Adaptation of a photovoltaic energy balance model for rooftop applications

In this study we adapted and evaluated the utility-scale photovoltaic energy balance model UCRC-Solar for rooftop scenarios. The most important modification is related to the sensible heat exchange. Different modifications are tested and evaluated against measured surface temperatures and power production of a rooftop photovoltaic (PV) module on a tilted roof in Braunschweig, Germany. Strong agreement with an RMSE of 2.7 K and 4 W, respectively, for highly varying meteorological conditions, is achieved by assuming boundary layer development over the PV modules which modulates their convective heat exchange with the atmosphere. The model error is related to the wind direction and turbulent kinetic energy. A higher model error is found for wind directions corresponding to upwind built structures with larger relative roughness. We deem the adapted model, UCRC-Solar(roof) to be suitable to calculate module temperatures and sensible heat fluxes of individual modules as well as the average of PV arrays on tilted roofs for different roof sizes. UCRC-Solar(roof) is applied to compare module temperatures, sensible heat fluxes and power production from urban, rooftop PV arrays to rural, ground-based modules. The main causal factor for elevated module temperature on rooftop PV modules relative to their rural, utility-scale counterparts is the smaller sensible heat flux at the lower side of the rooftop PV modules as a result of their proximity to the roof. UCRC-Solar(roof) has the potential to be integrated in micro-to macroscale meteorological and climate models as well as building energy simulation tools.

Automated summary

In this study, the UCRC-Solar model was adapted and evaluated for rooftop scenarios, with the most important modification related to sensible heat exchange. By assuming boundary layer development over the PV modules to modulate convective heat exchange, strong agreement was achieved for highly varying meteorological conditions. Model error was found to be related to wind direction and turbulent kinetic energy, with a higher error for upwind built structures with larger relative roughness. The adapted model was deemed suitable for calculating module temperatures and sensible heat fluxes on tilted roofs.