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W. Ludwig Kuhn has conducted trials using ultrasound in a specially built water channel in NTNU’s Waterpower Laboratory. To the right, master’s student Vera Gütle.

Ultrasound can save fish in hydropower rivers

Shooting sound waves through water can remove dissolved gas from hydropower production in rivers. 

The phenomenon of gas supersaturation occurs when air enters water-filled hydropower inlets and is then exposed to high pressure. 

When this water is subsequently released into the river below the hydropower plant, it is almost like opening a bottle of champagne, filling the river with bubbles.

There is so much air in the water that it can harm fish and other species. In the worst case, fish can die from gas bubble disease – a disease similar to decompression sickness in humans.

There are currently no requirements in place to monitor and limit gas supersaturation in rivers that are downstream of hydropower plants in Norway. However, studies have shown that this may be a problem in many more power plants than previously thought.

If requirements are introduced to avert the problem of gas supersaturation, this solution could help power companies avoid costly shutdowns of their plants when the problem occurs – as well as improve the environment.

Gas bubble disease in the tail fin of a salmon.

An ultrasound ‘speaker’

The researchers have conducted ultrasound trials in a specially built water channel in NTNU’s Waterpower Laboratory.

The technical solution is a type of speaker that creates ultrasound. It creates pressure waves in the water that cause dissolved gas molecules to accumulate and form bubbles. This phenomenon is called acoustic cavitation.

The bubbles join up, become larger, and rise to the surface. The method has been tested in the small and medium-sized laboratory, and W. Ludwig Kuhn shows in his doctoral thesis that the method immediately reduces gas saturation.

The salmon river Ekso, which clearly has gas-supersaturated water.

Collaboration has led to useful results

Collaboration with the energy industry and biologists has been important in implementing the project. Natural scientists have contributed to important knowledge about the consequences and extent of gas supersaturation. It has been necessary to discuss various solutions along the way with experts from the industry.

"At first, we thought we could place the ultrasound technology inside the suction pipe of the hydropower plant,” says Kuhn.

This was not well received by the industry, as there was too great a risk to install the equipment so close to the turbine. They feared it might affect the flow of water and thus affect other parts of the hydropower plant.

“Therefore, the conclusion is that the equipment should be placed in the river at the outlet of the hydropower plant,” he says.

This video shows how researchers created microbubbles in a specially built water channel at NTNU’s Waterpower Laboratory. It shows how sound creates pressure waves in the water that cause the bubbles to accumulate, grow larger, and rise to the surface. (Video: Juliet Landrø / HydroCen)

Large-scale testing in hydropower rivers

The researchers are now planning to conduct field trials to investigate the extent to which this technology can be used in the hydropower industry. Hydropower inlets are enormously large, transporting as much as one million litres of water per second.

The laboratory trials show that it is not a good idea to make a giant version of the ultrasound 'speaker’ (actually called a transducer), but that it would be more beneficial to use several smaller installations that collectively can do the job. When this is tested downstream of the hydropower plant, it will still be a major technical installation.

“A lot of equipment from different suppliers needs to be tested to ensure optimal functionality,” says Kuhn.

The ultrasound 'speaker’ creates pressure waves in the water that cause dissolved gas molecules to join up and form bubbles that rise to the surface.

Great potential

So far, the results in the DeGas project have exceeded all expectations. 

"The benefits to the hydropower industry have much greater potential than we thought at the outset,” says supervisor and project manager Ole Gunnar Dahlhaug. 

He is a professor at NTNU’s Department of Energy and Process Engineering.

“The method is efficient and is likely to have a relatively low cost in terms of installation, operation, and maintenance,” he says.

Collaborators have flocked to look at the trials conducted by W. Ludwig Kuhn in the DeGas project, which is linked to the HydroCen research centre.

“It has been fun to show the trials to our research and industry partners. Even the Minister of Energy Terje Aasland has stopped by. It's very useful to get input from many different sides,” says Kuhn. 

The project partners are: NTNU, Sintef, NINA, NORCE, EDRMedeso, Fornybar Norway, Hafslund Eco, Statkraft, Eviny, Otra Kraft, and Troms Kraft.


Kuhn et al. A potentially deadly problem gains attention due to climate changeHydro, 2023.

Kuhn et al. Evaluating natural degassing in a river to create a baseline for comparison to technical degassing methodsJournal of Physics: Conference Series, 2023. DOI: 10.1088/1742-6596/2629/1/012032

Li et al. Production of total dissolved gas supersaturation at hydropower facilities and its transport: A review,Water Research, vol. 223, 2022. DOI: 10.1016/j.watres.2022.119012

Pulg et al. Gassovermetning i vassdrag – en kunnskapsoppsummering (Gas supersaturation in draining systems - a summary of knowledge), NORCE LFI, 2018.

Sæle, A.G. Artificial gas supersaturated water from small hydropower plants: methods to detect air entrainment at intakesMaster thesis, University of Bergen, 2022.


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