Story URL: http://news.medill.northwestern.edu/chicago/news.aspx?id=214594
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Sensor green roof, solar  study

Courtesy of Nathaniel Wight

Study of solar panels over green and gravel roof conducted at the Bronx Design and Construction Academy in New York


What Chicago green roofs are missing out on

by Neel Tandan
Jan 24, 2013


Chicago is one of the leaders in green roofs in the nation, with more than 3.5 million square feet. But the city is not capitalizing on its environmentally friendly roofs, according to the latest research.

Studies are showing that solar panels, an increasingly popular renewable energy source, are more effective when placed above green roofs. Green roofs, building tops covered with living vegetation that can absorb rainwater and provide insulation, have also been proven to create a cooler microclimate in which solar panels thrive.

Nathaniel Wight, at Columbia University’s Department of Earth & Environmental Engineering, used models of the traditional rooftop and the green roof to look at changes in ambient temperature. Wight partnered with Marc Perez and and Christina Ho to collect  data in 2012 that showed the green roof model was able to maintain a cooler temperature. All three were graduate students at the time.

“There is a temperature variability,” said Wight, who now teaches ecology and physics at Bronx Design & Construction Academy. “The gravel roof is holding heat in it and raising the temperature, and the green roof is photosynthesizing it and absorbing it.”

Studies have shown that temperatures above 75 degrees can cause electrical resistance within a solar panel and slow down the flow of electrons and thus production.

Wight said by keeping solar panels about 18 inches above the green roof there is a 3 percent to 5 percent increase in efficiency.

Lucas Witmer, a doctoral student in energy and mineral engineering at Penn State University, developed a computer model to look at what he calls the “thermal relationship” between the green roof and the performance of solar panels.

“People hadn’t put the two together,” Witmer said, “how they interacted on a thermal level.”

Witmer’s study also found the green roof maintained lower temperatures for the solar panels. But he said that the relationship also goes the other way.

“The system is symbiotic,” Witmer said. “The panels benefit from the cooler microclimate and the green roof thrives from being in the shade.”

Witmer said that sedum, low-growing plants that are most typically used to cover green roofs, do very well in the shade of the solar panels. He also said the shade can contribute to lower cooling costs for the building.

“If a green roof reduces your ambient temperature around the panels, say 10 degrees,” Witmer said, “you still get maybe half a percent increase [in the panels’ efficiency]. But that little bit of a gain, it does mean money to the system owner.”

Similarly, Wight said his study means good things for consumers. “A gain in efficiency is a gain in efficiency,” Wight said. “It’s small but those are electrons and those electrons go back to the grid to be sold by the consumers or to your appliances to be used. There are definitely reasons why, as a consumer, you should be happy about that.”

In 2011 alone, more than 16 million square feet of green roof were installed in North America. Chicago, partially thanks to former Mayor Richard M. Daley, continues to lead in this category.

Jenny Babcock, Chicago’s Center for Green Technology’s programming director, said she hadn’t heard about this combination.

“We have green roofs with solar panels,” Babcock said. “But I don’t know of anyone that has one over the top of another.”

Keeping track of green roofs in the city can be difficult, with many of them located on top of private buildings and homes, she said.

Babcock said the city provides savings on green permit fees and a faster permitting process, along with state and federal incentives for solar.

But Chicago Sustainability 2015, Babcock said, provides no incentives for a combination of both solar and a green roof.

Seth Masia, editor of Solar Today Magazine and director of communications at the American Solar Energy Society, said that air circulation and wind can also cool solar panels, but he particularly likes the idea of using the green roof.

“The green roof approach, it’s elegant,” Masia said.

Masia said the savings of a big solar installation on top of a building like a warehouse could potentially amount to several thousand dollars.

“You’re coming out ahead in the long run,” Masia said.