Performance of Coanda-Effect Screens in a Cold Climate

This study assesses the performance of a Coanda-effect intake screen for a small hydropower plant during winter operations. The unique self-cleaning screen utilizes the Coanda-effect for withdrawing water. This screen consists of panels with 2-mm-wide horizontal wedge wires with 1mm spacing. Beneath the screen is a canal conveying water to the power plant intake. A broad understanding of the Coanda-effect screen's winter performance in a cold climate was obtained from experiments in a frost laboratory and a monitoring program consisting of hydrological measurements, an online web camera, and field observations of an intake structure in Norway. Various freezing processes were observed, and critical hydrological conditions for ice blockage of the Coanda-effect screen were evaluated. The Coanda-effect screen performed well under all normal conditions without snow or ice and was entirely self-cleaning. Two main types of ice formation were observed on the screen: type I where frazil ice particles stick to the screen wires but with openings remaining underneath the ice, and type II where solid ice forms between the wires at very low temperatures. During the winter of 2011-2012, there were a few periods with reduced intake capacity or complete blockage from ice. The complete ice blockage events were all related to supercooled water just upstream from the screen, thus indicating frazil ice and intake screen and air temperatures below 0 degrees C. The screen reopened without any operational interventions after all ice blockage events, and the intake performed well even during periods with solid ice cover over the screen, whereby water entered the intake through the screen wires underneath the ice cover. In spite of some periods with reduced intake capacity, the intake screen performed reasonably well in a cold climate.