Contrary to previous studies that predicted solar power installations would decrease temperatures around them by absorbing some of the sun’s energy, a study by a team of researchers from the University of Arizona and the University of Madison-Wisconsin indicates the opposite: Large solar power plants cause a photovoltaic heat island effect.
The study, titled “The Photovoltaic Heat Island Effect: Larger solar power plants increase local temperatures” and published in the journal Scientific Reports, explores how different types of ecosystems respond to changes in their land use and land cover in light of a changing climate. “We think this work is really important because we have seen semiarid environments around the globe undergo significant transitions in their structure — transitions to having more woody plant cover or desertification, or the intrusion of the built environment, for example,” explains Greg Barron-Gafford, assistant professor and associate director in the University of Arizona School of Geography & Development and co-author of the study.
In investigating these environments affected by large-scale power plants, the researchers wanted to find answers to questions such as how this new ecosystem absorbs and releases carbon dioxide. And how are nutrients and pollutants retained or released? How does the new collection of species interact? How does water cycle through that ecosystem? And — in reverse — how do these novel ecosystems influence the environment? “This is important because these are more and more often the environments that people directly experience,” says Barron-Gafford. “We have been doing this sort of work in natural, managed and urbanized ecosystems for more than a decade, and we wanted to use our same tools and expertise to examine a novel ‘built environment:’ large-scale photovoltaic areas.”
What motivated the study
“We had read about concerns over potential unintended side effects of renewable energy production that had led to residents being concerned about having large photovoltaic installations created nearby, including concerns that photovoltaic power plants could create a heat island effect,” says study’s other co-author, Mitchell Pavao-Zuckerman, assistant professor in the Department of Environmental Science and Technology and the Cluster for Sustainability in the Built Environment at the University of Maryland.
The researchers were at an impasse neither being able to support nor to counter these concerns because there were no studies to point to. A close look at the theory underlying heat island effects only garnered mixed predictions: Either that the panels’ shading creates cooler temperatures or that they introduced dark surfaces (a lower albedo). “This transition from a natural to a built environment mimics, in many ways, the transition from natural ecosystems to urban environments,” Pavao-Zuckerman says. If those transformations can create an urban heat island effect, they queried, might the creation of a large-scale PV installation create a PV heat island effect?
The team set out to find a place to study these contradicting theories empirically by conducting measurements in the local environment around a natural wildland and a larger PV installation.
What exactly is the solar heat island effect?
“We found temperatures over a PV power plant were regularly 3–4 degrees Celsius warmer than the nearby wildlands at night, confirming the presence of a PV heat island effect,” says Barron-Gafford. He adds that changing sun angles and background temperatures throughout the seasons cause varying degrees of this PV heat island effect varies, likely because of changing sun angles and background temperatures. While the PV heat island effect was detectable in the day, the real significant warming was found in the evening hours, partially because these large PV installations took longer to cool down in the nighttime hours. “We believe that this heat island effect results from the transition in how solar energy moves in and out of a PV installation versus a natural ecosystem,” Barron-Gafford says.
Plants transpire water vapor from photosynthesis into the atmosphere. The process is call latent heat exchange and is important here because incoming energy from the sun typically is either reflected back to the atmosphere or absorbed, stored and later re-radiated. “Basically, there are only two ways to ‘get rid’ of that heat — latent or sensible heat, and if you reduce one pathway, we should expect an increase in the other,” Barron-Gafford says, further explaining that within natural ecosystems, vegetation reduces heat gain and storage in soils through shading, and energy that is absorbed by vegetation and surface soils can be released as evapotranspiration – the combined water loss from soils (evaporation) and vegetation (transpiration). “This heat-dissipating latent energy exchange is dramatically reduced in a typical PV installation, potentially leading to greater heat absorption by soils in PV installations. This increased absorption, in turn, could increase soil temperatures and lead to greater sensible heat efflux.”
Findings contradict previous theories and models
Previous studies predicted that PV power generation would actually decrease overnight air temperatures. “The big difference between those studies and ours is the setting or context,” says Pavao-Zuckerman. “Previous modeling studies have asked how PV panels would impact the built environment, already urbanized spaces, while our study evaluated the impact of PV power plants as they transform native ecosystems. The key here is how PV interacts with land use changes to affect the overall inputs and outputs to the ecosystem transformations: in general cities already have significantly altered the landscape by removing vegetation and bringing in new surface types that affect temperatures by altering albedo, transpiration, and heat storage.”
The team argues that when asking questions about the net effects and benefits of PV installations, it is important to consider the local context of the broader ecosystem that is being transformed.
Impact of the new findings on the design of future solar panels and power plants
“More than anything, this study should serve as a reminder that we need to be mindful of the implications of our work on the landscape,” says Barron-Gafford. The results provide important actual field data from a semiarid environment, which the expert says is one of the most common biomes for siting large solar PV plants. “Yet, we still lack data on the size of the PV heat island effect.”
In their paper, the scientists note that the size of an urban heat island is determined by properties of the city, including population, spatial extent, and the geographic location. “We should similarly consider the spatial scale and geographic position of a PV installation when considering the presence and importance of the PV heat island effect,” Barron-Gafford notes. They have conducted preliminary research by running a transect of temperature sensors from the heart of a PV installation outward. Those results suggest that the PV heat island dissipates within about 30 m. “This might suggest that most of the heat generated simply rises away from the ground surface,” he says. To fully contextualize these findings in terms of global warming, the researchers says one needs to consider the relative significance of the globally averaged decrease in albedo due to PV power plants and their associated warming from the heat island effect against the carbon dioxide emission reductions associated with PV power plants. “For this reason, we do not see our findings to suggest any reasons to move away from PV as a valuable part of our renewable energy portfolio. That said, we should see if there are simple ways to mitigate this heat island effect.”
Written by Sandra Henderson, Research Editor, Solar Novus Today