With the price of solar power cratering by 85 percent during the 2010s, the question is no longer whether it’s economically feasible to deploy the technology on a large scale. Now it’s: Where can’t we put solar panels? Governments are handing out tax breaks to get people to install them at home, but we can also put them in the empty space around airports and over ugly parking lots, or slap them on rooftop gardens and in agricultural fields and grow crops under them, simultaneously generating power and food.
So how about laying a bunch of solar panels on reservoirs? Floating photovoltaic systems, also known as floatovoltaics, could be a powerful complement to the hydroelectric power already generated by a reservoir and save water by shading it and reducing evaporation.
A new study by an international team of researchers shows just how useful wide-scale floatovoltaics could be. They calculate that covering 30 percent of the surface of 115,000 reservoirs globally could generate 9,434 terawatt hours of power a year. That’s more than twice the energy the entire United States generates annually, and enough to fully power over 6,200 cities in 124 countries.
“That’s remarkable, this 9,434-terawatt-hours-per-year potential,” says J. Elliott Campbell, an environmental engineer at the University of California, Santa Cruz and coauthor of the paper, which was published today in Nature Sustainability. “It’s about 10 times today’s generation from solar. And solar is growing like crazy. If there was ever a time to ask where to put all this stuff, it’s now.”
Floatovoltaics work just like solar panels on land, only they’re … floating. Each one is a cluster or “island” of panels, built atop a buoyant mounting platform and anchored to the bottom of the water body by cables. Every other row of panels is a walkway for crews to do electrical maintenance or inspections.
The systems are of course built to resist rust, but so are terrestrial panels, which are exposed to rain. “The electrical system is really no different than a rooftop system or a ground mount system,” says Chris Bartle, director of sales and marketing at Ciel & Terre USA, which deploys floatovoltaic projects around the world. “We’ve taken essentially old technology from the marina world—docks and buoys and whatnot—and applied that to building a structure that an array of solar panels can be mounted to. It’s really as simple as that.”
They have an added engineering challenge, though, in that a reservoir’s water level can change dramatically during storms or droughts. There may be strong currents, as well as winds. So while the system is anchored to the lake bottom, there must be slack in the anchoring lines. “It allows the island to move around with the nature of the wind and the waves and water level variation,” says Bartle.
These islands shade water that would otherwise be exposed to relentless sunlight; if implemented worldwide, the study found that all those panels would save enough water to supply 300 million people each year. The reservoir water, in turn, actually makes the floatovoltaics more efficient at harvesting the sun’s energy. It cools them—like a human, solar cells can overheat.
In 2021, Campbell published another paper based on the same principle: If California spanned 4,000 miles of its canal system with panels, it would save 63 billion gallons of water from evaporation each year and provide half the new clean energy capacity the state needs to reach its decarbonization goals.
Because the US has so many reservoirs—some 26,000 in varying sizes, totaling 25,000 square miles of water—it would especially benefit from wide-scale floatovoltaics, the new study finds. If the country covered 30 percent of its reservoir area with floating panels, it could generate 1,900 terawatt hours of energy—about a fifth of the potential global total—while saving 5.5 trillion gallons of water a year.
China could manage 1,100 terawatt hours annually, followed by Brazil and India at 865 and 766, respectively. Egypt could deploy 100 square miles of floatovoltaics and generate 66 terawatt hours of electricity while saving over 200 billion gallons of water annually.
The study further found that 40 economically developing countries—including Zimbabwe, Myanmar, and Sudan—have more capacity for floatovoltaic power than current energy demand. (Though as they develop, that energy demand will go up.)
An additional upside of floatovoltaics is that many reservoirs are equipped with hydroelectric dams, so they already have the electrical infrastructure to ferry solar power to cities. The two power sources complement each other well, says Zhenzhong Zeng, of China’s Southern University of Science and Technology, a coauthor of the new paper. “The intermittency of solar energy is one of the main obstacles to its development. Hydroelectric power, which tends to be controlled, can make up for the shortfall at night when solar power does not work,” says Zeng. “Moreover, it can be combined with wind power, which is usually well-complemented to solar.”
Water savings will be all the more important as climate change supercharges droughts, like the historic one that’s been gripping the Western states. But even if a reservoir’s water level declines severely and hydroelectric generation begins to dip, floatovoltaics would still generate electricity. (However, more remote reservoirs without hydroelectric systems would need to connect their solar panels to the larger grid, which would increase costs.)
Floatovoltaics could also interface nicely with microgrids, says Sika Gadzanku, an energy technology and policy researcher at the National Renewable Energy Laboratory. These are divorced from a larger grid and use solar power to charge up batteries, which can, for example, power buildings at night. “If you maybe had a huge pond in a remote area, deploying floatovoltaics could look similar to just applying a solar-plus-battery project in some other remote area,” says Gadzanku, who wasn’t involved in the new paper but peer-reviewed it.
And it could benefit small communities in other ways, Gadzanku says: Installing a floating system on a local pond could save its water and might be cheaper than trying to connect a remote area to a bigger grid. “Expanding the grid is very expensive,” she says.
Putting panels over canals or reservoirs would make use of space that’s already been modified by people, and it wouldn’t require clearing additional land for huge solar farms. (Floatovoltaics can also be deployed on polluted water bodies, like industrial ponds.) “It takes about 70 times more land for solar than it does for a natural gas plant, for equal capacity,” says environmental engineer Brandi McKuin of the University of California, Merced, who coauthored the canal paper with Campbell but wasn’t involved in this new work. “If we’re going to reach these ambitious climate goals while also protecting biodiversity, we really need to look at these solutions that use the built environment.”
In recent years, floatovoltaics have graduated from smaller-scale projects to sprawling solar farms, like in Singapore’s Tengeh Reservoir, where the panels occupy an area equal to 45 football fields. As the systems scale up, “we really need additional research on what some of the potential impacts are, thinking about these water ecosystems,” says Gadzanku. For example, the shade might prevent the growth of aquatic plants, or the panels might cause problems for local waterfowl and migrating birds that rely on reservoirs as pitstops. It might be useful to determine, for instance, if there’s an optimal spacing of panels to allow species to freely move about the water.
While these projects alone won’t be able to provide whole metropolises with juice, they’ll help diversify the generation of power, making the grid more resilient as the renewables revolution gains speed. “Energy is such a big problem, we’re not going to have one silver bullet,” says Campbell. “We need floating photovoltaics and about a hundred other things to satisfy our energy needs.”