Jan Kleissl and his team at the University of California – San Diego Jacobs School of Engineering have what they believe are the first peer-reviewed measurements showing that solar photovoltaic panels on your roof can help insulate your house.
Using thermal imaging, they determined that during the day, a building’s ceiling was five degrees Fahrenheit cooler under solar panels than under an exposed roof. At night, the panels helped retain heat, reducing heating costs in the winter. Their paper was accepted for publication in the journal Solar Energy.
Kleissl and his team determined that the amount saved on cooling the building equaled a five percent discount on the price of the solar panels over the panels’ lifetime.
Data for the study was gathered with a thermal infrared camera over three days in April 2011 on the roof of the Powell Structural Systems Laboratory at the Jacobs School of Engineering. The building is equipped with tilted solar panels and solar panels that are flush with the roof. Some portions of the roof are not covered by panels.
The panels essentially act as roof shades, according to team member Anthony Dominguez. Rather than have the sun beating down onto the roof, which causes heat to be pushed through the roof and into the ceiling of the building, photovoltaic panels take the solar beating. Much of the heat is removed by wind blowing between the panels and the roof. The benefits are greater if there is an open gap where air can circulate between the building and the solar panel, so tilted panels provide more cooling. Also, the more efficient the solar panels, the bigger the cooling effect, said Kleissl. For the building the researchers analyzed, the panels reduced the amount of heat reaching the roof by about 38 percent.
Although the measurements took place over a limited period of time, Kleissl said he is confident his team developed a model that allows them to extrapolate their findings to predict cooling effects throughout the year. For example, in winter, the panels would keep the sun from heating up the building. But at night, they would also keep in whatever heat accumulated inside. For an area like San Diego, the two effects essentially cancel each other out, Kleissl said.
The idea for the study came about when Kleissl, Dominguez and a group of undergraduate students were preparing for an upcoming conference. They decided the undergraduates should take pictures of Powell’s roof with a thermal infrared camera. The data confirmed the team’s suspicion that the solar panels were indeed cooling the roof and the building’s ceiling, as well.
There are more efficient ways to passively cool buildings, such as reflective roof membranes. But, if you are considering installing solar photovoltaic, depending on your roof thermal properties, you can expect a large reduction in the amount of energy you use to cool your residence or business.
Bottom line: Researcher Jan Kleissl and his team at the UC San Diego Jacobs School of Engineering used thermal infrared imagery to show how tilted solar photovoltaic panels act as giant shades in the summer, keeping a building cooler, and serve as insulators in the winter, retaining heat in a building. Their study was published online on July 6, 2011 by the journal Solar Energy.