Green roofs and roof-mounted solar panels may initially appear as competitors for limited rooftop real estate, but researchers are finding that instead, they may be ideally suited to a symbiotic relationship.
The sun’s light energy (in photons) is the fuel that enables solar panels to output a stream of direct current for immediate use or for storage in a battery. However, there are few places subject to more extreme temperatures than a conventional rooftop, and heat is one of the primary factors that reduce the efficiency of roof-mounted solar panels. To be more specific, high rooftop temperatures increase the conductivity of a crystalline silicon panel’s semiconductor, which in turn inhibits charge separation and lowers the voltage of the solar cell. The higher the temperature, the less efficient the panel.
The absence of shading and moisture, and the presence of heat-absorbent surfaces on a rooftop can create hot, desertlike conditions. While this open exposure to sunlight makes it an ideal location for solar panels, this hot zone can decrease photovoltaic panel productivity by up to 25 percent. Installing a green roof, however, can improve solar panel efficiency by reducing rooftop temperatures. Traditional black roofs can reach temperatures of 158 F, and they have an enormous effect on temperatures in the building and at ground level. In comparison, the rooftop temperature of an extensive green roof rarely exceeds 77. (1)
There are also financial benefits to this collaboration. The urban heat island (UHI) effect is the temperature increase in urban centers caused when impermeable surfaces convert sunlight to heat. The elevated temperatures associated with UHI promote the production of harmful ground-level ozone. In addition, the disappearance of vegetation and the construction of tall buildings prohibits the occurrence of natural cooling processes such as evapotranspirationthe movement of water from the growing medium through the plants and into the air. High summer temperatures also create an enormous demand for interior air conditioning, which can cause problems for energy suppliers. It is projected that the cooling benefits of green roofs could save building owners in the city of Toronto, Canada, $37,130,000 annually in reduced energy costs. (2)
In addition to the cooling-efficiency symbiosis, racking systems for solar panels may be designed so that the green roof layers act as ballast, thereby saving the need for roof penetrations or concrete pavers. Companies such as Zinco in Germany have been installing these types of systems for years.
For their part, green roofs can increase the longevity of a roof’s waterproofing, thus helping to offset the expenses of removing and replacing solar panels when the waterproofing underneath has reached the end of its useful life. The drainage and root repellant layers can be designed to provide an additional protection to the roof membrane from the solar panel racking systems, should they move slightly in the wind and begin to erode the waterproofing underneath. Also, irrigated green roofs may provide additional benefits in excessive heat and drought conditions. Water collected during periods of high precipitation can be stored and redistributed during dry periods to sustain plant life and also to cool solar panels when needed.In return, solar panels can provide shelter for green roof components, helping to protect the vegetation and growing media from gusting winds, and they may create partial shading, decreasing excessive evapotranspiration that can dehydrate plants under extremely hot and dry circumstances.
Maximizing the symbiotic relationship between plants and solar arrays requires careful design, installation, and maintenance. As we move towards more sustainable, restorative buildings, our knowledge of how organic and inorganic technologies can work together will be critical to our long-term success. Because there are many factors involved in combining green roofs and solar arrays, these projects require a knowledge of systems integration that takes into account the components and stakeholders involved in a green roof. The success of a green roof project can be greatly aided by using a Green Roof Professional (GRP), an accreditation provided by Green Roofs for Healthy Cities. To become a GRP, individuals are tested on subjects including design, installation, waterproofing, drainage, plants, and growing media.
We are only just beginning to understand the ways in which these technologies may be integrated in different climates. Initial research presented at the Rio 02 World Climate & Energy Event in 2002 showed promising results for green roof and solar array integration, and North American researchers are now taking the lead on quantifying the exact benefits of the green roof and solar panel integration. Among them are David Sailor of Portland State University and professors Liat Margolis and Robert Wright from the University of Toronto, who will be presenting their research at the Eighth Annual CitiesAlive Green Roof & Wall Conference in Vancouver, British Columbia, from Nov. 30 to Dec. 3. For more information on the conference, which will focus on overcoming barriers to systems integration, visit citiesalive.org.
Steven W. Peck is the founder and president of Green Roofs for Healthy Cities and Damon van der Linde is its communications and research coordinator. You can learn more about the organization at greenroofs.org.
1. Bass, Brad. The Impact of Green Roofs on Toronto’s Urban Heat Island. Greening Rooftops for Sustainable Communities, 2003.2
2. Banting, Doug; Doshi, Hitesh; Li, James; Missios, Paul; with Au, Angela; Currie, Beth Anne; Verrati, Michael. Report on the Environmental Benefits and Costs of Green Roof Technology for the City of Toronto. 2005.