Quantifying outdoor heat stress potential due to city-wide installation of rooftop photovoltaic solar panels

Abstract

Urban heat is becoming increasingly severe in densely populated regions, particularly in the tropical climates, where average summer temperatures frequently reach > 34 °C [307.15 K]. As cities seek sustainable energy solutions, rooftop photovoltaic solar panels (RPVSPs) have gained popularity for their renewable energy potential. However, the microclimatic implications of RPVSPs on urban heat stress remain inadequately understood. This study addresses a critical study gap by being the first to systematically investigate RPVSP-induced heat stress taking the city scale analysis for Kolkata, India focusing on its effects on thermal comfort and the urban microclimate. By comparing buildings equipped with RPVSPs to those without, we analysed variations in key meteorological parameters and heat stress indicators, utilizing the universal thermal comfort index (UTCI) and heat index (HI) at city scale. The present study indicates that RPVSPs contribute to an increase in daytime ambient temperatures by approximately 1.6 °C and surface temperatures by about 3.4 °C, based on simulations incorporating improved surface and urban canopy parameters derived from the University of Texas–Global Building Heights for Urban Studies (UT-GLOBUS) dataset. These findings are consistent with our earlier study conducted without improved surface and urban canopy parameters, which reported that RPVSPs contributed to an increase in daytime ambient temperatures by approximately 1.5 °C and surface temperatures by around 3.2 °C. The results of this study further suggest that this increase in RPVSP-induced urban temperature can lead to a rise in extreme heat stress exposure, with 44.1 % of daytime hours classified as extreme heat stress (UTCI > 46 °C) for buildings with RPVSPs, compared to 18.5 % for non-RPVSP buildings at city scale. However, these panels also yield a decrease in extreme heat hours at night from 23.4 % to 19.1 %, indicating potential improvements in nighttime thermal comfort. The analysis of cooling degree hours (CDH) showed a 9.71 % increase in maximum daytime CDH, indicating a rise in cooling demand from 200.5 h to 220.6 h in Kolkata city where all buildings were considered equipped with RPVSPs. Nevertheless, a decrease of 7.70 % in maximum nighttime CDH, dropping from 130 h to 120 h, highlights the panels’ capacity to reduce heat retention during cooler hours. Additionally, this study also quantifies the dual role of RPVSPs in mitigating and amplifying thermal stress during Kolkata’s heatwaves. While daytime air conditioning (AC) energy demand averages 25.86 Wm⁻², RPVSPs generate 17.61 Wm⁻² covering 68.1 % of the cooling load. However, nighttime output drops to zero, despite a sustained demand of 19.4 Wm⁻². Overall, RPVSPs meet only 48.1 % of daily AC energy demand, with just 20–25 % coverage in dense, high-rise areas but up to 80 % in low-density zones. This study distinctively elucidates the dual role of RPVSPs in urban thermal dynamics, revealing how they can both exacerbate daytime heat stress while enhancing nighttime comfort. The findings reveal the need for adaptive urban planning that effectively integrates renewable energy solutions with strategies to mitigate urban heat.