Many countries are generating renewable energy using floating solar arrays, but by shielding the surface of bodies of water from solar radiation and wind, the technology could disrupt aquatic ecosystems, with mixed implications for flora and fauna.

In the first study measuring such potential effects of these "floating photovoltaics," published March 4 in Solar Energy, Great Britain researchers found that, in general, greater coverage lowers not only the temperature of a body of water, but also the amount of wind speed agitating the surface. Changing either of these variables can affect the organisms living there because they impact water stratification, or layering.

"There's this urgent need to decarbonize and switch to renewable energy sources, but very little was known about what the positive or negative environmental consequences of doing that are," said lead author Giles Exley, a Ph.D. student at Lancaster University. "Our inspiration came from this desire to further our understanding of that."

Solar power is a growing form of renewable energy. While many floating solar plants operate in Asia, by 2025, with private and government investment, global production of floating solar power is expected to increase by 10 gigawatts thanks to new projects in Brazil, the U.S., Tunisia and France. One gigawatt amounts to 3.1 million solar panels.

For the new study, researchers from Lancaster University and the University of Stirling investigated the physical effects of covering the surface of Windermere, the U.K.'s largest natural body of water, with these floating photovoltaics.

Exley and his coauthors created a computer simulation of Windermere using data from the U.K. Centre of Ecology & Hydrology. From there, they could virtually simulate floating solar coverage by changing the amount of wind and solar radiation hitting the water's surface and analyze how different levels of coverage affected water temperature and stratification.

Stratification occurs when a body of water neatly divides into different layers according to qualities such as temperature and oxygen concentration, with warm, oxygenated water on top and cold, less oxygenated water toward the bottom. Wind can agitate this top layer, allowing it to mix with the others, spreading oxygen and nutrients to lower depths. 

Usually, this happens naturally over the course of a year, with increasing stratification during warm months, before autumn breezes mix the waters back up again. During prolonged periods of stratification, however, lower layers can run out of oxygen, starving fish and other organisms that live there. 

"If all the oxygen is depleted, it can cause water quality issues, like the release of heavy metals from the bed sediments," Exley said in an interview with The Academic Times. "Fish might have to live in the cooler bottom waters, as they only have tolerance to a specific range of temperatures, and if there's no oxygen in the bottom layer, this poses a problem for these fish."

The team generally found that, by blocking winds that reduce stratification while lowering surface temperatures that increase it, wider coverage could ultimately shorten stratification periods.

However, as these variables can function in competition with each another, results varied depending on how much solar radiation and wind speed were reduced.

For example, even though colder temperatures can make a water body more homogeneous overall, stratification actually increased in some of these instances because of lower wind speeds. And in one extreme case, covering 90% of Windermere's surface with a floating solar array made it freeze, which wouldn't normally happen outside a simulation, according to Exley.

But these effects may change from water body to water body, and underscore the need to weigh these variables when considering floating solar and its suitability for a specific location. What occurred in the Windermere scenario may unfold differently at another lake in another climate zone. 

"We were surprised at how the output from the model was not entirely linear, that there are these potential combinations of wind speed and solar radiation reduction that could have the opposite effect of what we would expect," Exley said. "If anything, it's reinforced the drive to keep going to better understand the system."

The impact of the effects Exley discovered could be likened to those of climate change, only they would occur through cooling, rather than direct heating.

Some of these changes could even be a boon for water managers or recreational organizations. Some floating solar designs could reduce a water body's ability to stratify, weakening the growth of algae that photosynthesize on the warmer top layer of lakes, for example. This scenario could allow some floating solar arrays to double as an effective means to manage unwanted algae, depending on their location. 

While this study only examined the physical effects of floating photovoltaic coverage, Exley plans to investigate the biological effects as well, from algal development to species dynamics, in the future. He had also been conducting in-person experiments on a floating photovoltaic array, but COVID-19 has halted that project for now.

With this research, Exley hopes to show that there are a number of environmental and design concerns to account for when developing floating solar power.  

"This study has highlighted that there is this potentially big range in responses you could end up with: only a minor reduction in water temperature, or you could actually end up with more significant reductions," Exley said. "As with any renewable energy deployment, it's important to consider it on site-by-site basis."

The study, "Floating photovoltaics could mitigate climate change impacts on water body temperature and stratification," published March 4 in Solar Energy, was authored by Giles Exley, Alona Armstrong and Trevor Page, Lancaster University; and Ian D. Jones, University of Stirling.