Effects of Rooftop Photovoltaics on Building Cooling Demand and Sensible Heat Flux Into the Environment for an Installation on a White Roof

Abstract

Photovoltaic (PV) panels are commonly used for on-site generation of electricity in urban environments, specifically on rooftops. However, their implementation on rooftops poses potential (positive and negative) impacts on the heating and cooling energy demand of buildings, and on the surrounding urban climate. The adverse consequences can be compounded if PV is installed on top of an otherwise highly reflective (“white”) rooftop. This study investigates these impacts on a test building in Tempe, AZ, by directly measuring the temperature of all involved surfaces. These measurements are supplemented by whole-building energy simulations to model the energy implications for archetypical residential and retail buildings. This includes calculations of the ratio of the energy demand penalty to electricity generation as well as the net sensible heat flux to the ambient environment. Results indicate that the summertime cooling energy penalty due to blockage of outgoing longwave radiation can be 4.9—11.2% of the PV electricity generation. The addition of PV to the white roof resulted in a small decrease in the computed sensible heat flux at night, but a daytime increase in sensible flux by more than a factor of 10 (from less than 25 W/m2 for the white roof alone, to more than 250 W/m2 when PV is added to the roof). This study highlights the potential unintended consequences of rooftop PV under certain conditions and provides a broader perspective for building designers and urban planners.