Floating solar systems are considered a promising alternative to photovoltaics on land. But a long-term experiment in the USA now illustrates the dark sides of the “Floating PV”-and literally. If you cover smaller waters such as ponds with floating solar panels, the negative effects show almost immediately: the water becomes cooler, darker and significantly lesser. This is changing the degradation processes and on the seaside area there is more methane that drives the greenhouse gas output of the ponds up: The solar ponds exposed an average of around 27 percent more greenhouse gases than without the solar systems, as the team reports. However, the test ponds were 70 percent covered with solar panels – in Germany, on the other hand, only 15 percent would be allowed.
The electricity extraction using photovoltaics is an important component of the energy transition, but has a big disadvantage: it needs relatively large areas and is therefore often competing with other forms of land use. Alternatives such as roof and facade solar systems combined with agricultural cultivation Agri-Photovoltaic or the so-called “floating photovoltaic”. For the latter, floating solar panels are applied to reservoirs, ponds or cisterns and then produce solar flow on the water surface using photovoltaics. However, this does not remain without consequences for the waters: Because the solar system shadows the water, plankton and aquatic plants receive less light for their photosynthesis. In addition, the covering of the water surface inhibits the gas exchange between water and atmosphere. This is why, among other things, in Germany there is the requirement that a maximum of 15 percent of a water should be covered by floating photovoltaics.
Solar ponds in the extreme test
An experiment in the USA now shows that this restriction exists for a good reason. There, Nicholas Ray and his colleagues from Cornell University in New York for the first time examined how the biogeochemistry of smaller waters and in particular their greenhouse gas emissions changed due to floating solar systems. “Because worldwide, more than 90 percent of the Floating PV installations are carried out on reservoirs, lakes and ponds with less than a square kilometer,” said the team. The researchers used six 30 x 30 meter test ponds for their experiment of around 1.85 meters depth. Everyone had already been created in the 1960s, overgrown by aquatic plants, but did not contain any fish and resembled themselves in chemistry and biology. In the summer of 2023, the researchers then installed three of these ponds floating solar panels, which allows around 70 percent of the water surface to be significantly more than in this country. Ray and his colleagues pursued over two years, such as temperatures, oxygen content and the concentrations and delivery of greenhouse gases such as carbon dioxide (CO2) and methane (CH4) in the ponds with and without solar systems.
The result: A few days after the solar systems were installed, the first changes were shown. “We found immediate and persistent acceptance of the temperatures and the oxygen content,” reports the team. The water on the pond base quickly reached almost anoxic – oxygen -free – conditions. At the same time, the content of dissolved CO2 and methane also fell in the water. As a result, the diffuse greenhouse gas emissions of the ponds fell: around 24 percent in CO2 and 18 percent in methane. The main reason for this is the disabled gas exchange between water and air, as Ray and his colleagues explain. “If you cover the water with floating solar systems, you drastically reduce oxygen availability for organisms-and thus fundamentally intervene in the ecological processes,” says co-author Steven Grodsky from Cornell University. Planton, aquatic plants and the microbial decomposition on the pond base are severely impaired.
More greenhouse gas through methane bubbles
These changes also have an impact on the greenhouse gas emissions of the solar ponds. The changed chemistry of the water and the lack of oxygen is increasingly created on the ground of the water. This increases in the form of gas bubbles to the water surface and is released in the air in the still uncovered areas. In the experiment, the methane exhaust gasification from bubbles in the ponds with floating PV increased by 57 percent, as Ray and his team found. The result: ponds with solar systems became greenhouse gas sling: “The ponds with floating PV emitted 26.8 percent more greenhouse gases measured in CO2 equivalents than that without the solar systems,” report the researchers. This means that floating solar systems are not climate-neutral-at least not if small waters are largely covered with the PV panels. However: In the case of hydropower from reservoirs and solar systems on land, even more greenhouse gases can be released per area depending on the system, as the team explains.
In any case, the results of this experiment confirm that it makes sense to limit floating solar systems only to a small part of the water surface – as required in Germany. And you could also do a lot to reduce the negative effects, the researchers explain: “For example, if you tend to the solar cells and increase the distance between the PV panels and the water surface, this reduces the biogeochemical effects,” says Way and its Colleagues. As a result, more light and oxygen gets to the water than with flat systems. “In addition, ventilation devices that hang under the PV panels could reduce the oxygen loss and thus probably also the methane production,” the researchers write. Grodsky adds: “It is always a compromise. But only if we know exactly what is going on can we adapt the technology accordingly. ”
Source: Nicholas Ray (Cornell University, IThaca, NY) et al., Environmental Science & Technology, DOI: 10.1021/acs.4c06363