Observational study on the thermal performance of photovoltaic and cool-photovoltaic roofs during heatwaves in a semi-arid city

Extreme heatwaves are intensifying globally, yet observational evidence on the micro-climate effects of rooftop photovoltaics (PV) remains scarce, particularly in semi-arid regions. This study addresses this gap through a 46-day summer experimental investigation (July–August 2025) conducted in the semi-arid, valley-bound city of Lanzhou, north-west China. We compared the reference regular roof, photovoltaic roof (PV roof), and the cool roof integrated photovoltaic system (CPV roof), all employing double-glass modules, offering novel field-based insights into their thermal behavior under heatwaves. Results demonstrate a distinct diurnal asymmetry and vertical variation in cooling effects. Both PV and CPV roofs induced significant near-ground cooling during daytime (median: –0.49 to –0.77°C), with CPV being more effective. However, at heights above 1.5 m, PV roof maintained cooling while CPV caused slight warming. During nighttime, the thermal impact of both strategies was markedly reduced. Counterintuitively, CPV roof increased module operating temperatures by approximately 3°C than PV roof, indicating that the combination of a cool roof and PV modules does not constitute a linearly additive cooling benefit. All strategies reduced daytime roof surface temperature by 10–15°C. A robust micro-meteorological model confirmed that solar irradiance, air temperature, and wind speed dominate module heating, with PV warming twice as fast as air—affirming its role as an artificial heat island. The CPV roof showed heightened climate sensitivity, making its performance highly weather-dependent. These insights are critical for advancing sustainable city planning in a warming world.