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New research from India shows that rooftop PV system may have “unintended” consequences on temperartures in urban environments. Rooftop arrays, for example, may potentially lower nighttime temperatures by up to 0.6 C.
An international group of scientists has created a new model for the assessment of rooftop photovoltaic solar panels (RPVSPs) in urban microclimates.
The module utilizes the latest weather research and forecasting (WRF) model, integrating the building energy model (BEM) and the building effect parameterization (BEP) into it. The model was validated against ten observation stations in Kolkata, India, using experimentally validated models.
“While existing literature reports on the impact of RPVSP on the urban environment, most are based on in situ field experiments or building-scale simulations, lacking a comprehensive multicity-scale analysis. These studies also neglect convective heat transfer between the roof surface and the back of solar panels,” said the academics. “Our study addresses these gaps by incorporating new parameterizations for RPVSPs, including convective heat transfer, resulting in more aligned results with other studies incorporating similar considerations.”
The combined approach, named the WRF/BEP + BEM model, can calculate heat exchange, momentum, humidity, and turbulent kinetic energy flux between buildings and the outdoor environment under stable atmospheric conditions. It was initially tested in the Indian city of Kolkata and then validated in Sydney, Australia; Austin, Texas, USA; Athens, Greece; and Brussels, Belgium, to ensure that the findings are not limited to a specific climatic zone.
“Five experiments were conducted to assess the regional impact of extensive RPVSPs deployment during the current heatwave month in Kolkata. The control simulation used a roof albedo of 0.15 and no RPVSPs,” the group explained. “The experiments explored RPVSPs scenarios with coverage fractions of 0.25, 0.50, 0.75, and 1.0 on city rooftops. Standard RPVSP parameters, including albedo, conversion efficiency, and emissivity, were set to 0.11, 0.19, and 0.95, respectively.”
According to the data collected in Kolkata, RPVSPs can increase daytime near-surface air temperatures by up to 1.5 C, as they absorb approximately 90% of solar energy, converting up to approximately 20% of it into electricity, while the remainder contributes to their warming. At nighttime, on the other hand, full city PV coverage can reduce nighttime maximum near-surface air temperatures by up to 0.6 C. In peak heat hours, the roof surface temperature would rise by up to 3.2 C and have an average cooling of 1.4 at night.
The near-surface air temperatures were similar across the board. Sydney experienced a 0.8 C cooling at night and a 1.9 C rise during the day; Austin showed a cooling of 0.7 C and a rise of 1.8 C, while Athens had 0.4 C and 1.2 C, respectively. Brussels’s results showed a night cooling of 0.3 C and a day rise of 1.1 C.
“Our study also reveals that rooftop photovoltaic solar panels significantly alter urban surface energy budgets, near-surface meteorological fields, urban boundary layer dynamics, and sea breeze circulations,” the group added. “Elevated urban temperatures due to RPVSPs installation enhance lower atmospheric mixing and raise the planetary boundary layer (PBL) height by up to 615.6 m, reducing ground-level pollution.” PBL represents the lowest part of the atmosphere, which is directly influenced by the Earth’s surface.
The findings were presented in the study “Rooftop photovoltaic solar panels warm up and cool down cities,” published in Nature Cities. The research was conducted by Researchers from India’s University of Calcutta, the Indian Institute of Technology Kharagpur, Jadavpur University, the USA’s Massachusetts Institute of Technology (MIT), the University of Texas at Austin, China’s Chinese Academy of Sciences, and Australia’s University of New South Wales.
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