Variations in photovoltaic performance due to climate and low-slope roof choice

• The relationship of roof type and PV power output is a minor economic factor.
• Green roofs can increase PV power output in high temperatures.
• PV temperature coefficients at various solar irradiance values would be useful.

With urban space at a premium, roofs are being targeted as an opportunity to deploy sustainable energy technologies for buildings. This research evaluates the combination of green roofs and solar photovoltaics specifically through their temperature and electricity production relationship. Measurements over a one year study period from July 1, 2011 to June 30, 2012 from a large field project in Pittsburgh, Pennsylvania were used to determine the differences in power output from green and black roofs as well as to derive two regression functions for back-surface panel temperature and photovoltaic (PV) output. These estimation functions were applied to three different cities (San Diego, CA; Huntsville, AL; and Phoenix, AZ) chosen to represent a wide range of irradiance and temperature values. Based on the specific test-bed configuration, the green or black roof choice under the PV panels had little impact on the PV performance. The difference in magnitude of power generation for green roof–PV compared with black roof–PV assemblies was small (0.5%) corresponding to an annual loss of $9/60 panels in Pittsburgh and a benefit of approximately $8/60 panels per year in Phoenix. Results also suggest that sites consistently above 25 °C (77 °F) will most likely see a small, positive impact from a green roof–PV combination. Building managers and designers should consider this temperature and power output interaction a minor economic factor in roof decisions.