Green Roof + Solar Cells = Better Together?

Yes, that’s a whale skull! Whale skulls and solar panels – things you don’t normally see together.
Yes, that’s a whale skull! Whale skulls and solar panels – things you don’t normally see together.Courtesy OMSI
Researcher Nick Day checking on the solar panels.
Researcher Nick Day checking on the solar panels.Courtesy OMSI
You don’t really smell the whale bones drying in the sun, unless you’re close to them. I’m on the roof of Science Building 2 at Portland State University in Portland, Oregon. I’m not here to check out the whale bones (which are pretty cool!) but to see the PSU Photovoltaic Test Facility.

Plants and solar cells both need sunlight. Roofs can be great places for capturing sunlight if they aren’t shaded by tall trees or buildings. PSU professors Carl Wamser, David Sailor, and Todd Rosentiel wondered if putting green roofs and solar panels together could increase the effectiveness of both. Researchers from the Wamser, Sailor, Rosentiel, and Erik Johansson labs have been experimenting with different plants in the green roofs, irrigating the plants, and different roofing materials to see how they affect the power produced by the solar panels and how much energy the building uses.

As it gets hotter, photovoltaic solar panels become a little less effective. About 80% - 90% of the solar panels in use today in the US are crystalline silicon photovoltaic panels. Over temperatures of 25°C (77°F), these solar panels drop 0.4-0.5% in power for every 1°C that the temperature rises. How can green roofs help solar panels? Plants and soil give off water in a process called evapotranspiration. As the water evaporates, it cools the air. The PSU researchers wondered if that temperature drop is enough to cool solar panels. They are still analyzing the results, but the plants do cool the solar panels a little, with sedum plants doing a better job than a mix of sedums and grasses.

While analyzing the data, researchers Matt Smith and Hanny Selbak noticed that there was an unexplained power spike in the solar panels one day. After trying to figure out what caused that increase in power, they traced it to biologists in the Rosentiel lab irrigating the plants. The water ran over the solar panels on the way to the plants. The solar panels cooled, and their power went up. Serendipity!

From that observation, researchers from the labs devised an experiment where they pumped water continuously over a solar panel using a 7 watt aquarium pump. This cooled the panel from about 55°C (131°F) to 40°C (104°F). After subtracting the power used by the pump, the average net gain in power was around 5%. The cooling system cost about $15 in materials per panel. If a home owner with an average 3 kilowatt solar panel system used a similar cooling system, she could generate an additional 150 watts of power. That’s enough to run most televisions or computers.

Find out more about the green roof and solar panel project here: http://www.sciencebuzz.org/topics/green-roofs-solar-panels-better-together

Meet some of the researchers and find out why they do what they do here: http://www.sciencebuzz.org/topics/meet-scientist

Sources and Links:
To read this article click here:

Smith, Matthew K., H. Selbak, C.C. Wamser, N.U. Day, M. Krieske, D.J. Sailor, T.N. Rosentiel. Water Cooling Method to Improve the Performance of Field-Mounted, Insulated, and Concentrating Photovoltaic Modules. Journal of Solar Energy Engineering 2014; 136(3).

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