In the Pacific Northwest, salmon don’t stay still for long. They are born in fresh water, take off downstream to mature in the ocean and then, when they’re old enough, they head back upriver to spawn in their birthplace, starting the cycle again. Most people have seen pictures of bears snatching returning salmon out of the water as they leap back upriver, but the fish face obstacles on their way down, as well. Navigating hydropower dams is a big one.
Even if they manage to avoid the obstacle of the turbine blades, the turbulence leaves fish stunned and vulnerable to predators when they emerge at the end of the power plant. In hopes of giving fish an easier ride, engineers are working with GE Renewable Energy to build more fish-friendly hydro turbines in the Columbia River, which separates Oregon and Washington.
The high-tech project — which marries computer modeling with the latest turbine technology — could become a blueprint for hydropower plants around the globe. “We use a digital model, based on computer-fluid dynamics, to simulate the water flow through the turbines,” says Kristopher Toussaint, a hydraulic engineer with GE Renewable Energy. “We also have something that simulates a physical particle representing the fish to see, if it impacts components, what pressure that particle is exposed to, and so on.”
If you’re not a fish, hydro plants might look benign enough: They work by harnessing the force and pressure of water flowing from a high point to a lower point through chutes in a dam. Near the bottom of the chute sits a turbine. The water spins the turbine, which creates power that can be transmitted to businesses and homes. But this is also the main passage for the migrating fish.
Using the digital model, the team discovered that fish are particularly vulnerable to injuries when they pass through a first grid of stationary vanes, then another grid of movable vanes and finally through the blade channels of a runner in rotation. If they survive from the risk of strikes, they still have to bear the rapid pressure drop that occurs inside the runner. (See illustration below.)
To lessen the risk of strikes, one strategy is to align the two sets of vanes and shrink the gaps between rotating parts and stationary parts. Using modeling technology, engineers were able to figure out the optimal vane geometry to minimize the risk to the fish while keeping a smooth hydraulic passage for the flow of water. “Reducing the risk here for the fish is not detrimental to turbine efficiency,” says Laurent Bornard, a hydraulic consulting engineer at GE Renewable Energy.
To reduce the pressure drop in the runner, several strategies are usually possible, including changing the number of blades or the runner diameter to reduce the flow velocity and then increase the pressure.
But even if a salmon makes it past the vanes and the runner, its troubles aren’t over. To reach the river downstream of the power plant, it still has to go through the diffuser. “The flow deceleration inside the diffuser can create whirlpools and flow detachment that spin the fish around, disorienting them and making them easy prey for predators outside the power plant,” Bornard says. In order to prevent fish injury, the engineers typically optimize the flow characteristics delivered by the runner and seek, in some extreme cases, to modify the diffuser geometry to reduce the flow turbulence and make the fish passage as smooth as possible.
As a result of the design changes, engineers expect the survival rate of salmon to improve. And the further potential for the fish-friendly technology is, well, dizzying. “There are close to 10 power plants on the Snake and Columbia rivers that are especially concerned by fish passages during the salmon migration,” Bornard says.
GE is also working to make hydro turbines along the Mekong River in Southeast Asia safer for fish that pass through it.