PVs in place. This can ease installation and also prevent unecessary impacts on the waterproofing layer.

The addition of an array of PV panels can cause extreme shading underneath the panels. However, although this may stunt the growth of vegetation it actual provides a unique shady habitat. Research in Switzerland has shown that this can actually increase the number of species of invertebrates that able to inhabitat a green roof. Further more water run off form the surface of the panels can concentrate moisture at the foot of the panel allowing a greater diversity of vegetation to flourish than would be expected on a fully exposed extensive green roof.

Solar Thermal and Green Roofs

Whilst there is a positive benefit in terms of energy efficiency when green roofs and Photovoltaic panels are combined, there is a neutral effect in terms of heating water through the use of solar thermal panels.

Solar thermal evacuated Tubes, The Muse, London

However, depending on the type of solar thermal panel used, there can actually be a positive benefit on the vegetation and therefore the biodiversity value of a green roof. This is especially important where green roofs are planned in hot and dry climates.

Solar thermal panels that use evacuated tubes provide shade to the vegetation beneath the panels. They also allow rain to pass between the tubes thus providing added moisture. This can allow vegetation to be more varied and lusher.

Ecosystem Services, Solar Power and Green Roofs - the potential

One of the largest Green Roofs and Photovoltaic Arrays in Europe, Messe Hall, Basel, Switzerland

Whilst there is no conflict between the combined use of Solar Technology and green roofs, there is potential for the two technologies to be considered in more detail in terms of design. The Green Roof Consultancy is considering a number of design ideas to use 'forests' of various PV and solar thermal Panels to help create interesting green roof spaces. Not only will this embrace the need for the creation of renewable energy centres at roof level but also increase the interest of the green roof from both a visual and biodiversity perspective. Furthermore this concepts may actually provide an increase the wider benefits of green roofs on the Urban Heat Island Effect and storm water issues.

Dusty Gedge is director of Livingroofs.org and Lead Consultant with the Green Roof Consultancy