With wind and solar prices dropping, it can be easy to forget that two-thirds of the globe's renewable energy comes from hydropower. But hydro's future is muddled with ghostly costs—and sometimes dubious environmental consequences.
Most dams weren't, in fact, actually built for hydropower. They help stop floods and supply farms and families with water, but they're not generating electricity—especially in developing countries. Nearly half of Europe's large dams are primarily used for hydropower, but fewer than a sixth of Asian dams and a tenth of African dams generate substantial amounts of electricity—according to Tor Haakon Bakken, a civil engineer at the Norwegian University of Science and Technology (NTNU).
People like Bakken see such dams as opportunities. He's one of a few researchers proposing to retrofit old, non-generating dams by installing turbines at their bases. That, he thinks, would create electricity without adding an ecological burden.
Bakken's group and one of his graduate students, Nora Rydland Fjøsne, modeled theoretically doing just that for numerous dams in a part of southern Spain. Their study found that, in many cases, retrofitting was an economically viable approach.
Bakken hopes that hydro developers consider retrofitting before building anew. “I think, for the case in Spain, we have proven that this is both a technically and economically viable alternative," he says. “And I think it's the case in many other places too."
Even power-generating dams could also be productively retrofitted. The Brazilian Energy Research Office estimates that updating aging hydro plants could add 3 to 11 gigawatts of generating capacity—above and beyond Brazil's existing 87 GW hydropower base. Meanwhile, scientists at the National Renewable Energy Laboratory (NREL) in the US have proposed using reservoirs as beds for floating solar panels, something they think could theoretically generate terawatts of power.
But if you do need to build new hydropower facilities, other scientists believe the best course of action is to stay small: focus on establishing so-called “run-of-the-river" dams, which try to keep the river and its environmental conditions intact.
Craig and some of his colleagues from NREL and the private-sector Natel Energy modeled a sequence of run-of-the-river dams on a river in California, all linked such that they could be easily controlled. They found that this approach was, like retrofitting dams, economically viable.
Righting the course of old hydro
Hydropower's behemoth dams of old can singlehandedly rewrite the courses of rivers, creating winding reservoirs in their wake. They can prevent floods and supply water, but they can also displace countless communities upstream and constrict river flow downstream. Such dams disproportionately hurt rural and indigenous people who rely on rivers for a living.
Moreover, some hydro plants generate alarming amounts of greenhouse gases. The culprit, scientists now know, are those very reservoirs—and the biological material trapped underneath. “You're basically flooding a whole area that has all this vegetation on it that now is just decomposing underwater," says Ocko.
The result? Greenhouse gases. On top of carbon dioxide, it can generate methane, which—while not as long-lasting in the atmosphere—is more potent at warming. Reservoirs that are larger in surface area, and reservoirs that have warmer waters—such as those near the equator—are especially prone to burping up copious amounts of methane.
Take the Brazilian Amazon, for instance. “Over the last decades, basically all the potential for hydropower expansion that Brazil had close to consuming markets has been exhausted, and Amazonia became the new frontier," says Dailson José Bertassoli, Jr., a geochemist at the University of São Paulo.
But many dam reservoirs in the Amazon are comparable to fossil fuel plants in their capacity to generate greenhouse gases. So too are their counterparts in Western Africa. In fact, one Environmental Defense Fund study found that nearly seven percent of the 1500 hydro plants around the world they examined emit more greenhouse gases per unit energy than fossil fuel plants.
That's ample reason to eschew building new dams and instead look to what we have. “In terms of environmental impacts, it makes sense to focus on areas that already are not in their natural conditions," says Bertassoli.
So, in the face of its mounting environmental costs, does hydro have a future?
Yes, say climate scientists, but with a caveat. The key is to minimize the future of those large, greenhouse-gas-excreting reservoirs.
“I would never say that we should stay away from hydropower. From a climate perspective, I think we need all the solutions we can get." says Ocko. But, she adds, “We can't make an assumption and put it into this bucket of being a renewable energy source just like solar and wind. It's not."
This article appears in the September 2021 print issue as "Righting Hydropower's Course."