Modeling Synergistic Cooling Benefits of Photovoltaic Green Roofs Across U.S. Climates

Abstract

Photovoltaic green roofs (PV-GRs) have been found to improve solar panel efficiency, reduce building energy consumption, and counteract the urban heat island effect. Previous studies have examined these benefits through single-site experiments, but PV-GR performance varies across climates due to localized meteorological conditions. Furthermore, the effects of design parameters such as PV panel coverage and irrigation strategy on PV-GR performance remain underexplored. This study presents a flexible, physics-based modeling framework that simulates surface energy balance, heat transfer, and water transport throughout a PV-GR system. The PV-GR model is validated with field data from Beijing, China. Results show that PV-GR systems provide substantial roof cooling across five climates (Phoenix, Denver, New York, Miami, and Seattle) through two interacting mechanisms — evapotranspiration and panel shading — although these cooling mechanisms are not additive. This study finds that humidity governs which mechanism dominates the roof cooling: dry climates achieve more cooling from evapotranspiration, while humid climates benefit more from panel shading. Climates with high temperature variability also face heat-trapping at night with increasing panel coverage, offsetting daytime cooling gains. Analysis of irrigation scenarios finds that maintaining soil saturation at 30-40% is sufficient for evapotranspirative cooling across all studied climates, with minimal additional benefit at higher saturation levels. This is a critical finding because, ultimately, water availability emerges as the primary limiting factor for PV-GR implementation: dry climates require up to 15 times the average daily summer precipitation rate to maintain minimum irrigation thresholds, while humid climates can theoretically sustain irrigation from precipitation alone. These findings provide a framework for climate-informed PV-GR design and highlight the importance of water availability assessments prior to implementation.