Many people only think in terms of having either a green roof or solar power. In actual fact combining the two technologies is common in other countries such as Switzerland, Germany and Austria. Roofs combining green roofs and solar power are often referred to as Biosolar roofs.
Green roof and solar photovoltaic (PV) systems are two technologies that could contribute to sustainable building development and reduction of greenhouse gas emissions. Integrating them can enhance the way they work and their overall effectiveness by cooling and shading effects.
The results of various experiments show a positive outcome from this integration: green roof surface and soil temperatures are reduced from the shading and higher power output of PV panels is achieved from the cooling effect. Findings from year-round building energy simulation using EnergyPlus for a low-rise commercial building indicated that the energy consumption for air conditioning of the integrated system was slightly lower than the stand-alone system and the PV system on integrated approach generated 8.3% more electricity than the stand-alone option. The degree of benefit gained will depend on the system design and whether the optimum arrangement has been found for the particular building site.
There is considerable evidence that combining solar panels and vegetation can produce the following benefits:
- Solar/Photovoltaic panels can work more efficiently on a roof when installed over a green roof system. The green roof can have a cooling effect especially in the summer. If the micro-climate around the panels is too hot the panels may not work so well. Green roofs can help to keep ambient temperatures around the panels at or close to 25c which is the best temperature for solar panels to work efficiently.
- Installing A-frame panels is made easier on a green roof as it provides the ballast for holding the A-frames and panels in place. This also means that there is no impact on the waterproofing layer below.
- The diversity of vegetation and therefore fauna using the green roof should increase with the PV panels providing shaded areas underneath and rain run-off creating damper areas to the front and drier areas behind. This creates a ‘habitat mosaic’ which allows a wider variety of vegetation to flourish and this in turn can attract a wider range of butterflies, bees, beetles and other species.
It is very important however to design and install biosolar roofs correctly. If too much emphasis is put on the solar panel element of the installation and the green roof seen only as decoration the positives of combining the two can easily turn into negatives. One example would be where panels are placed too close together and entirely the wrong sort of plants take over.
Students at Kansas university compared three types of roof materials, where solar panels are mostly placed. The performances of the panels placed on white or reflective rooftops were compared to conventional (black) and vegetated rooftops.
They installed temperature, humidity and light sensors and a weather station to record conditions like wind speed. The sensors made recordings every five minutes for a year.
Current industry notions are that white, being the most reflective, is the most efficient; but the team found that the black roof performed better because the heat reflected by the white surface onto the panels was too intense.
Overall though they found that vegetative roofs performed best compared to the high-reflective and conventional roof material.
The panels installed over the green roof performed best, generating an average of 1.4 percent more energy as compared with those over the white and black roofs, said a press release by University of Kansas.
Unfortunately, more often than not architects decide to separate these two technologies on a roof. But as time goes on and the pressure mounts to meet renewable energy targets surely green roofs with solar energy generation will become part of a smart future.
Find out more about solar here.
Find out more about green roofs here.