Reducing ice accumulation on Coanda Screen intakes

Abstract

Coanda screen intakes has proved to be a reliable and cost-effective low maintenance solution for numerous small hydro (SHP) intakes in the past two decades. In Norway 50 intakes are installed since 2012, the first intake was at Dyrkorn SHP in western Norway. The concept is based on a fine meshed screen with transverse bars with a spacing from 0,5 – 3 mm, most of installed intakes have a space of 1 mm. The narrow spacing prevent most kind of floating debris to approach the real intake which is placed downstream of the screen. Due to the steep inclination of the screen all floating debris like leaves, grass, branches, ice floes etc will slide on the screen and be deposited on the downstream side of the screen structure. Sediments will be trapped or flushed over the intake screens like debris. International experience confirms that frazil ice accumulates on the screen itself in various different physical processes. Depending on the details of the screen ice accumulate in different ways and may cause blocking of the intake. This paper describes first the freezing process progress at the intake and how the ice by natural processes release from the intake after some time. Several winters with monitoring at the Dyrkorn intake give unique operational data from the ice accumulation and consequences for the hydropower production. Secondly, this paper describes how a general modification and two new innovations have reduced the ice accumulation on the screen. The innovations concentrate the flow on the intake in periods with low discharge, but the effect from siphoning water from the bottom of the pool may have even more positive effect to the reduced ice problems at the intake. In the end limitations for Coanda screen intakes will be discussed based on extreme climate conditions.