Floating photovoltaics strongly reduce water temperature: A whole-lake experiment.

Floating photovoltaics (FPVs), solar panels installed on floating structures in freshwater ecosystems such as lakes, represent a growing renewable technology aimed at decarbonizing the energy sector. However, robust empirical assessments of its environmental effects are still lacking. We used a Before-After-Control-Impact design replicated at the ecosystem level (n = 6 lakes: three lakes with FPV compared to three non-FPV lakes) to determine the global effects of FPV on water temperature over three years and allowing to isolate FPV effects from natural variability. Overall, we found that the presence of FPV strongly decreased annual water temperature (1.2 °C on average). The reduction in water temperature induced by FPV increased significantly with air temperature and differed between seasons, with stronger reductions (up to 3 °C) observed during warmest days of the year in spring and summer. In addition, the reduction in water temperature also occurred in areas of the lakes that were not covered by FPV. 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 C cycle, including greenhouse gas emissions. Therefore, the cascading effects of FPV on freshwater biodiversity and ecosystem functioning still need to be assessed.