The most important freshwater source in central North Carolina, the Yadkin River basin is home to some 38 rare species of fish and mollusks, like the Savannah lilliput and Carolina heelsplitter. The river also supports a series of hydroelectric dams that power more than 85,000 homes. While hydroelectric power is a bountiful source of clean energy, certain sites require upgrades to address low oxygen levels needed for a healthy ecosystem. Studies have shown that existing dams installed for energy generation, recreation, flood control and other key functions can deprive water of oxygen at most of its depth in certain conditions during the summer. This can lead to oxygen-free dead zones that can’t support fish and plant life deeper than a few feet below the surface.
But that story is now changing. One example is the first-of-a-kind installation of a new generation of GE Renewable Energy turbines by Cube Hydro Partners, which owns and operates the Yadkin hydro plants, among dozens of other facilities. Cube is in the midst of replacing three turbines installed decades ago by a former owner at its High Rock dam, the farthest upriver of its four Yadkin facilities. The GE machine, called the dissolved oxygen aerating turbine, is designed to feed in oxygen from a tube at an optimal point among its blades to reinvigorate the oxygen levels in the water as it flows downstream. Cube expects to complete the replacement of all three turbines at High Rock by September next year.
“The whole river downstream should benefit from this,” says Neal Simmons, vice president of Cube. With one turbine already installed this spring, the benefit has been immediate. The Yadkin River where Cube operates met state oxygen-level standards more than 95% of the time this summer, up from 57% with the original turbines. People who enjoy the river claim they’re seeing benefits, too, Simmons adds. “Anecdotally, fishermen say it’s some of the best fishing there’s been in years.”
Hydroelectric power is one of the world’s most utilized clean power sources, with 1,292 gigawatts (GW) installed worldwide, slightly more than the total generating capacity of the United States. But during the summer, water circulation in dammed water declines and the deeper levels of lakes can become starved of oxygen. As plants on the bottom essentially suffocate, the decay creates carbon dioxide that bubbles to the surface, reinforcing the reduction of oxygen. The problem can spread downstream when oxygen-deprived water passes through the hydro plant and pours into the river below.
The problem has come under increased scrutiny from regulators who, in states like North Carolina, now require hydro facilities to improve water oxygen levels for recertification. As a stopgap measure, some hydro producers pump oxygen directly from tanks into the water. “A typical plant requires a tractor-trailer load of oxygen a day,” Simmons says. “It’s extraordinarily expensive.”
The best solution would be to feed oxygen into the water while producing electricity — but to date, that’s a solution that has eluded turbine makers. Engineers tried, feeding turbines with oxygen harvested from the surface. But bubbles produced this way can be too large, so they float to the surface without dissolving in the deep water. To makes things worse, sending oxygen behind the turbine blades can reduce turbine efficiency by 10% or more.
But GE engineers designed an aerating turbine that sends smaller bubbles of dissolved oxygen into the water through naturally occurring low-pressure points in the turbine. By improving blade design and pumping the oxygen in front of the blade, GE significantly increases turbine efficiency. Simmons says aeration lowers the turbine’s efficiency by about 5% — making it still more efficient than the turbines Cube is replacing.
For low-demand periods when the turbines are not spinning and High Rock isn’t generating electricity, Cube has designed and installed its own proprietary system that can step in and oxygenate the Yadkin, too.
Cube is the first company to purchase and install the new aerating turbines. But more companies are looking at the technology, especially in the southeastern U.S., where hot summers and extensive damming make oxygen-poor water a widespread challenge.
“It has a very good environmental impact, and it’s a huge, huge win for us as an independent power producer,” Simmons adds. “This allows us to generate several thousands more megawatt-hours every year with the same amount of water.”