Do Floating Solar Panels Cut Greenhouse Gas Emissions?

The use of floating solar panels on freshwater bodies is an increasingly popular way of generating renewable energy. Such systems convert the sun’s rays into electricity while minimizing water evaporation. What’s more, they spare land that could be better used for agriculture or housing.

But like all energy systems, floating solar power, also known as floating photovoltaics or floatovoltaics, comes with environmental tradeoffs, says Nicholas Ray, an assistant professor in the School of Marine Science and Policy at the University of Delaware. In particular, does the benefits of clean energy from floatovoltaics offset the adverse impacts on the freshwater ecosystem beneath the panels? Answering that question was the focus of a study in Environmental Science & Technology published in December by Ray and colleagues at Cornell University, where Ray was a postdoc.

The study found that floating solar panels covering 70 percent of small man-made ponds increased greenhouse gas emissions by nearly 30 percent and rapidly decreased dissolved oxygen in the water. But the rise was lower than the researchers expected.

“I was expecting total emissions, like the whole pond emissions, to double—so it was much less,” Ray says.

And in general, floatovoltaics are expected to have a lower carbon footprint than traditional solar panels installed on land, the study concludes.The emissions can be mitigated if the floating solar panels are deployed in different scenarios—on larger bodies of water, on water bodies with less organic material than the grassy ponds in the study, or with oxygen pumps under the solar panels.

Floating Solar’s Global Power Potential

The World Bank estimates that floatovoltaics could potentially generate 400 gigawatts of electricity—double the worldwide capacity of existing solar panels. That’s a conservative estimate based on covering just 1 percent of man-made freshwater bodies with panels. Montana alone has a potential of 44 GW, according to a different study of federally owned reservoirs in the United States. There’s still a ways to go to reach those forecasts. In November, the energy data company Wood Mackenzie estimated that 1.7 GW of floating solar capacity would be added in 2024, rising to 6.7 GW this year and 13.5 GW in 2026.

In the United States, the largest array of floatovoltaics is on display on the Canoe Brook reservoir in Short Hills, N.J., where over 16,500 panels generate up to 8.9 megawatts in order to power the area’s water treatment plant. Most floating arrays are installed on smaller lakes and reservoirs.

Floating solar panels on small ponds increased methane and carbon dioxide emissions and decreased dissolved oxygen and the water temperature. Nicholas Ray, Meredith Holgerson, et al.

The Cornell study was the first to examine the effect of floatovoltaics on actual small ponds. In the summer of 2023, decommissioned solar panels from the floating solar company Ciel et Terre were placed on three ponds in Ithaca, N.Y. They covered 70 percent of the surface, the maximum coverage recommended by the company. Had the panels been functional, each pond would have generated the equivalent of over 52 megawatt-hours per year. Each square pond measured 30 by 30 meters, with an average depth of 1.85 meters.

Every few days during the study, researchers walked across the panels to the pond center. There, they sampled the water for dissolved oxygen and greenhouse gas concentrations, and measured the water temperature and the atmospheric greenhouse gas levels. Bubble traps, which are inverted funnels attached to graduated cylinders, captured methane bubbling to the water’s surface. In the three panel-free control ponds, they used canoes or kayaks for sample collection.

Within days of the panels being installed, greenhouse gas emissions started to rise, the study reports. But the sources of the emissions were unexpected, Ray says. Methane bubbles increased, but diffusive, or passive, carbon dioxide dropped, as did methane emissions through the water’s surface.

“So if we had not measured the [methane] bubbles, we actually would have said, ‘Oh, [floating solar] actually reduces emissions,’” he adds. The finding highlights the importance of doing field studies of renewable energy systems.

Water beneath the solar arrays was colder than elsewhere in the pond. There was also less dissolved oxygen in the water under the panels, likely caused by the panels blocking sunlight from reaching the photosynthetic microbes in the water.

Comparing greenhouse gas emissions between floating and land-based solar arrays is tricky. Land-based arrays often require removing carbon-trapping greenery like trees and shrubs, while plants at the bottom of a pond can die when shaded by a floating array.

Still, the estimated emissions due to floatovoltaics are small compared to hydropower or land-based solar arrays, the study concludes. Specifically, an array like the ones used in the Cornell ponds would create 2.6 grams of carbon dioxide equivalent emissions for every kilowatt-hour, while hydropower generates 10 grams, and land-based solar between 0 to 50 grams. Natural gas, by contrast, produces about 435 grams of carbon dioxide per kilowatt-hour.

For floatovoltaics, the “environmental impacts are likely fairly easy to mitigate,” Ray says. For example, adding aerators to the bottom of the floating arrays would likely cut emissions, he says.

Mitigating Methane Emissions in Ponds

It’s unsurprising that methane emissions would increase after material is placed on the surface of an organic-rich environment with lots of stored carbon, like the ponds featured in the Cornell experiment, says Lisa Stein, professor of climate change microbiology at University of Alberta, Canada, who wasn’t involved in the study.

Small, nutrient-rich water bodies such as small ponds may not be good candidates for solar coverage, she adds.

In her own work, Stein has experimented with decreasing methane production in freshwater bodies by using man-made floating islands. Each island, produced by Floating Islands International, contains a solar-powered oxygen pump that cycles air and methane through the water. Biomass on the floating island eats the methane as it arrives at the surface.

Industrial-scale solar panels could be padded with these floating wetlands to mitigate methane emissions, Stein says. Another option could be to filter the water by adding periphyton—the “gelatinous ooze that sits in the water,” Stein says—to the underside of the solar arrays.

Floatovoltaics might be best suited for fisheries or water-based farms where the ecosystem is degraded already, Ray says. Then, aquaculture voltaics as it’s called, could reduce emissions by producing power on site, he adds.