A floating PV system at the La Madone lake in southern France

Researchers from France have conducted a study into the impact of floating PV (FPV) power stations on water temperature in freshwater ecosystems. They have used a before-after-control-impact (BACI) framework to understand those differences.

The team measured six similar lakes, three of them with FPV and three without, over a period of three years.

“Although the effects of FPVs on water temperature are among the most frequently studied impacts of this emerging technology, most empirical research has primarily focused on small-scale pilot installations and short-term observations, strongly limiting the robustness of our knowledge,” they said.

“FPV power plants are becoming a conspicuous part of the landscape and, to date, long-term empirical assessments at the ecosystem scale of the global effects of FPVs on lake temperature, with robust experimental designs, are still lacking.”

This study was performed from December 2020 to December 2023 in six close gravel pit lakes located in the Garonne River floodplain, southwestern France. Temperature measurements were taken via buoys attached to a mooring with loggers deployed up to a depth of four meters. About one year into data collection, FPV was installed in three lakes, enabling a before-after analysis.

FPV power plants covered 40.3%, 51.5%, and 55.5% of the lakes, with an average coverage of 49.1%. Each FPV-covered lake, or “impact” lake, was paired with a “control” lake without FPV. They were paired based on their hydromorphological similarities, established using a principal component analysis (PCA) performed on environmental parameters. Those environmental parameters were surface-to-volume ratio, maximal depth, perimeter, and elevation.

Per the results, before FPV installation, water temperature averaged 15 C and 15.3 C in the FPV and non-FPV lakes, respectively; while after installation, they averaged 16.9 C and 18.1 C.

“Overall, the presence of FPV power plant decreased water temperature by 1.2 C compared to lakes without FPV power plant,” the academics highlighted.

This reduction in temperature changed significantly between seasons. A mean difference of 2.3 C between the control and impact lakes was recorded in spring, while in summer, the mean reduction was 1.9 C. The reduction was much lower in winter and autumn, with only 0.6 C of mean difference.

“On average, a reduction of water temperature of 0.08 C between FPV and non-FPV lakes was observed when air temperature increased by 1 C. When the average daily air temperature was <10 C, the predicted reduction of water temperature was very limited (<0.5 C). When the average daily air temperature was >30 C, the reduction of water temperature was >2 C. The highest differences in water temperature reached >3 C,” they added. “In lakes containing FPV power plants, water temperature measured in the area covered with FPV did not differ significantly from the water temperature measured in the uncovered area.”

In concluding their research, the team added that “in the context of climate warming, decreased water temperature in summer could benefit freshwater organisms, but these benefits could be counterbalanced by other negative impacts such as decreases in dissolved oxygen and modifications in the carbon cycle (C cycle), including greenhouse gas emissions. Therefore, the cascading effects of FPV on freshwater biodiversity and ecosystem functioning still need to be assessed.”