Coanda Intake

To avoid unwanted matter entering the penstock; debris (such as stones and branches), and living organisms (such as fish and eels), and being transported to the turbine it is necessary to use some form of grating or filter in front of the opening to the penstock.

Traditionally a grating has been placed under the surface of the water in front of the inlet to the penstock.

This is very simple as a starting point, but it has many disadvantages when the plant is operating. It is quite easy for the grating to get blocked by leaves and other debris, and regular attention is required to maintain production of electricity.

Another solution that has been developed for use with hydropower plants are Coanda screens. These screens are installed such that the water runs over the crest of the dam wall, and onto a screen, with fine openings, that ‘shaves’ off thin layers of water from the main stream of water. Any debris in the water is left on top of the screen and is continuously washed away by excess water. Any animal life continues on its way down the river without being drawn through the screen and into the penstock and onwards to the turbine.

The system was first used in the USA several decades ago, and first used in Norway in 2011 at Dyrkorn with screens supplied by Brødrene Dahl. The concept is proven to work very well.

The change from water running through, rather than over, the screen has numerous advantageous. It benefits the life of the river, the system is almost maintenance-free, it is kind to the environment and delivers excellent electricity production including when the river is frozen over.

Function

When water runs over the crest of the dam, it accelerates over a smooth curved plate before it crosses a screen of wires with (typically) 1.5mm wide openings between them.
Each wire in the screen shaves off a thin layer of water. All other material in the water is left trapped on top of the wires and washed down the river with the rest of the flow. If there is low flow, debris left on the screen, is washed off the next time there is sufficient discharge over the dam.

Many small streams make a large river. A screen measuring 1m x 1m can admit a flow of over 200 litres per second. To achieve higher capacity, several screens are mounted beside each other.

These screens perform all year round whenever there is water in the river. In the winter a crust of ice forms over the screen and water continues to flow through the screen underneath the ice. Experience suggests that actual electrical production is almost 100% of the theoretical maximum.

Disadvantages

The amount of water running over the dam is dependent on the patterns of precipitation. There are times when the flow in the river is greater than the capacity of the turbine, and the excess water runs past the intake and is ‘lost’ from production. With a traditional intake it is sometimes possible to regulate the water level behind the dam and save excess rainfall in a reservoir for production later. However, this is not necessarily good for the ecology of the river, with the water level in the reservoir changing significantly. With a Coanda intake it is more difficult to dam up the water for regulating, although it is still possible to achieve on a small scale. Such arrangements need to be approved by the river authorities.

Algae can grow on the screens in areas where there is farmland above the intake. This is easily removed manually with a brush. This maintenance can be undertaken during periods of low flow in the river.

If there is significant erosion of stone in the catchment area and there is much sand and silt in the water, the edges of the wires on the screens can get worn in the course of many years. This has been experienced in alpine regions in southern Europe, but in Norway the geology is such that this is not a significant problem.

The way the dam is built up such that the water runs over the crest and through the screens, means that the water running into the penstock has approximately 1.2m less head of pressure than the water behind the dam. For a hydroelectric plant with a large head of water this difference is negligible. For lower pressure projects lower screens can be used to reduce this loss.