Tilted solar panels (front), create a stronger cooling efffect than panels flush with the roof.
solar panels on top of your roof aren’t just providing clean power;
they are cooling your house, or your workplace, too, according to a team
of researchers led by Jan Kleissl, a professor of environmental
engineering at the UC San Diego Jacobs School of Engineering.
a study in an upcoming issue of the journal Solar Energy, Kleissl and
his team published what they believe are the first peer-reviewed
measurements of the cooling benefits provided by solar photovoltaic
panels. Using thermal imaging, researchers determined that during the
day, a building’s ceiling was 5 degrees Fahrenheit cooler under solar
panels than under an exposed roof. At night, the panels help hold heat
in, reducing heating costs in the winter.
“Talk about positive side-effects,” said Kleissl.
solar panels sprout on an increasing number of residential and
commercial roofs, it becomes more important to consider their impact on
buildings’ total energy costs, Kleissl said. His team determined that
the amount saved on cooling the building amounted to getting a 5 percent
discount on the solar panels’ price, over the panels’ lifetime. Or to
put it another way, savings in cooling costs amounted to selling 5
percent more solar energy to the grid than the panels are actually
producing— for the building researchers studied.
for the study was gathered over three days in April on the roof of the
Powell Structural Systems Laboratory at the Jacobs School of Engineering
with a thermal infrared camera. 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.
Jan Kleissl, a professor of environmental engineering at the UC San Diego Jacobs School of Engineering, poses with the solar panels his team studied.
panels essentially act as roof shades, said Anthony Dominguez, the
graduate student lead on the project. Rather than the sun beating down
onto the roof, which causes heat to be pushed through the roof and
inside the ceiling of the building, photovoltaic panels take the solar
beating. Then 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
researchers analyzed, the panels reduced the amount of heat reaching the
roof by about 38 percent.
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
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.
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.
are more efficient ways to passively cool buildings, such as reflective
roof membranes,” said Kleissl. “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.”
Left: A Google Earth image of the Powell Structural Systems Laboratory with a tilted solar panel array on the north side and a flush solar panel array on the center of the roof. Right: A thermal infrared image of the ceiling of the Powell Structural Systems Laboratory, taken at 5:10 p.m. April 19, 2009. The color bar shows temperatures in degrees Kelvin. The footprint of the tilted solar panel array is visible as a cool area in the center of the image.
additional funding became available, Kleissl said his team could
develop a calculator that people could use to predict the cooling effect
on their own roof and in their own climate-specific area. To further
increase the accuracy of their models, researchers also could compare
two climate-controlled, identical buildings in the same neighborhood,
one with solar panels, the other without.
study was funded by a NASA Graduate Student Research Program
fellowship. Kleissl’s research is funded by the National Science
Foundation, California Public Utilities Commission, the Department of
Energy and the California Energy Commission. The authors thank the staff
of the Powell Structural Lab, especially Andrew Gunthardt, for making
the building available for the study.