Low-Energy Mixing in Algal Culture Raceways

Abstract

A project in Honolulu to use marine algae mass for energy has demonstrated that the use of angled plates in shallow (100–150 mm) raceways, where cultures of marine algae are grown, significantly increased algae production. The reasons for higher productivity are not fully understood, but the increased production rates were clearly demonstrated. Accepting the premise that angled plates in shallow raceways significantly increased algal production, the research objectives of a complementary project described here were to study the mixing relationships in shallow (100 mm) water flows with low velocity (∼0.3 m/s) that are created by variously shaped and positioned submerged plates and to identify the optimum energy efficiency. Based on six test runs with combinations of three to five 100 mm square and triangular plates set at 10° and 20° angles to the water flow, the velocities directly behind the test plates were consistently higher than those in between for up to 4.5 m downstream (thus creating desired mixing conditions); the 20° angled plates created a notably higher degree of mixing than those at 10°; the use of three 100 mm plates (particularly the square-shaped ones) provided adequate mixing in comparison to five plates; the energy loss between the plate section and a section 4.5 m downstream was essentially negligible; and the drag force on the plates was relatively insignificant regardless of the plate shape or angle of attact.