Modeling impacts of roof reflectivity, integrated photovoltaic panels and green roof systems on sensible heat flux into the urban environment

This study presents results of a modeling effort to explore the role that sustainable roofing technologies play in impacting the rooftop energy balance, and the resultant net sensible heat flux into the urban atmosphere with a focus on the summertime urban heat island. The model has been validated using data from a field experiment conducted in Portland Oregon. Roofing technologies explored include control dark membrane roof, a highly reflective (cool) roof, a vegetated green roof, and photovoltaic (PV) panels elevated above various base roofs. Energy balance models were developed, validated with experimental measurements, and then used to estimate sensible fluxes in cities located in six climate zones across the US. On average the black roof and black roof with PV have the highest peak daily sensible flux to the environment, ranging from 331 to 405 W/m(2). The addition of PV panels to a black roof had a negligible effect on the peak flux, but decreased the total flux by an average of 11%. Replacing a black roof with a white or green roof resulted in a substantial decrease in the total sensible flux. Results indicate that if a black membrane roof is replaced by a PV-covered white or a PV-covered green roof the corresponding reduction in total sensible flux is on the order of 50%. The methodology developed for this analysis provides a foundation for evaluating the relative impacts of roof design choices on the urban climate and should prove useful in guiding urban heat island mitigation efforts. (C) 2011 Elsevier Ltd. All rights reserved.