Developers interested in making their buildings more sustainable typically face a choice: solar panels or a green roof? There isn't, presumably, room for both.
Some University of Toronto researchers are challenging that assumption. This summer,
with the help of many government and private sector partners, they're launching a study looking at whether the two can be combined—at the possibility of installing one roof that uses both vegetation and solar panels. The bonus: if the researchers' hypothesis is correct, they won't just be making dual use of the same space; the cumulative effect of combining the technologies will provide greater environmental benefits than using them separately.
"Solar photo voltaics operate best when they are not overheated," explains Liat Margolis, director of UofT's Green Roof Innovation Testing (GRIT) Lab. "Ideally [the panels] would be in a relatively cool climate, but sunny; conversely when they are overheated their energy production drops. The hypothesis is that ...if the vegetation actually cools the air, that could improve the performance of the solar panels."
Basically: because green roofs create a cooling effect through the evaporation they facilitate, they will keep the solar panels above cooler, and thereby—so the theory goes—keep those panels working more efficiently.
The GRIT Lab is running the experiment on the roof of 230 College Street; it includes 40 solar panels installed two and four feet above a layer of vegetation. The study is still in the early stages: Margolis says they anticipate about a year of calibration and testing, and hope to begin collecting data next spring. They'll gather results for three growing seasons, to have a data sample that accounts for variations in the weather. (This summer's cool temperatures would likely yield different results than a much hotter summer might, for instance.)
The basic benefit of solar panels—energy generation—can be appealing over the long term, but since even the best solar panels are only about 18 per cent efficient, it can take eight to 10 years to reap the financial rewards of installing them.
Green roofs, meanwhile, provide other environmental benefits, such as stormwater management, and the reduction of flooding and erosion. This too is a tough sell, though: while these are genuine environmental concerns, they are generally managed by municipal governments rather than building owners. However, Margolis says, "I think water performance will become more and more of a factor as the public becomes more aware of the issue."
As we experience more major storm events, in other words, the incentive to use green roofs to mitigate storm effects will grow. The ultimate hope is that the combination of the two technologies will create a better business case for installing them both, and make it easier for developers to pursue environmentally friendlier projects by allowing them to see the financial impact of doing so more quickly.
Writer: Hamutal Dotan
Source: Liat Margolis, director, Green Roof Innovation Testing Lab
Photo: Courtesy of the John H. Daniels Faculty of Architecture, Landscape, and Design.