According to the research team Anthony Dominguez, Jan Kleissl, and Jeffrey C. Luvall at the University of California-San Diego’s Jacobs School of Engineering, PV modules can do more than just generate electricity. They can also help reduce a building’s annual cooling load.
In a first-of-its-kind study, published in Solar Energy, the official journal of the International Solar Energy Society, thermal imaging and temperature sensors were used to measure the effect of PV modules on roof and ceiling temperatures. Data for the study was gathered over three days on a university building equipped with both shallow-tilted (4.4°) and flush-mounted PV modules.
The study found that inside the building, the ceiling was up to 5°F cooler in areas under the tilted PV modules than under the exposed roof (and about 2.5°F cooler under the flush-mounted array). The team developed a model to extrapolate the findings and predict the cooling effects throughout the year. The reduction in total annual cooling load for the PV-covered roof was estimated to be 38%. They also determined that the cooling effect amounted to long-term savings in energy costs—the annual equivalent of increasing the PV array energy contribution by 4%.
Other findings include that the daily temperature swings for the roof under the array were about half that of the exposed roof, which can reduce overall thermal stress on the roof structure. In winter, the arrays can reduce the passive heating of the building, increasing heat load during the day. However, the extra “roof” layer reduces heat loss at night. Overall, the winter effects in daytime and nighttime cancel each other out. Consequently, the researchers concluded that a PV array (for the San Diego climate and building type) can reduce the annual cooling load, without increasing the annual heating load.
The actual impact of an array on a building’s heating and cooling load will vary depending on building type, climate zone, and temperature preferences, as well as existing roof insulation and reflectance. Kleissl and his team hope to secure funding to create a tool that can calculate the effect, on a house-by-house basis.