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A new study by international researchers shows that rooftop photovoltaic systems can have “unintended” consequences on the temperature of urban environments. Rooftop installations, for example, could reduce night-time temperatures by up to 0.6°C.
An international group of scientists has created a new model for the evaluation of rooftop photovoltaic solar panels (RPVSP) in urban microclimates.
The module uses the latest Weather Research and Forecasting (WRF) model, integrating the Building Energy Model (BEM) and Building Effect Parameterization (BEP). The model has been validated at ten observation stations in Kolkata, India, using experimentally validated models.
“While existing literature reports on the impact of RPVSP on the urban environment, most of it is based on in-situ field experiments or building-scale simulations, lacking a comprehensive multi-city scale analysis. These studies also fail to account for convective heat transfer between the roof surface and the back of the solar panels,” the academics explain. “Our study addresses these gaps by incorporating new parameterizations for RPVSPs, including convective heat transfer, leading to results more aligned with other studies that incorporate similar considerations.”
The combined approach, called the WRF/BEP + BEM model, can calculate heat exchange, momentum, humidity and turbulent kinetic energy flux between buildings and the outside 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 results are not limited to a specific climate zone.
“Five experiments were conducted to evaluate the regional impact of extensive deployment of RPVSPs during the current heatwave month in Kolkata. A roof albedo of 0.15 and no RPVSPs were used in the control simulation,” the group explains. “The experiments explored RPVSP scenarios with coverage fractions of 0.25, 0.50, 0.75 and 1.0 on urban rooftops. Standard parameters of RPVSPs such as albedo, conversion efficiency and emissivity were set at 0.11, 0.19 and 0.95, respectively.”
According to data collected in Kolkata, RPVSPs can increase near-surface air temperatures during the day by up to 1.5°C, as they absorb about 90% of solar energy, converting up to about 20% into electricity, while the rest contributes to its heating. At night, in contrast, the citys full PV coverage can reduce maximum nighttime near-surface air temperatures by up to 0.6°C. During peak hours, roof surface temperatures would increase by up to 3.2°C and would have an average cooling of 1.4°C at night.
Surface air temperatures were similar across the board. Sydney experienced a 0.8°C cooling at night and a 1.9°C increase during the day; Austin showed a 0.7°C cooling and a 1.8°C increase, while Athens had 0.4°C and 1.2°C, respectively. Results from Brussels showed a 0.3°C night-time cooling and a 1.1°C daytime increase.
“Our study also reveals that rooftop solar PV panels significantly alter urban surface energy balances, near-surface meteorological fields, urban boundary layer dynamics, and sea breeze circulations,” the group adds. “The elevated urban temperatures due to the installation of RPVSP enhance lower atmospheric mixing and raise the height of the planetary boundary layer (PBL) up to 615.6 m, reducing ground-level pollution.” The PBL represents the lowest part of the atmosphere, which is directly influenced by the Earth’s surface.
The results are presented in the study “ Rooftop photovoltaic solar panels warm up and cool down cities ”, published in Nature Cities . The research was carried out by researchers from the University of Calcutta (India), the Indian Institute of Technology Kharagpur, Jadavpur University, the Massachusetts Institute of Technology (MIT) (USA), the University of Texas at Austin, the Chinese Academy of Sciences and the University of New South Wales (Australia). |
