LOW-ENERGY MIXING IN ALGAL CULTURE RACEWAYS

A project in Honolulu to use marine algae mass for energy has demonstr ated that the use of angled plates in shallow (100-150 mm) raceways, w here cultures of marine algae are grown, significantly increased algae production. The reasons for higher productivity are not fully underst ood, but the increased production rates were clearly demonstrated. Acc epting the premise that angled plates in shallow raceways significantl y increased algal production, the research objectives of a complementa ry project described here were to study the mixing relationships in sh allow (100 mm) water flows with low velocity (similar to 0.3 m/s) that are created by variously shaped and positioned submerged plates and t o 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 degrees and 20 degrees 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 cond itions); the 20 degrees angled plates created a notably higher degree of mixing than those at 10 degrees; the use of three 100 mm plates (pa rticularly the square-shaped ones) provided adequate mixing in compari son to five plates; the energy loss between the plate section and a se ction 4.5 m downstream was essentially negligible; and the drag force on the plates was relatively insignificant regardless of the plate sha pe or angle of attact.