Shooting sound waves through water can remove dissolved gas that results from hydropower production in rivers, avoiding harm to fish, according to scientists at the Norwegian University of Science and Technology (NTNU).
Gas supersaturation occurs when air enters water-filled hydropower inlets and is then exposed to high pressure. When this water is released into the river below the plant, it’s 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 no requirements to monitor and limit gas supersaturation in rivers downstream of hydropower plants in Norway, but studies have shown that this may be a problem in many more power plants than previously thought. If requirements are introduced to avert gas supersaturation, this solution could help power companies avoid costly shutdowns of their plants when the problem occurs – as well as improve the environment.
An ultrasound “speaker”
The scientists have conducted ultrasound trials in a specially built water channel in NTNU’s Waterpower Laboratory.
The technical solution is a type of speaker (called a transducer) that creates ultrasound. It creates pressure waves in the water that cause dissolved gas molecules to accumulate and form bubbles (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.
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, and being able to discuss various solutions along the way with experts from the industry has been essential,” said Kuhn.
“At first we thought we could place the ultrasound technology inside the suction pipe at the hydropower plant. This was not well-received by the industry, because it 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 hydropower plant outlet.”
Large-scale testing
The scientists are 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 1 million liters of water per second. Laboratory trials show that it is not a good idea to make a giant version of the transducer but that it would be more beneficial to use several smaller installations that collectively can do the job.
“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.”
The DeGas project is linked to the HydroCen research center.
The project partners are NTNU, Sintef, NINA, NORCE, EDRMedeso, Fornybar Norway, Hafslund Eco, Statkraft, Eviny, Otra Kraft and Troms Kraft.
