REFLECTING THE EFFECTS OF PHOTOVOLTAIC PANELS
Adding reflectors to rooftop photovoltaic installations could boost solar activity and help with temperature control.
Written by Kayt Sukel
Photovoltaic (PV) panels convert sunlight into electricity with remarkable efficiency. Studies have shown that when PV installations are installed on the tops of buildings, they not only generate power as they collect solar energy but also help to decrease the energy load required to heat or cool buildings thanks to the shade they provide.
Now, researchers from the University of Dayton have demonstrated that the addition of reflectors add to that cumulative savings by increasing energy collection while also promoting greater energy savings thanks to adding to the overall radiative heating and cooling effects.
Rydge Mulford, an assistant professor at the University of Dayton’s Department of Mechanical and Aerospace Engineering, said the idea for the study came to his team as his graduate student Habeeb Alasadi was finishing up his doctorate.
“His graduate work had involved both reflectors for PV panels and then PV panels on rooftops,” Mulford said. “As he came toward the end of his Ph.D., we thought it would be interesting to put those things together to see how it might augment the PV and potentially shade the rooftop.”
The research group calculated rooftop heat flux and temperature for building rooftop installations that included PV panels and reflectors for three locations: Phoenix, Ariz.; Boise, Idaho; and Dayton, Ohio.
“We selected Phoenix because it’s a place with significant solar resources, as well as extreme heat loads on buildings because it’s more or less a cloudless environment,” Mulford explained. “You have a lot of solar load in the summer and winter and we wanted to demonstrate the potential for solar augmentation and also see how much we could reduce the building’s energy requirements by shading the rooftop.”
Dayton, which tends to be thick with cloud cover in the winter months, was chosen for the opposite reason. The team wanted to see whether there would be less heat loss.
“We chose Boise because it’s kind of in-between the two,” Mulford added.
The energy balance on a photovoltaic-shaded roof. Graphic: Rydge Mulford
“When it comes to solar, we need to find synergy with other systems, be that a building or crops.”
—Rydge Mulford, assistant professor in the Department of Mechanical and Aerospace Engineering at the University of Dayton
Five-point daily average of energy savings and additional energy loads of three cities (Dayton, Ohio; Boise, Idaho; and Phoenix). A positive value indicates a decrease in building energy load when compared with an unshaded rooftop. Graphic: Rydge Mulford
The group calculated saved energy load, additional energy load, PV power output, rooftop heat flux, and the ratio of positive building energy impacts to negative building energy impacts (or utility factor) based on rooftop absorptivity and reflector area for the three different locations. They found the reflectors not only augmented PV output but also assisted with thermal building control. Their calculations found a building with a combination of PV panels and reflectors on the roof can get a 15 percent boost to its utility factor compared to one without the reflectors, while Dayton can see even greater savings with a 22 percent increase to its score.
“I knew that the reflectors would act as a barrier for radiative heating into the building itself. In summer, they prevent solar input into the rooftop which decreases the amount of cooling that the building requires,” Mulford said. “What I didn’t expect was the influence of the reflectors in the winter months. The reflectors were also able to decrease the amount of heating required in the building by shielding the roof from losing radiation to the clouds or, on the off chance of a clear night’s sky, preventing it from losing that energy to space.”
While Mulford’s research is now primarily focused on agrivoltaic applications, looking for new ways to harness solar energy to promote crop growth, he hopes that others who read this study will see that solar energy research does not have to solely focus on developing new materials. While those innovative materials are always welcome, systems level analysis of how panels might benefit other systems is just as important, he added.
“When it comes to solar, we need to find synergy with other systems, be that a building or crops,” Mulford said. “Instead of thinking of solar on their own, we need to think of how they can be integrated. The more we can do so, the easier it will be to justify the work financially and politically—and the easier it will be to find ways to sustainably generate power for a wide variety of applications.”
Kayt Sukel is a technology writer and author in Houston.
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