^{PSU Researchers Study Relationship of Green Roofs and Solar Panels – Photo from solar.pdx.edu}

A symbiotic relationship might exist from solar panels that benefit from the decrease in temperature that green roofs provide, when the two are combined, while green roofs benefit from solar panels that provide shade in hot summer months. Since Portland is known for having many buildings that boast green roofs and solar panels, who thought that combining the two would make for a perfect match.

The Portland State University researchers are looking into the effects of combining single-cell silicon photovoltaic (PV) solar panels with green roof technology. The project they came up with, located on the roof of PSU’s Science Building 2, consists of four pans totaling 720 square feet of planting area for the green roofs with PV panels donated by SolarWorld partially shading the back half of the pans. The project is funded in part by a grant from the National Science Foundation of more than $600,000 for three years, and was installed in 2010.

The data collected from panels is posted online for everyone to see. You can chart by year, week, or by 48 hours. It’s actually kind of neat to see historical data of total collected energy during winter months. If you notice how much energy is collected during summer months, and if you were to compare that to your own utility bill, would you be able to stay within that energy budget?

One of the advantages of green roofs is that they absorb water that would flow off a traditional roof and into the sewers. In a 2002 report looking into the water retention capabilities of two green roofs in Portland, BES calculated that 69 percent of all rainfall was absorbed.

Other advantages of green roofs include:

• Increased building insulation – reduced energy costs
• Reduced urban heat island effect
• Absorption of pollutants from rainwater
• Extended roof life

More relevant to the PSU project, green roofs lower the temperature of the air around them. The researchers believe that since the silicone in a PV panel is a semiconductor, and semiconductors are more efficient when they’re cooler, locating arrays over the green roofs might increase their output.

On the other side of the equation, the PV panels provide shade for the plants. While the rain may seem as if it never stops during the Portland winters, it can be hard to come by in the summer when average monthly rainfall is approximately three inches and 30-60 day stretches without any rain at all are common.

Because of the lack of moisture during the summer, green roofs in Portland are usually planted with sedum, a succulent plant native to the area that holds moisture and weathers the summers better than many other plants. They still suffer heat damage, though. By shading the plants, the PV panels protect them and allow them to stay healthier and more efficient year-round. The researchers think the efficiency of the roofs and panels can be increased even more through irrigation and greater biodiversity on the green roof portion.

To explore the idea that biodiversity can improve the efficiency of the green roofs and the solar panels, the researchers planted two of the pans with a sedum mix, and two of the pans with a mixture of sedum and herbaceous plants such as sage, thyme, grasses, and yarrow. Over the summer, the team has been irrigating one sedum pan and one mixed pan, while letting the other two dry out naturally.