3rd July 2014

Rooftop PV has added benefit of passive cooling: UCSD study

A study by the University of California, San Diego (UCSD) has concluded that PV modules keep buildings significantly cooler in summer through passive solar cooling. The UCSD researchers concluded that rooftops with PV installed kept buildings up to 5 °C cooler during the day than areas without. For their case study at the Jacobs School of Engineering UCSD conducted in 2011, this equates to a reduction in the annual cooling load of 38% and amounts to cost savings equivalent to 5% of the total cost of the PV system over its lifetime. The team of researchers was led by Jan Kleissl, a professor of environmental engineering at the UC San Diego Jacobs School of Engineering

The 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.

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.

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