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New research conducted in Hong Kong confirms that rooftop photovoltaic systems contribute significantly to rising daytime temperatures in urban environments. The scientists explained that while their findings highlight important considerations regarding urban resilience, they should not detract from the overall benefits of photovoltaic systems for energy sustainability and carbon reduction. Researchers at the Hong Kong University of Science and Technology (HKUST) have studied the impact of rooftop photovoltaic systems on urban microclimates and temperatures, and found that these installations, despite their potential to improve energy sustainability, could inadvertently exacerbate urban heat. The research was conducted on HKUSTs humid subtropical campus, which has been home to several photovoltaic systems since 2020. The initiative included the installation of more than 8,000 solar panels at 50 locations on campus, generating up to 3 million kWh of electricity. “During a visit to the PV-equipped rooftops, the researchers observed that the panels were extremely hot, seemingly hot enough to fry an egg,” Chen Liutao, the study’s lead author, told pv magazine . “This observation inspired the team to deploy environmental sensors on several rooftops to monitor air and surface temperatures and investigate the potential thermal impacts of rooftop PV systems.” The results revealed that rooftops equipped with photovoltaic systems experienced higher ambient air temperatures compared to conventional rooftops. During the hottest month of July, the air temperature above the photovoltaic rooftops was, on average, 1.3°C higher than that of conventional rooftops without air conditioning (AC) units, and 0.7°C higher than that of those with AC units. In 15-minute averages, the temperature difference reached as much as 5.2°C at midday. The researchers also analyzed the thermal behavior of the photovoltaic panels themselves, finding that under intense solar radiation during heat waves, the panel surfaces could reach temperatures of up to 65.8°C, with monthly averages 9.7°C higher than concrete surfaces. This warming effect was attributed to the panels high solar absorptivity and low thermal inertia. Heat is dissipated to the surrounding air through convection. At night, the photovoltaic panels cooled faster than conventional rooftops, slightly lowering the surrounding air temperature by less than 1°C. To better understand how photovoltaic-induced microclimate changes affect buildings energy demands, the research team conducted EnergyPlus simulations for a rooftop office space under a photovoltaic-equipped roof. Although 50% photovoltaic coverage could offset 71% of the buildings energy consumption through electricity generation, the ambient warming caused by the photovoltaic panels offset the shading benefits. In July, this resulted in a net 1.5% increase in cooling energy demand. Beyond the energy impacts, the study highlighted critical health risks during power outages. Photovoltaic-induced heating extended periods of “Extreme Danger” heat exposure by 29.8%, increasing the risk to building occupants during heat waves. The researchers emphasized that targeted use of photovoltaic-generated energy for cooling during periods of greatest danger could transform these systems from passive heat amplifiers into resilience assets. The study also suggested the need for innovative photovoltaic system designs to mitigate adverse microclimatic effects. Promising solutions include hybrid green roof-photovoltaic systems, phase-change materials, and photovoltaic-integrated thermal systems. These technologies could simultaneously reduce panel surface temperatures and improve energy conversion efficiency, balancing energy production with urban heat mitigation. “Rooftop photovoltaic systems are not just energy infrastructure; they are active participants in urban microclimatic systems,” Liutao said. “Their successful integration requires a dual optimization: maximizing generation potential while intelligently managing localized environmental impacts.” The findings also highlighted the variability in the thermal effects of photovoltaic systems depending on factors such as panel tilt angle, array spacing, and roof base properties. The researchers called for further simulations and experiments in different climates and configurations to better understand and address these challenges. “Its important to note that while these findings highlight important considerations regarding urban resilience, they should not detract from the overall benefits of PV systems for energy sustainability and carbon reduction,” Liutao concluded. “The goal of this study is to inform improved designs and deployment strategies, ensuring that rooftop PV systems can deliver both environmental and social benefits without unintended trade-offs.” The full results of the study are detailed in the article “ The Resilience Paradox of Rooftop PV: Building Cooling Penalties and Heat Risks,” recently published in Building and Environment . In October, another international group of scientists presented a new model for evaluating rooftop photovoltaics in urban microclimates. The modeling showed that rooftop photovoltaic arrays can have unintended consequences on temperatures in urban environments. According to data collected in India, rooftop systems 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% into electricity, while the remainder contributes to its heating. At night, however, complete photovoltaic coverage of the city can reduce maximum nighttime near-surface air temperatures by up to 0.6°C. During peak hours, rooftop surface temperatures would increase by up to 3.2°C, with an average cooling of 1.4°C at night. “Our study also reveals that rooftop solar photovoltaic panels significantly alter urban surface energy balances, near-surface meteorological fields, urban boundary layer dynamics, and sea breeze circulations,” the research group noted at the time. |
