Rooftop photovoltaic solar panels warm up and cool down cities
Authors: Khan Ansar , Khorat Samiran , Khatun Rupali , Anand Prashant , Santamouris Mat , Nigoyi Dev 
Published: November 2023

The large-scale deployment of rooftop photovoltaic solar panels (RPVSPs) may increase the risk of urban overheating due to a thermal convection developing between RPVSPs and roof surface. Therefore, it is crucial to develop a scientific understanding of the implications of large-scale RPVSPs in urban settings. This study examines the impact of RPVSPs on the urban environment in the lower atmosphere through urban-resolving regional climate modeling for the Kolkata metropolitan area (KMA).In this study, a new physical parameterization of the RPVSPs system based on model physics and integrated with a multilayer urban canopy model (a multilayer building energy model) has been used. Here the urban canopy model is further fully integrated with the Weather Research and Forecasting (WRF) model. To evaluate the impact of RPVSPs on the urban environment, it has been assumed that RPVSPs arrays are parallel, detachable from the roof with a height of 0.3 m, and consist of a single thin layer (6.55 mm). The results suggest that large-scale adoption of RPVSPs can significantly increase urban temperatures during the day, but it typically cools the urban environment at night. While daytime near-surface air temperatures can rise by up to 1.5 °C during summer heatwave events in urban areas, it has also been observed that RPVSPs can decrease nighttime near-surface air temperatures by up to 0.6°C when rooftops are 100% covered by RPVSPs. Extensive RPVSPs adoption can lead to an increase in urban surface skin temperatures of up to 3.2°C during peak hours, with an average cooling effect of up to 1.4°C during summer heatwaves at night. Additionally, the extensive adoption of RPVSPs shows higher near-surface temperatures with lower relative humidity and results in increased outdoor thermal stress in the urban environment. The distribution of near-surface air temperatures over the urban domain strongly depends on its synoptic meteorological conditions and advection flow strength. Further, Large-scale RPVSPs deployment can increase sensible heat flux and latent heat flux by 241.6 Wm-²and 35.3 Wm-², respectively. RPVSPs generating convective heat flow with airflow alternation results in warming of the urban surface that impact on the moisture transport and evaporation rate, affecting the local heat flux dynamics. Furthermore, the higher urban surface skin temperatures caused by RPVSPs enhance mixing in the lower atmospheric boundary layer, leading to accelerated wind speeds in urbanized regions. Another interesting finding is that the onset of sea breeze circulation occurs earlier in the afternoon due to regional low-pressure effects within a deeper planetary boundary layer (PBL) height and offshore synoptic winds above the atmospheric boundary layer. Finally, large-scale RPVSPs significantly warm the urban surface by increasing sensible heat flow and concomitant turbulence in the lower atmosphere, resulting in an increase in PBL height by up to 535.6 m in the most aggressive scenario (RPVSPs100%). This leads to lower pollution concentrations at ground level. The stronger vertical wind caused by large-scale RPVSPs indicates a stronger influence of convective rolls on the urban atmosphere during heatwave events. Additionally, we fully evaluated 30 case studies from local, national, and global scales to verify and compare the current study's findings. Overall, this study provides valuable insights for policymakers to plan and implement the deployment of large-scale RPVSPs in an informed manner.