Green Roofs or Solar Panels — why not both?
Demand for solar energy in a renewably-powered future could increase the value of open, sunny spaces. So, in thinking about the future of urban resilience and energy security, rooftops should become a coveted resource.
But there is more that a rooftop can be made into than a solar panel parking lot. Green roofs seem now to be an alternative route that can make a variety of beneficial and significant impacts, while the current number of photovoltaic panels required to make a meaningful input to the energy demand of a building is so high that it doesn’t seem like there can be space left for anything else to be kept on a roof.
Initiatives to create sustainable cities often assume that green roofs and rooftop gardens have to be separate from any space used for solar panels. It may turn out that by combining the two endeavors, the quality of both can be improved.
On their own, green roofs can provide a variety of benefits and services. In a dense urban space, they can mitigate the urban heat island effect by providing shade, and natural cooling via evapotranspiration. It can also act as natural insulation, and can reduce a building’s typical summer energy demands by up to 75%. In this way, green roofs also act as acoustic insulation, and can help reduce the noise pollution in busy parts of a city if positioned well.
A thoughtfully designed green roof intercepts rainfall and can mitigate surface runoff and flooding, which is a very important service in an urban place full of impervious surfaces. By reducing the direct wear and tear of weather on the roof itself, green roofs can also double or triple the lifespan of a typical roof. Green roofs can also improve air quality — in this EPA case study, green roof installations in Kansas City removed ninety pounds of particulate matter from the atmosphere.
Not only can these installations benefit the physical well-being of urban dwellers, they can also improve a community’s mental and emotional well being. The fact that green spaces can have a positive impact on people who encounter them is well researched and supported. It’s hard to find anyone who would be negatively impacted by these spaces — the animal world included. Though not a replacement for protecting ground-level habitat, a green roof as a type of mini-habitat can play a surprising role in supporting wildlife — for example, by providing a pitstop for migrating birds.
While the installation of a green roof may bring about a diverse set of benefits, it is not without cost. As a relatively new endeavor in cities, it is not known whether breaking even on a substantial green roof investment is even a possibility. An estimate from the University of Illinois places the break-even point at about 73 years, and there isn’t a studied green roof that has reached that age yet. And while it prevents run-off, cools the air, and basically absorbs air pollution, the Kansas City case study estimates that a green roof is only 40% as effective at preventing carbon dioxide from entering the atmosphere as the equivalent square footage of solar panels can.
Solar panels are a smart and sustainable addition to many rooftops. In fact, a roof full of solar panels has the potential to save 18 times more on energy costs than a green roof would in the same place. They are cheaper and more straight-forward to install, and remain low maintenance when compared to a green roof.
As solar technology currently stands, it is still a challenge to have both the budget and the roof space to make an installation of solar panels worth the investment. A small roofs with solar panels may only cover 2–5% of the energy usage for the building below. While the efficiency of solar energy technology is improving all the time, it may mean making costly updates in the near future to any panels installed now.
Some of the shortcomings and challenges of having either a green roof or a solar roof can be resolved or improved when these two project ideas are incorporated in a thoughtfully designed way. Nearby plants can actually improve the functioning of solar panels. For example, dirt and dust tend to reduce the efficacy of solar panels over time. By incorporating plants into the solar panel layout, the buildup of particulate matter on the panel surfaces is slowed down. A solar PV system can experience a decline in 0.3–0,4% efficacy for every 1 degree Celsius above 25, so as plants perform evapotranspiration and cool the air around them, they can improve the function of panels nearby. Greenery can also provide a type of ballast for solar panels, and provide stability to A-frame solar panels especially.
Similarly, solar panels can benefit the plants in a green roof installation. An installation of A-frame solar panels creates a mosaic of water and sun availability. Some areas are shadier behind the panels, and some areas at the base of a slanted panel collect more water. By creating a more diverse set of growing conditions for plants, a wider variety of plants can be grown, and a more diverse mini-habitat is a healthier one. In fact, research on the Queen Elizabeth Olympic Park solar panel-green roof combination showed that biodiversity was positively correlated with proximity to a panel.
The combination of solar panel and green roof installations as a concept has been dubbed the biosolar roof. It is an idea that is picking up traction around the world, while Britain, Switzerland, and Germany lead the way in biosolar roof research and innovation. Research from the Queen Elizabeth Olympic Park biosolar roof installation mentioned above recorded more than 90 plant species, and almost half of the invertebrate species observed were of conservation importance. In Switzerland, research from the biosolar roof on the Zurich University of Applied Sciences building documents how the use of plants and substrate with light-reflective properties can increase the yield measurements of solar panels by 17 percent.
There is a great amount of potential in solar panel roof installations and green roofs alike. But when combined, these concepts have the potential to make the most of urban rooftop space. As more biosolar roof projects are developed and researched, we may come to find that the solar panels and green roofs as separate projects should, in fact, be treated as inseparable.
