Rooftop solar panels impact day and night temperatures in cities, simulation study finds

Widespread coverage of building roofs with conventional photovoltaic solar panels can raise temperatures on hot days and lower them at night, according to new modeling.

The research led by Dr Ansar Khan of the University of Calcutta and co-authored by Professor Mattheos (Mat) Santamouris of UNSW Sydney Scientia used mesoscale (weather system) simulations due to the absence of observational data available for rooftop photovoltaic solar panels (RPVSP). to model their impact on local climatic conditions at the city scale.

They found that in a scenario with full RPVSP coverage in a city, urban temperature could increase during the day by up to 1.5°C during peak summer periods and decrease at night by up to 0.6°C.

The results, published in Nature Citiesdo not suggest that photovoltaics are not an important renewable energy solution in the transition away from fossil fuels. Instead, the researchers say it highlights the opportunity to develop integrated solutions for RPVSPs, such as reflective materials for roof and greenery combinations, to balance their many advantages with their potential disadvantages in an urban environment.

“Solar photovoltaic panels are an important renewable energy technology, but they can change the local conditions of cities when installed on large scales on rooftops,” says Professor Santamouris, Anita Lawrence Chair in High Architecture performance at UNSW Arts, Design & Architecture. “Understanding these changes is crucial for policymakers considering citywide use of RPVSP and the strategies needed to deploy them effectively.”

Daytime heating and nighttime cooling effects

For the study, the research team analyzed the impact of RPVSPs under different coverage scenarios in Kolkata, India. They also conducted RPVSP sensitivity studies specific to four other cities – Sydney, Austin, Athens and Brussels – to compare their results with those of Kolkata.

“We found that there is a linear association between increasing temperature and the percentage of roofs covered with photovoltaic panels,” explains Professor Santamouris. “In the maximum scenario of 100% coverage during the peak summer period, our data shows that RPVSP leads to a significant increase in daytime temperature.”

Professor Santamouris says the thermal effect of photovoltaic panels on 100% roof coverage would reduce many of the benefits of renewable energy. Estimates show that in Sydney, almost 40% of the electricity produced by photovoltaic panels is used to offset the overheating impact of additional cooling load, mainly air conditioning.

“When RPVSPs are installed on roofs, they absorb a significant amount of solar energy, converting part of it into electricity and thus generating heat,” explains Professor Santamouris. “This is mainly due to the lower albedo (reflectance) of the panels, but also the airflow over the top and bottom of the PV panels, which amplifies the heating effect.

“This means that during the peak summer period, the surface temperature can reach 70°C, making a cooling effect on urban temperature impossible.”

Conversely, radiant heat transfer at night leads to the surface temperature of PVs being lower than the urban temperature at night, thereby lowering the ambient air temperature. This is a crucial advantage as the impact of global warming is expected to be felt most intensely during nights, which could warm by around 4°C on average over the next century, says the professor. Santamouris.

The study also found that RPVSPs may have additional complex effects on other local conditions. In particular, they can reduce pollutant concentrations by increasing the planetary boundary layer – the lowest part of the atmosphere influenced by Earth’s surface conditions – leading to higher wind speeds and dilution of pollutants. pollutants.

“RPVSPs also improve the penetration of coastal sea breezes further inland into coastal cities, while reducing the concentration of pollutants at ground level,” explains Professor Santamouris. “This helps to some extent in offsetting the effect of increased temperature during the day and is important for cities like Kolkata where the problem of pollutants is very pressing.”

The promise of integrated solutions

Professor Santamouris says it is possible to create RPVSPs that mitigate their impacts on daytime heating while retaining their benefits through advanced cooling strategies. Additionally, innovations in advanced materials science, combined with RPVSPs, have significant potential to create cool photovoltaic systems that operate at lower temperatures and reduce thermal impacts.

In particular, Professor Santamouris advocates the use of hybrid photovoltaic systems integrating panels with a water-based thermal collection system to absorb excess heat for hot water production.

“It is already possible to cool the surface of photovoltaic installations by circulating water,” explains Professor Santamouris. “Designs that flow water behind the panels absorb excess heat and improve photovoltaic efficiency by lowering operating temperatures, while excess solar energy can be channeled to provide hot water at lower cost. cost.”

Highly reflective cool roof materials that bounce heat back rather than absorb it could also be used to increase energy production from nearby RPVSPs while mitigating local district heating during the day. Adding other heat sinks, such as greenery on roofs, can also improve photovoltaic efficiency.

“Combining photovoltaic panels with green roofs or cold roofs can increase the capacity of photovoltaic panels by up to 6-7% and significantly reduce surface temperatures,” explains Professor Santamouris.

“If we want to continue implementing rooftop PV panels, we need to seriously consider these integrated solutions to maximize the efficiency of RPVSP and also address urban heat challenges.”