Impact of roof-top photovoltaics on the urban microclimate: High-resolution LES study using the PALM model system

Cities account for approximately 2% of the global land area but consume 60 to 85% of the world's energy and produce 70% of the world's carbon dioxide emissions. In recent years, photovoltaic (PV) electricity energy generation has grown exponentially. Roof-top PV deployments offer several advantages, such as local clean energy generation, energy independence as well as efficient space usage. The installation of roof-top PV alters the urban surface properties and thus influences surface-atmosphere interactions. Previous studies considered only the impact of solar panels on urban air temperatures from an energy balance and mesoscale approach. However, these models do not resolve the complex flow around buildings, not to mention PV panels, which plays an important role in momentum and thermal exchange processes. Therefore, we developed a new parametrization in the PALM model system based on the effective albedo method in which PV modules are represented by an change in surface albedo only. This effective albedo is a function of the PV module albedo and its energy conversion efficiency. This method will be refined into a more sophisticated PV representation in the course of our current research project. PALM is a Large-Eddy Simulation (LES) model capable of resolving turbulence and heat transfer processes in resolved street canyons, i.e. in urban microscale simulations. In this study we employ PALM to investigate the impact of roof-top PV on the urban microclimate, outdoor thermal comfort, indoor energy demand, and CO₂-equivalent savings. Furthermore, we conduct sensitivity studies regarding PV deployment height, synoptic forcings, PV roof coverage, and conversion efficiency. All simulations are embedded in a validated framework of representative local climate zones, which enables comparability and generalizability of the results. Our work can contribute to urban planning, as we examine under which conditions PV enhances urban heat or can be used as urban adaptation measure.