A reservoir is many things: a source of drinking water, a playground for swimmers, a refuge for migrating birds. But if you ask solar-power enthusiasts, a reservoir is also not realizing its full potential. That open water could be covered with buoyant panels, a burgeoning technology known as floating photovoltaics, aka “floatovoltaics.” They could simultaneously gather energy from the sun and shade the water, reducing evaporation — an especially welcome bonus where droughts are getting worse.
Now, scientists have crunched the numbers and found that if humans deployed floatovoltaics in a fraction of lakes and reservoirs around the world — covering just 10 percent of the surface area of each — the systems could collectively generate four times the amount of power the United Kingdom uses in a year. The effectiveness of so-called FPVs would vary from country to country, but their research found that some could theoretically supply all their electricity this way, including Ethiopia, Rwanda, and Papua New Guinea.
“The countries around the world that we saw gain the most from these FPVs were these low-latitude, tropical countries that did not have a high energy demand in the first place,” said Iestyn Woolway, an Earth system scientist at Bangor University and lead author of a new paper describing the findings in the journal Nature Water. “It meant that if only a small percentage of their lakes — this 10 percent — was covered by FPVs, it could be enough to fuel the energy demand of the entire country.”
For developing countries, floatovoltaics could be especially powerful as a means of generating clean electricity. Instead of building out more planet-warming infrastructure running on fossil fuels, like gas-fired power plants, fledgling economies could run panels on land and water, in addition to other renewables like wind and hydropower. With solar power comes autonomy: Utilities don’t have to rely on shipments of fossil fuels, but can tap into the abundant power of the sun.
Floatovoltaic solar panels — which have been proliferating globally, from California to France to Taiwan — are the same ones found on a rooftop. “It’s the same electrical system, same panels, same inverters,” said Chris Bartle, director of sales and marketing at Ciel and Terre USA, which is deploying floatovoltaic systems. “We’re just providing a structure that floats to mount that electrical system.” The solar rafts are anchored either to the bottom of the water body or to the shore, or both, to keep them from wandering.
In many ways, solar panels and bodies of water can benefit one another. Photovoltaics get less efficient the hotter they get, so having them floating on a lake or reservoir helps cool them off. “Because of the cooling effect, we see increased efficiency of the systems,” said Sika Gadzanku, a researcher at the National Renewable Energy Laboratory in Colorado, who studies floatovoltaics but wasn’t involved in the new research. Returning the favor, the panels provide shading, reducing evaporation. If floatovoltaics are spread across a reservoir, that could mean more water would be available for drinking.
If a reservoir is equipped with a dam for hydroelectric generation, the floatovoltaics could hook into that existing transmission infrastructure. (Countries like Kenya, for instance, are building out more of this hydroelectric infrastructure already.) That could save local governments money because they wouldn’t need to string new transmission lines from the floatovoltaics to the nearest city. In the event of a drought, when water levels drop too low to generate hydropower, the panels could still operate as backup power.
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To do their new modeling, Woolway and his colleagues began with over a million lakes and reservoirs around the world big enough and deep enough for floatovoltaics. Then they whittled those down based on critical qualities. For one, the body of water couldn’t dry up, beaching the panels, or freeze over for more than six months a year, entombing the panels in ice and damaging them. The lake couldn’t be protected by law, either, like as a natural refuge. And the site had to be near a human population that could use the generated power.
A remote lake, by contrast, would require long transmission lines to connect a faraway city to the floatovoltaics. This doesn’t necessarily rule out the technology for more remote communities of people living near an otherwise suitable lake. In fact, floatovoltaics could be particularly potent there as a way to provide clean energy. These cases just weren’t included in the scope of this modeling.
Regardless, all those characteristics considered, the team ended up with 68,000 feasible locations in 163 countries. They found that on average, countries could meet 16 percent of their energy demand with floatovoltaics, but some places could generate a lot more. In Bolivia, for instance, floatovoltaics could provide up to 87 percent of national electricity demand, and in Tonga, they could meet 92 percent. The potential is much lower in the United States, however, meeting just 4 percent of energy demand — even though the country has a plethora of large lakes and reservoirs, overall energy usage is extremely high. In less-sunny climes, like northern Europe, the effectiveness of floatovoltaics drops, but Finland could still satisfy 17 percent of its electricity demand with floating panels.
“The regions or the countries that we saw had the highest potential had these two critical variables, in that they were close to the equator, or were at high elevations, so they received high amounts of incoming solar radiation,” Woolway said. “And secondly, they had large water bodies.”
Covering 10 percent of a 100-square-mile lake, for instance, would end up with a lot more solar panels than covering the same percentage of a 10-square-mile lake. “We considered 10 percent to be a reasonable surface area coverage without having a devastating impact on the ecology and the biodiversity,” Woolway said. “If you were to cover the surface 90 percent with solar panels, there would be no light going into the water itself.”
This is where the new science of floatovoltaics gets tricky, as there’s still little data on the potential environmental and social downsides of these floating systems. Scientists are investigating, for example, whether the floats might leach harmful chemicals or microplastics into the water.
And keep in mind that these ecosystems are solar-powered, too: Light fuels the growth of aquatic vegetation, which feeds all kinds of other organisms. If a floatovoltaic system cuts off too much of that light, it might reduce the food supply, and hinder plants’ ability to produce oxygen. “You’re changing light penetration, and that’s the most fundamental physical variable for an aquatic ecosystem,” said Rafael Almeida, a freshwater ecosystem scientist at the University of Texas Rio Grande Valley, who studies floatovoltaics but wasn’t involved in the new study. “If you don’t have enough light, and you’re reducing oxygen concentrations in that system, and that may cascade through the food web, potentially impacting fish.” At the same time, early research suggests that the panels can counter the growth of harmful algal blooms that make water dangerous for people to drink.
Scientists are still trying to figure out what amount of coverage can still produce enough power to justify the monetary cost of deploying floatovoltaics, while not incurring ecological costs. Each body of water is its own unique universe of chemical and biological interactions, so the same coverage on two different lakes might have dramatically different effects. “Would 10 percent be enough to cascade into system-wide changes?” asks Almeida. “These are things that we really don’t know.”
Researchers also need more data on how effective the panels are at reducing evaporation, and therefore how much water a given system might actually save. “What we are yet to fully understand is that so many of the existing floating solar systems that have tried to collect data on this have been smaller,” Gadzanku said. “So it is more: How do potential evaporation savings scale as you build larger systems?”
Humans rely on bodies of water in many ways other than for drinking. Subsistence fishers rely on them for food. And owners of lakefront properties might bristle if they think floating solar panels would cut their property values.
Still, Almeida says, this new research identifies where floatovoltaics might work, and how much energy they might provide given local conditions. “I think that now what we need,” said Almeida, “is understanding — out of these suitable sites — which ones are really the low-hanging fruits.”
