Conducting a study and field experiments, a doctoral student in the School of Architecture and Design at the University of Kansas (KU) shows that PV panels installed over a green roof perform an average of 1.5% better, compared with panels over highly reflective or black surfaces.
Typically, the efficiency of a photovoltaic panel is measured under standard testing conditions, in a temperature of 77° Fahrenheit (25° C). With every degree the temperature increases, performance decreases. So KU doctoral student Mohammed Alshayeb wanted to find a way to extract the heat from the panels when the temperature rises above 77° F. Since most solar panels are installed on rooftops, he compared the effect of three different roof materials on the panels’ performance: highly reflective (white), conventional (black) and vegetated (green). The last roof type, a complete drained roof (CDR), was mostly covered with sedum, planted in trays.
Alshayeb says in the past, roofing choice has not been considered an important enough factor in estimating the performance or the payback of a PV system. Government incentives are being awarded based on the size of the system and the amount of electricity that will be generated. Therefore, he emphasizes the importance of finding ways to maximize the energy output. “In this study, we conducted experiments to compare the performance of PV panels installed over a green roof, a conventional roof and a highly reflective roof,” the KU doctoral student says.
The rooftop testbed
Alshayeb built his testbed over the roof of the Center of Design Research at the University, installing a solar panel monitoring system over the green roof as well as nearby white and black sections.
The goal was to measure temperature differences and energy output on PV panels installed over the three different roofing types as well as the energy output. Temperature and relative humidity sensors were installed under each PV panel. A weather station was installed to measure ambient temperature, relative humidity, solar radiation and wind speed and direction. Enphase microinverters measured the energy produced by each panel. The sensors and monitoring devises recorded measurements every five minutes, collecting data for one year.
“The result, in short, was that PV panels over a green roof have a lower underside surface temperature than those installed over a white roof or a black roof,” says Alshayeb, adding that while the differences in temperature varied throughout the year, the fact that the PV panels over the green roof always recorded a lower temperature than the panels installed over the white and the black roof when the temperature was above 25° C remained a constant. “In terms of energy output, whenever there are PV panel underside surface temperature differences, there are differences in energy output. The differences can reach up to about 5%, but the average difference over the whole year is around 1.5%. This is for our specific site and installation.”
Alshayeb shares that several of his finding were surprising to him. One stands out in particular: Although standard practice favors white roofs over black surfaces, Alshayeb’s test results showed that the efficiency of the panels installed over the white section of the roof actually declined more, due to the heat the white surface reflected up toward the panels. “I was expecting to get almost similar results between the PV panels installed over the highly reflective roof and the ones installed over the green roof.”
According to Alshayeb’s faculty adviser, Associate Professor of Architecture Jae Chang, his student’s research is the most comprehensive study in this subject area that he is aware of.
The study’s potential impact on future PV installations
In conclusion, Alshayeb notes that roofing types should absolutely be considered as a factor in estimating the expected energy output of PV panels: “Even though more studies need to be conducted to reach a solid conclusion regarding which roofing type has the best performance, it is clear that roofing has an impact,” he says.
Alshayeb projects that the distance of the PV panels from the roof surface has an impact on the performance. In his current study, the height of the PV panels in reference to the roof was a constant. “Monitoring the performance of a PV system with different heights from the roof will likely answer more questions,” he reckons. “The results we have right know are for this specific site and installation. We are looking to run more experimental studies at different sites, with different weather conditions and different PV configurations.”
Written by Sandra Henderson, Research Editor, Solar Novus Today